Identification of a new variant CYP2D6 allele lacking the codon encoding Lys-281: possible association with the poor metabolizer phenotype.

A variant CYP2D6(C) P450 protein was found in a liver characterized by deficient microsomal metabolism of bufuralol and sparteine, prototypical substrates for the debrisoquine-sparteine drug oxidation polymorphism. This protein was present at decreased levels in liver and had a slightly different relative mobility on SDS-polyacrylamide gels. The cDNA cloning and sequencing of the variant, designated CYP2D6(C), revealed that its mRNA lacked a single codon resulting in deletion of Lys281. This was the result of a three base pair deletion at the 3' end of CYP2D6 exon 5. The CYP2D6(C) P450, produced in HepG2 cells using vaccinia virus mediated cDNA expression displayed Km values toward bufuralol, debrisoquine and sparteine that were not significantly different from wild type CYP2D6. These data suggest that the poor metabolizer phenotype in livers expressing CYP2D6(C) is not due to a catalytically defective enzyme but perhaps due to decreased levels of the P450 protein in microsomal membranes. Low microsomal CYP2D6(C) contents could result from deficient membrane insertion or decreased stability of the P450 protein. A polymerase chain reaction-based procedure, developed to detect CYP2D6(C) alleles, indicates that this variant probably represents less than 1.5% of all CYP2D6 alleles.