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Pathogenic short tandem repeat (STR) expansions cause over 20 neurodegenerative diseases. To determine the contribution of STRs in sporadic amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), we used ExpansionHunter, REviewer, and polymerase chain reaction validation to assess 21 neurodegenerative disease-associated STRs in whole-genome sequencing data from 608 patients with sporadic ALS, 68 patients with sporadic FTD, and 4703 matched controls. We also propose a data-derived outlier detection method for defining allele thresholds in rare STRs. Excluding C9orf72 repeat expansions, 17.6% of clinically diagnosed ALS and FTD cases had at least one expanded STR allele reported to be pathogenic or intermediate for another neurodegenerative disease. We identified and validated 162 disease-relevant STR expansions in C9orf72 (ALS/FTD), ATXN1 [spinal cerebellar ataxia type 1 (SCA1)], ATXN2 (SCA2), ATXN8 (SCA8), TBP (SCA17), HTT (Huntington's disease), DMPK [myotonic dystrophy type 1 (DM1)], CNBP (DM2), and FMR1 (fragile-X disorders). Our findings suggest clinical and pathological pleiotropy of neurodegenerative disease genes and highlight their importance in ALS and FTD.

Once a gene mutation that is causal of amyotrophic lateral sclerosis (ALS) and/or frontotemporal dementia (FTD) is identified in a family, relatives may decide to undergo predictive genetic testing to determine whether they are at risk of developing disease. Recent advances in gene discovery have led to a pressing need to better understand the implications of predictive genetic testing. Here we review the uptake of genetic counselling, predictive and reproductive testing, and the factors that impact the decision to undergo testing, for consideration in clinical practice. The literature suggests that the factors impacting the decision to undergo testing are complex due to the nature of these diseases, absence of available preventative medical treatment and variable age of onset in mutation carriers. Gaining further insight into the decision-making process and the impact of testing is critical as we seek to develop best-practice guidelines for predictive testing for familial ALS and FTD.