The progressive rod-cone degeneration (PRCD) protein is secreted through the conventional ER/Golgi-dependent pathway

Retinitis pigmentosa (RP) is the most common form of hereditary retinal degeneration. Mutations of the PRCD gene are associated with RP in both dogs and humans. To date, four distinct PRCD mutations have been reported worldwide. Here we report the clinical phenotype of another patient with PRCD mutations, carrying the known p.R18X mutation and a novel missense mutation, p.P25T. This mutation affects a highly conserved amino acid, is predicted to be damaging by several prediction tools, and was not found in the public databases or in 115 ethnically-matched control individuals. The phenotype of this patient resembles that of previously reported patients with PRCD mutations, including bull's eye maculopathy, which appears to be a hallmark of the PRCD-induced phenotype. PRCD encodes for a 54 amino acids long protein with unknown function. The first 20 amino acids appear to encode for a signal peptide (SP), suggesting that PRCD is a secreted protein. To study PRCD secretion, C-terminally myc-tagged PRCD was expressed in cultured cells. Cells and conditioned media were analyzed by Western blot. PRCD was found in both cell extracts and media. However, a truncated PRCD protein lacking the first 20 amino acids was present only in cell extracts and not in media, confirming that PRCD extracellular secretion is mediated by its N-terminal SP. To characterize the secretory pathway of PRCD, various pharmacological agents which interfere with transport of proteins through the ER and Golgi to the plasma membrane were used. PRCD secretion was significantly inhibited by all tested pharmacological agents, confirming that it is secreted through the classic ER/Golgi-dependent secretory pathway. We tested the effect of two mutations on the PRCD protein, and found that p.C2Y, but not p.P25T, affects protein stability, and that neither mutation affects secretion. Our data suggest that PRCD functions as a secreted protein. These findings shed a new light on PRCD function and the etiology of RP.