Genetic signature consistent with selection against the CYP3A4*1B allele in non-African populations

Cytochrome P450 3A enzymes (CYP3A) play a major role in the metabolism of steroid hormones, drugs and other chemicals, including many carcinogens. The individually variable CYP3A expression, which remains mostly unexplained, has been suggested to affect clinical phenotypes. We investigated the CYP3A locus in five ethnic groups. The degree of linkage disequilibrium (LD) differed among ethnic groups, but the most common alleles of the conserved LD regions were remarkably similar. Non-African haplotypes are few; for example, only four haplotypes account for 80% of common European Caucasian alleles. Large LD blocks of high frequencies were suggestive of selection. Accordingly, European Caucasian and Asian cohorts each contained a block of single nucleotide polymorphism (SNPs) with very high P excess values. The overlap between these blocks in these two groups contained only two of the investigated 26 SNPs and one of them was the CYP3A4* 1B allele. The region centromeric of CYP3A4* 1B exhibited high haplotype homozygosity in European Caucasians as opposed to African-Americans. CYP3A4* 1B showed a moderate effect on CYP3A4 mRNA and protein expression, as well as on CYP3A activity assessed as V max of testosterone 6β-hydroxylation in a liver bank. Our data are consistent with a functional relevance of CYP3A4* 1B and with selection against this allele in non-African populations. The elimination of CYP3A4* 1B involved different parts of the CYP3A locus in European Caucasians and Asians. Because CYP3A4 is involved in the vitamin D metabolism, rickets may have been the underlying selecting factor

Evidence of CYP3A Allosterism In Vivo: Analysis of Interaction Between Fluconazole and Midazolam

The allosteric effect of fluconazole (effector) on the formation of 1’-hydroxymidazolam (1’-OH-MDZ) and 4-hydroxymidazolam (4-OH-MDZ) from the CYP3A4/5 substrate, midazolam (MDZ), was examined in healthy volunteers. Following pre-treatment of fluconazole, AUC(4-OH)/AUC(MDZ) increased 35–62%, while AUC(1’-OH)/AUC(MDZ) decreased 5–37%; AUC(1’-OH)/AUC(4-OH) ratio decreased 46–58% by fluconazole and had no association with CYP3A5 genotype. 1’-OH-MDZ formation in vitro was more susceptible than 4-OH-MDZ formation to inhibition by fluconazole. Fluconazole decreased the intrinsic formation clearance ratio of 1’-OH-MDZ/4-OH-MDZ to an extent that was quantitatively comparable to in vivo observations. The elimination clearance of midazolam metabolites appeared unaffected by fluconazole. This study demonstrated that fluconazole alters midazolam product formation both in vivo and in vitro in a manner consistent with an allosteric interaction. The 1'-OH-MDZ/4-OH-MDZ ratio may serve as a biomarker of such interactions between midazolam, CYP3A4/5 and other putative effectors