Characterization of human CYP2G genes: widespread loss-of-function mutations and genetic polymorphism

CYP2G1 is an abundant, olfactory mucosa-specific cytochrome P450 enzyme active in the metabolism of sex steroids and xenobiotic substrates in mammalian animals. Two different human CYP2G genes, CYP2GP1 and CYP2GP2, were characterized in the present study. Polymorphisms in these genes were also studied. CYP2GP1 contained a single nucleotide deletion in exon 2 (deltaC) and a 2.4-kb deletion between exons 3 and 7 (deltaE4-6), whereas CYP2GP2 contained a nonsense mutation in exon 1 and another in exon 3. The coding region sequences in exons 1-3 and 7-9 of the two genes were 96.7% identical. Both genes were localized to human chromosome 19, and Southern blot analysis of human genomic DNA did not detect any additional copies of the CYP2G gene. The occurrence of these loss-of-function mutations was analysed by polymerase chain reaction-based genotyping in more than 200 individuals. The deltaE4-6 deletion in CYP2GP1 was detected in 94% of subjects (either homozygous or heterozygous), and an allele which does not contain this deletion was detected in 11.6% of individuals. The nonsense mutation in CYP2GP2 exon 3 was detected in 86% of individuals (either homozygous or heterozygous); however, a potentially functional CYP2GP2 allele based on the absence of the nonsense mutation in exon 3 was also detected in 31% of individuals. These results indicate that a functional CYP2G allele is rare in humans. Analysis of the allelic distribution in different ethnic groups suggested that a functional CYP2G allele, if present, is more likely to be found in Black and Hispanic subjects.