Mutations in the beta1,3-N-acetylgalactosaminyltransferase 2 (B3GALNT2) gene can lead to impaired glycosylation of α-dystroglycan, which, in turn, causes congenital muscular dystrophy (CMD). The clinical phenotypes of CMD are broad, and there are only a few reports of CMD worldwide.

This report describes the cases of two children with CMD caused by B3GALNT2 gene mutation. The main manifestations of the two cases were abnormal walking posture, language development delay, and abnormal development of the white matter. Case 2 also had unreported symptoms of meningocele and giant arachnoid cyst. Both cases had compound heterozygous mutations of the B3GALNT2 gene, each containing a truncated mutation and a missense mutation, and three of the four loci had not been reported. Nineteen patients with CMD caused by B3GALNT2 gene mutation were found in the literature. Summary and analysis of the characteristics of CMD caused by B3GALNT2 gene mutation showed that 100% of the cases had nervous system involvement. Head magnetic resonance imaging often showed abnormal manifestations, and more than half of the children had eye and muscle involvement; some of the gene-related symptoms were self-healing.

B3GALNT2 gene can be used as one of the candidate genes for screening CMD, cognitive development retardation, epilepsy, and multiple brain developmental malformations in infants.

Congenital muscular dystrophy (CMD) refers to the primary and progressive myopathy that occurs at birth or within a few months after birth, usually causing considerable burden to the child and their families and society. The main clinical manifestations of CMD include premature myasthenia, hypodystonia, motor development retardation, and joint contracture with or without central nervous system involvement. The creatinase level tends to increase to varying degrees, myogenic damage can be seen on electromyography, and muscular dystrophy is the most common muscle pathology. Ge et al[1] reported that the prevalence rate of CMD in China was 0.017-0.083/100000. CMD is a group of hereditary diseases with strong heterogeneity caused by anti-dystroglycan (DG) deficiency. DG is a protein that is widely expressed in a variety of tissues, including the brain, Schwann cells of peripheral nerves, neuromuscular junction, muscle, liver, lung, and skin[2]. DG is cleaved into α-DG and β-DG after translation. The core structure of α-DG is dumbbell-shaped, with a spherical C-terminal and an N-terminal at the two ends, respectively, while the middle is connected by a mucoprotein-like structure. α-DG undergoes O-linked glycosylation, which rarely occurs in vivo[3], and then forms an anti-dystrophin-glycoprotein complex with β-DG. Then, it binds with ligands containing a laminin globular domain in the extracellular matrix, such as laminin, perlecan, agglutinin, axon protein, and pikacurin, to maintain muscle cell integrity[4], thus playing an important role in brain development[5]. α-DG has many glycosylation sites, short glycoprotein fragments, complex functions, and diverse pathogenic genes[6]. Once the glycosylation of α-DG is impaired, it can trigger many pathophysiological processes, such as cell migration disorder[7] or α-dystroglycanopathy (α-DGP)[8]. Recent studies[9] have shown that the gene located in 1q42.3 containing 64292 base pairs is one of its pathogenic genes, which encode beta1,3-N-acetylgalactosaminyltransferase2 (B3GALNT2). The deficiency of this enzyme can lead to the damage of α-DG glycosylation, consequently impairing the function of α-DG and affecting cell proliferation, migration, differentiation, and maintenance of tissue integrity, resulting in α-DGP[10]. Its pathogenic type was autosomal recessive inheritance.

At present, there are few reports about the B3GALNT2 gene causing CMD, and the relationship between genotype and phenotype is still not fully understood. In this paper, the pathogenic characteristics of this gene in two cases of α-DGP caused by B3GALNT2 gene mutation found in our hospital are summarized and discussed in view of previous reports.

The final diagnosis for both cases was B3GALNT2 gene mutation-related α-DGP. Case 2 was also diagnosed with a giant arachnoid cyst.

Both cases received coenzyme Q10 and levocarnitine. The giant arachnoid cyst in case 2 was resected.

This study reports two cases of CMD caused by B3GALNT2 gene mutation. Both cases had an abnormal gait and significant delays in language development. Head MRI was performed for both children and revealed the presence of white matter lesions in the brain. The present report suggested that the cognitive language retardation and white matter lesions in CMD were caused by B3GALNT2 gene mutation. New symptoms of meningocele and giant arachnoid cyst were also encountered in one of our cases. To our knowledge, this is the first study to use electrophysiological examination of the audiovisual conduction pathway in children with this type of gene mutation. The findings suggest that even if audiovisual function was normal, the evoked audiovisual potential should be examined to investigate for potential abnormalities. The white matter lesions in case 2 were gradually absorbed and improved with age. He recovered well after surgical resection of the giant arachnoid cyst, and the meningocele gradually self-healed. These results suggested that some of the symptoms associated with the gene mutation might have a tendency to self-heal, which was consistent with the report of Hedberg et al[11]. Further studies are required to determine whether this is related to the peak age of gene expression. Both of the present cases had compound heterozygous mutations, the gene mutation sites were from their parents, and the mutations were compatible with autosomal recessive inheritance.

Case 1 had two compound heterozygous mutations in the B3GALNT2 gene; one of them was c.1068dupT (p.D357_D358delinsX), which was a nonsense (zero-effect) mutation that may lead to loss of gene function. The frequency in the normal population database was 0.00010, indicating that this was a low-frequency variation (PM2). The bioinformatics protein function prediction software packages SIFT, PolyPhen_2, and REVEL all predicted it as unknown. According to the American College of Medical Genetics and Genomics (ACMG) guidelines, the variation was preliminarily determined as a suspected pathogenic variation. The other was c.40G>C (p.G14R), which was a missense mutation. It was (-) in the normal population database, which indicated a low-frequency variation (PM2). It existed in trans with another pathogenic variation (PM3). SIFT, PolyPhen_2, and REVEL predicted it as harmful, harmful, and benign, respectively. According to the ACMG guidelines, the variation was preliminarily determined as being of unknown clinical significance. In case 2, there were two compound heterozygous mutations in the B3GALNT2 gene: One was c.261-2A>G, which was located in the classical splicing site (± 1, 2). It was likely to affect the normal splicing of mRNA and lead to protein truncation. The frequency in the normal population database was 0.00004, which showed that this was a low-frequency variation (PM2). According to the ACMG guidelines, this variation was preliminarily determined to be a suspected pathogenic variation (PS1). The other site was c.979G>A (p.D327N), which was a known PS1 in the Human Gene Mutation Database, and the frequency of variation in the normal population was 0.00003 (PM2). In the transposition, the suspected pathogenicity variation c.261-2A>G (PM3) was detected. SIFT, PolyPhen2, and Mutation Taster all predicted that the mutation was harmful (PP3). According to the ACMG guidelines, the variation was preliminarily determined to be a suspected PS1. According to the clinical symptoms and gene results, both children were diagnosed as having CMD caused by the B3GALNT2 gene mutation. Each child had one missense mutation, and each had a mutation leading to protein truncation. Case 1 had a frameshift mutation leading to protein truncation, whereas case 2 had a splicing mutation leading to protein truncation, suggesting that truncation mutation may be related to a severe clinical phenotype.

In this study, data from two hospitalized patients and data of 19 patients obtained from a search of the previous literature were collected, yielding data from a total of 21 patients with α-DGP caused by B3GALNT2 gene mutation. Clinical manifestations included common symptoms such as nervous system, muscular dystrophy, eye development malformation, and rare symptoms such as sensorineural deafness. Moreover, 100% of the 21 patients had nervous system involvement, and delayed cognitive and motor development. In severe cases, patients had no cognitive and motor development, and most cognitive disorders included language retardation. Patients with a mild lesion on MRI only had periventricular white matter lesions, while patients with severe lesions seen on MRI had brainstem and/or cerebellar dysplasia and neuronal migration disorders. Among the 21 patients, 63.2% had elevated creatinase that was mostly between 300 and 1740 U/L, and 26.6% of the patients had eye involvement. The common symptoms were optic nerve atrophy and eye fissure deformities (excessive small or large). Patients with extensive nervous system and muscle involvement had severe ocular symptoms such as glaucoma, cataracts, and even blindness.

There were 12 pairs of alleles in 21 patients involving 15 Loci. Among them, c.979G>A (p.D327N) appeared three times, and c.822_823dup (pI276Lfs*26) appeared twice, suggesting that the above two sites may be focal mutations. It was further speculated that the severity of the B3GANLT2 gene mutation was still related to its mutation type and site. All four patients with biallelic truncated mutations in sporadic cases 2, 3, 7, and 19 had neuronal migration disorders (polymicrogyria or cobblestone-like surface with no gyri). Pedigree 1 and sporadic cases 8 and 21 had severe truncated mutations of a single allele (the sites were all before the 103^rd amino acid), and they also had neuronal migration disorders or developmental brain malformations. Pedigree 3 and sporadic case 20 had mild truncated mutations of a single allele (the sites were all after the 276^th amino acid), and they had only white matter involvement. Other forms of mutation had relatively mild symptoms. The above results suggested that patients with a truncated mutation of the gene located more at the front of the mutation site had more severe symptoms. The missense mutation was often associated with relatively mild symptoms. Sporadic type 1 is special, in that it is a compound heterozygous missense mutation that is associated with severe symptoms such as polymicrogyria malformation and eye and muscle involvement. Further studies are required to study the mechanism underlying sporadic type 1 mutation.

This article reports two cases of CMD caused by B3GALNT2 gene mutation. Each case carried a truncated mutation and a missense mutation, while three of the four variants have not been previously reported; thus, our findings in this case report expand the gene database. Case 2 had two new phenotypes: Giant arachnoid cyst, which recovered well after surgical resection, and meningocele after birth, which gradually self-healed. These data broadened the phenotypic spectrum of the gene. Clinical characteristics of the gene mutation are summarized and analyzed through a literature search, providing clues for clinicians to diagnose and treat this gene mutation. Due to the limitation of a small number of patients, absence of functional verification, and lack of evidence at the cellular level, the clinical characteristics and pathogenic mechanisms of this mutation were not completely clarified. Further studies at a clinical level are required to support our findings.