DAG1 and muscular dystrophy: O-mannosylation of α-dystroglycan is required for its proper function and when disrupted is a significant cause of a subset of congenital muscular dystrophies referred to as secondary dystroglycanopathies (Barresi and Campbell, 2006; Freeze, 2007; Muntoni et al., 2011; Mercuri and Muntoni, 2012; Dobson et al., 2013; Wells, 2013; Praissman and Wells, 2014).