Mice with endogenous <scp>TDP</scp> ‐43 mutations exhibit gain of splicing function and characteristics of amyotrophic lateral sclerosis

TDP-43 (encoded by the gene <i>TARDBP</i>) is an RNA binding protein central to the pathogenesis of amyotrophic lateral sclerosis (ALS). However, how <i>TARDBP</i> mutations trigger pathogenesis remains unknown. Here, we use novel mouse mutants carrying point mutations in endogenous <i>Tardbp</i> to dissect TDP-43 function at physiological levels both <i>in vitro</i> and <i>in vivo</i> Interestingly, we find that mutations within the C-terminal domain of TDP-43 lead to a gain of splicing function. Using two different strains, we are able to separate TDP-43 loss- and gain-of-function effects. TDP-43 gain-of-function effects in these mice reveal a novel category of splicing events controlled by TDP-43, referred to as "skiptic" exons, in which skipping of constitutive exons causes changes in gene expression. <i>In vivo</i>, this gain-of-function mutation in endogenous <i>Tardbp</i> causes an adult-onset neuromuscular phenotype accompanied by motor neuron loss and neurodegenerative changes. Furthermore, we have validated the splicing gain-of-function and skiptic exons in ALS patient-derived cells. Our findings provide a novel pathogenic mechanism and highlight how TDP-43 gain of function and loss of function affect RNA processing differently, suggesting they may act at different disease stages.