Molecular studies in breast cancer susceptibility genes

Abstract

Breast cancer is the most common malignancy among women, with a cumulative risk of approximately 12% by the age of 85. Of the breast cancer cases, 5-10% are hereditary. To date, several breast cancer susceptibility genes have been believed to play variant roles in the occurrence of familial breast cancer, demonstrating the presence of genetic heterogeneity. These genes include BRCA1 (17q), BRCA2 (13q), TP53 (17p), ATM (11q), PTEN/MMAC1 (10q) and STK11/LKB1 (19p). Mutations in BRCA1 and BRCA2 appear to explain the majority of hereditary breast cancer. To determine the mutation spectrum of breast cancer susceptibility genes in the Swedish population and search for additional breast cancer susceptibility genes, a series of mutation analyses were conducted in the genes aforementioned using a variant of techniques including protein truncating test (PTT), DNA sequencing, single-strand conformational polymorphism (SSCP) analysis, heteroduplex analysis (HA), Southern blot and LOH analysis.

Since our previous studies have identified no TP53 germline mutations and a low (<10%) BRCA1 mutation frequency, we decided to screen for germline mutations in the subsequent cloned BRCA2 gene. In the study, only two BRCA2 germline mutations and one polymorphic stop codon were identified in 162 families (mutation frequency <5%). (Paper I)

The rather low mutation frequencies in these genes promoted us to screen for germline mutations in other candidate genes including ATM, PTEN/MMAC1, STK11/LKB1. First, a cohort of 100 families were screened for AT carriers and one mutation was identified. However, the observed frequency (1%) fall into the frequency range (0.2% - 1%) of AT carriers. Further LOH studies excluded the tumor suppressor role of ATM in familial breast cancer. (Paper II)

Subsequently, PTEN mutation analysis was conducted by PTT in a subset of 136 breast cancer families. No mutation was found. However, some mutation(s) in this gene could be missed since PTT was designed to detect only truncation mutations which is not dominant in PTEN. (Paper III)

Since a previous study suggested that LOH on 19p was involved in familial breast cancer, and STK11 is a tumor suppressor gene on 19p responsible for Peutz-Jeghers syndrome (PJS) with elevated risks of several cancer types including breast cancer, we selectively conducted STK11 mutation screening in 14 breast cancer families with 19p LOH and one PJS family. No mutation was identified. (Paper IV)

These data imply that additional gene(s) may be responsible for the disease in the rest of the families. Hunting for new candidate genes is clearly essential. A following LOH study on chromosome 17p has revealed a candidate region for one additional predisposing BRCA gene. (Paper V)

Although BRCA1 and BRCA2 are supposed to play major roles in familial breast cancer, the mutation frequencies vary widely among populations. The frequencies are also associated with the criteria used in the studies for the recruitment of the families. Actually, BRCA1 and BRCA2 explain only a very limited proportion of Swedish breast cancer families in our studies. Our data indicate that the contribution of BRCA 1 and BRCA2 mutations in Swedish population is not considerable. In addition, the roles of germline mutations in TP53, ATM, PTEN and STK11 are also minimal in familial breast cancer. The contributions of these genes to breast cancer probably are only limited to the cases existing in corresponding rare cancer syndromes associated with an increased risk of breast cancer. There must be additional gene(s) associated with the remaining breast cancer families. Our LOH study suggested that a region distal to TP53 locus on 17p could be a candidate locus involved in familial breast cancer.