'Dusty core disease' (DuCD): expanding morphological spectrum of RYR1 recessive myopathies

Abstract

Several morphological phenotypes have been associated to RYR1-recessive myopathies. We recharacterized the RYR1-recessive morphological spectrum by a large monocentric study performed on 54 muscle biopsies from a large cohort of 48 genetically confirmed patients, using histoenzymology, immunohistochemistry, and ultrastructural studies. We also analysed the level of RyR1 expression in patients' muscle biopsies. We defined "dusty cores" the irregular areas of myofibrillar disorganisation characterised by a reddish-purple granular material deposition with uneven oxidative stain and devoid of ATPase activity, which represent the characteristic lesion in muscle biopsy in 54% of patients. We named Dusty Core Disease (DuCD) the corresponding entity of congenital myopathy. Dusty cores had peculiar histological and ultrastructural characteristics compared to the other core diseases. DuCD muscle biopsies also showed nuclear centralization and type1 fibre predominance. Dusty cores were not observed in other core myopathies and centronuclear myopathies. The other morphological groups in our cohort of patients were: Central Core (CCD: 21%), Core-Rod (C&R:15%) and Type1 predominance "plus" (T1P+:10%). DuCD group was associated to an earlier disease onset, a more severe clinical phenotype and a lowest level of RyR1 expression in muscle, compared to the other groups. Variants located in the bridge solenoid and the pore domains were more frequent in DuCD patients. In conclusion, DuCD is the most frequent histopathological presentation of RYR1-recessive myopathies. Dusty cores represent the unifying morphological lesion among the DuCD pathology spectrum and are the morphological hallmark for the recessive form of disease.

Keywords: Central Core Disease; Centronuclear myopathy; Congenital Myopathy; Dusty Core Disease; RYR1 recessive; Ryanodine receptor.

Fig. 1

DuCD morphological spectrum. Dusty cores: irregular areas of reddish-purple granular material deposition at GT stain (a, d, g and i) corresponding to areas of devoid ATPase activity (c, f and h) and decreased or increased enzymatic activity at oxidative stains (b, e and l), sometime occurring in the same core side-by-side (b and e; arrows) or concentric with “targetoid” appearance (b, i and l; arrowheads). Note type 1 fibre uniformity (c, f and h) and prominent nuclear internalization (d and g). (a-c: p12, 28 years; d-f: p11, 48 years; g and h: p19, 4 years; i and l: p24, 1 year)

Fig. 2

Immunoistochemestry and Immunofluorescence of DuCD and CCD. IHC showing positive immunostaining for desmin (a and g) myotilin (b and h) and αB-cristallim (c and i) in both central cores (a-c) and dusty cores (g-i). Serial images of NADH and immunofluorescence for RyR1 and DHPR showing positive signal in central cores (e and f) and positive, but unmatching signal in dusty cores (m and n). (a-c: p26, 32 years; d-f: p40, 25 years; g-i: p13, 28 years; l-n: p7, 76 year)

Fig. 3

Ultrastructural findings of DuCD. Large irregular areas of sarcomeric disorganization with longer longitudinal axis and shorter transversal axis (a) containing abundant electrodense longitudinally-smeared material (arrows) and thickened short Z-line darker fragments (arrowheads), in longitudinal (a and b) and trasversal (c) sections. Smaller areas of sarcomeric disorganization (few sarcomeres) were alsodetected (d). Areas of disorganization occupying the entire muscle fibre in width with centralized/internalised nuclei (e). Several triads duplication or multiplication were detected inside dusty cores (black arrows). (a, d and e: p10, 34 years; b: p17, 34 years; c and f: p7, 76 year)

Fig. 4

Clinical, morphological and genetic correlation. RyR1 expression was significantly lower in patients with an earlier onset (<1 year) (a) and more severe clinical presentation (b). The lowest RyR1 expression was observed in DuCD group compared to CCD, C&R and T1P+ groups alone (c) or together (d)

Fig. 5

Inhomogeneous composition of dusty cores. Transversal section of dusty core by EM (a) showing sub-area of accumulated osmophilic material (red line) close to pale sub-area (yellow line), possibly corresponding to the peculiar aspect of increased (red line) and decreased (yellow line) enzymatic activity at NADH (b)

Fig. 6

Star-shaped dusty cores and ultrastructural finding diagram. Longitudinal section of star-like shaped dusty core by EM (a). Three-dimensional representation of dusty cores inside muscle fibre (b,1). Superficial slides showing small areas of disorganization corresponding to the peripheral-side of dusty core (b,2), leading to a possible misdiagnosis with minicores. Deeper analysis revealing the real size of disorganization (b,3)