Sporadic centronuclear myopathy with muscle pseudohypertrophy, neutropenia, and necklace fibres due to a DNM2 mutation

Lymphocyte cytosolic protein 1 (L-plastin) I232F mutation impairs granulocytic proliferation and causes neutropenia

Neutrophils migrate into inflamed tissue, engage in phagocytosis, and clear pathogens or apoptotic cells. These processes require well-coordinated events involving the actin cytoskeleton. We describe a child with severe neutropenia and episodes of soft tissue infections and pneumonia. Bone marrow examination showed granulocytic hypoplasia with dysplasia. Whole-exome sequencing revealed a de novo heterozygous missense mutation in LCP1, which encodes the F-actin-binding protein Lymphocyte Cytosolic Protein 1. To determine its pathophysiological significance, we stably transduced cells with doxycycline-inducible wild-type LCP1 and LCP1 I232F lentiviral constructs. We observed dysplastic granulocytic 32D cells expressing LCP1 I232F cells. These cells showed decreased proliferation without a block in differentiation. In addition, expression of LCP1 I232F resulted in a cell cycle arrest at the G2/M phase, but it did not lead to increased levels of genes involved in apoptosis or the unfolded protein response. Both 32D and HeLa cells expressing mutant LCP1 displayed impaired cell motility and invasiveness. Flow cytometry showed increased F-actin. However, mutant LCP1-expressing 32D cells exhibited normal oxidative burst upon stimulation. Confocal imaging and subcellular fractionation revealed diffuse intracellular localization of LCP1, but only the mutant form was found in the nucleus. We conclude that LCP1 is a new gene involved in granulopoiesis, and the missense variant LCP1 I232F leads to neutropenia and granulocytic dysplasia with aberrant actin dynamics. Our work supports a model of neutropenia due to aberrant actin regulation.

Update on Congenital Myopathies in Adulthood

Congenital myopathies (CMs) constitute a group of heterogenous rare inherited muscle diseases with different incidences. They are traditionally grouped based on characteristic histopathological findings revealed on muscle biopsy. In recent decades, the ever-increasing application of modern genetic technologies has not just improved our understanding of their pathophysiology, but also expanded their phenotypic spectrum and contributed to a more genetically based approach for their classification. Later onset forms of CMs are increasingly recognised. They are often considered milder with slower progression, variable clinical presentations and different modes of inheritance. We reviewed the key features and genetic basis of late onset CMs with a special emphasis on those forms that may first manifest in adulthood.

New variant of necklace fibres display peculiar lysosomal structures and mitophagy

Here, we describe a new variant of necklace fibres with specific myopathological features that have not been described thus far. They were observed in two patients, from two independent families with identical DNM2 (dynamin 2) mutation (c.1106 G > A (p.Arg369Gln)), displaying mildly heterogeneous clinical phenotypes. The variant is characterized by lysosomal inclusions, arranged in a necklace pattern, containing homogenous material, devoid of myonuclei. The so-called necklace region has a certain characteristic distance to the sarcolemma. Electron microscopy, including three dimensional reconstructions of serial section images highlights their ultrastructural properties and relation to neighbouring organelles. This new pattern is compared to the previously reported patterns in muscle biopsies containing necklace fibres associated with MTM1- and DNM2-mutations.

An Overview of Congenital Myopathies

PURPOSE OF REVIEW

This article uses a case-based approach to highlight the clinical features as well as recent advances in molecular genetics, muscle imaging, and pathophysiology of the congenital myopathies.

RECENT FINDINGS

Congenital myopathies refer to a heterogeneous group of genetic neuromuscular disorders characterized by early-onset muscle weakness, hypotonia, and developmental delay. Congenital myopathies are further classified into core myopathies, centronuclear myopathies, nemaline myopathies, and congenital fiber-type disproportion based on the key pathologic features found in muscle biopsies. Genotype and phenotype correlations are hampered by the diverse clinical variability of the genes responsible for congenital myopathies, ranging from a severe neonatal course with early death to mildly affected adults with late-onset disease. An increasing number of genes have been identified, which, in turn, are associated with overlapping morphologic changes in the myofibers. Precise genetic diagnosis has important implications for disease management, including family counseling; avoidance of anesthetic-related muscle injury for at-risk individuals; monitoring for potential cardiac, respiratory, or orthopedic complications; as well as for participation in clinical trials or potential genetic therapies.

SUMMARY

Collaboration with neuromuscular experts, geneticists, neuroradiologists, neuropathologists, and other specialists is needed to ensure accurate and timely diagnosis based on clinical and pathologic features. An integrated multidisciplinary model of care based on expert-guided standards will improve quality of care and optimize outcomes for patients and families with congenital myopathies.

Congenital myopathies: not only a paediatric topic

PURPOSE OF REVIEW

This article reviews adult presentations of the major congenital myopathies - central core disease, multiminicore disease, centronuclear myopathy and nemaline myopathy - with an emphasis on common genetic backgrounds, typical clinicopathological features and differential diagnosis.

RECENT FINDINGS

The congenital myopathies are a genetically heterogeneous group of conditions with characteristic histopathological features. Although essentially considered paediatric conditions, some forms - in particular those due to dominant mutations in the skeletal muscle ryanodine receptor (RYR1), the dynamin 2 (DNM2), the amphiphysin 2 (BIN1) and the Kelch repeat-and BTB/POZ domain-containing protein 13 (KBTBD13) gene - may present late into adulthood. Moreover, dominant RYR1 mutations associated with the malignant hyperthermia susceptibility trait have been recently identified as a common cause of (exertional) rhabdomyolysis presenting throughout life. In addition, improved standards of care and development of new therapies will result in an increasing number of patients with early-onset presentations transitioning to the adult neuromuscular clinic. Lastly, if nemaline rods are the predominant histopathological feature, acquired treatable conditions have to be considered in the differential diagnosis.

SUMMARY

Recently identified genotypes and phenotypes indicate a spectrum of the congenital myopathies extending into late adulthood, with important implications for clinical practice.

Necklace fibers as histopathological marker in a patient with severe form of X-linked myotubular myopathy

X-linked myotubular myopathy due to mutations in the MTM1 gene is classically characterized by a severe neonatal phenotype and a typical muscle biopsy presenting globular and centrally located nuclei in muscle myofibers. Recently, four patients with mild late-onset form have been described, a male with a hemizygous mutation and three females with heterozygous mutations in the MTM1 gene. The muscle biopsies were performed at 13-35 years of age and a new histological marker, the necklace fibers, was described. Here, we report two siblings with the pathogenic c.664 C>T mutation in the MTM1 gene, presenting a severe muscle weakness and respiratory impairment requiring ventilatory support since the first months of life until death, at the age of 36 months and 5 months. In the older brother the muscle biopsy, performed at the age of 30 months, showed almost 100% of necklace fibers, which were not present in the younger one submitted to muscle biopsy at 5 months of age. Our findings confirm the necklace fibers can be a histopathological finding of MTM1 myopathies, even in the severe neonatal form, and suggest that the necklace fibers appear or increase in number over time.