Milone, M

ABSTRACT Background Agrin‐congenital myasthenic syndrome ( AGRN ‐CMS) is a rare, heterogeneous genetic disorder of the neuromuscular transmission that can present from infancy to adulthood. The clinical phenotype includes distal weakness mimicking distal myopathies. Additionally, electromyography and muscle biopsy may demonstrate myopathic features, increasing the diagnostic challenge. Mitochondrial dysfunction has not been reported in the muscle of AGRN ‐CMS patients. Methods We report the clinical, electrophysiological, radiological, myopathological, and genetic findings of a patient with AGRN‐ CMS. Results A 47‐year‐old male presented at age 18 with sudden onset of eyelid ptosis, weakness, and fatigue, followed by dysphagia, dyspnea on exertion, exercise intolerance, and myalgias. Three sisters had similar symptoms. Neurological examination showed lower limb weakness, predominantly affecting the calf muscles with associated atrophy. Tendon reflexes were normal but absent at the ankles. Creatine kinase levels were normal. Electromyography demonstrated myopathic changes in distal muscles with fibrillation potentials in the gastrocnemius. 2 Hz‐Repetitive nerve stimulation of the fibular nerve revealed a 32% decrement in the extensor digitorum brevis without facilitation. Tibialis anterior biopsy showed non‐specific myopathic changes, neurogenic features, and histological evidence of mitochondrial dysfunction, as suggested by the scattered cytochrome c oxidase‐negative fibers. Whole exome sequencing detected a homozygous pathogenic AGRN variant (c.5012G>A, p.Arg1671Gln). No pathogenic or suspected pathogenic variants were detected in muscle mitochondrial DNA or in nuclear genes affecting mitochondrial function. Conclusions This patient expands the pathological spectrum of AGRN ‐CMS to include mitochondrial dysfunction and highlights the importance of low‐frequency repetitive nerve stimulation in the assessment of patients with distal weakness for differentiating myopathies from CMS.

ABSTRACT Autoantibody internalization has been implicated in autoimmune disease pathogenesis, yet its mechanisms, and generality across different diseases, cell types, and affected tissues remain poorly defined. Using bulk RNA sequencing, we identified reproducible, autoantibody-specific transcriptomic signatures consistent with autoantigen dysfunction in muscle biopsies from patients with anti-Mi2 dermatomyositis and anti-PM/Scl scleromyositis across independent cohorts. Electroporation of purified patient IgG into primary cultures of healthy cells was sufficient to induce the corresponding transcriptomic programs in vitro . Direct immunofluorescence demonstrated immunoglobulin internalization into subcellular compartments matching the localization of the autoantigen in different affected tissues. Spatial transcriptomic analyses revealed that antibody-secreting cells translocated cytoplasmic material (i.e., immunoglobulin RNA) into adjacent affected cells expressing autoantibody-specific transcripts. The disease-specific transcripts were present not only in muscle fibers, but also in other cells, including macrophages, endothelial cells, and fibroblasts. Autoantibody-induced transcriptomic programs were associated with cell damage and autoantibody-specific reactive inflammatory programs, including activation of type I interferon and TGF-ß1 signaling in anti-Mi2 dermatomyositis and activation of type II interferon in anti-PM/Scl scleromyositis. Antibody internalization was also observed in different tissues from patients with other autoimmune diseases, including anti-U1RNP mixed connective tissue disease, anti-Ku overlap syndrome, and anti-Scl70 systemic sclerosis. Together, these findings establish autoantibody internalization as a shared pathogenic mechanism across diverse autoimmune diseases, providing a unifying framework for conditions driven by autoantibodies against intracellular antigens.

ABSTRACT Objectives T‐cell large granular lymphocytic leukemia (T‐LGLL), reported in up to 58% of inclusion body myositis (IBM) patients, is a rare leukemia of cytotoxic or less commonly helper T cells. The range of myopathies in T‐LGLL and the impact of coexisting T‐LGLL in IBM are not well understood. Our objectives are to investigate the spectrum of myopathies in patients with T‐LGLL and compare the clinical, serological, pathological, and transcriptomic features of IBM patients with and without T‐LGLL. Methods We reviewed two Mayo Clinic cohorts: (1) T‐LGLL patients evaluated for myopathies (2003–2018), and (2) IBM patients tested for T‐LGLL via T‐cell receptor gene rearrangement or flow cytometry (2016–2022). We also compared transcriptomic profiles of IBM muscle biopsies with and without T‐LGLL. Results Of 447 T‐LGLL patients, 13 (2.9%) had myopathies, IBM being the most common ( n = 7). Of 43 IBM patients, 9 (20.9%) had T‐LGLL, with 5 diagnosed prior to the onset of weakness. Clinical and pathological features were largely similar between IBM patients with and without T‐LGLL, except for milder finger flexor weakness and more frequent neutropenia (55.6% vs. 0%, p < 0.001) in the T‐LGLL group. However, transcriptomic analysis of muscle tissues revealed a more immunologically active profile, with upregulation of T‐cell and macrophage markers, elevated levels of IFN‐γ gene and IFN‐γ–inducible genes, heightened cytokine activity, and enhanced immunoglobulin production in IBM patients with T‐LGLL. Interpretation IBM is the most common myopathy in T‐LGLL patients. While clinico‐sero‐pathological features were largely similar, transcriptomic differences suggest IBM with T‐LGLL may represent a distinct, more immune‐driven subtype.

The heat shock protein B8 (HSPB8) is one of the small heat shock proteins (sHSP or HSPB) and is a ubiquitous protein in various organisms, including humans. It is highly expressed in skeletal muscle, heart, and neurons. It plays a crucial role in identifying misfolding proteins and participating in chaperone-assisted selective autophagy (CASA) for the removal of misfolded and damaged, potentially cytotoxic proteins. Mutations in HSPB8 can cause distal hereditary motor neuropathy (dHMN), Charcot–Marie–Tooth (CMT) disease type 2L, or myopathy. The disease can manifest from childhood to mid-adulthood. Most missense mutations in the N-terminal and α-crystallin domains of HSPB8 lead to dHMN or CMT2L. Frameshift mutations in the C-terminal domain (CTD), resulting in elongation of the HSPB8 C-terminal, cause myopathy with myofibrillar pathology and rimmed vacuoles. Myopathy and motor neuropathy can coexist. HSPB8 frameshift mutations in the CTD result in HSPB8 mutant aggregation, which weakens the CASA ability to direct misfolded proteins to autophagic degradation. Cellular and animal models indicate that HSPB8 mutations drive pathogenesis through a toxic gain-of-function mechanism. Currently, no cure is available for HSPB8-associated neuromuscular disorders, but numerous therapeutic strategies are under investigation spanning from small molecules to RNA interference to exogenous HSPB8 delivery.

AbstractValosin‐containing protein (VCP)‐associated multisystem proteinopathy (MSP) is a rare genetic disorder with abnormalities in the autophagy pathway leading to various combinations of myopathy, bone diseases, and neurodegeneration. Ninety percent of patients with VCP‐associated MSP have myopathy, but there is no consensus‐based guideline. The goal of this working group was to develop a best practice set of provisional recommendations for VCP myopathy which can be easily implemented across the globe. As an initiative by Cure VCP Disease Inc., a patient advocacy organization, an online survey was initially conducted to identify the practice gaps in VCP myopathy. All prior published literature on VCP myopathy was reviewed to better understand the different aspects of management of VCP myopathy, and several working group sessions were conducted involving international experts to develop this provisional recommendation. VCP myopathy has a heterogeneous clinical phenotype and should be considered in patients with limb‐girdle muscular dystrophy phenotype, or any myopathy with an autosomal dominant pattern of inheritance. Genetic testing is the only definitive way to diagnose VCP myopathy, and single‐variant testing in the case of a known familial VCP variant, or multi‐gene panel sequencing in undifferentiated cases can be considered. Muscle biopsy is important in cases of diagnostic uncertainty or lack of a definitive pathogenic genetic variant since rimmed vacuoles (present in ~40% cases) are considered a hallmark of VCP myopathy. Electrodiagnostic studies and magnetic resonance imaging can also help rule out disease mimics. Standardized management of VCP myopathy will optimize patient care and help future research initiatives.

GDP-mannose pyrophosphorylase B (GMPPB) is a cytoplasmic protein that catalyzes the formation of GDP-mannose. Impaired GMPPB function reduces the amount of GDP-mannose available for the O-mannosylation of α-dystroglycan (α-DG) and ultimately leads to disruptions of the link between α-DG and extracellular proteins, hence dystroglycanopathy. GMPPB-related disorders are inherited in an autosomal recessive manner and caused by mutations in either a homozygous or compound heterozygous state. The clinical spectrum of GMPPB-related disorders spans from severe congenital muscular dystrophy (CMD) with brain and eye abnormalities to mild forms of limb-girdle muscular dystrophy (LGMD) to recurrent rhabdomyolysis without overt muscle weakness. GMPPB mutations can also lead to the defect of neuromuscular transmission and congenital myasthenic syndrome due to altered glycosylation of the acetylcholine receptor subunits and other synaptic proteins. Such impairment of neuromuscular transmission is a unique feature of GMPPB-related disorders among dystroglycanopathies. LGMD is the most common phenotypic presentation, characterized by predominant proximal weakness involving lower more than upper limbs. Facial, ocular, bulbar, and respiratory muscles are largely spared. Some patients demonstrate fluctuating fatigable weakness suggesting neuromuscular junction involvement. Patients with CMD phenotype often also have structural brain defects, intellectual disability, epilepsy, and ophthalmic abnormalities. Creatine kinase levels are typically elevated, ranging from 2 to >50 times the upper limit of normal. Involvement of the neuromuscular junction is demonstrated by the decrement in the compound muscle action potential amplitude on low-frequency (2–3 Hz) repetitive nerve stimulation in proximal muscles but not in facial muscles. Muscle biopsies typically show myopathic changes with variable degrees of reduced α-DG expression. Higher mobility of β-DG on Western blotting represents a specific feature of GMPPB-related disorders, distinguishing it from other α-dystroglycanopathies. Patients with clinical and electrophysiologic features of neuromuscular transmission defect can respond to acetylcholinesterase inhibitors alone or combined with 3,4 diaminopyridine or salbutamol.