P.9.7 Skeletal muscle biopsy reappraisal in nebulin-related nemaline myopathy

Abstract

Nemaline myopathy (NM) is a congenital disorder associating muscle weakness with rods in muscle biopsy. Clinical presentation is variable spanning from severe to mild forms. Seven genes are associated with NM. The nebulin (NEB) gene is the most commonly mutated accounting for 50% of cases. We performed a muscle morphological analysis of thirteen NEB-mutated patients with different clinical forms to define pathological patterns and clinical-morphological correlations. Four groups were identified according to clinical severity and age at biopsy. Group 1 ( n = 4) comprises severe/lethal NM and biopsy in first days. Group 2 ( n = 3) comprehends severe NM and biopsy after one month. Group 3 ( n = 3) comprises typical NM and biopsy in childhood, and group 4 ( n = 2) patients with mild NM and biopsy in adulthood. Molecular diagnosis was done using dHPLC/Sanger-sequencing in six patients and next-generation sequencing in seven. Biopsies underwent histoenzymological, immunohistochemical and ultrastructural analysis. Fiber type distribution, rod characteristics, distribution and localization were investigated. All patients presented NEB mutations consistent with AR inheritance. G1 showed type 2 predominance and scattered squared rods in 1/3 of fibers. Ultrastructural analysis revealed high percentage of fibers with sarcomeric disarray. G2 showed a variable pattern of fiber type distribution spanning from slight type 2 predominance to type 1 uniformity. Rods presented variable distribution and shape. Ultrastructural analysis revealed rare fibers with sarcomeric disarray. In contrast, G3 and G4 presented a homogeneous type 1 uniformity associated with well-delimited subsarcolemmal and/or cytoplasmic elongated rods without sarcomeric alterations. In conclusions we (1) identified an unreported association of type 2 predominance in muscle biopsy of patients presenting severe NEB-related NM; (2) suggest a direct correlation between the association of sarcomeric disarray and clinical severity.