HOMOLOGOUS UNEQUAL CROSS-OVER INVOLVING A 2.8-KB DIRECT REPEAT AS A MECHANISM FOR THE GENERATION OF ALLELIC VARIANTS OF THE HUMAN CYTOCHROME-P450 CYP2D6 GENE

The cytochrome P450 enzyme debrisoquine 4-hydroxylase (CYP2D6) metabol izes many different classes of commonly used drugs, In Caucasian popul ations, 5-10% are classified as poor metabolizers (PM) due to autosoma l recessive inheritance of two mutant CYP2D6 null alleles, In contrast , up to 5% may demonstrate ultrarapid metabolism (UM) of debrisoquine caused by inherited amplification of functional CYP2D6 genes in the CY P2D locus, Poor metabolizer subjects may develop toxic plasma concentr ations and adverse drug reactions, whereas UMs may suffer from therape utic failure, Moreover, mutant CYP2D6 alleles have been implicated as a predictor of susceptibility for diseases such as cancer and neurolog ical disorders, The break points and molecular mechanisms involved in the generation of the PM-associated CYP2D6(D) gene deletion allele and the UM-related CYP2D6 amplification have not been clarified, Here we demonstrate the presence of a 2.8 kb repeated region (CYP-REP) which f lanks the active CYP2D6 gene in the wild type allele. The CYP-REP unit may by itself predispose to homologous unequal cross-over and contain s an Alu element and a tandem 10 bp direct repeat, which could both se rve as hotspots for recombination, The break points of the CYP2D6(D) d eletion allele are present within the repeated 2.8 kb region, but the exact positions are non-determinable due to perfect recombination of t he misaligned, homologous CYP-REP elements, We also propose that the a lleles with multiple copies of CYP2D6, which represent the first examp le of inherited amplification of an active gene in man, can be explain ed by unequal cross-over events involving the CYP-REP units, In our mo del, the CYP2D6 deletion and amplification alleles are reciprocal to e ach other, generated through homologous unequal recombination of non-a llelic CYP-REP elements.