Measurement of Myonuclear Accretion In Vitro and In Vivo.

Adult skeletal muscle stem cells (MuSC) are the regenerative precursors of myofibers and also have an important role in myofiber growth, adaptation, and maintenance by fusing to the myofibers-a process referred to as "myonuclear accretion." Due to a focus on MuSC function during regeneration, myofibers remain a largely overlooked component of the MuSC niche influencing MuSC fate. Here, we describe a method to directly measure the rate of myonuclear accretion in vitro and in vivo using ethynyl-2'-deoxyuridine (EdU)-based tracing of MuSC progeny. This method supports the dissection of MuSC intrinsic and myofiber-derived factors influencing myonuclear accretion as an alternative fate of MuSCs supporting myofiber homeostasis and plasticity.

Inaugural dropped head syndrome and camptocormia in inflammatory myopathies: a retrospective study.

OBJECTIVES: Inaugural axial muscle involvement, defined as dropped head syndrome (DHS) and/or camptocormia (CC), is poorly described in inflammatory myopathies (IM). This study aimed to further characterize IM patients with inaugural DHS/CC, their outcome and care management. METHODS: This retrospective study included IM patients diagnosed between 2000 and 2021. The main inclusion criterion was IM revealed by axial muscle deficit (DHS/CC). RESULTS: Twenty-seven patients were included; median (IQR) age at first symptoms was 66.0 years (55.5-75.0); 21 were female (77.8%). There were nine IBM, 33.3%, nine overlap myositis (OM, 33.3%), five DM, 18.5%, two immune checkpoint inhibitor-related myositis (7.4%), one focal myositis (3.7%) and one myositis with anti-Hu antibodies (3.7%). Age at first symptoms was ≤70 years in 16 patients (59.3%), including all DM patients and 8/9 OM patients (88.9%). In this group, partial remission of the disease was obtained in 9/16 (56.3%) and complete remission in 1/16 patients (6.3%); regression of DHS/CC was achieved in 3/16 patients (18.8%). Conversely, in the group of 11 patients aged >70 years at first symptoms, there were eight IBM (72.7%). Partial remission was obtained in 5/11 patients (45.5%), the disease was stable in 6/11 patients (54.5%); no complete remission was obtained nor regression of DHS/CC. CONCLUSION: The analysis of IM patients with inaugural DHS/CC delineates two groups of patients according to the age at first symptoms in terms of clinical and outcome specificities, and proposes an adapted diagnostic and care management approach to prevent long-term complications.

The new missense G376V-TDP-43 variant induces late-onset distal myopathy but not amyotrophic lateral sclerosis.

TDP-43-positive inclusions in neurons are a hallmark of several neurodegenerative diseases including familial amyotrophic lateral sclerosis (fALS) caused by pathogenic TARDBP variants as well as more common non-Mendelian sporadic ALS (sALS). Here we report a G376V-TDP-43 missense variant in the C-terminal prion-like domain of the protein in two French families affected by an autosomal dominant myopathy but not fulfilling diagnostic criteria for ALS. Patients from both families presented with progressive weakness and atrophy of distal muscles, starting in their 5th-7th decade. Muscle biopsies revealed a degenerative myopathy characterized by accumulation of rimmed (autophagic) vacuoles, disruption of sarcomere integrity and severe myofibrillar disorganization. The G376 V variant altered a highly conserved amino acid residue and was absent in databases on human genome variation. Variant pathogenicity was supported by in silico analyses and functional studies. The G376 V mutant increased the formation of cytoplasmic TDP-43 condensates in cell culture models, promoted assembly into high molecular weight oligomers and aggregates in vitro, and altered morphology of TDP-43 condensates arising from phase separation. Moreover, the variant led to the formation of cytoplasmic TDP-43 condensates in patient-derived myoblasts and induced abnormal mRNA splicing in patient muscle tissue. The identification of individuals with TDP-43-related myopathy but not ALS implies that TARDBP missense variants may have more pleiotropic effects than previously anticipated and support a primary role for TDP-43 in skeletal muscle pathophysiology. We propose to include TARDBP screening in the genetic work-up of patients with late-onset distal myopathy. Further research is warranted to examine the precise pathogenic mechanisms of TARDBP variants causing either a neurodegenerative or myopathic phenotype.

Autosomal recessive pathogenic MSTO1 variants in hereditary optic atrophy.

Gerber et al report 2 autosomal recessive pathogenic Misato homolog 1 (MSTO1) variants causing hereditary optic atrophy and raise concerns about a previously identified dominant variant of MSTO1 by Gal et al (2017).

Hypersensitivity infusion-associated reactions induced by enzyme replacement therapy in a cohort of patients with late-onset Pompe disease: An experience from the French Pompe Registry.

BACKGROUND AND OBJECTIVES: Pompe disease is a rare hereditary glycogen storage disorder due to lysosomal acid alpha-glucosidase deficiency. Enzyme replacement therapy (ERT) is the only available treatment. Infusion-associated reactions (IAR) are challenging since there are no guidelines for ERT rechallenge after a drug hypersensitivity reaction (DHR) in Pompe disease. The objective of the present study was to describe IAR and their management in late-onset Pompe disease (LOPD) patients in France, and to discuss the various possibilities of ERT rechallenge. METHODS: An exhaustive screening of LOPD patients receiving ERT between 2006 and 2020 from the 31-participating hospital-based or reference centers was performed. The patients who had presented at least one hypersensitivity IAR (=DHR) episode were included. Demographic characteristics of the patients, IAR onset and timing, were retrospectively collected from the French Pompe Registry. RESULTS: Fifteen patients among 115 treated LOPD patients in France presented at least 1 IAR; 80.0% were women. Twenty-nine IAR were reported; 18 (62.1%) IAR were Grade I reactions, 10 (34.5%) IAR were Grade II, and 1 (3.4%) IAR was Grade III. IgE-mediated hypersensitivity was found in 2/15 patients (13.3%). The median [IQR] time from ERT introduction to the first IAR was 15.0 months [11.0-24.0]. ERT was safely and effectively re-introduced either with premedication alone, or in combination with either modified regimen or desensitization protocol, in all 9 rechallenged patients; including in patients with IgE-mediated hypersensitivity, in the patient with the Grade III reaction, as well as in patients with very high anti-GAA titer. DISCUSSION: Based on the results herein and previous reports, we discuss premedication and modified regimen for Grade I reactions, and desensitization in Grade II and III reactions. In conclusion, ERT-induced IAR can be safely and effectively managed with a modified regimen or desensitization protocol in LOPD patients.

Occurrence of focal myositis during Behçet's disease: The identification of a specific vasculitis-associated focal myopathy.

AIMS: This study aimed to report the association of focal myositis (FM) and Behçet's disease (BD) and to analyse the main characteristics of such an association. METHODS: This is a retrospective multicentre study of patients with BD and FM (BD + FM+ group) and those without FM (BD - FM+ group). Clinical, laboratory, radiological, pathological, treatment and outcome data were analysed. RESULTS: The BD + FM+ group included 10 patients; the median [interquartile range] age at BD diagnosis was 25 [16-35] years, and at FM diagnosis, it was 30 [26-42] years. The diagnosis of BD preceded FM in the majority of cases (n = 8/10). FM occurrence was associated with BD flare-ups in three cases. The creatine kinase levels remained normal or slightly increased. Histological analyses identified relatively preserved muscle tissue, associated with vasculitis (n = 5/6). All patients required treatment; most patients relapsed (n = 9/10). The BD - FM+ group included 35 patients. A comparison of the groups identified a trend towards a younger median age at diagnosis of FM among those with BD (p = 0.063) and more frequent focal muscle swelling in the BD + FM+ group (p = 0.029). The pathological analysis identified significantly less frequent muscle alterations in the BD + FM+ group (muscle fibre size heterogeneity, p = 0.021; necrosis, p = 0.007; and fibrosis, p = 0.027). BD + FM+ patients had a higher frequency of relapse (p = 0.003) and systematic treatment (p = 0.042). CONCLUSIONS: FM occurring during BD appears to be part of the systemic vasculitis process and presents as a vasculitis-associated focal myopathy with a specific clinico-histological pattern. Patients with this association require long-term follow-up and adapted management. This case series also highlights the need for research on BD diagnostic criteria in cases of FM.

Revisiting idiopathic eosinophilic myositis: towards a clinical-pathological continuum from the muscle to the fascia and skin.

OBJECTIVES: Idiopathic inflammatory myopathies are mainly defined by inflammatory infiltrates within the muscle (lymphocytes and macrophages). Eosinophil muscle infiltration has been described in idiopathic eosinophilic myositis (IEM) and rarely in EF. This study aimed to further delineate the nosological frame of idiopathic eosinophil muscle infiltration through the exhaustive analysis of IEM and EF patients. METHODS: This multicentre retrospective case series included IEM patients diagnosed between 2000 and 2022. IEM inclusion criteria were eosinophilic muscle infiltration with myositis pathological features, after the exclusion of differential diagnoses. An additional group of EF patients diagnosed between 2016 and 2022 was constituted. Inclusion criteria were an EF diagnosis and fascia thickening with inflammatory infiltrate. RESULTS: A total of 20 IEM cases and 10 EF cases were included. The median (interquartile range) age at diagnosis was 65 (49-70) years; there were 18 males. Data analysis delineated four subgroups: focal EM (FEM, n = 3), diffuse EM (DEM, n = 6), eosinophilic myofasciitis (EMF, n = 11) and EF (n = 10). FEM represented a limited and benign form of myositis. DEM cases presented objective muscle impairment with eosinophilic muscle infiltration. EMF patients presented subjective muscle impairment (myalgia, 55%), fasciitis (on histology and/or imaging), eosinophilic muscle infiltration and frequent hypereosinophilia (55%). EF patients presented myalgia (50%), muscle lesions on histology with fascia-restricted inflammatory infiltrates with (60%) or without (40%) eosinophils. CONCLUSIONS: The analysis of IEM and EF patient characteristics delineates four subgroups (FEM, DEM, EMF and EF) in terms of clinical, laboratory, imaging, pathological and outcome specificities, and proposes an adapted diagnostic and care management approach.

Involvement of Type I Interferon Signaling in Muscle Stem Cell Proliferation During Dermatomyositis.

BACKGROUND AND OBJECTIVES: The idiopathic inflammatory myopathy dermatomyositis is an acquired disease that involves muscle, lung, and skin impairments. Patients with dermatomyositis show a wide range of severity of proximal skeletal muscle weakness, associated with inflammatory infiltrates, vasculitis, capillary dropout, and perifascicular myofiber atrophy. Muscles of patients with dermatomyositis show signs of muscle regeneration. Because muscle stem cells (MuSCs) are responsible for myofiber repair, we wondered whether the proliferative properties of MuSCs are altered in dermatomyositis muscle. We investigated the role of type I interferon (IFN-I) in this process because dermatomyositis is associated with sustained inflammation with high IFN-I levels. METHODS: MuSCs isolated from normal muscles and those from adult and juvenile patients with dermatomyositis were grown in culture and analyzed in vitro for their proliferating properties, myogenic capacities, and senescence. Gain- and loss-of-function experiments were performed to assess the role of IFN-I signaling in the proliferative capacities of MuSCs. RESULTS: MuSCs derived from 8 adult patients with dermatomyositis (DM-MuSCs) (5 severe form and 3 mild form, established from histologic evaluation), from 3 patients with juvenile dermatomyositis, and from normal muscle were used to analyze their myogenesis in vitro. DM-MuSCs exhibited strongly reduced proliferating capacities as compared with healthy MuSCs (-31% to -43% for mild and severe dermatomyositis, respectively), leading to poor myotube formation (-36% to -71%). DM-MuSCs were enriched in senescent, ß-galactosidase-positive cells, partly explaining the proliferation defect. Gain- and loss-of-function experiments were performed to assess the role of IFN-I on the proliferative capacity of MuSCs. High concentrations of IFN-I decreased the proliferation of healthy MuSCs. Similarly, conditioned medium from DM-MuSCs decreased the proliferation of healthy MuSCs (-15% to -22%), suggesting the delivery of an autocrine effector. Pharmacologic blockade of IFN signaling (using ruxolitinib or anti-IFN receptor antibodies) in DM-MuSCs rescued their proliferation up to the control values. DISCUSSION: These results show that autocrine IFN-I signaling prevents MuSC expansion, leading to muscle repair deficit. This process may explain the persistent muscle weakness observed in patients with severe dermatomyositis.