We report a patient with a typical presentation of CMS due to compound heterozygous mutations in GFPT1. The onset of CMS in patients harboring a GFPT1 mutation usually occurs under 10 years of age (2, 25). In this study, the patient started experiencing limb weakness at 4 years old accompanied by limb fatigue after exercise. RNS revealed decrements in abductor digiti minimi and abductor pollicis brevis, and EMG assessment suggested myogenic damage. The proband responded well to an acetylcholinesterase inhibitor, and the phenotypes of the proband were consistent with previous reports of CMS (26–28). Thus, myasthenia gravis or CMS was initially suspected. However, he did not experience drooping eyelids or blurred vision, and did not have alleviation of symptoms in the morning and aggravation of symptoms in the evening, which are important characteristics of myasthenia gravis (29). The proband had pneumonia and underwent tracheotomy due to difficulty breathing. Liu et al. (30) indicated that patients with CMS have repeated apnea and become ventilator-dependent, and need a tracheomtomy to alleviate symptoms as was the case in our study. The proband and his parents tested negative for acetylcholine receptor (AchR) antibodies. Furthermore, MRI in the proband showed prominent fatty infiltration in the thigh muscles. Thus, the patient was diagnosed with CMS. Symptoms of CMS markedly improved with treatment of 60 mg pyridostigmine, which was taken orally three times a day.
CMS has several distinctive clinical features including the presence of tubular aggregates (TAs) on muscle biopsy, increased levels of serum CK, proximal limb muscle weakness and fatigability starting in childhood (31). It is still unclear whether TAs are pathological structures or show compensatory reactions to a variety of pathogenic events, such as familial myopathies and other myopathies of uncertain etiology (32, 33). Previous results have paved the way for understanding the etiology of this rare neuromuscular disorder that may be considered “TA myopathy with synaptopathy” (20). In the study by Zhang et al. (34), muscle biopsy revealed massive TAs within the muscle fibers of a patient with CMS. However, Guergueltcheva et al. (35) reported two patients with clinical features of CMS and GFPT1 mutations, but in whom TAs were not conclusively detected. These two patients were diagnosed with CMS at 7 and 19 years old, respectively (35). In addition, Finlayson et al. (36) reported a patient diagnosed with CMS at the age of 6, while TAs in muscle biopsies were not present in this proband. Muscle biopsy of the 30-year-old proband in this study showed no TAs in the myofibers, indicating that TA expression may differ in different muscles or during the lifetime of patients.
A genetic test identified two missense mutations (c.331C>T; p.Arg111Cys and c.1428G>C; p.Lys476Asn) in GFPT1. The c.331C>T; p.Arg111Cys variant has been previously reported in a compound heterozygous state (2, 20), and has been recorded in the ClinVar dataset and the gnomAD global population dataset, whereas the c.1428G>C; p.Lys476Asn variant has not been recorded in these two datasets. The c.331C>T; p.Arg111Cys variant was first found in two families, while functional analysis showed that enzymatic activities in the wild-type GFPT1 group were not different from those in the mutant GFPT1 (c.331C>T; p.Arg111Cys) group (2). In addition, Bauche et al. (20) reported a patient with CMS due to compound heterozygous mutations in GFPT1 (c.331C>T; p.Arg111Cys). Moreover, the variant was classified as ‘pathogenic/likely pathogenic’ according to the ClinVar dataset, and was seen at a frequency of 0.05% in the Latino population in the gnomAD global population dataset. According to the current data, the variant was classified as a likely pathogenic variant. The novel mutation c.1428G>C; p.Lys476Asn occurred at a nucleotide position that is highly conserved among 100 vertebrates. In addition, this variant led to a change in the amino acid sequence, as a highly conserved lysine residue was replaced, according to analysis using the UCSC Genome Browser. Moreover, Splice AI software predicted that this mutation could result in a deficiency in a variable splice donor site. Previous studies have indicated that mutations that disrupt the splicing code may contribute to a variety of diseases (37, 38). In addition, the novel variant c.1428G>C; p.Lys476Asn in GFPT1 was predicted to likely be deleterious by using the Pathogenicity Island Prediction Software. However, the pathogenicity of the mutation has not been assessed, so we classified this variant as ‘uncertain significance’.
GFPT1 is the rate-limiting enzyme in the hexosamine biosynthetic pathway, which contributes to the production of UDP-N-acetylglucosamine (UDP-GlcNAc), a donor substrate for O-linked and N-linked glycosylation of various proteins (39). In addition, GFPT1 provides important glycosylated groups to proteoglycan, glycoprotein, and glycolipid synthesis (40). Muscle, skeletal receptor tyrosine-protein kinase, AChR subunits, and integrins are the key proteins in the NMJ, and exert biological functions via glycosylation (41). Bauche et al. (20) showed that GFPT1 mutations typically result in decreased GFPT1 levels, which lead to abnormal muscle protein glycosylation. Senderek et al. (2) found that GFPT1 knockdown led to histological changes in zebrafish muscle with delayed maturation of the NMJ. In addition, a previous study showed that inhibiting GFPT1 enzymatic activity or small interfering RNA-mediated silencing of GFPT1 expression downregulated AChR expression on the cell surface in vitro, thus affecting neuromuscular transmission at NMJs (19). However, the molecular basis of CMS is not fully understood. Therefore, further studies on the molecular mechanisms of this disorder are needed, which may provide valuable information that inform current therapies.
CMS is a rare inherited condition, that is very difficult to diagnose (42). In this study, the patient began experiencing symptoms of CMS at 4 years old, but was not diagnosed with CMS until 30 years old. Here, we described the symptoms, evaluation, and management of a patient with CMS, and identified two compound heterozygous mutations. These findings may lead to the earlier diagnosis of this disease and an earlier start to treatment.