Characterization of the common genetic defect in humans deficient in debrisoquine metabolism

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Abstract
In population studies of individuals given the antihypertensive drug debrisoquine1,3, two distinct phenotypes have been described: extensive metabolizers excrete 10–200 times more of the urinary metabolite 4-hydroxydebrisoquine than poor metabolizers. In family studies the poor-metabolizer phenotype behaves as an auto-somal recessive trait with an incidence between 5% and 10% in the white population of Europe and North America1–4, and extends to the deficient metabolism of more than 20 commonly prescribed drugs1–4. Clinical studies have shown that such individuals are at high risk for the development of adverse side effects from these and probably many other drugs1–6. Here we show that poor meta-bolizers have negligible amounts of the cytochrome P450 enzyme P450dbl. We have cloned the human P450dbl complementary DNA and expressed it in mammalian cell culture. Furthermore, by directly cloning and sequencing cDNAs from several poor-metabolizer livers, we have identified three variant messenger RNAs that are products of mutant genes producing incorrectly spliced dbl pre-mRNA, providing a molecular explanation for one of man's most commonly defective genes (frequency of mutant alleles 35-43%).