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

High CDT without clinical context: beware of the variant

Carbohydrate-deficient transferrin (CDT) is a performant biomarker used for the diagnosis of chronic alcohol abuse. Here, we describe the case of a 39-year-old male of Tamil ethnicity who had extremely elevated (20%) CDT using capillary electrophoresis (but without glycoforms profile analysis), putting his driving license regranting at risk. However, the patient had no symptoms of chronic alcohol abuse, normal mean corpuscular volume and gamma-glutamyl transferase, and did not admit to any alcohol consumption. Re-analysis by N-Latex CDT immunoassay revealed a CDT at 1.7%. Further investigation by whole-exome sequencing revealed a c.1295A>G missense variant at the heterozygous state on the TFgene. This variant is characterized by an amino-acid change at a consensus sequence forN-glycosylation. Therefore, half of the patient transferrin proteins were lacking a completeN-glycan chain out of two, despite no alcohol consumption. This also explains the discrepancies between the techniques, as the N-Latex antibodies did not recognize the mutated sequence. In conclusion, this case highlights the importance of comparing laboratory results between themselves and the clinical description, the absolute requirement for glycoforms profile analysis before delivering results, and the necessity to confirm intriguing results by another technique in a specialized laboratory.

Variation of the serum N-glycosylation during the pregnancy of a MPI-CDG patient

For the first time the glycosylation of a patient with a MPI-CDG during pregnancy is monitored. MPI-CDG, is characterised by a deficiency in mannose-6-phosphate isomerase (MPI) leading to a reduced pool of glycosylation precursors, impairing the biosynthesis of N-glycans leading to N-glycosylation defects. The abnormal N-glycosylation profile with an elevation of asialotransferrin and disialotransferrin, typical of CDG type I, is assessable by transferrin isoelectrofocusing. Oral D-mannose supplementation for MPI-CDG patients has been widely used and improves clinical manifestations. The glycosylation of a MPI-CDG patient during pregnancy without mannose supplementation was studied using carbohydrate deficient transferrin (CDT) assay, transferrin isoelectrofocusing (IEF) and mass spectrometry of total serum N-glycans. A general improvement of the glycosylation profile of the patient due to a better transfer of the glycan precursors as well as an increase of the triantennary glycans (and sialylation) was observed. In conclusion, in the absence of mannose supplementation, the previously observed glycosylation abnormality of the MPI-CDG patient was corrected. The molecular mechanism underlying this N-glycosylation rescue during MPI-CDG pregnancy further needs to be investigated.

Identification through exome sequencing of the first PMM2-CDG individual of Mexican mestizo origin

Congenital Disorders of Glycosylation (CDG) are scarcely reported from Latin America. We here report on a Mexican mestizo with a multi-systemic syndrome including neurological involvement and a type I transferrin (Tf) isoelectric focusing (IEF) pattern. Clinical exome sequencing (CES) showed known compound missense variants in PMM2 c.422G > A (p.R141H) and c.395 T > C (p.I132T), coding for the phosphomanomutase 2 (PMM2). PMM2 catalyzes the conversion of mannose-6-P to mannose-1-P required for the synthesis of GDP-Man and Dol-P-Man, donor substrates for glycosylation reactions. This is the third reported Mexican CDG patient and the first with PMM2-CDG. PMM2 has been recently identified as one of the top 10 genes carrying pathogenic variants in a Mexican population cohort.

Congenital disorders of glycosylation

Congenital disorders of glycosylation (CDG) is a genetically heterogeneous and clinically polymorphic group of diseases caused by defects in various enzymes, the synthesis and processing of N-linked glycans or oligosaccharides into glycoproteins. Approximately half of all proteins expressed in cells are glycosylated to achieve their full functionality. Basically there are 2 variants of glycosylation: N-glycosylation and O-glycosylation. N-glycans are bound to the amide group of aspartine, whereas O-glycans are bonded to the hydroxyl group of serine or threonine. Synthesis of N-glycans occurs in 3 stages: the formation of nucleotide-linked sugars, assembly (in the cytosol and endoplasmic reticulum) and treatment (in the Golgi apparatus). Synthesis of O-glycans occurs mainly in the Golgi apparatus. The most frequently identified types of CDG are associated with a defect in the N-glycosylation pathway. CDGs are typically multisystem disorders with varying clinical manifestations such as hepatomegaly, cholestasis, liver failure, developmental delay, hypotonia, convulsions, facial dysmorphism and gastrointestinal disorders. Also histological findings showed liver fibrosis, malformation of the ducts, cirrhosis, and steatosis. CDGs typically present in the first months of life, and about 20% of patients do not survive to 5 years. The first line of CDG screening is based on the analysis of N-glycosylation of transf ferin. Exome sequencing or targeted gene panel is used for diagnosis. Several CDG subtypes are amenable to teraphy with mannose and galactose.

Receptor-mediated selective impairment of insulin-like growth factor-1 activity in congenital disorders of glycosylation patients

BACKGROUND

Congenital disorders of glycosylation (CDG) patients share a basic feature of protein hypoglycosylation. Activity of growth factors and their receptors, glycoproteins playing a pivotal role during child development, remains unexplored in CDG patients.

METHODS

Peripheral blood lymphocytes (PBL) isolated from 9 CDG patients and 12 healthy controls were cultured in the presence of fetal bovine serum (FBS), platelet-derived growth factor (PDGF), basic fibroblast growth factor (bFGF), and insulin-like growth factor-1 (IGF-1), and BrdU incorporation was measured. Levels of plasma IGF-1 and PBL IGF-1 receptor (IGF-1R) and its glycosylation were detected using immunoassay and western blot.

RESULTS

CDG patients showed significantly less (P < 0.01) serum-induced 5'-Bromo-2'-deoxyuridine (BrdU) incorporation in PBL than in controls. PDGF-/FGF-stimulated BrdU incorporation showed no difference in patients and controls, whereas IGF-1-induced DNA synthesis was significantly (P < 0.01) less in patients. Plasma IGF-1 levels and PBL IGF-1 receptor protein were significantly (P < 0.01) reduced in patients as compared to controls. IGF-1 receptor in PBL of all CDG patients had significantly (P < 0.01) reduced carbohydrate content when compared with control.

CONCLUSIONS

These results show selective impairment of IGF-1-induced DNA synthesis in lymphocytes of CDG patients through decreased gene expression and hypoglycosylation of the IGF-1 receptor.

Biological roles of glycans

Simple and complex carbohydrates (glycans) have long been known to play major metabolic, structural and physical roles in biological systems. Targeted microbial binding to host glycans has also been studied for decades. But such biological roles can only explain some of the remarkable complexity and organismal diversity of glycans in nature. Reviewing the subject about two decades ago, one could find very few clear-cut instances of glycan-recognition-specific biological roles of glycans that were of intrinsic value to the organism expressing them. In striking contrast there is now a profusion of examples, such that this updated review cannot be comprehensive. Instead, a historical overview is presented, broad principles outlined and a few examples cited, representing diverse types of roles, mediated by various glycan classes, in different evolutionary lineages. What remains unchanged is the fact that while all theories regarding biological roles of glycans are supported by compelling evidence, exceptions to each can be found. In retrospect, this is not surprising. Complex and diverse glycans appear to be ubiquitous to all cells in nature, and essential to all life forms. Thus, >3 billion years of evolution consistently generated organisms that use these molecules for many key biological roles, even while sometimes coopting them for minor functions. In this respect, glycans are no different from other major macromolecular building blocks of life (nucleic acids, proteins and lipids), simply more rapidly evolving and complex. It is time for the diverse functional roles of glycans to be fully incorporated into the mainstream of biological sciences.

N-linked glycosylation of Kv1.2 voltage-gated potassium channel facilitates cell surface expression and enhances the stability of internalized channels

KEY POINTS

Kv1.2 and related voltage-gated potassium channels have a highly conserved N-linked glycosylation site in the first extracellular loop, with complex glycosylation in COS-7 cells similar to endogenous Kv1.2 glycosylation in hippocampal neurons. COS-7 cells expressing Kv1.2 show a crucial role of this N-linked glycosylation in the forward trafficking of Kv1.2 to the cell membrane. Although both wild-type and non-glycosylated mutant Kv1.2 channels that have reached the cell membrane are internalized at a comparable rate, mutant channels are degraded at a faster rate. Treatment of wild-type Kv1.2 channels on the cell surface with glycosidase to remove sialic acids also results in the faster degradation of internalized channels. Glycosylation of Kv1.2 is important with respect to facilitating trafficking to the cell membrane and enhancing the stability of channels that have reached the cell membrane.

ABSTRACT

Studies in cultured hippocampal neurons and the COS-7 cell line demonstrate important roles for N-linked glycosylation of Kv1.2 channels in forward trafficking and protein degradation. Kv1.2 channels can contain complex N-linked glycans, which facilitate cell surface expression of the channels. Additionally, the protein stability of cell surface-expressed Kv1.2 channels is affected by glycosylation via differences in the degradation of internalized channels. The present study reveals the importance of N-linked complex glycosylation in boosting Kv1.2 channel density. Notably, sialic acids at the terminal sugar branches play an important role in dampening the degradation of Kv1.2 internalized from the cell membrane to promote its stability.

Hypertrophic cardiomyopathy with cardiac rupture and tamponade caused by congenital disorder of glycosylation type Ia

Hypertrophic cardiomyopathy (HCM) is a rare presenting feature of congenital disorder of glycosylation type Ia (CDG-Ia). We report two female siblings with CDG-Ia and cardiomyopathy. Patient no. 1 died at 12 days of age from cardiac rupture and tamponade, which has not previously been reported in CDG-Ia. The second patient died at 2 months of age from HCM. The severe cardiac manifestations seen in our patients emphasize the importance of early cardiac assessment in all patients with CDG-Ia.

The clinical spectrum of phosphomannomutase 2 deficiency (CDG-Ia)

Congenital disorders of glycosylation are a clinically and genetically heterogeneous group of disorders resulting from abnormal glycosylation of various glycoconjugates. The first description of congenital disorders of glycosylation was published in the early 80s and once screening tests for glycosylation disorders (CDGs) became readily available, CDG-Ia became the most frequently diagnosed CDG subtype. CDG-Ia is pan-ethnic and the spectrum of the clinical manifestations is still evolving: it spans from severe hydrops fetalis and fetal loss to a (nearly) normal phenotype. However, the most common presentation in infancy is of a multisystem disorder with central nervous system involvement.

Activity-dependent heteromerization of the hyperpolarization-activated, cyclic-nucleotide gated (HCN) channels: role of N-linked glycosylation

Formation of heteromeric complexes of ion channels via co-assembly of different subunit isoforms provides an important mechanism for enhanced channel diversity. We have previously demonstrated co-association of the hyperpolarization activated cyclic-nucleotide gated (HCN1/HCN2) channel isoforms that was regulated by network (seizure) activity in developing hippocampus. However, the mechanisms that underlie this augmented expression of heteromeric complexes have remained unknown. Glycosylation of the HCN channels has been implicated in the stabilization and membrane expression of heteromeric HCN1/HCN2 constructs in heterologous systems. Therefore, we used in vivo and in vitro systems to test the hypothesis that activity modifies HCN1/HCN2 heteromerization in neurons by modulating the glycosylation state of the channel molecules. Seizure-like activity (SA) increased HCN1/HCN2 heteromerization in hippocampus in vivo as well as in hippocampal organotypic slice cultures. This activity increased the abundance of glycosylated HCN1 but not HCN2-channel molecules. In addition, glycosylated HCN1 channels were preferentially co-immunoprecipitated with the HCN2 isoforms. Provoking SA in vitro in the presence of the N-linked glycosylation blocker tunicamycin abrogated the activity-dependent increase of HCN1/HCN2 heteromerization. Thus, hippocampal HCN1 molecules have a significantly higher probability of being glycosylated after SA, and this might promote a stable heteromerization with HCN2.

Investigation of human protein variants and their frequency in the general population

Genetic variations and posttranslational modifications give rise to structural diversity in fully expressed human proteins. Structural modifications can also be induced during the life cycle of a protein and can lead to impaired functioning and pathological conditions. Although a large number of protein modifications have been discovered thus far, their incidence among the general population has not been determined. Here we show that human proteins exhibit a wide range of modifications present at various frequencies in the general population. The screening of 1,000 individuals from four geographical regions in the United States for five plasma proteins revealed the existence of 27 protein modifications. Some variants, such as those resulting from oxidation and single amino acid terminal truncations, were observed in the majority of individuals, whereas point mutations and extensive sequence truncations were detected in only a few individuals. Gender correlations were observed for two protein modifications. The data obtained reveal the extent of structural diversity in the general populace and represent the first such catalogue of structural protein modifications. Systematic studies of this kind will help redefine the normal human proteome and reveal the effects of these modifications in pathological processes.

The glycosylation state of Kv1.2 potassium channels affects trafficking, gating, and simulated action potentials

We presented evidence previously that decreasing the glycosylation state of the Kv1.1 potassium channel modified its gating by a combined surface potential and a cooperative subunit interaction mechanism and these effects modified simulated action potentials. Here we continued to test the hypothesis that glycosylation affects channel function in a predictable fashion by increasing and decreasing the glycosylation state of Kv1.2 channels. Compared with Kv1.2, increasing the glycosylation state shifted the V(1/2) negatively with a steeper G-V slope, increased activation kinetics with little change in deactivation kinetics or in their voltage-dependence, and decreased the apparent level of C-type inactivation. Decreasing the glycosylation state had essentially the opposite effects and shifted the V(1/2) positively with a shallower G-V slope, decreased activation kinetics (and voltage-dependence), decreased deactivation kinetics, and increased the apparent level of C-type inactivation. Single channel conductance was not affected by the different glycosylation states of Kv1.2 tested here. Hyperpolarized or depolarized shifts in V(1/2) from wild type were apparently due to an increased or decreased level of channel sialylation, respectively. Data and modeling suggested that the changes in activation properties were mostly predictable within and between channels and were consistent with a surface potential mechanism, but those on deactivation properties were not predictable and were more consistent with a conformational mechanism. Moreover the effect on the deactivation process appeared to be channel-type dependent as well as glycosylation-site dependent. The glycosylation state of Kv1.2 also affected action potentials in simulations. In addition, preventing N-glycosylation decreased cell surface Kv1.2 expression levels by approximately 40% primarily by increasing partial endoplasmic reticulum retention and this effect was completely rescued by Kv1.4 subunits, which are glycosylated, but not by cytoplasmic Kvbeta2.1 subunits. The nonglycosylated Kv1.2 protein had a similar protein half-life as the glycosylated protein and appeared to be folded properly. Thus altering the native Kv1.2 glycosylation state affected its trafficking, gating, and simulated action potentials. Differential glycosylation of ion channels could be used by excitable cells to modify cell signaling.

Congenital disorders of N-glycosylation including diseases associated with O- as well as N-glycosylation defects

The congenital disorders of N-glycosylation (CDG), a steadily increasing group of multi-systemic disorders, have severe clinical implications in infancy and early childhood. The various inborn errors responsible adversely affect N-glycosylation of lysosomal proteins because of either failing assembly of lipid-linked (LL) oligosaccharides (OS) in the endoplasmic reticulum, CDG Type I, or faulty processing of the asparagines (N)-linked OS in the ER and in the Golgi, CDG Type II. The overlap of phenotypes precludes specific clinical delineation. Isoelectric focusing (IEF) of plasma transferrin remains a valuable, albeit imperfect, screening tool. IEF of plasma ApoC-III protein, introduced O-glycosylation defects that delineated some new CDGs due to mutations of both N- and O-glycosylation. Only CDG-Ib is amenable to treatment with free mannose supplementation. Hence, early specific diagnosis of any one entity is crucial for genetic counseling and elective preventive measures.

Structure of Leishmania mexicana Phosphomannomutase Highlights Similarities with Human Isoforms

Phosphomannomutase (PMM) catalyses the conversion of mannose-6-phosphate to mannose-1-phosphate, an essential step in mannose activation and the biosynthesis of glycoconjugates in all eukaryotes. Deletion of PMM from Leishmania mexicana results in loss of virulence, suggesting that PMM is a promising drug target for the development of anti-leishmanial inhibitors. We report the crystallization and structure determination to 2.1 A of L. mexicana PMM alone and in complex with glucose-1,6-bisphosphate to 2.9 A. PMM is a member of the haloacid dehalogenase (HAD) family, but has a novel dimeric structure and a distinct cap domain of unique topology. Although the structure is novel within the HAD family, the leishmanial enzyme shows a high degree of similarity with its human isoforms. We have generated L. major PMM knockouts, which are avirulent. We expressed the human pmm2 gene in the Leishmania PMM knockout, but despite the similarity between Leishmania and human PMM, expression of the human gene did not restore virulence. Similarities in the structure of the parasite enzyme and its human isoforms suggest that the development of parasite-selective inhibitors will not be an easy task.

Characterization of N-glycosylation consensus sequences in the Kv3.1 channel

N-Glycosylation is a cotranslational and post-translational process of proteins that may influence protein folding, maturation, stability, trafficking, and consequently cell surface expression of functional channels. Here we have characterized two consensus N-glycosylation sequences of a voltage-gated K+ channel (Kv3.1). Glycosylation of Kv3.1 protein from rat brain and infected Sf9 cells was demonstrated by an electrophoretic mobility shift assay. Digestion of total brain membranes with peptide N glycosidase F (PNGase F) produced a much faster-migrating Kv3.1 immunoband than that of undigested brain membranes. To demonstrate N-glycosylation of wild-type Kv3.1 in Sf9 cells, cells were treated with tunicamycin. Also, partially purified proteins were digested with either PNGase F or endoglycosidase H. Attachment of simple-type oligosaccharides at positions 220 and 229 was directly shown by single (N229Q and N220Q) and double (N220Q/N229Q) Kv3.1 mutants. Functional measurements and membrane fractionation of infected Sf9 cells showed that unglycosylated Kv3.1s were transported to the plasma membrane. Unitary conductance of N220Q/N229Q was similar to that of the wild-type Kv3.1. However, whole cell currents of N220Q/N229Q channels had slower activation rates, and a slight positive shift in voltage dependence compared to wild-type Kv3.1. The voltage dependence of channel activation for N229Q and N220Q was much like that for N220Q/N229Q. These results demonstrate that the S1-S2 linker is topologically extracellular, and that N-glycosylation influences the opening of the voltage-dependent gate of Kv3.1. We suggest that occupancy of the sites is critical for folding and maturation of the functional Kv3.1 at the cell surface.

Identification of glycoconjugates in the urine of a patient with congenital disorder of glycosylation by high-resolution mass spectrometry

More than 150 molecular species were detected in a single glycoconjugate fraction obtained from urine of a congenital disorders of glycosylation (CDG) patient by use of high-resolution FT-ICR MS. With respect to its high-mass accuracy and resolving power, FT-ICR MS represents an ideal tool for analysis of single components in complex glycoconjugate mixtures obtained from body fluids. The presence of overlapping nearly isobaric ionic species in glycoconjugate mixtures obtained from CDG patient's urine was postulated from fragmentation data of several precursor ions obtained by nanoESI Q-TOF CID. Their existence was confirmed by high-resolution/high-mass accuracy FT-ICR MS detection. High-resolution FT-ICR mass spectra can, therefore, be generally considered for glycoscreening of complex mixture samples in a single stage. From the accurate molecular ion mass determinations the composition of glycoconjugate species can be identified. Particular enhancement of identification is offered by computer-assisted calculations in combination with monosaccharide building block analysis, which can be extended by considerations of non-carbohydrate modifications, such as amino acids, phosphates and sulfates. Taking advantage of this strategy, the number of compositions assigned to mass peaks was significantly increased in a fraction obtained from urine by size exclusion and anion exchange chromatography.

Effects of Kv1.1 channel glycosylation on C-type inactivation and simulated action potentials

Kv1.1 channels are brain glycoproteins that play an important role in repolarization of action potentials. In previous work, we showed that lack of N-glycosylation, particularly lack of sialylation, of Kv1.1 affected its macroscopic gating properties and slowed activation and C-type inactivation kinetics and produced a depolarized shift in the steady-state activation curve. In our current study, we used single channel analysis to investigate voltage-independent C-type inactivation in both Kv1.1 and Kv1.1N207Q, a glycosylation mutant. Both channels underwent brief and long-lived closures, and the lifetime and frequency of the long-lived closed states were voltage-independent and similar for both channels. We found that, as in macroscopic measurements, Kv1.1N207Q exhibited a approximately 8 mV positive shift in its single channel fractional open time (fo) and a shallower fo-voltage slope compared with Kv1.1. Data suggested that C-type inactivation reflected the equilibration time with at least two slow voltage-independent long-lived closed states that followed the rapid activation process. In addition, data simulation indicated that the C-type inactivation process reflected the equilibration time between the open state and at least two long-lived closed states. Moreover, the faster macroscopic current decay in Kv1.1 mostly reflected a slower equilibration time in these channels as compared with Kv1.1N207Q. Finally, action potential simulations indicated that the N207Q mutation broaden the action potential and decreased the interspike interval. The shape of the action potential was not significantly affected by C-type inactivation, however, for a given channel, C-type inactivation increased the interspike interval. Data and simulations suggested that excitable cells could use differences in K(+) channel glycosylation degree as an additional mechanism to increase channel functional diversity which could modify cell excitability.

Investigating diversity in human plasma proteins

Plasma proteins represent an important part of the human proteome. Although recent proteomics research efforts focus largely on determining the overall number of proteins circulating in plasma, it is equally important to delineate protein variations among individuals, because they can signal the onset of diseases and be used as biological markers in diagnostics. To date, there has been no systematic proteomics effort to characterize the breadth of structural modifications in individual proteins in the general population. In this work, we have undertaken a population proteomics study to define gene- and protein-level diversity that is encountered in the general population. Twenty-five plasma proteins from a cohort of 96 healthy individuals were investigated through affinity-based mass spectrometric assays. A total of 76 structural forms/variants were observed for the 25 proteins within the samples cohort. Posttranslational modifications were detected in 18 proteins, and point mutations were observed in 4 proteins. The frequency of occurrence of these variations was wide-ranged, with some modifications being observed in only one sample, and others detected in all 96 samples. Even though a relatively small cohort of individuals was investigated, the results from this study illustrate the extent of protein diversity in the human population and can be of immediate aid in clinical proteomics/biomarker studies by laying a basal-level statistical foundation from which protein diversity relating to disease can be evaluated.

Glycosylation affects the protein stability and cell surface expression of Kv1.4 but Not Kv1.1 potassium channels. A pore region determinant dictates the effect of glycosylation on trafficking

Kv1.1 and Kv1.4 potassium channels are plasma membrane glycoproteins involved in action potential repolarization. We have shown previously that glycosylation affects the gating function of Kv1.1 and that a pore region determinant of Kv1.1 and Kv1.4 affects their cell surface trafficking negatively or positively, respectively. Here we investigated the role of N-glycosylation of Kv1.1 and Kv1.4 on their protein stability, cellular localization pattern, and trafficking to the cell surface. We found that preventing N-glycosylation of Kv1.4 decreased its protein stability, induced its high partial intracellular retention, and decreased its cell surface protein levels, whereas it had little or no effect on these parameters for Kv1.1. Exchanging a trafficking pore region determinant between Kv1.1 and Kv1.4 reversed these effects of glycosylation on these chimeric channels. Thus it appeared that the Kv1.4 pore region determinant and the sugar tree attached to the S1-S2 linker showed some type of dependence in promoting proper trafficking of the protein to the cell surface, and this dependence can be transferred to chimeric Kv1.1 proteins that contain the Kv1.4 pore. Understanding the different trafficking programs of Kv1 channels, and whether they are altered by glycosylation, will highlight the different posttranslational mechanisms available to cells to modify their cell surface ion channel levels and possibly their signaling characteristics.

Detailed glycan analysis of serum glycoproteins of patients with congenital disorders of glycosylation indicates the specific defective glycan processing step and provides an insight into pathogenesis

The fundamental importance of correct protein glycosylation is abundantly clear in a group of diseases known as congenital disorders of glycosylation (CDGs). In these diseases, many biological functions are compromised, giving rise to a wide range of severe clinical conditions. By performing detailed analyses of the total serum glycoproteins as well as isolated transferrin and IgG, we have directly correlated aberrant glycosylation with a faulty glycosylation processing step. In one patient the complete absence of complex type sugars was consistent with ablation of GlcNAcTase II activity. In another CDG type II patient, the identification of specific hybrid sugars suggested that the defective processing step was cell type-specific and involved the mannosidase III pathway. In each case, complementary serum proteome analyses revealed significant changes in some 31 glycoproteins, including components of the complement system. This biochemical approach to charting diseases that involve alterations in glycan processing provides a rapid indicator of the nature, severity, and cell type specificity of the suboptimal glycan processing steps; allows links to genetic mutations; indicates the expression levels of proteins; and gives insight into the pathways affected in the disease process.

Glycosylation affects rat Kv1.1 potassium channel gating by a combined surface potential and cooperative subunit interaction mechanism

The effect of glycosylation on Kv1.l potassium channel function was investigated in mammalian cells stably transfected with Kv1.l or Kv1.1N207Q. Macroscopic current analysis showed that both channels were expressed but Kv1.1N207Q, which was not glycosylated, displayed functional differences compared with wild-type, including slowed activation kinetics, a positively shifted V 1/2, a shallower slope for the conductance versus voltage relationship, slowed C-type inactivation kinetics, and a reduced extent of and recovery from C-type inactivation. Kv1. 1N207Q activation properties were also less sensitive to divalent cations compared with those of Kv1.l. These effects were largely due to the lack of trans-Golgi added sugars, such as galactose and sialic acid, to the N207 carbohydrate tree. No apparent change in ionic current deactivation kinetics was detected inKv1.1N207Q compared with wild-type. Our data, coupled with modelling, suggested that removal of the N207 carbohydrate tree had two major effects. The first effect slowed the concerted channel transition from the last dosed state to the open state without changing the voltage dependence of its kinetics. This effect contributed to the G-V curve depolarization shift and together with the lower sensitivity to divalent cations suggested that the carbohydrate tree and its negatively charged sialic acids affected the negative surface charge density on the channel's extracellular face that was sensed by the activation gating machinery. The second effect reduced a cooperativity factor that slowed the transition from the open state to the dosed state without changing its voltage dependence. This effect accounted for the shallower G-V slope, and contributed to the depolarized G-V shift, and together with the inactivation changes it suggested that the carbohydrate tree also affected channel conformations. Thus N-glycosylation, and particularly terminal sialylation, affected Kv1.l gating properties both by altering the surface potential sensed by the channel's activation gating machinery and by modifying conformational changes regulating cooperative subunit interactions during activation and inactivation. Differences in glycosylation pattern among closely related channels may contribute to their functional differences and affect their physiological roles.

Structure of beta-antithrombin and the effect of glycosylation on antithrombin's heparin affinity and activity

Antithrombin is a member of the serpin family of protease inhibitors and the major inhibitor of the blood coagulation cascade. It is unique amongst the serpins in that it circulates in a conformation that is inactive against its target proteases. Activation of antithrombin is brought about by a conformational change initiated upon binding heparin or heparan sulphate. Two isoforms exist in the circulation, alpha-antithrombin and beta-antithrombin, which differ in the amount of glycosylation present on the polypeptide chain; beta-antithrombin lacks the carbohydrate present at Asn135 in alpha-antithrombin. Of the two forms, beta-antithrombin has the higher affinity for heparin and thus functions as the major inhibitor in vivo even though it is the less abundant form. The reason for the differences in heparin affinity between the alpha and beta-forms have been shown to be due to the additional carbohydrate changing the rate of the conformational change. Here, we describe the most accurate structures of alpha-antithrombin and alpha-antithrombin+heparin pentasaccharide reported to date (2.6A and 2.9A resolution, respectively, both re-refinements using old data), and the structure of beta-antithrombin (2.6A resolution). The new structures have a remarkable degree of ordered carbohydrate and include parts of the antithrombin chain not modeled before. The structures have allowed a detailed comparison of the conformational differences between the three. They show that the structural basis of the lower affinity for heparin of alpha-antithrombin over beta-antithrombin is due to the conformational change that occurs upon heparin binding being sterically hindered by the presence of the additional bulky carbohydrate at Asn135.

Biochemical markers of alcohol use in pregnant women

OBJECTIVES

To describe the serious health consequences of alcohol (ethanol) use, especially as they relate to pregnancy and the development of fetal alcohol syndrome (FAS) and fetal alcohol effects (FAE). The classic markers of alcohol exposure, including blood/breath alcohol, gamma-glutamyl transferase (gammaGT), mean corpuscular volume (MCV), hemoglobin-associated acetaldehyde (HAA) and carbohydrate deficient transferrin (CDT), are valuable and their methods of analysis are reviewed.

CONCLUSIONS

Since both FAS and FAE represent two of the leading preventable causes of mental retardation and birth defects, identification of alcohol use early in pregnancy is important to avoid adverse fetal outcomes. Unfortunately, the diagnosis of FAS and FAE is usually made after birth, when alcohol damage has become irreversible and permanent. The clinical laboratory can help prevent this damage and make a valuable contribution in assessing prenatal alcohol use. The clinical utility of blood/breath alcohol, gammaGT, MCV, HAA and CDT in alcohol use identification, especially in pregnancy, is substantial. Although none of the markers singularly has adequate sensitivity and specificity for screening, their diagnostic utility increases when measured as a panel. This is especially true in detecting alcohol use in pregnancy where the presence of several positive markers was correlated with the presence of alcohol-related fetal effects.

A deletion-insertion mutation in the phosphomannomutase 2 gene in an African American patient with congenital disorders of glycosylation-Ia

Congenital disorders of glycosylation (CDG) are a group of metabolic disorders with multisystemic involvement characterized by abnormalities in the synthesis of N-linked oligosaccharides. The most common form, CDG-Ia, resulting from mutations in the gene encoding the enzyme phosphomannomutase (PMM2), manifests with severe abnormalities in psychomotor development, dysmorphic features and visceral involvement. While this disorder is panethnic, we present the first cases of CDG-Ia identified in an African American family with two affected sisters. The proband had failure to thrive in infancy, hypotonia, ataxia, cerebellar hypoplasia and developmental delay. On examination, she also exhibited strabismus, inverted nipples and an atypical perineal fat distribution, all features characteristic of CDG-Ia. Direct sequencing demonstrated that the patient had a unique genotype, T237M/c.565-571 delAGAGAT insGTGGATTTCC. The novel deletion-insertion mutation, which was confirmed by subcloning and sequencing of each allele, introduces a stop codon 11 amino acids downstream from the site of the deletion. The presence of this deletion-insertion mutation at cDNA position 565 suggests that this site in the PMM2 gene may be a hotspot for chromosomal breakage.

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.

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.

Genetic defects in N-glycosylation and cellular diversity in mammals

Glycoproteins in mammalian cells are modified with complex-type aspargine-linked glycans of variable chain lengths and composition. Observations of mice carrying mutations in glycosyltransferase genes imply that N-glycan structures regulate T-cell receptor clustering and hence sensitivity to agonists. We argue that the heterogeneity inherent in N-glycosylation contributes to cellular diversity and, thereby, to adaptability in the immune system.

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.

Genomic organization of the human phosphomannose isomerase (MPI) gene and mutation analysis in patients with congenital disorders of glycosylation type Ib (CDG-Ib)

CDG-Ib is the "gastro-intestinal" type of the congenital disorders of glycosylation (CDG) and a potentially treatable disorder. It has been described in patients presenting with congenital hepatic fibrosis and protein losing enteropathy. The symptoms result from hypoglycosylation of serum- and other glycoproteins. CDG-Ib is caused by a deficiency of mannose-6-phosphate isomerase (synonym: phosphomannose isomerase, EC 5.3.1.8), due to mutations in the MPI gene. We determined the genomic structure of the MPI gene in order to simplify mutation detection. The gene is composed of 8 exons and spans only 5 kb. Eight (7 novel) different mutations were found in seven patients with a confirmed phosphomannose isomerase deficiency, analyzed in the context of this study: six missense mutations, a splice mutation and one insertion. In the last, the mutation resulted in an unstable transcript, and was hardly detectable at the mRNA level. This emphasizes the importance of mutation analysis at the genomic DNA level.

The joys of HexNAc. The synthesis and function of N- and O-glycan branches

This review covers discoveries made over the past 30-35 years that were important to our understanding of the synthetic pathway required for initiation of the antennae or branches on complex N-glycans and O-glycans. The review deals primarily with the author's contributions but the relevant work of other laboratories is also discussed. The focus of the review is almost entirely on the glycosyltransferases involved in the process. The following topics are discussed. (1) The localization of the synthesis of complex N-glycan antennae to the Golgi apparatus. (2) The "evolutionary boundary" at the stage in N-glycan processing where there is a change from oligomannose to complex N-glycans; this switch correlates with the appearance of multicellular organisms. (3) The discovery of the three enzymes which play a key role in this switch, N-acetylglucosaminyltransferases I and II and mannosidase II. (4) The "yellow brick road" which leads from oligomannose to highly branched complex N-glycans with emphasis on the enzymes involved in the process and the factors which control the routes of synthesis. (5) A short discussion of the characteristics of the enzymes involved and of the genes that encode them. (6) The role of complex N-glycans in mammalian and Caenorhabditis elegans development. (7) The crystal structure of N-acetylglucosaminyltransferase I. (8) The discovery of the enzymes which synthesize O-glycan cores 1, 2, 3 and 4 and their elongation.

Analysis of multiple mutations in the hALG6 gene in a patient with congenital disorder of glycosylation Ic

Congenital disorder of glycosylation Ic is caused by mutations in the hALG6 gene that encodes an alpha-1,3 glucosyltransferase. This enzyme is required for the addition of the first glucose residue to the lipid-linked oligosaccharide precursor for N-linked glycosylation. Here we describe the biochemical and molecular analysis of a patient with three mutations in the hALG6 gene. The maternal allele has an intronic G --> A mutation resulting in skipping of exon3 (IVS3 + 5G > A). This produces a nonfunctional enzyme as shown by its inability to restore normal glycosylation in a Saccharomyces cerevisiae strain lacking a functional ALG6. The paternal allele has two mutations. One is a deletion of three bases (895-897delATA) leading to an in-frame deletion of isoleucine 299 (delI299) located in a transmembrane domain. The second mutation on the same allele 911T > C causes a F304S change. When expressed in the ALG6 deficient yeast strain, this allele restores glycosylation but the mRNA is unstable or inefficiently transcribed, contributing to the impaired glycosylation in the patient.

Molecular basis of carbohydrate-deficient glycoprotein syndromes type I with normal phosphomannomutase activity

Carbohydrate deficient glycoprotein syndromes (CDGS) are inherited disorders in glycosylation. Isoelectric focusing of serum transferrin is used as a biochemical indicator of CDGS; however, this technique cannot diagnose the molecular defect. Even though phosphomannomutase (PMM) deficiency accounts for the great majority of known CDGS cases (CDGS type Ia), newly discovered cases have significantly different clinical presentations than the PMM-deficient patients. These differences arise from other defects affecting the biosynthesis of N-linked oligosaccharides in the endoplasmic reticulum and in the Golgi compartment. The most notable is the loss of phosphomannose isomerase (PMI) (CDGS type Ib). It causes severe hypoglycemia, protein-losing enteropathy, vomiting, diarrhea, and congenital hepatic fibrosis. In contrast to PMM-deficiency, there is no developmental delay nor neuropathy. Most symptoms in the PMI-deficient patients can be successfully treated with dietary mannose supplements. Another defect is the lack of glucosylation of the lipid-linked oligosaccharide precursor. The clinical features of this form of CDGS are milder, but similar to, PMM-deficient patients. Yeast genetic and biochemical techniques were critical in unraveling these disorders since many of the defective genes were known in yeast and corresponding mutants were available for complementation. Yeast strains carrying mutations in the homologous genes are likely to provide conclusive identification of the primary defects in novel CDGS types that affect the synthesis and transfer of precursor oligosaccharides.