AMT

AMT encodes aminomethyltransferase (T-protein), a mitochondrial enzyme of the glycine cleavage system that catalyzes the transfer of the aminomethyl moiety from the H-protein lipoyl arm to tetrahydrofolate, yielding 5,10-methylene-tetrahydrofolate and ammonia as part of glycine catabolism (PMID:8188235). The human AMT gene spans approximately 6 kb with nine exons at chromosome 3p21.2-p21.1 and encodes a protein highly conserved across mammals (PMID:8188235, PMID:10894947). Biallelic loss-of-function mutations in AMT—predominantly missense but also frameshift and splice-site variants—cause nonketotic hyperglycinemia (NKH), with residual AMT enzyme activity correlating with attenuated rather than classic disease presentation (PMID:16450403, PMID:27362913, PMID:12948742).

1. 1994 High

   Establishing the genomic structure and chromosomal location of human AMT resolved the gene architecture encoding the glycine cleavage T-protein, enabling subsequent mutation screening in NKH patients.

   Evidence Genomic library screening, restriction mapping, sequencing, primer extension, and FISH mapping to 3p21.2-p21.1

   PMID:8188235

   Open questions at the time

   • No disease-causing mutations identified at this stage
   • Promoter elements described computationally but not functionally validated
   • No crystal structure of the human protein
2. 2000 Medium

   Characterization of canine and murine AMT orthologs demonstrated high sequence conservation (88% identity to human) and identical nine-exon gene architecture, supporting functional conservation of aminomethyltransferase across mammals.

   Evidence Genomic cloning, sequencing, and gene mapping of canine (chr 20q15.1–q15.2) and murine Amt genes

   PMID:10894947 PMID:12487019

   Open questions at the time

   • No functional assays performed in animal models to test enzymatic equivalence
   • No animal model of AMT deficiency generated
3. 2001 Medium

   Identification of recurring AMT mutations (R320H, IVS7-1G>A) in unrelated NKH families established that loss of T-protein function directly causes nonketotic hyperglycinemia, distinguishing AMT-related NKH from GLDC-related NKH.

   Evidence Sanger sequencing and PCR/restriction enzyme screening across NKH patient cohorts from multiple unrelated families

   PMID:11139253 PMID:11286506

   Open questions at the time

   • Functional impact of individual mutations not demonstrated by in vitro enzyme assays
   • Relative contribution of AMT vs. GLDC mutations to NKH prevalence not yet quantified in large cohorts
4. 2003 Medium

   Demonstrating that AMT mutations can produce residual glycine cleavage activity and atypical NKH presentations revealed that AMT genotype modulates disease severity, not just presence or absence of disease.

   Evidence Enzyme activity assays in liver and lymphoblasts combined with mutation identification in patients with unusual biochemical profiles

   PMID:12948742

   Open questions at the time

   • Structural basis for why specific mutations retain partial activity unknown
   • No recombinant expression system used to quantify individual mutant enzyme kinetics
5. 2006 High

   Systematic sequencing of all three glycine cleavage genes across 69 NKH families quantified AMT mutations as the cause in ~16% of families, with missense mutations predominating, delineating the genetic landscape of AMT-related NKH.

   Evidence Complete coding-region sequencing of AMT, GLDC, and GCSH in a 69-family NKH cohort

   PMID:16450403

   Open questions at the time

   • Deep intronic and regulatory variants not assessed
   • No functional validation of newly identified missense variants
6. 2016 High

   Large-scale international genotype-phenotype analysis across 578 NKH families confirmed that AMT genotype predicts attenuated vs. classic NKH in a subset of patients, solidifying the correlation between residual T-protein function and clinical outcome.

   Evidence Sequencing and CNV analysis with phenotype correlation in a multi-center cohort of 578 families

   PMID:27362913

   Open questions at the time

   • Mechanism by which specific AMT genotypes produce residual activity not structurally resolved
   • No therapeutic strategies targeting residual AMT activity tested
7. 2022 Medium

   Discovery of novel compound heterozygous frameshift mutations in a Chinese NKH patient expanded the AMT mutational spectrum and confirmed autosomal recessive inheritance in a previously underrepresented population.

   Evidence Whole-exome sequencing with Sanger validation and family segregation analysis

   PMID:35646099

   Open questions at the time

   • Single case report; population frequency of these alleles unknown
   • No functional assay of the truncated proteins

• The structural basis for differential residual activity of AMT missense variants, the precise mechanism of AMT's interaction with H-protein within the glycine cleavage complex, and whether pharmacological chaperones or gene therapy can rescue partial AMT deficiency remain unresolved.
• No high-resolution structure of human AMT in complex with H-protein
• No reconstituted in vitro system measuring kinetics of individual disease-causing mutants
• No animal model of AMT-deficient NKH established for therapeutic testing