Faisal Fecto

Mutations in UBQLN2 cause dominant X-linked juvenile and adult-onset ALS and ALS/dementia

Amyotrophic lateral sclerosis (ALS) is a paralytic and usually fatal disorder caused by motor-neuron degeneration in the brain and spinal cord. Most cases of ALS are sporadic but about 5–10% are familial. Mutations in superoxide dismutase 1 (SOD1), TAR DNA-binding protein (TARDBP, also known as TDP43) and fused in sarcoma (FUS, also known as translocated in liposarcoma (TLS)) account for approximately 30% of classic familial ALS. Mutations in several other genes have also been reported as rare causes of ALS or ALS-like syndromes. The causes of the remaining cases of familial ALS and of the vast majority of sporadic ALS are unknown. Despite extensive studies of previously identified ALS-causing genes, the pathogenic mechanism underlying motor-neuron degeneration in ALS remains largely obscure. Dementia, usually of the frontotemporal lobar type, may occur in some ALS cases. It is unclear whether ALS and dementia share common aetiology and pathogenesis in ALS/dementia. Here we show that mutations in UBQLN2, which encodes the ubiquitin-like protein ubiquilin 2, cause dominantly inherited, chromosome-X-linked ALS and ALS/dementia. We describe novel ubiquilin 2 pathology in the spinal cords of ALS cases and in the brains of ALS/dementia cases with or without UBQLN2 mutations. Ubiquilin 2 is a member of the ubiquilin family, which regulates the degradation of ubiquitinated proteins. Functional analysis showed that mutations in UBQLN2 lead to an impairment of protein degradation. Therefore, our findings link abnormalities in ubiquilin 2 to defects in the protein degradation pathway, abnormal protein aggregation and neurodegeneration, indicating a common pathogenic mechanism that can be exploited for therapeutic intervention.

FUS-immunoreactive inclusions are a common feature in sporadic and non-SOD1 familial amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal disorder of motor neuron degeneration. Most cases of ALS are sporadic (SALS), but about 5 to 10% of ALS cases are familial (FALS). Recent studies have shown that mutations in FUS are causal in approximately 4 to 5% of FALS and some apparent SALS cases. The pathogenic mechanism of the mutant FUS‐mediated ALS and potential roles of FUS in non‐FUS ALS remain to be investigated.

Scapuloperoneal spinal muscular atrophy and CMT2C are allelic disorders caused by alterations in TRPV4

Scapuloperoneal spinal muscular atrophy (SPSMA) and hereditary motor and sensory neuropathy type IIC (HMSN IIC, also known as HMSN2C or Charcot-Marie-Tooth disease type 2C (CMT2C)) are phenotypically heterogeneous disorders involving topographically distinct nerves and muscles. We originally described a large New England family of French-Canadian origin with SPSMA and an American family of English and Scottish descent with CMT2C. We mapped SPSMA and CMT2C risk loci to 12q24.1–q24.31 with an overlapping region between the two diseases. Further analysis reduced the CMT2C risk locus to a 4-Mb region. Here we report that SPSMA and CMT2C are allelic disorders caused by mutations in the gene encoding the transient receptor potential cation channel, subfamily V, member 4 (TRPV4). Functional analysis revealed that increased calcium channel activity is a distinct property of both SPSMA- and CMT2C-causing mutant proteins. Our findings link mutations in TRPV4 to altered calcium homeostasis and peripheral neuropathies, implying a pathogenic mechanism and possible options for therapy for these disorders.

Frameshift and novel mutations in FUS in familial amyotrophic lateral sclerosis and ALS/dementia

Objective: Amyotrophic lateral sclerosis (ALS) is a progressive paralytic disorder caused by degeneration of motor neurons. Mutations in the FUS gene were identified in patients with familial ALS (FALS) and patients with sporadic ALS (SALS) from a variety of genetic backgrounds. This work further explores the spectrum of FUS mutations in patients with FALS and patients with FALS with features of frontotemporal dementia (FALS/FTD) or parkinsonism and dementia (FALS/PD/DE). Methods: All exons of the FUS gene were sequenced in 476 FALS index cases negative for mutations in SOD1 and TARDBP. A total of 561–726 controls were analyzed for genetic variants observed. Clinical data from patients with FUS mutations were compared to those of patients with known SOD1 and TARDBP mutations. Results: We identified 17 FUS mutations in 22 FALS families, 2 FALS/FTD families, and 1 FALS/PD/DE family from diverse genetic backgrounds; 11 mutations were novel. There were 4 frameshift, 1 nonsense, and 1 possible alternate splicing mutation. Patients with FUS mutations appeared to have earlier symptom onset, a higher rate of bulbar onset, and shorter duration of symptoms than those with SOD1 mutations. Conclusions: FUS gene mutations are not an uncommon cause in patients with FALS from diverse genetic backgrounds, and have a prevalence of 5.6% in non-SOD1 and non-TARDBP FALS, and ∼4.79% in all FALS. The pathogenicity of some of these novel mutations awaits further studies. Patients with FUS mutations manifest earlier symptom onset, a higher rate of bulbar onset, and shorter duration of symptoms.

Identification of TMEM230 mutations in familial Parkinson's disease

Parkinsons disease is the second most common neurodegenerative disorder without effective treatment. It is generally sporadic with unknown etiology. However, genetic studies of rare familial forms have led to the identification of mutations in several genes, which are linked to typical Parkinsons disease or parkinsonian disorders. The pathogenesis of Parkinsons disease remains largely elusive. Here we report a locus for autosomal dominant, clinically typical and Lewy body–confirmed Parkinsons disease on the short arm of chromosome 20 (20pter-p12) and identify TMEM230 as the disease-causing gene. We show that TMEM230 encodes a transmembrane protein of secretory/recycling vesicles, including synaptic vesicles in neurons. Disease-linked TMEM230 mutants impair synaptic vesicle trafficking. Our data provide genetic evidence that a mutant transmembrane protein of synaptic vesicles in neurons is etiologically linked to Parkinsons disease, with implications for understanding the pathogenic mechanism of Parkinsons disease and for developing rational therapies.

Making Connections: Pathology and Genetics Link Amyotrophic Lateral Sclerosis with Frontotemporal Lobe Dementia

Over the last couple of decades, there has been a growing body of clinical, genetic, and histopathological evidence that similar pathological processes underlie amyotrophic lateral sclerosis (ALS) and some types of frontotemporal lobe dementia (FTD). Even though there is great diversity in the genetic causes of these disorders, there is a high degree of overlap in their histopathology. Genes linked to rare cases of familial ALS and/or FTD, like FUS, TARDBP, OPTN, and UBQLN2 may converge onto a unifying pathogenic pathway and thereby provide novel therapeutic targets common to a spectrum of etiologically diverse forms of ALS and ALS–FTD. Additionally, there are major loci for ALS–FTD on chromosomes 9p and 15q. Identification of causative genetic alterations at those loci will be an important step in understanding the pathogenesis of juvenile- and adult-onset ALS and ALS–FTD. Interactions between TDP-43, FUS, optineurin, and ubiquilin 2 need to be studied to understand their common molecular pathways. Future efforts should also be directed towards generation and characterization of in vivo models to dissect the pathogenic mechanisms of these diseases. Such efforts will rapidly accelerate the discovery of new drugs that regulate accumulation of pathogenic proteins and their downstream consequences.

TRPV4 mutations and cytotoxic hypercalcemia in axonal Charcot-Marie-Tooth neuropathies

Objective: To improve understanding of TRPV4-associated axonal Charcot-Marie-Tooth (CMT) neuropathy phenotypes and their debated pathologic mechanism. Methods: A total of 17 CMT2C phenotypic families with vocal cord and diaphragmatic involvement and 36 clinically undifferentiated CMT2 subjects underwent sequencing analysis of the coding region of TRPV4. Functional studies of mutant proteins were performed using transiently transfected cells for TRPV4 subcellular localization, basal and stimulated Ca2+ channel analysis, and cell viability assay with or without channel blockade. Results: Two TRPV4 mutations R232C and R316H from 17 CMT2C families were identified in the ankyrin repeat domains. The R316H is a novel de novo mutation found in a patient with CMT2C phenotype. The family with R232C mutation had individuals with and without vocal cord and diaphragm involvement. Both mutant TRPV4 proteins had normal subcellular localization in HEK293 and HeLa cells. Cells transfected with R232C and R316H displayed increased intracellular Ca2+ levels and reversible cell death by the TRPV channel antagonist, ruthenium red. Conclusion: TRPV4 ankyrin domain alterations including a novel de novo mutation cause axonal CMT2. Individuals with the same mutation may have nondistinct CMT2 or have phenotypic CMT2C with vocal cord paresis. Reversible hypercalcemic gain-of-function of mutant TRPV4 instead of loss-of-function appears to be pathologically important. The reversibility of cell death by channel blockade provides an attractive area of investigation in consideration of treatable axonal degeneration.

Differential Involvement of Optineurin in Amyotrophic Lateral Sclerosis With or Without SOD1 Mutations

BACKGROUND Mutations in optineurin have recently been linked to amyotrophic lateral sclerosis (ALS). OBJECTIVE To determine whether optineurin-positive skeinlike inclusions are a common pathologic feature in ALS, including SOD1 -linked ALS. DESIGN Clinical case series. SETTING Academic referral center. SUBJECTS We analyzed spinal cord sections from 46 clinically and pathologically diagnosed ALS cases and ALS transgenic mouse models overexpressing ALS-linked SOD1 mutations G93A or L126Z. RESULTS We observed optineurin-immunoreactive skeinlike inclusions in all the sporadic ALS and familial ALS cases without SOD1 mutation, but not in cases with SOD1 mutations or in transgenic mice overexpressing the ALS-linked SOD1 mutations G93A or L126Z. CONCLUSION The data from this study provide evidence that optineurin is involved in the pathogenesis of sporadic ALS and non- SOD1 familial ALS, thus supporting the hypothesis that these forms of ALS share a pathway that is distinct from that of SOD1-linked ALS.

UBQLN2/P62 cellular recycling pathways in amyotrophic lateral sclerosis and frontotemporal dementia

Recent findings highlight a pathologic and functional convergence in amyotrophic lateral sclerosis (ALS) and amyotrophic lateral sclerosis with frontotemporal dementia (ALS‐FTD) at the level of protein recycling and disposal. Genes linked to rare cases of familial ALS and ALS‐FTD, like UBQLN2, OPTN, SQSTM1/p62, and VCP, may converge onto a unifying pathogenic pathway and thereby provide novel therapeutic targets common to a spectrum of etiologically diverse forms of ALS and ALS‐FTD. Interactions between these genes need to be further explored to understand their common molecular pathways. Future efforts should be directed toward generation and characterization of in vivo models to dissect the pathogenic mechanisms of ALS and ALS‐FTD and the role of protein degradation pathways, both centrally, at the cell body, and peripherally, at the level of the synapse. Such efforts will rapidly accelerate the discovery of new drugs that regulate accumulation of pathogenic proteins and their downstream consequences in ALS and ALS‐FTD and, possibly, other neurodegenerative diseases as well. Muscle Nerve, 2012

Mutant TRPV4-mediated Toxicity Is Linked to Increased Constitutive Function in Axonal Neuropathies

Mutations in TRPV4 have been linked to three distinct axonal neuropathies. However, the pathogenic mechanism underlying these disorders remains unclear. Both gain and loss of calcium channel activity of the mutant TRPV4 have been suggested. Here, we show that the three previously reported TRPV4 mutant channels have a physiological localization and display an increased calcium channel activity, leading to increased cytotoxicity in three different cell types. Patch clamp experiments showed that cells expressing mutant TRPV4 have much larger whole-cell currents than those expressing the wild-type TRPV4 channel. Single channel recordings showed that the mutant channels have higher open probability, due to a modification of gating, and no change in single-channel conductance. These data support the hypothesis that a “gain of function” mechanism, possibly leading to increased intracellular calcium influx, underlies the pathogenesis of the TRPV4-linked axonal neuropathies, and may have immediate implications for designing rational therapies.