ALG9

ALG9 encodes an ER lumen-oriented alpha-1,2-mannosyltransferase that builds the dolichol-PP-linked oligosaccharide precursor used for protein N-glycosylation (PMID:8692962, PMID:15987956). It is unique among ER-luminal glycosyltransferases in catalyzing the addition of two distinct alpha-1,2-linked mannose residues to the lipid-linked oligosaccharide using dolichylphosphomannose as donor, acting at two separate steps of the assembly pathway (PMID:15987956). Loss of ALG9 function arrests precursor maturation, causing accumulation of lipid-linked Man6GlcNAc2 and Man8GlcNAc2 intermediates and transfer of these truncated oligosaccharides onto nascent glycoproteins (PMID:8692962, PMID:15148656, PMID:15945070); the truncated glycans still enter the glucosylation/deglucosylation ER quality control cycle, and the resulting Man8GlcNAc2 isomer C promotes enhanced degradation of misfolded glycoproteins (PMID:19451548). Disrupted N-glycosylation impairs the maturation and biogenesis of the substrate glycoprotein polycystin-1 (PMID:31395617). Loss-of-function ALG9 variants cause congenital disorder of glycosylation type IL, established through patient LLO accumulation and yeast complementation of disease-associated missense mutations (PMID:15148656, PMID:15945070, PMID:35839600), and biallelic loss in cyst tissue links ALG9 to polycystic kidney and liver disease via a second-hit mechanism (PMID:31395617, PMID:37761895).

1. 1996 High

   Established ALG9 as a component of stepwise ER lipid-linked oligosaccharide assembly by showing that its loss blocks the pathway at a defined intermediate.

   Evidence Yeast gene deletion with in vivo LLO analysis and secreted-protein glycosylation assay

   PMID:8692962

   Open questions at the time

   • Did not directly demonstrate enzymatic activity or donor/acceptor specificity
   • Human ortholog function not yet addressed
2. 2004 High

   Connected human ALG9 deficiency to disease by showing that a homozygous missense mutation abolishes mannosyltransferase function and causes transfer of incomplete precursors to protein.

   Evidence LLO analysis in patient fibroblasts plus yeast complementation of the E523K variant

   PMID:15148656

   Open questions at the time

   • Did not resolve which specific mannose-addition steps are catalyzed
   • Substrate glycoproteins affected not identified
3. 2005 High

   Defined ALG9's distinctive catalytic role as adding two distinct alpha-1,2-mannose residues at separate steps, unlike single-addition ER glycosyltransferases.

   Evidence Biochemical LLO analysis with in vivo and in vitro glycosyltransferase assays and genetic complementation in yeast

   PMID:15945070 PMID:15987956

   Open questions at the time

   • Structural basis for dual-step specificity not determined
   • No high-resolution enzyme structure
4. 2009 High

   Showed the downstream consequence of truncated glycan transfer: equal-efficiency transfer of Man6 and Man8 intermediates that nonetheless engage ER quality control and bias misfolded glycoprotein degradation.

   Evidence Protein transfer efficiency, quality control cycle, and glycoprotein degradation assays in patient fibroblasts

   PMID:19451548

   Open questions at the time

   • Specific endogenous glycoproteins driving phenotype not defined
   • Quantitative impact on secretory proteome unresolved
5. 2019 Medium

   Identified polycystin-1 as a glycoprotein whose maturation depends on ALG9, linking the glycosylation defect to a specific clinically relevant substrate.

   Evidence Cell-based PC1 maturation and glycosylation assays after Alg9 inactivation

   PMID:31395617

   Open questions at the time

   • Single-lab cell-based system
   • In vivo relevance to cystogenesis not directly shown
6. 2022 Medium

   Clarified a disease mechanism at the protein level by showing a missense variant destabilizes ALG9 enzyme despite elevated mRNA, producing the characteristic LLO accumulation.

   Evidence RT-PCR, Western blot, and LLO analysis in patient fibroblasts for the L487P variant

   PMID:35839600

   Open questions at the time

   • Single variant and single lab
   • Degradation pathway for mutant enzyme not defined
7. 2023 Low

   Provided in-tissue evidence for a two-hit mechanism by showing absence of ALG9 protein in cyst tissue from a heterozygous carrier.

   Evidence Immunohistochemistry on liver cyst tissue with exome sequencing and in silico modeling

   PMID:37761895

   Open questions at the time

   • Single patient with IHC only and no functional reconstitution
   • Somatic loss of heterozygosity inferred indirectly

• How ALG9 enzyme architecture enables sequential addition at two distinct LLO positions, and how truncated-glycan transfer mechanistically drives polycystic phenotypes in vivo, remain unresolved.
• No high-resolution structure of the enzyme
• No in vivo animal model linking glycosylation defect to cystogenesis
• Full substrate glycoprotein repertoire uncharacterized