Association of West African ancestry, reproductive factors, and deprivation with incidence of triple-negative breast cancer among Black women in the U.S.

1609 Background: Black women have the highest incidence of triple negative breast cancer (TNBC) of any racial or ethnic group in the U.S. The TNBC incidence rate among Black women varies substantially by state of residence (Sung H, et al, JAMA Oncol 2023), and the underlying factors driving state level variation are unknown. Genetic ancestry of West Africans and U.S.-born Black Americans (whose ancestry is primarily admixed West African) is significantly different from East Africans, and approximately 10% of the U.S. Black population identify as African immigrants. We used state level data on the number of Black residents who identify as East African immigrants to estimate the proportion of the Black population in that state with West African ancestry (defined as not East African immigrant), and we conducted a mediation analysis of state TNBC incidence data to investigate whether West African ancestry, reproductive patterns, and socioeconomic deprivation influence the relationship between race and TNBC incidence. Methods: We obtained state level TNBC incidence rates for Black women (2011-2021) from the U.S. Cancer Statistics Public Use database. State level data on country of birth from the 2020 U.S. census, rates of breastfeeding and fertility for Black residents, and socioeconomic indicators (the 2015 official poverty measure (OPM) and Multidimensional Deprivation Index (MDI)) were obtained from the U.S. Census Bureau. Correlations between TNBC incidence rates and variables of interest were tested with Spearman’s correlation test. Causal mediation analysis of TNBC incidence rate differences was performed by estimating coefficients for direct and indirect effects with linear regression models. We calculated estimates of the proportion mediated with 95% confidence intervals (CI) accounting for a priori confounders and potential effect modification at the state level. Results: State TNBC incidence rates among Black residents were inversely correlated with the proportion of residents identifying as East African immigrants (r = -0.42, p = .006) and the rate of breastfeeding (r = -0.35, p = .03). There was no correlation with fertility rates, OPM or MDI. In unadjusted analyses, East African immigrant proportion at the state level mediated 15.3% (p = 0.01) of the differences in TNBC incidence rates. After adjustment for rates of breastfeeding, fertility, and socioeconomic indicators, East African immigrant proportion was associated with 30.1% (p = 0.02) of the difference in TNBC incidence rates for Black women at the state level. Conclusions: Proportion of East African immigrants and rate of breastfeeding are inversely correlated with TNBC incidence rates in Black women. These factors transmit a portion of state level differences in TNBC incidence, suggesting that West African ancestry partially mediates the higher incidence of TNBC in Black women.