腫瘤抑制基因p53, BRCA1, BRCA2於台灣乳癌之分子流行病學研究

Abstract

Abstract p53 gene mutation is a very frequent event in many human cancers. Analysis of p53 gene mutations can also provide clues to the etiology of tumor formation. The wild-type p53 has two common polymorphic variants from a single-base-pair substitution at codon 72, where either CCC encodes proline or CGC encodes arginine. Somatic mutation of BRCA1 or BRCA2 have been thought to be rare in breast cancers, though common allelic deletions in the BRCA1 locus (17q21) or BRCA2 locus (13q12-q13) imply an important role of somatic mutation in these tumors. Reasons of the reported rare incidence could be related to very few studies focusing on the mutational analysis of BRCA1or BRCA2 in sporadic tumors. The present study was conducted to investigate the p53 mutations, p53 codon 72 polymorphism, BRCA1 and BRCA2 mutations in patients with breast cancer from Taiwan. We also compare the genetic abnormalities between different areas and ethnicities. Tumor and blood samples from different cohorts including 119 patients to 175 patients respectively undergoing mastectomy for breast cancer were evaluated. The mutational statuses of the p53 gene (exons 5–8), exon 2 of BRCA1 which containing the translation initiation codon and BRCA2 gene in exon11, the largest exon harboring the RAD51 interacting BRC domains which are critical for BRCA2 function were screened by polymerase chain reaction-single strand conformation polymorphism analysis followed by direct sequencing. The p53 codon 72 allele status was also examined by restriction fragment length polymorphism (RFLP) from PMN cell of blood. Of all 119 cases of breast carcinoma, 26 mutations of the p53 gene were found in 22 cases (18.5%). Among these mutations, 78% (20/26) were point mutations with the majority of those being missense mutations (75%, 15 of 20 mutations) and the other 22% (6/26) were frameshift mutations. No significant correlation between p53 mutations and clinicopathological features was found, including HER2 status. Moreover, our results disclosed distinct mutation spectra in excess transversions to transitions (15/21, 71.4% vs. 6/21, 28.6%) with GC to CG dominant (6/15, 40%). Mutation hot spots we identified at codons 167, 185, 186, 210, 265 and 295 have rarely been documented in the literature. In BRCA1 mutation analysis, there were 25 mutations of BRCA1 gene in 21 cases (21/175, 12%). Among them, 84.6% (22/26) of BRCA1 mutations occurred at splicing region of exon 2. Five recurrent mutations occurred in IVS1 and in nucleotides -20 and -63 each harbored 3 mutations respectively. Interestingly, the mutation of BRCA1 was associated with pathological grade 2 (p=0.034). As for BRCA2 mutation status, there were 20 mutations of BRCA2 gene in exon 11 in 15 cases (15/175, 8.6%). Most mutations we identified were point mutation (19/20, 95%), except for one nucleotide insertion. Furthermore, among these mutations, missense mutations comprised 80% (16/20) of the BRCA2 mutations. All mutations we found were novel mutation after searched in the BIC database. There were three recurrent mutations at codon 1904; and two recurrent mutations at 1907, 1936, 1937 and 1968, respectively. The mutations were associated with ductal carcinoma in situ (PZ0.038) and borderline with low histological grade (PZ0.072). Besides, there were three cases possessing multiple mutations in the region we studied and one of them demonstrated aggressive lymph node metastasis. For the p53 codon72 polymorphism, there were 29 Pro/Pro (17.1%), 83 Arg/Pro (48.8%) and 58 Arg/Arg (34.1%) with the allele frequencies 0.59 for the Arg72 and 0.41 for Pro72 allele respectively. Our results are consistent with that there was more Pro72 allele (40.6% in Asian vs. 26.4% in Caucasian) and twice the incidence of p53 codon 72 Pro/Pro homozygotes (18% in Asian vs. 8% in Caucasian) among the Asian population. Furthermore when further correlation of the polymorphism with the BRCA1 mutation status , the BRCA1 somatic mutation associated with the Arg/Pro genotype significantly (81% vs. 44.3%; P = 0.018). Our findings showed that p53, BRCA1 and BRCA2 genes mutations might contribute to the pathogenesis of breast carcinoma in Taiwan. Furthermore, the events of Somatic mutations in BRCA1 and BRCA2 genes were associated with some specific histopathological features. In addition, the different mutation spectrum with high transversions in G:C to C:G may imply that the exogenous mutagens outweigh the endogenous processes in breast cancer in patients in Taiwan. The higher frequency of the p53 Arg/Pro genotype in breast cancer with BRCA1 mutation detected in this study in Taiwan suggests a possible selection role of p53 Arg72Pro polymorphism in malignant transformation and may constitute a risk factor for breast cancers, especially in breast cancer with BRCA1 somatic mutation.