rs776746 — CYP3A5 *3

CYP3A5*3 — The Transplant Pharmacogenetics Game-Changer

CYP3A5 is a member of the cytochrome P450 superfamily, metabolizing approximately 37% of clinically used drugs¹¹ 37% of clinically used drugs
The CYP3A subfamily is one of the most versatile drug biotransformation systems. While its close relative CYP3A4 dominates hepatic metabolism, CYP3A5 is the predominant CYP3A enzyme expressed in kidneys, intestines, and other extrahepatic tissues. The CYP3A5*3 allele (rs776746, 6986A>G)²² CYP3A5*3 allele (rs776746, 6986A>G)
Located in intron 3 of the CYP3A5 gene on chromosome 7q22.1 is a splice site variant that has become the poster child for pharmacogenomics-guided immunosuppressant dosing.

This single nucleotide change from A to G creates a cryptic splice acceptor site in intron 3. The spliceosome machinery, faced with competing splice signals, incorrectly incorporates intronic sequence into the mature mRNA. This pseudo-exon contains a premature stop codon³³ pseudo-exon contains a premature stop codon
The alternatively spliced isoform has an insertion from intron 3, which alters the reading frame and results in a premature termination codon, triggering nonsense-mediated mRNA decay. The result: individuals homozygous for CYP3A5*3 produce virtually no functional CYP3A5 protein — they're classified as "non-expressors."

The Mechanism

The 6986A>G substitution creates an intron/exon sequence (CAG/TA)⁴⁴ 6986A>G substitution creates an intron/exon sequence (CAG/TA)
Creating a pseudo-exon with a splice acceptor site in intron 3 that competes with the authentic exon 4 splice acceptor (CAG/AA). Upstream, a U2 snRNP branch point sequence (AAAGAG) mimics the reference branch point (AATCAG), further stabilizing the aberrant splicing event. When the spliceosome chooses the cryptic site, the resulting transcript includes 102 nucleotides of intronic sequence, shifting the reading frame and introducing a stop codon 27 amino acids downstream. The truncated protein lacks the critical heme-binding domain and catalytic machinery required for enzyme function.

Interestingly, conditional CYP3A5*3 expression has been observed⁵⁵ conditional CYP3A5*3 expression has been observed
Salt-sensitive cellular mechanisms regulate splicing and conditional expression of CYP3A5*3 transcripts, suggesting that cellular stressors affecting renal cation transport may occasionally shift splicing back to the correct exon 4 site. This salt-sensitivity may explain inconsistent associations with hypertension across studies.

The Evidence

The clinical impact of CYP3A5*3 is best established for tacrolimus, the mainstay immunosuppressant after solid organ transplantation⁶⁶ tacrolimus, the mainstay immunosuppressant after solid organ transplantation
CPIC Level A evidence for tacrolimus dosing based on CYP3A5 genotype. The 2015 CPIC guideline recommends 1.5-2 times higher starting doses⁷⁷ 2015 CPIC guideline recommends 1.5-2 times higher starting doses
CYP3A5 expressers require increased doses to achieve target blood concentrations for CYP3A5 expressers (*1/*1 and *1/*3 genotypes) compared to non-expressers (*3/*3). This isn't a subtle effect: non-expressers achieve dose-adjusted trough concentrations 1.8-2.5 times higher than expressers.

A meta-analysis of kidney transplant recipients⁸⁸ meta-analysis of kidney transplant recipients
Significantly lower concentration/dose ratios among CYP3A5*1 allele carriers at weeks 1-2 and months 1, 3, 6, and 12 found that CYP3A5*1 carriers consistently required higher tacrolimus doses across all time points post-transplant. The expresser genotype was also associated with higher risk of acute rejection (due to delayed achievement of therapeutic levels) and potentially increased chronic nephrotoxicity. In vitro studies show 8-fold higher CYP3A5 content⁹⁹ In vitro studies show 8-fold higher CYP3A5 content
In African Americans, CYP3A5*1/*3 individuals had eight-fold higher mean kidney microsomal CYP3A5 content and 18-fold higher catalytic activity in kidney microsomes from *1/*3 individuals versus *3/*3 non-expressers.

Evidence is also accumulating for sirolimus and midazolam. CYP3A5 genotype influences sirolimus dose requirements¹⁰¹⁰ CYP3A5 genotype influences sirolimus dose requirements
CYP3A5 genotype has significant influence on sirolimus metabolism, though the effect is less pronounced than for tacrolimus since CYP3A4 is the major metabolizing enzyme for sirolimus. For cyclosporine, the data are conflicting — most studies don't support a relationship¹¹¹¹ most studies don't support a relationship
Most studies do not support a relationship between CYP3A5 genotype and cyclosporine disposition, though renal CYP3A5 expression may influence local generation of nephrotoxic metabolites.

Practical Implications

If you're taking tacrolimus after kidney, liver, heart, or lung transplantation, your CYP3A5 genotype is among the strongest predictors of your dose requirements. Non-expressers (*3/*3) typically achieve target trough levels on standard starting doses (0.1-0.15 mg/kg/day), while expressers may need 1.5-2 times that dose. Genotype-guided dosing reduces time to therapeutic range¹²¹² Genotype-guided dosing reduces time to therapeutic range
Dose alterations based on CYP3A5 genotype may result in faster achievement of target concentrations with fewer dose adjustments, though therapeutic drug monitoring remains essential regardless of genotype.

Beyond transplantation, CYP3A5 status may affect response to midazolam (a benzodiazepine used for sedation), vincristine (chemotherapy — CYP3A5 non-expressers may have increased neurotoxicity risk¹³¹³ CYP3A5 non-expressers may have increased neurotoxicity risk
Increased risk of vincristine neurotoxicity associated with low CYP3A5 expression genotype), and potentially some statins, though clinical evidence for drugs other than tacrolimus is less robust.

The dramatic population frequency differences for CYP3A5*3 are striking: ~90% of Europeans but only ~33% of Africans carry the *3 allele¹⁴¹⁴ ~90% of Europeans but only ~33% of Africans carry the *3 allele
In White populations, estimated allele frequency 0.82-0.95; African Americans 0.33. This means roughly 80% of Europeans are CYP3A5 non-expressers versus only 10% of Africans. East Asians fall in between at ~75% *3 allele frequency. This frequency gradient correlates with distance from the equator¹⁵¹⁵ frequency gradient correlates with distance from the equator
CYP3A5*3 frequency ranged from 0.06 in Yorubans (Nigeria) to 0.96 in Basques, correlating with population distance from equator, suggesting evolutionary selection related to salt retention and blood pressure regulation.

Interactions

CYP3A5 status interacts with CYP3A4 variants (particularly CYP3A4*22, rs35599367¹⁶¹⁶ CYP3A4*22, rs35599367
CYP3A4*22 results in up to 50% reduction in mRNA expression and enzyme activity) to determine total CYP3A metabolic capacity. Individuals who are both CYP3A5 non-expressers (*3/*3) and carry reduced-function CYP3A4 variants have the lowest total CYP3A activity and require the most dramatic dose reductions for CYP3A substrates.

For tacrolimus specifically, donor (graft) CYP3A5 genotype matters as much or more than recipient genotype in liver transplantation, since hepatic CYP3A5 expression in the transplanted liver¹⁷¹⁷ hepatic CYP3A5 expression in the transplanted liver
Donor CYP3A5 genotype influences tacrolimus disposition, particularly in liver transplant drives first-pass metabolism. In kidney transplantation, recipient genotype dominates because tacrolimus is dosed orally and intestinal/hepatic recipient CYP3A5 determines bioavailability.

Co-administration of CYP3A inhibitors (azole antifungals, macrolide antibiotics, grapefruit juice) or inducers (rifampin, St. John's wort, some anticonvulsants) can override genetic effects. Azole antifungals preferentially inhibit CYP3A4¹⁸¹⁸ Azole antifungals preferentially inhibit CYP3A4
The extent of itraconazole inhibition is greater in CYP3A5 non-expressors due to relatively CYP3A4-specific inhibition, so CYP3A5 expressers may experience less dramatic drug interactions than non-expressers when these inhibitors are added.