rs1934963 — CYP2C9

CYP2C9 rs1934963 — Deep Intronic Variant Affecting Metabolizer Status

CYP2C9 is one of the most clinically important drug-metabolizing enzymes in the human body, responsible for clearing roughly 15-20% of all clinically used pharmaceuticals ¹¹ CYP2C9 substrate drugs include warfarin, phenytoin, NSAIDs, and multiple sulfonylurea antidiabetics. The well-known functional variants CYP2C9*2 (rs1799853) and *3 (rs1057910) reduce enzyme activity by 50-90% and are the subject of CPIC and FDA pharmacogenomic guidance for warfarin and phenytoin. rs1934963 adds a deeper layer to this picture: a deep intronic variant sitting 2,674 nucleotides into intron 7 (NM_000771.4:c.961+2674T>C) that has emerged as an independent signal for variable drug response in CYP2C9-substrate medications.

The Mechanism

Unlike CYP2C9*2 and *3, which directly alter the enzyme's active site through amino acid substitutions, rs1934963 lies within a non-coding intronic region and causes no protein change. Deep intronic variants can affect drug metabolism through several mechanisms: altered pre-mRNA splicing²² Cryptic splice sites in deep introns can be activated by single-nucleotide changes, diverting a fraction of transcripts toward aberrant mRNA isoforms, altered regulatory element binding affecting transcription factor access, or linkage disequilibrium with uncharacterized functional variants in the same genomic region. CYP2C9 is located on the minus strand of chromosome 10 (10q23.33); the plus-strand alleles T (reference) and C (alternate) correspond to the dbSNP-reported genotypes. The precise molecular mechanism by which the C allele influences CYP2C9 activity has not been established in functional biochemistry studies.

The Evidence

The primary pharmacogenomic evidence comes from a 2025 Indian cohort study of 144 type 2 diabetes mellitus patients stratified by HbA1c-defined drug response Mohanty et al. 2025³³ Mohanty et al. 2025
Mohanty IR et al. Association of CYP2C9, CYP2C19, CYP2C8, CYP2A6, and CYP3A4 gene polymorphism with drug response in an Indian cohort of T2DM. J Diabetes Metab Disord, 2025. The study found that rs1934963 polymorphism in CYP2C9 was significantly associated with drug response (P=0.001 across CYP2C9 genotypes), with wild-type T/T carriers demonstrating higher response rates to diabetes medications. The effect was observed alongside the established CYP2C9 variants rs2298037 and rs1057910 (*3), suggesting rs1934963 may contribute to the variance in CYP2C9 metabolizer phenotype beyond what is captured by *2 and *3 alone.

This finding is promising but must be treated cautiously: the study was small (n=144), single-centre, and the abstract does not provide individual allele frequency data or effect sizes for rs1934963 alone. No replication study and no functional characterization study have yet been published for this specific variant. It has no entry in ClinVar and is not listed as part of any named CYP2C9 star allele in PharmVar, meaning it falls outside current CPIC/DPWG pharmacogenomic guidelines.

The C allele frequency is approximately 20% globally, with modest variation across ancestries (European ~20%, African ~22%, South Asian ~15%, East Asian ~11%). This common-variant frequency is consistent with a low-penetrance modifier of CYP2C9 metabolizer status rather than a high-impact loss-of-function variant.

Practical Actions

Given the emerging evidence, individuals carrying one or two copies of the C allele should be aware that their CYP2C9 activity may be modified beyond what is predicted by the well-characterized *2 and *3 alleles. The most actionable implication is for sulfonylurea antidiabetic drugs (glipizide, glibenclamide, tolbutamide) — all CYP2C9 substrates. Reduced clearance at standard doses raises the risk of hypoglycemia, particularly in the context of any other CYP2C9 reducing factor (co-medications, other variant alleles). NSAIDs metabolized by CYP2C9 (ibuprofen, celecoxib) and warfarin may similarly be affected, though direct clinical data for these drug classes at rs1934963 are not yet available.

Interactions

rs1934963 operates within the same gene as CYP2C9*2 (rs1799853) and *3 (rs1057910). Carrying the C allele at rs1934963 alongside a *2 or *3 allele could create a compound metabolizer phenotype where total CYP2C9 activity is further reduced beyond what either variant predicts alone. No compound action study has directly measured this combination, but the pharmacological logic is consistent: multiple CYP2C9 activity-reducing variants on the same or different chromosomes sum their effect on warfarin clearance and NSAID exposure. Full CYP2C9 diplotype assessment (including all known variants) provides the most accurate metabolizer prediction for clinical dosing decisions.