Atypical TDP-43 protein expression in an ALS pedigree carrying a p.Y374X truncation mutation in TARDBP

Stress-induced TDP-43 nuclear condensation causes splicing loss of function and STMN2 depletion

TDP-43 protein is dysregulated in several neurodegenerative diseases, which often have a multifactorial nature and may have extrinsic stressors as a "second hit." TDP-43 undergoes reversible nuclear condensation in stressed cells including neurons. Here, we demonstrate that stress-inducible nuclear TDP-43 condensates are RNA-depleted, non-liquid assemblies distinct from the known nuclear bodies. Their formation requires TDP-43 oligomerization and ATP and is inhibited by RNA. Using a confocal nanoscanning assay, we find that amyotrophic lateral sclerosis (ALS)-linked mutations alter stress-induced TDP-43 condensation by changing its affinity to liquid-like ribonucleoprotein assemblies. Stress-induced nuclear condensation transiently inactivates TDP-43, leading to loss of interaction with its protein binding partners and loss of function in splicing. Splicing changes are especially prominent and persisting for STMN2 RNA, and STMN2 protein becomes rapidly depleted early during stress. Our results point to early pathological changes to TDP-43 in the nucleus and support therapeutic modulation of stress response in ALS.

C-terminal frameshift variant of TDP-43 with pronounced aggregation-propensity causes rimmed vacuole myopathy but not ALS/FTD

Neuronal TDP-43-positive inclusions are neuropathological hallmark lesions in frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). Pathogenic missense variants in TARDBP, the gene encoding TDP-43, can cause ALS and cluster in the C-terminal prion-like domain (PrLD), where they modulate the liquid condensation and aggregation properties of the protein. TDP-43-positive inclusions are also found in rimmed vacuole myopathies, including sporadic inclusion body myositis, but myopathy-causing TDP-43 variants have not been reported. Using genome-wide linkage analysis and whole exome sequencing in an extended five-generation family with an autosomal dominant rimmed vacuole myopathy, we identified a conclusively linked frameshift mutation in TDP-43 producing a C-terminally altered PrLD (TDP-43p.Trp385IlefsTer10) (maximum multipoint LOD-score 3.61). Patient-derived muscle biopsies showed TDP-43-positive sarcoplasmic inclusions, accumulation of autophagosomes and transcriptomes with abnormally spliced sarcomeric genes (including TTN and NEB) and increased expression of muscle regeneration genes. In vitro phase separation assays demonstrated that TDP-43Trp385IlefsTer10 does not form liquid-like condensates and readily forms solid-like fibrils indicating increased aggregation propensity compared to wild-type TDP-43. In Drosophila TDP-43p.Trp385IlefsTer10 behaved as a partial loss-of-function allele as it was able to rescue the TBPH (fly ortholog of TARDBP) neurodevelopmental lethal null phenotype while showing strongly reduced toxic gain-of-function properties upon overexpression. Accordingly, TDP-43p.Trp385IlefsTer10 showed reduced toxicity in a primary rat neuron disease model. Together, these genetic, pathological, in vitro and in vivo results demonstrate that TDP-43p.Trp385IlefsTer10 is an aggregation-prone partial loss-of-function variant that causes autosomal dominant vacuolar myopathy but not ALS/FTD. Our study genetically links TDP-43 proteinopathy to myodegeneration, and reveals a tissue-specific role of the PrLD in directing pathology.

A patient carrying a heterozygous p.Asn267Ser TARDBP missense mutation diagnosed as ALS and only involving lower motor neurons

Amyotrophic lateral sclerosis (ALS) is a rare neurodegenerative disease involving upper motor neurons (UMN) and lower motor neurons (LMN), which can be caused by mutations of pathogenic genes such as superoxide dismutase 1 (SOD1), sarcoma fusion (FUS), and TAR-DNA binding protein (TARBDP/TDP-43). Among these pathogenic genes, TARBDP mutation accounts for approximately 1% of sporadic ALS (sALS). The clinical phenotype of ALS is heterogeneous owing to different mutant genes and sites. Here, we report a case of sALS from China, the pathogenic site (c.800A > G) of TARDBP in this patient was identified by whole-exome sequencing. But his clinical symptoms involve only the LMN, presented with progressive limb weakness, and dyspnea, without obvious limb muscle atrophy. We considered this patient as a possible LMN-dominant ALS variant and this report further explores the genotype-phenotype correlations of ALS10. Furthermore, interestingly, the pathogenic site in this person was previously reported in a Parkinson's disease (PD) patient and frontotemporal dementia (FTD) patient. Our findings illustrate the clinical heterogeneity and the types of diseases which carry p.Asn267Ser TDP-43 mutation were broadened furtherly. Meanwhile, considering that the range of neurodegenerative diseases associated with this mutant site may be expanding, the mechanism of different neurodegenerative changes mediated by the same pathogenic site still needs to be further studied.