The major genetic cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) is a C9orf72 GC repeat expansion. Proposed mechanisms by which the expansion causes c9FTD/ALS include toxicity from repeat-containing RNA and from dipeptide repeat proteins translated from these transcripts. To investigate the contribution of poly(GR) dipeptide repeat proteins to c9FTD/ALS pathogenesis in a mammalian in vivo model, we generated mice that expressed GFP-(GR) in the brain. GFP-(GR) mice developed age-dependent neurodegeneration, brain atrophy, and motor and memory deficits through the accumulation of diffuse, cytoplasmic poly(GR). Poly(GR) co-localized with ribosomal subunits and the translation initiation factor eIF3η in GFP-(GR) mice and, of importance, in c9FTD/ALS patients. Combined with the differential expression of ribosome-associated genes in GFP-(GR) mice, these findings demonstrate poly(GR)-mediated ribosomal distress. Indeed, poly(GR) inhibited canonical and non-canonical protein translation in HEK293T cells, and also induced the formation of stress granules and delayed their disassembly. These data suggest that poly(GR) contributes to c9FTD/ALS by impairing protein translation and stress granule dynamics, consequently causing chronic cellular stress and preventing cells from mounting an effective stress response. Decreasing poly(GR) and/or interrupting interactions between poly(GR) and ribosomal and stress granule-associated proteins may thus represent potential therapeutic strategies to restore homeostasis.

A hallmark pathological feature of the neurodegenerative diseases amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) is the depletion of RNA-binding protein TDP-43 from the nucleus of neurons in the brain and spinal cord1. A major function of TDP-43 is as a repressor of cryptic exon inclusion during RNA splicing2–4. Single nucleotide polymorphisms in UNC13A are among the strongest hits associated with FTD and ALS in human genome-wide association studies5,6, but how those variants increase risk for disease is unknown. Here we show that TDP-43 represses a cryptic exon-splicing event in UNC13A. Loss of TDP-43 from the nucleus in human brain, neuronal cell lines and motor neurons derived from induced pluripotent stem cells resulted in the inclusion of a cryptic exon in UNC13A mRNA and reduced UNC13A protein expression. The top variants associated with FTD or ALS risk in humans are located in the intron harbouring the cryptic exon, and we show that they increase UNC13A cryptic exon splicing in the face of TDP-43 dysfunction. Together, our data provide a direct functional link between one of the strongest genetic risk factors for FTD and ALS (UNC13A genetic variants), and loss of TDP-43 function.

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease affecting motor neurons and other neuronal cells, leading to severe disability and eventually death from ventilatory failure. It has a prevalence of 5 in 100,000, with an incidence of 1.7 per 100,000, reflecting short average survival. The pathogenesis is incompletely understood, but defects of RNA processing and protein clearance may be fundamental. Repeat expansions in the chromosome 9 open reading frame 72 gene (C9orf72) are the most common known genetic cause of ALS and are seen in approximately 40% of patients with a family history and approximately 10% of those without. No environmental risk factors are proved to be causative, but many have been proposed, including military service. The diagnosis of ALS rests on a history of painless progressive weakness coupled with examination findings of upper and lower motor dysfunction. No diagnostic test is yet available, but electromyography and genetic tests can support the diagnosis. Care for patients is best provided by a multidisciplinary team, and most interventions are directed at managing symptoms. Two medications with modest benefits have Food and Drug Administration approval for the treatment of ALS: riluzole, a glutamate receptor antagonist, and, new in 2017, edaravone, a free radical scavenger. Many other encouraging treatment strategies are being explored in clinical trials for ALS; herein we review stem cell and antisense oligonucleotide gene therapies.

INTRODUCTION: Frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) are fatal neurodegenerative diseases that share clinical and neuropathological features. Furthermore, the most common genetic cause of both FTD and ALS is a GGGGCC (G4C2) repeat expansion in the C9orf72 gene. This repeat expansion leads to several abnormalities, including C9orf72 haploinsufficiency, the accumulation of repeat RNA, and the production of five aggregation-prone proteins composed of repeating dipeptides. However, the contribution of these abnormalities to disease pathogenesis remains unresolved. RATIONALE: Among the five dipeptide repeat proteins nonconventionally translated from expanded G4C2 repeats, proline-arginine (PR) repeat proteins [poly(PR) proteins] have proven especially toxic in various model systems. Their involvement in C9orf72-associated FTD and ALS (c9FTD/ALS) has nevertheless been questioned because poly(PR) pathology is relatively infrequent in c9FTD/ALS patient brains. Postmortem tissues, however, represent end-stage disease and do not necessarily reflect early events in the disease process. Therefore, we generated mice that express poly(PR) in the brain to evaluate the temporal consequences of its expression in a mammalian in vivo model. More specifically, we engineered mice to express green fluorescent protein (GFP)–conjugated (PR)50 (a 50-repeat PR protein) or GFP via intracerebroventricular administration of adeno-associated viral vectors and then performed behavioral, pathological, and transcriptomic characterizations of poly(PR) mice in comparison with control GFP mice. RESULTS: We found that ~60% of poly(PR)- expressing mice died by 4 weeks of age and had significantly decreased brain and body weights at death compared with age-matched GFP control mice. Poly(PR) mice that escaped premature death developedmotor andmemory impairments, likely as a consequence of their progressive brain atrophy, neuron loss, loss of poly(PR)-positive cells, and gliosis. In investigating the mechanisms by which poly(PR) caused neurodegeneration and functional deficits, we found that poly(PR) localized to heterochromatin (highly condensed regions of transcriptionally silent chromatin) and caused abnormal histone H3 methylation, features that we also detected in brain tissues from patients with c9FTD/ALS. Additionally, we observed aberrations in nuclear lamins and heterochromatin protein 1α (HP1α), key proteins thatmaintain heterochromatin structure and regulate gene silencing. Nuclear lamina invaginations and decreased HP1a protein expression were seen in poly(PR)-positive cells in poly(PR) mice, and in vitro studies demonstrated that poly(PR) disrupted HP1α liquid phases. Because poly(PR)-induced histone H3 posttranslational modifications, lamin invaginations, and decreased HP1α levels could profoundly affect gene expression, we compared transcriptome profiles between control and poly(PR) mice. As well as analyzing differentially expressed genes, we examined repetitive element expression given that repetitive...

No treatment for frontotemporal dementia (FTD), the second most common early-onset dementia, is available but therapeutics are being investigated to target the two main proteins associated with FTD pathological subtypes: TDP-43 (FTLD-TDP) and tau (FTLD-tau). Testing potential therapies in clinical trials is hamstrung by our inability to distinguish between patients with FTLD-TDP and FTLD-tau. Therefore, we evaluated truncated stathmin-2 (STMN2) as a proxy of TDP-43 pathology, given reports that TDP-43 dysfunction causes truncated STMN2 accumulation. Truncated STMN2 accumulated in human iPSC-derived neurons depleted of TDP-43, but not in those with pathogenic TARDBP mutations in the absence of TDP-43 aggregation or loss of nuclear protein. In RNA-seq analyses of human brain samples from the NYGC ALS cohort, truncated STMN2 RNA was confined to tissues and disease sub-types marked by TDP-43 inclusions. Lastly, we validated that truncated STMN2 RNA is elevated in the frontal cortex of a cohort of FTLD-TDP cases but not in controls or cases with progressive supranuclear palsy (PSP), a type of FTLD-tau. Further, in FTLD-TDP, we observed significant associations of truncated STMN2 RNA with phosphorylated TDP-43 levels and an earlier age of disease onset. Overall, our data uncovered truncated STMN2 as a marker for TDP-43 dysfunction in FTD.

TAR DNA-binding protein 43 (TDP-43) inclusions are a pathological hallmark of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS), including cases caused by G4C2 repeat expansions in the C9orf72 gene (c9FTD/ALS). Providing mechanistic insight into the link between C9orf72 mutations and TDP-43 pathology, we demonstrated that a glycine-arginine repeat protein [poly(GR)] translated from expanded G4C2 repeats was sufficient to promote aggregation of endogenous TDP-43. In particular, toxic poly(GR) proteins mediated sequestration of full-length TDP-43 in an RNA-independent manner to induce cytoplasmic TDP-43 inclusion formation. Moreover, in GFP-(GR)200 mice, poly(GR) caused the mislocalization of nucleocytoplasmic transport factors and nuclear pore complex proteins. These mislocalization events resulted in the aberrant accumulation of endogenous TDP-43 in the cytoplasm where it co-aggregated with poly(GR). Last, we demonstrated that treating G4C2 repeat–expressing mice with repeat-targeting antisense oligonucleotides lowered poly(GR) burden, which was accompanied by reduced TDP-43 pathology and neurodegeneration, including lowering of plasma neurofilament light (NFL) concentration. These results contribute to clarification of the mechanism by which poly(GR) drives TDP-43 proteinopathy, confirm that G4C2-targeted therapeutics reduce TDP-43 pathology in vivo, and demonstrate that alterations in plasma NFL provide insight into the therapeutic efficacy of disease-modifying treatments.

Synopsis Muscle biopsy is a commonly ordered diagnostic procedure, used by clinicians who evaluate patients with weakness suspected to be caused by muscle disease. This article reviews the indications for a muscle biopsy, and then serves as a step-by-step guide reviewing the processes of muscle selection through to interpreting the biopsy report. The goal of this article is to aid the clinician in preparing for a muscle biopsy procedure so that they may avoid common pitfalls and obtain optimal results from this minimally invasive procedure. We review the basic anatomical structure of normal muscle to provide a foundation for understanding common patterns of pathologic change observed in muscle disease, and then present both common and disease-specific histopathologic findings, focused for illustrative purposes on a select group of neuromuscular diseases.

Objective: To explore the putative connection between inclusion body myopathy, Paget disease, frontotemporal dementia (IBMPFD) and motor neuron disease (MND).Methods: Clinical, genetic, and EMG characterization of 17 patients from 8 IBMPFD families.Results: Limb weakness was the most common clinical manifestation (present in 15 patients, median onset age 38 years, range 25-52), with unequivocal evidence of upper motor neuron dysfunction in 3. EMG, abnormal in all 17, was purely neurogenic in 4, purely myopathic in 6, and mixed neurogenic/myopathic in 7. Cognitive/behavioral impairment was detected in at least 8. Mutations in VCP (R155H, R159G, R155C) were identified in 6 families, and in hnRNPA2B1 (D290V) in another family. The genetic cause in the eighth family has not yet been identified. Conclusion:Mutations in at least 4 genes may cause IBMPFD, and its phenotypic spectrum extends beyond IBM, Paget disease, and frontotemporal dementia (FTD). Weakness, the most common and disabling manifestation, may be caused by muscle disease or MND. The acronym IBMPFD is, therefore, insufficient to describe disorders due to VCP mutations or other recently identified IBMPFD-associated genes. Instead, we favor the descriptor multisystem proteinopathy (MSP), which encompasses both the extended clinical phenotype and the previously described prominent pathologic feature of protein aggregation in affected tissues. The nomenclature MSP1, MSP2, and MSP3 may be used for VCP-, HNRNPA2B1-, and HNRNPA1-associated disease, respectively. Genetic defects in MSP implicate a range of biological mechanisms including RNA processing and protein homeostasis, both with potential relevance to the pathobiology of more common MNDs such as amyotrophic lateral sclerosis (ALS) and providing an additional link between ALS and FTD. Neurology Inclusion body myopathy (IBM) with Paget disease and frontotemporal dementia (IBMPFD) is a rare multisystem degenerative disorder named after the organ systems originally recognized to be affected-muscle, bone, and brain. Mutations in the valosin-containing protein (VCP) gene were the first identified cause of IBMPFD, 1,2 but reports of families without linkage to chromosome 9 established the genetic heterogeneity of the disorder.3,4 It has recently emerged that mutations in the HNRNPA2B1 (chromosome 7) and HNRNPA1 (chromosome 12) genes account for some families with IBMPFD. 5 We first raised the possibility of a connection between IBMPFD and amyotrophic lateral sclerosis (ALS) after mutations in the VCP gene were identified in patients with familial ALS.6 Interestingly, the original report of IBMPFD 7 almost 30 years ago described it as a "familial disorder of combined lower motor neuron degeneration and skeletal disorganization"; the presence of fasciculations, EMG evidence of chronic reinnervation, and muscle biopsy showing grouped atrophy all pointing toward a primarily neurogenic process. This family was subsequently found to have an R155Q mutation in the VCP gene. 8 Since the original description, var...