Poly ADP-Ribose Signaling is Dysregulated in Huntington’s Disease Patients

Huntington’s disease (HD) is an autosomal dominant genetic neurodegenerative disease caused by a CAG expansion in the Huntingtin (HTT) gene, translating to an expanded polyglutamine tract in the huntingtin (HTT) protein. Age at disease onset is correlated 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. Recent clinical studies show elevated DNA damage in HD, even at the premanifest stage of disease. One of the major DNA repair nodes influencing neurodegenerative disease is the PARP pathway. Accumulation of poly ADP-ribose (PAR), produced by PARP1 and PARP2, has been implicated in the pathology of Alzheimer’s and Parkinson’s diseases, as well as autosomal recessive cerebellar ataxia. We report that HD mutation carriers have lower cerebrospinal fluid PAR levels than healthy controls, starting at the premanifest stage. Patient-derived fibroblasts have reduced PARP1/2 activity and elevated DNA damage, while elevated PAR levels are only revealed upon inhibition of PAR degradation. These phenotypes are rescued by moderate huntingtin level reduction via the huntingtin-lowering splice modulator drug, LMI070 (Branaplam). As a direct mechanism, 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. These results provide insight into a very early molecular mechanism of HD, suggesting possible targets in HD to design early preventive therapies. ### Competing Interest Statement RT is a consultant to Novartis AG. 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.