Thesis (MSc (Genetics))--University of Stellenbosch, 2005. / Breast cancer is a major cause of morbidity and mortality as it is the most common invasive cancer in women worldwide. The lifetime risk for South African women to develop breast cancer is one in 31. A family history of the disease is a well-established risk factor and germline mutations in the BRCA1 (breast cancer one) and BRCA2 (breast cancer two) tumour suppressor genes markedly increase the risk of developing breast cancer. A few hundred mutations spanning the entire coding sequences of both genes have already been reported. Numerous other breast cancer susceptibility loci have been identified, but results from association studies are discrepant. The checkpoint kinase gene, CHEK2, and specifically the CHEK2*1100delC variant has, however, consistently been implicated as a candidate low-penetrance breast cancer allele. To date, few comprehensive molecular-genetic studies have been completed for the various South African breast cancer populations. The aim of this study was to determine the BRCA1 and BRCA2 mutation spectrum and prevalence in two South African populations, namely Mixed Ancestry and Caucasian. The frequency of the CHEK2*1100delC mutation was also investigated. The patient group comprised 101 unrelated patients (98 women and 3 men), presenting with invasive breast cancer. Patients with a moderate family history of breast cancer (n=48) were screened for the CHEK2*1100delC allele and the coding sequences of the BRCA1 (partly completed in a previous study) and BRCA2 genes. Patients without a family history of the disease (n=53) were only screened for the CHEK2*1100delC allele. Mutation detection was done using combined single-strand conformation polymorphism and heteroduplex analysis (SSCP/HA), followed by DNA sequencing of the identified variants. Due to its size (~5kb), exon 11 of BRCA2 was sequenced directly after amplification, in seven overlapping fragments. Three deleterious BRCA1 mutations, 1623_1627delTTAAA, E881X and 5313delC have previously been identified in three patients from the study population. No additional pathogenic mutations have been detected in this gene during this study. Two deleterious BRCA2 mutations, 6677_6678insTA and 8162delG, were identified in two and three patients respectively. Overall, BRCA1 and BRCA2 mutations have been identified in 17% of the Mixed Ancestry patients and in 15.8% of the Caucasian patients. Together BRCA1 and BRCA2 mutations account for 16.7% of breast cancer in the study population. In addition, a number of silent polymorphisms as well as variants of unknown functional significance, both known and novel, were identified. The E881X variant, which has been reported as an Afrikaner founder mutation (Reeves et al. 2004), was identified in one patient of Mixed Ancestry, but none of the published European founder mutations have been detected in our patient group. This suggests a unique mutation spectrum for South African breast cancer patients. The prevalence of the BRCA2 mutations, 8162delG and 6677_6678insTA, has to be elucidated within a larger study group. Haplotype analysis will reveal whether these patients have a common ancestor. Our findings do not suggest the presence of the CHEK2 variant in South African breast cancer patients, but a larger study population has to be analysed to confirm this. The results of this study are in agreement with those from other populations, indicating that less than 20% of breast cancers that occur in individuals with a moderate-risk for developing breast cancer are due to BRCA1 and BRCA2 mutations. By determining the contribution of BRCA1 and BRCA2 mutations to breast cancer in this group of patients, one can assess the appropriateness of predictive or diagnostic DNA testing in the clinical setting.