| Summary: | The aberrant glycosylation of α-dystroglycan is associated with a subset of clinically heterogeneous muscular dystrophies collectively referred to as dystroglycanopathies. Mutations in seven genes are currently known to result in these autosomal recessive disorders. To define the mutation frequency and clinical phenotypes associated with mutations in five of these genes, I analysed a large cohort of patients. This study redefined the clinical spectrum associated with mutations in POMT1, POMT2, POMGNT1, FKTN and LARGE. Mutations in these genes were detected in approximately a third of all cases indicating that, after the exclusion of FKRP involvement, the majority of patients with a dystroglycanopathy harbour mutations in novel genes. In order to identify novel genes in these disorders I applied a candidate gene approach to the whole genome. Genes known to be causative in dystroglycanopathy animal models and those predicted to function within the pathway of dystroglycan glycosylation were investigated. Mutation screening indicated that defects in GYLTL1B, β3GNT1, WWP1, GYG1, GYG2, MGAT5B and DAG1 are not frequent causes of human dystroglycanopathy. In order to evaluate the function of a subset of candidate genes, I optimised a cell culture model of dystroglycanopathy. Using a ribonucleotide interference based strategy, the expression of candidate genes were modulated in a mouse myogenic cell line. The down regulation of β3gnt1 induced a marked reduction in the glyco-epitope present on α-dystroglycan. This finding confirms β3gnt1 as a key enzyme functioning in the pathway of α-dystroglycan glycosylation. In addition, I combined the use of high density single nucleotide polymorphism genotyping platforms, expression microarrays and sequencing data to investigate the cause of disease in a small consanguineous pedigree. In summary, this data set has refined genotype-phenotype correlations of known causative genes as well as investigated the involvement of novel loci in muscular dystrophies with defective glycosylation of dystroglycan.
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