Clinico-pathological features and mutational spectrum of 16 nemaline myopathy patients from a Chinese neuromuscular center

Nemaline Myopathy Type 6 Caused by Variants in the KBTBD13 Gene: A Cross-Sectional Study of 24 Patients

Background and Objectives

Nemaline myopathy type 6 (NEM6) is the most prevalent type of nemaline myopathy in the Netherlands. Because a detailed clinical characterization is not available yet, we here provide a detailed assessment of 24 patients.

Methods

In this cross-sectional study, we performed a full clinical assessment (medical history and neurologic examination) in patients with NEM6. Patient demographics, causative variants in the KBTBD13 gene, creatine kinase levels, and the results of previous muscle biopsies were collected. We evaluated experienced health-related quality of life, fatigue severity, prevalence of falls, balance control (Mini-Balance Evaluation Systems Test [Mini-BESTest]), functional motor score (Motor Function Measure [MFM]), and 6-minute walk distance. We used transcranial magnetic stimulation to assess muscle relaxation kinetics.

Results

Twenty-four patients were included (19 women [19-76 years]; 5 men [25-57 years]). Key patient-reported symptoms since childhood were muscle weakness (n = 23; 96%), slowness of movements (n = 23; 96%), and difficulties with running (n = 20; 83%). Axial, proximal, and distal muscles showed mild weakness in most patients. Health-related quality of life was significantly lower, and there was a significantly increased fatigue severity compared with controls. Prospectively, in a period of 100 days, 8 patients (33%) fell at least 1 time, of whom 5 patients (21%) fell 2 times or more. The median total score on the Mini-BESTest was 24 (21.0-26.0 [interquartile range]) of 28 and the median total percentage on the MFM was 91% (83.5-95.3), both considered to be mildly abnormal. The 6-minute walk distance was below the lower limit of normal in 4 patients (17%). All patients with NEM6 showed a markedly reduced muscle relaxation rate with a median of 6.5 [4.9-8.1] s-1 (lower limit of normal is 10.1 s-1).

Discussion

This cross-sectional study in patients with NEM6 shows a relatively mild clinical phenotype and mildly abnormal functional tests. However, patients report an important impact on the daily activities, which is illustrated by functional difficulties, reduced quality of life, increased fatigue severity, and increased prevalence of falls. This might be related to delayed muscle relaxation. This study provides a comprehensive overview of the clinical presentation and functional limitations in patients with NEM6.

Case report: A novel ACTA1 variant in a patient with nemaline rods and increased glycogen deposition

Background

Congenital myopathies are a group of heterogeneous inherited disorders, mainly characterized by early-onset hypotonia and muscle weakness. The spectrum of clinical phenotype can be highly variable, going from very mild to severe presentations. The course also varies broadly resulting in a fatal outcome in the most severe cases but can either be benign or lead to an amelioration even in severe presentations. Muscle biopsy analysis is crucial for the identification of pathognomonic morphological features, such as core areas, nemaline bodies or rods, nuclear centralizations and congenital type 1 fibers disproportion. However, multiple abnormalities in the same muscle can be observed, making more complex the myopathological scenario.

Case presentation

Here, we describe an Italian newborn presenting with severe hypotonia, respiratory insufficiency, inability to suck and swallow, requiring mechanical ventilation and gastrostomy feeding. Muscle biopsy analyzed by light microscopy showed the presence of vacuoles filled with glycogen, suggesting a metabolic myopathy, but also fuchsinophilic inclusions. Ultrastructural studies confirmed the presence of normally structured glycogen, and the presence of minirods, directing the diagnostic hypothesis toward a nemaline myopathy. An expanded Next Generation Sequencing analysis targeting congenital myopathies genes revealed the presence of a novel heterozygous c.965 T > A p. (Leu322Gln) variant in the ACTA1 gene, which encodes the skeletal muscle alpha-actin.

Conclusion

Our case expands the repertoire of molecular and pathological features observed in actinopathies. We highlight the value of ultrastructural examination to investigate the abnormalities detected at the histological level. We also emphasized the use of expanded gene panels in the molecular analysis of neuromuscular patients, especially for those ones presenting multiple bioptic alterations.

Clinico-pathological and gene features of 15 nemaline myopathy patients from a single Chinese neuromuscular center

BACKGROUND

Nemaline myopathy, the most common of the congenital myopathies, is caused by various genetic mutations. In this study, we attempted to investigate the clinical features, muscle pathology and genetic features of 15 patients with nemaline myopathy.

RESULTS

Among the 15 patients, there were 9 (60.00%) males and 6 (40.00%) females, and 9 (60.00%) of them came from three families respectively. The age of seeing a doctor ranged from 9 to 52 years old, the age of onset was from 5 to 23 years old, and the duration of disease ranged from 3 to 35 years. Ten out of the 15 patients had high arched palate and elongated face. Only one patient had mild respiratory muscle involvement and none had dysphagia. Muscle biopsies were performed in 9 out of the 15 patients. Pathologically, muscle fibers of different sizes, atrophic muscle fibers and compensatory hypertrophic fibers could be found, and occasionally degenerated and necrotic muscle fibers were observed. Different degrees of nemaline bodies aggregation could be seen in all 9 patients. The distribution of type I and type II muscle fibers were significantly abnormal in patients with nemaline myopathy caused by NEB gene, however, it was basically normal in patients with nemaline myopathy caused by TPM3 gene and ACTA1 gene. Electron microscopic analysis of 6 patients showed that nemaline bodies aggregated between myofibrils were found in 5(83.33%) cases, and most of them were located near the Z band, but no intranuclear rods were found. The gene analysis of 15 NM patients showed that three NM-related genes were harbored, including 11 (73.33%) patients with NEB, 3 (20.00%) patients with TPM3, and 1 (6.67%) patient with ACTA1, respectively. A total of 12 mutation sites were identified and included 10 (83.33%) mutations in exon and 2(16.67%) mutations in intron.

CONCLUSIONS

The clinical phenotype of nemaline myopathy is highly heterogeneous. Muscle pathology shows that nemaline bodies aggregation is an important feature for the diagnosis of NM. NEB is the most frequent causative gene in this cohort. The splicing mutation, c.21522 + 3A > G may be the hotspot mutation of the NEB gene in Chinese NM patients.