SRRM4 expression and the loss of REST activity may promote the emergence of the neuroendocrine phenotype in castration-resistant prostate cancer

Purpose: The neuroendocrine phenotype is associated with the development of metastatic castration-resistant prostate cancer (CRPC). Our objective was to characterize the molecular features of the neuroendocrine phenotype in CRPC. Experimental Design: Expression of chromogranin A (CHGA), synaptophysin (SYP), androgen receptor (AR), and prostate-specific antigen (PSA) was analyzed by IHC in 155 CRPC metastases from 50 patients and in 24 LuCaP prostate cancer patient-derived xenografts (PDX). Seventy-one of 155 metastases and the 24 LuCaP xenograft lines were analyzed by whole-genome microarrays. REST splicing was verified by PCR. Results: Coexpression of CHGA and SYP in >30% of cells was observed in 22 of 155 metastases (9 patients); 11 of the 22 metastases were AR⁺/PSA⁺ (6 patients), 11/22 were AR-/PSA-(4 patients), and 4/24 LuCaP PDXs were AR^-/PSA^-. By IHC, of the 71 metastases analyzed by whole-genome microarrays, 5 metastases were CHGA⁺/SYP⁺/AR^-, and 5 were CHGA⁺/SYP⁺/AR⁺. Only CHGA⁺/SYP⁺ metastases had a neuroendocrine transcript signature. The neuronal transcriptional regulator SRRM4 transcript was associated with the neuroendocrine signature in CHGA⁺/SYP⁺ metastases and all CHGA⁺/SYP⁺ LuCaP xenografts. In addition, expression of SRRM4 in LuCaP neuroendocrine xenografts correlated with a splice variant of REST that lacks the transcriptional repressor domain. Conclusions: (i) Metastatic neuroendocrine status can be heterogeneous in the same patient, (ii) the CRPC neuroendocrine molecular phenotype can be defined by CHGA⁺/SYP⁺ dual positivity, (iii) the neuroendocrine phenotype is not necessarily associated with the loss of ARactivity, and (iv) the splicing of REST by SRRM4 could promote the neuroendocrine phenotype in CRPC.