Pathogenesis and therapy of spinal and bulbar muscular atrophy (SBMA)

Reduced mesencephalic astrocyte-derived neurotrophic factor expression by mutant androgen receptor contributes to neurodegeneration in a model of spinal and bulbar muscular atrophy pathology

JOURNAL/nrgr/04.03/01300535-202509000-00027/figure1/v/2024-11-05T132919Z/r/image-tiff Spinal and bulbar muscular atrophy is a neurodegenerative disease caused by extended CAG trinucleotide repeats in the androgen receptor gene, which encodes a ligand-dependent transcription factor. The mutant androgen receptor protein, characterized by polyglutamine expansion, is prone to misfolding and forms aggregates in both the nucleus and cytoplasm in the brain in spinal and bulbar muscular atrophy patients. These aggregates alter protein-protein interactions and compromise transcriptional activity. In this study, we reported that in both cultured N2a cells and mouse brain, mutant androgen receptor with polyglutamine expansion causes reduced expression of mesencephalic astrocyte-derived neurotrophic factor. Overexpression of mesencephalic astrocyte-derived neurotrophic factor ameliorated the neurotoxicity of mutant androgen receptor through the inhibition of mutant androgen receptor aggregation. Conversely, knocking down endogenous mesencephalic astrocyte-derived neurotrophic factor in the mouse brain exacerbated neuronal damage and mutant androgen receptor aggregation. Our findings suggest that inhibition of mesencephalic astrocyte-derived neurotrophic factor expression by mutant androgen receptor is a potential mechanism underlying neurodegeneration in spinal and bulbar muscular atrophy.

Functional Outcome Measures to Optimize Drug Development in Spinal and Bulbar Muscular Atrophy: Results From a Meta-Analysis of the Global SBMA Dataset

BACKGROUND AND OBJECTIVES

Spinal and bulbar muscular atrophy (SBMA) is a rare, slowly progressive, and debilitating disease without effective treatments available. Lack of reliable biomarkers and sensitive outcome measures makes clinical research conduct challenging. The primary objective of this study was to identify clinically meaningful and statistically sensitive outcome measures enabling the evaluation of therapeutic interventions in late-stage clinical trials.

METHODS

This study was a meta-analysis of SBMA patient-level data from 6 observational studies conducted in Italy, South Korea, Denmark, United Kingdom, Japan, and United States. Patients with confirmed SBMA genetic diagnosis and differing severity were enrolled following individual site protocols. Routine assessments were performed longitudinally for approximately 3 years, including one or more clinical outcomes, such as SBMA functional rating scale (SBMAFRS), 6-minute walk test (6MWT), quantitative muscle testing (QMT), and Adult Myopathy Assessment Tool (AMAT). A modified scale, m-SBMAFRS, was derived by including only lower limb and trunk subscales having lower variability and larger effect size compared with the others. Changes from baseline at follow-up time points were calculated for all measures, and percent changes using random slope models were calculated to compare clinical measure performances. A survey conducted on 196 patients by the Coordination of Rare Diseases at Sanford (CoRDS), elucidating the impact of specific disease aspects on patients' lives, was also evaluated to corroborate these research outcomes.

RESULTS

This global SBMA dataset analyzed data from 278 men (mean age = 59.7 ± 10.8 years, mean disease duration = 17.7 ± 11.9 years). Patients progressed on SBMAFRS (-4.7 ± 6.2 points after 38 months with 1-year standard response mean [SRM] = 0.6) and 6MWT (distance walked decreased by -53.2 ± 87.0 meters after 26 months with 1-year SRM = 0.5). These measures showed lower variability and larger effect size than AMAT and QMT (1-year SRM = 0.1 and -0.2, respectively) and confirmed SBMA linear progression across a range of disease stages. The m-SBMAFRS also showed a significant yearly decline of 0.9 ± 1.5 points (SRM = 0.6) and more consistent performance with less variability across clinical sites. The CoRDS survey confirmed the relevance of lower limb strength and mobility, which correlated with higher quality-of-life metrics and were reported by patients as predominant disease issues.

DISCUSSION

We generated a comprehensive global SBMA dataset, enabling the identification of sensitive functional end points for clinical trials. Possible limitations relate to data collection nuances across sites that a single study protocol could override.

One case of spinal bulbar muscular atrophy misdiagnosed as polymyositis: Case report

RATIONALE

Spinal bulbar muscular atrophy (SBMA) is a rare X-linked recessive motor neuron degenerative disease. Due to the lack of specificity in its early clinical manifestations, SBMA is easily misdiagnosed. Herein, we present a case in which SBMA was misdiagnosed as polymyositis.

PATIENT CONCERNS

A 58-year-old patient began to develop symptoms of limb weakness 20 years ago and was admitted to the Second Hospital of Hebei Medical University 10 years ago without special treatment. Two years ago, the above symptoms worsened and he was admitted to Peking Union Medical College Hospital. The patient was misdiagnosed as polymyositis. According to the gene mutation characteristics of SBMA, the patient was diagnosed with SBMA.

DIAGNOSES

The result of the Kennedy gene test was positive, and the patient was diagnosed with Kennedy disease.

INTERVENTIONS

After the diagnosis of SBMA, the patient was given symptomatic treatment to alleviate the condition.

OUTCOMES

Conservative treatment after discharge was requested. It is recommended that patients avoid bucking to prevent complications.

LESSONS

This is a case of milder SBMA being misdiagnosed as polymyositis. For patients with weak limbs, the possibility of SBMA should be considered.

Exercise attenuates polyglutamine-mediated neuromuscular degeneration in a mouse model of spinal and bulbar muscular atrophy

BACKGROUND

Spinal and bulbar muscular atrophy (SBMA) is a hereditary neuromuscular disorder caused by the expansion of trinucleotide cytosine-adenine-guanine (CAG) repeats, which encodes a polyglutamine (polyQ) tract in the androgen receptor (AR) gene. Recent evidence suggests that, in addition to motor neuron degeneration, defective skeletal muscles are also the primary contributors to the pathogenesis in SBMA. While benefits of physical exercise have been suggested in SBMA, underlying mechanism remains elusive.

METHODS

We investigated the effect of running exercise in a transgenic mouse model of SBMA carrying human AR with 97 expanded CAGs (AR97Q). We assigned AR97Q mice to exercise and sedentary control groups, and mice in the exercise group received 1-h forced running wheel (5 m/min) 5 days a week for 4 weeks during the early stage of the disease. Motor function (grip strength and rotarod performance) and survival of each group were analysed, and histopathological and biological features in skeletal muscles and motor neurons were evaluated.

RESULTS

AR97Q mice in the exercise group showed improvement in motor function (~40% and ~50% increase in grip strength and rotarod performance, respectively, P < 0.05) and survival (median survival 23.6 vs. 16.7 weeks, P < 0.05) with amelioration of neuronal and muscular histopathology (~1.4-fold and ~2.8-fold increase in motor neuron and muscle fibre size, respectively, P < 0.001) compared to those in the sedentary group. Nuclear accumulation of polyQ-expanded AR in skeletal muscles and motor neurons was suppressed in the mice with exercise compared to the sedentary mice (~50% and ~30% reduction in 1C2-positive cells in skeletal muscles and motor neurons, respectively, P < 0.05). We found that the exercise activated 5'-adenosine monophosphate-activated protein kinase (AMPK) signalling and inhibited mammalian target of rapamycin pathway that regulates protein synthesis in skeletal muscles of SBMA mice. Pharmacological activation of AMPK inhibited protein synthesis and reduced polyQ-expanded AR proteins in C2C12 muscle cells.

CONCLUSIONS

Our findings suggest the therapeutic potential of exercise-induced effect via AMPK activation in SBMA.

Scalable, optically-responsive human neuromuscular junction model reveals convergent mechanisms of synaptic dysfunction in familial ALS

Neuromuscular junctions (NMJs) are specialized synapses that mediate communication between motor neurons and skeletal muscles and are essential for movement. The degeneration of this system can lead to symptoms observed in neuromuscular and motor neuron diseases. Studying these synapses and their degeneration has proven challenging. Prior NMJ studies heavily relied upon the use of mouse, chick, or isolated primary human cells, which have demonstrated limited fidelity for disease modeling. To enable the study of NMJ dysfunction and model genetic diseases, we, and others, have developed methods to generate human NMJs from pluripotent stem cells (PSCs), embryonic stem cells, and induced pluripotent stem cells. However, published studies have highlighted technical limitations associated with these complex in vitro NMJ models. In this study, we developed a robust PSC-derived motor neuron and skeletal muscle co-culture method, and demonstrated its sensitivity in modeling motor neuron disease. Our method spontaneously and reproducibly forms human NMJs. We developed multiwell-multielectrode array (MEA) parameters to quantify the activity of PSC-derived skeletal muscles, as well as measured the electrophysiological activity of functional human PSC-derived NMJs. We further leveraged our method to morphologically and functionally assess NMJs from the familial amyotrophic lateral sclerosis (fALS) PSCs, C9orf72 hexanucleotide (G4C2)n repeat expansion (HRE), SOD1 A5V , and TDP43 G298S to define the reproducibility and sensitivity of our system. We observed a significant decrease in the numbers and activity of PSC-derived NMJs developed from the different ALS lines compared to their respective controls. Furthermore, we evaluated a therapeutic candidate undergoing clinical trials and observed a variant-dependent rescue of functionality of NMJs. Our newly developed method provides a platform for the systematic investigation of genetic causes of NMJ neurodegeneration and highlights the need for therapeutic avenues to consider patient genotype.

Curbing Rhes Actions: Mechanism-based Molecular Target for Huntington's Disease and Tauopathies

A highly interconnected network of diverse brain regions is necessary for the precise execution of human behaviors, including cognitive, psychiatric, and motor functions. Unfortunately, degeneration of specific brain regions causes several neurodegenerative disorders, but the mechanisms that elicit selective neuronal vulnerability remain unclear. This knowledge gap greatly hinders the development of effective mechanism-based therapies, despite the desperate need for new treatments. Here, we emphasize the importance of the Rhes (Ras homolog-enriched in the striatum) protein as an emerging therapeutic target. Rhes, an atypical small GTPase with a SUMO (small ubiquitin-like modifier) E3-ligase activity, modulates biological processes such as dopaminergic transmission, alters gene expression, and acts as an inhibitor of motor stimuli in the brain striatum. Mutations in the Rhes gene have also been identified in selected patients with autism and schizophrenia. Moreover, Rhes SUMOylates pathogenic form of mutant huntingtin (mHTT) and tau, enhancing their solubility and cell toxicity in Huntington's disease and tauopathy models. Notably, Rhes uses membrane projections resembling tunneling nanotubes to transport mHTT between cells and Rhes deletion diminishes mHTT spread in the brain. Thus, we predict that effective strategies aimed at diminishing brain Rhes levels will prevent or minimize the abnormalities that occur in HD and tauopathies and potentially in other brain disorders. We review the emerging technologies that enable specific targeting of Rhes in the brain to develop effective disease-modifying therapeutics.

Long-term effects of the gait treatment using a wearable cyborg hybrid assistive limb in a patient with spinal and bulbar muscular atrophy: a case report with 5 years of follow-up

Background

Spinal and bulbar muscular atrophy (SBMA) is a progressive neuromuscular degenerative disease characterized by the degeneration of lower motor neurons in the spinal cord and brainstem and neurogenic atrophy of the skeletal muscle. Although the short-term effectiveness of gait treatment using a wearable cyborg hybrid assistive limb (HAL) has been demonstrated for the rehabilitation of patients with SBMA, the long-term effects of this treatment are unclear. Thus, this study aimed to investigate the long-term effects of the continued gait treatment with HAL in a patient with SBMA.

Results

A 68-year-old man with SBMA had lower limb muscle weakness and atrophy, gait asymmetry, and decreased walking endurance. The patient performed nine courses of HAL gait treatment (as one course three times per week for 3 weeks, totaling nine times) for ~5 years. The patient performed HAL gait treatment to improve gait symmetry and endurance. A physical therapist adjusted HAL based on the gait analysis and physical function of the patient. Outcome measurements, such as 2-min walking distance (2MWD), 10-meter walking test (maximal walking speed, step length, cadence, and gait symmetry), muscle strength, Revised Amyotrophic Lateral Sclerosis Functional Assessment Scale (ALSFRS-R), and patient-reported outcomes, were evaluated immediately before and after gait treatment with HAL for each course. 2MWD improved from 94 m to 101.8 m, and the ALSFRS-R gait items remained unchanged (score 3) for approximately 5 years. The patient could maintain walking ability in terms of gait symmetry, walking endurance, and independence walking despite disease progression during HAL treatment.

Conclusion

The long-term gait treatment with HAL in a patient with SBMA may contribute to the maintenance and improvement of the gait endurance and ability to perform activities of daily living. The cybernics treatment using HAL may enable patients to relearn correct gait movements. The gait analysis and physical function assessment by a physical therapist might be important to maximize the benefits of HAL treatment.

Novel Therapeutic Approaches in Inherited Neuropathies: A Systematic Review

The management of inherited neuropathies relies mostly on the treatment of symptoms. In recent years, a better understanding of the pathogenic mechanisms that underlie neuropathies has allowed for the development of disease-modifying therapies. Here, we systematically review the therapies that have emerged in this field over the last five years. An updated list of diseases with peripheral neuropathy as a clinical feature was created based on panels of genes used clinically to diagnose inherited neuropathy. This list was extended by an analysis of published data by the authors and verified by two experts. A comprehensive search for studies of human patients suffering from one of the diseases in our list yielded 28 studies that assessed neuropathy as a primary or secondary outcome. Although the use of various scales and scoring systems made comparisons difficult, this analysis identified diseases associated with neuropathy for which approved therapies exist. An important finding is that the symptoms and/or biomarkers of neuropathies were assessed only in a minority of cases. Therefore, further investigation of treatment efficacy on neuropathies in future trials must employ objective, consistent methods such as wearable technologies, motor unit indexes, MRI or sonography imaging, or the use of blood biomarkers associated with consistent nerve conduction studies.

Ligand-free mitochondria-localized mutant AR-induced cytotoxicity in spinal bulbar muscular atrophy

Spinal bulbar muscular atrophy (SBMA), the first identified CAG-repeat expansion disorder, is an X-linked neuromuscular disorder involving CAG-repeat-expansion mutations in the androgen receptor (AR) gene. We utilized CRISPR-Cas9 gene editing to engineer novel isogenic human induced pluripotent stem cell (hiPSC) models, consisting of isogenic AR knockout, control and disease lines expressing mutant AR with distinct repeat lengths, as well as control and disease lines expressing FLAG-tagged wild-type and mutant AR, respectively. Adapting a small-molecule cocktail-directed approach, we differentiate the isogenic hiPSC models into motor neuron-like cells with a highly enriched population to uncover cell-type-specific mechanisms underlying SBMA and to distinguish gain- from loss-of-function properties of mutant AR in disease motor neurons. We demonstrate that ligand-free mutant AR causes drastic mitochondrial dysfunction in neurites of differentiated disease motor neurons due to gain-of-function mechanisms and such cytotoxicity can be amplified upon ligand (androgens) treatment. We further show that aberrant interaction between ligand-free, mitochondria-localized mutant AR and F-ATP synthase is associated with compromised mitochondrial respiration and multiple other mitochondrial impairments. These findings counter the established notion that androgens are requisite for mutant AR-induced cytotoxicity in SBMA, reveal a compelling mechanistic link between ligand-free mutant AR, F-ATP synthase and mitochondrial dysfunction, and provide innovative insights into motor neuron-specific therapeutic interventions for SBMA.

HSF1 and Its Role in Huntington's Disease Pathology

PURPOSE OF REVIEW

Heat shock factor 1 (HSF1) is the master transcriptional regulator of the heat shock response (HSR) in mammalian cells and is a critical element in maintaining protein homeostasis. HSF1 functions at the center of many physiological processes like embryogenesis, metabolism, immune response, aging, cancer, and neurodegeneration. However, the mechanisms that allow HSF1 to control these different biological and pathophysiological processes are not fully understood. This review focuses on Huntington's disease (HD), a neurodegenerative disease characterized by severe protein aggregation of the huntingtin (HTT) protein. The aggregation of HTT, in turn, leads to a halt in the function of HSF1. Understanding the pathways that regulate HSF1 in different contexts like HD may hold the key to understanding the pathomechanisms underlying other proteinopathies. We provide the most current information on HSF1 structure, function, and regulation, emphasizing HD, and discussing its potential as a biological target for therapy.

DATA SOURCES

We performed PubMed search to find established and recent reports in HSF1, heat shock proteins (Hsp), HD, Hsp inhibitors, HSF1 activators, and HSF1 in aging, inflammation, cancer, brain development, mitochondria, synaptic plasticity, polyglutamine (polyQ) diseases, and HD.

STUDY SELECTIONS

Research and review articles that described the mechanisms of action of HSF1 were selected based on terms used in PubMed search.

RESULTS

HSF1 plays a crucial role in the progression of HD and other protein-misfolding related neurodegenerative diseases. Different animal models of HD, as well as postmortem brains of patients with HD, reveal a connection between the levels of HSF1 and HSF1 dysfunction to mutant HTT (mHTT)-induced toxicity and protein aggregation, dysregulation of the ubiquitin-proteasome system (UPS), oxidative stress, mitochondrial dysfunction, and disruption of the structural and functional integrity of synaptic connections, which eventually leads to neuronal loss. These features are shared with other neurodegenerative diseases (NDs). Currently, several inhibitors against negative regulators of HSF1, as well as HSF1 activators, are developed and hold promise to prevent neurodegeneration in HD and other NDs.

CONCLUSION

Understanding the role of HSF1 during protein aggregation and neurodegeneration in HD may help to develop therapeutic strategies that could be effective across different NDs.

The length of uninterrupted CAG repeats in stem regions of repeat disease associated hairpins determines the amount of short CAG oligonucleotides that are toxic to cells through RNA interference

Extended CAG trinucleotide repeats (TNR) in the genes huntingtin (HTT) and androgen receptor (AR) are the cause of two progressive neurodegenerative disorders: Huntington's disease (HD) and Spinal and Bulbar Muscular Atrophy (SBMA), respectively. Anyone who inherits the mutant gene in the complete penetrance range (>39 repeats for HD and 44 for SBMA) will develop the disease. An inverse correlation exists between the length of the CAG repeat and the severity and age of onset of the diseases. Growing evidence suggests that it is the length of uninterrupted CAG repeats in the mRNA rather than the length of poly glutamine (polyQ) in mutant (m)HTT protein that determines disease progression. One variant of mHTT (loss of inhibition; LOI) causes a 25 year earlier onset of HD when compared to a reference sequence, despite both coding for a protein that contains an identical number of glutamines. Short 21-22 nt CAG repeat (sCAGs)-containing RNAs can cause disease through RNA interference (RNAi). RNA hairpins (HPs) forming at the CAG TNRs are stabilized by adjacent CCG (in HD) or CUG repeats (in SBMA) making them better substrates for Dicer, the enzyme that processes CAG HPs into sCAGs. We now show that cells deficient in Dicer or unable to mediate RNAi are resistant to the toxicity of the HTT and AR derived HPs. Expression of a small HP that mimics the HD LOI variant is more stable and more toxic than a reference HP. We report that the LOI HP is processed by Dicer, loaded into the RISC more efficiently, and gives rise to a higher quantity of RISC-bound 22 nt sCAGs. Our data support the notion that RNAi contributes to the cell death seen in HD and SBMA and provide an explanation for the dramatically reduced onset of disease in HD patients that carry the LOI variant.

Mexiletine in spinal and bulbar muscular atrophy: a randomized controlled trial

Objective

Patients with spinal and bulbar muscular atrophy (SBMA) often experience muscular weakness under cold exposure.

Methods

In our previously conducted observational study, we assessed nerve conduction and grip strength to examine the effect of cold exposure on motor function, based on which we conducted a randomized controlled trial to evaluate the efficacy and safety of mexiletine hydrochloride in SBMA (MEXPRESS).

Results

In the observational study, 51 consecutive patients with SBMA and 18 healthy controls (HCs) were enrolled. Of the patients with SBMA, 88.0% experienced cold paresis. Patients with SBMA exhibited greater prolongation of ulnar nerve distal latency under cold (SBMA, 5.6 ± 1.1 msec; HC, 4.3 ± 0.6 msec; p <0.001); the change in the distal latencies between room temperature and cold exposure conditions correlated with the change in grip power. In the MEXPRESS trial, 20 participants took mexiletine or lactose, three times a day for 4 weeks with a crossover design. There was no difference in distal latencies at room temperature and under cold exposure between mexiletine and placebo groups as the primary endpoint. However, tongue pressure and 10‐sec grip and release test under cold exposure were improved in the mexiletine group. There were no serious adverse events throughout the study period.

Interpretation

Cold paresis is common and associated with prolongation of distal latency in SBMA. The results of the phase II clinical trial revealed that mexiletine showed short‐term safety, but it did not restore cold exposure‐induced prolongation of distal latency.

Label-free morphological sub-population cytometry for sensitive phenotypic screening of heterogenous neural disease model cells

Label-free image analysis has several advantages with respect to the development of drug screening platforms. However, the evaluation of drug-responsive cells based exclusively on morphological information is challenging, especially in cases of morphologically heterogeneous cells or a small subset of drug-responsive cells. We developed a novel label-free cell sub-population analysis method called “in silico FOCUS (in silico analysis of featured-objects concentrated by anomaly discrimination from unit space)” to enable robust phenotypic screening of morphologically heterogeneous spinal and bulbar muscular atrophy (SBMA) model cells. This method with the anomaly discrimination concept can sensitively evaluate drug-responsive cells as morphologically anomalous cells through in silico cytometric analysis. As this algorithm requires only morphological information of control cells for training, no labeling or drug administration experiments are needed. The responses of SBMA model cells to dihydrotestosterone revealed that in silico FOCUS can identify the characteristics of a small sub-population with drug-responsive phenotypes to facilitate robust drug response profiling. The phenotype classification model confirmed with high accuracy the SBMA-rescuing effect of pioglitazone using morphological information alone. In silico FOCUS enables the evaluation of delicate quality transitions in cells that are difficult to profile experimentally, including primary cells or cells with no known markers.

Exploring the Role of Posttranslational Modifications in Spinal and Bulbar Muscular Atrophy

Spinal and Bulbar Muscular Atrophy (SBMA) is an X-linked adult-onset progressive neuromuscular disease that affects the spinal and bulbar motor neurons and skeletal muscles. SBMA is caused by expansion of polymorphic CAG trinucleotide repeats in the Androgen Receptor (AR) gene, resulting in expanded glutamine tract in the AR protein. Polyglutamine (polyQ) expansion renders the mutant AR protein toxic, resulting in the formation of mutant protein aggregates and cell death. This classifies SBMA as one of the nine known polyQ diseases. Like other polyQ disorders, the expansion of the polyQ tract in the AR protein is the main genetic cause of the disease; however, multiple other mechanisms besides the polyQ tract expansion also contribute to the SBMA disease pathophysiology. Posttranslational modifications (PTMs), including phosphorylation, acetylation, methylation, ubiquitination, and SUMOylation are a category of mechanisms by which the functionality of AR has been found to be significantly modulated and can alter the neurotoxicity of SBMA. This review summarizes the different PTMs and their effects in regulating the AR function and discusses their pathogenic or protective roles in context of SBMA. This review also includes the therapeutic approaches that target the PTMs of AR in an effort to reduce the mutant AR-mediated toxicity in SBMA.

Mapping Motor Neuron Vulnerability in the Neuraxis of Male SOD1G93A Mice Reveals Widespread Loss of Androgen Receptor Occurring Early in Spinal Motor Neurons

Sex steroid hormones have been implicated as disease modifiers in the neurodegenerative disorder amyotrophic lateral sclerosis (ALS). Androgens, signalling via the androgen receptor (AR), predominate in males, and have widespread actions in the periphery and the central nervous system (CNS). AR translocates to the cell nucleus when activated upon binding androgens, whereby it regulates transcription of target genes via the classical genomic signalling pathway. We previously reported that AR protein is decreased in the lumbar spinal cord tissue of symptomatic male SOD1^G93A mice. Here, we further explored the changes in AR within motor neurons (MN) of the CNS, assessing their nuclear AR content and propensity to degenerate by endstage disease in male SOD1^G93A mice. We observed that almost all motor neuron populations had undergone significant loss in nuclear AR in SOD1^G93A mice. Interestingly, loss of nuclear AR was evident in lumbar spinal MNs as early as the pre-symptomatic age of 60 days. Several MN populations with high AR content were identified which did not degenerate in SOD1^G93A mice. These included the brainstem ambiguus and vagus nuclei, and the sexually dimorphic spinal MNs: cremaster, dorsolateral nucleus (DLN) and spinal nucleus of bulbocavernosus (SNB). In conclusion, we demonstrate that AR loss directly associates with MN vulnerability and disease progression in the SOD1^G93A mouse model of ALS.

Genetic architecture of motor neuron diseases

Motor neuron diseases (MNDs) are rare and frequently fatal neurological disorders in which motor neurons within the brainstem and spinal cord regions slowly die. MNDs are primarily caused by genetic mutations, and > 100 different mutant genes in humans have been discovered thus far. Given the fact that many more MND-related genes have yet to be discovered, the growing body of genetic evidence has offered new insights into the diverse cellular and molecular mechanisms involved in the aetiology and pathogenesis of MNDs. This search may aid in the selection of potential candidate genes for future investigation and, eventually, may open the door to novel interventions to slow down disease progression. In this review paper, we have summarized detailed existing research findings of different MNDs, such as amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), spinal bulbar muscle atrophy (SBMA) and hereditary spastic paraplegia (HSP) in relation to their complex genetic architecture.

Selective suppression of polyglutamine-expanded protein by lipid nanoparticle-delivered siRNA targeting CAG expansions in the mouse CNS

Polyglutamine (polyQ) diseases are inherited neurodegenerative disorders caused by expansion of cytosine-adenine-guanine (CAG)-trinucleotide repeats in causative genes. These diseases include spinal and bulbar muscular atrophy (SBMA), Huntington’s disease, dentatorubral-pallidoluysian atrophy, and spinocerebellar ataxias. Targeting expanded CAG repeats is a common therapeutic approach to polyQ diseases, but concomitant silencing of genes with normal CAG repeats may lead to toxicity. Previous studies have shown that CAG repeat-targeting small interfering RNA duplexes (CAG-siRNAs) have the potential to selectively suppress mutant proteins in in vitro cell models of polyQ diseases. However, in vivo application of these siRNAs has not yet been investigated. In this study, we demonstrate that an unlocked nucleic acid (UNA)-modified CAG-siRNA shows high selectivity for polyQ-expanded androgen receptor (AR) inhibition in in vitro cell models and that lipid nanoparticle (LNP)-mediated delivery of the CAG-siRNA selectively suppresses mutant AR in the central nervous system of an SBMA mouse model. In addition, a subcutaneous injection of the LNP-delivered CAG-siRNA efficiently suppresses mutant AR in the skeletal muscle of the SBMA mouse model. These results support the therapeutic potential of LNP-delivered UNA-modified CAG-siRNAs for selective suppression of mutant proteins in SBMA and other polyQ diseases.

Split hand and motor axonal hyperexcitability in spinal and bulbar muscular atrophy

OBJECTIVE

The 'split hand' sign refers to preferential wasting of the thenar and first dorsal interosseous muscles with relatively sparing of the hypothenar muscles in amyotrophic lateral sclerosis (ALS) and both cortical and spinal/peripheral excitotoxic mechanisms have been proposed. We aimed to study split hand and axonal excitability in spinal and bulbar muscular atrophy (SBMA) in which cortical motor neurons are intact.

METHODS

In 35 patients with genetically confirmed SBMA, 55 with ALS, 158 with other neuromuscular diseases and 90 normal controls; split hand was strictly determined by amplitudes of compound muscle action potentials. Nerve excitability testing of median motor axons was performed in 35 SBMA and 55 patients with ALS and 45 normal controls.

RESULTS

Split hand was as frequently found for patients with SBMA (57%) and ALS (62%), compared with disease (20%) and normal (0%) controls. Excitability testing showed that in both SBMA and ALS, strength-duration time constant was longer, and threshold changes in depolarising threshold electrotonus and superexcitability in the recovery cycle were greater than in normal controls (p<0.01).

CONCLUSIONS

Split hand is not specific to ALS and can be caused by the peripheral mechanism alone in SBMA, whereas the effect of upper motor neuron lesion cannot be excluded in ALS. Our results also suggest that SBMA and ALS share common axonal excitability changes; increased nodal persistent sodium and reduced potassium currents that may accelerate motor neuronal death and differently affect axons-innervating different muscles. Ion channel modulators could be a therapeutic option for both SBMA and ALS.

Disease mechanism, biomarker and therapeutics for spinal and bulbar muscular atrophy (SBMA)

Spinal and bulbar muscular atrophy (SBMA) is a hereditary neuromuscular disorder caused by CAG trinucleotide expansion in the gene encoding the androgen receptor (AR). In the central nervous system, lower motor neurons are selectively affected, whereas pathology of patients and animal models also indicates involvement of skeletal muscle including loss of fast-twitch type 2 fibres and increased slow-twitch type 1 fibres, together with a glycolytic-to-oxidative metabolic switch. Evaluation of muscle and fat using MRI, in addition to biochemical indices such as serum creatinine level, are promising biomarkers to track the disease progression. The serum level of creatinine starts to decrease before the onset of muscle weakness, followed by the emergence of hand tremor, a prodromal sign of the disease. Androgen-dependent nuclear accumulation of the polyglutamine-expanded AR is an essential step in the pathogenesis, providing therapeutic opportunities via hormonal manipulation and gene silencing with antisense oligonucleotides. Animal studies also suggest that hyperactivation of Src, alteration of autophagy and a mitochondrial deficit underlie the neuromuscular degeneration in SBMA and provide alternative therapeutic targets.

Muscle BDNF improves synaptic and contractile muscle strength in Kennedy's disease mice in a muscle-type specific manner

KEY POINTS

Muscle-derived neurotrophic factors may offer therapeutic promise for treating neuromuscular diseases. We report that a muscle-derived neurotrophic factor, BDNF, rescues synaptic and muscle function in a muscle-type specific manner in mice modelling Kennedy's disease (KD). We also find that BDNF rescues select molecular mechanisms in slow and fast muscle that may underlie the improved cellular function. We also report for the first time that expression of BDNF, but not other members of the neurotrophin family, is perturbed in muscle from patients with KD. Given that muscle BDNF had divergent therapeutic effects that depended on muscle type, a combination of neurotrophic factors may optimally rescue neuromuscular function via effects on both pre- and postsynaptic function, in the face of disease.

ABSTRACT

Deficits in muscle brain-derived neurotrophic factor (BDNF) correlate with neuromuscular deficits in mouse models of Kennedy's disease (KD), suggesting that restoring muscle BDNF might restore function. To test this possibility, transgenic mice expressing human BDNF in skeletal muscle were crossed with '97Q' KD mice. We found that muscle BDNF slowed disease, doubling the time between symptom onset and endstage. BDNF also improved expression of genes in muscle known to play key roles in neuromuscular function, including counteracting the expression of neonatal isoforms induced by disease. Intriguingly, BDNF's ameliorative effects differed between muscle types: synaptic strength was rescued only in slow-twitch muscle, while contractile strength was improved only in fast-twitch muscle. In sum, muscle BDNF slows disease progression, rescuing select cellular and molecular mechanisms that depend on fibre type. Muscle BDNF expression was also affected in KD patients, reinforcing its translational and therapeutic potential for treating this disorder.

The Role of Sex and Sex Hormones in Neurodegenerative Diseases

Neurodegenerative diseases (NDs) are a wide class of disorders of the central nervous system (CNS) with unknown etiology. Several factors were hypothesized to be involved in the pathogenesis of these diseases, including genetic and environmental factors. Many of these diseases show a sex prevalence and sex steroids were shown to have a role in the progression of specific forms of neurodegeneration. Estrogens were reported to be neuroprotective through their action on cognate nuclear and membrane receptors, while adverse effects of male hormones have been described on neuronal cells, although some data also suggest neuroprotective activities. The response of the CNS to sex steroids is a complex and integrated process that depends on (i) the type and amount of the cognate steroid receptor and (ii) the target cell type-either neurons, glia, or microglia. Moreover, the levels of sex steroids in the CNS fluctuate due to gonadal activities and to local metabolism and synthesis. Importantly, biochemical processes involved in the pathogenesis of NDs are increasingly being recognized as different between the two sexes and as influenced by sex steroids. The aim of this review is to present current state-of-the-art understanding on the potential role of sex steroids and their receptors on the onset and progression of major neurodegenerative disorders, namely, Alzheimer's disease, Parkinson's diseases, amyotrophic lateral sclerosis, and the peculiar motoneuron disease spinal and bulbar muscular atrophy, in which hormonal therapy is potentially useful as disease modifier.

Unveiling synapse pathology in spinal bulbar muscular atrophy by genome-wide transcriptome analysis of purified motor neurons derived from disease specific iPSCs

Spinal bulbar muscular atrophy (SBMA) is an adult-onset, slowly progressive motor neuron disease caused by abnormal CAG repeat expansion in the androgen receptor (AR) gene. Although ligand (testosterone)-dependent mutant AR aggregation has been shown to play important roles in motor neuronal degeneration by the analyses of transgenic mice models and in vitro cell culture models, the underlying disease mechanisms remain to be fully elucidated because of the discrepancy between model mice and SBMA patients. Thus, novel human disease models that recapitulate SBMA patients’ pathology more accurately are required for more precise pathophysiological analysis and the development of novel therapeutics. Here, we established disease specific iPSCs from four SBMA patients, and differentiated them into spinal motor neurons. To investigate motor neuron specific pathology, we purified iPSC-derived motor neurons using flow cytometry and cell sorting based on the motor neuron specific reporter, HB9^e438::Venus, and proceeded to the genome-wide transcriptome analysis by RNA sequences. The results revealed the involvement of the pathology associated with synapses, epigenetics, and endoplasmic reticulum (ER) in SBMA. Notably, we demonstrated the involvement of the neuromuscular synapse via significant upregulation of Synaptotagmin, R-Spondin2 (RSPO2), and WNT ligands in motor neurons derived from SBMA patients, which are known to be associated with neuromuscular junction (NMJ) formation and acetylcholine receptor (AChR) clustering. These aberrant gene expression in neuromuscular synapses might represent a novel therapeutic target for SBMA.

Src inhibition attenuates polyglutamine-mediated neuromuscular degeneration in spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is a neuromuscular disease caused by an expanded CAG repeat in the androgen receptor (AR) gene. Here, we perform a comprehensive analysis of signaling pathways in a mouse model of SBMA (AR-97Q mice) utilizing a phosphoprotein assay. We measure the levels of 17 phosphorylated proteins in spinal cord and skeletal muscle of AR-97Q mice at three stages. The level of phosphorylated Src (p-Src) is markedly increased in the spinal cords and skeletal muscles of AR-97Q mice prior to the onset. Intraperitoneal administration of a Src kinase inhibitor improves the behavioral and histopathological phenotypes of the transgenic mice. We identify p130Cas as an effector molecule of Src and show that the phosphorylated p130Cas is elevated in murine and cellular models of SBMA. These results suggest that Src kinase inhibition is a potential therapy for SBMA.

Long-term Effects of Androgen Deprivation in a Patient with Spinal and Bulbar Muscular Atrophy - A Case Report with 14 Years of Follow-up

Spinal and bulbar muscular atrophy (SBMA) is a progressive hereditary neuromuscular disease caused by the testosterone-dependent accumulation of pathogenic polyglutamine-expanded androgen receptor protein. A 41-year-old man with SBMA received the androgen deprivation agent leuprorelin acetate for 7 years in clinical trials and underwent castration following the trial. Suppression of testosterone levels for 14 years resulted in a slower disease progression, as measured prospectively with quantitative measurements, than the historical control data reported in previous studies. This suggests that long-term androgen deprivation delays disease progression in SBMA.

DNA methylation inhibitor attenuates polyglutamine-induced neurodegeneration by regulating Hes5

Spinal and bulbar muscular atrophy (SBMA) is a polyglutamine-mediated neuromuscular disease caused by a CAG repeat expansion in the androgen receptor (AR) gene. While transcriptional dysregulation is known to play a critical role in the pathogenesis of SBMA, the underlying molecular pathomechanisms remain unclear. DNA methylation is a fundamental epigenetic modification that silences the transcription of various genes that have a CpG-rich promoter. Here, we showed that DNA methyltransferase 1 (Dnmt1) is highly expressed in the spinal motor neurons of an SBMA mouse model and in patients with SBMA. Both genetic Dnmt1 depletion and treatment with RG108, a DNA methylation inhibitor, ameliorated the viability of SBMA model cells. Furthermore, a continuous intracerebroventricular injection of RG108 mitigated the phenotype of SBMA mice. DNA methylation array analysis identified hairy and enhancer of split 5 (Hes5) as having a CpG island with hyper-methylation in the promoter region, and the Hes5 expression was strongly silenced in SBMA. Moreover, Hes5 over-expression rescued the SBMA cells possibly by inducing Smad2 phosphorylation. Our findings suggest DNA hyper-methylation underlies the neurodegeneration in SBMA.

Disease Affects Bdnf Expression in Synaptic and Extrasynaptic Regions of Skeletal Muscle of Three SBMA Mouse Models

Spinal bulbar muscular atrophy (SBMA) is a slowly progressive, androgen-dependent neuromuscular disease in men that is characterized by both muscle and synaptic dysfunction. Because gene expression in muscle is heterogeneous, with synaptic myonuclei expressing genes that regulate synaptic function and extrasynaptic myonuclei expressing genes to regulate contractile function, we used quantitative PCR to compare gene expression in these two domains of muscle from three different mouse models of SBMA: the "97Q" model that ubiquitously expresses mutant human androgen receptor (AR), the 113Q knock-in (KI) model that expresses humanized mouse AR with an expanded glutamine tract, and the "myogenic" model that overexpresses wild-type rat AR only in skeletal muscle. We were particularly interested in neurotrophic factors because of their role in maintaining neuromuscular function via effects on both muscle and synaptic function, and their implicated role in SBMA. We confirmed previous reports of the enriched expression of select genes (e.g., the acetylcholine receptor) in the synaptic region of muscle, and are the first to report the synaptic enrichment of others (e.g., glial cell line-derived neurotrophic factor). Interestingly, all three models displayed comparably dysregulated expression of most genes examined in both the synaptic and extrasynaptic domains of muscle, with only modest differences between regions and models. These findings of comprehensive gene dysregulation in muscle support the emerging view that skeletal muscle may be a prime therapeutic target for restoring function of both muscles and motoneurons in SBMA.

Clinical manifestations and AR gene mutations in Kennedy's disease

Kennedy's disease, resulted from the expansion of a CAG repeat in exon 1 of androgen receptor (AR) gene, is a motor neuron degenerative disease in the brainstem and spinal cord with the slow development of facial, bulbar, and limb muscle degeneration. To investigate the clinical manifestations and gene mutations in Han Chinese patients with Kennedy's disease. The clinical manifestations of 5 male Han Chinese patients including 2 probands and their relatives from 2 families and 1 sporadic case were retrospectively studied. The CAG repeats in the first exon of AR were screened in 5 Han Chinese people including 2 probands and their healthy relatives from 2 families and 1 sporadic case by polymerase chain reaction (PCR) and direct sequencing. The average age at onset of Kennedy's disease was 48.20 ± 8.70 (mean ± SD) years and the average duration was 7.60 ± 5.32 years. All the patients showed slow onset and progressive weakness, wasting, and fasciculations of the whole body. Four patients demonstrated decreased fertility and 1 patient showed mild gynecomastia. Serum creatine kinase and testosterone levels were elevated mildly in 2 and 1 patients, respectively. The electromyogram showed neurogenic abnormalities. Muscle magnetic resonance demonstrated reduced muscle volume and fatty infiltration. Three different enlarged CAG domains were discovered in the 2 families and 1 sporadic patient with Kennedy's disease, and the CAG repeat number was 48, 43, and 44, respectively. The clinical manifestations of Kennedy's disease in Han Chinese middle-aged men were progressive weakness and atrophy in the bulbar and spinal muscles, occasionally demonstrating incomplete androgen insensitivity syndrome. These patients were also characterized with enlarged CAG repeat number in the first exon of AR, indicating that CAG number could be used in the diagnosis of Han Chinese patients with Kennedy's disease.

Autophagic and Proteasomal Mediated Removal of Mutant Androgen Receptor in Muscle Models of Spinal and Bulbar Muscular Atrophy

Spinal and bulbar muscular atrophy (SBMA) is an X-linked motoneuron disease (MND) caused by a mutant androgen receptor (AR) containing an elongated polyglutamine (polyQ) tract. ARpolyQ toxicity is triggered by androgenic AR ligands, which induce aberrant conformations (misfolding) of the ARpolyQ protein that aggregates. Misfolded proteins perturb the protein quality control (PQC) system leading to cell dysfunction and death. Spinal cord motoneurons, dorsal root ganglia neurons and skeletal muscle cells are affected by ARpolyQ toxicity. Here, we found that, in stabilized skeletal myoblasts (s-myoblasts), ARpolyQ formed testosterone-inducible aggregates resistant to NP-40 solubilization; these aggregates did not affect s-myoblasts survival or viability. Both wild type AR and ARpolyQ were processed via proteasome, but ARpolyQ triggered (and it was also cleared via) autophagy. ARpolyQ reduced two pro-autophagic proteins expression (BAG3 and VCP), leading to decreased autophagic response in ARpolyQ s-myoblasts. Overexpression of two components of the chaperone assisted selective autophagy (CASA) complex (BAG3 and HSPB8), enhanced ARpolyQ clearance, while the treatment with the mTOR independent autophagy activator trehalose induced complete ARpolyQ degradation. Thus, trehalose has beneficial effects in SBMA skeletal muscle models even when autophagy is impaired, possibly by stimulating CASA to assist the removal of ARpolyQ misfolded species/aggregates.

Trinucleotide Repeat Expansion Diseases, RNAi, and Cancer

Many neurodegenerative diseases are caused by unstable trinucleotide repeat (TNR) expansions located in disease-associated genes. siRNAs based on CAG repeat expansions effectively kill cancer cell lines in vitro through RNAi. They also cause significant reduction in tumor growth in a human ovarian cancer mouse model with no toxicity to the treated mice. This suggests that cancer cells are particularly sensitive to CAG TNR-derived siRNAs, and explains a reported inverse correlation between the length of CAG TNRs and reduced global cancer incidences in some CAG TNR diseases. This review discusses both mutant proteins and mutant RNAs as a cause of TNR diseases, with a focus on RNAi and its role in contributing to disease pathology and in suppressing cancer.

Study protocol for the MEXiletine hydrochloride administration trial: a placebo-controlled, randomised, double-blind, multicentre, crossover study of its efficacy and safety in spinal and bulbar muscular atrophy (MEXPRESS)

INTRODUCTION

Spinal and bulbar muscular atrophy (SBMA) is a slowly progressive neuromuscular disease. Cold exposure often leads to worsening of motor symptoms including paresis. Although mexiletine hydrochloride administration has been shown to be effective for the treatment of several muscular diseases, its effectiveness in SBMA has not been validated to date. The trial will test it as a symptomatic drug for cold paresis. This study is the first trial to evaluate the efficacy and safety of mexiletine hydrochloride administration in patients with SBMA.

METHODS AND ANALYSIS

A placebo-controlled, randomised, double-blind, multicentre, crossover clinical trial will be conducted to assess the safety and efficacy of mexiletine hydrochloride in patients with SBMA. The eligible patients will be assigned randomly in a 1:1 ratio to two groups in a double-blind manner. Participants will take mexiletine hydrochloride (300 mg/day) or a placebo orally three times a day for 4 weeks (period 1). After a 1-week washout period, participants will take the other drug for 4 weeks (period 2). The primary endpoint is the difference in distal latencies between room temperature and cold exposure conditions.

ETHICS AND DISSEMINATION

This study will be conducted in compliance with the Helsinki Declaration and the Ethical Guidelines for Medical and Health Research Involving Human Subjects by the Japanese government and has been approved by the ethics committee of Nagoya University Graduate School of Medicine, as a central institutional review board, and by each facility. The results will be disseminated in peer-reviewed journals and at scientific conferences.

TRIAL REGISTRATION NUMBER

UMIN000026150; Pre-results.

Preclinical progression of neurodegenerative diseases

Neurodegenerative diseases are disorders that are characterized by a progressive decline of the motor and/or cognitive function caused by a selective loss of neurons within the central nervous system. Recent advancements in the translational research have facilitated extensive insights into the molecular pathophysiology of neurodegenerative diseases. Nonetheless, a myriad of compounds that suppressed the disease progression in cellular and animal models did not exhibit efficacy in clinical trials. Perhaps, various biological, medical, and methodological factors could be attributed to unfavorable results of clinical trials of such disease-modifying therapies. Primarily, the fact that pathological changes at molecular and cellular levels precede the clinical onset by several years underscores a pressing need for the initiation of interventions before the emergence of neurological symptoms. Using exquisite biomarkers, recent studies revealed the preclinical and prodromal progression of pathophysiology, as well as compensatory brain responses in several neurodegenerative diseases. This review aims to discuss the recent advancement of biomarker studies on presymptomatic subjects and the perspective on a preventive trial of disease-modifying therapies for devastating neurological disorders.

Biomarker-based analysis of preclinical progression in spinal and bulbar muscular atrophy

OBJECTIVE

To identify a candidate biomarker reflecting biological changes during the preclinical progression of spinal and bulbar muscular atrophy (SBMA).

METHODS

We analyzed longitudinal changes in biochemical parameters obtained during health examinations before and after the diagnosis of SBMA. We estimated trajectories of clinical markers across years from the onset of weakness using linear mixed models and compared these trajectories with those estimated for male healthy controls and patients with amyotrophic lateral sclerosis (ALS) and Parkinson disease (PD). Moreover, we examined the relationship between serum creatinine level and the onset of symptoms using Kaplan-Meier curves.

RESULTS

Between October 2014 and October 2017, we enrolled 40 patients with genetically confirmed SBMA, 48 healthy controls, 25 patients with ALS, and 20 patients with PD. In patients with SBMA, we evaluated the patients' data for a period of 17.3 ± 7.5 years, including 11.4 ± 7.1 years of preclinical phase. Decreases in serum creatinine occurred >10 years before the onset. The mean serum creatinine concentration was 0.56 mg/dL at the onset of weakness in patients with SBMA compared to 0.88 ± 0.10 mg/dL on final evaluation in healthy controls. Serum levels of alanine transaminase and aspartate transaminase showed tendencies to increase in preclinical SBMA. These preclinical changes of biomarkers were not observed in either ALS or PD.

CONCLUSIONS

Our findings suggest that serum creatinine begins to decrease before the onset of clinical symptoms and is a biomarker for disease progression and the efficacy of therapeutics in preclinical SBMA.

The metabolic and endocrine characteristics in spinal and bulbar muscular atrophy

OBJECTIVE

Spinal and bulbar muscular atrophy (SBMA) is caused by an abnormal expansion of the CAG repeat in the androgen receptor gene. This study aimed to systematically phenotype a German SBMA cohort (n = 80) based on laboratory markers for neuromuscular, metabolic, and endocrine status, and thus provide a basis for the selection of biomarkers for future therapeutic trials.

METHODS

We assessed a panel of 28 laboratory parameters. The clinical course and blood biomarkers were correlated with disease duration and CAG repeat length. A subset of 11 patients was evaluated with body fat MRI.

RESULTS

Almost all patients reported muscle weakness (99%), followed by dysphagia (77%), tremor (76%), and gynecomastia (75%) as major complaints. Creatine kinase was the most consistently elevated (94%) serum marker, which, however, did not relate with either the disease duration or the CAG repeat length. Paresis duration and CAG repeat length correlated with dehydroepiandrosterone sulfate after correction for body mass index and age. The androgen insensitivity index was elevated in nearly half of the participants (48%).

CONCLUSIONS

Metabolic alterations in glucose homeostasis (diabetes) and fat metabolism (combined hyperlipidemia), and sex hormone abnormalities (androgen insensitivity) could be observed among SBMA patients without association with the neuromuscular phenotype. Dehydroepiandrosterone sulfate was the only biomarker that correlated strongly with both weakness duration and the CAG repeat length after adjusting for age and BMI, indicating its potential as a biomarker for both disease severity and duration and, therefore, its possible use as a reliable outcome measure in future therapeutic studies.

Spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is an adult-onset degenerative disorder of the neuromuscular system resulting in slowly progressive weakness and atrophy of the proximal limb and bulbar muscles. The disease is caused by the expansion of a CAG/glutamine tract in the amino-terminus of the androgen receptor. That SBMA exclusively affects males reflects the fact that critical pathogenic events are hormone-dependent. These include translocation of the polyglutamine androgen receptor from the cytoplasm to the nucleus and unfolding of the mutant protein. Studies of the pathology of SBMA subjects have revealed nuclear aggregates of the mutant androgen receptor, loss of lower motor neurons in the brainstem and spinal cord, and both neurogenic and myopathic changes in skeletal muscle. Mechanisms underlying disease pathogenesis include toxicity in both lower motor neurons and skeletal muscle, where effects on transcription, intracellular transport, and mitochondrial function have been documented. Therapies to treat SBMA patients remain largely supportive, although experimental approaches targeting androgen action or promoting degradation of the mutant androgen receptor protein or the encoding RNA are under active study.

X-Linked Spinal and Bulbar Muscular Atrophy: From Clinical Genetic Features and Molecular Pathology to Mechanisms Underlying Disease Toxicity

Spinal and Bulbar Muscular Atrophy (SBMA) is an inherited neuromuscular disorder caused by a CAG-polyglutamine (polyQ) repeat expansion in the androgen receptor (AR) gene. Unlike other polyQ diseases, where the function of the native causative protein is unknown, the biology of AR is well understood, and this knowledge has informed our understanding of how native AR function interfaces with polyQ-AR dysfunction. Furthermore, ligand-dependent activation of AR has been linked to SBMA disease pathogenesis, and has led to a thorough study of androgen-mediated effects on polyQ-AR stability, degradation, and post-translational modifications, as well as their roles in the disease process. Transcriptional dysregulation, proteostasis dysfunction, and mitochondrial abnormalities are central to polyQ-AR neurotoxicity, most likely via a 'change-of-function' mechanism. Intriguingly, recent work has demonstrated a principal role for skeletal muscle in SBMA disease pathogenesis, indicating that polyQ-AR toxicity initiates in skeletal muscle and results in secondary motor neuron demise. The existence of robust animal models for SBMA has permitted a variety of preclinical trials, driven by recent discoveries of altered cellular processes, and some of this preclinical work has led to human clinical trials. In this chapter, we review SBMA clinical features and disease biology, discuss our current understanding of the cellular and molecular basis of SBMA pathogenesis, and highlight ongoing efforts toward therapy development.

Long-term treatment with leuprorelin for spinal and bulbar muscular atrophy: natural history-controlled study

OBJECTIVE

To evaluate the prognosis and progression of spinal and bulbar muscular atrophy (SBMA), a rare X-linked motor neuron disorder caused by trinucleotide repeat expansion in the AR (androgen receptor) gene, after long-term androgen suppression with leuprorelin acetate treatment.

METHODS

In the present natural history-controlled study, 36 patients with SBMA treated with leuprorelin acetate for up to 84 months (leuprorelin acetate-treated group; LT group) and 29 patients with SBMA with no specific treatment (non-treated group; NT group) were analysed. Disease progression was evaluated by longitudinal quantitative assessment of motor functioning using the revised Amyotrophic Lateral Sclerosis Functional Rating Scale (ALSFRS-R), and the modified Norris score. In addition, we selected two major clinical endpoint events, namely the occurrence of pneumonia requiring hospitalisation and death, to evaluate disease prognosis following long-term leuprorelin acetate treatment.

RESULTS

In our analysis of the longitudinal disease progression using the random slope model, we observed a significant difference in the ALSFRS-R total score, the Limb Norris Score, and the Norris Bulbar Score (p=0.005, 0.026 and 0.020, respectively), with the LT group exhibiting a slower per-12-months decline compared with the NT group. As for the event analysis, the prognosis of the LT group was better in comparison to the NT group as for the event-free survival period (p=0.021).

CONCLUSION

Long-term treatment with leuprorelin acetate appears to delay the functional decline and suppress the incidence of pneumonia and death in subjects with SBMA.

The C. elegans VAPB homolog VPR-1 is a permissive signal for gonad development

VAMP/synaptobrevin-associated proteins (VAPs) contain an N-terminal major sperm protein domain (MSPd) that is associated with amyotrophic lateral sclerosis. VAPs have an intracellular housekeeping function, as well as an extracellular signaling function mediated by the secreted MSPd. Here we show that the C. elegans VAP homolog VPR-1 is essential for gonad development. vpr-1 null mutants are maternal effect sterile due to arrested gonadogenesis following embryo hatching. Somatic gonadal precursor cells and germ cells fail to proliferate fully and complete their respective differentiation programs. Maternal or zygotic vpr-1 expression is sufficient to induce gonadogenesis and fertility. Genetic mosaic and cell type-specific expression studies indicate that vpr-1 activity is important in the nervous system, germ line and intestine. VPR-1 acts in parallel to Notch signaling, a key regulator of germline stem cell proliferation and differentiation. Neuronal vpr-1 expression is sufficient for gonadogenesis induction during a limited time period shortly after hatching. These results support the model that the secreted VPR-1 MSPd acts at least in part on gonadal sheath cell precursors in L1 to early L2 stage hermaphrodites to permit gonadogenesis.

Highlighted Article:vpr-1 null mutants are sterile upon hatching, a defect rescued by the expression of MSPd from almost any tissue except for the somatic gonad itself. See also the companion paper by Schultz et al.

Kennedy disease (X-linked recessive bulbospinal neuronopathy): A comprehensive review from pathophysiology to therapy

Kennedy's disease, also known as spinal and bulbar muscular atrophy (SBMA), is a rare, adult-onset, X-linked recessive neuromuscular disease caused by expansion of a CAG repeat sequence in exon 1 of the androgen receptor gene (AR) encoding a polyglutamine (polyQ) tract. The polyQ-expanded AR accumulates in nuclei, and initiates degeneration and loss of motor neurons and dorsal root ganglia. While the disease has long been considered a pure lower motor neuron disease, recently, the presence of major hyper-creatine-kinase (CK)-emia and myopathic alterations on muscle biopsy has suggested the presence of a primary myopathy underlying a wide range of clinical manifestations. The disease, which affects male adults, is characterized by muscle weakness and atrophy localized proximally in the limbs, and bulbar involvement. Sensory disturbances are associated with the motor phenotype, but may be subclinical. The most frequent systemic symptom is gynecomastia related to androgen insensitivity, but other abnormalities, such as heart rhythm and urinary disturbances, have also been reported. The course of the disease is slowly progressive with normal life expectancy. The diagnosis of SBMA is based on genetic testing, with 38 CAG repeats taken as pathogenic. Despite several therapeutic attempts made in mouse models, no effective disease-modifying therapy is yet available, although symptomatic therapy is beneficial for the management of the weakness, fatigue and bulbar symptoms.

Kennedy's disease 1234 scale: Preliminary design and test

Kennedy's disease (KD), also known as spinal and bulbar muscular atrophy (SBMA), is a rare x-linked genetic disorder which is characterized by muscle weakness and atrophy. In previous clinical trials, KD patients had been assessed using the ALSFRS scale, which was specifically designed for ALS patients. However, the progression of KD is very slow, and thus, the ALSFRS does not accurately reflect changes in the clinical condition of KD patient. Here, we developed the KD 1234 scale which designed specially for KD. We evaluated KD 1234 and ALSFRS in 81 KD patients and 100 healthy age-matched male participants. Of 81 cases, 52 were followed up after 32months. Scale reliability was assessed using inter-rater reliability, split-half reliability and internal consistency reliability (Cronbach's α coefficient). Scale validity was evaluated using content and structure validity. Longitudinal analysis was performed. The results showed the KD 1234 scale was simple and easy to operate, with good reliability, validity and reactivity. We think this new scale can quantitatively evaluate the clinical condition of KD patients and any changes in this condition, and is thus suitable for clinical studies.

Correlation of insulin resistance and motor function in spinal and bulbar muscular atrophy

This study aimed to evaluate various metabolic parameters in patients with spinal and bulbar muscular atrophy (SBMA), to investigate the association between those indices and disease severity, and to explore the underlying molecular pathogenesis. We compared the degree of obesity, metabolic parameters, and blood pressure in 55 genetically confirmed SBMA patients against those in 483 age- and sex-matched healthy control. In SBMA patients, we investigated the correlation between these factors and motor functional indices. SBMA patients had lower body mass index, blood glucose, and Hemoglobin A1c, but higher blood pressure, homeostasis model assessment of insulin resistance (HOMA-IR, a marker of insulin resistance), total cholesterol, and adiponectin levels than the control subjects. There were no differences in visceral fat areas, high-density lipoprotein-cholesterol (HDL-C), or triglyceride levels in two groups. Revised amyotrophic lateral sclerosis functional rating scale (ALSFRS-R) correlated positively with HDL-C, but negatively with HOMA-IR. Through stepwise multiple regression analysis, we identified HOMA-IR as a significant metabolic determinant of ALSFRS-R. In biochemical analysis, we found that decreased expressions of insulin receptors, insulin receptor substrate-1 and insulin receptor-β, in autopsied muscles and fibroblasts of SBMA patients. This study demonstrates that SBMA patients have insulin resistance, which is associated with the disease severity. The expressions of insulin receptors are attenuated in the skeletal muscle of SBMA, providing a possible pathomechanism of metabolic alterations. These findings suggested that insulin resistance is a metabolic index reflecting disease severity and pathogenesis as well as a potential therapeutic target for SBMA.

Axonal Excitability in Amyotrophic Lateral Sclerosis : Axonal Excitability in ALS

Axonal excitability testing provides in vivo assessment of axonal ion channel function and membrane potential. Excitability techniques have provided insights into the pathophysiological mechanisms underlying the development of neurodegeneration and clinical features of amyotrophic lateral sclerosis (ALS) and related neuromuscular disorders. Specifically, abnormalities of Na+ and K+ conductances contribute to development of membrane hyperexcitability in ALS, thereby leading to symptom generation of muscle cramps and fasciculations, in addition to promoting a neurodegenerative cascade via Ca2+-mediated processes. Modulation of axonal ion channel function in ALS has resulted in significant symptomatic improvement that has been accompanied by stabilization of axonal excitability parameters. Separately, axonal ion channel dysfunction evolves with disease progression and correlates with survival, thereby serving as a potential therapeutic biomarker in ALS. The present review provides an overview of axonal excitability techniques and the physiological mechanisms underlying membrane excitability, with a focus on the role of axonal ion channel dysfunction in motor neuron disease and related neuromuscular diseases.

Decreased Peak Expiratory Flow Associated with Muscle Fiber-Type Switching in Spinal and Bulbar Muscular Atrophy

The aim of this study was to characterize the respiratory function profile of subjects with spinal and bulbar muscular atrophy (SBMA), and to explore the underlying pathological mechanism by comparing the clinical and biochemical indices of this disease with those of amyotrophic lateral sclerosis (ALS). We enrolled male subjects with SBMA (n = 40) and ALS (n = 25) along with 15 healthy control subjects, and assessed their respiratory function, motor function, and muscle strength. Predicted values of peak expiratory flow (%PEF) and forced vital capacity were decreased in subjects with SBMA compared with controls. In SBMA, both values were strongly correlated with the trunk subscores of the motor function tests and showed deterioration relative to disease duration. Compared with activities of daily living (ADL)-matched ALS subjects, %PEF, tongue pressure, and grip power were substantially decreased in subjects with SBMA. Both immunofluorescence and RT-PCR demonstrated a selective decrease in the expression levels of the genes encoding the myosin heavy chains specific to fast-twitch fibers in SBMA subjects. The mRNA levels of peroxisome proliferator-activated receptor gamma coactivator 1-alpha and peroxisome proliferator-activated receptor delta were up-regulated in SBMA compared with ALS and controls. In conclusion, %PEF is a disease-specific respiratory marker for the severity and progression of SBMA. Explosive muscle strength, including %PEF, was selectively affected in subjects with SBMA and was associated with activation of the mitochondrial biogenesis-related molecular pathway in skeletal muscles.

Polyglutamine androgen receptor-mediated neuromuscular disease

An expanded polyglutamine (polyQ) tract at the amino-terminus of the androgen receptor (AR) confers toxic properties responsible for neuronal and non-neuronal degeneration in spinal and bulbar muscular atrophy (SBMA), one of nine polyQ expansion diseases. Both lower motor neurons and peripheral tissues, including skeletal muscle, are affected, supporting the notion that SBMA is not a pure motor neuron disease but a degenerative disorder of the neuromuscular system. Here, we review experimental evidence demonstrating both nerve and muscle degeneration in SBMA model systems and patients. We propose that polyQ AR toxicity targets these components in a time-dependent fashion, with muscle pathology predominating early and motor neuron loss becoming more significant at late stages. This model of pathogenesis has important therapeutic implications, suggesting that symptoms arising from degeneration of nerve or muscle predominate at different points and that directed interventions targeting these components will be variably effective depending upon disease progression.

FUS interacts with nuclear matrix-associated protein SAFB1 as well as Matrin3 to regulate splicing and ligand-mediated transcription

FUS (Fused-in-Sarcoma) is a multifunctional DNA/RNA binding protein linked to familial amyotrophic lateral sclerosis/frontotemporal dementia (ALS/FTD). Since FUS is localized mainly in the nucleus with nucleo-cytoplasmic shuttling, it is critical to understand physiological functions in the nucleus to clarify pathogenesis. Here we report a yeast two-hybrid screening identified FUS interaction with nuclear matrix-associated protein SAFB1 (scaffold attachment factor B1). FUS and SAFB1, abundant in chromatin-bound fraction, interact in a DNA-dependent manner. N-terminal SAP domain of SAFB1, a DNA-binding motif, was required for its localization to chromatin-bound fraction and splicing regulation. In addition, depletion of SAFB1 reduced FUS’s localization to chromatin-bound fraction and splicing activity, suggesting SAFB1 could tether FUS to chromatin compartment thorough N-terminal DNA-binding motif. FUS and SAFB1 also interact with Androgen Receptor (AR) regulating ligand-dependent transcription. Moreover, FUS interacts with another nuclear matrix-associated protein Matrin3, which is muted in a subset of familial ALS cases and reportedly interacts with TDP-43. Interestingly, ectopic ALS-linked FUS mutant sequestered endogenous Matrin3 and SAFB1 in the cytoplasmic aggregates. These findings indicate SAFB1 could be a FUS’s functional platform in chromatin compartment to regulate RNA splicing and ligand-dependent transcription and shed light on the etiological significance of nuclear matrix-associated proteins in ALS pathogenesis.

Non-Cell-Autonomous Regulation of Retrograde Motoneuronal Axonal Transport in an SBMA Mouse Model

Defects in axonal transport are seen in motoneuronal diseases, but how that impairment comes about is not well understood. In spinal bulbar muscular atrophy (SBMA), a disorder linked to a CAG/polyglutamine repeat expansion in the androgen receptor (AR) gene, the disease-causing AR disrupts axonal transport by acting in both a cell-autonomous fashion in the motoneurons themselves, and in a non-cell-autonomous fashion in muscle. The non-cell-autonomous mechanism is suggested by data from a unique “myogenic” transgenic (TG) mouse model in which an AR transgene expressed exclusively in skeletal muscle fibers triggers an androgen-dependent SBMA phenotype, including defects in retrograde transport. However, motoneurons in this TG model retain the endogenous AR gene, leaving open the possibility that impairments in transport in this model also depend on ARs in the motoneurons themselves. To test whether non-cell-autonomous mechanisms alone can perturb retrograde transport, we generated male TG mice in which the endogenous AR allele has the testicular feminization mutation (Tfm) and, consequently, is nonfunctional. Males carrying the Tfm allele alone show no deficits in motor function or axonal transport, with or without testosterone treatment. However, when Tfm males carrying the myogenic transgene (Tfm/TG) are treated with testosterone, they develop impaired motor function and defects in retrograde transport, having fewer retrogradely labeled motoneurons and deficits in endosomal flux based on time-lapse video microscopy of living axons. These findings demonstrate that non-cell-autonomous disease mechanisms originating in muscle are sufficient to induce defects in retrograde transport in motoneurons.

Non-neural phenotype of spinal and bulbar muscular atrophy: results from a large cohort of Italian patients

OBJECTIVE

To carry out a deep characterisation of the main androgen-responsive tissues involved in spinal and bulbar muscular atrophy (SBMA).

METHODS

73 consecutive Italian patients underwent a full clinical protocol including biochemical and hormonal analyses, genitourinary examination, bone metabolism and densitometry, cardiological evaluation and muscle pathology.

RESULTS

Creatine kinase levels were slightly to markedly elevated in almost all cases (68 of the 73; 94%). 30 (41%) patients had fasting glucose above the reference limit, and many patients had total cholesterol (40; 54.7%), low-density lipoproteins cholesterol (29; 39.7%) and triglyceride (35; 48%) levels above the recommended values. Although testosterone, luteinising hormone and follicle-stimulating hormone values were generally normal, in one-third of cases we calculated an increased Androgen Sensitivity Index reflecting the presence of androgen resistance in these patients. According to the International Prostate Symptom Score (IPSS), 7/70 (10%) patients reported severe lower urinal tract symptoms (IPSS score >19), and 21/73 (30%) patients were moderately symptomatic (IPSS score from 8 to 19). In addition, 3 patients were carriers of an indwelling bladder catheter. Videourodynamic evaluation indicated that 4 of the 7 patients reporting severe urinary symptoms had an overt prostate-unrelated bladder outlet obstruction. Dual-energy X-ray absorptiometry scan data were consistent with low bone mass in 25/61 (41%) patients. Low bone mass was more frequent at the femoral than at the lumbar level. Skeletal muscle biopsy was carried out in 20 patients and myogenic changes in addition to the neurogenic atrophy were mostly observed.

CONCLUSIONS

Our study provides evidence of a wide non-neural clinical phenotype in SBMA, suggesting the need for comprehensive multidisciplinary protocols for these patients.