Structural Repair Strategies for LAMA2 MD using Linker Proteins

Abstract

LAMA2-related muscular dystrophy (LAMA2 MD or MDC1A) is caused by mutations in LAMA2, the gene encoding laminin-α2, the long arm of the heterotrimeric (α2, β1, γ1) protein laminin-211 (Lm-211). In most LAMA2 patients, biallelic mutations in LAMA2 result in the complete absence of Lm-211, the main laminin found in muscle basement membrane. As the cDNA encoding laminin-α2 is large (> 9kb), and the protein does not tolerate even subtle mutations because of its interaction with β1- and γ1-laminin chains, gene replacement or gene editing strategies are difficult to envisage. By studying the molecular mechanisms involved in the disease, we detect compensatory expression of laminin-α4, giving rise to Lm-411 (α4, β1, γ1) in LAMA2 patients’ biopsies and laminin-α2-deficient mice. Based on in vitro studies, Lm-411 was shown to be deficient in binding to muscle fibers and in forming a fully assembled basement membrane. In an attempt to restore muscle fiber binding and basement membrane assembly, we have designed two small, AAV-compatible linker proteins1. One linker, called mini-agrin (mag), works to mediate binding of Lm-411 to the muscle fiber via α-dystroglycan. The second linker, called αLNNd, involving the N-terminal part of laminin-α1 and the laminin-binding site of nidogen-1, allows polymerization of Lm-411. When expressed transgenically in dyW/dyW mice, a mouse model for LAMA2 MD, the two linkers ameliorate the muscle dystrophy, increase overall body weight and drastically extend lifespan. We will also report on our current efforts to evaluate the therapeutic potential of the linker proteins when expressed postnatally at different time points using AAV-mediated delivery to dyW/ dyW mice. These experiments aim to provide essential information for a possible translation of the linker strategy into LAMA2 MD patients.1 Reinhard. S. Lin, K. McKee, S.Meinen, M. Sury, P. Yurchenco, M. Rüegg. Effect of linker protein expression on LAMA2-related muscular dystrophy (MDC1A). Neuromuscular Disorders (2019), 29, S164.