Enhanced alpha7beta1 integrin prevents muscle disease in a mouse model of congenital muscular dystrophy

Abstract

Merosin Deficient Congenital Muscular Dystrophy Type 1A (MDC1A) is the most common form of Congenital Muscular Dystrophy (CMD). MDC1A accounts for approximately 40% of all CMD cases. CMD is estimated to have an incidence rate in the United States of 1/100,000 live births. MDC1A is a lethal disease which results from mutations in the LAMA2 gene leading to either a complete or partial absence of the protein laminin α2. The loss of laminin α2 protein in turn leads to a failure to produce laminin 211 and laminin 221. Laminin 211 is the major laminin component of the basal lamina of mature skeletal muscle while laminin 221 is primarily located at neuromuscular and myotendinous junctions. Patients with MDC1A display a spectrum of clinical signs which correlate with the type of mutation they carry in the LAMA2 gene. Patients who produce some laminin α2 or who produce a truncated form of the protein tend to have less severe clinical signs and longer life expectancies than patients who produce no laminin α2. Patients with classic MDC1A have mutations which result in either complete absence of or negligible production of laminin α2. Patients who produce no laminin α2 have the most severe clinical signs and shortest life expectancies. MDC1A patients display delayed motor milestones and unsupported sitting is often the maximal motor activity; they are often confined to a wheelchair at a young age. Feeding problems requiring feeding tube placement, and respiratory difficulties requiring positive pressure ventilation are common. It is not unusual for these patients to die within the first decade of life due to respiratory complications.Secondary to the loss of laminin α2, MDC1A patients display reduced levels of α7 integrin. The α7β1 integrin is one of the main laminin 211/221 receptors in skeletal muscle. The loss of α7 integrin is considered to be due to communication between the extracellular matrix and the cell surface receptors. Primary loss of the α7 integrin is rare and leads to a myopathy in affected individuals. The effect of the secondary loss of the α7 integrin in MDC1A patients has not been evaluated prior to this work. In order to determine the effect of α7 integrin loss, we enhanced α7 integrin expression in the skeletal muscle of the dyW-/- mouse model of MDC1A. The dyW-/- mouse is one of the most commonly used models of MDC1A and displays altered expression and localization of the α7 integrin with little integrin appropriately localized to the sarcolemma. We accomplished forced expression of α7 integrin by breeding the BX2-10i mouse and dyW-/- mouse over several generations to develop a mouse which lacked laminin α2 and overexpressed α7 integrin in the skeletal muscle. The BX2-10i mouse is a skeletal muscle specific overexpressor of α7BX2 integrin - one of the primary α7 integrin isoforms which binds with laminin in skeletal muscle. These animals contain a transgene which uses the muscle creatine kinase (MCK) promoter to drive α7BX2 integrin production specifically in skeletal muscle. The resultant mouse, designated as dyW-/-;itga7+, allowed us to determine the effect of reintroducing α7 integrin back into an MDC1A model, and thus, gave us insight into the role that reduced α7 integrin plays in the pathology of MDC1A.