Mutations in B3GALNT2 Cause Congenital Muscular Dystrophy and Hypoglycosylation of α-Dystroglycan

Abstract

Mutations in several known or putative glycosyltransferases cause glycosylation defects in α-dystroglycan (α-DG), an integral component of the dystrophin glycoprotein complex. The hypoglycosylation reduces the ability of α-DG to bind laminin and other extracellular matrix ligands and is responsible for the pathogenesis of an inherited subset of muscular dystrophies known as the dystroglycanopathies. By exome and Sanger sequencing we identified two individuals affected by a dystroglycanopathy with mutations in β-1,3-N-acetylgalactosaminyltransferase 2 (B3GALNT2). B3GALNT2 transfers N-acetyl galactosamine (GalNAc) in a β-1,3 linkage to N-acetyl glucosamine (GlcNAc). A subsequent study of a separate cohort of individuals identified recessive mutations in four additional cases that were all affected by dystroglycanopathy with structural brain involvement. We show that functional dystroglycan glycosylation was reduced in the fibroblasts and muscle (when available) of these individuals via flow cytometry, immunoblotting, and immunocytochemistry. B3GALNT2 localized to the endoplasmic reticulum, and this localization was perturbed by some of the missense mutations identified. Moreover, knockdown of b3galnt2 in zebrafish recapitulated the human congenital muscular dystrophy phenotype with reduced motility, brain abnormalities, and disordered muscle fibers with evidence of damage to both the myosepta and the sarcolemma. Functional dystroglycan glycosylation was also reduced in the b3galnt2 knockdown zebrafish embryos. Together these results demonstrate a role for B3GALNT2 in the glycosylation of α-DG and show that B3GALNT2 mutations can cause dystroglycanopathy with muscle and brain involvement.

B3GALNT2 is a gene associated with congenital muscular dystrophy with brain malformations

“…However, many patients with dystroglycanopathy still remain genetically unidentified. 5 Stevens et al 6 recently reported that mutations in the gene b-1,3-N-acetylgalactosaminyltransferase 2 (B3GALNT2; MIM 610194) cause congenital muscular dystrophy and hypoglycosylation of a-dystroglycan with, in most cases, severe brain involvement.…”

A POGLUT1 mutation causes a muscular dystrophy with reduced notch signaling and satellite cell loss

“…However, the agrinbinding activity to the patients' muscle extracts showed no difference compared with controls ( Fig 2B). Moreover, in skin fibroblasts from patients, the level of both functional a-dystroglycan glycosylation, examined by Western blot and flow cytometry (Stevens et al, 2013b), and its ability to bind laminin ( Fig 2B and Appendix Fig S5B) were normal, unlike known secondary dystroglycanopathies, which usually result in decreased functional a-dystroglycan glycosylation in both muscle and the skin fibroblasts (Willer et al, 2012;Carss et al, 2013;Stevens et al, 2013a). Finally, although basement membrane defects are commonly observed in dystroglycanopathies (Yamamoto et al, 1997;Goddeeris et al, 2013), transmission electron microscopy showed normal muscle ultrastructure in patients, with no alterations in basement membrane compaction (Fig 2C and Appendix Fig S5C).…”

Genetics and mechanisms leading to human cortical malformations

“…These genes include POMT1, POMT2, POMGNT1, FKTN, FKRP, LARGE, ISPD, GTDC2, TMEM5, POMK, B4GAT1, B3GALNT2 (see Table 4) [212,213,[215][216][217][218][219][220][221][222][223][224][225]. Other mutations are rare, described in only a few patients with WWS, and include genes encoding BM constituents, such as LAMB1, LAMC3 and COL4A1, and the transmembrane and tetratricopeptide repeat containing 3 gene, TMTC3 [215,218,226], coding for a protein for which its link is currently unclear.…”