Screening for genes that promote copper toxicity in a cell model of Wilson disease [HepG2 cells ATP7B-KO]

To find new genes that might be targeted for reduction of Copper (Cu) toxicity in Wilson disease we employed genome-wide shRNA screening in ATP7B-KO HepG2 cells, a bona fide WD cell model. Cells were screened using the Dharmacon™ Decode™ Pooled Lentiviral shRNA Screening Library targeting 18205 human genes (Dharmacon #RHS6083). The Lentiviral RNAi screening library is made of 10 pools of 9570 GIPZ short hairpin RNAs (shRNAs) packaged into high-titer lentiviral particles. Genomic DNA was isolated from the cells and read counts for each shRNAs were obtained by deep sequencing to determine the abundance of each shRNA in the Cu-treated cells compared to controls. This analysis allowed the identification of genes whose downregulation promoted survival of ATP7B-KO cells upon Cu overload. To exclude that some shRNAs would have been responsible for ATP7B-KO survival by suppressing generic proapoptotic genes, we run the same screening with the proapoptotic drug vinblastine instead of CuCl2. Genes, whose suppression allowed cell to resist Cu but not vinblastine were considered as Cu-specific hits and through this approach we identified PrP as a suitable target for reduction of Cu toxicity in WD. Indeed, we found that Cu accumulation in ATP7B-KO cells stimulates expression of PrP, which brings to toxic Cu overload while PrP suppression significantly reduces Cu accumulation and toxicity in ATP7B-deficient cells. In sum our data highlight an important regulatory role of PrP in copper metabolism and as a therapeutic tool for Wilson disease.