Spontaneous gene mutation encounters a paternal genetic variant

RATIONALE: Nemaline myopathy (NM) is a rare hereditary myopathy, most commonly caused by mutations in the NEB gene. This case highlights the diagnostic challenges and expands the mutational spectrum of the disease. PATIENT CONCERNS: A 21-year-old male presented with a history of progressive muscle weakness. The symptoms began in the distal lower limbs at age 3 and later involved proximal and masticatory muscles. DIAGNOSES: The patient was diagnosed with nemaline myopathy type 2. Diagnosis was based on electromyography suggesting myogenic damage, muscle magnetic resonance imaging (MRI) showing extensive atrophy and fatty infiltration, muscle biopsy revealing nemaline rods, and genetic testing confirming compound heterozygous NEB mutations (paternal c.21417 + 3A > G and de novo c.7272G > A). INTERVENTIONS: The patient received symptomatic treatment. OUTCOMES: Symptomatic treatment provided limited clinical improvement. This is the first report confirming the pathogenicity of the NEB c.7272G > A mutation. LESSONS: Genetic testing is essential for diagnosing NM. Clinicians should be aware of NM subtypes that initially present with masticatory muscle weakness. This case provides new evidence for genotype-phenotype correlation and is valuable for genetic counseling.