Laminin α2 chain-deficient congenital muscular dystrophy: Variable epitope expression in severe and mild cases

Objective: To characterize the expression of distinct fragments of laminin α2 chain in patients with partial laminin α2 chain deficiency and variable clinical severity. Background: Deficiency of laminin α2 chain caused by mutations of the LAMA2 gene on chromosome 6q2 account for approximately 50% of cases of congenital muscular dystrophy (CMD) in white patients. The complete absence of laminin α2 is usually associated with a severe phenotype affecting skeletal muscle and the peripheral and central nervous systems. Methods: Quantitative assessment of immunofluorescence to study the expression of C- and N-terminal portions of laminin α2 chain in five patients with partial laminin α2 chain deficiency and variable phenotype. All five patients showed abnormal T2 signal on brain MRI. Results: Immunohistochemistry of muscle specimens showed preserved or minimally reduced expression of the C-terminal region of the laminin α2 chain (67 to 74%), but a marked reduction of the N-terminal region in four patients (13 to 19%). One patient with a mild phenotype had a partial reduction (45%) of the C-terminal and the N-terminal (51%) portions of the laminin α2 chain. Two patients were unable to walk or sit, although the C-terminal portion of the laminin α2 chain was expressed at significant levels (67 to 74%). In contrast, two patients with a similar expression of the C-terminus (67 to 70%) had a milder phenotype and became ambulatory. It was impossible to predict the phenotypes in these four patients with a strong expression of the C-terminus and with low levels of the N-terminus based on the amount of protein expressed. In addition, the laminin β2 chain was moderately reduced (54 to 75%) in all patients with laminin α2 chain deficiency. A strong correlation between the amount of the C-terminus but not for the N-terminus and laminin β2 reduction could be observed. Conclusions: N-terminal antibodies to the laminin α2 chain provide a more precise immunohistochemical detection of partially laminin α2 chain-deficient CMD. The secondary reduction of laminin β2 chain may better define laminin α2 chain-deficient CMD. More data are needed to predict which portions of C- terminus and midrod region of the laminin α2 chain result in a semifunctional protein and a milder phenotype.