Two recent screens for copy-number variations in the entire human genome found 12.4 gene copy number variations per person, including 2.5% of individuals with gains between 7q21.1 and 7q22.1, the chromosomal location of
CYP3A4.
CYP3A4 is involved in the metabolism of approximately 50% of all drugs, including many
cancer chemotherapeutic agents.
CYP3A4 gene copy was determined in
DNA from 143 individuals: normal human livers, primary and secondary liver
tumors, human hepatic cell lines, and immortalized cell lines representing eight ethnically diverse populations.
CYP3A4 gene copy was normal in all but one sample, a primary human
hepatocellular carcinoma cell line (TONG/HCC). Southern blots of TONG/HCC
DNA revealed an approximate 10-fold increase in
CYP3A and a corresponding increase in
CYP3A mRNA expression and catalytic activity. Fluorescent in situ hybridization of TONG/HCC revealed specific amplification of the
CYP3A4 gene on chromosome 7q21 but no amplification of the MDR1 gene that localizes 11.9 Mb upstream of
CYP3A4. High resolution analysis of
DNA copy number by comparative genomic hybridization confirmed amplification at 7q21.3-7q22. The amplicon spanned 1.7 Mb and contained 30 known genes, including the entire
CYP3A locus. To determine whether
CYP3A4 expression affected chemotherapeutic toxicity, LLC-PK1 cells were transduced with adenoviruses expressing
CYP3A4 and P450
reductase.
CYP3A4 conferred resistance to
taxol,
vinblastine and
topotecan. These studies demonstrate that
CYP3A4 copy number differences do not contribute to the normal variation in
CYP3A4 expression.
Tumors with increased
CYP3A copy number (via amplification or increased chromosome 7q) would be expected to show reduced cytotoxicity to some chemotherapeutic drugs and potentially an increase in the outgrowth of
drug resistant
tumors.