Background: Large rearrangements account for 8% to 15% of deleterious BRCA mutations, although none have been characterized previously in individuals of Mexican ancestry. Methods: DNA from 106 Hispanic patients without an identifiable BRCA mutation by exonic sequence analysis was subjected to multiplexed quantitative differential PCR. One case of Native American and African American ancestry was identified via multiplex ligation-dependent probe amplification. Long-range PCR was used to confirm deletion events and to clone and sequence genomic breakpoints. Splicing patterns were derived by sequencing cDNA from reverse transcription-PCR of lymphoblastoid cell line RNA. Haplotype analysis was conducted for recurrent mutations. Results: The same deletion of BRCA1 exons 9 through 12 was identified in five unrelated families. Long-range PCR and

10015 Background: Large rearrangements are estimated to account for 5–10% of all mutations in BRCA1 and BRCA2. Prevalent founder rearrangement mutations have been described in European populations. We sought to identify rearrangements in the BRCA genes in a cohort of Hispanic patients. Methods: We identified 34 deleterious BRCA mutations via full sequence analysis among 110 unrelated high-risk Hispanic families enrolled in an IRB approved registry who underwent cancer risk assessment (CEBP 2005;14:1–6). DNA from 67 of 76 patients without an identifiable mutation was subjected to multiplexed quantitative differential PCR (MQDP, per B. Erickson and T. Scholl, Myriad Genetics Laboratory) for detection of large rearrangements. An apparent deletion of BRCA1 exons 9–12, indicated by a 50% loss of signal was identified by MQDP in 3 unrelated families (ASHG, 2005). Long range PCR resulted in the generation of a 2.7kb product in these samples, consistent with a deletion event of 15.1kb. This putative mutation was further characterized by cloning and sequencing the breakpoint in all 3 families. RNA splicing was evaluated by sequencing RT-PCR products from lymphoblastoid cell line RNA for each family. Results: Sequence analysis identified the breakpoint within Alu elements in introns 8 and 12, and all 3 unrelated families shared the same breakpoint. Analysis of cDNA demonstrated direct splicing of exons 8–13 predicting a frameshift mutation and premature truncation of the BRCA1 protein, thus confirming the deleterious nature of this mutation. Conclusion: We identified the same novel large deletion in three unrelated families of Mexican ancestry, suggesting potential founder effect. The frequency was 2.7% of the 110 high-risk Hispanic families screened for BRCA mutations, bringing the overall prevalence of deleterious mutations to 33.6%(37/110) in this cohort. This BRCA1 rearrangement may account for a substantial proportion of high-risk Hispanic families, and should be included in all subsequent studies of this ethnic group. No significant financial relationships to disclose.