Aromatase [
cytochrome P450 19 (
CYP19)] is a critical
enzyme for
estrogen biosynthesis, and
aromatase inhibitors are of increasing importance in the treatment of
breast cancer. We set out to identify and characterize genetic polymorphisms in the
aromatase gene,
CYP19, as a step toward pharmacogenomic studies of
aromatase inhibitors. Specifically, we "resequenced" all coding exons, all upstream untranslated exons plus their presumed core promoter regions, all exon-intron splice junctions, and a portion of the 3'-untranslated region of
CYP19 using 240
DNA samples from four ethnic groups. Eighty-eight polymorphisms were identified, resulting in 44 haplotypes. Functional genomic studies were done with the four nonsynonymous coding single nucleotide polymorphisms (cSNP) that we observed, two of which were novel. Those cSNPs altered the following
amino acids: Trp39Arg, Thr201Met, Arg264Cys, and Met364Thr. The Cys264, Thr364, and double variant Arg39Cys264
allozymes showed significant decreases in levels of activity and immunoreactive
protein when compared with the wild-type (WT)
enzyme after transient expression in COS-1 cells. A slight decrease in
protein level was also observed for the Arg39
allozyme, whereas Met201 displayed no significant changes in either activity or
protein level when compared with the WT
enzyme. There was also a 4-fold increase in apparent K(m) value for Thr364 with
androstenedione as substrate. Of the recombinant
allozymes, only the double mutant (Arg39Cys264) displayed a significant change from the WT
enzyme in inhibitor constant for the
aromatase inhibitors exemestane and
letrozole. These observations indicate that genetic variation in
CYP19 might contribute to variation in the pathophysiology of
estrogen-dependent disease.