Vacuoles, Often Containing Glycogen, Are a Consistent Finding in Hypokalemic Periodic Paralysis

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.

Autophagy is affected in patients with hypokalemic periodic paralysis: an involvement in vacuolar myopathy?

Hypokalemic periodic paralysis is an autosomal dominant, rare disorder caused by variants in the genes for voltage-gated calcium channel Ca_V1.1 (CACNA1S) and Na_V1.4 (SCN4A). Patients with hypokalemic periodic paralysis may suffer from periodic paralysis alone, periodic paralysis co-existing with permanent weakness or permanent weakness alone. Hypokalemic periodic paralysis has been known to be associated with vacuolar myopathy for decades, and that vacuoles are a universal feature regardless of phenotype. Hence, we wanted to investigate the nature and cause of the vacuoles. Fourteen patients with the p.R528H variation in the CACNA1S gene was included in the study. Histology, immunohistochemistry and transmission electron microscopy was used to assess general histopathology, ultrastructure and pattern of expression of proteins related to muscle fibres and autophagy. Western blotting and real-time PCR was used to determine the expression levels of proteins and mRNA of the proteins investigated in immunohistochemistry. Histology and transmission electron microscopy revealed heterogenous vacuoles containing glycogen, fibrils and autophagosomes. Immunohistochemistry demonstrated autophagosomes and endosomes arrested at the pre-lysosome fusion stage. Expression analysis showed a significant decrease in levels of proteins an mRNA involved in autophagy in patients, suggesting a systemic effect. However, activation level of the master regulator of autophagy gene transcription, TFEB, did not differ between patients and controls, suggesting competing control over autophagy gene transcription by nutritional status and calcium concentration, both controlling TFEB activity. The findings suggest that patients with hypokalemic periodic paralysis have disrupted autophagic processing that contribute to the vacuoles seen in these patients.