The nuclear ubiquitin ligase adaptor SPOP is a conserved regulator of C9orf72 dipeptide toxicity.

Abstract

A hexanucleotide repeat expansion in the <i>C9orf72</i> gene is the most common cause of inherited amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Unconventional translation of the <i>C9orf72</i> repeat produces dipeptide repeat proteins (DPRs). Previously, we showed that the DPRs PR50 and GR50 are highly toxic when expressed in <i>Caenorhabditis elegans</i>, and this toxicity depends on nuclear localization of the DPR. In an unbiased genome-wide RNA interference (RNAi) screen for suppressors of PR50 toxicity, we identified 12 genes that consistently suppressed either the developmental arrest and/or paralysis phenotype evoked by PR50 expression. All of these genes have vertebrate homologs, and 7 of 12 contain predicted nuclear localization signals. One of these genes was <i>spop-1</i>, the <i>C. elegans</i> homolog of <i>SPOP</i>, a nuclear localized E3 ubiquitin ligase adaptor only found in metazoans. <i>SPOP</i> is also required for GR50 toxicity and functions in a genetic pathway that includes <i>cul-3</i>, which is the canonical E3 ligase partner for <i>SPOP</i> Genetic or pharmacological inhibition of <i>SPOP</i> in mammalian primary spinal cord motor neurons suppressed DPR toxicity without affecting DPR expression levels. Finally, we find that knockdown of bromodomain proteins in both <i>C. elegans</i> and mammalian neurons, which are known <i>SPOP</i> ubiquitination targets, suppresses the protective effect of <i>SPOP</i> inhibition. Together, these data suggest a model in which <i>SPOP</i> promotes the DPR-dependent ubiquitination and degradation of <i>BRD</i> proteins. We speculate the pharmacological manipulation of this pathway, which is currently underway for multiple cancer subtypes, could also represent an entry point for therapeutic intervention to treat <i>C9orf72</i> FTD/ALS.