Abstract 1916: Peptidylarginine deiminase IV (PADI4) is a novel tumor suppressor that may contribute to regulation of breast cancer stem cell dynamics

Breast cancer is a global problem that accounts for almost a fourth of all cancers in women. Despite the many therapeutic strategies currently in use, treated patients often suffer from cancer relapse and metastasis due to the presence of a subpopulation of tumorigenic cells capable of self-renewal and termed cancer stem cells (CSCs). To address the properties of this critical sub-population, we utilized our recently-developed lentiviral CSC reporter carrying an Oct4/Sox2 response element coupled to a green fluorescent protein (SORE6-GFP) to detect and isolate the CSC population in a low-grade ER+ human breast cancer model, MCF10Ca1h, by FACS. Whole transcriptome sequencing analysis (RNA-seq) on the sorted cells identified Peptidylarginine deiminase IV (PADI4) as differentially expressed between CSCs and non-CSCs. PADI4 was recently shown to regulate citrullination of histone H1, displacing it from chromatin and resulting in global chromatin decondensation and pluripotency induction in embryonic stem cells (Christophorou, et al., 2014, Nature 507:104). Thus, we hypothesized that PADI4 may also regulate the stemness phenotype in breast cancer CSCs, likely through effects on chromatin structure. We showed that PADI4 protein expression is higher in luminal breast cancer cell lines than other subtypes, and we did shRNA knockdown (KD) of PADI4 in the luminal MCF10Ca1h model. PADI4 KD increased tumorosphere formation and clonogenicity indicating increased CSC population/activity. Additionally, MCF10Ca1h cells treated with a PADI4 selective inhibitor showed an increase in CSCs as assessed using the SORE6-GFP stem cell reporter. In vivo limiting dilution studies demonstrate that PADI4 KD increased tumor incidence at low cell inoculum, reflecting a statistically significant 2-4 fold increase in CSCs upon PADI4 KD. Our findings suggest that PADI4 may function as a tumor suppressor in luminal breast cancer. In support of this conclusion, deep deletions of PADI4 locus are found in 1% of human breast cancers in the TCGA datasets. Analysis of the effect of PADI4 KD on chromatin organization in the CSC and non-CSC compartments is ongoing. Citation Format: Nellie Moshkovich. Peptidylarginine deiminase IV (PADI4) is a novel tumor suppressor that may contribute to regulation of breast cancer stem cell dynamics [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1916. doi:10.1158/1538-7445.AM2017-1916