A primary dystroglycanopathy causing muscle-eye-brain disease with multicystic leukodystrophy: from cellular and biochemical analysis to a mouse model

The dystroglycan (DG) adhesion complex has a crucial role for muscle stability and is involved in a number of autosomal recessive neuromuscular disorders ranging from severe congenital (Muscle-Eye-Brain and Walker-Warburg syndrome) to milder limb-girdle muscular dystrophies (LGMD2P). The complex undergoes a still poorly understood molecular maturation/targeting pathway, in which a precursor molecule undergoes an early proteolytic event in the endoplasmic reticulum and is cleaved into two subunits, alpha (extracellular) and beta (transmembrane); this is followed by a multistep glycosylation process which mainly takes place within the Golgi apparatus. Most of the currently identified dystroglycanopathies depend on an impaired sugar decoration of the alpha-dystroglycan subunit that, when hypoglycosylated, shows a reduced binding affinity towards laminin-2 (dubbed as secondary dystroglycanopathies). However, lately a novel subgroup of primary dystroglycanopathies, depending on missense mutations of the dystroglycan core protein, is emerging. We have established an array of diverse tools for the analysis of primary dystroglycanopathies. Namely, the production of novel mice lines, the analysis of transfected cellular lines and the recombinant expression of isolated domains for biochemical and crystallization analysis, as well as computational biochemistry. Our cellular, microscopic and biochemical analysis of the murine counterpart (C667F) of the recently identified missense mutation C669F (inducing a severe muscle-eye-brain phenotype) located within the ectodomain of human beta-DG has allowed us to show that i) the processing of the alpha-beta DG precursor is altered; ii) the variant protein is mainly stuck in the ER and iii) there is some degree of oligomerization and/or structural organization within these C667F aggregates. Following the award of an AFM grant to study primary dystroglycanopathies, we have strongly invested into the generation of a first mouse model. The company GenOway is currently taking care of preparing the knocked-in gametes which will be employed at Bonn University for generating the desired heterozygous and homozygous mouse colony that will be used for morphologic and histologic analysis in the brain and muscle.