Abstract LB-199: Role of ASCL1 in neuroendocrine prostate cancer progression
Authors: Sinha Seema , Nyquist Michael D. , Corella Alexandra , Coleman Ilsa , Nelson Peter S. 
Published: August 2018

Abstract
INTRODUCTION: Metastatic prostate cancer is a lethal disease. Many patients develop a form of the disease resistant to Androgen Deprivation Therapy (ADT) called castration resistant prostate cancer (CRPC), and androgen refractory tumors emerge. A subset of these involve gaining small cell and neuroendocrine (NE) features, resulting in neuroendocrine prostate cancer (NEPC). The identification of key drivers and the mechanism of emergence is critical, as NEPC is associated with poor outcomes.
Of interest is the neuron lineage differentiation gene achaete-scute complex homologue 1 (ASCL1). Mouse models of small cell lung cancer (SCLC) show ASCL1 as a key promoter of tumor growth in vivo. While there is overlap between SCLC and NEPC (inactivation of RB1 and TP53, ASCL1 overexpression (OE)), it's unclear if ASCL1 contributes to transdifferentiation of prostate epithelium into NEPC. In addition to genetic drivers, defined soluble factors can promote reprogramming of fibroblasts into neurons. We thus hypothesize that ASCL1 in combination with soluble factors may promote transdifferentiation of prostate epithelium to NEPC.
METHODS: CRISPR technology is used to knockout (KO) RB1 and TP53 in LNCaP cells. Lentiviral vector is used to OE for ASCL1 or RFP in LNCaP or LNCap p53/RB1 KO cells. Gene expression for AR, AR target genes, neuronal, and NE genes is measured via qRT-PCR and Western. Regular FBS or neuronal media containing either FGF2/BDNF/NTF3 or all soluble factors is used for cell culture.
RESULTS: RNAseq data from metastatic AR+ CRPC, AR- NEPC, and SCLC tumors and cell lines shows OE of ASCL1 in AR- NEPC and SCLC. LNCaP ASCL1 OE cells show no change in cellular growth in FBS, CSS, or NM + supplements (BDNF, FGF2, NT3) compared to control. However, LNCaP p53/RB1 KO ASCL1 OE cells + 10μM Enzalutamide (ENZ) proliferated similar to the cells without any drug treatment when cultured in NM + sup (96h, p=0.999). Both LNCaP ± ASCL1 and LNCaP p53/RB1 KO ± ASCL1 cells grown in NM + sup show significantly decreased AR and AR target gene expression (~10 fold, p&lt; 0.05). FGF8 (~5 fold), SOX2 (&gt;300 fold), and SYP (&gt;10 fold) gene expression increased significantly (p&lt;0.05) in LNCaP ± ASCL1 grown in NM + sup. LNCaP p53/RB1 KO ± ASCL1 OE in NM + sup show ~3-6 fold increase in SOX2 and ~30-50 fold increase in FGF9 compared to cells cultured in FBS.
CONCLUSION: Loss of TP53 and RB1 are not sufficient to induce transdifferentiation to a NEPC state, but ASCL1 OE induced the expression of SOX2 and SYP that may trigger the process. Microenvironment effects operating through paracrine factors induced the expression of a NE gene program and reduced AR expression. Future studies will investigate the role of ASCL1 in promoting resistance to ENZ in prostate adenocarcinoma cells as well as its role in in vivo tumor growth. ASCL1, NEUROD1, and MYTL1 that are OE in NEPC and SCLC in combination with individual soluble factors will be used to delineate the mechanism required for the transdifferentiation process.
Citation Format: Seema Sinha, Michael D. Nyquist, Alexandra Corella, Ilsa Coleman, Peter S. Nelson. Role of ASCL1 in neuroendocrine prostate cancer progression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr LB-199.