Abstract A076: The role of PARP-1-mediated FoxA1 ADP-ribosylation in breast cancer biology
Authors: Camacho Cristel V. , Sundaresan Aishwarya , Nandu Tulip , Kraus W. Lee 
Published: February 2024

Abstract
Breast cancer is the most common cancer diagnosed in women. At the time of diagnosis, approximately 70% of breast cancers express Estrogen Receptor alpha (ERα), a key signature of luminal subtype tumors. Equally important in these cases is the expression of FoxA1, a subtype-specific transcription factor (TF) essential for ERα-mediated cell proliferation. FoxA1 is a key pioneer TF that assists ERα with the assembly of enhancers and the transcription of estrogen (E2)-regulated target genes. Together, the FoxA1-ERα axis is a major driver of cancer cell growth. Our recently published work implicates poly(ADP-ribose) polymerase 1 (PARP-1) and ADP-ribosylation (ADPRylation) in the regulation of E2-dependent transcriptional responses in ERα+ breast cancer cells. To determine the underlying mechanisms, we identified FoxA1 as a substrate of PARP-1-mediated ADPRylation. We hypothesize that (1) PARP-1 elicits its effects on E2 signaling and ERα enhancer function, in part, by ADPRylating FoxA1 at specific residues and (2) Loss of these regulatory ADPRylation events alters FoxA1 activity, which in turn can alter the assembly of ERα enhancers and transform the breast cancer cell transcriptome. Indeed, the sites of FoxA1 ADPRylation are recurrently mutated in breast cancers and are highly enriched in metastatic cases. Using an integrated set of biochemical, molecular, cell based, genomic and proteomic approaches, we have validated the ADPRylation of FoxA1 by PARP-1 in vitro and in vivo. We selected two FoxA1 ADPRylation sites, D226 and E255, for further analysis due to their positions in a mutation ‘hot spot’ region corresponding to the C-terminal forkhead DNA-binding domain. Mutation of these sites blocks FoxA1 ADPRylation and enhances cell proliferation in MCF-7 human breast cancer cells. Additionally, preliminary ChIP-seq analysis indicates that the binding of both ERα and FoxA1 are globally reduced at ERα enhancers in MCF-7 cells expressing the FoxA1 ADPRylation site mutants compared to wild-type but may also increase both at selected enhancers. While there is a global reduction in mutant FoxA1 binding near the canonical estrogen-regulated genes, interestingly, we observe an overall enhanced binding of FoxA1 D226N compared to wild-type outside of this canonical program. Collectively, these studies will (1) reveal new molecular endpoints modulated by PARP-1, providing new insights about the use of PARPi to treat ERα+ breast cancers, and (2) suggest new therapeutic strategies to disrupt the aberrant signaling pathways associated with mutant FoxA1 luminal breast cancers, and more importantly metastatic cases, which is of utmost clinical significance.
Citation Format: Cristel V. Camacho, Aishwarya Sundaresan, Tulip Nandu, W. Lee Kraus. The role of PARP-1-mediated FoxA1 ADP-ribosylation in breast cancer biology [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Breast Cancer Research; 2023 Oct 19-22; San Diego, California. Philadelphia (PA): AACR; Cancer Res 2024;84(3 Suppl_1):Abstract nr A076.