rs4646438 — CYP3A4 *6 (17776insA / frameshift)

CYP3A4*6 — A Null Allele That Silences the Body's Primary Drug-Metabolizing Enzyme

CYP3A4 is the single most important enzyme for drug metabolism in the human body. Sitting at the gateway between the gut and the bloodstream — in the intestinal wall and liver — it processes approximately 30–50% of all clinically prescribed medications¹¹ approximately 30–50% of all clinically prescribed medications
CYP3A4 and CYP3A5 together account for the majority of CYP3A-mediated metabolism. CYP3A4 predominates in the intestine and liver; CYP3A5 is more variable across tissues and populations. Immunosuppressants given after organ transplantation, opioid analgesics, benzodiazepines, many statins, antifungals, and calcium channel blockers all depend on CYP3A4 to determine how much drug reaches — and lingers in — the body after each dose.

rs4646438 defines the CYP3A4*6 allele²² CYP3A4*6 allele
Named according to the CYP Allele Nomenclature Committee convention; *1 is wild-type and higher numbers designate variant alleles identified in order of discovery, a frameshift insertion in exon 9 that destroys the enzyme entirely. This is not a subtle reduction in efficiency: in multiple independent in vitro experiments, the *6 protein produces zero detectable catalytic activity. Any CYP3A4 substrate that enters the body of a *6 carrier has one fewer working copy of the enzyme available to clear it.

The Mechanism

The CYP3A4 gene sits on the minus strand of chromosome 7 at position 99,766,412 (GRCh38)³³ minus strand of chromosome 7 at position 99,766,412 (GRCh38)
The gene runs in the reverse direction on the chromosome — the coding strand is the minus strand, so variants described in genomic coordinates use the complementary plus-strand alleles. CYP3A4*6 is caused by an adenine insertion at coding position 17776 within exon 9, described in plus-strand (genomic) terms as a T>TT tandem duplication at GRCh38 chr7:99,766,412.

This single extra nucleotide shifts the reading frame at codon 277 (p.Asp277fs). Every codon downstream of the insertion is scrambled — the ribosome reads a completely different amino acid sequence until it encounters a premature stop codon⁴⁴ premature stop codon
An in-frame stop codon generated by the shifted reading frame, causing the ribosome to halt translation early and produce a truncated protein that terminates the protein well before its 503-amino-acid full length. The mature CYP3A4 active site — including the heme-binding Cys-pocket and the substrate-access channel — lies downstream of position 277 and is missing entirely from the truncated *6 protein.

The result is a null allele. Lee and Goldstein (2005)⁵⁵ Lee and Goldstein (2005)
Lee SJ, Goldstein JA. Functionally defective or altered CYP3A4 and CYP3A5 single nucleotide polymorphisms and their detection with genotyping tests. Pharmacogenomics 2005;6(4):357-371 classified CYP3A4*6 as a "minimal function" allele — a status it shares only with CYP3A4*17 among the known CYP3A4 variants. This is distinct from the more common CYP3A4*22 (rs35599367), which reduces expression by ~50% through a splicing defect but leaves some enzyme present.

The Evidence

The original characterization by Hsieh et al. (2001)⁶⁶ Hsieh et al. (2001)
Hsieh KP, Lin YY, Cheng CL et al. Novel mutations of CYP3A4 in Chinese. Drug Metab Dispos 2001;29(3):268-273 sequenced CYP3A4 in 102 Chinese subjects and found the 17776insA allele heterozygously in one individual. Urinary 6β-hydroxycortisol-to-cortisol ratios — a well-validated phenotypic probe for in vivo CYP3A4 activity — were consistent with reduced enzyme function. The gene frequency in this initial sample was approximately 0.5%.

Subsequent in vitro functional studies have consistently confirmed complete loss of function. Cai et al. (2021)⁷⁷ Cai et al. (2021)
Cai Y, Lin Q, Jin Z et al. Evaluation of Recombinant CYP3A4 Variants on the Metabolism of Oxycodone In Vitro. Chem Res Toxicol 2021;34(1):127-134 expressed 30 CYP3A4 variants as recombinant proteins and measured metabolism of oxycodone to noroxycodone by mass spectrometry. CYP3A4*6 showed no detectable enzyme activity — zero formation of the measured metabolite — placing it in the null category alongside five other alleles. In a parallel study by Han et al. (2021)⁸⁸ Han et al. (2021)
Han M, Qian J, Ye Z et al. Functional assessment of the effects of CYP3A4 variants on acalabrutinib metabolism in vitro. Chem Biol Interact 2021;341:109464, CYP3A4*6 again completely lost catalytic function against the substrate acalabrutinib. A study by Lin et al. (2019)⁹⁹ Lin et al. (2019)
Lin QM, Li YH, Liu Q et al. Functional characteristics of CYP3A4 allelic variants on the metabolism of loperamide in vitro. Infect Drug Resist 2019;12:2985-2994 of 29 alleles against loperamide found that CYP3A4*6 displayed "extremely low activity or no activity" — the harshest category of impairment.

Population data from gnomAD¹⁰¹⁰ gnomAD
Genome Aggregation Database — aggregates exome and genome sequences from hundreds of thousands of individuals worldwide to estimate allele frequencies show that the *6 allele is globally rare, with an overall allele frequency of approximately 8.6×10⁻⁵ in gnomAD exomes. Notably, East Asian populations carry this allele at higher frequencies (~0.3% from ALFA East Asian data) compared to Europeans (~0.003%). The allele was identified in a Chinese cohort and appears to have originated in East Asian ancestry populations. Homozygous *6/*6 individuals would be extraordinarily rare.

Practical Actions

Because CYP3A4*6 is a null allele, not a partial reducer, its clinical implications are qualitatively different from the more common CYP3A4 variants. A heterozygous *6 carrier has one completely non-functional CYP3A4 allele and one normal allele — total functional CYP3A4 enzyme capacity is roughly halved at a molecular level. The degree of clinical impact depends on whether the wild-type allele alone can maintain adequate clearance, the co-presence of other CYP3A4-reducing variants, and any inhibitors in the environment.

For drugs with narrow therapeutic windows — tacrolimus and cyclosporine after organ transplantation, opioids for pain management, many antifungals — even a 50% reduction in CYP3A4 activity can produce dangerously elevated blood levels at standard doses. Transplant clinicians should note that CYP3A4*6 carriers require lower tacrolimus doses than extensive metabolizers and that therapeutic drug monitoring targets are especially important to establish early. For statins metabolized by CYP3A4 (simvastatin, lovastatin, atorvastatin), higher plasma levels increase myopathy risk; CYP3A4-independent statins (rosuvastatin, pravastatin) avoid this issue.

There are currently no CPIC or DPWG guidelines specific to CYP3A4*6, because the allele is too rare to power prospective clinical trials. Clinical management relies on standard therapeutic drug monitoring for narrow-therapeutic-index CYP3A4 substrates, combined with awareness that CYP3A4*6 carrier status predicts higher drug exposure than average.

Interactions

CYP3A4*6 interacts multiplicatively with other CYP3A-reducing variants. The most clinically important interaction is with CYP3A5*3 (rs776746) — the most common non-expressor allele of CYP3A5, present in ~85% of Europeans. CYP3A5 partially compensates for reduced CYP3A4 in tissues where it is expressed; a carrier who is also CYP3A5*3/*3 (no functional CYP3A5) has minimal combined CYP3A activity and belongs in the poor metabolizer tier functionally. Similarly, a *6 carrier who also carries CYP3A4*22 (rs35599367) on the other allele — one non-functional allele plus one ~50%-reduced allele — would have predicted overall CYP3A4 activity well below the intermediate range.

Environmental inhibitors (grapefruit juice, clarithromycin, ketoconazole, ritonavir) can reduce residual CYP3A4 activity by 3- to 8-fold. In a *6 heterozygote who already has one null allele, adding a strong CYP3A4 inhibitor can bring total clearance close to zero — raising drug exposure to dangerous levels particularly for tacrolimus, opioids, and benzodiazepines.