Abstract
Huntington disease (HD) is a genetic neurodegenerative disease caused by CAG expansion in the Huntingtin (HTT) gene, translating to an expanded polyglutamine tract in the huntingtin (HTT) protein. Age at disease onset correlates to CAG repeat length but varies by decades between individuals with identical repeat lengths. Genome-wide association studies link HD modification to DNA repair and mitochondrial health pathways. Clinical studies show elevated DNA damage in HD, even at the premanifest stage. A major DNA repair node influencing neurodegenerative disease is the PARP pathway. Accumulation of poly ADP-ribose (PAR) has been implicated in Alzheimer and Parkinson diseases, as well as cerebellar ataxia. We report that HD mutation carriers have lower cerebrospinal fluid PAR levels than healthy controls, starting at the premanifest stage. Human HD iPSC-derived neurons and patient- derived fibroblasts have diminished PAR response in the context of elevated DNA damage. We have defined a PAR-binding motif in huntingtin, detected huntingtin complexed with PARylated proteins in human cells during stress, and localized huntingtin to mitotic chromosomes upon inhibition of PAR degradation. Direct huntingtin PAR binding was measured by fluorescence polarization and visualized by atomic force microscopy at the single molecule level. While wild type and mutant huntingtin did not differ in their PAR binding ability, purified wild type huntingtin protein increased in vitro PARP1 activity while mutant huntingtin did not. These results provide insight into an early molecular mechanism of HD, suggesting possible targets for the design of early preventive therapies.
Significance statement A consensus on dysfunctional DNA repair has emerged in neurodegenerative disease research, with elevated poly ADP-ribose (PAR) signaling more recently implicated. In contrast, we have identified a deficient PAR response in Huntington’s disease (HD) patient spinal fluid samples and cells. This may be explained by the inability of huntingtin protein bearing the HD-causing mutation to stimulate production of PAR the way the wild type protein does. Since drugs that target PAR production and degradation have already been developed, these findings present an exciting avenue for therapeutic intervention for HD.
Competing Interest Statement
EJW reports consultancy / advisory board memberships with Annexon, Remix Therapeutics, Hoffman La Roche Ltd, Ionis Pharmaceuticals, PTC Therapeutics, Takeda, Teitur Trophics, Triplet Therapeutics and Vico Therapeutics. All honoraria for these consultancies were paid through the offices of UCL Consultants Ltd., a wholly owned subsidiary of University College London. FBR is a Medpace UK Ltd employee and was a University College London employee during the conduct of this study. FBR has provided consultancy services to G.L.G. and F. Hoffmann-La Roche Ltd. LMB currently holds consultancy contracts with Annexon Biosciences, Remix Therapeutics, and LoQus23 Therapeutics Ltd via UCL Consultants Ltd. RT has past consultancy with Novartis AG, PTC Therapeutics and Mitokinin LLC.
Footnotes
We have included additional data measuring DNA damage and PAR levels in the same cells to validate the subdued PAR response in HD cells, confirmed the reduced PARP inhibitor IC50 values in HD cells with an additional PARP inhibitor, and shown that wild type huntingtin protein stimulates PARP1 activity in vitro while mutant huntingtin does not, providing a possible mechanistic link to observations in HD patient CSF and cells.
