rs10925254 — MTR

MTR rs10925254 — A Regulatory Intronic Variant Linking Methyl Donor Flux to Craniofacial Development

Methionine synthase (MTR) performs one of the most consequential single-step reactions in human metabolism: it converts homocysteine¹¹ homocysteine
Homocysteine: a sulfur-containing amino acid that accumulates when the methylation cycle is impaired; elevated levels are neurotoxic and associated with cardiovascular disease and birth defects back to methionine using methylcobalamin (active vitamin B12) as a cofactor and 5-methyltetrahydrofolate (methylfolate) as the methyl donor. The methionine produced is then converted to S-adenosylmethionine (SAM)²² S-adenosylmethionine (SAM)
SAM: the universal methyl donor for DNA methylation, histone methylation, and over 200 methyltransferase reactions critical to embryonic gene regulation, the cell's primary currency for epigenetic regulation.

rs10925254 is a deep intronic variant in the MTR gene — it lies within intron sequence and does not alter the protein. It sits approximately 111 nucleotides into the intron in multiple transcript isoforms. Its clinical relevance emerges from a 2021 case-control study linking it, along with two sister intronic variants in the same gene, to significantly reduced risk of nonsyndromic cleft lip with or without cleft palate³³ nonsyndromic cleft lip with or without cleft palate
Nonsyndromic cleft lip/palate (NSCL/P): the most common craniofacial birth defect, affecting 1 in 700 births worldwide; "nonsyndromic" means it occurs without associated malformation syndromes (NSCL/P).

The Mechanism

Intronic variants can influence gene expression through several non-coding mechanisms: disrupting or creating splice regulatory signals, altering the binding of transcription factors or repressors embedded within intronic regulatory elements, or modifying mRNA secondary structure and stability. For rs10925254, the Salamanca et al. 2021 study found that the protective C allele correlates with reduced MTR expression in genomic expression databases — meaning it appears to act as a cis-regulatory element⁴⁴ cis-regulatory element
Cis-regulatory: influencing the expression of the gene on the same chromosome without altering its protein sequence that modestly dampens MTR transcription.

This creates a counterintuitive picture: lower MTR expression appears protective against cleft development. The authors hypothesize this may reflect altered SAM flux⁵⁵ SAM flux
SAM flux: the rate at which S-adenosylmethionine is produced and consumed; too much or too little can both disrupt the methylation reactions that regulate gene expression during embryogenesis during the critical embryonic window of palate closure. Craniofacial development is exquisitely sensitive to one-carbon metabolism — the longstanding protective effect of periconceptional folate supplementation on cleft risk independently confirms this pathway's developmental importance. The C allele's expression- dampening effect may tune SAM flux to optimal levels during this window.

The variant is in the same deep intronic cluster as rs10925239 and rs3768142, which showed identical OR and direction of effect in the same study. All three likely tag the same regulatory haplotype block rather than acting independently.

The Evidence

The primary evidence comes from a Chilean case-control study⁶⁶ Chilean case-control study
Salamanca C et al. Genetic variants in S-adenosyl-methionine synthesis pathway and nonsyndromic cleft lip with or without cleft palate in Chile. Pediatric Research, 2021 of 234 NSCL/P cases and 309 controls examining 18 SNPs across SAM synthesis pathway genes (AHCY, MTR, MTRR, MAT2A). Of these, three deep intronic MTR variants survived multiple-testing correction (q < 0.05): rs10925239 (OR 0.68, p=0.0032, q=0.0192), rs10925254 (OR 0.66, p=0.0018, q=0.0162), and rs3768142 (OR 0.66, p=0.0015, q=0.0097). All three showed the same protective direction and correlated with reduced MTR expression in database annotations.

Biological plausibility is supported by converging lines of evidence. A study by Fofou-Caillierez et al.⁷⁷ Fofou-Caillierez et al.
Fofou-Caillierez MB et al. Vitamin B-12 and liver activity and expression of methionine synthase are decreased in fetuses with neural tube defects. Am J Clin Nutr, 2019 found MTR activity, mRNA, and protein expression were all significantly reduced in fetal livers from neural tube defect cases (p=0.001, 0.016, and 0.003 respectively), with SAM levels tightly correlated with MTR activity and B12 status — directly confirming that MTR expression governs methyl donor supply during critical fetal development windows. A folate pathway study⁸⁸ folate pathway study
Blanton SH et al. Folate pathway and nonsyndromic cleft lip and palate. Birth Defects Res A, 2011 by Blanton et al. found MTR association with cleft risk specifically in Hispanic populations, with gene-gene interactions in the methionine arm of one-carbon metabolism.

At the gene level, the role of MTR in connecting B12 status, folate metabolism, and developmental methylation is well-established. Studies of the coding MTR variant A2756G (rs1805087) show associations with elevated NTD risk in some populations, and maternal MTR genotype⁹⁹ maternal MTR genotype
Mostowska A et al. Maternal MTR genotype contributes to the risk of non-syndromic cleft lip and palate in the Polish population. Clin Genet, 2006 has been associated with a 2.2-fold increased risk of having a child with cleft lip/palate, providing biological plausibility for expression-level effects at the same locus.

The evidence for rs10925254 is emerging: a single case-control study, population- limited to Chile, with no independent replication yet published.

Practical Actions

Because the variant is intronic and affects gene expression rather than enzyme kinetics, practical management focuses on ensuring the methylation cycle has adequate substrate regardless of MTR expression level. T allele carriers — who lack the expression-dampening C allele — have no direct enzyme defect to compensate for, but optimizing B12 and folate supply represents a reasonable precautionary measure. The form of supplementation matters: active B12 forms (methylcobalamin, hydroxocobalamin) and methylfolate are preferred because they enter the MTR reaction directly without requiring additional enzymatic conversion.

Homocysteine monitoring provides the most actionable functional readout: elevated homocysteine signals that overall methylation cycle throughput is suboptimal, regardless of which specific variant contributes.

Interactions

rs10925254 sits in the same deep intronic cluster as rs10925239 and rs3768142, and in the same gene as the coding MTR A2756G variant (rs1805087). Combined T allele burden across intronic MTR variants plausibly compounds any methylation cycle strain. Upstream, MTHFR (rs1801133 C677T, rs1801131 A1298C) controls methylfolate supply, and MTRR (rs1801394 A66G) reactivates oxidized B12 after each MTR cycle. Carriers of T alleles at rs10925254 alongside functional variants in MTHFR or MTRR have compounded demands on the methylation cycle as a whole.