A hexanucleotide repeat expansion in the C9orf72 gene is the most common cause of inherited amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Unconventional translation of the C9orf72 repeat produces dipeptide repeat proteins (DPRs). Previously, we showed that the DPRs (PR)50 and (GR)50 are highly toxic when expressed in C. elegans and this toxicity depends on nuclear localization of the DPR. In an unbiased genome-wide RNAi screen for suppressors of (PR)50 toxicity, we identified 12 genes that consistently suppressed either the developmental arrest and/or paralysis phenotype evoked by (PR)50 expression. All of these genes have vertebrate homologs and 7/12 contain predicted nuclear localization signals. One of these genes was
spop-1, the C. elegans homolog of SPOP, a nuclear localized E3 ubiquitin ligase adaptor only found in metazoans.
spop-1 is also required for (GR)50 toxicity and functions in a genetic pathway that includes
cul-3, which is the canonical E3 ligase partner for SPOP. Genetic or pharmacological inhibition of SPOP in mammalian primary spinal cord motor neurons suppressed DPR toxicity without affecting DPR expression levels. SPOP is commonly mutated in prostate, endometrial, and renal cancer. We find that CRISPR/Cas9 knock-in of two of the most common cancer-causing SPOP missense mutations also protect against (PR)50 toxicity, suggesting that similar mechanisms mediate the roles of SPOP in cancer and C9 toxicity. Finally, we find that genetic inhibition of
bet-1, the C. elegans homolog of the known SPOP ubiquitination targets BRD2/3/4, suppresses the protective effect of
spop-1 mutations. Together, these data suggest a model in which SPOP promotes the DPR-dependent ubiquitination and degradation of BRD proteins. We speculate the pharmacological manipulation of this pathway, which is currently underway for multiple cancer subtypes, could also represent a novel entry point for therapeutic intervention to treat C9 FTD/ALS.