The CYP3A4*18 genotype in the cytochrome P450 3A4 gene, a rapid metabolizer of sex steroids, is associated with low bone mineral density.

Osteoporosis is influenced by genetic factors. The interindividual variability in the activity of CYP3A, the metabolic enzyme of sex hormones, may result from genetic polymorphisms. In a study of 2,178 women of ages 40-79 years, the presence of the CYP3A4*18 variant was found to be significantly associated with low bone mass. In vitro functional analyses indicate that CYP3A4*18 is a gain-of-function mutation in sex steroid metabolism, resulting in rapid oxidation of estrogens and testosterone; in vivo pharmacokinetics using midazolam (MDZ) verify the altered activity of the CYP3A4*18, showing lower metabolic turnover in the mutant than in the wild type. Molecular modeling reveals the structural changes in the substrate recognition sites of CYP3A4*18 that can cause changes in enzymatic activity and that potentially account for the difference between the catalytic activities of estrogen and MDZ, depending on the genotype. The results indicate that a genetic variation in the CYP3A4 gene--as a gain-of-function mutation in the metabolism of certain CYP3A substrates, including sex steroids--may predispose individuals to osteoporosis.