Development of polyglucosan inclusions in skeletal muscle

Characterisation of phenotypic patterns in equine exercise-associated myopathies

BACKGROUND

Equine exercise-associated myopathies are prevalent, clinically heterogeneous, generally idiopathic disorders characterised by episodes of myofibre damage that occur in association with exercise. Episodes are intermittent and vary within and between affected horses and across breeds. The aetiopathogenesis is often unclear; there might be multiple causes. Poor phenotypic characterisation hinders genetic and other disease analyses.

OBJECTIVES

The aim of this study was to characterise phenotypic patterns across exercise-associated myopathies in horses.

STUDY DESIGN

Historical cross-sectional study, with subsequent masked case-control validation study.

METHODS

Historical clinical and histological features from muscle samples (n = 109) were used for k-means clustering and validated using principal components analysis and hierarchical clustering. For further validation, a blinded histological study (69 horses) was conducted comparing two phenotypic groups with selected controls and horses with histopathological features characterised by myofibrillar disruption.

RESULTS

We identified two distinct broad phenotypes: a non-classic exercise-associated myopathy syndrome (EAMS) subtype was associated with practitioner-described signs of apparent muscle pain (p < 0.001), reluctance to move (10.85, p = 0.001), abnormal gait (p < 0.001), ataxia (p = 0.001) and paresis (p = 0.001); while a non-specific classic RER subtype was not uniquely associated with any particular variables. No histological differences were identified between subtypes in the validation study, and no identifying histopathological features for other equine myopathies identified in either subtype.

MAIN LIMITATIONS

Lack of an independent validation population; small sample size of smaller identified subtypes; lack of positive control myofibrillar myopathy cases; case descriptions derived from multiple independent and unblinded practitioners.

CONCLUSIONS

This is the first study using computational clustering methods to identify phenotypic patterns in equine exercise-associated myopathies, and suggests that differences in patterns of presenting clinical signs support multiple disease subtypes, with EAMS a novel subtype not previously described. Routine muscle histopathology was not helpful in sub-categorising the phenotypes in our population.

Phenotypic and genotyping spectrum of two Iranian cases with RBCK1-associated polyglucosan body myopathy

Glycogen storage diseases (GSDs) are a group of metabolic disorders affecting glycogen metabolism, with polyglucosan body myopathy type 1 (PGBM1) being a rare variant linked to RBCK1 gene mutations. Understanding the clinical diversity of PGBM1 aids in better characterization of the disease. Two unrelated Iranian families with individuals exhibiting progressive muscle weakness underwent clinical evaluations, genetic analysis using whole exome sequencing (WES), and histopathological examinations of muscle biopsies. In one case, a novel homozygous RBCK1 variant was identified, presenting with isolated myopathy without cardiac or immune involvement. Conversely, the second case harbored a known homozygous RBCK1 variant, displaying a broader phenotype encompassing myopathy, cardiomyopathy, inflammation, and immunodeficiency. Histopathological analyses confirmed characteristic skeletal muscle abnormalities consistent with PGBM1. Our study contributes to the expanding understanding of RBCK1-related diseases, illustrating the spectrum of phenotypic variability associated with distinct RBCK1 variants. These findings underscore the importance of genotype-phenotype correlations in elucidating disease mechanisms and guiding clinical management. Furthermore, the utility of next-generation sequencing techniques in diagnosing complex neurogenetic disorders is emphasized, facilitating precise diagnosis and enabling tailored genetic counseling for affected individuals and their families.

Intrasarcoplasmic Polyglucosan Inclusions in Heart and Skeletal Muscles of Long-Finned Pilot Whales (Globicephala melas) may be Age-Related

Polysaccharide storage myopathies have been described in several animal species and are characterized by periodic acid-Schiff (PAS)-positive, diastase-resistant intrasarcoplasmic inclusions in myocytes. Skeletal and cardiac muscle samples from a subset of a single pod of stranded long-finned pilot whales (Globicephala melas) were evaluated by light and transmission electron microscopy. Twelve individuals demonstrated sporadic basophilic packets of PAS-positive, diastase-resistant complex polysaccharide material, either centrally or peripherally, in skeletal and cardiac myocytes. Few microscopic myopathic changes were found but included focal inflammation and internalized nuclei. Ultrastructurally, the inclusions consisted of loosely arranged, tangled filaments and were not membrane-bound, which is consistent with polyglucosan inclusions. Within skeletal muscle, the number of inclusions had a marginal statistically significant (P = 0.0536) correlation with length, as a proxy for age, suggesting that such inclusions in skeletal muscles may be age-related, although the cause remains unclear.

Vacuolar myopathy in an adult Warmblood horse

Histopathological interpretation of semimembranosus muscle samples from an adult Warmblood mare with clinical signs suggestive of exertional rhabdomyolysis and intermittent mild elevations in muscle enzyme activities revealed abundant sarcoplasmic vacuoles in all fibre-types containing fine, apparently proteinaceous debris. Vacuolar contents stained lightly with PAS, but did not appear to contain amylopectate, lipid or acid phosphatase and their periphery was unstained with dystrophin immunohistochemistry. Electron microscopy revealed that vacuoles were not membrane bound. No vacuoles were detected in muscle samples evaluated at post mortem following 4 months of rest. To our knowledge, this is the first report of a presumed primary vacuolar myopathy in a horse.

Intramuscular corpora amylacea adjacent to ileal low-grade neuroendocrine tumours (typical carcinoids): a light microscopic, immunohistochemical and ultrastructural study

AIMS

The purposes of this study are to (1) document the prevalence of intracytoplasmic inclusions adjacent to ileal well-differentiated neuroendocrine tumours (WNETs), (2) examine whether and how tumour and patient characteristics are associated with inclusions and (3) investigate their properties on special stains and electron microscopy in comparison with corpora amylacea (CA).

METHODS

We examined the resection slides from 26 ileal, 5 gastric and 5 rectal cases of WNET. Inclusions were readily identified with H&E staining. Histochemical, immunohistochemical and ultrastructural evaluations were performed on the block with the highest number of inclusions.

RESULTS

Intracytoplasmic inclusions occurred adjacent (<1 mm) to 15 of 26 (57.7%) ileal WNETs. Patients with and without inclusions were of similar mean ages (59.5 vs 57.4 years; p=0.88), but NETs with inclusions were larger than those without inclusions (3.3 vs 1.7 cm, p=0.03). Inclusions were neither associated with gastric (mean age=65 years, mean diameter=1.5 cm) or rectal WNETs (mean age=47.8 years, mean diameter=0.5 cm) (p=0.01), nor were they present >1 mm from ileal NETs. CA stained strongly for ubiquitin, DPAS and Alcian blue; faintly and peripherally for desmin and smooth muscle actin and negatively for calcium. Ultrastructurally, their appearance was consistent with filaments, some with cores of particle matter.

CONCLUSIONS

Our results suggest that these inclusions are virtually identical to CA and present adjacent to the majority of ileal WNET. They may be the result of a degenerative process, possibly due to chronic myocyte stress from an infiltrating slow growing tumour mass or local hormonal effects.

Allele copy number and underlying pathology are associated with subclinical severity in equine type 1 polysaccharide storage myopathy (PSSM1)

Equine type 1 polysaccharide storage myopathy (PSSM1), a common glycogenosis associated with an R309H founder mutation in the glycogen synthase 1 gene (GYS1), shares pathological features with several human myopathies. In common with related human disorders, the pathogenesis remains unclear in particular, the marked phenotypic variability between affected animals. Given that affected animals accumulate glycogen and alpha-crystalline polysaccharide within their muscles, it is possible that physical disruption associated with the presence of this material could exacerbate the phenotype. The aim of this study was to compare the histopathological changes in horses with PSSM1, and specifically, to investigate the hypothesis that the severity of underlying pathology, (e.g. vacuolation and inclusion formation) would (1) be higher in homozygotes than heterozygotes and (2) correlate with clinical severity. Resting and post-exercise plasma creatine kinase (CK) and aspartate aminotransferase (AST) enzyme activity measurements and muscle pathology were assessed in matched cohorts of PSSM1 homozygotes, heterozygotes or control horses. Median (interquartile range (IR)) resting CK activities were 364 (332–764) U/L for homozygotes, 301 (222–377) U/L for heterozygotes and 260 (216–320) U/L for controls, and mean (+/− SD) AST activity for homozygotes were 502 (+/116) U/L, for heterozygotes, 357 (+/−92) U/L and for controls, 311 (+/−64) U/L and were significantly different between groups (P = 0.04 and P = 0.01 respectively). Resting plasma AST activity was significantly associated with the severity of subsarcolemmal vacuolation (rho = 0.816; P = 0.01) and cytoplasmic inclusions (rho = 0.766; P = 0.01). There were fewer type 2× and more type 2a muscle fibres in PSSM1-affected horses. Our results indicate that PSSM1 has incomplete dominance. Furthermore, the association between plasma muscle enzyme activity and severity of underlying pathology suggests that physical disruption of myofibres may contribute to the myopathic phenotype. This work provides insight into PSSM1 pathogenesis and has implications for related human glycogenoses.

Comparative Skeletal Muscle Histopathologic and Ultrastructural Features in Two Forms of Polysaccharide Storage Myopathy in Horses

Polysaccharide storage myopathy (PSSM) has been found in more than 35 different horse breeds through identification of abnormal storage of polysaccharide in muscle biopsies. A dominant mutation in the glycogen synthase 1 gene ( GYS1) accounts for a substantial proportion of PSSM cases in at least 17 breeds, including Quarter Horses, but some horses diagnosed with PSSM by muscle histopathologic analysis are negative for the mutation. We hypothesized that a second distinct form of glycogen storage disease exists in GYS1 -negative horses with PSSM. The objectives of this study were to compare the histopathologic features, ultrastructure of polysaccharide, signalment, history, and presenting complaints of GYS1 -negative Quarter Horses and related breeds with PSSM to those of GYS1 -positive horses with PSSM. The total histopathologic score in frozen sections of skeletal muscle stained with hematoxylin and eosin, periodic acid Schiff (PAS) and amylase-PAS stains from 53 GYS1-negative horses did not differ from that of 52 GYS1 -positive horses. Abnormal polysaccharide was fine granular or homogenous in appearance (49/53; 92%), often amylase-sensitive (28/53; 53%), more commonly located under the sarcolemma, and consisting of β glycogen particles in GYS1 -negative horses. However, in GYS1 -positive horses, abnormal polysaccharide was usually coarse granular (50/52; 96%), amylase-resistant (51/52; 98%), more commonly cytoplasmic, and consisting of β glycogen particles or, in some myofibers, filamentous material surrounded by β glycogen particles. Retrospective analysis found that GYS1 -negative horses ( n = 43) were younger at presentation (4.9 ± 0.6 years vs. 6.7 ± 0.3 years for GYS1 -positive horses) and were more likely to be intact males than GYS1 -positive horses ( n = 160). We concluded that 2 forms of PSSM exist and often have distinctive abnormal polysaccharide. However, because evaluation of the histologic appearance of polysaccharide can be subjective and affected by age, the gold standard for diagnosis of PSSM at present would appear to be testing for the GYS1 mutation followed by evaluating muscle biopsy for characteristic abnormal polysaccharide in those horses that are negative for the mutation.