bioRxiv2025
BackgroundRecent evidence suggests that accumulation of mutant exon 1 protein (HTT1a) may be critical to HD pathogenesis, but the relation of this to differential regional and cellular vulnerability in HD is unknown. ObjectiveWe assessed the contribution of the accumulation of the mutant huntingtin HTT1a to the regional and cellular variation in HD brain pathology by determining if more vulnerable regions and neuron types were relatively enriched. MethodsWe performed immunolabeling using the novel monoclonal antibodies 11G2 and 1B12 against the C-terminal proline 90 (P90) neoepitope of huntingtin HTT1a, which detect accumulation of monomeric, oligomeric and aggregated mutant HTT1a, on forebrain of Q175 and R6/2 mice and human HD cases. ResultsDiffuse nuclear and aggregate immunolabeling increased in abundance in Q175 with age, with striatal projection neurons showing immunolabeling earlier than cortical neurons, and only neuropil immunolabeling prominent in pallidal regions. Nonetheless, some regions less affected in HD, such as hippocampus, were rich in mutant HTT1a as well. In humans, striatal immunolabeling was sparser than in mouse, and mainly in the neuropil, but sparser in striatal target areas. In human HD cortex, the P90 antibodies detected predominantly neuropil aggregates, which appeared to, in part, localize to dendrites. Immunostaining in mouse and human could be blocked with HTT1a target peptide, demonstrating antibody specificity. ConclusionsOur results indicate that mutant HTT1a burden appears to partly account for overall differential forebrain regional vulnerability in HD, but additional factors may contribute to vulnerability differences among forebrain regions and between specific neuron types. Plain Language SummaryHuntingtons disease (HD) is caused by a mutant gene that is passed from one generation to the next. The mutant gene causes production of a mutant variant of an otherwise valuable protein called huntingtin. This mutant protein is thought to gradually damage neurons in the brain, leading to such extensive loss in a part of the brain called the basal ganglia that movement is impaired. The disability is eventually so severe, it proves fatal. It has been a mystery as to why the mutant protein might cause brain damage, but more so to some brain areas than others. One important recent clue has emerged from studies of the abnormal huntingtin protein that cells with the mutation make. Namely, rather than make a huntingtin protein of full size, as occurs normally, cells instead make only an abbreviated form of the huntingtin protein, but one that contains the abnormality. We hypothesized that if this mutant fragment is particularly toxic and the cause of the neuron damage in HD, then those brain regions that are most injured in the disease should accumulate more of this mutant fragment early in disease. We evaluated this hypothesis by examining accumulation of this mutant fragment, using a staining method selective for the fragment, in histological specimens from the brains of humans with HD and mice engineered to possess the same mutant gene as in the human disease. We found to a large extent that it is the case that those brain regions that showed the most disease-related damage, such as the basal ganglia, also accumulated the mutant huntingtin protein fragment the most. This fragment thus does appear to be toxic for neurons. Our work suggests that therapeutic efforts for HD should be directed at preventing its production or accumulation.