L-tyrosine for treatment of an infant with nemaline rod myopathy.

Nemaline rod myopathy is an extremely rare muscle disease responsible for hypotonia and poor muscle strength in infants. The disease has variable phenotypic presentations across different ages, ranging from neonatal to the adult onset and from severe to asymptomatic varieties. Clinical features, muscle biopsy and genetic testing help in diagnosis. The histopathological examination shows the presence of rod-like structures or nemaline bodies in muscles. Management remains mainly supportive, and currently, there is no available curative treatment. This case report describes an infant presenting with gross hypotonia, poor handling of secretions and multiple extubation failures who was diagnosed by clinical exome sequencing. The patient harboured compound heterozygous variants in the NEB gene suggestive of nemaline rod myopathy. The newborn showed significant improvement in muscle strength after he was started on dietary L-tyrosine supplementation. This case highlights the emerging role of L-tyrosine in the supportive care of infants with nemaline rod myopathy.

Characterization of NEB pathogenic variants in patients reveals novel nemaline myopathy disease mechanisms and omecamtiv mecarbil force effects.

Nebulin, a critical protein of the skeletal muscle thin filament, plays important roles in physiological processes such as regulating thin filament length (TFL), cross-bridge cycling, and myofibril alignment. Pathogenic variants in the nebulin gene (NEB) cause NEB-based nemaline myopathy (NEM2), a genetically heterogeneous disorder characterized by hypotonia and muscle weakness, currently lacking curative therapies. In this study, we examined a cohort of ten NEM2 patients, each with unique pathogenic variants, aiming to understand their impact on mRNA, protein, and functional levels. Results show that pathogenic truncation variants affect NEB mRNA stability and lead to nonsense-mediated decay of the mutated transcript. Moreover, a high incidence of cryptic splice site activation was found in patients with pathogenic splicing variants that are expected to disrupt the actin-binding sites of nebulin. Determination of protein levels revealed patients with either relatively normal or markedly reduced nebulin. We observed a positive relation between the reduction in nebulin and a reduction in TFL, or reduction in tension (both maximal and submaximal tension). Interestingly, our study revealed a pathogenic duplication variant in nebulin that resulted in a four-copy gain in the triplicate region of NEB and a much larger nebulin protein and longer TFL. Additionally, we investigated the effect of Omecamtiv mecarbil (OM), a small-molecule activator of cardiac myosin, on force production of type 1 muscle fibers of NEM2 patients. OM treatment substantially increased submaximal tension across all NEM2 patients ranging from 87 to 318%, with the largest effects in patients with the lowest level of nebulin. In summary, this study indicates that post-transcriptional or post-translational mechanisms regulate nebulin expression. Moreover, we propose that the pathomechanism of NEM2 involves not only shortened but also elongated thin filaments, along with the disruption of actin-binding sites resulting from pathogenic splicing variants. Significantly, our findings highlight the potential of OM treatment to improve skeletal muscle function in NEM2 patients, especially those with large reductions in nebulin levels.

Tirasemtiv enhances submaximal muscle tension in an Acta1:p.Asp286Gly mouse model of nemaline myopathy.

Nemaline myopathies are the most common form of congenital myopathies. Variants in ACTA1 (NEM3) comprise 15-25% of all nemaline myopathy cases. Patients harboring variants in ACTA1 present with a heterogeneous disease course characterized by stable or progressive muscle weakness and, in severe cases, respiratory failure and death. To date, no specific treatments are available. Since NEM3 is an actin-based thin filament disease, we tested the ability of tirasemtiv, a fast skeletal muscle troponin activator, to improve skeletal muscle function in a mouse model of NEM3, harboring the patient-based p.Asp286Gly variant in Acta1. Acute and long-term tirasemtiv treatment significantly increased muscle contractile capacity at submaximal stimulation frequencies in both fast-twitch extensor digitorum longus and gastrocnemius muscle, and intermediate-twitch diaphragm muscle in vitro and in vivo. Additionally, long-term tirasemtiv treatment in NEM3 mice resulted in a decreased respiratory rate with preserved minute volume, suggesting more efficient respiration. Altogether, our data support the therapeutic potential of fast skeletal muscle troponin activators in alleviating skeletal muscle weakness in a mouse model of NEM3 caused by the Acta1:p.Asp286Gly variant.

Pharmacological Inhibition of Myostatin in a Mouse Model of Typical Nemaline Myopathy Increases Muscle Size and Force.

Nemaline myopathy is one of the most common non-dystrophic congenital myopathies. Individuals affected by this condition experience muscle weakness and muscle smallness, often requiring supportive measures like wheelchairs or respiratory support. A significant proportion of patients, approximately one-third, exhibit compound heterozygous nebulin mutations, which usually give rise to the typical form of the disease. Currently, there are no approved treatments available for nemaline myopathy. Our research explored the modulation of myostatin, a negative regulator of muscle mass, in combating the muscle smallness associated with the disease. To investigate the effect of myostatin inhibition, we employed a mouse model with compound heterozygous nebulin mutations that mimic the typical form of the disease. The mice were treated with mRK35, a myostatin antibody, through weekly intraperitoneal injections of 10 mg/kg mRK35, commencing at two weeks of age and continuing until the mice reached four months of age. The treatment resulted in an increase in body weight and an approximate 20% muscle weight gain across most skeletal muscles, without affecting the heart. The minimum Feret diameter of type IIA and IIB fibers exhibited an increase in compound heterozygous mice, while only type IIB fibers demonstrated an increase in wild-type mice. In vitro mechanical experiments conducted on intact extensor digitorum longus muscle revealed that mRK35 augmented the physiological cross-sectional area of muscle fibers and enhanced absolute tetanic force in both wild-type and compound heterozygous mice. Furthermore, mRK35 administration improved grip strength in treated mice. Collectively, these findings indicate that inhibiting myostatin can mitigate the muscle deficits in nebulin-based typical nemaline myopathy, potentially serving as a much-needed therapeutic option.

Sporadic late onset nemaline myopathy with concurrent dermatological symptoms responding to immunosuppressive treatment.

BACKGROUND: Sporadic late onset nemaline myopathy is a rare, progressive muscle disease, presenting in adulthood, mainly affecting proximal limb and bulbar muscles. Muscle biopsies show characteristic nemaline rods. The putative mechanism is considered immune-related. Other manifestations aside from neuromuscular symptoms have not been described previously. CASE PRESENTATION: We present a case with atypical sporadic late onset nemaline myopathy (SLONM) of a non-HIV, non-MGUS subtype, where skin manifestations preceded neuromuscular symptoms, and a residual thymus with the histology of thymic follicular hyperplasia was detected during the diagnostic workup. Thorough dermatological investigations could not explain the skin presentations. Muscle biopsy revealed variation in fiber diameter, ragged-red and COX-negative fibers associated with discrete fibrosis. Electron microscopy detected atrophic muscle fibres with disorganization of the myofibrils, nemaline rods and abnormal mitochondria. Single-fiber EMG suggested signs of a neuromuscular transmission defect, EMG showed signs of myopathy. Analyses of antibodies associated with myasthenia gravis were negative. The patient showed improvement after intravenous immunoglobulin treatment regarding both the skin and the muscle symptoms. CONCLUSIONS: Our case highlights the heterogeneity of SLONM with its varied spectrum of presentation. A unique combination of dermatological symptoms and SLONM could be seen with skin lesions as primary presenting symptoms. An association can be considered between the different manifestations, presumably based on immune etiology, where immunosuppressive therapy has been beneficial.

Acute and chronic tirasemtiv treatment improves in vivo and in vitro muscle performance in actin-based nemaline myopathy mice.

Nemaline myopathy, a disease of the actin-based thin filament, is one of the most frequent congenital myopathies. To date, no specific therapy is available to treat muscle weakness in nemaline myopathy. We tested the ability of tirasemtiv, a fast skeletal troponin activator that targets the thin filament, to augment muscle force-both in vivo and in vitro-in a nemaline myopathy mouse model with a mutation (H40Y) in Acta1. In Acta1H40Y mice, treatment with tirasemtiv increased the force response of muscles to submaximal stimulation frequencies. This resulted in a reduced energetic cost of force generation, which increases the force production during a fatigue protocol. The inotropic effects of tirasemtiv were present in locomotor muscles and, albeit to a lesser extent, in respiratory muscles, and they persisted during chronic treatment, an important finding as respiratory failure is the main cause of death in patients with congenital myopathy. Finally, translational studies on permeabilized muscle fibers isolated from a biopsy of a patient with the ACTA1H40Y mutation revealed that at physiological Ca2+ concentrations, tirasemtiv increased force generation to values that were close to those generated in muscle fibers of healthy subjects. These findings indicate the therapeutic potential of fast skeletal muscle troponin activators to improve muscle function in nemaline myopathy due to the ACTA1H40Y mutation, and future studies should assess their merit for other forms of nemaline myopathy and for other congenital myopathies.

Chemotherapy-based approach is the preferred treatment for sporadic late-onset nemaline myopathy with a monoclonal protein.

Sporadic late-onset nemaline myopathy (SLONM) associated with monoclonal protein (MP) is a rare disease with an aggressive, and often fatal course. Whether SLONM + MP represents a malignancy or dysimmune disease remains unclear. Currently, two main approaches are used to treat SLONM + MP: nonchemotherapy-based treatment (immunosuppression, intravenous immunoglobulins, plasmapheresis and plasma exchange) or chemotherapy with or without autologous stem cell transplantation. Due to the rare occurrence of the disease, the best treatment modality is unknown. We analyzed treatment and outcomes in a large cohort of 53 patients with SLONM + MP: four our own patients and 49 cases from published literature. Neurological improvement in the nonchemotherapy group (N = 25) was observed in 52% of patients: 8% reached marked improvement, 8% moderate response, 36% mild response; none reached complete remission (CR). In the chemotherapy group (N = 28), neurological improvement was seen in 86% of patients: 46% reached CR, 25% marked response, 11% moderate response and 4% mild response. The best neurological improvement correlated with deep hematological remission. Mean time to best response in the chemotherapy group was 8 months versus 21 months in the nonchemotherapy group (P < .001). Overall survival was higher in patients in the chemotherapy group. A chemotherapy approach should be the preferred treatment for patients with SLOMN + MP with the goal to reach complete hematologic remission. Based on the clinical, morphological peculiarities, aggressive disease course and superior clinical benefits of chemotherapy over nonchemotherapy, SLONM + MP should be considered as a hematological malignancy with the presence of MP of clinical rather than undetermined significance.

[A case of sporadic late-onset nemaline myopathy associated with myasthenia gravis positive for anti-titin antibody and anti-Kv1.4 antibody].

A 66-year-old woman who had myasthenia gravis (MG) admitted for type II respiratory failure and right heart failure. Although she had neither ptosis, eye movement disorder, nor diplopia, she had orbital muscles weakness, reduction of gag reflex, dysarthria, dysphagia, and mild proximal muscle weakness. Blood tests showed anti-striated muscle antibodies (anti-titin antibody and anti-Kv1.4 antibody). A muscle biopsy of the left biceps showed a marked variation in fiber size, mild mononuclear cell infiltration was seen surrounding blood vessels in perimysium and nemaline bodies in some fibers. Immunohistochemical stains showed many muscle fibers express HLA-ABC. The patient was diagnosed as sporadic late-onset nemaline myopathy (SLONM) with MG, and treated by tacrolimus. After treatment, her respiratory function gradually improved and she discharged. In the case of atypical MG, measurement of anti-striated muscle antibody or muscle biopsy should be considered.

Omecamtiv mecarbil lowers the contractile deficit in a mouse model of nebulin-based nemaline myopathy.

Nemaline myopathy (NEM) is a congenital neuromuscular disorder primarily caused by nebulin gene (NEB) mutations. NEM is characterized by muscle weakness for which currently no treatments exist. In NEM patients a predominance of type I fibers has been found. Thus, therapeutic options targeting type I fibers could be highly beneficial for NEM patients. Because type I muscle fibers express the same myosin isoform as cardiac muscle (Myh7), the effect of omecamtiv mecarbil (OM), a small molecule activator of Myh7, was studied in a nebulin-based NEM mouse model (Neb cKO). Skinned single fibers were activated by exogenous calcium and force was measured at a wide range of calcium concentrations. Maximal specific force of type I fibers was much less in fibers from Neb cKO animals and calcium sensitivity of permeabilized single fibers was reduced (pCa50 6.12 ±0.08 (cKO) vs 6.36 ±0.08 (CON)). OM increased the calcium sensitivity of type I single muscle fibers. The greatest effect occurred in type I fibers from Neb cKO muscle where OM restored the calcium sensitivity to that of the control type I fibers. Forces at submaximal activation levels (pCa 6.0-6.5) were significantly increased in Neb cKO fibers (~50%) but remained below that of control fibers. OM also increased isometric force and power during isotonic shortening of intact whole soleus muscle of Neb cKO mice, with the largest effects at physiological stimulation frequencies. We conclude that OM has the potential to improve the quality of life of NEM patients by increasing the force of type I fibers at submaximal activation levels.

[Nemaline rod myopathy treated with L-tyrosine to relieve symptoms in a neonate]. / Miopatía nemalínica tratada con L-tirosina para aliviar los síntomas en un recién nacido.

Nemaline myopathy (NM) is a heterogeneous disorder defined by the presence of rod-shaped structures known as nemaline bodies or rods. The diagnosis is based on muscle weakness, combined with visualization of nemaline bodies on muscle biopsy. There is no curative treatment for nemaline myopathy. Therapeutic strategies for this condition are symptomatic and empirical. Herein, we present a newborn with severe respiratory failure and generalized muscle weakness, who was diagnosed as NM by muscle biopsy. The patient experienced remarkable decrease in sialorrhea and improvement of spontaneous movements after L-tyrosine treatment. This case is presented to emphasize the importance of muscle biopsy in the differential diagnosis of severe hypotonia during neonatal period and a possible benefit of L-tyrosine supplementation for decreasing sialorrhea and restoring muscle strength.

La miopatía nemalínica es un trastorno heterogéneo definido por la presencia de estructuras con forma de bastones, conocidas como cuerpos nemalínicos (o bastones de nemalina). El diagnóstico se funda en la debilidad muscular, además de la visualización de cuerpos nemalínicos en la biopsia muscular. La miopatía nemalínica no tiene cura. Las estrategias terapéuticas para este trastorno son sintomáticas y empíricas. En este artículo, presentamos el caso de una recién nacida con insuficiencia respiratoria grave y debilidad muscular generalizada, a la que se le diagnosticó miopatía nemalínica a través de la biopsia muscular. La paciente tuvo una notable disminución de la sialorrea y una mejora de los movimientos espontáneos después del tratamiento con L-tirosina. Este caso se presenta para destacar la importancia de la biopsia muscular en el diagnóstico diferencial de la hipotonía grave durante el período neonatal y el posible beneficio del aporte suplementario de L-tirosina para disminuir la sialorrea y restaurar la fuerza muscular.

L-tyrosine supplementation does not ameliorate skeletal muscle dysfunction in zebrafish and mouse models of dominant skeletal muscle α-actin nemaline myopathy.

L-tyrosine supplementation may provide benefit to nemaline myopathy (NM) patients, however previous studies are inconclusive, with no elevation of L-tyrosine levels in blood or tissue reported. We evaluated the ability of L-tyrosine treatments to improve skeletal muscle function in all three published animal models of NM caused by dominant skeletal muscle α-actin (ACTA1) mutations. Highest safe L-tyrosine concentrations were determined for dosing water and feed of wildtype zebrafish and mice respectively. NM TgACTA1D286G-eGFP zebrafish treated with 10 µM L-tyrosine from 24 hours to 6 days post fertilization displayed no improvement in swimming distance. NM TgACTA1D286G mice consuming 2% L-tyrosine supplemented feed from preconception had significant elevations in free L-tyrosine levels in sera (57%) and quadriceps muscle (45%) when examined at 6-7 weeks old. However indicators of skeletal muscle integrity (voluntary exercise, bodyweight, rotarod performance) were not improved. Additionally no benefit on the mechanical properties, energy metabolism, or atrophy of skeletal muscles of 6-7 month old TgACTA1D286G and KIActa1H40Y mice eventuated from consuming a 2% L-tyrosine supplemented diet for 4 weeks. Therefore this study yields important information on aspects of the clinical utility of L-tyrosine for ACTA1 NM.

Sporadic late-onset nemaline myopathy with monoclonal gammopathy of undetermined significance (SLONM-MGUS): An alternative treatment using cyclophosphamide-thalidomide-dexamethasone (CTD) regimen.

Sporadic late-onset nemaline myopathy with monoclonal gammopathy of undetermined significance is a rare subacute adult-onset myopathy. Without appropriate treatment, the prognosis is unfavorable and can be fatal. Various efficacious treatment options have been reported. High dose melphalan followed by autologous stem cell transplantation is the most used option with favorable outcome. Nevertheless, potentially safer alternative regimens await exploration. Here, we report the case of sporadic late-onset nemaline myopathy with monoclonal gammopathy of undetermined significance in a 33-year-old man with significant clinical improvement and complete remission of monoclonal gammopathy after 5 cycles of cyclophosphamide, thalidomide, and dexamethasone regimen. The regimen may be considered as an alternative option for patients with sporadic late-onset nemaline myopathy with monoclonal gammopathy of undetermined significance that are ineligible for upfront high-dose melphalan with autologous stem cell transplantation or that are being treated in resource-limited settings. Longer-term follow-up is needed to determine the long-term effectiveness of the cyclophosphamide, thalidomide, and dexamethasone regimen.

Sporadic late-onset nemaline myopathy: clinical, pathology and imaging findings in a single center cohort.

Sporadic late-onset nemaline myopathy (SLONM) is a rare acquired myopathy characterized by rapid-onset proximal weakness in late adulthood, and the presence of nemaline bodies on muscle biopsy. In recent years, several therapeutic interventions, including immunomodulating agents and autologous stem cell transplantation, have shown variable degrees of efficacy in different patients, but no consensus has been reached to allow an effective tailoring of treatments in this severe disease. We performed a retrospective evaluation of clinical, pathological, laboratory, muscle MRI, and follow-up data of SLONM patients diagnosed in the period 2010-2015 in our neuromuscular center. Six patients (three males and three females) were identified. Average time elapsed from the onset of symptoms to referral to the neuromuscular specialist was 23.7 months. Monoclonal gammopathy was detectable in five patients. Nemaline bodies were detected in all the patients, and their abundance correlated with clinical severity. Signs of cardiac involvement were present in all the patients to different extents. Muscle MRI showed a preferential involvement of neck extensors, paraspinal, gluteal, hamstring and soleus muscles. All patients were treated with prednisone and repeated courses of intravenous immunoglobulins, and a favorable outcome was reached in five patients. Our experience confirms that SLONM is clinically characterized by subacute proximal and axial muscle weakness. Time to referral was relatively long and should be reduced with increasing awareness of the disease. Muscle MRI could be of help as a diagnostic tool to identify this potentially treatable myopathy. Cardiac evaluation should be warranted in all SLONM patients to detect subclinical heart involvement.

Clinical, histological and radiological responses to methylprednisolone in HIV-associated rod myopathy.

Skeletal muscle involvement as a neurologic manifestation in individuals with HIV is rare, especially as rod myopathy. We describe a 41-year-old male with HIV infection who presented progressive proximal muscle weakness and limb-girdle atrophy. A muscle magnetic resonance image showed bilateral fatty infiltration and post-contrast enhancement in the arm and thigh muscles. The muscle biopsy revealed intracytoplasmic aggregates with appearance of nemaline rod bodies with Gomori trichrome staining and electron microscopy in most fibers. The patient underwent six cycles of intravenous methylprednisolone pulses, presenting clinical improvement. Post-treatment muscle biopsy showed fewer nemaline bodies and muscle magnetic resonance image depicted a pronounced reduction of muscular edema. These findings corroborate that deposition of nemaline bodies in these patients might be related to an immune response triggered by the virus.

KLHL40-related nemaline myopathy with a sustained, positive response to treatment with acetylcholinesterase inhibitors.

Congenital myopathies are a group of inherited muscle disorders characterized by hypotonia, weakness and a non-dystrophic muscle biopsy with the presence of one or more characteristic histological features. Neuromuscular transmission defects have recently been reported in several patients with congenital myopathies (CM). Mutations in KLHL40 are among the most common causes of severe forms of nemaline myopathy. Clinical features of affected individuals include fetal akinesia or hypokinesia, respiratory failure, and swallowing difficulties at birth. Muscle weakness is usually severe and nearly half of the individuals have no spontaneous antigravity movement. The average age of death has been reported to be 5 months in a recent case series. Herein we present a case of a patient with a nemaline myopathy due to KLHL40 mutations (c.604delG, p.Ala202Argfs*56 and c.1513G>C, p.Ala505Pro) with an impressive and prolonged beneficial response to treatment with high-dose pyridostigmine. Myasthenic features or response to ACEI have not previously been reported as a characteristic of nemaline myopathy or KLHL40-related myopathy.