Discussion:
In this case report, we describe an infant with an early-onset NEM and a morphological phenotype characterized by the presence of nemaline rods which was confirmed ultrastructurally. Using exome sequencing analysis, we revealed a heterozygous missense mutation, c.43G>C (p.Asp15His), in the TMP3 gene. This mutation was not present in both parents indicating that the mutation has occurred de novoand suggests a pathogenic mutation associated with autosomal dominant NEM1.
Childhood NEM is associated with a wide range of phenotypes from more benign congenital conditions which present early and either progresses slowly or not at all [7, 8] to severe weakness and debilitating functional impairment. The TPM3 gene is one of ten genes (TPM3, NEB, ACTA1, TNNT1, TPM2, CFL2, KBTBD13, KLHL40, KLHL41 andLMOD3 ) currently associated with NEM [9]. In contrast to our case which showed a normal pregnancy history with normal fetal movements, it has been reported that hydramnion and decreased fetal movements are the most frequent symptoms of NEM during pregnancy [5].At birth, the clinical findings of NEM are inconsistent and nonspecific however, severe hypotonia, especially involving the proximal limb muscles and those of the face, neck, and trunk may be noted [5]. Respiratory difficulties can be a prominent and worrisome finding at birth due to diaphragmatic muscle weakness [4, 5]. In their paper that studied the clinical and pathological features of 28 Chinese patients with NEM, Yin and colleagues demonstrated that hypotonia was observed in most patients [10]. This is in line with our case which showed signs of hypotonia since birth, but with the ability to spontaneously move all limbs in response to external stimuli. Additionally, respiratory problems appear to be a consistent finding at birth due to diaphragmatic muscle weakness [4, 5]. In an accordance with these 28 cases of NEM [10], the current case showed normal creatine kinase level. Side-to-side rolling was the most complex volitional motor function achieved in our case and in a similar case reported by Kiiskiet al [11].
In the case reported by Kiiski et al, the Gomori trichrome stain identified red-staining inclusions in several fibres which were confirmed to be nemaline rods by electron microscopy [12]. Additionally, with the modified Gomori trichrome stain, Tsujihataet al [13] found numerous dark red granular deposits in many fibres. In the current case, and as judged by myosin and ATPase stains, the small fibres were of histochemical type 1 and the normal-sized fibres consisting of a mixture of type 2 (predominantly) and type 1, and the rod bodies were observed in both type 1 and scattered type2 fibres [13].
Tan and colleagues [14] described a NEM case with a homozygous nonsense mutation in TPM3 with severe NEM1 phenotype, showing extremely delayed and impaired motor development, except for rolling over. This patient showed type 1 fibre hypotrophy, mild predominance of type 2 fibres, and nemaline bodies were only present in type 1 fibres. In contrast to the current case, that case showed no feeding problems. Kiiski et al reported that muscle biopsy showed fibre size and rod variations in a population of hypotrophic muscle fibres expressing slow myosin, often with internal nuclei, and abnormal immune labelling that revealed many hybrid fibres [11].
Family history and clinical study of the parents helped in establishing a diagnosis in the current case. There were no known cases of muscle disease in the immediate family and the parents’ clinical investigations and genetic testing were normal. The possibility of germline mosaicism in either parent has not been ruled out and current literature estimates the recurrence risk for de novo variants as 0.011% to 28.5% (PMID 30397338). Genetic counseling is both important and recommended to advice on future pregnancies as is thorough monitoring with repetitive ultrasound examinations to assess fetal parameters.