Somatic TARDBP variants identified in human brain with FTLD-TDP-C disrupt morphology and microglia numbers in zebrafish

Abstract Pathogenic germline variants in TARDBP can cause amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration with TDP-43 pathology (FTLD-TDP). Recently, we identified somatic TARDBP variants in the brains of two individuals with semantic dementia and FTLD-TDP type C pathology. These two somatic variants (L41F and R42H) were identified in the N-terminal domain of TARDBP , whereas the vast majority of functional studies so far have focused on C-terminal variants, in which other known pathogenic germline variants are located. We assessed the pathogenicity of the somatic N-terminal variants in vivo using zebrafish in direct comparison to germline TARDBP variants causing ALS and FTLD-TDP (P112H, K263E, and I383V). In line with in silico predictions, we show that somatic variant L41F is poorly tolerated in in vivo toxicity assays, resulting in a toxic gain-of-function when ubiquitously expressed. Furthermore, L41F showed substantial loss-of-function in rescue experiments performed in tardbp -deficient background. Germline variant K263E showed similar outcomes, while somatic variant R42H and germline variant I383V demonstrated lower levels of toxicity and loss-of-function in these assays. An increase in microglia was detected with overexpression of R42H mRNA compared to wild-type, indicating a potential immunogenic effect. Additionally, we created a novel germline N-terminal tardbp single amino acid deletion (G37del) in zebrafish, which caused severe morphological abnormalities. In conclusion, these data provide evidence of the pathogenicity of N-terminal variants, including L41F and R42H, found as somatic variants in FTLD-TDP C, and suggest that possibly germline variants in this domain may not be compatible with life. This might contribute to the absence of pathogenic N-terminal variants in patients, and further reinforces the importance of considering somatic contributions in FTLD-TDP.