Hereditary breast cancer in an underserved population: Hispanic BRCA mutations

CN06-02

Breast cancer is the most commonly diagnosed cancer in Hispanic women, and is the leading cancer cause of death in this population. Mutations in the BRCA genes are associated with 5-10% of breast cancer cases. Large rearrangements, deletions or duplications of a portion of a gene that are not detectable by the standard sequencing test, account for up to 15% of deleterious BRCA mutations. The lifetime risk of developing breast cancer in individuals with a BRCA mutation may be as high as 85%.
 There is little information on BRCA mutations in the Hispanic population. Our previous studies have provided the foundation for the current project. We reported on the frequency of BRCA mutations in a largely immigrant Hispanic high-risk clinic population residing in Southern California in which deleterious mutations were detected in 31% of 110 unrelated families (CEBP 2005. 14:1666). Interestingly, BRCA1 185delAG, a founder mutation seen in approximately 1% of individuals of Ashkenazi Jewish ancestry [1], was observed in 3.6% (4/110) of families and represented 11.7% (4/34) of BRCA mutations identified in this cohort. Our Hispanic 185delAG carrier families share the same haplotype from D17s1320 through BRCA1, as do two reference Ashkenazi Jewish families. This finding, along with identification of the 185delAG mutation among several apparently non-Jewish families in Spain [2-6], in a high-risk Mexican cohort [7] and in one Chilean family [8], as well as data from a founder cohort (established~1598) in the rural San Louis Valley in Colorado [9], provide further biologic evidence that the 185delAG mutation may have arisen before the widespread geographic and cultural separation of the Jews in the early Common Era that gave rise to the development of distinct Ashkenazi, Asian, North African and Sephardic (Iberian) Jewish subpopulations. The 185delAG families identified among the Spanish cohorts described above [2-6], are likely descendants of the estimated 150,000 conversos who remained in Spain through the Inquisition and assimilated into the non-Jewish population, while the carriers reported in our Hispanic-American cohort, as well as those identified in Mexico, Chile and the San Luis Valley of Colorado, are likely descendants of conversos or crypto-Jews who emigrated to the Americas in the late 15th Century and over generations assimilated into the larger Hispanic society.
 We then analyzed the remaining patients without an identifiable mutation for large BRCA rearrangements and identified 3/67 (4.5%) unrelated families with the same BRCA1 deletion of exons 9-12 mutation. DNA from a total of 106 Hispanic patients without an identifiable BRCA mutation by standard commercially available techniques was analyzed by multiplexed quantitative differential PCR (MQDP), a method to identify large rearrangements. Long range PCR was used to confirm deletion events and to clone and sequence genomic breakpoints, and splicing patterns were derived by sequencing cDNA from the RNA of affected individuals. The extent of shared chromosome markers (e.g. haploype analysis) was conducted to confirm ancestral links among unrelated families with the same mutation. A 3-primer assay was designed for detection of the BRCA1 del ex9-12 mutation based on the deletion breakpoints, identified within repetitive sequence elements that are prone to rearrangements, AluSp in intron 8 and AluSx in intron 12. The same deletion of BRCA1 exons 9-12 was identified in 5/106 unrelated families, 4 of Mexican ancestry and 1 of African and Native American ancestry. Analysis of cDNA demonstrated direct splicing of exon 8 to exon 13 resulting in a frameshift mutation and premature truncation of the BRCA1 protein, typical of deleterious mutations. Haplotype analysis of this mutation confirmed common ancestry, supporting the idea of possible founder effect of Mestizo or Amerindian origin. A manuscript detailing the identification and characterization of the BRCA1 deletion of exons 9-12 mutation was recently published (CEBP, 2007. 16: 1615)
 The overall scientific goals of our studies are to obtain comprehensive data on the prevalence and nature of BRCA mutations among Hispanic families in California as an efficient approach to genetic cancer risk assessment in women of Hispanic ancestry, and to obtain a critical dataset to guide future studies. The populations being studied include 2 population-based cohorts with and without breast cancer: the Multiethnic cohort (MEC) and La Puente Latino Eye Study (LALES) cohort and high-risk clinic-based cohorts: the City of Hope Cancer Screening & Prevention Program (CSPPN) and cohorts in Albuquerque, NM and within the NCI-supported Breast Cancer Cooperative Family Registry.
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