Myopathic aggregation-prone variants in the TDP-43 prion-like domain: genetics paving the way.

While neuropathological and genetic studies have established the crucial involvement of TDP-43 proteinopathy in the pathogenesis of amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD) and related neurodegenerative disorders, multiple studies have described the presence of TDP-43 inclusions in muscular disorders, including inclusion body myositis but also other related rimmed vacuole myopathies. In addition, TAR DNA-binding protein-43 (TDP-43) has been reported to be essential in normal muscle physiology as it is implicated in the formation of so-called amyloid-like myogranules during normal muscle regeneration after injury. However, genetic evidence supporting a primary role for TDP-43 proteinopathy in muscle disease has been missing. In the present review we highlight recent landmark discoveries linking novel pathogenic TDP-43 variants [p.(W385IfsX10) and p.(G376V)] within the prion-like domain with unusual aggregation-propensity and muscle rather than neuronal pathology. We discuss these studies in the context of known TDP-43-related pathways in ALS/FTD pathogenesis and show how they challenge some widely accepted views such as ALS as a pure neurogenic presynaptic neuromuscular disease and the direct correlation between TDP-43 aggregation-propensity and neurotoxicity. Finally, we discuss TDP-43 as part of a growing list of RNA-binding proteins including hnRNPA2B1 and hnRNPA1 as genetic causes of myopathies and relate this to the idea of 'multisystem proteinopathy'.