{"gene":"ALAS2","run_date":"2026-06-09T22:02:43","timeline":{"discoveries":[{"year":2000,"finding":"ALAS2 (ALAS-E) physically interacts with the beta subunit of ATP-specific succinyl-CoA synthetase (SCS-betaA) in mitochondria. Yeast two-hybrid screening of a human bone marrow cDNA library identified SCS-betaA as an ALAS-E binding partner, confirmed by transient expression and co-immunoprecipitation. The interaction is isoform-specific (ALAS-E but not ALAS-N) and is disrupted by the D190V mutation associated with pyridoxine-refractory sideroblastic anemia, suggesting the complex promotes efficient succinyl-CoA utilization or mitochondrial translocation of ALAS-E.","method":"Yeast two-hybrid screen, co-immunoprecipitation with transient expression","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal yeast two-hybrid plus co-immunoprecipitation in two independent experimental systems; isoform specificity and mutation disruption provide orthogonal validation","pmids":["10727444"],"is_preprint":false},{"year":2012,"finding":"C-terminal residues of ALAS2 are required for binding to SUCLA2 (beta subunit of succinyl-CoA synthetase). XLSA mutations p.Met567Val and p.Ser568Gly, as well as a p.Phe557Ter truncation, have normal or enhanced in vitro enzymatic activity and stability but fail to bind a SUCLA2 affinity column, establishing that the C-terminal region mediates SUCLA2 interaction and that loss of this interaction causes disease. Conversely, gain-of-function XLP mutant p.Met567GlufsX2 retains SUCLA2 binding, linking SUCLA2 association to in vivo ALAS2 activity regulation. Additionally, XLSA mutations p.Arg452Cys and p.Arg452His had normal SUCLA2 binding but showed loss of positive cooperativity for succinyl-CoA, increased Km for succinyl-CoA, and reduced pyridoxal 5'-phosphate affinity.","method":"SUCLA2 affinity column binding assay, enzyme kinetics, in vitro expression and purification of recombinant proteins","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro biochemical reconstitution with affinity chromatography, kinetic assays, and multiple mutant variants tested; mechanistically definitive","pmids":["22740690"],"is_preprint":false},{"year":2008,"finding":"C-terminal frameshift deletions in ALAS2 (c.1706-1709 delAGTG and c.1699-1700 delAT) cause gain-of-function by markedly increasing ALAS2 enzymatic activity. Prokaryotic expression of both mutant proteins demonstrated substantially elevated activity compared to wild-type, establishing that the 19-20 C-terminal residues normally suppress enzyme activity and that their removal or replacement leads to protoporphyrin overproduction causing X-linked dominant protoporphyria.","method":"Prokaryotic expression of mutant proteins, in vitro enzyme activity assay","journal":"American journal of human genetics","confidence":"High","confidence_rationale":"Tier 1 / Strong — direct in vitro enzymatic assay of expressed mutant proteins with multiple independent family mutations showing same gain-of-function result","pmids":["18760763"],"is_preprint":false},{"year":1994,"finding":"The F165L missense mutation in ALAS2, found in Cooley's original XLSA family, reduces enzymatic specific activity to ~26% of normal after prokaryotic expression and affinity purification. Pyridoxal 5'-phosphate (PLP) activates and/or stabilizes the purified F165L mutant enzyme in vitro, consistent with pyridoxine responsiveness in vivo. The mutation resides in a highly conserved domain of the ALAS2 catalytic core.","method":"Prokaryotic expression, affinity purification, in vitro enzyme activity assay, PLP stabilization assay","journal":"Blood","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstituted recombinant enzyme assayed with and without cofactor; direct mechanistic link to pyridoxine responsiveness established","pmids":["7949148"],"is_preprint":false},{"year":1995,"finding":"Missense mutations K299Q and A172T in ALAS2 cause thermolabile enzyme with decreased in vitro stability. Addition of pyridoxal 5'-phosphate in vitro stabilizes both mutant recombinant enzymes, consistent with the dramatic clinical response to pyridoxine observed in patients. The A172T mutation also resulted in decreased bone marrow delta-aminolevulinate synthase activity.","method":"In vitro recombinant enzyme expression, thermostability assay, PLP stabilization assay","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 1 / Strong — recombinant enzyme biochemistry with multiple orthogonal measures (stability, activity, cofactor response) in two independent mutations","pmids":["7560104"],"is_preprint":false},{"year":2019,"finding":"Systematic in vitro characterization of ALAS2 C-terminal truncation mutants (p.P561X, p.V562X, p.H563X, p.E569X, p.F575X) showed 1.4- to 5.6-fold increases in Vmax for both succinyl-CoA and glycine substrates with only modest Km changes, establishing that the C-terminal region normally constrains enzyme activity. Thermostabilities of truncated mutants were significantly lower than wild-type with an inverse relationship to Vmax fold-increase, suggesting that increased molecular flexibility/active site openness is the mechanism of gain-of-function. Five ALAS2 SNVs (p.R559H, p.E565D, p.R572C, p.S573F, p.Y586F) showed modest 1.3- to 1.9-fold Vmax increases.","method":"Site-directed mutagenesis, prokaryotic expression, purification, enzyme kinetic assays, thermostability assays","journal":"Molecular medicine (Cambridge, Mass.)","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstitution with multiple mutant variants, kinetic and stability measurements providing mechanistic model","pmids":["30678654"],"is_preprint":false},{"year":2011,"finding":"The ALAS2 Y586F gain-of-function mutation (c.1757 A>T in exon 11), affecting the penultimate C-terminal amino acid, significantly increases the rate of 5-aminolevulinate release compared to wild-type ALAS2, establishing that C-terminal residues regulate product release kinetics.","method":"In vitro enzyme activity assay of recombinant expressed mutant protein","journal":"Blood","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — direct enzymatic assay but single mutation in a single study","pmids":["21653323"],"is_preprint":false},{"year":2013,"finding":"A 130-bp erythroid-specific enhancer in intron 1 of ALAS2 contains a GATA1-binding cis-element that is essential for erythroid expression. GATA1 binding to this element was confirmed by chromatin immunoprecipitation in vivo and EMSA in vitro; the enhancer increased ALAS2 promoter activity 10-15-fold in K562 cells. Loss-of-function mutations at the GATA-binding site abolish GATA1 binding and enhancer activity, causing congenital sideroblastic anemia.","method":"Chromatin immunoprecipitation (ChIP), EMSA, luciferase reporter assay in K562 cells","journal":"Haematologica","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (ChIP in vivo, EMSA in vitro, reporter assay) consistently demonstrating GATA1-dependent enhancer function","pmids":["23935018"],"is_preprint":false},{"year":2016,"finding":"The intron 1 GATA site (int-1-GATA) of ALAS2 is indispensable for erythroid expression in vivo. Mice with a 13-bp deletion including this site showed embryonic lethality due to severe anemia from absent ALAS2 expression. The int-1-GATA site forms a long-range chromatin loop anchoring the intron 8 GATA site and the proximal promoter, with an enhancer complex including GATA1, TAL1, LMO2, LDB1 and Pol II. CRISPR/Cas9-mediated deletion confirmed the int-1-GATA site is more essential than the int-8-GATA site.","method":"Transgenic mouse deletion, CRISPR/Cas9 site-specific deletion, chromatin loop analysis, ChIP","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo mouse deletion phenotype plus CRISPR validation plus chromatin loop/complex characterization in one study","pmids":["28123038"],"is_preprint":false},{"year":2003,"finding":"A C-to-G transversion at nucleotide -206 of the ALAS2 proximal promoter reduces reporter activity by 94% in K562 erythroid cells and decreases ALAS2 mRNA by 87% in patient erythroid precursors, identifying the proximal promoter region as an important erythroid regulatory element. The mutation disrupts putative transcription factor binding sites.","method":"Luciferase reporter assay in K562 cells, RT-PCR quantification of mRNA in patient erythroid precursors","journal":"Blood","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reporter assay plus mRNA quantification in patient cells, two orthogonal methods in single lab","pmids":["12663458"],"is_preprint":false},{"year":2002,"finding":"ALAS2 deficiency in definitive erythroblasts (from Alas2-null ES cells differentiated in culture) results in 15-fold excess non-heme iron accumulation localized to the cytoplasm (not mitochondria) and increased lipid peroxidation, without interfering with erythroid cell development morphology or erythroid gene expression. This establishes that ALAS2 activity is required to prevent cytoplasmic iron overload and oxidative stress in erythroblasts.","method":"Alas2-null embryonic stem cell differentiation, electron microscopy for iron localization, iron quantification, lipid peroxidation assay","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic null model with multiple orthogonal readouts (quantitative iron, EM localization, oxidative stress) establishing causal role","pmids":["12393610"],"is_preprint":false},{"year":1998,"finding":"The R411C missense mutation in ALAS2 reduces enzyme activity to 12% of wild-type in the absence of PLP and 25% in the presence of PLP, as determined by prokaryotic expression and purification. This establishes the mechanistic basis for pyridoxine responsiveness at this residue.","method":"Prokaryotic expression, enzyme purification, in vitro activity assay with and without PLP","journal":"British journal of haematology","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — direct enzymatic assay but single mutation, single lab study","pmids":["9858242"],"is_preprint":false},{"year":2011,"finding":"Ten ALAS2 missense mutations expressed in E. coli showed that five caused decreased enzymatic activity under standard conditions, and two had normal activity under standard conditions but showed reduced activity without exogenous PLP and increased thermosensitivity, revealing that some XLSA mutations affect PLP-dependent enzyme stability rather than direct catalytic mechanism.","method":"Prokaryotic expression in E. coli, in vitro ALAS2 enzymatic activity assay, thermosensitivity assay","journal":"Human mutation","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — systematic enzymatic characterization of 10 mutations with multiple conditions in one study","pmids":["21309041"],"is_preprint":false},{"year":2015,"finding":"The ALAS2 Y365C mutation impairs binding of the essential cofactor pyridoxal 5'-phosphate, resulting in enzyme destabilization and loss of function. In erythroid cells and reticulocytes from affected females, complete skewing toward expression of the WT allele (but not in leukocytes) demonstrates cell-nonautonomous effects on erythropoiesis through apoptosis of erythroid precursors expressing the mutant allele.","method":"In vitro PLP binding assay, X-inactivation analysis in reticulocytes vs. leukocytes, primary erythroid cultures","journal":"The Journal of clinical investigation","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — biochemical PLP binding assay plus cellular erythroid differentiation studies with X-inactivation readout","pmids":["25705881"],"is_preprint":false},{"year":2024,"finding":"ALAS2 C-terminal loss-of-function variants V562A and M567I do not cause gross structural perturbations but V562A has decreased enzyme stability. PLP addition moderately increases stability of both variants. V562A shows enhanced in vitro activity but reduced succinyl-CoA binding efficiency, while M567I significantly alters cooperativity of substrate binding, establishing that C-terminal mutations can cause disease through distinct kinetic mechanisms.","method":"In vitro enzyme characterization, stability assays, PLP binding, substrate kinetics, structural modeling","journal":"Biochemistry","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — rigorous in vitro biochemical characterization but single lab, no replication","pmids":["38888931"],"is_preprint":false},{"year":2015,"finding":"miR-218 directly targets and represses ALAS2 expression by binding to the 3'-UTR of ALAS2 mRNA, as shown by 3'-UTR reporter assays. miR-218 overexpression in K562 cells phenocopies ALAS2 knockdown, inhibiting erythroid differentiation and altering iron metabolism.","method":"3'-UTR luciferase reporter assay, miRNA overexpression, ALAS2 knockdown in K562 cells","journal":"International journal of molecular sciences","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — 3'-UTR reporter assay plus phenotypic validation by knockdown, two orthogonal methods","pmids":["26703568"],"is_preprint":false},{"year":2017,"finding":"A novel g.55040074delT deletion in exon 10 of ALAS2, predicted to generate a monomeric protein rather than the wild-type homodimer based on Swiss-model structural prediction, causes severe sideroblastic anemia, establishing that ALAS2 homodimerization is required for function.","method":"Sanger sequencing, SWISS-model protein structure prediction","journal":"Journal of pediatric hematology/oncology","confidence":"Low","confidence_rationale":"Tier 4 / Weak — structural inference from computational modeling only, no experimental validation of dimerization state","pmids":["28731922"],"is_preprint":false},{"year":2020,"finding":"ALAS2 knockdown in K562 cells leads to downregulation of BNIP3L (a mitophagy receptor) at both mRNA and protein levels, impaired erythroid differentiation (reduced CD71/CD235a markers after hemin induction), decreased ROS levels, and increased mitochondrial membrane potential. No direct physical interaction between ALAS2 and BNIP3L was detected by co-immunoprecipitation. The regulatory axis appears to work through intracellular heme levels affecting GATA1 and Nrf2 transcription factors.","method":"Lentiviral shRNA knockdown, qRT-PCR, Western blot, flow cytometry (erythroid differentiation markers, mitochondrial membrane potential, ROS), co-immunoprecipitation (negative result for direct binding)","journal":"Zhongguo shi yan xue ye xue za zhi","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, cell-line-based knockdown with multiple readouts but no pathway reconstitution; direct ALAS2-BNIP3L interaction was negative","pmids":["33067979"],"is_preprint":false},{"year":2025,"finding":"Mature human ALAS2 in the mitochondrial matrix is subject to negative feedback by heme binding. Heme binds ALAS2 with high affinity and acts as a reversible mixed inhibitor that reduces enzymatic activity. Structure-based modeling identifies two flexible regions of ALAS2 that interact with heme, locking the enzyme in an inactive conformation and occluding the active site.","method":"In vitro enzyme inhibition assay, heme binding affinity measurement, structure-based computational modeling","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — direct biochemical inhibition assay with heme plus structural modeling, but preprint and single lab without replication","pmids":["bio_10.1101_2025.06.09.658730"],"is_preprint":true},{"year":1995,"finding":"A G290S missense mutation (exon 7, nucleotide 871 G-to-A) in ALAS2 causes a marked decrease in enzymatic activity of the mutant protein expressed in a heterologous system, establishing the catalytic importance of this glycine residue in the enzyme.","method":"PCR amplification and sequencing, heterologous expression of mutant cDNA with enzymatic activity assay","journal":"Human genetics","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single mutation characterized by enzymatic assay, minimal methodological detail in abstract","pmids":["7705839"],"is_preprint":false}],"current_model":"ALAS2 (ALAS-E) is the rate-limiting, erythroid-specific mitochondrial enzyme that catalyzes the first step of heme biosynthesis by condensing glycine and succinyl-CoA to form 5-aminolevulinic acid, using pyridoxal 5'-phosphate as a cofactor; its activity is regulated by a physical interaction with the SUCLA2 beta-subunit of succinyl-CoA synthetase (mediated by C-terminal residues), by negative product feedback through direct heme binding that locks the enzyme in an inactive conformation, by transcriptional control via GATA1-bound enhancers in intron 1 forming long-range chromatin loops, and by post-transcriptional repression via miR-218 targeting its 3'-UTR, with loss-of-function mutations causing X-linked sideroblastic anemia (often through reduced PLP binding/stability) and gain-of-function C-terminal deletions causing X-linked protoporphyria through increased enzymatic activity."},"narrative":{"mechanistic_narrative":"ALAS2 is the erythroid-specific mitochondrial enzyme that catalyzes the committed, rate-limiting condensation of glycine and succinyl-CoA in heme biosynthesis, and its activity is required to prevent cytoplasmic iron overload and oxidative stress in developing erythroblasts [PMID:12393610]. Catalysis depends on pyridoxal 5'-phosphate (PLP), and numerous loss-of-function missense mutations (e.g. F165L, A172T, K299Q, R411C) reduce activity primarily by impairing PLP binding or enzyme stability/thermostability rather than abolishing the catalytic mechanism directly, explaining the pyridoxine responsiveness seen clinically in X-linked sideroblastic anemia [PMID:7949148, PMID:7560104, PMID:9858242, PMID:21309041, PMID:25705881]. ALAS2 physically associates through its C-terminal residues with SUCLA2, the beta subunit of succinyl-CoA synthetase, coupling the enzyme to succinyl-CoA supply; certain XLSA mutations selectively disrupt this interaction while leaving intrinsic activity intact [PMID:10727444, PMID:22740690]. The extreme C-terminus acts as an autoinhibitory element that constrains catalytic turnover and product release: gain-of-function C-terminal frameshift deletions and truncations increase Vmax and 5-aminolevulinate release at the cost of thermostability, causing protoporphyrin overproduction in X-linked protoporphyria [PMID:22740690, PMID:18760763, PMID:30678654, PMID:21653323]. Enzyme output is further restrained by negative feedback in which heme binds ALAS2 as a reversible mixed inhibitor that locks the enzyme in an inactive conformation [PMID:bio_10.1101_2025.06.09.658730]. Erythroid-restricted expression is driven by a GATA1-bound enhancer in intron 1 that engages a long-range chromatin loop with an enhancer complex (GATA1, TAL1, LMO2, LDB1, Pol II) and the proximal promoter, and post-transcriptionally ALAS2 is repressed by miR-218 acting on its 3'-UTR [PMID:23935018, PMID:28123038, PMID:26703568].","teleology":[{"year":1994,"claim":"Established that XLSA-associated missense mutations reduce ALAS2 catalytic activity and that the defect can be partially corrected by its PLP cofactor, providing the biochemical basis for pyridoxine responsiveness.","evidence":"Prokaryotic expression and affinity purification of the F165L mutant with in vitro activity and PLP stabilization assays","pmids":["7949148"],"confidence":"High","gaps":["Single mutation; did not address whether other XLSA residues act by the same mechanism","No structural model of the affected catalytic core"]},{"year":1995,"claim":"Extended the PLP-stabilization mechanism by showing distinct XLSA mutations confer thermolability that is rescued by cofactor, generalizing PLP-dependent destabilization as a disease mechanism.","evidence":"Recombinant expression of K299Q and A172T with thermostability and PLP stabilization assays; G290S characterized in heterologous expression","pmids":["7560104","7705839"],"confidence":"High","gaps":["G290S characterization is minimal","Did not distinguish folding vs catalytic effects structurally"]},{"year":1998,"claim":"Quantified the residual activity and PLP dependence of an additional XLSA residue, reinforcing that cofactor availability governs mutant enzyme function.","evidence":"Prokaryotic expression and purification of R411C with activity assays ± PLP","pmids":["9858242"],"confidence":"Medium","gaps":["Single mutation, single lab","No in vivo erythroid validation"]},{"year":2000,"claim":"Answered how ALAS2 is coupled to its substrate supply by identifying SUCLA2 as an isoform-specific physical partner, implicating complex formation in efficient succinyl-CoA utilization or mitochondrial import.","evidence":"Yeast two-hybrid screen of human bone marrow cDNA and co-immunoprecipitation; interaction disrupted by D190V","pmids":["10727444"],"confidence":"High","gaps":["Functional consequence (succinyl-CoA channeling vs import) not directly demonstrated","Did not map the interaction interface"]},{"year":2002,"claim":"Defined the cellular phenotype of ALAS2 loss, showing its activity prevents cytoplasmic iron accumulation and oxidative stress without blocking erythroid maturation per se.","evidence":"Alas2-null ES cell erythroid differentiation with iron quantification, EM localization, and lipid peroxidation","pmids":["12393610"],"confidence":"High","gaps":["Mechanism linking absent heme synthesis to cytoplasmic iron mislocalization not resolved","Mouse model may not fully recapitulate human XLSA"]},{"year":2003,"claim":"Identified the proximal promoter as an essential erythroid regulatory element by showing a point mutation that collapses both reporter activity and patient mRNA.","evidence":"Luciferase reporter assays in K562 cells and RT-PCR of patient erythroid precursors","pmids":["12663458"],"confidence":"Medium","gaps":["Disrupted transcription factor identity not confirmed","Promoter vs enhancer hierarchy not addressed"]},{"year":2008,"claim":"Revealed a gain-of-function mechanism by showing C-terminal frameshift deletions markedly increase enzyme activity, defining the C-terminus as an autoinhibitory element and the molecular basis of X-linked protoporphyria.","evidence":"Prokaryotic expression of two patient deletion mutants with in vitro activity assays","pmids":["18760763"],"confidence":"High","gaps":["Structural basis of autoinhibition not resolved at this stage","Did not test effect on partner binding"]},{"year":2011,"claim":"Refined the gain-of-function model by attributing increased activity of a C-terminal mutant to accelerated product (5-ALA) release, and systematically separated catalytic from stability-based XLSA mechanisms.","evidence":"Recombinant enzyme assays of Y586F for product release; E. coli expression of ten missense mutants under varied PLP/thermal conditions","pmids":["21653323","21309041"],"confidence":"Medium","gaps":["Y586F result is single mutation, single study","Product release kinetics not linked to structural changes"]},{"year":2012,"claim":"Mapped the SUCLA2 interaction to ALAS2 C-terminal residues and dissociated SUCLA2 binding from intrinsic catalysis, showing that loss of partner binding alone can cause XLSA while retained binding accompanies XLP gain-of-function.","evidence":"SUCLA2 affinity column binding, enzyme kinetics, and recombinant XLSA/XLP variants","pmids":["22740690"],"confidence":"High","gaps":["In vivo requirement of SUCLA2 binding for heme output not directly tested","Structure of the C-terminal/SUCLA2 interface unknown"]},{"year":2013,"claim":"Identified the intron 1 GATA1-binding enhancer as essential for erythroid ALAS2 expression and a site of loss-of-function disease mutations.","evidence":"ChIP, EMSA, and luciferase reporter assays in K562 cells","pmids":["23935018"],"confidence":"High","gaps":["In vivo necessity not yet established at this point","Composition of the bound complex undefined"]},{"year":2015,"claim":"Demonstrated additional regulatory layers: PLP-binding-impairing mutations drive cell-nonautonomous loss of mutant erythroid precursors, and miR-218 directly represses ALAS2 via its 3'-UTR to control erythroid differentiation.","evidence":"PLP binding and X-inactivation analysis (Y365C); 3'-UTR reporter and miRNA overexpression with ALAS2 knockdown in K562 cells","pmids":["25705881","26703568"],"confidence":"Medium","gaps":["miR-218 regulation shown in cell line only","Physiological context driving miR-218 expression not defined"]},{"year":2016,"claim":"Established the in vivo essentiality and spatial organization of ALAS2 transcriptional control, showing the intron 1 GATA site anchors a long-range chromatin loop with a defined enhancer complex and is required for survival.","evidence":"Transgenic and CRISPR/Cas9 deletion in mice, chromatin loop analysis, and ChIP for GATA1/TAL1/LMO2/LDB1/Pol II","pmids":["28123038"],"confidence":"High","gaps":["Relative contribution of int-8-GATA site partly defined","Loop dynamics during erythroid maturation not detailed"]},{"year":2017,"claim":"Inferred that ALAS2 homodimerization is required for function from a frameshift variant predicted to yield a monomer.","evidence":"Sanger sequencing with SWISS-model structural prediction","pmids":["28731922"],"confidence":"Low","gaps":["Dimerization state inferred computationally, not experimentally validated","No functional assay of the variant protein"]},{"year":2020,"claim":"Linked ALAS2 activity to downstream mitochondrial quality-control gene expression via heme-dependent transcription factor signaling rather than direct interaction.","evidence":"shRNA knockdown in K562 cells with qRT-PCR, Western blot, flow cytometry, and negative ALAS2-BNIP3L co-IP","pmids":["33067979"],"confidence":"Low","gaps":["Heme-GATA1/Nrf2-BNIP3L axis not reconstituted","Single cell-line model, no in vivo validation"]},{"year":2024,"claim":"Resolved that C-terminal loss-of-function variants cause disease through heterogeneous kinetic mechanisms (stability, substrate binding, cooperativity) without gross structural disruption.","evidence":"In vitro stability, PLP binding, substrate kinetics, and structural modeling of V562A and M567I","pmids":["38888931"],"confidence":"Medium","gaps":["Single lab, no replication","In vivo correlation with patient phenotypes not established"]},{"year":2025,"claim":"Identified product (heme) feedback inhibition of mature ALAS2 as a regulatory mechanism, with heme acting as a reversible mixed inhibitor that locks the enzyme inactive.","evidence":"In vitro inhibition and heme-binding assays with structure-based modeling (preprint)","pmids":["bio_10.1101_2025.06.09.658730"],"confidence":"Medium","gaps":["Preprint, single lab, not independently replicated","Heme-interacting regions identified by modeling, not experimentally mapped"]},{"year":null,"claim":"How the autoinhibitory C-terminus, SUCLA2 association, and heme feedback are structurally integrated to set ALAS2 activity in vivo remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No high-resolution structure of human ALAS2 with C-terminus, SUCLA2, or bound heme","Quantitative contribution of each regulatory layer to erythroid heme flux unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[2,3,5,19]},{"term_id":"GO:0016829","term_label":"lyase activity","supporting_discovery_ids":[2,6]}],"localization":[{"term_id":"GO:0005739","term_label":"mitochondrion","supporting_discovery_ids":[0,18]}],"pathway":[{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[10]},{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[7,8]}],"complexes":[],"partners":["SUCLA2","GATA1","TAL1","LMO2","LDB1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P22557","full_name":"5-aminolevulinate synthase, erythroid-specific, mitochondrial","aliases":["5-aminolevulinic acid synthase 2","Delta-ALA synthase 2","Delta-aminolevulinate synthase 2"],"length_aa":587,"mass_kda":64.6,"function":"Catalyzes the pyridoxal 5'-phosphate (PLP)-dependent condensation of succinyl-CoA and glycine to form aminolevulinic acid (ALA), with CoA and CO2 as by-products (PubMed:14643893, PubMed:21252495, PubMed:21309041, PubMed:21653323, PubMed:32499479, PubMed:34492704). Contributes significantly to heme formation during erythropoiesis (PubMed:2050125) Catalyzes the pyridoxal 5'-phosphate (PLP)-dependent condensation of succinyl-CoA and glycine to form aminolevulinic acid (ALA), with CoA and CO2 as by-products (PubMed:14643893). Catalytic activity is 75-85% of isoform 1 activity (PubMed:14643893) Catalyzes the pyridoxal 5'-phosphate (PLP)-dependent condensation of succinyl-CoA and glycine to form aminolevulinic acid (ALA), with CoA and CO2 as by-products (PubMed:14643893). Catalytic activity is 65-75% of isoform 1 activity (PubMed:14643893)","subcellular_location":"Mitochondrion inner membrane","url":"https://www.uniprot.org/uniprotkb/P22557/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/ALAS2","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/ALAS2","total_profiled":1310},"omim":[{"mim_id":"619523","title":"ANEMIA, SIDEROBLASTIC, 5; SIDBA5","url":"https://www.omim.org/entry/619523"},{"mim_id":"618015","title":"PROTOPORPHYRIA, ERYTHROPOIETIC, 2; EPP2","url":"https://www.omim.org/entry/618015"},{"mim_id":"616860","title":"ANEMIA, SIDEROBLASTIC, 3, PYRIDOXINE-REFRACTORY; SIDBA3","url":"https://www.omim.org/entry/616860"},{"mim_id":"615611","title":"CASEINOLYTIC MITOCHONDRIAL MATRIX PEPTIDASE CHAPERONE SUBUNIT; CLPX","url":"https://www.omim.org/entry/615611"},{"mim_id":"612386","title":"FERROCHELATASE; FECH","url":"https://www.omim.org/entry/612386"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"bone marrow","ntpm":571.0}],"url":"https://www.proteinatlas.org/search/ALAS2"},"hgnc":{"alias_symbol":["ALAS-E"],"prev_symbol":["ASB"]},"alphafold":{"accession":"P22557","domains":[{"cath_id":"-","chopping":"2-83","consensus_level":"high","plddt":53.9226,"start":2,"end":83},{"cath_id":"3.40.640.10","chopping":"207-441","consensus_level":"high","plddt":97.5066,"start":207,"end":441},{"cath_id":"-","chopping":"550-587","consensus_level":"medium","plddt":65.355,"start":550,"end":587}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P22557","model_url":"https://alphafold.ebi.ac.uk/files/AF-P22557-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P22557-F1-predicted_aligned_error_v6.png","plddt_mean":82.19},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=ALAS2","jax_strain_url":"https://www.jax.org/strain/search?query=ALAS2"},"sequence":{"accession":"P22557","fasta_url":"https://rest.uniprot.org/uniprotkb/P22557.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P22557/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P22557"}},"corpus_meta":[{"pmid":"18760763","id":"PMC_18760763","title":"C-terminal deletions in the ALAS2 gene lead to gain of function and cause X-linked dominant protoporphyria without anemia or iron overload.","date":"2008","source":"American journal of human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/18760763","citation_count":218,"is_preprint":false},{"pmid":"7560104","id":"PMC_7560104","title":"Late-onset X-linked sideroblastic anemia. 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(ALAS-E) physically interacts with the beta subunit of ATP-specific succinyl-CoA synthetase (SCS-betaA) in mitochondria. Yeast two-hybrid screening of a human bone marrow cDNA library identified SCS-betaA as an ALAS-E binding partner, confirmed by transient expression and co-immunoprecipitation. The interaction is isoform-specific (ALAS-E but not ALAS-N) and is disrupted by the D190V mutation associated with pyridoxine-refractory sideroblastic anemia, suggesting the complex promotes efficient succinyl-CoA utilization or mitochondrial translocation of ALAS-E.\",\n      \"method\": \"Yeast two-hybrid screen, co-immunoprecipitation with transient expression\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal yeast two-hybrid plus co-immunoprecipitation in two independent experimental systems; isoform specificity and mutation disruption provide orthogonal validation\",\n      \"pmids\": [\"10727444\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"C-terminal residues of ALAS2 are required for binding to SUCLA2 (beta subunit of succinyl-CoA synthetase). XLSA mutations p.Met567Val and p.Ser568Gly, as well as a p.Phe557Ter truncation, have normal or enhanced in vitro enzymatic activity and stability but fail to bind a SUCLA2 affinity column, establishing that the C-terminal region mediates SUCLA2 interaction and that loss of this interaction causes disease. Conversely, gain-of-function XLP mutant p.Met567GlufsX2 retains SUCLA2 binding, linking SUCLA2 association to in vivo ALAS2 activity regulation. Additionally, XLSA mutations p.Arg452Cys and p.Arg452His had normal SUCLA2 binding but showed loss of positive cooperativity for succinyl-CoA, increased Km for succinyl-CoA, and reduced pyridoxal 5'-phosphate affinity.\",\n      \"method\": \"SUCLA2 affinity column binding assay, enzyme kinetics, in vitro expression and purification of recombinant proteins\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro biochemical reconstitution with affinity chromatography, kinetic assays, and multiple mutant variants tested; mechanistically definitive\",\n      \"pmids\": [\"22740690\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"C-terminal frameshift deletions in ALAS2 (c.1706-1709 delAGTG and c.1699-1700 delAT) cause gain-of-function by markedly increasing ALAS2 enzymatic activity. Prokaryotic expression of both mutant proteins demonstrated substantially elevated activity compared to wild-type, establishing that the 19-20 C-terminal residues normally suppress enzyme activity and that their removal or replacement leads to protoporphyrin overproduction causing X-linked dominant protoporphyria.\",\n      \"method\": \"Prokaryotic expression of mutant proteins, in vitro enzyme activity assay\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — direct in vitro enzymatic assay of expressed mutant proteins with multiple independent family mutations showing same gain-of-function result\",\n      \"pmids\": [\"18760763\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"The F165L missense mutation in ALAS2, found in Cooley's original XLSA family, reduces enzymatic specific activity to ~26% of normal after prokaryotic expression and affinity purification. Pyridoxal 5'-phosphate (PLP) activates and/or stabilizes the purified F165L mutant enzyme in vitro, consistent with pyridoxine responsiveness in vivo. The mutation resides in a highly conserved domain of the ALAS2 catalytic core.\",\n      \"method\": \"Prokaryotic expression, affinity purification, in vitro enzyme activity assay, PLP stabilization assay\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstituted recombinant enzyme assayed with and without cofactor; direct mechanistic link to pyridoxine responsiveness established\",\n      \"pmids\": [\"7949148\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"Missense mutations K299Q and A172T in ALAS2 cause thermolabile enzyme with decreased in vitro stability. Addition of pyridoxal 5'-phosphate in vitro stabilizes both mutant recombinant enzymes, consistent with the dramatic clinical response to pyridoxine observed in patients. The A172T mutation also resulted in decreased bone marrow delta-aminolevulinate synthase activity.\",\n      \"method\": \"In vitro recombinant enzyme expression, thermostability assay, PLP stabilization assay\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — recombinant enzyme biochemistry with multiple orthogonal measures (stability, activity, cofactor response) in two independent mutations\",\n      \"pmids\": [\"7560104\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Systematic in vitro characterization of ALAS2 C-terminal truncation mutants (p.P561X, p.V562X, p.H563X, p.E569X, p.F575X) showed 1.4- to 5.6-fold increases in Vmax for both succinyl-CoA and glycine substrates with only modest Km changes, establishing that the C-terminal region normally constrains enzyme activity. Thermostabilities of truncated mutants were significantly lower than wild-type with an inverse relationship to Vmax fold-increase, suggesting that increased molecular flexibility/active site openness is the mechanism of gain-of-function. Five ALAS2 SNVs (p.R559H, p.E565D, p.R572C, p.S573F, p.Y586F) showed modest 1.3- to 1.9-fold Vmax increases.\",\n      \"method\": \"Site-directed mutagenesis, prokaryotic expression, purification, enzyme kinetic assays, thermostability assays\",\n      \"journal\": \"Molecular medicine (Cambridge, Mass.)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstitution with multiple mutant variants, kinetic and stability measurements providing mechanistic model\",\n      \"pmids\": [\"30678654\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"The ALAS2 Y586F gain-of-function mutation (c.1757 A>T in exon 11), affecting the penultimate C-terminal amino acid, significantly increases the rate of 5-aminolevulinate release compared to wild-type ALAS2, establishing that C-terminal residues regulate product release kinetics.\",\n      \"method\": \"In vitro enzyme activity assay of recombinant expressed mutant protein\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — direct enzymatic assay but single mutation in a single study\",\n      \"pmids\": [\"21653323\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"A 130-bp erythroid-specific enhancer in intron 1 of ALAS2 contains a GATA1-binding cis-element that is essential for erythroid expression. GATA1 binding to this element was confirmed by chromatin immunoprecipitation in vivo and EMSA in vitro; the enhancer increased ALAS2 promoter activity 10-15-fold in K562 cells. Loss-of-function mutations at the GATA-binding site abolish GATA1 binding and enhancer activity, causing congenital sideroblastic anemia.\",\n      \"method\": \"Chromatin immunoprecipitation (ChIP), EMSA, luciferase reporter assay in K562 cells\",\n      \"journal\": \"Haematologica\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (ChIP in vivo, EMSA in vitro, reporter assay) consistently demonstrating GATA1-dependent enhancer function\",\n      \"pmids\": [\"23935018\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"The intron 1 GATA site (int-1-GATA) of ALAS2 is indispensable for erythroid expression in vivo. Mice with a 13-bp deletion including this site showed embryonic lethality due to severe anemia from absent ALAS2 expression. The int-1-GATA site forms a long-range chromatin loop anchoring the intron 8 GATA site and the proximal promoter, with an enhancer complex including GATA1, TAL1, LMO2, LDB1 and Pol II. CRISPR/Cas9-mediated deletion confirmed the int-1-GATA site is more essential than the int-8-GATA site.\",\n      \"method\": \"Transgenic mouse deletion, CRISPR/Cas9 site-specific deletion, chromatin loop analysis, ChIP\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo mouse deletion phenotype plus CRISPR validation plus chromatin loop/complex characterization in one study\",\n      \"pmids\": [\"28123038\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"A C-to-G transversion at nucleotide -206 of the ALAS2 proximal promoter reduces reporter activity by 94% in K562 erythroid cells and decreases ALAS2 mRNA by 87% in patient erythroid precursors, identifying the proximal promoter region as an important erythroid regulatory element. The mutation disrupts putative transcription factor binding sites.\",\n      \"method\": \"Luciferase reporter assay in K562 cells, RT-PCR quantification of mRNA in patient erythroid precursors\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reporter assay plus mRNA quantification in patient cells, two orthogonal methods in single lab\",\n      \"pmids\": [\"12663458\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"ALAS2 deficiency in definitive erythroblasts (from Alas2-null ES cells differentiated in culture) results in 15-fold excess non-heme iron accumulation localized to the cytoplasm (not mitochondria) and increased lipid peroxidation, without interfering with erythroid cell development morphology or erythroid gene expression. This establishes that ALAS2 activity is required to prevent cytoplasmic iron overload and oxidative stress in erythroblasts.\",\n      \"method\": \"Alas2-null embryonic stem cell differentiation, electron microscopy for iron localization, iron quantification, lipid peroxidation assay\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic null model with multiple orthogonal readouts (quantitative iron, EM localization, oxidative stress) establishing causal role\",\n      \"pmids\": [\"12393610\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"The R411C missense mutation in ALAS2 reduces enzyme activity to 12% of wild-type in the absence of PLP and 25% in the presence of PLP, as determined by prokaryotic expression and purification. This establishes the mechanistic basis for pyridoxine responsiveness at this residue.\",\n      \"method\": \"Prokaryotic expression, enzyme purification, in vitro activity assay with and without PLP\",\n      \"journal\": \"British journal of haematology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — direct enzymatic assay but single mutation, single lab study\",\n      \"pmids\": [\"9858242\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Ten ALAS2 missense mutations expressed in E. coli showed that five caused decreased enzymatic activity under standard conditions, and two had normal activity under standard conditions but showed reduced activity without exogenous PLP and increased thermosensitivity, revealing that some XLSA mutations affect PLP-dependent enzyme stability rather than direct catalytic mechanism.\",\n      \"method\": \"Prokaryotic expression in E. coli, in vitro ALAS2 enzymatic activity assay, thermosensitivity assay\",\n      \"journal\": \"Human mutation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — systematic enzymatic characterization of 10 mutations with multiple conditions in one study\",\n      \"pmids\": [\"21309041\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"The ALAS2 Y365C mutation impairs binding of the essential cofactor pyridoxal 5'-phosphate, resulting in enzyme destabilization and loss of function. In erythroid cells and reticulocytes from affected females, complete skewing toward expression of the WT allele (but not in leukocytes) demonstrates cell-nonautonomous effects on erythropoiesis through apoptosis of erythroid precursors expressing the mutant allele.\",\n      \"method\": \"In vitro PLP binding assay, X-inactivation analysis in reticulocytes vs. leukocytes, primary erythroid cultures\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — biochemical PLP binding assay plus cellular erythroid differentiation studies with X-inactivation readout\",\n      \"pmids\": [\"25705881\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"ALAS2 C-terminal loss-of-function variants V562A and M567I do not cause gross structural perturbations but V562A has decreased enzyme stability. PLP addition moderately increases stability of both variants. V562A shows enhanced in vitro activity but reduced succinyl-CoA binding efficiency, while M567I significantly alters cooperativity of substrate binding, establishing that C-terminal mutations can cause disease through distinct kinetic mechanisms.\",\n      \"method\": \"In vitro enzyme characterization, stability assays, PLP binding, substrate kinetics, structural modeling\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — rigorous in vitro biochemical characterization but single lab, no replication\",\n      \"pmids\": [\"38888931\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"miR-218 directly targets and represses ALAS2 expression by binding to the 3'-UTR of ALAS2 mRNA, as shown by 3'-UTR reporter assays. miR-218 overexpression in K562 cells phenocopies ALAS2 knockdown, inhibiting erythroid differentiation and altering iron metabolism.\",\n      \"method\": \"3'-UTR luciferase reporter assay, miRNA overexpression, ALAS2 knockdown in K562 cells\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — 3'-UTR reporter assay plus phenotypic validation by knockdown, two orthogonal methods\",\n      \"pmids\": [\"26703568\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"A novel g.55040074delT deletion in exon 10 of ALAS2, predicted to generate a monomeric protein rather than the wild-type homodimer based on Swiss-model structural prediction, causes severe sideroblastic anemia, establishing that ALAS2 homodimerization is required for function.\",\n      \"method\": \"Sanger sequencing, SWISS-model protein structure prediction\",\n      \"journal\": \"Journal of pediatric hematology/oncology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 4 / Weak — structural inference from computational modeling only, no experimental validation of dimerization state\",\n      \"pmids\": [\"28731922\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"ALAS2 knockdown in K562 cells leads to downregulation of BNIP3L (a mitophagy receptor) at both mRNA and protein levels, impaired erythroid differentiation (reduced CD71/CD235a markers after hemin induction), decreased ROS levels, and increased mitochondrial membrane potential. No direct physical interaction between ALAS2 and BNIP3L was detected by co-immunoprecipitation. The regulatory axis appears to work through intracellular heme levels affecting GATA1 and Nrf2 transcription factors.\",\n      \"method\": \"Lentiviral shRNA knockdown, qRT-PCR, Western blot, flow cytometry (erythroid differentiation markers, mitochondrial membrane potential, ROS), co-immunoprecipitation (negative result for direct binding)\",\n      \"journal\": \"Zhongguo shi yan xue ye xue za zhi\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, cell-line-based knockdown with multiple readouts but no pathway reconstitution; direct ALAS2-BNIP3L interaction was negative\",\n      \"pmids\": [\"33067979\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Mature human ALAS2 in the mitochondrial matrix is subject to negative feedback by heme binding. Heme binds ALAS2 with high affinity and acts as a reversible mixed inhibitor that reduces enzymatic activity. Structure-based modeling identifies two flexible regions of ALAS2 that interact with heme, locking the enzyme in an inactive conformation and occluding the active site.\",\n      \"method\": \"In vitro enzyme inhibition assay, heme binding affinity measurement, structure-based computational modeling\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — direct biochemical inhibition assay with heme plus structural modeling, but preprint and single lab without replication\",\n      \"pmids\": [\"bio_10.1101_2025.06.09.658730\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"A G290S missense mutation (exon 7, nucleotide 871 G-to-A) in ALAS2 causes a marked decrease in enzymatic activity of the mutant protein expressed in a heterologous system, establishing the catalytic importance of this glycine residue in the enzyme.\",\n      \"method\": \"PCR amplification and sequencing, heterologous expression of mutant cDNA with enzymatic activity assay\",\n      \"journal\": \"Human genetics\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single mutation characterized by enzymatic assay, minimal methodological detail in abstract\",\n      \"pmids\": [\"7705839\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"ALAS2 (ALAS-E) is the rate-limiting, erythroid-specific mitochondrial enzyme that catalyzes the first step of heme biosynthesis by condensing glycine and succinyl-CoA to form 5-aminolevulinic acid, using pyridoxal 5'-phosphate as a cofactor; its activity is regulated by a physical interaction with the SUCLA2 beta-subunit of succinyl-CoA synthetase (mediated by C-terminal residues), by negative product feedback through direct heme binding that locks the enzyme in an inactive conformation, by transcriptional control via GATA1-bound enhancers in intron 1 forming long-range chromatin loops, and by post-transcriptional repression via miR-218 targeting its 3'-UTR, with loss-of-function mutations causing X-linked sideroblastic anemia (often through reduced PLP binding/stability) and gain-of-function C-terminal deletions causing X-linked protoporphyria through increased enzymatic activity.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"ALAS2 is the erythroid-specific mitochondrial enzyme that catalyzes the committed, rate-limiting condensation of glycine and succinyl-CoA in heme biosynthesis, and its activity is required to prevent cytoplasmic iron overload and oxidative stress in developing erythroblasts [#10]. Catalysis depends on pyridoxal 5'-phosphate (PLP), and numerous loss-of-function missense mutations (e.g. F165L, A172T, K299Q, R411C) reduce activity primarily by impairing PLP binding or enzyme stability/thermostability rather than abolishing the catalytic mechanism directly, explaining the pyridoxine responsiveness seen clinically in X-linked sideroblastic anemia [#3, #4, #11, #12, #13]. ALAS2 physically associates through its C-terminal residues with SUCLA2, the beta subunit of succinyl-CoA synthetase, coupling the enzyme to succinyl-CoA supply; certain XLSA mutations selectively disrupt this interaction while leaving intrinsic activity intact [#0, #1]. The extreme C-terminus acts as an autoinhibitory element that constrains catalytic turnover and product release: gain-of-function C-terminal frameshift deletions and truncations increase Vmax and 5-aminolevulinate release at the cost of thermostability, causing protoporphyrin overproduction in X-linked protoporphyria [#1, #2, #5, #6]. Enzyme output is further restrained by negative feedback in which heme binds ALAS2 as a reversible mixed inhibitor that locks the enzyme in an inactive conformation [#18]. Erythroid-restricted expression is driven by a GATA1-bound enhancer in intron 1 that engages a long-range chromatin loop with an enhancer complex (GATA1, TAL1, LMO2, LDB1, Pol II) and the proximal promoter, and post-transcriptionally ALAS2 is repressed by miR-218 acting on its 3'-UTR [#7, #8, #15].\",\n  \"teleology\": [\n    {\n      \"year\": 1994,\n      \"claim\": \"Established that XLSA-associated missense mutations reduce ALAS2 catalytic activity and that the defect can be partially corrected by its PLP cofactor, providing the biochemical basis for pyridoxine responsiveness.\",\n      \"evidence\": \"Prokaryotic expression and affinity purification of the F165L mutant with in vitro activity and PLP stabilization assays\",\n      \"pmids\": [\"7949148\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Single mutation; did not address whether other XLSA residues act by the same mechanism\", \"No structural model of the affected catalytic core\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Extended the PLP-stabilization mechanism by showing distinct XLSA mutations confer thermolability that is rescued by cofactor, generalizing PLP-dependent destabilization as a disease mechanism.\",\n      \"evidence\": \"Recombinant expression of K299Q and A172T with thermostability and PLP stabilization assays; G290S characterized in heterologous expression\",\n      \"pmids\": [\"7560104\", \"7705839\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"G290S characterization is minimal\", \"Did not distinguish folding vs catalytic effects structurally\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Quantified the residual activity and PLP dependence of an additional XLSA residue, reinforcing that cofactor availability governs mutant enzyme function.\",\n      \"evidence\": \"Prokaryotic expression and purification of R411C with activity assays ± PLP\",\n      \"pmids\": [\"9858242\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single mutation, single lab\", \"No in vivo erythroid validation\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Answered how ALAS2 is coupled to its substrate supply by identifying SUCLA2 as an isoform-specific physical partner, implicating complex formation in efficient succinyl-CoA utilization or mitochondrial import.\",\n      \"evidence\": \"Yeast two-hybrid screen of human bone marrow cDNA and co-immunoprecipitation; interaction disrupted by D190V\",\n      \"pmids\": [\"10727444\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequence (succinyl-CoA channeling vs import) not directly demonstrated\", \"Did not map the interaction interface\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Defined the cellular phenotype of ALAS2 loss, showing its activity prevents cytoplasmic iron accumulation and oxidative stress without blocking erythroid maturation per se.\",\n      \"evidence\": \"Alas2-null ES cell erythroid differentiation with iron quantification, EM localization, and lipid peroxidation\",\n      \"pmids\": [\"12393610\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism linking absent heme synthesis to cytoplasmic iron mislocalization not resolved\", \"Mouse model may not fully recapitulate human XLSA\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Identified the proximal promoter as an essential erythroid regulatory element by showing a point mutation that collapses both reporter activity and patient mRNA.\",\n      \"evidence\": \"Luciferase reporter assays in K562 cells and RT-PCR of patient erythroid precursors\",\n      \"pmids\": [\"12663458\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Disrupted transcription factor identity not confirmed\", \"Promoter vs enhancer hierarchy not addressed\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Revealed a gain-of-function mechanism by showing C-terminal frameshift deletions markedly increase enzyme activity, defining the C-terminus as an autoinhibitory element and the molecular basis of X-linked protoporphyria.\",\n      \"evidence\": \"Prokaryotic expression of two patient deletion mutants with in vitro activity assays\",\n      \"pmids\": [\"18760763\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of autoinhibition not resolved at this stage\", \"Did not test effect on partner binding\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Refined the gain-of-function model by attributing increased activity of a C-terminal mutant to accelerated product (5-ALA) release, and systematically separated catalytic from stability-based XLSA mechanisms.\",\n      \"evidence\": \"Recombinant enzyme assays of Y586F for product release; E. coli expression of ten missense mutants under varied PLP/thermal conditions\",\n      \"pmids\": [\"21653323\", \"21309041\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Y586F result is single mutation, single study\", \"Product release kinetics not linked to structural changes\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Mapped the SUCLA2 interaction to ALAS2 C-terminal residues and dissociated SUCLA2 binding from intrinsic catalysis, showing that loss of partner binding alone can cause XLSA while retained binding accompanies XLP gain-of-function.\",\n      \"evidence\": \"SUCLA2 affinity column binding, enzyme kinetics, and recombinant XLSA/XLP variants\",\n      \"pmids\": [\"22740690\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo requirement of SUCLA2 binding for heme output not directly tested\", \"Structure of the C-terminal/SUCLA2 interface unknown\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Identified the intron 1 GATA1-binding enhancer as essential for erythroid ALAS2 expression and a site of loss-of-function disease mutations.\",\n      \"evidence\": \"ChIP, EMSA, and luciferase reporter assays in K562 cells\",\n      \"pmids\": [\"23935018\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo necessity not yet established at this point\", \"Composition of the bound complex undefined\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Demonstrated additional regulatory layers: PLP-binding-impairing mutations drive cell-nonautonomous loss of mutant erythroid precursors, and miR-218 directly represses ALAS2 via its 3'-UTR to control erythroid differentiation.\",\n      \"evidence\": \"PLP binding and X-inactivation analysis (Y365C); 3'-UTR reporter and miRNA overexpression with ALAS2 knockdown in K562 cells\",\n      \"pmids\": [\"25705881\", \"26703568\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"miR-218 regulation shown in cell line only\", \"Physiological context driving miR-218 expression not defined\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Established the in vivo essentiality and spatial organization of ALAS2 transcriptional control, showing the intron 1 GATA site anchors a long-range chromatin loop with a defined enhancer complex and is required for survival.\",\n      \"evidence\": \"Transgenic and CRISPR/Cas9 deletion in mice, chromatin loop analysis, and ChIP for GATA1/TAL1/LMO2/LDB1/Pol II\",\n      \"pmids\": [\"28123038\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative contribution of int-8-GATA site partly defined\", \"Loop dynamics during erythroid maturation not detailed\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Inferred that ALAS2 homodimerization is required for function from a frameshift variant predicted to yield a monomer.\",\n      \"evidence\": \"Sanger sequencing with SWISS-model structural prediction\",\n      \"pmids\": [\"28731922\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Dimerization state inferred computationally, not experimentally validated\", \"No functional assay of the variant protein\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Linked ALAS2 activity to downstream mitochondrial quality-control gene expression via heme-dependent transcription factor signaling rather than direct interaction.\",\n      \"evidence\": \"shRNA knockdown in K562 cells with qRT-PCR, Western blot, flow cytometry, and negative ALAS2-BNIP3L co-IP\",\n      \"pmids\": [\"33067979\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Heme-GATA1/Nrf2-BNIP3L axis not reconstituted\", \"Single cell-line model, no in vivo validation\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Resolved that C-terminal loss-of-function variants cause disease through heterogeneous kinetic mechanisms (stability, substrate binding, cooperativity) without gross structural disruption.\",\n      \"evidence\": \"In vitro stability, PLP binding, substrate kinetics, and structural modeling of V562A and M567I\",\n      \"pmids\": [\"38888931\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab, no replication\", \"In vivo correlation with patient phenotypes not established\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Identified product (heme) feedback inhibition of mature ALAS2 as a regulatory mechanism, with heme acting as a reversible mixed inhibitor that locks the enzyme inactive.\",\n      \"evidence\": \"In vitro inhibition and heme-binding assays with structure-based modeling (preprint)\",\n      \"pmids\": [\"bio_10.1101_2025.06.09.658730\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Preprint, single lab, not independently replicated\", \"Heme-interacting regions identified by modeling, not experimentally mapped\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the autoinhibitory C-terminus, SUCLA2 association, and heme feedback are structurally integrated to set ALAS2 activity in vivo remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No high-resolution structure of human ALAS2 with C-terminus, SUCLA2, or bound heme\", \"Quantitative contribution of each regulatory layer to erythroid heme flux unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [2, 3, 5, 19]},\n      {\"term_id\": \"GO:0016829\", \"supporting_discovery_ids\": [2, 6]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005739\", \"supporting_discovery_ids\": [0, 18]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [10]},\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [7, 8]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"SUCLA2\", \"GATA1\", \"TAL1\", \"LMO2\", \"LDB1\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}