Introduction
Merosin deficient congenital muscular dystrophy type 1A (MDC1A) or laminin-α2 related muscular dystrophy (OMIM entry, #607855) is one of the most common forms of congenital muscular dystrophy (CMD), and accounts for 30% of all cases of CMD in European countries (1). CMD may appear during early onset or later in a patient's life, and is characterized by proximal weakness, contractures, delayed motor development, white matter abnormalities and spiral scoliosis (2,3). CMD is a hereditary disorder caused by recessive mutations in the LAMA2 gene, which is located on chromosome 6q22-2, spans over 260 kb and is comprised of 65 exons. LAMA2 encodes the laminin-α2 chain which attaches with laminin-β1 and laminin-γ1 chain to form the heterotrimeric protein laminin 211. Laminin 211 is a primary component of the skeletal muscle basement membrane and the extracellular matrix (1,4,5). The protein interacts with other matrix macromolecules and contributes to cell differentiation, cell movement and tissue phenotypes (4). The majority of reported genetic variations of MDC1A are homozygous or compound heterozygous variants (6,7). De novo variations in contrast are not common events and only a few have been reported in patients with MDC1A (8-10).
It is estimated that the prevalence of MDC1A is 1-9/1,000,000 individuals, and accounts for 1-6% of all CMD cases (1,2). MDC1A is more common in Caucasians and rarer in the Asian population (2). However, the prevalence of the disease in the Asian population may be higher than expected due to a lack of widespread availability of appropriate diagnostic testing (11). In the present study, the case of a Vietnamese male child exhibiting clinical signs of muscular dystrophy is reported. The child was previously undiagnosed due to his rare clinical presentation and a lack of appropriate testing. Whole exome sequencing (WES) was performed on the patient and his parents. WES showed that the proband harbored two variants in the LAMA2 gene. Genotype-phenotype analysis suggested that the patient had classical early onset MDC1A. Additionally, the de novo variant was suggested to serve a contributive role in the development of the disease. To the best of our knowledge, the present study is the first to report a case of MDC1A in a Vietnamese patient.
Discussion
In the present study, two variations in the LAMA2 gene of a male proband who exhibited clinical manifestations of CMD were identified. The missense variant (NM_000426.3:c.1964T>C) is located in the N-terminal domain of laminin-α2. A previous study found that half of the variants detected in a cohort of 43 patients with MDC1A were located in the N-terminal domain (6). Therefore, this may suggest that this domain is either more vulnerable than the other domains to mutations, is more sensitive to mutations or that it exhibits a crucial function on the part of the protein. WES analysis of the proband and his parents alongside database analyses indicated that NM_000426.3:c.1964T>C was de novo and a novel variant. A splice site variant (NG_008678.1:c.3556-13T>A) in the proband and his mother was also detected. This variant and its mRNA splicing effect (premature termination of transcription) have been previously described in two Chinese patients with MDC1A (6).
Together, these two variants likely caused a deficit in laminin-α2 function, chain which is a primary component of a trimeric basement membrane glycoprotein (1,4,5). Mutations in the LAMA2 gene result in defective function of the laminin protein with consequential poor muscle fiber adhesion and degeneration (31). Phenotype-genotype analysis indicated that this proband was associated with classical early onset LAMA2-associated muscular dystrophy. Therefore, these two variants were submitted to the Global Variome Shared LOVD database (databases.lovd.nl/shared/individuals/00208524).
The missense variant was searched against in multiple databases and was not found to have been previously reported. Therefore, the present case report may be the first report underlining genetic variations in a Vietnamese individual with MDC1A. In addition to the rare nature of MDC1A, a lack of common symptoms in the child, such as increased levels of CK and ophthalmoplegia, and the genetic heterogeneity of this disease made diagnosis difficult. CK levels are often elevated in patients with MDC1A during the early years of a patient's life and tend to decrease with age (32). The proband in the present report exhibited normal levels of CK at the age of 14. Several studies have reported normal CK levels in patients with MDC1A (6,11,33). Therefore, age should be taken into account when interpreting the CK level in each patient with CMD (34). In the present study, the patient was suspected to suffer from X-linked adrenoleukodystrophy owing to his abnormal brain MRI scan. Genetic testing of the ABCD1 gene, defects of which cause X-linked adrenoleukodystrophy were performed. However, there was no evidence of mutations found and the patient remained undiagnosed prior to WES testing (data not shown).
MDC1A exhibits a wide spectrum of genetic and clinical heterogeneity. Immunohistochemistry (IHC) is a first-tier test used to determine CMD-associated diseases. Commercially available antibodies, such as clone 5H2, clone Mer3/22B2 and clone 4H8-2 are available for performing laminin protein assays (7). However, as MDC1A is extremely rare in Asian populations, including the Vietnamese, IHC using these specific antibodies for laminin-α2 is often not viable. Therefore, in addition to clinical features (phenotypes, brain MRI, EMG and muscle morphology), molecular genetic testing, such as WES, is highly recommended (7,35). Additionally, WES should be performed for both the patient and their parents, as it assists in improving our understanding of the diagnostic yield for genetically heterogeneous disorders (36), and may result in the identification of novel variants which contribute to this disease (37). With the increase in the use and availability of next generation sequencing techniques, novel variants/mutations associated with MDC1A have been uncovered (7). The newly discovered variant in the present study adds to the genetic spectrum of the disease, and also suggests the contribution of a de novo event to the autosomal recessive inheritance of MDC1A. Therefore, de novo events should be taken into consideration to accurately determine the occurrence risk of this disease in future offspring.