Importance of muscle light microscopic mitochondrial subsarcolemmal aggregates in the diagnosis of respiratory chain deficiency

Identification of two novel RRM2B variants associated with autosomal recessive progressive external ophthalmoplegia in a family with pseudodominant inheritance pattern

RRM2B encodes the p53-inducible small subunit (p53R2) of ribonucleotide reductase, a key protein for mitochondrial DNA (mtDNA) synthesis. Pathogenic variants in this gene result in familial mitochondrial disease in adults and children, secondary to a maintenance disorder of mtDNA. This study describes two patients, mother and son, with early-onset chronic progressive external ophthalmoplegia (PEO). Skeletal muscle biopsy from the latter was examined: cytochrome c oxidase (COX)-negative fibres were shown, and molecular studies revealed multiple mtDNA deletions. A next-generation sequencing gene panel for nuclear-encoded mitochondrial maintenance genes identified two unreported heterozygous missense variants (c.514 G > A and c.682 G > A) in the clinically affected son. The clinically affected mother harboured the first variant in homozygous state, and the clinically unaffected father harboured the remaining variant in heterozygous state. In silico analyses predicted both variants as deleterious. Cell culture studies revealed that patients' skin fibroblasts, but not fibroblasts from healthy controls, responded to nucleoside supplementation with enhanced mtDNA repopulation, thus suggesting an in vitro functional difference in patients' cells. Our results support the pathogenicity of two novel RRM2B variants found in two patients with autosomal recessive PEO with multiple mtDNA deletions inherited with a pseudodominant pattern.

Succinate Dehydrogenase B (SDHB) Immunohistochemistry for the Evaluation of Muscle Biopsies

Succinate dehydrogenase (SDH) is a key mitochondrial enzyme complex composed of 4 subunits. SDH histochemistry is routinely utilized in the assessment of muscle biopsies to reveal underlying pathology such as subsarcolemmal mitochondrial aggregates. In this study, we evaluated the utility of succinate dehydrogenase B (SDHB) immunohistochemistry (IHC) in 27 muscle biopsies, including 13 mitochondrial myopathies (MMs), 9 inflammatory myopathies, and 5 controls. SDHB IHC was performed on formalin-fixed, paraffin-embedded tissue sections with a mouse monoclonal antibody (Abcam 21A11AE7) in parallel with histochemical SDH stains on a fresh-frozen tissue. In all muscle biopsies, SDHB IHC exhibited granular immunoreactivity and highlighted the dark type 1 and lighter type 2 staining pattern observed by histochemistry. In all cases of MM, SDHB IHC showed subsarcolemmal granular aggregates involving the entire periphery of the fibers that were more distinct than those seen by SDH histochemistry. In 3 extraocular muscle biopsies, SDHB immunoreactive speckles of various sizes were distributed throughout the entire sarcoplasm that were more prominent than those seen on SDH histochemistry. Subsarcolemmal and cytoplasmic granular aggregates seen on SDHB IHC correlated with mitochondrial pathology on electron microscopy. In cases of inflammatory myopathy, there was diffuse sarcoplasmic SDHB immunoreactivity in degenerating fibers, but no evidence of subsarcolemmal aggregates. This study demonstrates that SDHB IHC is highly sensitive and specific in the identification of MM. The automation, reproducibility, and cost efficiency of SDHB IHC offer advantages over the labor-intensive histochemical method requiring frozen sections. As this technique is performed on formalin-fixed, paraffin-embedded tissues, it can be easily applied for retrospective studies.

Myopathology of Adult and Paediatric Mitochondrial Diseases

Mitochondria are dynamic organelles ubiquitously present in nucleated eukaryotic cells, subserving multiple metabolic functions, including cellular ATP generation by oxidative phosphorylation (OXPHOS). The OXPHOS machinery comprises five transmembrane respiratory chain enzyme complexes (RC). Defective OXPHOS gives rise to mitochondrial diseases (mtD). The incredible phenotypic and genetic diversity of mtD can be attributed at least in part to the RC dual genetic control (nuclear DNA (nDNA) and mitochondrial DNA (mtDNA)) and the complex interaction between the two genomes. Despite the increasing use of next-generation-sequencing (NGS) and various omics platforms in unravelling novel mtD genes and pathomechanisms, current clinical practice for investigating mtD essentially involves a multipronged approach including clinical assessment, metabolic screening, imaging, pathological, biochemical and functional testing to guide molecular genetic analysis. This review addresses the broad muscle pathology landscape including genotype–phenotype correlations in adult and paediatric mtD, the role of immunodiagnostics in understanding some of the pathomechanisms underpinning the canonical features of mtD, and recent diagnostic advances in the field.