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1.  Dissecting functions of the conserved oligomeric Golgi tethering complex using a cell free assay 
Traffic (Copenhagen, Denmark)  2013;15(1):12-21.
Vesicle transport sorts proteins between compartments and is thereby responsible for generating the non-uniform protein distribution along the eukaryotic secretory and endocytic pathways. The mechanistic details of specific vesicle targeting are not yet well characterised at the molecular level. We have developed a cell free assay that reconstitutes vesicle targeting utilising the recycling of resident enzymes within the Golgi apparatus. The assay has physiological properties, and could be used to show that the two lobes of the conserved oligomeric Golgi tethering complex play antagonistic roles in trans-Golgi vesicle targeting. Moreover, we can show that the assay is sensitive to several different congenital defects that disrupt Golgi function and therefore cause glycosylation disorders. Consequently, this assay will allow mechanistic insight into the targeting step of vesicle transport at the Golgi, and could also be useful for characterising some novel cases of congenital glycosylation disorders.
PMCID: PMC3892563  PMID: 24102787
Golgi apparatus; vesicle tethering; conserved oligomeric Golgi complex; congenital disorders of glycosylation; cell free reconstitution; glycosylation enzyme sorting
2.  Severe, fatal multisystem manifestations in a patient with dolichol kinase-congenital disorder of glycosylation 
Molecular genetics and metabolism  2013;110(4):484-489.
Congenital disorders of glycosylation are a group of metabolic disorders with an expansive and highly variable clinical presentation caused by abnormal glycosylation of proteins and lipids. Dolichol kinase (DOLK) catalyzes the final step in biosynthesis of dolichol phosphate (Dol-P), which is the oligosaccharide carrier required for protein N-glycosylation. Human DOLK deficiency, also known as DOLK-CDG or CDG-Im, results in a syndrome that has been reported to manifest with dilated cardiomyopathy of variable severity. A male neonate born to non-consanguineous parents of Palestinian origin presented with dysmorphic features, genital abnormalities, talipes equinovarus, and severe, refractory generalized seizures. Additional multi-systemic manifestations developed including dilated cardiomyopathy, hepatomegaly, severe insulin-resistant hyperglycemia, and renal failure, which were ultimately fatal at age 9 months. Electrospray ionization mass spectrometric (ESI-MS) analysis of transferrin identified a type I congenital disorder of glycosylation; next-generation sequencing demonstrated homozygous p.Q483K DOLK mutations that were confirmed in patient fibroblasts to result in severely reduced substrate binding and catalytic activity. This patient expands the phenotype of DOLK-CDG to include anatomic malformations and multi-systemic dysfunction.
PMCID: PMC3909743  PMID: 24144945
Congenital disorder of glycosylation; DOLK-CDG; Dolichol kinase deficiency; Renal failure; Hepatic dysfunction; Insulin-resistant hyperglycemia
3.  Mutations in NGLY1 Cause an Inherited Disorder of the Endoplasmic Reticulum-Associated Degradation (ERAD) Pathway 
The endoplasmic reticulum-associated degradation (ERAD) pathway is responsible for the translocation of misfolded proteins across the ER membrane into the cytosol for subsequent degradation by the proteasome. In order to understand the spectrum of clinical and molecular findings in a complex neurological syndrome, we studied a series of eight patients with inherited deficiency of N-glycanase 1 (NGLY1), a novel disorder of cytosolic ERAD dysfunction.
Whole-genome, whole-exome or standard Sanger sequencing techniques were employed. Retrospective chart reviews were performed in order to obtain clinical data.
All patients had global developmental delay, a movement disorder, and hypotonia. Other common findings included hypo- or alacrima (7/8), elevated liver transaminases (6/7), microcephaly (6/8), diminished reflexes (6/8), hepatocyte cytoplasmic storage material or vacuolization (5/6), and seizures (4/8). The nonsense mutation c.1201A>T (p.R401X) was the most common deleterious allele.
NGLY1 deficiency is a novel autosomal recessive disorder of the ERAD pathway associated with neurological dysfunction, abnormal tear production, and liver disease. The majority of patients detected to date carry a specific nonsense mutation that appears to be associated with severe disease. The phenotypic spectrum is likely to enlarge as cases with a more broad range of mutations are detected.
PMCID: PMC4243708  PMID: 24651605
NGLY1; alacrima; choreoathetosis; seizures; liver disease
4.  Mutations in STT3A and STT3B cause two congenital disorders of glycosylation 
Human Molecular Genetics  2013;22(22):4638-4645.
We describe two unreported types of congenital disorders of glycosylation (CDG) which are caused by mutations in different isoforms of the catalytic subunit of the oligosaccharyltransferase (OST). Each isoform is encoded by a different gene (STT3A or STT3B), resides in a different OST complex and has distinct donor and acceptor substrate specificities with partially overlapping functions in N-glycosylation. The two cases from unrelated consanguineous families both show neurologic abnormalities, hypotonia, intellectual disability, failure to thrive and feeding problems. A homozygous mutation (c.1877T > C) in STT3A causes a p.Val626Ala change and a homozygous intronic mutation (c.1539 + 20G > T) in STT3B causes the other disorder. Both mutations impair glycosylation of a GFP biomarker and are rescued with the corresponding cDNA. Glycosylation of STT3A- and STT3B-specific acceptors is decreased in fibroblasts carrying the corresponding mutated gene and expression of the STT3A (p.Val626Ala) allele in STT3A-deficient HeLa cells does not rescue glycosylation. No additional cases were found in our collection or in reviewing various databases. The STT3A mutation significantly impairs glycosylation of the biomarker transferrin, but the STT3B mutation only slightly affects its glycosylation. Additional cases of STT3B-CDG may be missed by transferrin analysis and will require exome or genome sequencing.
PMCID: PMC3888133  PMID: 23842455
5.  Congenital disorder of glycosylation due to DPM1 mutations presenting with dystroglycanopathy-type congenital muscular dystrophy 
Molecular genetics and metabolism  2013;110(3):345-351.
Congenital disorders of glycosylation (CDG) are rare genetic defects mainly in the post-translational modification of proteins via attachment of carbohydrate chains. We describe an infant with the phenotype of a congenital muscular dystrophy, with borderline microcephaly, hypotonia, camptodactyly, severe motor delay, and elevated creatine kinase. Muscle biopsy showed muscular dystrophy and reduced α-dystroglycan immunostaining with glycoepitope-specific antibodies in a pattern diagnostic of dystroglycanopathy. Carbohydrate deficient transferrin testing showed a pattern pointing to a CDG type I. Sanger sequencing of DPM1 (dolichol-P-mannose synthase subunit 1) revealed a novel Gly>Val change c.455 G>T missense mutation resulting in p.Gly152Val) of unknown pathogenicity and deletion/duplication analysis revealed an intragenic deletion from exons 3 to 7 on the other allele. DPM1 activity in fibroblasts was reduced by 80%, while affinity for the substrate was not depressed, suggesting a decrease in the amount of active enzyme. Transfected cells expressing tagged versions of wild type and the p.Gly152Val mutant displayed reduced binding to DPM3, an essential, non-catalytic subunit of the DPM complex, suggesting a mechanism for pathogenicity. The present case is the first individual described with DPM1-CDG (CDG-Ie) to also have clinical and muscle biopsy findings consistent with dystroglycanopathy.
PMCID: PMC3800268  PMID: 23856421
congenital disorder of glycosylation; dystroglycanopathy; congenital muscular dystrophy; DPM1; DPM1-CDG; CDG-Ie; mutation
6.  Identification of the first COG-CDG patient of Indian Origin 
Molecular genetics and metabolism  2010;102(3):364-367.
Mutations in the Conserved Oligomeric Golgi (COG) complex give rise to type II congenital disorders of glycosylation (CDG). Thus far, mutations have been identified in 6 of the 8 COG subunits. Here we present data identifying a previously reported CDG-IIx case from Singapore as a new COG4 patient with 2 novel mutations leading to p.E233X and p.L773R; with p.E233X being a de novo mutation. As a result, COG4 protein expression was dramatically reduced, while expression of the other subunits remained unaffected. Analysis of serum N-glycans revealed deficiencies in both sialylation and galactosylation. Furthermore, patient fibroblasts have impaired O-glycosylation. Importantly, patient fibroblasts exhibited a delay in Brefeldin A (BFA) induced retrograde transport, a common characteristic seen in COG deficiencies.
PMCID: PMC3058693  PMID: 21185756
N-Glycosylation; Congenital Disorders of Glycosylation; COG4
7.  Deficiency of the Cog8 Subunit in Normal and CDG-Derived Cells Impairs the Assembly of the COG and Golgi SNARE Complexes 
Traffic (Copenhagen, Denmark)  2013;14(10):1065-1077.
Multiple mutations in different subunits of the tethering complex Conserved Oligomeric Golgi (COG) have been identified as a cause for Congenital Disorders of Glycosylation (CDG) in humans. Yet, the mechanisms by which COG mutations induce the pleiotropic CDG defects have not been fully defined. By detailed analysis of Cog8 deficiency in either HeLa cells or CDG-derived fibroblasts, we show that Cog8 is required for the assembly of both the COG complex and the Golgi Stx5-GS28-Ykt6-GS15 and Stx6-Stx16-Vti1a-VAMP4 SNARE complexes. The assembly of these SNARE complexes is also impaired in cells derived from a Cog7-deficient CDG patient. Likewise, the integrity of the COG complex is also impaired in Cog1-, Cog4-and Cog6-depleted cells. Significantly, deficiency of Cog1, Cog4, Cog6 or Cog8 distinctly influences the production of COG subcomplexes and their Golgi targeting. These results shed light on the structural organization of the COG complex and its subcellular localization, and suggest that its integrity is required for both tethering of transport vesicles to the Golgi apparatus and the assembly of Golgi SNARE complexes. We propose that these two key functions are generally and mechanistically impaired in COG-associated CDG patients, thereby exerting severe pleiotropic defects.
PMCID: PMC4084554  PMID: 23865579
CDG; COG complex; Golgi; SNARE; tethering
Molecular genetics and metabolism  2009;96(4):268-272.
Increasing intracellular mannose-6-phosphate (Man-6-P) was previously reported to reduce the amount of the major lipid linked oligosaccharide (LLO) precursor of N-glycans; a loss that might decrease cellular N-glycosylation. If so, providing dietary mannose supplements to glycosylation-deficient patients might further impair their glycosylation. To address this question, we studied the effects of exogenous mannose on intracellular levels of Man-6-P, LLO, and N-glycosylation in human and mouse fibroblasts. Mannose (500μM) did not increase Man-6-P pools in human fibroblasts from controls or from patients with Congenital Disorders of Glycosylation (CDG), who have 90–95% deficiencies in either phosphomannomutase (CDG-Ia) or phosphomannose isomerase (MPI) (CDG-Ib), enzymes that both use Man-6-P as a substrate. In the extreme case of fibroblasts derived from Mpi null mice (<0.001% MPI activity), intracellular Man-6-P levels greatly increased in response to exogenous mannose, and this produced a dose-dependent decrease in the steady state level of the major LLO precursor. However, LLO loss did not decrease total protein N-glycosylation or that of a hypoglycosylation indicator protein, DNaseI. These results make it very unlikely that exogenous mannose could impair N-glycosylation in glycosylation-deficient CDG patients.
PMCID: PMC2676341  PMID: 19157945
9.  Molecular and Clinical Characterization of a Moroccan Cog7 Deficient Patient 
Molecular genetics and metabolism  2007;91(2):201-204.
Mutations in the N-linked glycosylation pathway cause rare autosomal recessive defects known as Congenital Disorders of Glycosylation (CDG). A previously reported mutation in the Conserved Oligomeric Golgi complex gene, COG7, defined a new subtype of CDG in a Tunisian family. The mutation disrupted the hetero-octomeric COG complex and altered both N- and O- linked glycosylation. Here we present clinical and biochemical data from a second family with the same mutation.
PMCID: PMC1941618  PMID: 17395513
N-glycosylation; Cog7; Congenital Disorders of Glycosylation
10.  Heparan sulfate and syndecan-1 are essential in maintaining murine and human intestinal epithelial barrier function 
Patients with protein-losing enteropathy (PLE) fail to maintain intestinal epithelial barrier function and develop an excessive and potentially fatal efflux of plasma proteins. PLE occurs in ostensibly unrelated diseases, but emerging commonalities in clinical observations recently led us to identify key players in PLE pathogenesis. These include elevated IFN-γ, TNF-α, venous hypertension, and the specific loss of heparan sulfate proteoglycans from the basolateral surface of intestinal epithelial cells during PLE episodes. Here we show that heparan sulfate and syndecan-1, the predominant intestinal epithelial heparan sulfate proteoglycan, are essential in maintaining intestinal epithelial barrier function. Heparan sulfate– or syndecan-1–deficient mice and mice with intestinal-specific loss of heparan sulfate had increased basal protein leakage and were far more susceptible to protein loss induced by combinations of IFN-γ, TNF-α, and increased venous pressure. Similarly, knockdown of syndecan-1 in human epithelial cells resulted in increased basal and cytokine-induced protein leakage. Clinical application of heparin has been known to alleviate PLE in some patients but its unknown mechanism and severe side effects due to its anticoagulant activity limit its usefulness. We demonstrate here that non-anticoagulant 2,3-de-O-sulfated heparin could prevent intestinal protein leakage in syndecan-deficient mice, suggesting that this may be a safe and effective therapy for PLE patients.
PMCID: PMC2117765  PMID: 18064305
11.  Glycosylation diseases: Quo vadis? 
Biochimica et biophysica acta  2008;1792(9):925-930.
About 250 to 500 glycogenes (genes that are directly involved in glycan assembly) are in the human genome representing about 1–2% of the total genome. Over 40 human congenital diseases associated with glycogene mutations have been described to date. It is almost certain that the causative glycogene mutations for many more congenital diseases remain to be discovered. Some glycogenes are involved in the synthesis of only a specific protein and/or a specific class of glycan whereas others play a role in the biosynthesis of more than one glycan class. Mutations in the latter type of glycogene result in complex clinical phenotypes that present difficult diagnostic problems to the clinician. In order to understand in biochemical terms the clinical signs and symptoms of a patient with a glycogene mutation, one must understand how the glycogene works. That requires, first of all, determination of the target protein or proteins of the glycogene followed by an understanding of the role, if any, of the glycogene-dependent glycan in the functions of the protein. Many glycogenes act on thousands of glycoproteins. There are unfortunately no general methods to identify all the potentially large number of glycogene target proteins and which of these proteins are responsible for the mutant phenotypes. Whereas biochemical methods have been highly successful in the discovery of glycogenes responsible for many congenital diseases, it has more recently been necessary to use other methods such as homozygosity mapping. Accurate diagnosis of many recently discovered diseases has become difficult and new diagnostic procedures must be developed. Last but not least is the lack of effective treatment for most of these children and of animal models that can be used to test new therapies.
PMCID: PMC3927646  PMID: 19061954
Congenital disease; Glycosylation; Congenital muscular dystrophy; Glycogene; Protein targets of glycosylation; Glycoprotein synthesis; Classification problem; Diagnostic problem; Therapeutic problem; Look into the future
12.  CDG nomenclature: Time for a change! 
Biochimica et biophysica acta  2009;1792(9):825-826.
PMCID: PMC3917312  PMID: 19765534
13.  Sweet solution 
The Journal of Cell Biology  2002;158(4):615-616.
Sugar pills are usually placebos, but Smith et al. (2002, this issue) use one to rescue designer mice unable to make GDP-Fucose. Dietary fucose enters a salvage pathway and spares the mice. Sound simple? Not so. Unknown genetic factors determine life or death.
PMCID: PMC2174005  PMID: 12186848
14.  Endoglycosidase and Glycoamidase Release of N-Linked Glycans 
Nearly all proteins entering the lumen of the endoplasmic reticulum (ER) become glycosylated en route to a cellular organelle, the plasma membrane, or the extracellular space. Many glycans can be attached to proteins, but the most common are the N-linked glycans (oligosaccharides). These chains are added very soon after a protein enters the ER, but they undergo extensive remodeling (processing), especially in the Golgi. Processing changes the sensitivity of the N-glycan to enzymes that cleave entire sugar chains or individual monosaccharides, which also changes the migration of the protein on SDS gels. These changes can be used to indicate when a protein has passed a particular subcellular location. This unit details some of the methods used to track a protein as it trafficks from the ER to the Golgi toward its final location.
PMCID: PMC3869378  PMID: 20069534
ER/Golgi; oligosaccharide; glycan; N-glycosylation; glycosidase; intracellular trafficking
15.  Pubertal development in ALG6 deficiency (congenital disorder of glycosylation type Ic) 
Molecular genetics and metabolism  2011;103(1):10.1016/j.ymgme.2011.01.016.
Information on the hypothalamic pituitary ovarian axis in congenital disorders of glycosylation (CDG) females is scarce. Varying hormonal profiles and degrees of virilization in CDG females suggest a spectrum of yet unidentified mechanisms affected by impaired N-glycosylation. We describe an ALG6D woman who completed puberty with normal gonadotropins and testosterone levels, no virilization, and regular menses. Hormonal follow-up of CDG females is necessary to improve our understanding of the role of glycosylation in pubertal development.
PMCID: PMC3869397  PMID: 21334936
CDG; Puberty; Glycosylation; Hormone; ALG6; Aromatase
16.  RFT1 Deficiency in Three Novel CDG Patients 
Human mutation  2009;30(10):10.1002/humu.21085.
The medical significance of N-glycosylation is underlined by a group of inherited human disorders called Congenital Disorders of Glycosylation (CDG). One key step in the biosynthesis of the Glc3Man9Glc-NAc2-PP-dolichol precursor, essential for N-glycosylation, is the translocation of Man5GlcNAc2-PP-dolichol across the endoplasmic reticulum membrane. This step is facilitated by the RFT1 protein. Recently, the first RFT1-deficient CDG (RFT1-CDG) patient was identified and presented a severe N-glycosylation disorder. In the present study, we describe three novel CDG patients with an RFT1 deficiency. The first patient was homozygous for the earlier reported RFT1 missense mutation (c.199C4T; p.R67C), whereas the two other patients were homozygous for the missense mutation c.454A4G (p.K152E) and c.892G4A (p.E298 K), respectively. The pathogenic character of the novel mutations was illustrated by the accumulation of Man5GlcNAc2-PP-dolichol and by reduced recombinant DNase 1 secretion. Both the glycosylation pattern and recombinant DNase 1 secretion could be normalized by expression of normal RFT1 cDNA in the patients’ fibroblasts. The clinical phenotype of these patients comprised typical CDG symptoms in addition to sensorineural deafness, rarely reported in CDG patients. The identification of additional RFT1-deficient patients allowed to delineate the main clinical picture of RFT1-CDG and confirmed the crucial role of RFT1 in Man5GlcNAc2-PPdolichol translocation.
PMCID: PMC3869400  PMID: 19701946
glycosylation; CDG; RFT1; dolichol
17.  Insufficient ER-stress response causes selective mouse cerebellar granule cell degeneration resembling that seen in congenital disorders of glycosylation 
Molecular Brain  2013;6:52.
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.
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.
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.
PMCID: PMC3907076  PMID: 24305089
Cerebellar granule cells; Congenital disorders of glycosylation; Cortical neurons; Endoplasmic reticulum stress; GRP78/BiP; Neurodegeneration; Phosphomannomutase 2
18.  Dolichol phosphate mannose synthase (DPM1) mutations define congenital disorder of glycosylation Ie (CDG-Ie) 
Journal of Clinical Investigation  2000;105(2):191-198.
Congenital disorders of glycosylation (CDGs) are metabolic deficiencies in glycoprotein biosynthesis that usually cause severe mental and psychomotor retardation. Different forms of CDGs can be recognized by altered isoelectric focusing (IEF) patterns of serum transferrin (Tf). Two patients with these symptoms and similar abnormal Tf IEF patterns were analyzed by metabolic labeling of fibroblasts with [2-3H]mannose. The patients produced a truncated dolichol-linked precursor oligosaccharide with 5 mannose residues, instead of the normal precursor with 9 mannose residues. Addition of 250 μΜ mannose to the culture medium corrected the size of the truncated oligosaccharide. Microsomes from fibroblasts of these patients were approximately 95% deficient in dolichol-phosphate-mannose (Dol-P-Man) synthase activity, with an apparent Km for GDP-Man ∼6-fold higher than normal. DPM1, the gene coding for the catalytic subunit of Dol-P-Man synthase, was altered in both patients. One patient had a point mutation, C274G, causing an R92G change in the coding sequence. The other patient also had the C274G mutation and a 13-bp deletion that presumably resulted in an unstable transcript. Defects in DPM1 define a new glycosylation disorder, CDG-Ie.
PMCID: PMC377427  PMID: 10642597
19.  Golgi Glycosylation and Human Inherited Diseases 
The Golgi factory receives custom glycosylates and dispatches its cargo to the correct cellular locations. The process requires importing donor substrates, moving the cargo, and recycling machinery. Correctly glycosylated cargo reflects the Golgi's quality and efficiency. Genetic disorders in the specific equipment (enzymes), donors (nucleotide sugar transporters), or equipment recycling/reorganization components (COG, SEC, golgins) can all affect glycosylation. Dozens of human glycosylation disorders fit these categories. Many other genes, with or without familiar names, well-annotated pedigrees, or likely homologies will join the ranks of glycosylation disorders. Their broad and unpredictable case-by-case phenotypes cross the traditional medical specialty boundaries. The gene functions in patients may be elusive, but their common feature may include altered glycosylation that provide clues to Golgi function. This article focuses on a group of human disorders that affect protein or lipid glycosylation. Readers may find it useful to generalize some of these patient-based, translational observations to their own research.
The Golgi glycosylates and sorts intracellular protein and lipid cargos. Impaired performance by mutated Golgi resident proteins creates severe and highly variable pathologies.
PMCID: PMC3181031  PMID: 21709180
20.  Inhibition of N-Linked Glycosylation Disrupts Receptor Tyrosine Kinase Signaling in Tumor Cells 
Cancer research  2008;68(10):3803-3809.
Receptor tyrosine kinases (RTK) are therapeutic targets for the treatment of malignancy. However, tumor cells develop resistance to targeted therapies through the activation of parallel signaling cascades. Recent evidence has shown that redundant or compensatory survival signals responsible for resistance are initiated by nontargeted glycoprotein RTKs coexpressed by the cell. We hypothesized that disrupting specific functions of the posttranslational machinery of the secretory pathway would be an effective strategy to target both primary and redundant RTK signaling. Using the N-linked glycosylation inhibitor, tunicamycin, we show that expression levels of several RTKS (EGFR, ErbB2, ErbB3, and IGF-IR) are exquisitely sensitive to inhibition of N-linked glycosylation. Disrupting this synthetic process reduces both cellular protein levels and receptor activity in tumor cells through retention of the receptors in the endoplasmic reticulum/Golgi compartments. Using U251 glioma and BXPC3 pancreatic adenocarcinoma cell lines, two cell lines resistant to epidermal growth factor receptor–targeted therapies, we show that inhibiting N-linked glycosylation markedly reduces RTK signaling through Akt and radiosensitizes tumor cells. In comparison, experiments in nontransformed cells showed neither a reduction in RTK-dependent signaling nor an enhancement in radiosensitivity, suggesting the potential for a therapeutic ratio between tumors and normal tissues. This study provides evidence that enzymatic steps regulating N-linked glycosylation are novel targets for developing approaches to sensitize tumor cells to cytotoxic therapies.
PMCID: PMC3696581  PMID: 18483264
21.  A zebrafish model of PMM2-CDG reveals altered neurogenesis and a substrate-accumulation mechanism for N-linked glycosylation deficiency 
Molecular Biology of the Cell  2012;23(21):4175-4187.
PMM2-CDG patients have phosphomannomutase (Pmm2) deficiency, with developmental and N-linked glycosylation defects attributed to depletion of mannose-1-phosphate and downstream lipid-linked oligosaccharides (LLOs). This, the first PMM2-CDG zebrafish model, shows, unexpectedly, that accumulation of the Pmm2 substrate mannose-6-phosphate explains LLO deficiency.
Congenital disorder of glycosylation (PMM2-CDG) results from mutations in pmm2, which encodes the phosphomannomutase (Pmm) that converts mannose-6-phosphate (M6P) to mannose-1-phosphate (M1P). Patients have wide-spectrum clinical abnormalities associated with impaired protein N-glycosylation. Although it has been widely proposed that Pmm2 deficiency depletes M1P, a precursor of GDP-mannose, and consequently suppresses lipid-linked oligosaccharide (LLO) levels needed for N-glycosylation, these deficiencies have not been demonstrated in patients or any animal model. Here we report a morpholino-based PMM2-CDG model in zebrafish. Morphant embryos had developmental abnormalities consistent with PMM2-CDG patients, including craniofacial defects and impaired motility associated with altered motor neurogenesis within the spinal cord. Significantly, global N-linked glycosylation and LLO levels were reduced in pmm2 morphants. Although M1P and GDP-mannose were below reliable detection/quantification limits, Pmm2 depletion unexpectedly caused accumulation of M6P, shown earlier to promote LLO cleavage in vitro. In pmm2 morphants, the free glycan by-products of LLO cleavage increased nearly twofold. Suppression of the M6P-synthesizing enzyme mannose phosphate isomerase within the pmm2 background normalized M6P levels and certain aspects of the craniofacial phenotype and abrogated pmm2-dependent LLO cleavage. In summary, we report the first zebrafish model of PMM2-CDG and uncover novel cellular insights not possible with other systems, including an M6P accumulation mechanism for underglycosylation.
PMCID: PMC3484097  PMID: 22956764
22.  A zebrafish model of congenital disorders of glycosylation with phosphomannose isomerase deficiency reveals an early opportunity for corrective mannose supplementation 
Disease Models & Mechanisms  2012;6(1):95-105.
Individuals with congenital disorders of glycosylation (CDG) have recessive mutations in genes required for protein N-glycosylation, resulting in multi-systemic disease. Despite the well-characterized biochemical consequences in these individuals, the underlying cellular defects that contribute to CDG are not well understood. Synthesis of the lipid-linked oligosaccharide (LLO), which serves as the sugar donor for the N-glycosylation of secretory proteins, requires conversion of fructose-6-phosphate to mannose-6-phosphate via the phosphomannose isomerase (MPI) enzyme. Individuals who are deficient in MPI present with bleeding, diarrhea, edema, gastrointestinal bleeding and liver fibrosis. MPI-CDG patients can be treated with oral mannose supplements, which is converted to mannose-6-phosphate through a minor complementary metabolic pathway, restoring protein glycosylation and ameliorating most symptoms, although liver disease continues to progress. Because Mpi deletion in mice causes early embryonic lethality and thus is difficult to study, we used zebrafish to establish a model of MPI-CDG. We used a morpholino to block mpi mRNA translation and established a concentration that consistently yielded 13% residual Mpi enzyme activity at 4 days post-fertilization (dpf), which is within the range of MPI activity detected in fibroblasts from MPI-CDG patients. Fluorophore-assisted carbohydrate electrophoresis detected decreased LLO and N-glycans in mpi morphants. These deficiencies resulted in 50% embryonic lethality by 4 dpf. Multi-systemic abnormalities, including small eyes, dysmorphic jaws, pericardial edema, a small liver and curled tails, occurred in 82% of the surviving larvae. Importantly, these phenotypes could be rescued with mannose supplementation. Thus, parallel processes in fish and humans contribute to the phenotypes caused by Mpi depletion. Interestingly, mannose was only effective if provided prior to 24 hpf. These data provide insight into treatment efficacy and the broader molecular and developmental abnormalities that contribute to disorders associated with defective protein glycosylation.
PMCID: PMC3529342  PMID: 22899857
23.  Targeted PCR-based enrichment and next generation sequencing for diagnostic testing of congenital disorders of glycosylation (CDG) 
Genetics in Medicine  2011;13(11):921-932.
Congenital disorders of glycosylation (CDG) are a heterogeneous group of disorders caused by deficient glycosylation, primarily affecting the N-linked pathway. It is estimated that over 40% of CDG patients lack a confirmatory molecular diagnosis. The purpose of this study was to improve molecular diagnosis for CDG by developing and validating a next generation sequencing (NGS) panel for comprehensive mutation detection in 24 genes known to cause CDG.
NGS validation was performed on 12 positive control CDG patients. These samples were blinded as to the disease causing mutations. Both RainDance and Fluidigm platforms were used for sequence enrichment and targeted amplification. The SOLiD platform was used for sequencing the amplified products. Bioinformatic analysis was performed using NextGENe® software.
The disease causing mutations were identified by NGS for all 12 positive controls. Additional variants were also detected in three controls that are known or predicted to impair gene function and may contribute to the clinical phenotype.
We conclude that development of NGS panels in the diagnostic laboratory where multiple genes are implicated in a disorder is more cost-effective and will result in improved and faster patient diagnosis compared with a gene-by-gene approach. Recommendations are also provided for data analysis from the NGS-derived data in the clinical laboratory, which will be important for the widespread use of this technology.
PMCID: PMC3398737  PMID: 21811164
congenital disorders of glycosylation; next generation sequencing; molecular diagnostic testing; target enrichment; bioinformatics
24.  Potent, Selective, and Orally Available Benzoisothiazolone Phosphomannose Isomerase Inhibitors as Probes for Congenital Disorder of Glycosylation Ia 
Journal of medicinal chemistry  2011;54(10):3661-3668.
We report the discovery and validation of a series of benzoisothiazolones as potent inhibitors of phosphomannose isomerase (PMI), an enzyme which converts mannose-6-phosphate (Man-6-P) into fructose-6-phosphate (Fru-6-P), and more importantly, competes with phosphomannomutase 2 (PMM2) for Man-6-P, diverting this substrate from critical protein glycosylation events. In Congenital Disorder of Glycosylation type Ia, PMM2 activity is compromised, thus PMI inhibition is a potential strategy for the development of therapeutics. High-throughput screening (HTS) and subsequent chemical optimization led to the identification of a novel class of benzoisothiazolones as potent PMI inhibitors having little or no PMM2 inhibition. Two complimentary synthetic routes were developed enabling the critical structural requirements for activity to be determined, and the compounds were subsequently profiled in biochemical and cellular assays to assess efficacy. The most promising compounds were also profiled for bioavailability parameters including metabolic stability, plasma stability, and permeability. The pharmacokinetic profile of a representative of this series was also assessed, demonstrating the potential of this series for in vivo efficacy when dosed orally in disease models.
PMCID: PMC3437750  PMID: 21539312
25.  Molecular Imaging of N-linked Glycosylation Suggests Glycan Biosynthesis is a Novel Target for Cancer Therapy 
Redundant receptor tyrosine kinase (RTK) signaling is a mechanism for therapeutic resistance to EGFR inhibition. A strategy to reduce parallel signaling by co-expressed RTKs is inhibition of N-linked glycosylation (NLG), an endoplasmic reticulum (ER) co-translational protein modification required for receptor maturation and cell surface expression. We therefore investigated the feasibility of blocking NLG in vivo to reduce over-expression of RTKs.
Experimental design
We developed a model system to dynamically monitor NLG in vitro and in vivo using bioluminescent imaging techniques. Functional imaging of NLG is accomplished with a luciferase reporter (ER-LucT) modified for ER-translation and glycosylation. After in vitro validation, this reporter was integrated with D54 glioma xenografts to perform non-invasive imaging of tumors, and inhibition of NLG was correlated with RTK protein levels and tumor growth.
The ER-LucT reporter demonstrates the ability to sensitively and specifically detect NLG inhibition. Using this molecular imaging approach we performed serial imaging studies to determine safe and efficacious in vivo dosing of the GlcNAc-1-phosphotransferase inhibitor, tunicamycin, which blocks N-glycan precursor biosynthesis. Molecular analyses of tunicamycin treated tumors showed reduced levels of EGFR and Met, two RTKs over-expressed in gliomas. Furthermore, D54 and U87MG glioma xenograft tumor experiments demonstrated significant reductions in tumor growth following NLG inhibition and radiation therapy, consistent with an enhancement in tumor radiosensitivity.
This study suggests NLG inhibition is a novel therapeutic strategy for targeting EGFR and RTK signaling in both gliomas and other malignant tumors.
PMCID: PMC3413408  PMID: 20413434
Glycosylation; Radiation; EGFR; Met

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