Frontotemporal dementia (FTD) is the second most common type of inherited dementia following Alzheimer’s
disease. FTD is caused by the progressive neurodegeneration of cells in the frontal and temporal lobe of the
cerebral cortex. Expansion of a GGGGCC (G4C2) sequence in the first intron of the C9orf72 gene is the most
common genetic cause of FTD and is responsible for ~25% of cases. The mechanisms by which expansion of
the G4C2 sequence lead to neurodegeneration of specific neurons is incompletely understood. G4C2 RNA is
transcribed in both sense and antisense directions and both RNA strands can undergo an unusual type of
translation called Repeat Associated non-AG dependent translation (RANT). RANT of the sense and antisense
G4C2 RNA produces five distinct dipeptide repeat proteins (DPRs), two of which (PR and GR) confer strong
toxicity in multiple model systems. To better understand the pathogenesis of C9orf72-mediated FTD, we
generated C. elegans models expressing pure DPRs. Both PR and GR were toxic in worms and caused
neurodegeneration. To define genes and pathways causing toxicity, we performed an unbiased genetic
suppressor screen and discovered several highly conserved genes that blocked PR50 toxicity. One highly
conserved suppressor is the nuclear E3 ligase adaptor SPOP. SPOP is widely studied in cancer since SPOP
missense mutations are a major genetic cause of prostate and endometrial cancer. However, SPOP has never
been linked to a neurodegenerative disease until now. The role of SPOP in DPR toxicity is conserved, since
both SPOP genetic knockdown and an SPOP small molecule inhibitor blocks DPR toxicity in mammalian primary
neurons. One major SPOP target in cancer is BRD2/3/4, which are bromodomain-containing transcriptional
regulatory proteins. We found that inhibition of the BRD homolog bet-1 suppresses the ability of SPOP mutants
to protect against DPR toxicity. Based on these findings, we hypothesize that the SPOP pathway, which is
currently being targeted for the treatment of cancer, may also underlie neurodegenerative pathology in C9
disease. To test this hypothesis, we will: 1) determine whether DPRs directly interact with SPOP to modulate
known pathological pathways, such as defective nuclear transport and stress granule formation; 2) delineate the
mechanism by which SPOP, BRD, and possibly other substrates mediate DPR toxicity; and 3) determine if SPOP
is a ‘druggable’ target for neuroprotection against DPRs in mammalian neurons. Our studies will interrogate a
novel pathway associated with C9 disease using a diversity of approaches and experimental model systems.
The discovery of this novel ubiquitination system could lead to new therapeutic insights for this incurable form of
dementia.
