Re: modification of clinical presentation of prostate tumors by a novel genetic variant in CYP3A4.

In the recent study on a new variant of the cytochrome P450 3A4 (CYP3A4) gene by Rebbeck et al. (1), the authors did not provide data that demonstrate an alteration of CYP3A4 function as a consequence of the polymorphism. They suggested that the variant allele might alter disposition of the androgenic substrates of CYP3A4 as a result of decreased enzymatic activity. Worse clinical presentation of prostate cancer(1) and a decreased risk for treatment-related leukemia (2) were reported to be associated with the variant allele, the former probably due to increased bioavailability of testosterone and the latter probably due to reduced production of leukemogenic metabolites of anticancer drugs. Thus, we investigated the possible relationship between CYP3A4 genotypes and nifedipine oxidation activity, a prototype reaction of the encoded enzyme, by use of a human liver microsome systemin vitro. Fifteen liver samples from Caucasian transplant donors were obtained from the National Disease Research Interchange (Philadelphia, PA) through the Biomedical Research Institute, Human and Animal Bridge Discussion Group (Chiba, Japan). The nifedipine oxidation activity and the expression levels of CYP3A4 protein of the samples have been reported elsewhere (3). A variant sequence of the CYP3A4 gene was distinguished from wild-type by a nested polymerase chain reaction–restriction fragment length polymorphism assay by the use of genomic DNA prepared from the liver samples (Fig. 1, A). The institutional review board of St. Marianna University School of Medicine has approved the study. The genotyping analysis revealed one homozygote and four heterozygotes for the variant. The remaining 10 subjects were homozygous for the wild-type allele. No apparent relationships between the genotypes and the activity or amount of CYP3A4 were found (Fig. 1, B). Moreover, we examined the distribution of the new polymorphism in the Japanese population. DNA samples were prepared from 128 unrelated, healthy volunteers (median age, 22 years; range, 21–40 years) who had given written informed consent for their blood to be used in this study; the study had obtained approval from the Ethics Committee of Nagoya University School of Medicine. Although the frequency of the variant allele is 9.4% in Caucasian individuals(1), the population study revealed that all of the Japanese subjects were homozygous for wild-type. In our in vitro analysis, we did not f i nd any re la t i onsh ip be tween the CYP3A4 genotype and the level of nifedipine oxidation activity, which seems inconsistent with the findings presented by Rebbeck et al. (1). The reason for the disagreement is unclear. This may be due to the small sample size in our study. Alternatively, the variant allele may be in linkage disequilibrium with another mutation or gene that more strongly influences prostate carcinogenesis and leukemogenic drug effects. However, CYP3A4 activity is known to exhibit wide interindividual variability due to physiologic factors (e.g., age, food), pathologic conditions (e.g., hepatic diseases), environmental factors (e.g., smoking), and concomitant drug intake (e.g., steroids, anticonvulsants, antifungal agents), aside from any genetic factors (6). We consider the difference in enzymatic activity between the CYP3A4 genotypes, if any, as too small to explain such a wide variation in the observed activity. It cannot be ruled out that the genetic variant in a 58 regulatory element might be related to a variability in the inducibility of CYP3A4 by such agents as rifampin or anticonvulsants. It is also possible that the induction of CYP3A4 in subjects with the variant allele might be weak compared with those with wildtype, given exposure to the same amount of inducer. This analysis demonstrates a great difference in the distribution of the variant allele between Caucasian and Japanese populations. Recently, increased attention has been focused on racial variability in drug disposition and sensitivity. The recognized ethnic difference in the distribution of pharmacogenet ic polymorphisms should be important in determining drug dosages in different populations. Because the variant allele examined here does not seem to be strongly related to CYP3A4