Identification and functional analysis of a defect in the human ALG9 gene: definition of congenital disorder of glycosylation type IL

Comparative analysis of Saccharomyces cerevisiae WW domains and their interacting proteins

BACKGROUND

The WW domain is found in a large number of eukaryotic proteins implicated in a variety of cellular processes. WW domains bind proline-rich protein and peptide ligands, but the protein interaction partners of many WW domain-containing proteins in Saccharomyces cerevisiae are largely unknown.

RESULTS

We used protein microarray technology to generate a protein interaction map for 12 of the 13 WW domains present in proteins of the yeast S. cerevisiae. We observed 587 interactions between these 12 domains and 207 proteins, most of which have not previously been described. We analyzed the representation of functional annotations within the network, identifying enrichments for proteins with peroxisomal localization, as well as for proteins involved in protein turnover and cofactor biosynthesis. We compared orthologs of the interacting proteins to identify conserved motifs known to mediate WW domain interactions, and found substantial evidence for the structural conservation of such binding motifs throughout the yeast lineages. The comparative approach also revealed that several of the WW domain-containing proteins themselves have evolutionarily conserved WW domain binding sites, suggesting a functional role for inter- or intramolecular association between proteins that harbor WW domains. On the basis of these results, we propose a model for the tuning of interactions between WW domains and their protein interaction partners.

CONCLUSION

Protein microarrays provide an appealing alternative to existing techniques for the construction of protein interaction networks. Here we built a network composed of WW domain-protein interactions that illuminates novel features of WW domain-containing proteins and their protein interaction partners.

Altered glycan structures: the molecular basis of congenital disorders of glycosylation

Congenital disorders of glycosylation (CDG) are a group of diseases that affect glycoprotein biogenesis. Eighteen different types of CDG have been defined genetically. They result from deficiencies in either the biosynthesis of oligosaccharide precursors or specific steps of N-glycan assembly, resulting in the absence or structural alteration of N-glycan chains. These diseases have a broad range of clinical phenotypes and affect nearly every organ system, with special emphasis on normal brain development and the multiple functions of the nervous, hepatic, gastrointestinal and immune systems. Although most of the deficiencies observed in CDG patients are only partial, the severity of the clinical manifestations signifies the relevance of protein N-glycosylation and shows the importance of defined glycan structures.

An evolving view of the eukaryotic oligosaccharyltransferase

Asparagine-linked glycosylation (ALG) is one of the most common protein modification reactions in eukaryotic cells, as many proteins that are translocated across or integrated into the rough endoplasmic reticulum (RER) carry N-linked oligosaccharides. Although the primary focus of this review will be the structure and function of the eukaryotic oligosaccharyltransferase (OST), key findings provided by the analysis of the archaebacterial and eubacterial OST homologues will be reviewed, particularly those that provide insight into the recognition of donor and acceptor substrates. Selection of the fully assembled donor substrate will be considered in the context of the family of human diseases known as congenital disorders of glycosylation (CDG). The yeast and vertebrate OST are surprisingly complex hetero-oligomeric proteins consisting of seven or eight subunits (Ost1p, Ost2p, Ost3p/Ost6p, Ost4p, Ost5p, Stt3p, Wbp1p, and Swp1p in yeast; ribophorin I, DAD1, N33/IAP, OST4, STT3A/STT3B, Ost48, and ribophorin II in mammals). Recent findings from several laboratories have provided overwhelming evidence that the STT3 subunit is critical for catalytic activity. Here, we will consider the evolution and assembly of the eukaryotic OST in light of recent genomic evidence concerning the subunit composition of the enzyme in diverse eukaryotes.

CDG-IL: an infant with a novel mutation in the ALG9 gene and additional phenotypic features

We describe the second case of congenital disorder of glycosylation type IL (CDG-IL) caused by deficiency of the ALG9 a1,2 mannosyltransferase enzyme. The female infant's features included psychomotor retardation, seizures, hypotonia, diffuse brain atrophy with delayed myelination, failure to thrive, pericardial effusion, cystic renal disease, hepatosplenomegaly, esotropia, and inverted nipples. Lipodystrophy and dysmorphic facial features were absent. Magnetic resonance imaging of the brain showed volume loss in the cerebral hemispheres and cerebellum and delayed myelination. Laboratory investigations revealed low levels of multiple serum proteins including antithrombin III, factor XI, and cholesterol. Hypoglycosylation was confirmed by the typical CDG type 1 pattern of serum transferrin analyzed by isoelectric focusing. A defect in the ALG9 enzyme was suggested by the accumulation of the DolPP-GlcNAc2Man6 and DolPP-GlcNAc2Man8 in the patient's fibroblasts and confirmed by mutation analysis: the patient is homozygous for the ALG9 mutation p.Y286C. The causal effect of the mutation was shown by complementation assays in alg9 deficient yeast cells. The child described here further delineates the clinical spectrum of CDG-IL and confirms the significant clinical overlap amongst CDG subtypes.

Metabolic mimics: the disorders of N-linked glycosylation

N-linked glycosylation is essential for normal cellular function. Defects have now been described in eighteen genes that participate in the process. All give rise to complex multisystem diseases which, with a few exceptions, primarily involve the nervous system. Frequent features of these disorders include developmental delay, ataxia, seizures, stroke-like episodes, recurrent infections, coagulopathy and dysmorphism. Most cases can be detected by screening carbohydrate-deficient transferrin, but definitive diagnosis requires enzymatic and molecular confirmation, frequently in collaboration with a research glycobiologist.

Patients with unsolved congenital disorders of glycosylation type II can be subdivided in six distinct biochemical groups

Defects in the biosynthesis of N- and core 1 O-glycans may be found by isoelectric focusing (IEF) of plasma transferrin and apolipoprotein C-III (apoC-III). We hypothesized that IEF of transferrin and apoC-III in combination with sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) of apoC-III may provide a classification for congenital disorders of glycosylation (CDG) patients. We analyzed plasma from 22 patients with eight different and well-characterized CDG subtypes and 19 cases with unsolved CDG. Transferrin IEF (TIEF) has been used to distinguish between N-glycan assembly (type 1 profile) and processing (type 2 profile) defects. We differentiated two different CDG type 2 TIEF profiles: The "asialo profile" characterized by elevated levels of asialo- and monosialotransferrin and the "disialo profile" characterized by increased levels of disialo- and trisialotransferrin. ApoC-III IEF gave two abnormal profiles ("apoC-III(0)" and "apoC-III(1)" profiles). The results for the eight established CDG forms exactly matched the theoretical expectations, providing a validation for the study approach. The combination of the three electrophoretic techniques was not additionally informative for the CDG-Ix patients as they had normal apoC-III IEF patterns. However, the CDG-IIx patients could be further subdivided into six biochemical subgroups. The robustness of the methodology was supported by the fact that three patients with similar clinical features ended in the same subgroup and that another patient, classified in the "CDG-IIe subgroup," turned out to have a similar defect. Dividing the CDG-IIx patients in six subgroups narrows down drastically the options of the primary defect in each of the subgroups and will be helpful to define new CDG type II defects.

ALG9 mannosyltransferase is involved in two different steps of lipid-linked oligosaccharide biosynthesis

N-linked protein glycosylation follows a conserved pathway in eukaryotic cells. The assembly of the lipid-linked core oligosaccharide Glc3Man9GlcNAc2, the substrate for the oligosaccharyltransferase (OST), is catalyzed by different glycosyltransferases located at the membrane of the endoplasmic reticulum (ER). The substrate specificity of the different glycosyltransferase guarantees the ordered assembly of the branched oligosaccharide and ensures that only completely assembled oligosaccharide is transferred to protein. The glycosyltransferases involved in this pathway are highly specific, catalyzing the addition of one single hexose unit to the lipid-linked oligosaccharide (LLO). Here, we show that the dolichylphosphomannose-dependent ALG9 mannosyltransferase is the exception from this rule and is required for the addition of two different alpha-1,2-linked mannose residues to the LLO. This report completes the list of lumen-oriented glycosyltransferases required for the assembly of the LLO.

A combined defect in the biosynthesis of N- and O-glycans in patients with cutis laxa and neurological involvement: the biochemical characteristics

Based on our preliminary observation of abnormal glycosylation in a cutis laxa patient, nine cutis laxa patients were analyzed for congenital defects of glycosylation (CDG). Isoelectric focusing of plasma transferrin and apolipoproteinC-III showed that three out of nine patients had a defect in the biosynthesis of N-glycans and core 1 mucin type O-glycans, respectively. Mass spectrometric N-glycan analyses revealed a relative increase of glycans lacking sialic acid and glycans lacking sialic acid and galactose residues. Mutation analysis of the fibulin-5 gene (FBLN5), which has been reported in cases of autosomal recessive cutis laxa, revealed no mutations in the patients' DNA. Evidence is presented that extracellular matrix (ECM) proteins of skin are likely to be highly glycosylated with N- and/or mucin type O-glycans by using algorithms for predicting glycosylation. The conclusions in this study were that the clinical phenotype of autosomal recessive cutis laxa seen in three patients is not caused by mutations in the FBLN5 gene. Our findings define a novel form of CDG with cutis laxa and neurological involvement due to a defect in the sialylation and/or galactosylation of N- and O-glycans. Improper glycosylation of ECM proteins of skin may form the pathophysiological basis for the cutis laxa phenotype.

Analysis of glycosylation in CDG-Ia fibroblasts by fluorophore-assisted carbohydrate electrophoresis: implications for extracellular glucose and intracellular mannose 6-phosphate

Phosphomannomutase (PMM) deficiency causes congenital disorder of glycosylation (CDG)-Ia, a broad spectrum disorder with developmental and neurological abnormalities. PMM converts mannose 6-phosphate (M6P) to mannose-1-phosphate, a precursor of GDP-mannose used to make Glc(3)Man(9)GlcNAc(2)-P-P-dolichol (lipid-linked oligosaccharide; LLO). LLO, in turn, is the donor substrate of oligosaccharyltransferase for protein N-linked glycosylation. Hepatically produced N-linked glycoproteins in CDG-Ia blood are hypoglycosylated. Upon labeling with [(3)H]mannose, CDG-Ia fibroblasts have been widely reported to accumulate [(3)H]LLO intermediates. Since these are thought to be poor oligosaccharyltransferase substrates, LLO intermediate accumulation has been the prevailing explanation for hypoglycosylation in patients. However, this is discordant with sporadic reports of specific glycoproteins (detected with antibodies) from CDG-Ia fibroblasts being fully glycosylated. Here, fluorophore-assisted carbohydrate electrophoresis (FACE, a nonradioactive technique) was used to analyze steady-state LLO compositions in CDG-Ia fibroblasts. FACE revealed that low glucose conditions accounted for previous observations of accumulated [(3)H]LLO intermediates. Additional FACE experiments demonstrated abundant Glc(3)Man(9)GlcNAc(2)-P-P-dolichol, without hypoglycosylation, CDG-Ia fibroblasts grown with physiological glucose. This suggested a "missing link" to explain hypoglycosylation in CDG-Ia patients. Because of the possibility of its accumulation, the effects of M6P on glycosylation were explored in vitro. Surprisingly, M6P was a specific activator for cleavage of Glc(3)Man(9)GlcNAc(2)-P-P-dolichol. This led to futile cycling the LLO pathway, exacerbated by GDP-mannose/PMM deficiency. The possibilities that M6P may accumulate in hepatocytes and that M6P-stimulated LLO cleavage may account for both hypoglycosylation and the clinical failure of dietary mannose therapy with CDG-Ia patients are discussed.

Clinical and biochemical presentation of siblings with COG-7 deficiency, a lethal multiple O- and N-glycosylation disorder

Congenital disorders of glycosylation (CDG) represent a group of inherited multiorgan diseases caused by defects in the biosynthesis of glycoproteins. We report on two dysmorphic siblings with severe liver disease who died at the age of a few weeks. Increased activities of lysosomal enzymes in plasma were found, though total sialic acid in plasma was strongly decreased. Isoelectric focusing of serum sialotransferrins showed a type 2-like CDG pattern. Some of the known CDG subtypes were excluded. O-Glycosylation was investigated by isoelectric focusing of apolipoprotein C-III, which showed increased fractions of hyposialylated isoforms. In a consecutive study a defect in the conserved oligomeric Golgi complex was established at the level of subunit COG-7, leading to disruption of multiple glycosylation functions of the Golgi. This report on patients with a new variant of CDG, due to a multiple Golgi defect, emphasizes in addition to sialotransferrins the importance of analysis of a serum O-linked glycoprotein, e.g. apolipoprotein C-III, in unclassified CDG-X cases.