INHERITED AMPLIFICATION OF AN ACTIVE GENE IN THE CYTOCHROME-P450 CYP2D LOCUS AS A CAUSE OF ULTRARAPID METABOLISM OF DEBRISOQUINE

Deficient hydroxylation of debrisoquine is an autosomal recessive trai t that affects almost-equal-to 7% of the Caucasian population. These i ndividuals (poor metabolizers) carry deficient CYP2D6 gene variants an d have an impaired metabolism of several commonly used drugs. The oppo site phenomenon also exists, and certain individuals metabolize the dr ugs very rapidly, resulting in subtherapeutic plasma concentrations at normal doses. In the present study, we have investigated the molecula r genetic basis for ultrarapid metabolism of debrisoquine. Restriction fragment length polymorphism analysis of the CYP2D locus in two famil ies with very rapid metabolism of debrisoquine [metabolic ratio (MR) f or debrisoquine = 0.01-0.1] revealed the variant CYP2D6 gene CYP2D6L. EcoRI RFLP and Xba I pulsed-field gel electrophoresis analyses showed that this gene had been amplified 12-fold in three members (father and his two children) of one of the families, and two copies were present among members of the other family. The CYP2D6L gene had an open readi ng frame and carried two mutations causing amino acid substitutions: o ne in exon 6, yielding an Arg-296 --> Cys exchange and one in exon 9 c ausing Ser-486 --> Thr. The MR of subjects carrying one copy of the CY P2D6L gene did not significantly differ from that of those with the wi ld-type gene, indicating that the structural alterations were not of i mportance for the catalytic properties of the gene product. Examinatio n of the MR among subjects carrying wild-type CYP2D6, CYP2D6L, or defi cient alleles revealed a relationship between the number of active gen es and MR. The data show the principle of inherited amplification of a n active gene. Furthermore, the finding of a specific haplotype with t wo or more active CYP2D6 genes allows genotyping for ultrarapid drug m etabolizers. This genotyping could be of predictive value for individu alized and more efficient drug therapy.