rs55897648 — CYP2E1 *3 (Val389Ile)

CYP2E1*3 missense variant with no demonstrated change in enzyme activity for acetaminophen, ethanol, or industrial solvent substrates

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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.

Drug Interactions

acetaminophen dose_adjustment literature
isoniazid dose_adjustment literature
benzene altered_metabolism literature
halothane altered_metabolism literature

Genotype Interpretations

What each possible genotype means for this variant.

GG Normal Metabolizer Normal

Wild-type CYP2E1 at the *3 position — standard enzyme function

You carry two copies of the reference G allele at rs55897648. This is the wild-type CYP2E1*1 genotype at this position. About 99.8% of people globally share this result. CYP2E1 functions at normal capacity for metabolizing acetaminophen, ethanol, isoniazid, volatile anesthetics, and environmental solvents such as benzene.

AG CYP2E1*3 Carrier Carrier

One copy of CYP2E1*3 — no demonstrated change in enzyme function

In vitro characterization by Hanioka et al. (2003 and 2010) expressed CYP2E1*3 as a recombinant protein and measured activity using chlorzoxazone 6-hydroxylation and benzene/toluene metabolism as substrates. Kinetic parameters (Km, Vmax, intrinsic clearance) were comparable to wild-type CYP2E1*1 across all substrates tested. This places CYP2E1*3 in contrast to CYP2E1*2 (Arg76His), which shows roughly 2.7–3-fold higher catalytic efficiency.

Because the variant is so rare, no sufficiently large clinical cohort study has been conducted to confirm or refute these in vitro findings with human outcome data. The practical implication is that CYP2E1-mediated drug metabolism risks should be assessed primarily through co-existing variants (especially rs2515641), through environmental inducers (alcohol, fasting), and through the pharmacological context (co-substrates, doses) rather than through CYP2E1*3 carrier status per se.

AA CYP2E1*3 Homozygote Homozygous

Two copies of CYP2E1*3 — extremely rare, functional impact unclear

Because the AA genotype is so vanishingly rare, it has not been the subject of dedicated clinical research. Extrapolating from the in vitro heterozygous data, it is plausible that homozygous Val389Ile also shows no meaningful difference from wild-type kinetics. However, the small CADD score difference between the A and T alternate alleles (0.013 vs 0.128) does suggest the Val389Ile variant is predicted to have minimal functional impact computationally as well.

Standard CYP2E1 substrate precautions — conservative acetaminophen dosing, liver enzyme monitoring during isoniazid therapy, awareness of occupational benzene and solvent exposure — represent reasonable prudence given the theoretical uncertainty about the true clinical phenotype of homozygous CYP2E1*3.