Genetic Risks for Breast Cancer in African American Women: Analysis of Candidate Single Nucleotide Polymorphisms from Association Studies

Background: African American breast cancer patients present with more aggressive disease than Caucasian women. They are also more likely to experience earlier onset of illness and tumors that are resistant to current treatment options due to their high grade and negative hormone receptor status. These attributes are correlated with greater mortality. The different frequencies of these indicators in African American and Caucasian patients have been associated with genetic susceptibility and genetically driven progression of breast cancer. However, these studies have focused on well-known breast cancer genes, not the low-penetrance mutations identified by genome-wide association studies (GWAS).Aims: This pilot case-control replication study investigated both the overall and attribute-based breast cancer risk of African American women associated with three single nucleotide polymorphisms (SNPs) selected through previously published association studies, including a genome-wide association study by Gold et al. (2008), and predicted high odds ratios and high minor allele frequencies in the African American population compared to the original study populations. We hypothesized that rs2180341 and rs7776136, located in 6q22.33 and associated with risk in a GWAS of Ashkenazi Jewish familial breast cancer cases, would contribute to overall risk in a case-control cohort of African American women. We also hypothesized that rs3741378, a SNP in SIPA1 previously associated with hormone-receptor negative tumors, would contribute to this outcome in the same cohort. These SNPs were additionally studied for relevance to other clinical outcomes.Methods: Two hundred and eighteen cases and 105 controls from the University of Chicago Cancer Risk Clinic were genotyped for these three SNPs using SNaPshot multiplex. Clinical information, including age of diagnosis, family history, hormone receptor status, survival, and tumor grade and stage, was also collected. Statistical analysis was performed using Pearson's χ2 test, Fisher's exact test, logistic regression and supplementary tests in Stata.Results: Unlike other populations, the African American population showed allelic odds ratios of approximately 1.0 for all SNPs tested, indicating no statistically significant association between these SNPs and breast cancer risk. We also found no association with age of diagnosis, hormone receptor status, tumor stage and grade. However, the minor allele of rs2180341 was associated with decreased survival in patients with a hazard ratio of 3.991 (p=0.044, 95% CI: 1.038-15.337) for the homozygous minor allele genotype.Conclusion: This pilot study represents one of the first forays into the translation of breast cancer GWAS results to populations of African ancestry. Based on our preliminary findings, future studies on a larger cohort of breast cancer cases of African ancestry will be needed to identify risk factors that may be particular to this patient population.

Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 3068.

Parity and breastfeeding are protective against breast cancer in Nigerian women

As the relation between reproductive factors and breast cancer risk has not been systematically studied in indigenous women of sub-Saharan Africa, we examined this in a case–control study in Nigeria. In-person interviews were conducted using structured questionnaires to collect detailed reproductive history in 819 breast cancer cases and 569 community controls between 1998 and 2006. Logistic regression was used to estimate odds ratios (ORs) and 95% confidence intervals (CI). Compared with women with menarcheal age <17 years, the adjusted OR for women with menarcheal age ⩾17 years was 0.72 (95% CI: 0.54–0.95, P=0.02). Parity was negatively associated with risk (P-trend=0.02) but age at first live birth was not significant (P=0.16). Importantly, breast cancer risk decreased by 7% for every 12 months of breastfeeding (P-trend=0.005). It is worth noting that the distribution of reproductive risk factors changed significantly from early to late birth cohorts in the direction of increasing breast cancer incidence. Our findings also highlight the heterogeneity of breast cancer aetiology across populations, and indicate the need for further studies among indigenous sub-Saharan women.

Prevalence of BRCA1 and BRCA2 mutations among clinic-based African American families with breast cancer

To define the prevalence and relative contributions of BRCA1 and BRCA2 mutations among African American families with breast cancer, we analyzed 28 DNA samples from patients identified through two oncology clinics. The entire coding regions of BRCA1 and BRCA2 were screened by protein truncation test, heteroduplex analysis, or single-stranded conformation polymorphism followed by DNA sequencing of variant bands. Deleterious protein-truncating BRCA1 and BRCA2 mutations were identified in five patients or 18% of the entire cohort. Only 8% (1 of 13) of women with a family history of breast cancer, but no ovarian cancer, had mutations. The mutation rates were higher for women from families with a history of breast cancer and at least one ovarian cancer (three of six, 50%). One woman with a family history of undocumented cancers was also found to carry a deleterious mutation in BRCA2. The spectrum of mutations was unique in that one novel BRCA1 mutation (1625del5) and three novel BRCA2 mutations (1536del4, 6696delTC, and 7795delCT) were identified. No recurrent mutations were identified in this cohort, although one BRCA2 (2816insA) mutation had been previously reported. In addition, two BRCA1 and four BRCA2 missense mutations of unknown significance were identified, one of which was novel. Taken together with our previous report on recurrent mutations seen in unrelated families, we conclude that African Americans have a unique mutation spectrum in BRCA1 and BRCA2 genes, but recurrent mutations are likely to be more widely dispersed and therefore not readily identifiable in this population.