rs1021737 — CTH CTH Ser403Ile

CTH Ser403Ile — When the H₂S Pathway Fails to Protect the Heart

Your cardiovascular system relies on a steady supply of hydrogen sulfide (H₂S), a gaseous signaling molecule produced mainly by an enzyme called cystathionine gamma-lyase (CTH/CSE)¹¹ cystathionine gamma-lyase (CTH/CSE)
also abbreviated CSE; encoded by the CTH gene on chromosome 1 in the liver and vascular wall. CTH sits at the junction of the transsulfuration pathway — the metabolic route that converts methionine into cysteine, consuming homocysteine along the way. H₂S produced by CTH acts as a vasodilator, an anti-inflammatory mediator, and a cardioprotective signal. The rs1021737 variant (c.1208G>T, p.Ser403Ile) is a common missense change in CTH's active domain that shifts the transsulfuration balance — with consequences that appear most sharply in women and in contexts of cardiovascular stress.

The Mechanism

CTH is a pyridoxal-5-phosphate (PLP)-dependent enzyme²² pyridoxal-5-phosphate (PLP)-dependent enzyme
PLP is the active form of vitamin B6, required as a cofactor for all transsulfuration enzymes that cleaves cystathionine to yield cysteine, α-ketobutyrate, and ammonia — with H₂S generated as a byproduct of cysteine catabolism. The Ser403Ile substitution replaces a serine residue (hydroxyl side chain, hydrophilic) with isoleucine (branched aliphatic, hydrophobic) near the enzyme's C-terminus.

Importantly, in vitro kinetic studies³³ in vitro kinetic studies
steady-state enzyme parameters Km and Vmax measured in purified recombinant protein show that Ser403Ile does not reduce CTH catalytic efficiency per se — Km, Vmax, and PLP cofactor loading are unchanged compared to wild-type. This distinguishes the polymorphism from pathogenic CTH mutations (e.g. T67I, Q240E) that directly impair catalysis. The mechanism by which TT homozygotes accumulate homocysteine likely involves altered protein stability, changed substrate channeling within the transsulfuration pathway, or a yet-uncharacterized regulatory effect on the enzyme's in-vivo activity under physiological conditions.

The downstream consequence is a relative insufficiency in two cardioprotective outputs of the transsulfuration pathway: (1) less homocysteine cleared through the cystathionine route, raising plasma total homocysteine (tHcy), and (2) potentially reduced H₂S bioavailability. Elevated tHcy damages vascular endothelium through oxidative stress, impairs nitric oxide signaling, promotes thrombosis, and accelerates atherosclerosis. H₂S exerts the opposite effects — it relaxes vascular smooth muscle, reduces inflammation, and protects cardiomyocytes during ischemia-reperfusion injury.

The Evidence

The foundational association was established by Wang et al. 2004⁴⁴ Wang et al. 2004
Wang J, Huff AM, Spence JD, Hegele RA. Single nucleotide polymorphism in CTH associated with variation in plasma homocysteine concentration. Clin Genet. 2004;65(6):483–486. in 496 Caucasian subjects. TT homozygotes had significantly higher mean plasma total homocysteine than GG or GT carriers, with an effect size comparable to that of the well-established MTHFR C677T variant. This places CTH Ser403Ile alongside MTHFR as a meaningful genetic determinant of plasma homocysteine — a finding that was replicated in subsequent studies.

The most striking clinical evidence comes from a 2022 Swedish case-control study Söderström et al. 2022⁵⁵ Söderström et al. 2022
CTH G1208T and MTHFR A1298C polymorphisms are associated with a higher risk of a first myocardial infarction with fatal outcome among women. Drug Metab Pers Ther. 2022. of first myocardial infarction. In women, the CTH G1208T variant (rs1021737 T allele carriers) showed a striking OR of 3.14 [95% CI 1.16–8.54] for fatal MI in heterozygotes, rising to OR 3.22 [1.22–8.51] in the dominant model. Critically, this association was entirely sex-specific: no comparable effect was found in men, and neither genotype was associated with non-fatal MI in either sex. The MTHFR A1298C variant showed a similar sex-specific pattern, suggesting that transsulfuration-pathway impairment preferentially raises fatal cardiovascular risk in women — possibly because estrogen normally supports transsulfuration enzyme activity and H₂S production, making women more vulnerable to CTH impairment.

H₂S bioavailability was further implicated by Rajpal et al. 2018⁶⁶ Rajpal et al. 2018
Total sulfane sulfur bioavailability reflects ethnic and gender disparities in cardiovascular disease. Redox Biol. 2018;15:480–489. in 324 CVD patients and controls. The CTH T allele was significantly more common in CVD patients vs controls, and Caucasian females with CVD had markedly lower acid-labile sulfide (a key H₂S pool) than controls — pointing to CTH-mediated H₂S insufficiency as a sex-stratified cardiovascular mechanism.

The variant does not appear to affect essential hypertension risk in Chinese Han populations (Li et al. 2008; n=993; p>0.05 for both allele and genotype associations), nor was it significantly associated with preeclampsia in Caucasians (Mrozikiewicz et al. 2015). Its effects may be most pronounced in the context of cumulative cardiovascular risk and the sex-hormone environment.

Practical Actions

For TT homozygotes and GT carriers — particularly women — the actionable focus is: (1) driving homocysteine levels down through the methylation pathway (B-vitamins), (2) monitoring homocysteine as a cardiovascular biomarker, and (3) supporting H₂S production through dietary sulfur amino acid intake. The CTH pathway requires vitamin B6 (as PLP) as its essential cofactor; ensuring adequate B6 status is especially important when CTH function may be compromised.

Because CTH Ser403Ile impairs homocysteine clearance through the transsulfuration route, the MTHFR re-methylation pathway becomes more important as an alternative route. This means active B12 and methylfolate availability become critical backstops — especially for individuals who also carry MTHFR C677T (rs1801133) or A1298C (rs1801131), where the re-methylation route is also impaired.

Interactions

CTH rs1021737 operates downstream of MTHFR in the one-carbon/homocysteine cycle. The transsulfuration pathway (CTH's route) and the re-methylation pathway (MTHFR's route) are the two main homocysteine disposal routes. If both are impaired — by CTH TT and MTHFR C677T TT simultaneously — homocysteine has nowhere to go, creating a severely elevated tHcy state. This dual-impairment combination is the most clinically relevant interaction for rs1021737 and warrants a compound action.

MTRR rs1801394 (methionine synthase reductase) supports the re-methylation pathway; MTRR risk genotypes further reduce the pressure on the transsulfuration route. Any combination of CTH TT + folate pathway impairment should be flagged for aggressive B-vitamin optimization and homocysteine monitoring.