P53 mutations in breast and ovarian carcinomas from BRCA1 and 2 mutation carriers and noncarriers

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Breast and ovarian carcinomas occurring in carriers of BRCA1 and 2 gene mutations may have a distinct pathway of molecular pathogenesis from those occurring in noncarriers. Data from murine models suggest that the p53 gene, which is involved in initiating cell cycle arrest and apoptosis in response to DNA damage, may be important in the tumorigenesis of BRCA1 and 2-associated cancers, and its loss of function could be a early critical event in the malignant transformation of cells defective for BRCA1 and 2 genes. Therefore, breast and ovarian tumors from carriers of BRCA1 and 2 alterations might be expected to exhibit a high rate of somatic p53 mutations.

An analysis was carried out on 84 Italian hereditary breast and/or ovarian families to evaluate the frequency of BRCA1 and 2 mutations by PTT and PCR-SSCP. 21 out of 84 families showed disease-associated BRCA germline mutations: 15 probands (18%) had BRCA1 mutations and 6 patients (7%) presented alterations in the BRCA2 gene. In addition, 80% of mutations found in the BRCA1 gene and 33% of alterations in the BRCA2 result in a premature termination of translation. The frequency of p53 mutations was then evaluated in 40 tumor DNAs from 33 out of 84 families analysed for BRCA1 and 2 gene alterations (8 were BRCA1 mutation-carriers and 25 were noncarriers).

The tumor DNAs were screened for alterations in the DNA-binding domain of the p53 gene (exon 5 through 8) using PCR-SSCP. Direct sequencing was performed on gene fragments that showed altered mobility in the PCR-SSCP pattern. Mutations in the p53 gene were detected in 4 out of 8 tumors from BRCA1 and 2 mutation carriers (50%) versus 2 out of 32 carcinomas from noncarriers (6%). All the observed mutations were single-base-pair substitutions. One alteration affected the splice donor site of exon 6, and it was present in a BRCA-negative family as germline mutation. The expression of p53 protein in BRCA-carrier and noncarrier tumors was analysed by immunohistochemistry using the mouse monoclonal antibody DO-7. The same analysis was also performed in a consecutive series of 72 sporadic tumors as a control group. 5 out of 8 BRCA-carrier tumors (63%) and 2 out of 32 BRCA-noncarrier carcinomas (6%) were positive for p53 staining. Finally, 25 out of 72 sporadic tumors (35%) had p53-positive immunostaining. A significantly higher frequency of p53 mutation and overexpression was found in the BRCA-associated tumors. Our data are in keeping with the postulate that loss of p53 checkpoint control is important in the molecular pathogenesis of breast and ovarian carcinomas in carriers of BRCA1 and 2 mutations.