Assessing healthy breast tissue for evidence of ancestry-dependent molecular contributions to TNBC disparities

In some diseases, people of a given sex or ancestry experience a higher incidence, more aggressive biology, and different survival. The differences frequently persist after accounting for modifiable risk factors, suggesting underlying intrinsic causes. However, genomic studies did not uncover DNA changes (i.e., mutations) that can

adequately explain these differences. Our data show that the transcriptome strongly contributes to dispari- ties. We posit that studying the transcriptome of healthy tissues can reveal a predisposition of some ancestries to future aggressive biology and to disparities after disease is diagnosed in these tissues.

The project will leverage findings relating to three classes of short RNAs: the isoforms of microRNAs (miRNAs) known as isomiRs, the fragments of transfer RNAs (tRNAs) known as tRFs, and the fragments of ribosomal RNAs (rRNAs) known as rRFs. The expression of these RNAs depends on personal attributes (e.g., biological

sex, ancestry) and context (e.g., tissue type, disease type). Additionally, all three RNA classes regulate messen- ger RNA abundance and, by extension protein abundance. Because of their properties, these three RNA classes are ideal tools for investigating the molecular basis of disparities. Published findings in multiple diseases support

this view. Three additional observations are relevant for this project: 1. In triple-negative breast cancer (TNBC), tumors from African American (AA) patients show evidence of ancestry-based (transcriptomic, regulatory) differences and more aggressive biology. 2. In TNBC, normal-tissue-adjacent-to-the-tumor (NAT) specimens, too, from AA patients show evidence of

ancestry-based differences and more aggressive biology. 3. NAT is an interim state between healthy tissue and tumor, and distant from both endpoints. Since these ancestry-based differences are found at two distinct time points (i.e., NAT and tumors), it is reason- able to assume that these differences have their origin further back in time, presumably in the healthy tissue. We

hypothesize that studying isomiRs, tRFs, and rRFs in healthy tissues can uncover a predisposition of some ancestries to future aggressive biology in those tissues. We will test a specific instance of our hypothesis using truly normal breast specimens from healthy women from three ancestries: AA, Hispanic White (HW), and non-Hispanic white (NHW). We chose these ancestries

because AA and HW women have a higher TNBC incidence, higher metastasis rates, and worse survival than NHW women. The specimens are already available through the Komen Tissue Bank at Indiana University. While this project focuses on a single tissue type, we conjecture that general principles underlie the ancestry-,

sex-, and age-dependent differences that are linked to isomiRs, tRFs, and rRFs. The principles would apply to additional tissue, sex, age, and ancestry combinations. If successful, the project will generate foundational re- sults that will inform and guide subsequent focused mechanistic studies of disparities.