Dystroglycanopathy via the DAG1 Gene

Dystroglycan is the central component of the muscle dystrophin-glycoprotein complex ([DGC] Ervasti et al. Nature 345:315-319, 1990, Michele and Campbell. J Biol Chem 278:15457-15460, 2003). Beta-dystroglycan spans the sarcolemma, binding dystrophin intracellulary and alpha-dystroglycan extracellularly. Alpha-dystroglycan is connected to the basal lamina by way of linkage to laminin (Ibraghimov-Beskrovnaya et al. Nature 355:696-702, 1992). Alpha-dystroglycan binds additional proteins with laminin G-like domains such as agrin and perlecan in muscle and neurexin in brain (Sugita et al. J Cell Biol 154:435-445, 2001). The gene DAG1 (OMIM 128239) encodes one precursor peptide that is cleaved post-translationally to form the alpha and beta subunits of dystroglycan. Both subunits undergo further modification by N- and O-linked glycosylation; however, alpha-dystroglycan undergoes extensive O-linked glycosylation (Ibraghimov-Beskrovnaya et al. Hum Mol Genet 2:1651-1657, 1993). The glycosylation status of dystroglycan is critical for ligand binding as well as for pathogenesis (Michele et al. Nature 418:417-422, 2002; Muntoni et al. Curr Opin Neurol 17:205-209, 2004; Barresi and Campbell. J Cell Sci 119:199-207, 2006).

Alpha- and beta-dystroglycan are encoded by the DAG1 gene located on chr 3p21. The 895 residue precursor peptide is directed to the endoplasmic reticulum via a 29 amino acid N-terminal signal peptide. Cleavage of the precursor peptide occurs at p.Ser654 to yield core alpha- and beta-dystroglycan proteins. A single transmembrane domain (residues 751-774) directs beta-dystroglycan to span the sarcolemma. Interaction between dystrophin and beta-dystroglycan occurs at a PPXY motif (p.Pro828_p.Tyr831) at the C-terminus of beta dystroglycan. The predicted mass of alpha-dystroglycan core protein and the corresponding protein isolated from tissue differ because of extensive species- and tissue-specific O-linked glycosylation of the central mucin domain between residues 312 and 485 (Barresi and Campbell, 2006). Studies in mouse demonstrate that constitutional DAG1 variants are lethal in the early embryonic period secondary to disruption of Reichert’s membrane (Williamson et al. Hum Mol Genet 6:831-841, 1997; Henry and Campbell Cell 95:859-870, 1998). Transgenic animals in which expression of the knockout DAG1 gene construct is limited to the brain show structural malformations like those seen in patients with congenital muscular dystrophies, as well as loss of high affinity binding to laminin (Moore et al. Nature 418:422-425). One patient with limb girdle muscular dystrophy and severe cognitive impairment was found to be homozygous for a DAG1 variant (p.Thr192Met, Hara et al. New Eng J Med 364: 939-946, 2011).

At this time there is no known clinical utility for this test. Analytical and clinical sensitivity are unknown because there are no reported genotype-phenotype correlations for dystroglycan variants in humans. Findings generated from this test should not alone be considered as diagnostic.

This test provides full coverage of all coding exons of the DAG1 gene plus 10 bases of flanking noncoding DNA in all available transcripts along with other non-coding regions in which pathogenic variants have been identified at PreventionGenetics or reported elsewhere. We define full coverage as >20X NGS reads or Sanger sequencing. PGnome panels typically provide slightly increased coverage over the PGxome equivalent. PGnome sequencing panels have the added benefit of additional analysis and reporting of deep intronic regions (where applicable).

Dependent on the sequencing backbone selected for this testing, discounted reflex testing to any other similar backbone-based test is available (i.e., PGxome panel to whole PGxome; PGnome panel to whole PGnome).