"In the largest dataset to date that has good representation of tumors from black women, we did not find much difference between the somatic mutations driving tumors in black and white women," he added. "Yet black women were more likely to develop aggressive molecular subtypes of breast cancer. Now we provide data showing that differences in germline genetics may be responsible for up to 40 percent of the likelihood of developing one tumor subtype versus another."
The study used DNA data collected from 930 women - 154 of predominantly African ancestry and 776 of European ancestry - available through The Cancer Genome Atlas (TCGA), established by the National Cancer Institute and the National Human Genome Research Institute. The researchers combed through the data methodically, looking for racial differences in germline variations (normal DNA), somatic mutations (tumor acquired), subtypes of breast cancers, survival time, as well as gene expression, protein expression and DNA methylation patterns.
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"Most significantly," explained first author Dezheng Huo, MD, PhD, associate professor of public health sciences at the University of Chicago, "we observed a higher genetic contribution to estrogen-receptor negative breast cancer in blacks."
Black women were more likely to get these highly aggressive cancers. This is one of the first studies to connect genetics to this racial difference in tumor subtype frequencies.
The study also revealed 142 genes that showed differences in expression levels according to race. One gene, CRYBB2 (Crystallin Beta B2), was consistently higher in tumors from black patients within each breast cancer subtype, as well as in normal tissues, suggesting it may be a race-specific gene.
The researchers also found somatic mutations in 13 genes or DNA segments that differed in frequency in tumors from black and white women. One of them, a mutated gene called TP53, was more common in black women (52%) than white women (31%) and was a strong predictor of disease recurrence.
"Despite the relatively short follow-up time in the TCGA dataset, we were able to detect a significant racial disparity in patient survival using breast cancer-free interval as the endpoint between patients of African and European ancestries," said co-first author Hai Hu, PhD, vice president for research at the Chan Soon-Shiong Institute of Molecular Medicine at Windber. "Most of the worst outcomes came from basal-like subtype breast cancer patients of African Ancestry."
