Walker-Warburg syndrome (WWS) is an autosomal recessive developmental disorder characterized by congenital muscular dystrophy, brain malformation, and structural eye abnormalities. WWS is due to defects in protein O-mannosyltransferase 1 (POMT1), which catalyzes the transfer of mannose to protein to form O-mannosyl glycans. POMT1 has been shown to require co-expression of another homologue, POMT2, to have activity. In the present study, mutations in POMT1 genes observed in patients with WWS were duplicated by site-directed mutagenesis. The mutant genes were co-expressed with POMT2 in Sf9 cells and assayed for protein O-mannosyltransferase activity. Expression of all mutant proteins was confirmed by Western blot, but the recombinant proteins did not show any protein O-mannosyltransferase activity. The results indicate that mutations in the POMT1 gene result in a defect of protein O-mannosylation in WWS patients. This may cause failure of binding between alpha-dystroglycan and laminin or other molecules in the extracellular matrix and interrupt normal muscular function and migration of neurons in developing brain.
"The mutated forms of the POMT1 (protein-O-mannosyltransferase 1), POMT2 (protein-O-mannosyltransferase 2), POMGnT1 (protein O-mannose N-acetylglucosaminyltransferase 1), FKTN (fukutin), FKRP (fukutin-related protein) and LARGE (like-acetylglucosaminyltransferase) genes were originally reported in patients with distinct phenotypes [18–22], and therefore defects of those glycosyltransferases were initially associated with different clinical conditions. In recent years, a wider spectrum of phenotypes with different severities have been reported for mutations in the POMT1 [23,24], POMT2 [25,26] and the FKTN gene [27,28], whereas mutations affecting FKRP [29,30], POMGnT1 [31–33] and LARGE [34,35] have been described to have less phenotypical variation. However, a study of 81 CMD patients from Italy could only detect 43 homozygous or heterocompound mutations (53%) in those six genes , which allows the presumption that even more, as yet unidentified, genes are involved in the post-translational modification of α-DG. "
[Show abstract][Hide abstract] ABSTRACT: Congenital muscular dystrophies have a broad spectrum of genotypes and phenotypes and there is a need for a better biochemical understanding of this group of diseases in order to aid diagnosis and treatment. Several mutations resulting in these diseases cause reduced O-mannosyl glycosylation of glycoproteins, including α-dystroglycan. The enzyme POMGnT1 (protein-O-mannose N-acetylglucosaminyltransferase 1; EC 2.4.1.-) catalyses the transfer of N-acetylglucosamine to O-linked mannose of α-dystroglycan. In the present paper we describe the biochemical characterization of 14 clinical mutants of the glycosyltransferase POMGnT1, which have been linked to muscle-eye-brain disease or similar conditions. Truncated mutant variants of the human enzyme (recombinant POMGnT1) were expressed in Escherichia coli and screened for catalytic activity. We find that three mutants show some activity towards mannosylated peptide substrates mimicking α-dystroglycan; the residues affected by these mutants are predicted by homology modelling to be on the periphery of the POMGnT1 surface. Only in part does the location of a previously described mutated residue on the periphery of the protein structure correlate with a less severe disease mutant.
"Three POMT1 mutations that are causative for WWS, including two missense mutations G76R and V428D, have been demonstrated to result in loss of enzymatic activity in cellular expression assays . However, several mutations leading to milder forms of CMD or limb girdle muscular dystrophy (G65R, A200P, and W582C) also show complete loss of enzymatic activity in this cellular assay  . "
[Show abstract][Hide abstract] ABSTRACT: The dystroglycanopathies are a group of inherited muscular dystrophies that have a common underlying mechanism, hypoglycosylation of the extracellular receptor α-dystroglycan. Many of these disorders are also associated with defects in the central nervous system and the eye. Defects in α-dystroglycan may also play a role in cancer progression. This review discusses the six dystroglycanopathy genes identified so far, their known or proposed roles in dystroglycan glycosylation and their relevance to human disease, and some of animal models now available for the study of the dystroglycanopathies.
Biochimica et Biophysica Acta (BBA) - Molecular Basis of Disease 09/2009; 1792(9-1792):853-861. DOI:10.1016/j.bbadis.2009.06.003 · 4.88 Impact Factor
"This finding suggests that α-DG is a potential target of POMT1 and POMT2 and that hypoglycosylation of α-DG may be a pathomechanism of WWS. In fact, POMT1 mutations found in WWS patients led to a defect of protein O-mannosyltransferase activity even when the defective POMT1 was coexpressed with wild-type POMT2 (Akasaka-Manya et al., 2004). In Drosophila, functional dPOMT1 and dPOMT2 are required for normal muscle development (Ichimiya et al., 2004). "
[Show abstract][Hide abstract] ABSTRACT: Mammalian O-mannosylation, although an uncommon type of protein modification, is essential for normal brain and muscle development. Defective O-mannosylation causes congenital muscular dystrophy with abnormal neuronal migration [Walker-Warburg syndrome (WWS)]. Here, we have identified and cloned rat Pomt1 and Pomt2, which are homologues of human POMT1 and POMT2, with identities of 86 and 90%, respectively, at the amino acid level. Coexpression of both genes was found to be necessary for enzymatic activity, as is the case with human POMT1 and POMT2. Northern blot and reverse transcriptase polymerase chain reaction (RT-PCR) analyses revealed that rat Pomt1 and Pomt2 are expressed in all tissues but most strongly in testis. In situ hybridization histochemistry of rat brain revealed that Pomt1 and Pomt2 mRNA are coexpressed in neurons (dentate gyrus and CA1-CA3 region of the hippocampus and cerebellar Purkinje cells). Two transcription-initiation sites were observed in rat Pomt2, resulting in two forms: a testis form and a somatic form. The two forms had equal protein O-mannosyltransferase activity when coexpressed with rat Pomt1. Coexpression studies also showed that the human and rat protein O-mannosyltransferases are interchangeable, providing further evidence for the closeness of their structures.
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