PMM (PMM1), the human homologue of SEC53 or yeast phosphomannomutase, is localized on chromosome 22q13

Enzymatic and structural characterization of HAD5, an essential phosphomannomutase of malaria-causing parasites

The malaria-causing parasite Plasmodium falciparum is responsible for over 200 million infections and 400,000 deaths per year. At multiple stages during its complex life cycle, P. falciparum expresses several essential proteins tethered to its surface by glycosylphosphatidylinositol (GPI) anchors, which are critical for biological processes such as parasite egress and reinvasion of host red blood cells. Targeting this pathway therapeutically has the potential to broadly impact parasite development across several life stages. Here, we characterize an upstream component of parasite GPI anchor biosynthesis, the putative phosphomannomutase (PMM) (EC 5.4.2.8), HAD5 (PF3D7_1017400). We confirmed the PMM and phosphoglucomutase activities of purified recombinant HAD5 by developing novel linked enzyme biochemical assays. By regulating the expression of HAD5 in transgenic parasites with a TetR-DOZI-inducible knockdown system, we demonstrated that HAD5 is required for malaria parasite egress and erythrocyte reinvasion, and we assessed the role of HAD5 in GPI anchor synthesis by autoradiography of radiolabeled glucosamine and thin layer chromatography. Finally, we determined the three-dimensional X-ray crystal structure of HAD5 and identified a substrate analog that specifically inhibits HAD5 compared to orthologous human PMMs in a time-dependent manner. These findings demonstrate that the GPI anchor biosynthesis pathway is exceptionally sensitive to inhibition in parasites and that HAD5 has potential as a specific, multistage antimalarial target.

A mutant of phosphomannomutase1 retains full enzymatic activity, but is not activated by IMP: Possible implications for the disease PMM2-CDG

The most frequent disorder of glycosylation, PMM2-CDG, is caused by a deficiency of phosphomannomutase activity. In humans two paralogous enzymes exist, both of them require mannose 1,6-bis-phosphate or glucose 1,6-bis-phosphate as activators, but only phospho-mannomutase1 hydrolyzes bis-phosphate hexoses. Mutations in the gene encoding phosphomannomutase2 are responsible for PMM2-CDG. Although not directly causative of the disease, the role of the paralogous enzyme in the disease should be clarified. Phosphomannomutase1 could have a beneficial effect, contributing to mannose 6-phosphate isomerization, or a detrimental effect, hydrolyzing the bis-phosphate hexose activator. A pivotal role in regulating mannose-1phosphate production and ultimately protein glycosylation might be played by inosine monophosphate that enhances the phosphatase activity of phosphomannomutase1. In this paper we analyzed human phosphomannomutases by conventional enzymatic assays as well as by novel techniques such as 31P-NMR and thermal shift assay. We characterized a triple mutant of phospomannomutase1 that retains mutase and phosphatase activity, but is unable to bind inosine monophosphate.

Molecular cloning and functional analysis of the phosphomannomutase (PMM) gene from Dendrobium officinale and evidence for the involvement of an abiotic stress response during germination

Phosphomannomutase (PMM, EC 5.4.2.8) catalyzes the interconversion of mannose-6-phosphate to mannose-1-phosphate, the precursor for the synthesis of GDP-mannose. In this study, the complementary DNA (cDNA) of the Phosphomannomutase (PMM) gene was initially cloned from Dendrobium officinale by RACE method. Transient transform result showed that the DoPMM protein was localized in the cytoplasm. The DoPMM gene was highly expressed in the stems of D. officinale both in vegetative and reproductive developmental stages. The putative promoter was cloned by TAIL-PCR and used for searched cis-elements. Stress-related cis-elements like ABRE, TCA-element, and MBS were found in the promoter regions. The DoPMM gene was up-regulated after treatment with abscisic acid, salicylic acid, cold, polyethylene glycol, and NaCl. The total ascorbic acid (AsA) and polysaccharide content in all of the 35S::DoPMM Arabidopsis thaliana transgenic lines #1, #2, and #5 showed a 40, 39, and 31% increase in AsA and a 77, 22, and 39% increase in polysaccharides, respectively more than wild-type (WT) levels. All three 35S::DoPMM transgenic lines exhibited a higher germination percentage than WT plants when seeded on half-strength MS medium supplemented with 150 mM NaCl or 300 mM mannitol. These results provide genetic evidence for the involvement of PMM genes in the biosynthesis of AsA and polysaccharides and the mediation of PMM genes in abiotic stress tolerance during seed germination in A. thaliana.

Insufficient ER-stress response causes selective mouse cerebellar granule cell degeneration resembling that seen in congenital disorders of glycosylation

BACKGROUND

Congenital disorders of glycosylation (CDGs) are inherited diseases caused by glycosylation defects. Incorrectly glycosylated proteins induce protein misfolding and endoplasmic reticulum (ER) stress. The most common form of CDG, PMM2-CDG, is caused by deficiency in the cytosolic enzyme phosphomannomutase 2 (PMM2). Patients with PMM2-CDG exhibit a significantly reduced number of cerebellar Purkinje cells and granule cells. The molecular mechanism underlying the specific cerebellar neurodegeneration in PMM2-CDG, however, remains elusive.

RESULTS

Herein, we report that cerebellar granule cells (CGCs) are more sensitive to tunicamycin (TM)-induced inhibition of total N-glycan synthesis than cortical neurons (CNs). When glycan synthesis was inhibited to a comparable degree, CGCs exhibited more cell death than CNs. Furthermore, downregulation of PMM2 caused more CGCs to die than CNs. Importantly, we found that upon PMM2 downregulation or TM treatment, ER-stress response proteins were elevated less significantly in CGCs than in CNs, with the GRP78/BiP level showing the most significant difference. We further demonstrate that overexpression of GRP78/BiP rescues the death of CGCs resulting from either TM-treatment or PMM2 downregulation.

CONCLUSIONS

Our results indicate that the selective susceptibility of cerebellar neurons to N-glycosylation defects is due to these neurons' inefficient response to ER stress, providing important insight into the mechanisms of selective neurodegeneration observed in CDG patients.

Report of interstitial 22q13.1q13.2 microduplication in two siblings with distinctive dysmorphic features, heart defect and mental retardation

We present two siblings (a boy and a girl) with a submicroscopic 4 Mb duplication at 22q13.1q13.2. Both children manifested infantile hypotonia and delayed motor milestones, congenital heart defect, growth deficiency, and strikingly similar and distinctive craniofacial dysmorphism including brachycephaly, blepharophimosis, short broad-based nose and wide mouth with thin upper lip. The boy had also a submucous cleft palate. Both had fair skin and hair compared with their parents. Both had moderate mental retardation associated with a short attention span. A 4-Mb interstitial duplication at 22q13.1q13.2 was detected by whole genome microarray comparative genomic hybridisation (array CGH) in both children. The duplication was confirmed by fluorescence in situ hybridisation (FISH) analysis. Their parents had normal array CGH results. FISH analysis revealed that the father was a carrier of a balanced interchromosomal submicroscopic insertion of 22q13 into chromosome 11q23, explaining the unbalanced aberration detected in both children. This report narrows down the critical region at 22q13.1q13.2, which is associated with mental retardation, pre- and post-natal growth retardation, hippocampal malformation, psychiatric symptoms such as short attention span and facial dysmorphism including hypertelorism, epicanthal folds and low set/abnormal ears.

Ontogeny of D-mannose transport and metabolism in rat small intestine

Oral mannose therapy is used to treat congenital disorders of glycosylation caused by a deficiency in phosphomannose isomerase. The segmental distribution and ontogenic regulation of D-mannose transport, phosphomannose isomerase, and phosphomannose mutase is investigated in the small intestine of fetuses, newborn, suckling, 1-month-old, and adult rats. The small intestine transports D-mannose by both Na(+)-dependent and Na(+)-independent transport mechanisms. The activities of both systems normalized to intestinal weight peak at birth and thereafter they decreased. In all the ages tested, the activity of the Na(+)-independent mechanism was higher than that of the Na(+)/mannose transport system. At birth, the Na(+)-independent D-mannose transport in the ileum was significantly higher than that in jejunum. Phosphomannose isomerase activity and mRNA levels increased at 1 month, and the values in the ileum were lower than in jejunum. Phosphomannose mutase activity in jejunum increased during the early stages of life, and it decreased at 1 month old, as does the amount of mannose incorporated into glycoproteins, whereas in the ileum, they were not affected by age. The phosphomannose isomerase/phosphomannose mutase activity ratio decreased at birth and during the suckling period, and increased at 1 month old. In conclusion, intestinal D-mannose transport activity and metabolism were affected by ontogeny and intestinal segment.

Chapter 5: rab proteins and their interaction partners

The Ras superfamily consists of over 150 low molecular weight proteins that cycle between an inactive guanosine diphosphate (GDP)-bound state and an active guanosine triphosphate (GTP)-bound state. They are involved in a variety of signal transduction pathways that regulate cell growth, intracellular trafficking, cell migration, and apoptosis. Several methods have been devised to detect and characterize the interacting partners of small GTPases with the aim of better understanding their physiological function in normal cells and tumor cells. The Rab (Ras analog in brain) proteins form the largest family within the Ras superfamily. Rab proteins regulate vesicular trafficking pathways, behaving as membrane-associated molecular switches. The guanine nucleotide-binding status of Rab proteins is modulated by three different classes of regulatory proteins, which have been extensively studied for the Rab molecules but also for other subfamilies of the Ras superfamily. Furthermore, numerous effector molecules have been isolated especially for the Rab subfamily of proteins, which interact via a Rab-binding domain (RBD) and are recruited afterwards to specific sub-cellular compartments by the Rab proteins.

Mammalian phosphomannomutase PMM1 is the brain IMP-sensitive glucose-1,6-bisphosphatase

Glucose 1,6-bisphosphate (Glc-1,6-P(2)) concentration in brain is much higher than what is required for the functioning of phosphoglucomutase, suggesting that this compound has a role other than as a cofactor of phosphomutases. In cell-free systems, Glc-1,6-P(2) is formed from 1,3-bisphosphoglycerate and Glc-6-P by two related enzymes: PGM2L1 (phosphoglucomutase 2-like 1) and, to a lesser extent, PGM2 (phosphoglucomutase 2). It is hydrolyzed by the IMP-stimulated brain Glc-1,6-bisphosphatase of still unknown identity. Our aim was to test whether Glc-1,6-bisphosphatase corresponds to the phosphomannomutase PMM1, an enzyme of mysterious physiological function sharing several properties with Glc-1,6-bisphosphatase. We show that IMP, but not other nucleotides, stimulated by >100-fold (K(a) approximately 20 mum) the intrinsic Glc-1,6-bisphosphatase activity of recombinant PMM1 while inhibiting its phosphoglucomutase activity. No such effects were observed with PMM2, an enzyme paralogous to PMM1 that physiologically acts as a phosphomannomutase in mammals. Transfection of HEK293T cells with PGM2L1, but not the related enzyme PGM2, caused an approximately 20-fold increase in the concentration of Glc-1,6-P(2). Transfection with PMM1 caused a profound decrease (>5-fold) in Glc-1,6-P(2) in cells that were or were not cotransfected with PGM2L1. Furthermore, the concentration of Glc-1,6-P(2) in wild-type mouse brain decreased with time after ischemia, whereas it did not change in PMM1-deficient mouse brain. Taken together, these data show that PMM1 corresponds to the IMP-stimulated Glc-1,6-bisphosphatase and that this enzyme is responsible for the degradation of Glc-1,6-P(2) in brain. In addition, the role of PGM2L1 as the enzyme responsible for the synthesis of the elevated concentrations of Glc-1,6-P(2) in brain is established.

Identification and characterization of interacting partners of Rab GTPases by yeast two-hybrid analyses

Much of our knowledge of the mechanisms governing vesicular transport has come from the combination of genetic and biochemical approaches that have identified Ypt/Rab guanosine 5'-triphosphatases (GTPases) as key components of transport processes in both yeast and mammalian cells. More recently, research has focused on establishing the complex protein-protein interactions necessary for the regulation of vesicular transport by a variety of methods, including the yeast two-hybrid interaction assay. A central component of the signaling pathway regulated by Ypt/Rab proteins is the GTPase cycle, in which the proteins cycle between an active guanosine 5'-triphosphate (GTP)-bound form and an inactive guanosine 5'-diphosphate (GDP)-bound form. Alterations in the conformation of the Ypt/Rab proteins when either GTP or GDP is bound specify the interaction of effector proteins and influence membrane binding. Our work has focused on identifying interacting partners for the GTPases Rab1 and Rab6 and their isoforms, which regulate transport steps between the endoplasmic reticulum and Golgi in mammalian cells. We have employed both active (GTP-bound) and inactive (GDP-bound) Rab1 and Rab6 mutants to identify potential new interacting proteins using the yeast two-hybrid system and have verified these interactions using alternative methods.

Molecular and functional analysis of phosphomannomutase (PMM) from higher plants and genetic evidence for the involvement of PMM in ascorbic acid biosynthesis in Arabidopsis and Nicotiana benthamiana

Phosphomannomutase (PMM) catalyzes the interconversion of mannose-6-phosphate and mannose-1-phosphate. However, systematic molecular and functional investigations on PMM from higher plants have hitherto not been reported. In this work, PMM cDNAs were isolated from Arabidopsis, Nicotiana benthamiana, soybean, tomato, rice and wheat. Amino acid sequence comparisons indicated that plant PMM proteins exhibited significant identity to their fungal and mammalian orthologs. In line with the similarity in primary structure, plant PMM complemented the sec53-6 temperature sensitive mutant of Saccharomyces cerevisiae. Histidine-tagged Arabidopsis PMM (AtPMM) purified from Escherichia coli converted mannose-1-phosphate into mannose-6-phosphate and glucose-1-phosphate into glucose-6-phosphate, with the former reaction being more efficient than the latter one. In Arabidopsis and N. benthamiana, PMM was constitutively expressed in both vegetative and reproductive organs. Reducing the PMM expression level through virus-induced gene silencing caused a substantial decrease in ascorbic acid (AsA) content in N. benthamiana leaves. Conversely, raising the PMM expression level in N. benthamiana using viral-vector-mediated ectopic expression led to a 20-50% increase in AsA content. Consistent with this finding, transgenic expression of an AtPMM-GFP fusion protein in Arabidopsis also increased AsA content by 25-33%. Collectively, this study improves our understanding on the molecular and functional properties of plant PMM and provides genetic evidence on the involvement of PMM in the biosynthesis of AsA in Arabidopsis and N. benthamiana plants.

The normal phenotype of Pmm1-deficient mice suggests that Pmm1 is not essential for normal mouse development

Phosphomannomutases (PMMs) are crucial for the glycosylation of glycoproteins. In humans, two highly conserved PMMs exist: PMM1 and PMM2. In vitro both enzymes are able to convert mannose-6-phosphate (mannose-6-P) into mannose-1-P, the key starting compound for glycan biosynthesis. However, only mutations causing a deficiency in PMM2 cause hypoglycosylation, leading to the most frequent type of the congenital disorders of glycosylation (CDG): CDG-Ia. PMM1 is as yet not associated with any disease, and its physiological role has remained unclear. We generated a mouse deficient in Pmm1 activity and documented the expression pattern of murine Pmm1 to unravel its biological role. The expression pattern suggested an involvement of Pmm1 in (neural) development and endocrine regulation. Surprisingly, Pmm1 knockout mice were viable, developed normally, and did not reveal any obvious phenotypic alteration up to adulthood. The macroscopic and microscopic anatomy of all major organs, as well as animal behavior, appeared to be normal. Likewise, lectin histochemistry did not demonstrate an altered glycosylation pattern in tissues. It is especially striking that Pmm1, despite an almost complete overlap of its expression with Pmm2, e.g., in the developing brain, is apparently unable to compensate for deficient Pmm2 activity in CDG-Ia patients. Together, these data point to a (developmental) function independent of mannose-1-P synthesis, whereby the normal knockout phenotype, despite the stringent conservation in phylogeny, could be explained by a critical function under as-yet-unidentified challenge conditions.

Tissue distribution of the murine phosphomannomutases Pmm1 and Pmm2 during brain development

The most common type of the congenital disorders of glycosylation, CDG-Ia, is caused by mutations in the human PMM2 gene, reducing phosphomannomutase (PMM) activity. The PMM2 mutations mainly lead to neurological symptoms, while other tissues are only variably affected. Another phosphomannomutase, PMM1, is present at high levels in the brain. This raises the question why PMM1 does not compensate for the reduced PMM2 activity during CDG-Ia pathogenesis. We compared the expression profile of the murine Pmm1 and Pmm2 mRNA and protein in prenatal and postnatal mouse brain at the histological level. We observed a considerable expression of both Pmms in different regions of the embryonic and adult mouse brain. Surprisingly, the expression patterns were largely overlapping. This data indicates that expression differences on the cellular and tissue level are an unlikely explanation for the absence of functional compensation. These results suggest that Pmm1 in vivo does not exert the phosphomannomutase-like activity seen in biochemical assays, but either acts on as yet unidentified specific substrates or fulfils entirely different functions.

Chromosomal location of genes for novel glutenin subunits and gliadins in wild emmer wheat (Triticum turgidum L. var. dicoccoides)

The glutenin and gliadin proteins of wild emmer wheat, Triticum turgidum L. var. dicoccoides, have potential for improvement of durum wheat ( T. turgidum L. var. durum) quality. The objective of this study was to determine the chromosomes controlling the high molecular weight (HMW) glutenin subunits and gliadin proteins present in three T. turgidum var. dicoccoides accessions (Israel-A, PI-481521, and PI-478742), which were used as chromosome donors in Langdon durum- T. turgidum var. dicoccoides (LDN-DIC) chromosome substitution lines. The three T. turgidum var. dicoccoides accessions, their respective LDN-DIC substitution lines, and a number of controls with known HMW glutenin subunits were analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), urea/SDS-PAGE, and acid polyacrylamide gel electrophoresis (A-PAGE). The results revealed that all three T. turgidum var. dicoccoides accessions possess Glu-A1 alleles that are the same as or similar to those reported previously. However, each T. turgidum var. dicoccoides accession had a unique Glu-B1 allele. PI-478742 had an unusual 1Bx subunit, which had mobility slightly slower than the 1Ax subunit in 12% SDS-PAGE gels. The subunits controlled by chromosome 1B of PI-481521 were slightly faster in mobility than the subunits of the Glu-B1n allele, and the 1By subunit was identified as band 8. The 1B subunits of Israel-A had similar mobility to subunits 14 and 16. The new Glu-B1 alleles were designated as Glu-B1be in Israel-A, Glu-B1bf in PI-481521, and Glu-B1bg in PI-478742. Results from A-PAGE revealed that PI-481521, PI-478742, and Israel-A had eight, 12, and nine unique gliadin bands, respectively, that were assigned to specific chromosomes. The identified glutenin subunits and gliadin proteins in the LDN-DIC substitution lines provide the basis for evaluating their effects on end-use quality, and they are also useful biochemical markers for identifying specific chromosomes or chromosome segments of T. turgidum var. dicoccoides.

Congenital disorders of glycosylation

Congenital disorders of glycosylation (CDG) are a rapidly growing group of genetic diseases that are due to defects in the synthesis of glycans and in the attachment of glycans to other compounds. Most CDG are multisystem diseases that include severe brain involvement. The CDG causing sialic acid deficiency of N-glycans can be diagnosed by isoelectrofocusing of serum sialotransferrins. An efficient treatment, namely oral D-mannose, is available for only one CDG (CDG-Ib). In many patients with CDG, the basic defect is unknown (CDG-x). Glycan structural analysis, yeast genetics, and knockout animal models are essential tools in the elucidation of novel CDG. Eleven primary genetic glycosylation diseases have been discovered and their basic defects identified: six in the N-glycan assembly, three in the N-glycan processing, and two in the O-glycan (glycosaminoglycan) assembly. This review summarizes their clinical, biochemical, and genetic characteristics and speculates on further developments in this field.

Glycosylation defects and virulence phenotypes of Leishmania mexicana phosphomannomutase and dolicholphosphate-mannose synthase gene deletion mutants

Leishmania parasites synthesize an abundance of mannose (Man)-containing glycoconjugates thought to be essential for virulence to the mammalian host and for viability. These glycoconjugates include lipophosphoglycan (LPG), proteophosphoglycans (PPGs), glycosylphosphatidylinositol (GPI)-anchored proteins, glycoinositolphospholipids (GIPLs), and N-glycans. A prerequisite for their biosynthesis is an ample supply of the Man donors GDP-Man and dolicholphosphate-Man. We have cloned from Leishmania mexicana the gene encoding the enzyme phosphomannomutase (PMM) and the previously described dolicholphosphate-Man synthase gene (DPMS) that are involved in Man activation. Surprisingly, gene deletion experiments resulted in viable parasite lines lacking the respective open reading frames (DeltaPMM and DeltaDPMS), a result against expectation and in contrast to the lethal phenotype observed in gene deletion experiments with fungi. L. mexicana DeltaDPMS exhibits a selective defect in LPG, protein GPI anchor, and GIPL biosynthesis, but despite the absence of these structures, which have been implicated in parasite virulence and viability, the mutant remains infectious to macrophages and mice. By contrast, L. mexicana DeltaPMM are largely devoid of all known Man-containing glycoconjugates and are unable to establish an infection in mouse macrophages or the living animal. Our results define Man activation leading to GDP-Man as a virulence pathway in Leishmania.

Functional significance of PMM2 mutations in mildly affected patients with congenital disorders of glycosylation Ia

PURPOSE

Congenital disorders of glycosylation (CDG) result from mutations in N-glycan biosynthesis. Mutations in phosphomannomutase (PMM2) cause CDG-Ia. Here, we report four clinically mild patients and their mutations in PMM2.

METHODS

Analysis of the PMM2 cDNA and gene revealed the mutations affecting the glycosylation efficiency.

RESULTS

The patients have 30% to 50% normal PMM activity in fibroblasts due to different mutations in PMM2, and we studied the effect of each mutation on the PMM activity in a Saccharomyces cerevisiae expression system.

CONCLUSIONS

Each patient carried a severe mutation that decreased the PMM activity to less than 10% as well as a relatively mild mutation. A new mutation, deletion of base 24, changed the reading frame. The C9Y, C241S, and L32R mutations showed 27% to 45% activity when expressed in the eukaryotic expression system, and the more severe D148N was shown to be thermolabile.

Identification and localization of two mouse phosphomannomutase genes, Pmm1 and Pmm2

Phosphomannomutases catalyze the reversible conversion of mannose 6-phosphate to mannose 1-phosphate. In humans, two different isozymes have recently been identified, PMM1 and PMM2. We have previously shown that mutations in the PMM2 gene cause the most frequent type of the congenital disorders of glycosylation, CDG-Ia. Here, we present data on the two mouse orthologous genes, Pmm1 and Pmm2. The chromosomal localization of the two mouse genes has been determined. We also present the gene structure and the exon-intron organization of Pmm1 and Pmm2. Pmm1 maps to mouse chromosome 15, Pmm2 to chromosome 16. These chromosomal regions are syntenic with regions on human chromosomes 22 and 16, respectively. The Pmm1 gene is composed of eight exons and spans approximately 9.5 kb. The genomic structure is extremely well conserved between the human and mouse gene. The Pmm2 gene consists of eight exons and spans a larger genomic region ( approximately 20 kb). An alignment of the human and mouse protein sequences confirms the conservation among this family of phosphomannomutases. The two mouse genes are expressed in many tissues, but the expression pattern is slightly different between Pmm1 and Pmm2. The most striking difference is the high expression of Pmm1 in brain tissue, whereas Pmm2 is only weakly expressed in this tissue.

Functional analysis of novel mutations in a congenital disorder of glycosylation Ia patient with mixed Asian ancestry

Congenital disorders of glycosylation (CDG) are caused by autosomal recessive mutations in genes affecting N-glycan biosynthesis. Mutations in the PMM2 gene, which encodes the enzyme phosphomannomutase (mannose 6-phosphate <--> mannose 1-phosphate), give rise to the most common form: CDG-Ia. These patients typically present with dysmorphic features and neurological abnormalities, cerebellar hypoplasia, ataxia, hypotonia, and coagulopathy, in addition to feeding problems. However, the clinical symptoms vary greatly. The great majority of known CDG-Ia patients are of European descent where the most common mutant alleles originated. This ethnic bias can also be explained by lack of global awareness of the disorder. Here we report an Asian patient with prominent systemic features that we diagnosed with CDG-Ia resulting from two new mutations in the PMM2 gene (310C --> G resulting in L104V and an intronic mutation IVS1-1G --> A). The latter mutation seems to result in lower mRNA levels, and the L104V has been functionally analyzed in a yeast expression system together with known mutations. The Filipino and Cambodian origins of the parents show that CDG-Ia mutations occur in these ethnic groups as well as in Caucasians.

Congenital disorders of glycosylation: glycosylation defects in man and biological models for their study

Several inherited disorders affecting the biosynthetic pathways of N-glycans have been discovered during the past years. This review summarizes the current knowledge in this rapidly expanding field and covers the molecular bases of these disorders as well as their phenotypical consequences.

Mutations in PMM2 that cause congenital disorders of glycosylation, type Ia (CDG-Ia)

The PMM2 gene, which is defective in CDG-Ia, was cloned three years ago [Matthijs et al., 1997b]. Several publications list PMM2 mutations [Matthijs et al., 1997b, 1998; Kjaergaard et al., 1998, 1999; Bjursell et al., 1998, 2000; Imtiaz et al., 2000] and a few mutations have appeared in case reports or abstracts [Crosby et al., 1999; Kondo et al., 1999; Krasnewich et al., 1999; Mizugishi et al., 1999; Vuillaumier-Barrot et al., 1999, 2000b]. However, the number of molecularly characterized cases is steadily increasing and many new mutations may never make it to the literature. Therefore, we decided to collate data from six research and diagnostic laboratories that have committed themselves to a systematic search for PMM2 mutations. In total we list 58 different mutations found in 249 patients from 23 countries. We have also collected demographic data and registered the number of deceased patients. The documentation of the genotype-phenotype correlation is certainly valuable, but is out of the scope of this molecular update. The list of mutations will also be available online (URL: http://www.kuleuven. ac.be/med/cdg) and investigators are invited to submit new data to this PMM2 mutation database.

Genotypes and phenotypes of patients in the UK with carbohydrate-deficient glycoprotein syndrome type 1

18 UK patients (14 families) have been diagnosed with the carbohydrate-deficient glycoprotein syndrome (CDGS), type 1, on the basis of their clinical symptoms and/or abnormal electrophoretic patterns of serum transferrin. Eleven out of the 16 infants died before the age of 2 years. Patients from 12 families had a typical type 1 transferrin profile but one had a variant profile and another, who had many of the clinical features of CDGS type 1, had a normal profile. Eleven of the patients (10 families) with the typical type 1 profile had a deficiency of phosphomannomutase (PMM), (CDGS type 1a) but there was no correlation between residual enzyme activity and severity of disease. All these patients were compound heterozygotes for mutations in the phosphomannomutase (PMM2) gene, with 7 out of the 10 families having the common R141H mutation. Eight different mutations were found, including three novel ones. There was no correlation between genotype and phenotype, although siblings had similar phenotypes. Three patients, including the one with the normal transferrin profile, did not have a deficiency of phosphomannomutase or phosphomannose isomerase (CDGS 1b).

Carbohydrate-deficient glycoprotein syndromes

Four types of carbohydrate-deficient glycoprotein syndrome have been described, and the cause of two of them has been found. The symptoms and signs of these syndromes are described, with variations that occur at different ages. The commonest is type Ia with an autosomal recessive form of inheritance, and the gene responsible has been mapped to 16p. The typical pathology is atrophy of the cerebellum and brainstem, sometimes also involving the cortex, although both the pathology and the biochemical deficiencies vary between different types of syndrome. The diagnosis depends firstly on recognising the clinical features, including the presence of complications such as thyroid disorders. Then biochemical tests can be carried out, especially chromatographic carbohydrate-deficient transferrin assay and isoelectric focusing of serum transferrin. The prognosis depends on the complications, renal, hepatic, and cardiac, but affected children will be severely handicapped. Therefore treatment consists mainly of coping with the complications, and supporting the child and the family. Oral infusion of mannose can be effective in type Ib disease.

[Carbohydrate-deficient blood glycoprotein syndrome]

Carbohydrate-deficient glycoprotein syndrome (CDGS) is a newly delineated group of inherited multisystemic disorders associated with abnormal glycosylation of a number of serum glycoproteins. Several types have been described on the basis of clinical presentation and biochemical changes of the glycosylation of serum transferrin and attributed to different enzymatic defects; their clinical presentations are fully different and a clinical heterogeneity is observed within a same type of CDGS. Patients with CDGS type la usually present with neurologic (hypotonia, strabismus and cerebellar hypoplasia) and cutaneous (inverted nipples, abnormal distribution of adipose tissue) abnormalities, together with multivisceral involvement (digestive, hepatic, cardiac, renal). However, neurologic and cutaneous symptoms may be absent, so that CDGS must be looked for in case of unexplained organ failure such as isolated liver insufficiency, cardiomyopathy, pericarditis, tubulopathy, nephrotic syndrome, vascular accident or retinitis pigmentosa. Patients with CDGS type Ib present with liver disease, enteropathy and hypoglycemia without neurologic involvement. These patients are successfully treated with oral mannose administration emphasizing the importance of making the diagnosis. Patients with CDGS type Ic present with mild psychomotor retardation and seizures. Patients with CDGS type II have psychomotor retardation association with severe gastrointestinal disorder, dysmorphic features and abnormal electroretinogram. Other types (III, IV) are less clearly defined and the clinical presentation includes convulsive encephalopathy. Biological abnormalities such as mild hepatic cytolysis, hematologic and hormonal abnormalities are consistently observed in CDGS type I, as well as renal hyperechogeneity, leading one to look for this syndrome when they are associated. Until now, only four enzymatic deficiencies have been identified (types Ia, Ib, Ic, II).

Cloning and characterization of complementary DNA encoding human N-acetylglucosamine-phosphate mutase protein

Endothelial cells express erythropoietin receptor (EpoR) and are responsive to erythropoietin (Epo). Upon ligand binding, EpoR activates multiple signaling cascades. Identification of genes expressed in response to Epo is important for understanding the molecular nature of the signals. Applying the differential display approach, an effective method for analysis of gene expression, we identified five differentially expressed mRNAs. In this study, we cloned human N-acetylglucosamine-phosphate mutase from a human microvascular endothelial cell (HMVEC) cDNA library using one of the differentially expressed fragments as a probe. The nucleotide (nt) sequence analysis of the longest clone displayed a 2 kb cDNA fragment and encodes a protein of approximately 542 amino acids with a predicted MW of approximately 60 kDa. Northern blotting and reverse transcriptase-polymerase chain reaction analysis revealed an upregulation of the N-acetylglucosamine-phosphate mutase mRNA after 2 h of stimulation of cells with Epo. This gene was shown to be variably expressed in human tissues and is located on chromosome 6. These studies demonstrate that the expression of N-acetylglucosamine-phosphate mutase mRNA responds to cytokines, and the presence of a 10 aa motif similar to the putative active site of several hexose-phosphate mutases provides a basis for future studies of the role of this gene in the regulation of Epo-stimulated endothelial cell proliferation.

Carbohydrate-deficient glycoprotein syndrome type IA (phosphomannomutase-deficiency)

The carbohydrate-deficient glycoprotein or CDG syndromes (OMIM 212065) are a recently delineated group of genetic, multisystem diseases with variable dysmorphic features. The known CDG syndromes are characterized by a partial deficiency of the N-linked glycans of secretory glycoproteins, lysosomal enzymes, and probably also membranous glycoproteins. Due to the deficiency of terminal N-acetylneuraminic acid or sialic acid, the glycan changes can be observed in serum transferrin or other glycoproteins using isoelectrofocusing with immunofixation as the most widely used diagnostic technique. Most patients show a serum sialotransferrin pattern characterized by increased di- and asialotransferrin bands (type I pattern). The majority of patients with type I are phosphomannomutase deficient (type IA), while in a few other patients, deficiencies of phosphomannose isomerase (type IB) or endoplasmic reticulum glucosyltransferase (type IC) have been demonstrated. This review is an update on CDG syndrome type IA.

Microheterogeneity of Serum Glycoproteins in Patients with Chronic Alcohol Abuse Compared with Carbohydrate-deficient Glycoprotein Syndrome Type I

Background: Chronic alcohol abuse alters the normal N-glycosylation of transferrin, producing the carbohydrate-deficient transferrin isoforms. This alteration could be similar to that present in patients with carbohydrate-deficient glycoprotein syndrome type 1 (CDG1). We thus compared the alterations of N-glycans present in patients with alcoholism and patients with CDG1.

Methods: The N-glycans of serum glycoproteins were compared in sera of patients with alcoholism, patients with CDG1, and controls by two-dimensional electrophoresis, neuraminidase, peptide:N-glycosidase F, and endoglycosidase F2 treatments. A specific antibody directed against the amino acid sequence surrounding the N-432 N-glycosylation site of transferrin was prepared (SZ-350 antibody).

Results: In patients with alcoholism, the abnormal transferrin and α1-antitrypsin isoforms were devoid of a variable number of entire N-glycan moieties and were identical with those present in CDG1. In the serum of patients with alcoholism, this finding was less pronounced than in CDG1. In contrast to CDG1, there was no decrease in clusterin or serum amyloid P in patients with alcoholism. The SZ-350 antibody recognized only transferrin isoforms with one or no N-glycan moieties.

Conclusion: Antibodies directed against specific N-glycosylation sites of glycoproteins could be useful for developing more specific immunochemical tests for the diagnosis of chronic alcohol abuse.

Missense mutations in the phosphomannomutase 2 gene of two Japanese siblings with carbohydrate-deficient glycoprotein syndrome type I

Carbohydrate-deficient glycoprotein syndrome type I (CDG1) is an autosomal recessive disorder characterized by severe nervous system involvement and a carbohydrate moiety deficiency in N-linked glycoproteins. Clinical symptoms are psychomotor retardation, stroke-like episodes or hemorrhagic episodes, hepatic dysfunction, polyneuropathy, and cerebellar ataxia. Marked atrophy of the cerebellar hemispheres and pons is recognizable on CT scan or MRI. CDGI has been mapped to human chromosome 16p by linkage studies. Recently, missense mutations in the gene for phosphomannomutase (PMM2) have been detected in Caucasian patients with CDG1. We studied DNA mutations in PMM2 in a Japanese family with CDG1. DNA sequencing of PMM2 in the siblings showed missense mutations of maternal origin in exon 5 and of paternal origin in exon 8. No such mutations were detected in 50 unrelated healthy Japanese. These findings suggest that the PMM2 is responsible for CDG1 in the Japanese as well as in Caucasians, and CDG1 may be the diagnosis in OPCA of neonatal onset, more often than currently thought.

Human glycosylation disorders and sugar supplement therapy

Some genetic defects in protein glycosylation can be treated effectively with dietary supplements of monosaccharides. An easy screening test and non-toxic therapy for potentially lethal disorders should encourage physicians to search for more patients with glycosylation disorders. It should also stimulate research on the occurrence and availability of monosaccharides used for glycoconjugate synthesis and for vertebrate models to study their utilization.

Carbohydrate-deficient glycoprotein syndromes: inborn errors of protein glycosylation

The carbohydrate-deficient glycoprotein (CDG) syndromes (CDGS) are a series of autosomal recessive enzyme deficiencies which result in incomplete glycosylation of plasma proteins. CDGS types Ia and Ib have been related to deficiencies of phosphomannomutase and phosphomannose isomerase, respectively, while CDGS type II results from a deficiency of N-acetylglucosaminyltransferase II. Secondary CDG syndromes are associated with galactosaemia and hereditary fructose intolerance. The diagnosis of CDGS is most easily made by studying the glycoforms of suitable marker proteins using either electrophoresis or isoelectric focusing. This paper reviews the structure of the glycan chains of proteins and structural alterations in CDGS. It also outlines analytical techniques which are useful in the laboratory study of protein glycoforms and the diagnosis of CDGS.

Disorders in protein glycosylation and potential therapy: tip of an iceberg?

Genetic defects in glycoprotein metabolism usually result in neurologic symptoms, but newly discovered defects in glycoprotein biosynthesis (the carbohydrate-deficient glycoprotein syndromes) also present as severe gastrointestinal disorders with hypoglycemia, protein-losing enteropathy, and hepatic pathology. Glycosylation disorders may be more widespread than previously thought and can be detected by using a simple, but underutilized, serum test. Some patients may benefit from promising dietary therapies now in clinical trials.

A new class of phosphotransferases phosphorylated on an aspartate residue in an amino-terminal DXDX(T/V) motif

When incubated with their substrates, human phosphomannomutase and L-3-phosphoserine phosphatase are known to form phosphoenzymes with chemical characteristics of an acyl-phosphate. The phosphorylated residue in phosphomannomutase has now been identified by mass spectrometry after reduction of the phosphoenzyme with tritiated borohydride and trypsin digestion. It is the first aspartate in a conserved DVDGT motif. Replacement of either aspartate of this motif by asparagine or glutamate resulted in complete inactivation of the enzyme. The same mutations performed in the DXDST motif of L-3-phosphoserine phosphatase also resulted in complete inactivation of the enzyme, except for the replacement of the second aspartate by glutamate, which reduced the activity by only about 40%. This suggests that the first aspartate of the motif is also the phosphorylated residue in L-3-phosphoserine phosphatase. Data banks contained seven other phosphomutases or phosphatases sharing a similar, totally conserved DXDX(T/V) motif at their amino terminus. One of these (beta-phosphoglucomutase) is shown to form a phosphoenzyme with the characteristics of an acyl-phosphate. In conclusion, phosphomannomutase and L-3-phosphoserine phosphatase belong to a new phosphotransferase family with an amino-terminal DXDX(T/V) motif that serves as an intermediate phosphoryl acceptor.

Lack of homozygotes for the most frequent disease allele in carbohydrate-deficient glycoprotein syndrome type 1A

Carbohydrate-deficient-glycoprotein syndrome type 1 (CDG1; also known as "Jaeken syndrome") is an autosomal recessive disorder characterized by defective glycosylation. Most patients show a deficiency of phosphomannomutase (PMM), the enzyme that converts mannose 6-phosphate to mannose 1-phosphate in the synthesis of GDP-mannose. The disease is linked to chromosome 16p13, and mutations have recently been identified in the PMM2 gene in CDG1 patients with a PMM deficiency (CDG1A). The availability of the genomic sequences of PMM2 allowed us to screen for mutations in 56 CDG1 patients from different geographic origins. By SSCP analysis and by sequencing, we identified 23 different missense mutations and 1 single-base-pair deletion. In total, mutations were found on 99% of the disease chromosomes in CDG1A patients. The R141H substitution is present on 43 of the 112 disease alleles. However, this mutation was never observed in the homozygous state, suggesting that homozygosity for these alterations is incompatible with life. On the other hand, patients were found homozygous for the D65Y and F119L mutations, which must therefore be mild mutations. One particular genotype, R141H/D188G, which is prevalent in Belgium and the Netherlands, is associated with a severe phenotype and a high mortality. Apart from this, there is only a limited relation between the genotype and the clinical phenotype.

Availability of specific sugars for glycoconjugate biosynthesis: a need for further investigations in man

We review the metabolism of specific sugars used for protein glycosylation, focusing on the fate of exogenously provided sugars. Theoretically, all glycoprotein sugars can derive from glucose, but previous studies show that other exogenous sugars can be incorporated into glycoproteins. From data obtained in congenital galactosemia, exogenous galactose may be important for correct glycosylation. Contrary to galactose, the metabolism of other sugars seems to depend on insulin regulation: stimulation of their endogenous production in diabetic subjects might participate in some diabetic complications, precluding the need for an exogenous supply. The metabolic fate of these sugars is different according to the administration route and exogenous supply may be important either in enteral nutrition or in some clinical situations as has been suggested for sialic acid in the newborn. Data in man are too sparse to reach firm conclusions, implying a need for further investigations. Our preliminary results in animals and man demonstrate that stable isotope methodology allows one to trace glycoprotein sugar metabolism in nutritionally relevant conditions with accuracy and sensitivity, using doses of specific sugars well below toxic levels.

Abnormal metabolism of mannose in families with carbohydrate-deficient glycoprotein syndrome type 1

Patients with carbohydrate-deficient glycoprotein syndrome (CDGS) Type 1 underglycosylate many glycoproteins by failing to add entire N-linked carbohydrate chains to them. The primary defect in these patients has been reported as a > 90% deficiency in phosphomannomutase activity (PMM), the enzyme that converts mannose-6-phosphate to mannose-1-phosphate. This lesion reduces both the amount and the size of the lipid-linked oligosaccharide precursor. We have now analyzed the activity of PMM and the level of glycosylation in cultured fibroblasts as well as the level of blood mannose in seven CDGS Type 1 patients and their parents. All of these patients were approximately 95% deficient in PMM activity and their parents had an average of 51% of control PMM activity. Furthermore, parental fibroblasts showed reduced glycosylation and a higher proportion of truncated N-linked chains compared to those made by control fibroblasts. Addition of 0.25 mM mannose to the culture medium corrected both the underglycosylation and size of the oligosaccharide chains in CDGS Type 1 patients and their parents. Finally, serum from CDGS patients had considerably reduced mannose levels (5-40 microM) compared to normal controls (40-80 microM) and some parents were below normal (16-103 microM). These results suggest that the reduced blood mannose level is a consequence of the PMM deficiency. This is the first inherited disorder in human metabolism that shows a decrease in available mannose. Increasing blood mannose levels might correct some protein underglycosylation in these patients.

Comparison of PMM1 with the phosphomannomutases expressed in rat liver and in human cells

Carbohydrate-deficient glycoprotein syndrome type I (CDGI) is most often due to phosphomannomutase deficiency; paradoxically, the human phosphomannomutase gene PMM1 is located on chromosome 22, whereas the CDGI locus is on chromosome 16. We show that phosphomannomutases present in rat or human liver share with homogeneous recombinant PMM1 several kinetic properties and the ability to form an alkali- and NH2OH-sensitive phosphoenzyme with a subunit mass of approximately 30,000 Mr. However, they have a higher affinity for the activator mannose-1,6-bisphosphate than PMM1 and are not recognized by anti-PMM1 antibodies, indicating that they represent a related but different isozyme. Phosphomannomutases belong to a novel mutase family in which the active residue is a phosphoaspartyl or a phosphoglutamyl.

Mutations in PMM2, a phosphomannomutase gene on chromosome 16p13, in carbohydrate-deficient glycoprotein type I syndrome (Jaeken syndrome)

Carbohydrate-deficient glycoprotein syndrome type 1 (CDG1 or Jaeken syndrome) is the prototype of a class of genetic multisystem disorders characterized by defective glycosylation of glycoconjugates. It is mostly a severe disorder which presents neonatally. There is a severe encephalopathy with axial hypotonia, abnormal eye movements and pronounced psychomotor retardation, as well as a peripheral neuropathy, cerebellar hypoplasia and retinitis pigmentosa. The patients show a peculiar distribution of subcutaneous fat, nipple retraction and hypogonadism. There is a 20% lethality in the first years of life due to severe infections, liver insufficiency or cardiomyopathy. CDG1 shows an autosomal recessive mode of inheritance and has been mapped to chromosome 16p. Most patients show a deficiency of phosphomannomutase (PMM)8, an enzyme necessary for the synthesis of GDP-mannose. We have cloned the PMM1 gene, which is on chromosome 22q13 (ref.9). We now report the identification of a second human PMM gene, PMM2, which is located on 16p13 and which encodes a protein with 66% identity to PMM1. We found eleven different missense mutations in PMM2 in 16 CDG1 patients from different geographical origins and with a documented phosphomannomutase deficiency. Our results give conclusive support to the biochemical finding that the phosphomannomutase deficiency is the basis for CDG1.