Direct Measure of DNA Damage in Huntington Disease Reveals Elevated Oxidative Genotoxic Stress and Dysfunctional DNA Repair

DNA damage repair (DDR) pathway proteins are genetic modifiers of Huntington disease (HD) age of onset and severity. Deficient DDR in HD at the first steps of poly ADP-ribosylation makes DNA damage quantification by traditional downstream DNA damage response markers inaccurate. Repair Assisted Damage Detection (RADD) allows for an accurate direct assessment of oxidative DNA damage, a DDR pathway in which huntingtin protein directly participates. Using RADD, we show that human HD-derived cells have elevated oxidative DNA damage and assess the effect of relevant HD therapeutic targets in rescuing this phenotype. Using huntingtin protein level lowering, we define a dysfunctional role of mutant huntingtin in oxidative DDR. We show that ataxia-telangiectasia mutated (ATM) signaling is deficient and that ATM inhibition rescues elevated oxidative DNA damage in HD cells. In contrast, we show that N6-furfuryladenine (N6FFA) treatment, to increase huntingtin phosphorylation within the amino terminal N17 domain (p-N17), is not effective at restoring HD DDR but reveals dysfunctional N6FFA mediated DDR signaling. We propose a model in which elevated DNA damage arises from both aberrant mutant huntingtin involvement in oxidative DDR and the impairment of oxidative DDR pathways, such as ATM kinase activity, N6FFA processing, and poly ADP-ribose signaling in HD. This highlights the importance of using direct measures of DNA damage such as RADD, rather than measures of a DNA damage response pathway.