To identify novel genes associated with ALS, we undertook two lines of investigation. We carried out a genome-wide association study comparing 20,806 ALS cases and 59,804 controls. Independently, we performed a rare variant burden analysis comparing 1,138 index familial ALS cases and 19,494 controls. Through both approaches, we identified kinesin family member 5A (KIF5A) as a novel gene associated with ALS. Interestingly, mutations predominantly in the N-terminal motor domain of KIF5A are causative for two neurodegenerative diseases: hereditary spastic paraplegia (SPG10) and Charcot-Marie-Tooth type 2 (CMT2). In contrast, ALS-associated mutations are primarily located at the C-terminal cargo-binding tail domain and patients harboring loss-of-function mutations displayed an extended survival relative to typical ALS cases. Taken together, these results broaden the phenotype spectrum resulting from mutations in KIF5A and strengthen the role of cytoskeletal defects in the pathogenesis of ALS.

Variants of UNC13A, a critical gene for synapse function, increase the risk of amyotrophic lateral sclerosis and frontotemporal dementia1–3, two related neurodegenerative diseases defined by mislocalization of the RNA-binding protein TDP-434,5. Here we show that TDP-43 depletion induces robust inclusion of a cryptic exon in UNC13A, resulting in nonsense-mediated decay and loss of UNC13A protein. Two common intronic UNC13A polymorphisms strongly associated with amyotrophic lateral sclerosis and frontotemporal dementia risk overlap with TDP-43 binding sites. These polymorphisms potentiate cryptic exon inclusion, both in cultured cells and in brains and spinal cords from patients with these conditions. Our findings, which demonstrate a genetic link between loss of nuclear TDP-43 function and disease, reveal the mechanism by which UNC13A variants exacerbate the effects of decreased TDP-43 function. They further provide a promising therapeutic target for TDP-43 proteinopathies.

Of the many inherited Charcot-Marie-Tooth peripheral neuropathies, type 2D (CMT2D) is caused by dominant point mutations in the gene GARS, encoding glycyl tRNA synthetase (GlyRS). Here we report a dominant mutation in Gars that causes neuropathy in the mouse. Importantly, both sensory and motor axons are affected, and the dominant phenotype is not caused by a loss of the GlyRS aminoacylation function. Mutant mice have abnormal neuromuscular junction morphology and impaired transmission, reduced nerve conduction velocities, and a loss of large-diameter peripheral axons, without defects in myelination. The mutant GlyRS enzyme retains aminoacylation activity, and a loss-of-function allele, generated by a gene-trap insertion, shows no dominant phenotype in mice. These results indicate that the CMT2D phenotype is caused not by reduction of the canonical GlyRS activity and insufficiencies in protein synthesis, but instead by novel pathogenic roles for the mutant GlyRS that specifically affect peripheral neurons.

Genetic background significantly affects phenotype in multiple mouse models of human diseases, including muscular dystrophy. This phenotypic variability is partly attributed to genetic modifiers that regulate the disease process. Studies have demonstrated that introduction of the γ-sarcoglycan-null allele onto the DBA/2J background confers a more severe muscular dystrophy phenotype than the original strain, demonstrating the presence of genetic modifier loci in the DBA/2J background. To characterize the phenotype of dystrophin deficiency on the DBA/2J background, we created and phenotyped DBA/2J-congenic Dmdmdx mice (D2-mdx) and compared them with the original, C57BL/10ScSn-Dmdmdx (B10-mdx) model. These strains were compared with their respective control strains at multiple time points between 6 and 52 weeks of age. Skeletal and cardiac muscle function, inflammation, regeneration, histology and biochemistry were characterized. We found that D2-mdx mice showed significantly reduced skeletal muscle function as early as 7 weeks and reduced cardiac function by 28 weeks, suggesting that the disease phenotype is more severe than in B10-mdx mice. In addition, D2-mdx mice showed fewer central myonuclei and increased calcifications in the skeletal muscle, heart and diaphragm at 7 weeks, suggesting that their pathology is different from the B10-mdx mice. The new D2-mdx model with an earlier onset and more pronounced dystrophy phenotype may be useful for evaluating therapies that target cardiac and skeletal muscle function in dystrophin-deficient mice. Our data align the D2-mdx with Duchenne muscular dystrophy patients with the LTBP4 genetic modifier, making it one of the few instances of cross-species genetic modifiers of monogenic traits.

Homologues of the SHARPIN (SHANK-associated RH domain-interacting protein) gene have been identified in the human, rat and mouse genomes. SHARPIN and its homologues are expressed in many tissues. SHARPIN protein forms homodimers and associates with SHANK in the post-synaptic density of excitatory neurotransmitters in the brain. SHARPIN is hypothesized to have roles in the crosslinking of SHANK proteins and in enteric nervous system function. We demonstrate that two independently arising spontaneous mutations in the mouse Sharpin gene, cpdm and cpdm Dem , cause a chronic proliferative dermatitis phenotype, which is characterized histologically by severe inflammation, eosinophilic dermatitis and defects in secondary lymphoid organ development. These are the first examples of disease-causing mutations in the Sharpin gene and demonstrate the importance of SHARPIN protein in normal immune development and control of inflammation.

Limb girdle muscular dystrophy type 2B and Miyoshi myopathy are clinically distinct forms of muscular dystrophy that arise from defects in the dysferlin gene. Here, we report two novel lines of dysferlin-deficient mice obtained by (a) gene targeting and (b) identification of an inbred strain, A/J, bearing a retrotransposon insertion in the dysferlin gene. The mutations in these mice were located at the 3' and 5' ends of the dysferlin gene. Both lines of mice lacked dysferlin and developed a progressive muscular dystrophy with histopathological and ultrastructural features that closely resemble the human disease. Vital staining with Evans blue dye revealed loss of sarcolemmal integrity in both lines of mice, similar to that seen in mdx and caveolin-3 deficient mice. However, in contrast to the latter group of animals, the dysferlin-deficient mice have an intact dystrophin glycoprotein complex and normal levels of caveolin-3. Our findings indicate that muscle membrane disruption and myofiber degeneration in dysferlinopathy were directly mediated by the loss of dysferlin via a new pathogenic mechanism in muscular dystrophies. We also show that the mutation in the A/J mice arose between the late 1970s and the early 1980s, and had become fixed in the production breeding stocks. Therefore, all studies involving the A/J mice or mice derived from A/J, including recombinant inbred, recombinant congenic and chromosome substitution strains, should take into account the dysferlin defect in these strains. These new dysferlin-deficient mice should be useful for elucidating the pathogenic pathway in dysferlinopathy and for developing therapeutic strategies.

Duchenne and Becker muscular dystrophy (DMD and BMD) are X-linked recessive diseases caused by defective expression of dystrophin. The mdx mouse, an animal model for DMD, has a mutation that eliminates expression of the 427K muscle and brain isoforms of dystrophin. Although these animals do not display overt muscle weakness or impaired movement, the diaphragm muscle of the mdx mouse is severely affected and shows progressive myofibre degeneration and fibrosis which closely resembles the human disease. Here we explore the feasibility of gene therapy for DMD by examining the potential of a full-length dystrophin transgene to correct dystrophic symptoms in mdx mice. We find that expression of dystrophin in muscles of transgenic mdx mice eliminates the morphological and immunohistological symptoms of muscular dystrophy. In addition, overexpression of dystrophin prevents the development of the abnormal mechanical properties associated with dystrophic muscle without causing deleterious side effects. Our results provide functional evidence for the feasibility of gene therapy for DMD.

Neural activity actively regulates muscle gene expression. This regulation is crucial for specifying muscle functionality and synaptic protein expression. How neural activity is relayed into nuclei and connected to the muscle transcriptional machinery, however, is not known. Here we identify the histone deacetylase HDAC4 as the critical linker connecting neural activity to muscle transcription. We found that HDAC4 is normally concentrated at the neuromuscular junction (NMJ), where nerve innervates muscle. Remarkably, reduced neural input by surgical denervation or neuromuscular diseases dissociates HDAC4 from the NMJ and dramatically induces its expression, leading to robust HDAC4 nuclear accumulation. We present evidence that nuclear accumulated HDAC4 is responsible for the coordinated induction of synaptic genes upon denervation. Inactivation of HDAC4 prevents denervation-induced synaptic acetylcholine receptor (nAChR) and MUSK transcription whereas forced expression of HDAC4 mimics denervation and activates ectopic nAChR transcription throughout myofibers. We determined that HDAC4 executes activity-dependent transcription by regulating the Dach2-myogenin transcriptional cascade where inhibition of the repressor Dach2 by HDAC4 permits the induction of the transcription factor myogenin, which in turn activates synaptic gene expression. Our findings establish HDAC4 as a neural activity-regulated deacetylase and a key signaling component that relays neural activity to the muscle transcriptional machinery.The histone deacetylase HDAC4 and its closely related family member HDAC5 have been implicated in muscle differentiation by virtue of their efficient binding and inhibition of MEF2, a master transcription factor critical for muscle differentiation (1). The potent transcriptional repressor activity of HDAC4 and HDAC5, in turn, is negatively regulated by calcium/calmodulindependent kinase (CaMK) 3 (2, 3). CaMK-mediated phosphorylation promotes 14-3-3 association and nuclear export of HDAC4 and HDAC5, resulting in MEF2 activation. The active intracellular trafficking of HDAC4 and related class IIA HDACs was considered a key regulatory mechanism for MEF2 transcriptional activity. As potent repressors of MEF2, HDAC4 and HDAC5 were logically proposed to act as inhibitors of skeletal muscle differentiation, and their nuclear exclusion induced by CaMK was thought to allow differentiation to proceed (2). Surprisingly, our previous analysis, however, showed that HDAC4 became concentrated to rather than excluded from the nuclei upon C2C12 myotube formation (3). This unexpected finding is incompatible with a simple model that nuclear HDAC4 prevents muscle differentiation. Instead, the import of HDAC4 into nuclei of myotubes likely reflects a specific function of HDAC4 in the differentiated myofiber. However, any potential role for HDAC4 in the regulation of muscle function in vivo has not been established.