Molecular Function and Mechanism of ARID4B in ERalpha Signaling and Breast Cancer

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Project Summary/Abstract Over 70% of breast cancer are ERα+ and endocrine therapy is the standard treatment for these patients. Unfortunately, resistance to endocrine therapy develops over time and remains a major problem. The cause for resistance is not fully understood, but aberrant ERα activation is an underlying factor. Therefore, identifying new component of the ERα signaling and understanding its role in breast cancer holds a great promise for treatment of ERα+ cancer and will particularly benefit patients with advanced and metastatic tumors that are refractory to current endocrine therapies. Our preliminary results showed that chromatin remodeling protein AT-rich interaction domain 4B (ARID4B) is an essential transcription co-activator (not a co-repressor) for ERα and mammary gland-specific ablation of Arid4b inhibits tumorigenesis. Analyses of large-scale genomic datasets (TCGA and other breast cancer cohorts), genome-wide transcriptome, and IHC analyses showed that ARID4B is amplified and its expression elevated in ERα+ breast cancer. Interestingly, high ARID4B expression in ERα+ (but not ERα-) breast cancers is associated with increased risk of cancer recurrence and decreased survival, suggesting that ARID4B is involved in the cancer development and progression to therapy resistance. To gain mechanistic insight into its function, preliminary results revealed that ARID4B is recruited to the promoters of ERα target genes that are far away from the identified ERα-bound enhancers. In addition, DHX9 was identified as a novel ARID4B-interacting protein and is involved in ERα activation. DHX9 is an NTP-dependent helicase capable of resolving transcription-coupled ‘R-loops’ that is inhibitory to productive transcription. Based on these findings, our central hypothesis is that ARID4B activates ERα signaling to promote tumorigenesis and endocrine therapy resistance by: (1) mediating promoter-enhancer looping via interaction with ERα and (2) by recruiting DHX9 to resolve the transcription-coupled R-loops on promoters and enhancers to promote productive transcription. In Aim 1, the in vivo function of ARID4B in tumor initiation, cancer growth, and endocrine therapy resistance will be investigated using several breast cancer cells in tumor xenograft models, patient-derived resistant tumors in PDX, and a newly generated ARID4B knock-in mouse model. In Aim 2, we will investigate how ARID4B-mediated enhancer-promoter looping regulates ERα activation and identify the important functional domain(s) on ARID4B to provide in-depth understanding of ARID4B-ERα and ARID4B-DHX9 interactions. Furthermore, we will investigate whether resolution of “R-loops” on the promoter and enhancer by DHX9 is required for the activation of ERα. Finally, we will demonstrate that targeting the ARID4B-DHX9 axis by suppressing DHX9 expression or activity inhibits tumorigenesis and resistance to antiestrogens. Our study will fill the crucial gap in understanding the function and mechanism underlying the emerging oncogenic activity of ARID4B and establish ARID4B-DHX9 axis as a therapeutic target for breast cancer.