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Abstract Intermediate-length repeat expansions in ATAXIN-2 (ATXN2) are the strongest genetic risk factor for amyotrophic lateral sclerosis (ALS). At the molecular level, ATXN2 intermediate expansions enhance TDP-43 toxicity and pathology. However, whether this triggers ALS pathogenesis at the cellular and functional level remains unknown. Here, we combine patient-derived and mouse models to dissect the effects of ATXN2 intermediate expansions in an ALS background. iPSC-derived motor neurons from ATXN2-ALS patients show altered stress granules, neurite damage and abnormal electrophysiological properties compared to healthy control and other familial ALS mutations. In TDP-43 Tg -ALS mice, ATXN2-Q33 causes reduced motor function, NMJ alterations, neuron degeneration and altered in vitro stress granule dynamics. Furthermore, gene expression changes related to mitochondrial function and inflammatory response are detected and confirmed at the cellular level in mice and human neuron and organoid models. Together, these results define pathogenic defects underlying ATXN2-ALS and provide a framework for future research into ATXN2-dependent pathogenesis and therapy.

ABSTRACT Objective To summarize and evaluate evidence pertaining to the clinical, genetic, histopathological, and neuroimaging correlates of cognitive and behavioral dysfunction in amyotrophic lateral sclerosis (ALS). Methodology We comprehensively reviewed the literature on cognitive and behavioral manifestations of ALS, narrating findings from both cross‐sectional and longitudinal studies. We discussed knowledge gaps in the evidence base and key limitations affecting studies to date, before formulating a framework for future research paradigms aimed at investigating clinicopathological correlates of neuropsychological dysfunction in ALS. Results Studies have demonstrated clinical associations with cognitive dysfunction in ALS e.g., bulbar‐onset of symptoms, pathological associations (extramotor TDP‐43 deposition), and imaging associations (frontotemporal involvement). The most common behavioral deficit, apathy, is highly associated with verbal fluency, but longitudinal studies assessing behavioral dysfunction in ALS are comparatively lacking. Conclusion Longitudinal studies have been helpful in identifying several potential correlates of cognitive and behavioral dysfunction but have frequently been confounded by selection bias and inappropriate testing platforms. This review provides a framework for more robust assessment of clinicopathological associations of neuropsychological abnormalities in ALS in the future, advocating for greater utilization of pre‐symptomatic C9orf72 repeat expansion‐carrying cohorts.

Background C9orf72 hexanucleotide repeat expansions are the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) in European populations. Variable disease penetrance between families presents a challenge for genetic counselling of at-risk relatives and reduces the predictive utility of testing asymptomatic relatives. We have developed a novel model for estimating penetrance in individual families affected by C9orf72 using available family history information, allowing the calculation of personalised risk estimates.Methods Published aggregated age-of-onset data for C9orf72-related ALS/FTD were used to generate age-related cumulative relative risks for at-risk relatives within pedigrees. Age-related relative risks are combined with a priori chance of individuals carrying an expansion based on known pedigree information. Penetrance is calculated as a number of affected individuals divided by the sum of cumulative age-related risks of relatives being affected by 80 years.Results This method allows family-specific penetrance to be estimated from family history and at-risk relatives’ personalised age-related ALS/FTD risks to be calculated and illustrated graphically. Penetrance reduces as the number and age of at-risk unaffected relatives increases.Conclusions Family history remains the best indicator of penetrance in C9orf72 expansion carriers. Calculating family-specific penetrance can aid genetic counselling by allowing at-risk relatives a more accurate understanding of their individual risk.