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A novel laminin alpha2 isoform in severe laminin alpha2 deficient congenital muscular dystrophy
E Pegoraro1, M Fanin, C P Trevisan
1Department of Neurological and Psychiatric Sciences, University of Padova, Italy. elenap@ux1.unipd.it
Objectives:
Laminin alpha2 deficiency presents at birth with muscle weakness, hypotonia, and usually asymptomatic white matter signal on MRI. Few patients with laminin alpha2 deficiency have been described with seizures and structural brain abnormalities. The reason for the variation in the severity of the clinical phenotype in congenital muscular dystrophy (CMD) with laminin alpha2 deficiency is not known.
Methods:
A patient with CMD with partial laminin alpha2 presenting with brain structural abnormalities and untreatable generalized and partial complex seizure was studied. Alternative laminin alpha2 splicing was studied by single-strand conformational polymorphism/sequencing analysis.
Results:
A novel laminin alpha2 isoform was identified. Nonsense laminin alpha2 mutations (stop codons) were inherited from both parents; however, one of the nonsense mutations was in a region of exon 31, which is alternatively spliced. The alternatively spliced isoform excluded one of the stop codon mutations, and was thus able to produce normal laminin alpha2 corresponding to this isoform. Laminin alpha2 immunofluorescence showed that this isoform was not evenly distributed at the muscle fiber basal lamina, but preferentially localized in discrete areas. Laminin alpha5, beta1, gamma1, and nidogen showed decreased expression by immunofluorescence.
Conclusions:
The severity of this patient's phenotype may be due to overexpression of the exon 31-spliced laminin alpha2 isoform. Exon 31 lies in the IIIA domain of the laminin alpha2 protein, just proximal to the triple coil-coiled region. It is possible that chain assembly is impaired by this isoform, resulting in a loss of possible rescue mechanisms.
Insights
Congenital muscular dystrophy (CMD) with laminin alpha2 deficiency can cause severe symptoms. A novel laminin alpha2 isoform, due to alternative splicing, may explain the varied and severe phenotypes observed in patients.
Area of Science:
- Neurology
- Genetics
- Biochemistry
Background:
- Laminin alpha2 deficiency causes congenital muscular dystrophy (CMD), typically presenting with muscle weakness and hypotonia.
- While usually asymptomatic on MRI, some patients exhibit seizures and structural brain abnormalities, indicating phenotypic variability.
- The underlying genetic and molecular reasons for this clinical spectrum in laminin alpha2 deficiency remain unclear.
Purpose of the Study:
- Investigate the cause of severe phenotype in a patient with congenital muscular dystrophy (CMD) and partial laminin alpha2 deficiency.
- Analyze alternative splicing of the laminin alpha2 gene to understand its role in disease presentation.
- Determine the molecular basis for the observed brain abnormalities and intractable seizures.
Main Methods:
- Studied a patient with CMD, partial laminin alpha2 deficiency, brain abnormalities, and complex seizures.
- Employed single-strand conformational polymorphism and sequencing to analyze alternative laminin alpha2 splicing.
- Utilized immunofluorescence to assess laminin alpha2 isoform distribution and expression of associated proteins.
Main Results:
- Identified a novel laminin alpha2 isoform resulting from alternative splicing of exon 31.
- Discovered that this alternatively spliced isoform excluded a nonsense mutation, allowing partial production of normal laminin alpha2.
- Observed abnormal, localized distribution of the novel laminin alpha2 isoform and decreased expression of laminin alpha5, beta1, gamma1, and nidogen.
Conclusions:
- The severe phenotype, including brain abnormalities and seizures, may be linked to the overexpression of the exon 31-spliced laminin alpha2 isoform.
- This specific isoform, located near the protein's triple coil-coiled region, might impair chain assembly.
- Impaired chain assembly could disrupt normal laminin alpha2 function, potentially explaining the loss of rescue mechanisms and disease severity.