A genetic study on familial breast cancer predisposing genes

Abstract

Breast cancer is the most common malignant disease among women in the western world and 10% of all breast cancer is assumed to constitute hereditary cases. Two major genes, BRCA1 and BRCA2, can only explain a fraction of familial breast cancer. The ATM carriers have high risk of developing breast cancer. To evaluate the genetic roles of other susceptibility genes that may contribute to breast cancer, we used a PCR-SSCP method to screen for mutations in the ATM gene in sporadic and familial breast cancer cases. No somatic mutations were identified in the ATM gene in 38 sporadic breast cancer cases although LOH were detected in the region flanking ATM in 47% of the patients. Five constitutive rare polymorphisms were reported with a frequency between 0.005 and 0.23 (Paper I). A further study of 88 familial breast cancers revealed three germline mutations in the ATM gene. Two mutations had previously been found in A-T patients and one new mutation was a 1 bp insertion (Paper II). Our studies rule out ATM as a major susceptibility gene for breast cancer.

To investigate the possibility of an epigenetic mechanism underlying the inactivation of the ATM gene in leukemia patients, where no mutations were found, DNA methylation in the promoter region of the ATM with 67 CpG islands was analyzed using bisulphite genomic sequencing. No hypermethylation was found in 9 T-PLL patients who did not show ATM or Tp53 mutations. Six rare nucleotide substitutions in the ATM gene in T-ALL were detected, which were also identified in the germline DNA, indicating constitutive polymorphisms (paper III).

Another candidate susceptibility gene, BACH1, was studied for germline mutations by using PCR-SSCP in 29 breast cancer families exhibiting positive NPL with microsatellite markers flanking BRCA1, and an additional 95 breast cancer cases with a family history of breast cancer. No germline mutations were identified in any breast cancer patients. This result excludes BACH1 as a frequently altered gene predisposing to familial breast cancer and does not support it as a breast cancer susceptibility gene (paper IV).

Linkage analysis can pinpoint putative moderate penetrance genes and association studies are being used to search for low penetrance genes or other factors increasing the risk of developing breast cancer. A previous study used LOH to identify a candidate region on 17p in familial breast tumors and another study using comparative genome hybridization (CGH) pinpointed a region on 13q. To evaluate the potential contribution of these putative loci on chromosome 17p and 13q in 102 Stockholm breast cancer families without detected BRCA1 and BRCA2 mutations, we performed LOH study and targeted linkage analysis. Two loci with high frequency of LOH were found on 17p, one spanned the Tp53 gene and the other was distal to Tp53. Linkage analysis either did not support any of the two loci on 17p as predisposing to familial breast cancer (paper V). Nor was there any support for a putative breast cancer susceptibility gene on 13q (paper V1).