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 heterogeneous disorder influenced by complex genetic factors. We previously described intermediate alleles (IAs; 27–35 CAG repeats) 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 repeats. We hypothesized that HTT IAs induce gene expression deregulation, including altered microRNA (miRNA) profiles, leading to altered disease progression. METHODS: We investigated the impact of HTT IAs on LOAD progression by genotyping HTT CAG repeats in a cohort of 323 LOAD patients and 335 healthy controls. Comprehensive histopathological and molecular analyses were performed on caudate nucleus samples from a matched 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 compared to non-carriers. 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. This phenotype was characterized by heightened diffuse HTT immunoreactivity and an advanced maturation of tau pathology, specifically a pronounced increase in neuronal tau 3R burden and 3R tau-enriched ghost tangles. Interestingly, this pathological state was associated with alterations in key splicing factors, including decreased SRSF6 levels and increased nuclear FUS-SFPQ complex assembly. Analysis of microRNA (miRNA) profiling in the caudate nucleus revealed that the LOAD-associated miRNA dysregulation was significantly amplified in HTT IA carriers, identifying a signature of five miRNAs (miR-100-5p, miR-218-5p, miR-27b-3p, miR-487-3p, and miR-9-3p). In silico modeling based on high-confidence sequence homology demonstrated that these miRNAs are predicted to target components of the nuclear spliceosome machinery, such as SRSF family, along with MAPT and HTT genes, suggesting a direct link to the observed tauopathy. CONCLUSIONS: Our findings underscore that HTT IAs as critical modifiers in LOAD progression through an miRNA-mediated dysregulation of splicing and proteostasis. Thus, identifying HTT IAs through routine blood genetic screening offers a practical, non-invasive biomarker for patient stratification, taking a step forward to personalized therapeutic strategies in LOAD.