SPECTRUM OF MUTATION AND FREQUENCY OF ALLELIC DELETION OF THE P53 GENE IN OVARIAN-CANCER

Background: The p53 gene encodes a nuclear phosphoprotein present in l ow levels in normal human cells. The wild-type form of this protein fu nctions to restrain inappropriate cellular proliferation. Approximatel y one half of human epithelial ovarian cancers have mutations in the p 53 gene and overexpress the mutant protein product. Deletion of one al lele of the p53 gene also frequently occurs in these cancers. Purpose: We sought to define the spectrum of mutations in the p53 gene in epit helial ovarian cancer with respect to both the specific codons involve d and the type of mutations observed. We also examined the frequency o f allelic deletion of the p53 gene in cancers containing p53 gene muta tions. Methods: Tissue samples from the epithelial ovarian cancers of 62 patients were obtained during initial laparotomy. Histologic examin ation was done to ensure that the experimental samples used in this st udy contained more than 75% cancer cells. Total RNA was extracted from these samples and separately from matched control noncancerous region s of the surgical specimen or white blood cells. The purified RNAs wer e reverse transcribed to generate cDNA copies of exons 4-10 of the p53 gene. Two rounds of polymerase chain reaction (PCR) were conducted to produce enough template for DNA sequence analysis of the regions of i nterest within the p53 gene. Dideoxy sequencing of at least two indepe ndent productions of each amplified DNA template was done to confirm t he validity of the mutations found. Allelic deletions were identified by PCR and gel electrophoretic techniques to examine three polymorphis ms within the p53 gene in cancer-normal DNA pairs. Results: We identif ied 45 mutations in exons 5-8 of the p53 gene, where mutations frequen tly have been found in other cancer types. An additional mutation was identified in exon 4. Overall, 72% of the mutations were transitions, 24% transversions, and 4% microdeletions. Allelic deletion of the othe r p53 allele was seen in 67% of ovarian cancers in which a p53 mutatio n was present. Germ-line p53 mutations were not found in any patients whose cancers had p53 mutations. Conclusions and Implications: Like p5 3 mutations in other types of human cancers, those in epithelial ovari an cancers are diverse and occur frequently in exons 5-8. The predomin ance of transition mutations suggests that p53 mutations in ovarian ca ncer arise because of spontaneous errors in DNA synthesis and repair r ather than the direct interaction of carcinogens with DNA. These molec ular data are consistent with data from epidemiologic studies that hav e failed to demonstrate a convincing relationship between exposure to environmental carcinogens and the development of ovarian cancer.