Diagnostic Challenges of Neuromuscular Disorders after Whole Exome Sequencing

Abstract

Background: Whole-exome sequencing (WES) facilitates the diagnosis of hereditary neuromuscular disorders. To achieve an accurate diagnosis, physicians should interpret the genetic report carefully along with clinical information and examinations. We described our experience with (1) clinical validation in patients with variants found using WES and (2) a diagnostic approach for those with negative findings from WES.

Methods: WES was performed on patients with the clinical impression of hereditary neuromuscular disorders. Information on clinical manifestations, neurological examination, electrodiagnostic studies, histopathology of muscle and nerve, and laboratory tests were collected.

Results: Forty-one patients (Male/Female: 18/23, age of onset: 34.5±15.9) accepted WES and were categorized into four scenarios: (1) patients with a positive WES result, (2) patients with an inconclusive WES result but supporting clinical data, (3) negative findings from WES, but a final diagnosis after further work-up, and (4) undetermined etiology from WES and in further work-ups. The yield rate of the initial WES was 63.4% (26/41). Among these, seventeen patients had positive WES result, while the other nine patients had inconclusive WES result but supporting clinical data. Notably, in the fifteen patients with negative findings from WES, four patients (26.7%) achieved a diagnosis after further workup: tumor-induced osteomalacia, metabolic myopathy with pathogenic variants in mitochondrial DNA, microsatellite expansion disease, and vasculitis-related neuropathy. The etiologies remained undetermined in eleven patients (myopathy: 7, neuropathy: 4) after WES and further workup.

Conclusions: It is essential to design genotype-guided molecular studies to correlate the identified variants with their clinical features. For patients who had negative findings from WES, acquired diseases, mitochondrial DNA disorders and microsatellite expansion diseases should be considered.

Keywords: Whole-exome sequencing; motor neuron disease; myopathy; neuromuscular disorders; neuropathy.

Fig. 1

Algorithm of the study.

Fig. 2

Muscle and bone histopathology in patient 28 with tumor-induced osteomalacia. The staining of hematoxylin and eosin (H&E) (A) and myosin adenosine triphosphatase at pH 4.3 (B) showed compacted muscle fibers with scattered atrophic fibers in the biopsy at left biceps brachii. The bone tumor pathology showed spindle tumor cells in myxohyalinized fibrotic stroma with eosinophilic matrix-like materials accompanying the bony components in H&E staining. (C&D) Focal bony destruction, woven bone formation and grungy calcification were also noted. The immunohistochemical staining of CD56 (E) and SATB2 (F) was strongly positive. Whole-body bone scintigraphy revealed a bone tumor with markedly increased osteoblastic activity at the right proximal femur. (G) Additionally, there were multiple posttraumatic changes in the thoracic cage, pubic bones, and bilateral femurs.

Fig. 3

Muscle histopathology in patient 29 with mitochondrial myopathy. Hematoxylin and eosin staining showed variated muscle fiber sizes, degenerative and atrophic muscle fiber sizes, and nuclear internalization in the biopsy at the left biceps brachii. There were muscle fibers with variated cytoplasmic vacuoles. The vacuole was strongly positive for Oil Red O staining (B). In the staining of nicotinamide adenine dinucleotide-tetrazolium reductase (NADH-TR) (C) and succinate dehydrogenase (D), dark staining was noted in some muscle fibers (arrowhead). Vacuoles were also seen in the NADH-TR staining ( ^*).

Fig. 4

Combined muscle and nerve biopsy in patient 31. The staining of hematoxylin and eosin (H&E) (A) and nicotinamide adenine dinucleotide-tetrazolium reductase (B) showed grouped atrophy in the right peroneal brevis muscle. The biopsy in the right superficial peroneal nerve did not show thickening of small vessels, lymphocyte infiltration, red blood cell extravasation in H&E (C-E) and Masson’s trichrome staining (F).