O-glycan assembly starts in the Golgi, and the initiating event is the addition of  a monosaccharide to serine or threonine residues. This monosaccharide can be GalNAc (in case of a mucin type O-glycan), but it can also be fucose, galactose, glucose, mannose, GlcNAc and xylose.  Eight core structures of mucin type O-glycans can be distinguished depending on the following monosaccharide(s) and/or its binding. These core structures are further elongated or modified by a.o. acetylation, fucosylation, polylactosamine extension, sialylation and sulfatation. O-glycans are less branched than most N-glycans.

Protein O-mannosyltransferase 1 deficiency (Walker Warburg syndrome, limb-girdle muscular dystrophy 2K, intermediate phenotype)

phenotype

Three different phenotypes have been associated with this defect. The first is Walker-Warburg syndrome, a rare genetic disorder with brain and eye dysgenesis associated with congenital muscular dystrophy. This neuronal migration disorder usually runs a fatal course before the age of one year. Psychomotor development is absent. The brain lesions consist of cobblestone lissencephaly, agenesis of the corpus callosum, cerebellar hypoplasia, hydrocephaly, and sometimes encephalocoele. In this disorder there is an aberrant glycosylation of alfa-dystroglycan, an external membrane protein expressed in muscle, brain and other tissues. Most glycans of this heavily glycosylated protein seem to be O- linked via mannose and control the interaction with extracellular matrix proteins. Disrupted glycosylation of alfa-dystroglycan (and probably other glycoproteins) results in loss of this interaction and hence in progressive muscle degeneration and abnormal neuronal migration in the brain. The enzyme catalyses the first step in the synthesis of the O-mannose linked core Gal-beta1-4GlcNAc-beta1-2Man-O-Ser/Thr. This defect is only present in a subgroup (ca 20%) of patients with Walker-Warburg syndrome(4). Another much milder phenotype caused by this defect is limb-girdle muscular dystrophy with mental retardation and microcephaly, but without eye and brain abnormalities (3). The third phenotype shows a severity in between the two others: a congenital muscular dystrophy with early onset, severe motor disability, microcephaly, and mental retardation not associated with structural brain changes on neuroimaging (12).

genotype

references new review

Protein O-mannosyltransferase 2 deficiency (Walker-Warburg syndrome)

phenotype

Together with protein O-mannosyltransferase 1 deficiency, fukutin deficiency and fukutin related protein deficiency, this disorder accounts for about one third of patients with Walker-Warburg syndrome. Both POMT1 and POMT2 are required to achieve protein O-mannosyltranferase activity (65).

genotype

references new review

O-mannosyl-beta-1,2-N-acetylglucosaminyltransferase 1 deficiency (muscle-eye-brain disease)

phenotype

This is a neuronal migration/ congenital muscular dystrophy syndrome similar to Walker-Warburg syndrome, but less severe. The defective protien catalyses the second step in the synthesis of the O-mannosylglycan core (75).

genotype

references new review

phenotype

Hereditary familial tumoral calcinosis (HFTC) is characterized by hyperphosphatemia, heterotopic calcifications, unresponsiveness to PTH and increased renal tubular  phosphate reabsorbtion. The levels of 1,25 dihydroxy vitamin D are inapproriately normal or elevated. Dental abnormalities are often associated. The disease is allelic with hyperphosphatemia-hyperostosis syndrome, characterized by painfull swellings of the long bones and radiological evidence for periostal reactions and cortical hyperostosis.

genotype

the gene involved in this disease is GALNT3 (UDP-N-acetyl-alpha-D-galactosamine: polypeptide N-acetylgalactosaminyltransferase 3), a glycosyltransferase responsible for initiating mucin-type O-glycosylation. Mutations in GALNT3 were identified in at least eight families. Members of three families carried a homozyous splice site mutation (c.1524+1 G>A) disrupting a donor splice site consensus sequence of intron 7, leading to an in frame deletion of 44 amino acid residues (24,64). One family showed two compound heterozygous mutations: a nonsens mutation c. 484 C>T (R162X) in exon 1 and a splice site mutation c.1524+5 G>A in intron 7 (64). This nonsense mutation (R162X) was also found in combination with another splice site muation c. 515-2 A>T in intron 1 which disrupts a  consensus splice acceptor sit and which is predicted to lead to the skipping of exon 2 (b). Two families with different homozygous nonsense muations were described: c.1387 A>T (K463X) in exon 6 (a) and c.1774 C>T (Q592X) in exon9 (d). In a 25 year old patient with tumoral calcinosis two heterozygous missense mutations c.815 C>A (T272K) and c.1076 G>A (T359K) were identified (c). This brings the total to eight different mutations: 3 different splice site mutations (one in intron 1, two in intron7), three different nonsense mutations (R162X, K463X and Q592X) and two different missense mutations (T272K and T359K).

(24) Frishberg Y, Topaz O, Bergman R, Behar D, Fisher D, Gordon D, Richard G, Sprecher E. Identification of a recurrent mutation in GALNT3 demonstrates that hyperostosis-hyperphosphatemia syndrome and familial tumoral calcinosis are alllelic disorders. J Mol Med 2005;83: 33-38.

(64) Topaz O, Shurman DL, Bergman R, Indelman M, Ratajczak P, Mizrachi M, Khamaysi Z, Behar D, Petronius D, Friedman V, Zelikovic I, Raimer S, Metzker A, Richard G, Sprecher E. Mutations in GALNT3, encoding a protein involved in O-linked glycosylation, cause familial tumoral calcinosis. Nat Genet 2004; 36:579-581

a. Familial tumoral calcinosis and testicular microlithiasis associated with a  new mutation of GALNT3 in a white family. Campagnoli MF, Pucci A, Garelli E, Carando A, Defilippi C, Lala R, Ingrosso G, Dianzani I, Forni M and Ramenghi U. J Clin Pathol 2006; 59:440-442

b. A novel GALNT3 mutation in a pseudoautosomal dominant form of tumora calcinosis: evidence that this disorder is autosomal recessive. Ishikawa S, Lyles KW, Econs MJ. J Clin Endocrin Metab 2005; 90:2420-2423.

c. Tumoral calcinosis presenting with eyelid calcifications due to novel missense muations in the glycosyl transferase domain of the GALNT3 gene. Ishikawa S, Imel EA, Sorenson AH, Severe R, Knudson P, Harris GJ, Shaker JL, Econs MJ. J Clin Endocrin Metab  2006;91: 4472-4475.

d. Hyperphosphatemic familial tumoral calcinosis caused by a mutation in GALNT3 in a European kindred. Specktor P, Cooper JG, Indelman M, Sprecher E. Hum Genet 2006; 51: 487-490.

phenotype

The first patient which has been reported with this so-called progeroid variant of Ehlers-Danlos syndrome presented with aged appearance, developmental delay, dwarfism, psychomotor retardation, macrocephaly, hyperlaxity of the joints and loose, elastic skin. A defect was found in the common linkage region of glycosaminoglycans and, more specifically, in the attachmetn of the first galactose to xylose. Galactosyltransferase I activity was less than 5% of normal in the patient's fibroblasts(127).

genotype

The cDNA for the galactosyltransferase (XGALT-1 or B4GALT7) was only recently isolated by searching an EST database (113). The gene shows high homology (38%) to the sqv3 gene of Caenorhabditis elegans, and its product shows specific activity of galactosyltransferase on p-nitro-phenyl-beta-D-xylopyranoside with beta-1,4-linkage. The patient was compound heterozygous for A186D and L206P mutations in the gene. The former is a mild mutation, and the mutant protein retains approximatively 50% of activity. However, the latter is a severe mutation with no residual activity of the mutant protein (113).

old review references

phenotype

This defect has been identied in patients with the multiple exostoses syndrome, which has an autosomal dominant inheritance (158) (MIM133700,133701). More than 1000 patients have been reported with this syndrome. Its estimated prevalence is 1/50000. It is charactreized by diaphyseal juxtaepiphyseal cartilage capped outgrowths that cause deformity. They are often present at birth but usually not diagnosed until early childhood. Their growth slows at adolescence and stops in adulthood. They are also most prominent at the ends of long bones. There is a 3% incidence of sarcoma from these lesions. Complications may arise from compression of peripheral nerves and blood vessels (158). The basic defects is in the Golgi-localized EXT1/EXT2 complex that has glucuronyltransferase as well as N-acetyl-D-glucosaminyltransferase activities and that catalyses the polymerization of heparan sulfate (84,108).

genotype

The EXT1 and EXT2 loci were localized using linkage analysis in large families with the autosomal dominant hereditary multiple exostoses, and the genes were identified by positional cloning. EXT1 is located on chromosome 8q23-q24 and EXT2 is located on 11p11-p12. A third EXT locus, which is probably only involved in rare instances (EXT3), lies on chromosome 19p (162). The availability of the genes did not lead investigators to the identification of the biological function because the sequence did not show any recognized functional domains or structural motifs. Recent functional data revealed that both proteins are in fact glycosyltranferases, which ware involved in heparan sulfate biosynthesis (94,108). EXT1 and EXT2 encode homologous proteins of 746 and 718 amino acids, respectively (4,134,161). The overal identity is 31%. These genes are large: EXT1 contains 11 exons and spans more than 250 kb; EXT2 contains 16 exons and spans more than 100 kb (30,96). The drosophila homologue of EXT1 is the tout-velu gene, which has a role in hedgehog (hh) signaling (12). Indian hh is one of the mammalian homologues of hh, and it regulates cartilage differentiation. The current model for EXT involvement in the development of exostoses states that the mutation in the glycosyltransferase(s) impairs the synthesis of a glycosaminoglycan that normally regulates the diffusion of Indian hh and thus disrupts the negative feedback loop of chondrocyte differentiation. Interestingly, both the EXT1 and EXT2 regions show loss of heterozygosity in EXT-related and non-EXT-related chondrosarcomas (18). Almost 50 different EXT1 and 25 EXT2 mutations have been reported (162). In the case of EXT1, 73% of all mutations cause an interruption of the ORF and premature termination of the EXT1 protein. There are nonsense and splice site mutations as well as small deletions and insertions. The other mutations are missense mutations or in-frame deletions and insertions. Only one mutation has been observed in more thatn one family, and it may be a recurrent mutation. For EXT2, a comparable mutation spectrum has been observed, also with a majority of nonsense, splice, and frameshift mutations as well as only a few known missense mutations and no recurrent mutations.