rs4997557 — CYP2A6

CYP2A6 Thr294Ile — Rare Variant Slowing Nicotine Breakdown

CYP2A6 is the liver's primary nicotine-metabolising enzyme, responsible for converting approximately 80% of absorbed nicotine to cotinine. The rate at which your body clears nicotine after each cigarette determines how quickly cravings return, how many cigarettes you smoke per day, and how well nicotine replacement therapy (NRT) or varenicline work for you. Variants that reduce CYP2A6 activity are among the most replicated genetic influences on smoking behaviour identified to date.

rs4997557 introduces a missense change (p.Thr294Ile) in the CYP2A6 coding sequence on chromosome 19. The variant is extremely rare globally — absent from most large population datasets — so direct pharmacokinetic studies on this specific position do not yet exist. Its predicted functional impact is inferred from the structural importance of Thr294 and the established pattern of reduced-activity CYP2A6 variants described elsewhere in the gene.

The Mechanism

CYP2A6 belongs to the cytochrome P450 ¹¹ Cytochrome P450 enzymes are haem-containing oxidases that perform phase I metabolism — adding or exposing a reactive group on the drug molecule to prepare it for conjugation and excretion superfamily. The enzyme active site is well-characterised: Thr294 sits in the substrate-recognition region, and a hydrophobic substitution to isoleucine (Thr294Ile) is predicted to alter substrate positioning and reduce oxidative turnover. The net effect — slower nicotine C-oxidation — mirrors the phenotype caused by other characterised reduced-function CYP2A6 alleles (*2, *9, *12, *17) ²² CYP2A6*2 = rs1801272 L160H (complete loss of function); *9 = rs28399433 promoter variant (reduced expression); *12 = gene-conversion hybrid (reduced activity).

The Evidence

Direct evidence for this specific variant is limited by its rarity. The functional impact is inferred from the extensive literature on CYP2A6 reduced-activity alleles as a class.

The most comprehensive systematic review by Jones et al. (2022)³³ Jones et al. (2022)
Jones et al. Nicotine Metabolism Predicted by CYP2A6 Genotypes in Relation to Smoking Cessation. Nicotine Tob Res, 2022 analysed 34 studies and found that European-ancestry individuals with genetically reduced CYP2A6 activity were more than twice as likely to quit smoking without pharmacotherapy compared with normal metabolisers (summary OR 2.05, 95% CI 1.23–3.42). However, NRT attenuated this advantage — an observation consistent with the hypothesis that slower metabolisers already maintain nicotine levels between cigarettes more efficiently and therefore derive less incremental benefit from continuous low-dose NRT delivery.

Glatard et al. (2017)⁴⁴ Glatard et al. (2017)
Glatard et al. Association of nicotine metabolism and sex with relapse following varenicline and NRT. Exp Clin Psychopharmacol, 2017 found that normal metabolisers showed stronger benefit from varenicline over NRT (hazard ratio 0.33 for relapse), while slow metabolisers showed comparable outcomes on both treatment types — suggesting treatment selection should be informed by metaboliser status.

Beyond nicotine, CYP2A6 is the principal clearance mechanism for letrozole⁵⁵ letrozole
an aromatase inhibitor used in hormone receptor-positive breast cancer, an aromatase inhibitor used in breast cancer treatment. Desta et al. (2011) demonstrated >10-fold interpatient variability in plasma letrozole concentrations that was significantly associated with CYP2A6 genotype (P < 0.0001). Reduced-function alleles lead to higher letrozole exposure, which may increase both efficacy and adverse effects (particularly arthralgia and bone density loss).

CYP2A6 also activates tegafur, a prodrug of the chemotherapy agent fluorouracil. Reduced CYP2A6 activity results in lower conversion to the active 5-FU, potentially reducing efficacy of tegafur-based regimens.

Practical Implications

If you carry the rare A allele at rs4997557, your CYP2A6 enzyme is likely less active than average. For smoking and nicotine, this means nicotine clears more slowly between cigarettes, maintaining blood levels that suppress withdrawal and reduce the drive to smoke. If you use nicotine replacement patches or gum, the therapeutic advantage may be smaller than for faster metabolisers. Varenicline, which does not rely on CYP2A6 for its own pharmacokinetics, may be the preferred cessation aid. If you are being prescribed letrozole for breast cancer, your oncologist should be aware of your genotype, as your plasma levels will likely be higher than predicted by standard dosing, warranting closer monitoring for side effects.

Interactions

CYP2A6 activity is further modulated by co-occurring variants in the same gene. rs1801272 (CYP2A6*2, L160H) is a well-characterised complete loss-of-function allele. rs28399433 (CYP2A6*9, promoter) reduces expression by ~30%. Carrying rs4997557 alongside either of these variants on the other allele would compound the reduction in enzyme activity. Tobacco smoke also auto-induces CYP2A6 expression, meaning that smokers who quit may initially process nicotine more slowly as induction subsides — an interaction that can mimic the slow-metaboliser phenotype transiently during early cessation.