rs2515641 — CYP2E1

CYP2E1 — The Silent Variant That Turns Down Your Drug-Detox Engine

CYP2E1 (cytochrome P450 2E1) is the liver enzyme responsible for metabolizing a remarkably diverse set of substrates: acetaminophen (paracetamol), isoniazid (a front-line antibiotic for tuberculosis), ethanol at high concentrations, volatile anesthetics such as halothane, and a range of industrial solvents including benzene, styrene, and trichloroethylene. The rs2515641 variant in exon 8 is [synonymous | a synonymous variant changes the DNA sequence but not the amino acid in the resulting protein] — it substitutes thymine for cytosine at position 1263 in the coding sequence (c.1263C>T), leaving phenylalanine at position 421 unchanged (p.Phe421=). Yet despite producing no amino acid change, it demonstrably reduces how much CYP2E1 the body makes.

The Mechanism

Synonymous variants were once dismissed as functionally inert, but rs2515641 illustrates how altered [codon usage | Codon usage bias: different codons for the same amino acid can differ in translation speed and mRNA stability] can reshape enzyme expression. The c.1263C>T substitution changes a common codon to a rarer one; the result is reduced mRNA stability and slower translation elongation, yielding lower CYP2E1 at both the transcript and protein level. Chen et al. (2020)¹¹ Chen et al. (2020)
Chen K, Guo R, Wei C. Synonymous mutation rs2515641 affects CYP2E1 mRNA and protein expression and susceptibility to drug-induced liver injury. Pharmacogenomics, 2020;21(7):459-470. demonstrated this directly in HepG2 hepatoma cells transfected with lentiviral vectors carrying either the C (wild-type) or T (variant) allele: cells expressing the T allele showed significantly lower CYP2E1 mRNA and protein, and the response of CYP2E1 expression to acetaminophen or triptolide challenge was dramatically altered. rs2515641 is also in complete linkage disequilibrium with [rs2070676 (CYP2E1*1B) | rs2070676, also called CYP2E1*1B, is an intronic variant frequently co-inherited with rs2515641], a variant used as a tag SNP for this genomic region across diverse populations.

The Evidence

For isoniazid toxicity, the most directly clinically documented effect, Yu et al. (2019)²² Yu et al. (2019)
Yu YY et al. Association of Drug Metabolic Enzyme Genetic Polymorphisms and Adverse Drug Reactions in Patients Receiving Rifapentine and Isoniazid Therapy for Latent Tuberculosis. IJERPH, 2019 enrolled 377 patients on a rifapentine-isoniazid regimen for latent tuberculosis. Those carrying the CT or TT genotype had 1.85–1.90-fold increased odds of developing adverse drug reactions (OR 1.850, 95% CI 1.193–2.870 for CT; OR 1.903, 95% CI 1.250–2.898 for CT+TT combined; p=0.003). The T allele itself conferred a 1.70-fold increase (95% CI 1.200–2.421). Isoniazid is metabolized by CYP2E1 into hepatotoxic intermediates; reduced enzyme capacity paradoxically increases systemic isoniazid exposure, raising toxicity risk.

For anti-TB hepatitis more broadly, Tang et al. (2013)³³ Tang et al. (2013)
Tang S et al. Cytochrome P450 2E1 gene polymorphisms/haplotypes and anti-tuberculosis drug-induced hepatitis in a Chinese cohort. PLoS One, 2013 found a minor allele frequency of 20.8% in hepatitis cases vs 18.2% in 356 matched controls (n=4,304 total TB cohort), but this difference was not statistically significant, suggesting that rs2515641 alone does not dominate risk in all populations. Effect size appears to differ substantially between Chinese and Taiwanese cohorts, likely reflecting population-level LD differences and co-exposure patterns.

For acetaminophen, the in vitro Chen et al. data suggest that reduced CYP2E1 expression in T carriers produces less [NAPQI | N-acetyl-p-benzoquinone imine (NAPQI): the toxic metabolite of acetaminophen responsible for liver injury at overdose], the toxic oxidative metabolite responsible for acetaminophen-induced liver failure. However, this does not mean T carriers are fully protected — CYP3A4 and CYP1A2 can compensate at high doses. No large clinical outcome study has confirmed this in humans for this specific SNP.

Population-level [haplotype analysis | Lee et al. (2008) examined 11 CYP2E1 polymorphisms in 2,657 individuals from 50 populations] shows markedly different T-allele frequencies across ancestry groups, with African populations carrying the T allele at ~59% — far above European (~12%) or East Asian (~17%) frequencies — making ancestry a critical variable when interpreting clinical studies predominantly drawn from Han Chinese or European cohorts.

Practical Actions

T allele carriers — particularly CT and TT individuals — face a measurably higher risk of adverse reactions when prescribed isoniazid-containing regimens. Informing a prescribing clinician or infectious disease specialist before starting isoniazid-containing tuberculosis therapy allows proactive liver enzyme monitoring. Dose timing and duration of exposure are key modifiable variables.

For acetaminophen, the practical implication is that standard labelled doses remain appropriate, but T carriers should be especially cautious with high-dose or chronic acetaminophen use, and should avoid stacking it with alcohol, which competes for the same pathway. CYP2E1 is also induced by chronic alcohol use, meaning T carriers who drink regularly may paradoxically upregulate their enzyme back toward normal levels.

Interactions

CYP2E1 expression is strongly induced by chronic ethanol, fasting/ketosis, isoniazid itself, and obesity. A T carrier whose CYP2E1 is already reduced at baseline can have that reduction partially reversed by alcohol-induced enzyme induction. Conversely, combining isoniazid with alcohol in a T carrier amplifies both the metabolic burden and the hepatotoxicity risk. Concurrent use of other CYP2E1 substrates (chlorzoxazone, halothane, certain solvents) with isoniazid or acetaminophen creates additive competitive inhibition that the T carrier's lower enzymatic reserve may not accommodate.