HYPOVITAMINOSIS D PROMOTES MED 12-ASSOCIATED GENOMIC INSTABILITY IN UTERINE FIBROIDS

Uterine fibroids (UFs; leiomyomas) are the most important benign neoplastic threat to women's healthworldwide, but disproportionately affect women of color, particularly African American (AA) women, who have athreefold higher incidence rate and relative risk of UFs than Caucasian (CC) women. While the underlyingcause for this risk disparity is not fully understood, recent studies implicate hypovitaminosis D as a majorcontributor. Thus, AA women have a tenfold increased risk of vitamin D deficiency compared to CC women,and as we first reported, UF risk is inversely correlated with 25-hydroxy vitamin D serum levels. Nonetheless, itis not clear whether and how the processes that drive UF formation and determine relative risk are geneticallyor biochemically linked. In this regard, we and others have identified somatic mutations in the transcriptionalMediator subunit MED12 as the dominant drivers of UFs, accounting for ~70% of tumors. Notably, MED12-mutant UFs are characterized by significant chromosomal loss and rearrangement, suggesting genomicinstability as a driving force in tumor progression. Herein, we clarify the molecular basis for mutant MED12-driven genomic instability, and further identify vitamin D3 receptor signaling as a likely suppressor of thisprocess. We show that MED12-mutant UF stem cells (SCs) accumulate high levels of unrepaired DNA double-strand breaks (DSBs) through downregulation of key DNA damage response (DDR) and repair genes. Notably,we find the vitamin D3/receptor axis to be a variable modulator of MED12-regulated DDR gene expression.Thus, we show that reduced vitamin D3/receptor signaling suppresses, while elevated signaling activates,DDR genes downregulated in MED12-mutant UF SCs. Based on these findings, we hypothesize thathypovitaminosis D exacerbates DNA damage accumulation and genomic instability arising in MED12-mutantUFs, leading to enhanced tumor progression and burden. Accordingly, we propose that vitamin D3, throughreparation of an impaired DDR will provide therapeutic benefit in MED12-mutant tumors. To test thesehypotheses, we will (1) Elucidate the molecular basis of genomic instability in MED12-mutant UFs. We willdetermine if DSB accumulation in MED12-mutant UF SCs derives from defects in DNA damage-inducedcheckpoint signaling and repair and/or R-loop-induced replication stress. (2) Investigate the relationshipbetween vitamin D3 and MED12 in UF genome maintenance. We will ask whether and how vitamin D3signaling strength modulates the DDR defects in MED12-mutant UF SCs, relate this activity to patient race andserum vitamin D levels, and elucidate the mechanism by which the vitamin D3/receptor axis and MED12coordinately control the DDR network at the genomic and epigenomic levels; (3) Examine the therapeuticpotential of vitamin D3 in a preclinical mouse model of human UFs. Using a renal capsule mouse model ofhuman UFs, we will evaluate vitamin D3 and its potent non-hypercalcemic analogs for therapeutic efficacy,safety, and mechanism of action, including impact on tumor DNA damage load and DDR gene networks.