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Access Type
WSU Access
Date of Award
January 2021
Degree Type
Thesis
Degree Name
M.S.
Department
Molecular Biology and Genetics
First Advisor
Michael A. Tainsky
Abstract
Breast cancer represents a cancer disparity in African Americans in which incidence is lower than in Caucasian women, but mortality is higher. Germline variants contribute to this disparity and 5-10% of breast cancer cases are thought to result from these inherited factors. Hereditary Breast and Ovarian Cancer Syndrome (HBOC) is characterized by an increased risk of breast, ovarian, pancreatic and, to a lesser extent, a few other cancers. HBOC is caused by pathogenic variants in genes involved in DNA repair and cell cycle regulation the riskiest of which occur in the BRCA1 and BRCA2 genes. Patients suspected of hereditary cancer undergo genetic testing and identified variants are classified according to evidence supporting or opposing the variant as the cause of the condition. A classification of a variant of unknown significance represents a middle ground in which insufficient or conflicting evidence neither implicates nor exonerates the variant as the cause. As these variants are of limited clinical utility, efforts are made to reclassify them. As part of an effort to investigate the contribution of such variants, genetic testing results, mostly from testing of BRCA1 and BRCA2 only, from a cohort of African American women diagnosed with breast cancer were accessed. The BRCA1 S1613C missense variant was found in two unrelated women that tested negative for pathogenic variants in BRCA1 and BRCA2. This variant is rare and African-specific and several in silico prediction tools predict this variant to be damaging to protein function. Classifying such a variant is difficult because a missense variant is not as obviously damaging as a nonsense or frameshift variant and the rarity complicates the use of case/control or segregation data. Thus, functional assays which interrogate the effects of variants on protein function are crucial to classifying such variants. Given BRCA1’s important role in facilitating accurate repair of DNA double-strand breaks through homologous recombination, this function was leveraged to examine the effect of the S1613C variant. In HeLa cells, the S1613C variant did not perturb BRCA1 function in homologous recombination supporting a classification as likely benign in this context. Conversely, in U2OS cells the S1613C variant had an intermediate effect on homologous recombination. A closer examination revealed that U2OS cells contain a truncating, gain of function mutation in the PPM1D gene that encodes the WIP1 phosphatase which is involved in counteracting the DNA damage response. Amplification or somatic mutation of WIP1 occurs in several cancer types including breast and ovarian cancer and WIP1 modulates homologous recombination. This provides support for the WIP1 truncation as a possible modifier of penetrance and has potential implications in cancer risk assessment and treatment of cancers in carriers of the BRCA1 S1613C variant and WIP1 amplification or mutation.
Recommended Citation
Depoorter, Douglas Gerald, "Functional Evaluation Of The Brca1 S1613c Variant Of Unknown Significance In Hereditary Breast And Ovarian Cancer Syndrome" (2021). Wayne State University Theses. 843.
https://digitalcommons.wayne.edu/oa_theses/843