Connecting HTT intermediate alleles and microRNA dysregulation to enhanced tauopathy in Late-Onset Alzheimer's Disease

Background Late-onset Alzheimer´s disease (LOAD) is a heterogenous disorder influenced by genetic factors. In fact, we have previously described intermediate alleles ( IAs ) in the huntingtin ( HTT ) gene as potential modifiers in around 6% of AD population. The caudate nucleus, the most affected region in Huntington's disease, is highly sensitive to these HTT CAG expansions, as they can induce epigenetic changes, including altered microRNA profiles. All this implies a potential source of gene expression deregulation, affecting disease onset and/or progression in LOAD patients with HTT IAs . Methods We investigated the impact of HTT IAs on LOAD progression and neuropathology using a comprehensive approach, genotyping HTT CAG repeats in 323 LOAD patients and 335 healthy controls and further performing histopathological and molecular analyses on caudate nucleus samples in a small subcohort (6 healthy controls, 14 LOAD non- HTT IA carriers, and 13 LOAD HTT IA carriers). Results HTT IAs carriers patients exhibited decreased survival after disease onset, suggesting accelerated progression. Histopathologically, while LOAD patients showed increased soluble HTT levels and altered tau pathology compared to controls, these changes were consistently and markedly exacerbated in HTT IA carriers, characterized by heightened diffuse HTT immunoreactivity, pronounced tau 3R isoform imbalance, and increased 3R tau-enriched ghost tangles. Interestingly, this pathological exacerbation was supported by alterations in key splicing factors, including decreased SRSF6 and increased FUS-SFPQ complex formation. Analysis of microRNA (miRNA) profiling in the caudate nucleus revealed not only a LOAD-associated miRNA dysregulation that was significantly amplified in HTT IA carriers, but also five HTT IA signature miRNAs (miR-100-5p, miR-218-5p, miR-27b-3p, miR-487-3p, and miR-9-3p). In silico analyses, supported by network modeling and direct target validation, demonstrated that altered miRNAs target components of the nuclear spliceosome machinery, such as SRSF family, along with MAPT and HTT genes, suggesting a direct link to the observed tau 3R/4R imbalance. Conclusions Our findings underscore that HTT IAs as critical players in LOAD progression through an intricate network involving miRNA-mediated dysregulation of splicing. Thus, identifying HTT IAs through routine blood genetic screening offers a practical, non-invasive biomarker for patient stratification, taking a step closer to personalized therapeutic strategies in LOAD.