NEM6, KBTBD13-Related Congenital Myopathy: Myopathological Analysis in 18 Dutch Patients Reveals Ring Rods Fibers, Cores, Nuclear Clumps, and Granulo-Filamentous Protein Material

Skeletal muscle immunohistochemistry of acquired and hereditary myopathies

PURPOSE OF REVIEW

The continued development in the field of immunohistochemistry (IHC) has improved the ability to diagnose muscle diseases. Many hereditary diseases are diagnosed by the absence or abnormal localization of proteins. Detection of secondary pathological protein expression is also used in diagnostics, and to study disease processes. We relate and discuss recent reports, where IHC has been an important tool in the investigation of muscle diseases.

RECENT FINDINGS

In idiopathic inflammatory myopathies, IHC has extended its role to diagnose subgroups. This is most evident concerning immune-mediated necrotizing myopathy and antisynthetase syndrome. The availability of new antibodies has increased the sensitivity of a muscle biopsy to diagnose several hereditary myopathies. The introduction of protein restoration therapies in muscular dystrophies also comes with the need to detect and measure protein levels. For the study of disease processes at the protein level, in both acquired and hereditary myopathies IHC, often combined with gene studies, PCR-based methods, western blotting and electron microscopy, continues to bring forth interesting results.

SUMMARY

IHC is an integrated tool in muscle pathology, where recent studies contribute to improved diagnostic skills and increased insights into disease processes.

A review of core myopathy: central core disease, multiminicore disease, dusty core disease, and core-rod myopathy

Core myopathies are clinically, pathologically, and genetically heterogeneous muscle diseases. Their onset and clinical severity are variable. Core myopathies are diagnosed by muscle biopsy showing focally reduced oxidative enzyme activity and can be pathologically divided into central core disease, multiminicore disease, dusty core disease, and core-rod myopathy. Although RYR1-related myopathy is the most common core myopathy, an increasing number of other causative genes have been reported, including SELENON, MYH2, MYH7, TTN, CCDC78, UNC45B, ACTN2, MEGF10, CFL2, KBTBD13, and TRIP4. Furthermore, the genes originally reported to cause nemaline myopathy, namely ACTA1, NEB, and TNNT1, have been recently associated with core-rod myopathy. Genetic analysis allows us to diagnose each core myopathy more accurately. In this review, we aim to provide up-to-date information about core myopathies.

Recent advances in nemaline myopathy

The nemaline myopathies constitute a large proportion of the congenital or structural myopathies. Common to all patients is muscle weakness and the presence in the muscle biopsy of nemaline rods. The causative genes are at least twelve, encoding structural or regulatory proteins of the thin filament, and the clinical picture as well as the histological appearance on muscle biopsy vary widely. Here, we suggest a renewed clinical classification to replace the original one, summarise what is known about the pathogenesis from mutations in each causative gene to the forms of nemaline myopathy described to date, and provide perspectives on pathogenetic mechanisms possibly open to therapeutic modalities.