Introduction
Merosin collectively is a term that refers to a group of laminins that share the alpha2 subunit encoded by the LAMA2 gene. Laminins are a family of high molecular weight glycoproteins that function as extracellular matrix components of the structural basement membrane. They are heterotrimers composed of homologous alpha, beta, and gamma polypeptide chains. Individually, these chains have a distinct organization of structural domains and can exhibit biological activities, including self-assembly and protein interaction. The alpha2 subunit is present in laminin 2 (merosin) and laminin 4 (s-merosin), two laminin types that play an integral role in skeletal muscle.[1]
Fundamentals
Laminins alongside other proteins are present in the extracellular matrix and basement membrane of specific tissues such as muscle. It is within this matrix where they predominantly regulate cellular growth, motility, adhesion, and structural stability through protein-protein interactions.[1]
Molecular
The LAMA2 gene, which encodes for the laminin-alpha2 subunit, is located on chromosome 6q22.33. LAMA2 expresses in a wide variety of tissues, including skeletal muscle, astrocytes, pericytes of capillaries in the brain, and Schwann cells. Fagerberg et al. revealed the expression of the LAMA2 protein-encoding gene via a quantitative transcriptomics analysis of over 27 different human tissues including but not limited to: pancreas, heart, fat, thyroid.[2][3]
Function
The laminin-alpha2 subunit expresses in numerous tissues, including skeletal muscle, astrocytes, pericytes of capillaries in the brain, and Schwann cells.[2] It is believed to play a role in the attachment, migration, and cellular organization of tissues during embryonic development through interactions with other extracellular matrix components.
Using thin-section fracture labeling, cryo ultramicrotomy, and immunohistochemistry, research has localized the laminin-alpha2 chain to the basal lamina of all cerebral blood vessels. However, localization signals were absent or non-reactive in the choroid plexus and meningeal blood vessels. Villanova et al. thus hypothesized that the laminin-alpha2 subunit might be significant in the selective filtration ability of the blood-brain barrier based on the chain's localized expression.[4]
Additionally, mutations of the LAMA2 gene and merosin carry implications in the pathophysiology of merosin-deficient congenital muscular dystrophy, which is also known as LAMA2-related muscular dystrophy. Over 100 mutations in this gene have links with this disorder, with most of these mutations arising in the early-onset variant of the disease.[5]
Testing
Mutations of the LAMA2 gene account for the etiology of merosin-deficient congenital muscular dystrophy (MDC1A) or LAMA2-related muscular dystrophy (LAMA2 MD). Identification of these mutations has been revealed through numerous molecular diagnostic techniques, including array-based comparative genomic hybridization (array CGH) and next-generation sequencing (NGS), to improve clinical management and guide genetic counseling.
CGH was developed to identify copy-number variations (CNV) within a genome. This technique hybridizes differentially labeled genomic DNA from a test and reference source to arrays of genomic clones or metaphase chromosomes. The labeled DNA binds to its respective locus of origin, and the resulting fluorescent intensity is directly proportional to the ratio of CNV relative to the corresponding DNA of the two sources. CGH aims to compare two genomic DNA samples from two different but often related sources and identify differences within whole or subchromosomal regions. CGH provides a measure of detecting chromosomal differences between two sources as well as an approach to associating disease phenotypes to the localization of specific genes. CGH is, however, limited as it can only detect unbalanced chromosomal aberrations as reciprocal translocations, inversions, or ring chromosomes do not affect changing CNV.[6]
Next-generation sequencing allows for high-throughput data collection and analysis of mutations by detecting nucleotide substitutions and small insertions/deletions (indels). NGS sequences millions of DNA fragments in parallel and utilizes bioinformatic analytical programs to piece the numerous fragments relative to the known human genome. These fragments are often sequenced multiple times to ensure high fidelity accuracy.[7]
In the clinical setting, MDC1A patients will have elevated serum levels of creatine kinase (CK). Diagnostic imaging such as magnetic resonance imaging (MRI) with diffusion-weighted imaging may also reveal abnormal intensities in white matter. The clinical diagnosis may warrant direct genetic testing rather than an invasive muscle biopsy.[8][9]
Clinical Significance
Merosin, which refers to the laminins that contain the alpha2 subunit encoded by LAMA2, play a key role in extracellular matrix composition and signaling. Recent research has begun to find novel uses and functions of the alpha2 subunit, which may elucidate the pathogenesis, diagnosis, and treatment of certain diseases.
Bacterial Superantigen Binding
T cell activation, initiated by T cell receptor (TCR) binding to peptide-MHC complexes or antibody-mediated crosslinking of TCR, requires functional LCK and ZAP-70 proteins. Staphylococcal antigens, however, can fully activate T cells through an alternative LCK-independent pathway. This alternative pathway involves an interaction between the superantigen and a Galpha11-containing G-protein coupled receptor (GPCR) that activates phospholipase beta (PLCbeta) and subsequent T cell activation and IL-2 production. Complementary binding assays and ligand-receptor capture technology (LRC-TriCEPS) have identified a laminin-alpha2 subunit as an important component of this process. The laminin-alpha2 subunit bridges the superantigen to the GPCR, and researchers have proposed it as a coreceptor target for novel therapeutic strategies.[10]
Muscular Dystrophy
Mutations in LAMA2 contribute to the pathogenesis of merosin-deficient congenital muscular dystrophy (MDC1A) or LAMA2-related muscular dystrophy (LAMA2 MD), a rare autosomal recessive disorder characterized by neonatal hypotonia, muscle weakness secondary to demyelination of peripheral nerves, and elevated levels of creatine kinase.[5] MDC1A is one of the most common forms of congenital muscular dystrophy in Western countries. Genomic sequencing has identified numerous mutations and variants of the LAMA2 gene in patients with MDC1A and LAMA2 MD, thus indicating that phenotypic expression of the diseases exists within a variable spectrum.[11][12] A majority of patients will exhibit normal intellectual and speech development; however, reports of cases of mental retardation do exist. Additionally, there is an estimated 6 to 8% of reported cases that have also had epilepsy, often with seizures displaying no particular pattern.[13]
Medullary Sponge Kidney Disease
Laminin-alpha 2 subunit has also been proposed as a potential urine-specific biomarker for medullary sponge kidney (MSK) disease, according to Fabry et al. ’s proteomic studies. MSK is a rare malformation of the kidney associated with recurrent nephrolithiasis and nephrocalcinosis. Currently, this disease gets diagnosed through time-consuming and expensive clinical tests such as urography as no molecular diagnostic biomarkers currently exist. Their study found laminin-alpha2 subunits in urine samples of patients with MSK and have proposed the protein as a potential candidate MSK biomarker for early and convenient diagnosis of the disease.[14]