rs55897648 — CYP2E1 *3 (Val389Ile)

CYP2E1*3 (Val389Ile) — A Named Pharmacogenomics Allele Without a Demonstrated Clinical Effect

CYP2E1 (cytochrome P450 2E1) is the liver enzyme responsible for metabolizing a diverse and clinically important set of substrates: acetaminophen¹¹ acetaminophen
paracetamol, converted by CYP2E1 to NAPQI — the toxic metabolite responsible for liver damage at overdose doses, ethanol at high concentrations, front-line anti-tuberculosis drugs including isoniazid, volatile anesthetics such as halothane and enflurane, and environmental toxins including benzene, styrene, and trichloroethylene. The CYP2E1*3 allele, defined by the rs55897648 G>A substitution at chromosome 10 position 133,537,760 (GRCh38), results in a valine-to-isoleucine change at position 389 (p.Val389Ile). Despite carrying a formal star allele designation in pharmacogenomics nomenclature, this is one of a handful of CYP2E1 coding variants that has been functionally characterised and found to have no meaningful impact on enzyme function.

The Mechanism

The Val389Ile substitution replaces a nonpolar [valine | a small nonpolar amino acid] with the structurally similar [isoleucine | also nonpolar, slightly bulkier side chain] at position 389, which lies in the enzyme's substrate-binding region but not within a catalytically critical residue. This conservative substitution apparently preserves the three-dimensional architecture of the active site. Hanioka et al. (2003)²² Hanioka et al. (2003)
Hanioka N et al. Functional characterization of three human cytochrome p450 2E1 variants with amino acid substitutions. Xenobiotica, 2003;33(6):575-86. expressed wild-type CYP2E1 and three variants — including CYP2E1.3 (Val389Ile) and CYP2E1.4 (Val179Ile) — as recombinant proteins and measured chlorzoxazone [6-hydroxylation | CYP2E1 activity is routinely measured using chlorzoxazone as a probe substrate — the enzyme converts it to 6-hydroxychlorzoxazone] as a functional readout. CYP2E1.3 showed protein levels and catalytic activity equivalent to wild-type; the variant that did show altered function (a roughly 2.7–3.0-fold higher catalytic efficiency) was the CYP2E1.2 (Arg76His) variant — a different SNP entirely.

The Evidence

The 2010 follow-up by the same group — Hanioka et al. (2010)³³ Hanioka et al. (2010)
Hanioka N et al. Functional characterization of human cytochrome P4502E1 allelic variants: in vitro metabolism of benzene and toluene by recombinant enzymes expressed in yeast cells. Arch Toxicol, 2010;84(5):363-71. — extended the analysis to environmental toxins, measuring benzene hydroxylation and toluene methylhydroxylation in a yeast expression system. The Km, Vmax, and intrinsic clearance values for CYP2E1*3 (Val389Ile) were comparable to wild-type across both substrates. This is notable because benzene bioactivation by CYP2E1 is linked to bone marrow toxicity and leukemia risk, yet CYP2E1*3 carriers do not appear to be at altered risk via this particular mechanism. Similarly, a Northern Spanish cohort study (Celorrio et al. 2012⁴⁴ (Celorrio et al. 2012
Celorrio D et al. A comparison of Val81Met and other polymorphisms of alcohol metabolising genes in patients and controls in Northern Spain. Alcohol, 2012;46(5):427-31.) typed rs55897648 alongside several other CYP2E1 and ALDH2 polymorphisms in alcohol-use cases and controls, finding no heterozygosity for this SNP in their 172-patient Spanish cohort — consistent with a European minor allele frequency of roughly 0.2%.

No published study has identified a significant clinical association between CYP2E1*3 and acetaminophen hepatotoxicity, alcohol-related liver disease, isoniazid adverse reactions, anesthetic toxicity, or cancer risk. Absence of evidence is not evidence of absence — the extreme rarity of the A allele (0.14–0.23% globally) means that adequately powered clinical studies have not been feasible. The functional null hypothesis (no effect) is supported by in vitro characterisation, but the variant cannot be considered functionally proven benign in a clinical sense.

Practical Actions

For the vast majority of carriers — who will have one copy of the A allele (GA genotype, approximately 0.23% prevalence in Europeans) — the in vitro data suggest no change in CYP2E1-mediated drug and toxin processing. Standard dosing for acetaminophen, standard monitoring during isoniazid therapy, and standard precautions around occupational chemical exposure all apply. The clinical guidance for CYP2E1-mediated risks in this individual rests more heavily on environmental and pharmacological inducers — alcohol (upregulates CYP2E1 protein), fasting/ketosis (stabilises CYP2E1), and concurrent substrate use — than on this specific missense variant.

The most clinically actionable CYP2E1 genetic variant is the synonymous exon-8 SNP [rs2515641 | CYP2E1 exon 8 synonymous variant that reduces mRNA and protein expression, associated with ~1.9-fold higher isoniazid adverse drug reaction risk], which demonstrably reduces CYP2E1 expression and has published clinical associations. Carriers of *3 who want a complete picture of their CYP2E1 pharmacogenomics should also check their rs2515641 result.

Interactions

Because CYP2E1*3 does not appear to alter enzymatic function, gene-gene interactions that depend on reduced CYP2E1 capacity (such as the NAT2 slow-acetylator / low CYP2E1 combination for isoniazid toxicity) are less likely to be relevant here. Chronic alcohol use, fasting, and obesity independently upregulate CYP2E1 protein regardless of genotype. Concurrent substrate competition (acetaminophen + isoniazid, or acetaminophen + ethanol) represents a pharmacological interaction that applies broadly to all CYP2E1 genotypes.