Mutations in MAGT1 lead to a glycosylation disorder with a variable phenotype.

Congenital disorders of glycosylation (CDG) are a group of rare metabolic diseases, due to impaired protein and lipid glycosylation. We identified two patients with defective serum transferrin glycosylation and mutations in the MAGT1 gene. These patients present with a phenotype that is mainly characterized by intellectual and developmental disability. MAGT1 has been described to be a subunit of the oligosaccharyltransferase (OST) complex and more specifically of the STT3B complex. However, it was also claimed that MAGT1 is a magnesium (Mg2+) transporter. So far, patients with mutations in MAGT1 were linked to a primary immunodeficiency, characterized by chronic EBV infections attributed to a Mg2+ homeostasis defect (XMEN). We compared the clinical and cellular phenotype of our two patients to that of an XMEN patient that we recently identified. All three patients have an N-glycosylation defect, as was shown by the study of different substrates, such as GLUT1 and SHBG, demonstrating that the posttranslational glycosylation carried out by the STT3B complex is dysfunctional in all three patients. Moreover, MAGT1 deficiency is associated with an enhanced expression of TUSC3, the homolog protein of MAGT1, pointing toward a compensatory mechanism. Hence, we delineate MAGT1-CDG as a disorder associated with two different clinical phenotypes caused by defects in glycosylation.

International clinical guidelines for the management of phosphomannomutase 2-congenital disorders of glycosylation: Diagnosis, treatment and follow up.

Phosphomannomutase 2 (PMM2-CDG) is the most common congenital disorder of N-glycosylation and is caused by a deficient PMM2 activity. The clinical presentation and the onset of PMM2-CDG vary among affected individuals ranging from a severe antenatal presentation with multisystem involvement to mild adulthood presentation limited to minor neurological involvement. Management of affected patients requires a multidisciplinary approach. In this article, a systematic review of the literature on PMM2-CDG was conducted by a group of international experts in different aspects of CDG. Our managment guidelines were initiated based on the available evidence-based data and experts' opinions. This guideline mainly addresses the clinical evaluation of each system/organ involved in PMM2-CDG, and the recommended management approach. It is the first systematic review of current practices in PMM2-CDG and the first guidelines aiming at establishing a practical approach to the recognition, diagnosis and management of PMM2-CDG patients.

Use of Endoglycosidase H as a diagnostic tool for MAN1B1-CDG patients.

Congenital disorders of glycosylation (CDG) are heterogeneous group of genetic protein and lipid glycosylation abnormalities. With some 33 reported patients, MAN1B1-CDG belongs to the more frequent causes of CDG-II. MAN1B1 encodes an α1,2-mannosidase that removes the terminal mannose residue from the middle branch. Several methods have been proposed to characterize the glycosylation changes. In MAN1B1-CDG, the abnormal accumulating N-glycan structures are mostly absent or found in trace amounts in total human serum. To overcome this issue, in this study, we present a straightforward procedure based on the use of Endo-ß-N-acetylglucosaminidase H to easily diagnose MAN1B1-CDG patients and mannosidase defects.

Manganese-induced turnover of TMEM165.

TMEM165 deficiencies lead to one of the congenital disorders of glycosylation (CDG), a group of inherited diseases where the glycosylation process is altered. We recently demonstrated that the Golgi glycosylation defect due to TMEM165 deficiency resulted from a Golgi manganese homeostasis defect and that Mn2+ supplementation was sufficient to rescue normal glycosylation. In the present paper, we highlight TMEM165 as a novel Golgi protein sensitive to manganese. When cells were exposed to high Mn2+ concentrations, TMEM165 was degraded in lysosomes. Remarkably, while the variant R126H was sensitive upon manganese exposure, the variant E108G, recently identified in a novel TMEM165-CDG patient, was found to be insensitive. We also showed that the E108G mutation did not abolish the function of TMEM165 in Golgi glycosylation. Altogether, the present study identified the Golgi protein TMEM165 as a novel Mn2+-sensitive protein in mammalian cells and pointed to the crucial importance of the glutamic acid (E108) in the cytosolic ELGDK motif in Mn2+-induced degradation of TMEM165.

What is new in CDG?

Congenital disorders of glycosylation (CDG) are one group among the disorders of glycosylation. The latter comprise defects associated with hypoglycosylation but also defects with hyperglycosylation. Genetic diseases with hypoglycosylation can be divided in primary congenital disorders of glycosylation (CDG) and in genetic diseases causing secondary hypoglycosylation. This review covers the human CDG highlights from the last 3 years (2014-2016) following a summary of the actual status of CDG. It expands on 23 novel CDG namely defects in SLC39A8, CAD, NANS, PGM3, SSR4, POGLUT1, NUS1, GANAB, PIGY, PIGW, PIGC, PIGG, PGAP1, PGAP3, VPS13B, CCDC115, TMEM199, ATP6AP1, ATP6V1A, ATP6V1E1, TRAPPC11, XYLT1 and XYLT2. Besides, it discusses novel phenotypes of known CDG (DHDDS-CDG, ALG9-CDG, EXT2-CDG, PIGA-CDG, PIGN-CDG), the elucidation of putative glycosyltransferase disorders as O-mannosylglycan synthesis disorders (TMEM5-CDG, ISPD-CDG, FKTN-CDG, FKRP-CDG), a novel CDG mechanism, advances in diagnosis, pathogenesis, treatment and finally an updated list of the 104 known CDG.

MAN1B1 deficiency: an unexpected CDG-II.

Congenital disorders of glycosylation (CDG) are a group of rare metabolic diseases, due to impaired protein and lipid glycosylation. In the present study, exome sequencing was used to identify MAN1B1 as the culprit gene in an unsolved CDG-II patient. Subsequently, 6 additional cases with MAN1B1-CDG were found. All individuals presented slight facial dysmorphism, psychomotor retardation and truncal obesity. Generally, MAN1B1 is believed to be an ER resident alpha-1,2-mannosidase acting as a key factor in glycoprotein quality control by targeting misfolded proteins for ER-associated degradation (ERAD). However, recent studies indicated a Golgi localization of the endogenous MAN1B1, suggesting a more complex role for MAN1B1 in quality control. We were able to confirm that MAN1B1 is indeed localized to the Golgi complex instead of the ER. Furthermore, we observed an altered Golgi morphology in all patients' cells, with marked dilatation and fragmentation. We hypothesize that part of the phenotype is associated to this Golgi disruption. In conclusion, we linked mutations in MAN1B1 to a Golgi glycosylation disorder. Additionally, our results support the recent findings on MAN1B1 localization. However, more work is needed to pinpoint the exact function of MAN1B1 in glycoprotein quality control, and to understand the pathophysiology of its deficiency.

Diagnostic serum glycosylation profile in patients with intellectual disability as a result of MAN1B1 deficiency.

Congenital disorders of glycosylation comprise a group of genetic defects with a high frequency of intellectual disability, caused by deficient glycosylation of proteins and lipids. The molecular basis of the majority of the congenital disorders of glycosylation type I subtypes, localized in the cytosol and endoplasmic reticulum, has been solved. However, elucidation of causative genes for defective Golgi glycosylation (congenital disorders of glycosylation type II) remains challenging because of a lack of sufficiently specific diagnostic serum methods. In a single patient with intellectual disability, whole-exome sequencing revealed MAN1B1 as congenital disorder of glycosylation type II candidate gene. A novel mass spectrometry method was applied for high-resolution glycoprofiling of intact plasma transferrin. A highly characteristic glycosylation signature was observed with hybrid type N-glycans, in agreement with deficient mannosidase activity. The speed and robustness of the method allowed subsequent screening in a cohort of 100 patients with congenital disorder of glycosylation type II, which revealed the characteristic glycosylation profile of MAN1B1-congenital disorder of glycosylation in 11 additional patients. Abnormal hybrid type N-glycans were also observed in the glycoprofiles of total serum proteins, of enriched immunoglobulins and of alpha1-antitrypsin in variable amounts. Sanger sequencing revealed MAN1B1 mutations in all patients, including severe truncating mutations and amino acid substitutions in the alpha-mannosidase catalytic site. Clinically, this group of patients was characterized by intellectual disability and delayed motor and speech development. In addition, variable dysmorphic features were noted, with truncal obesity and macrocephaly in ∼65% of patients. In summary, MAN1B1 deficiency appeared to be a frequent cause in our cohort of patients with unsolved congenital disorder of glycosylation type II. Our method for analysis of intact transferrin provides a rapid test to detect MAN1B1-deficient patients within congenital disorder of glycosylation type II cohorts and can be used as efficient diagnostic method to identify MAN1B1-deficient patients in intellectual disability cohorts. In addition, it provides a functional confirmation of MAN1B1 mutations as identified by next-generation sequencing in individuals with intellectual disability.

Glycosylation disorders of membrane trafficking.

During evolution from prokaryotic to eukaryotic cells, compartmentalization of cellular functions has been achieved with a high degree of complexity. Notably, all secreted and transmembrane proteins travel through endoplasmic reticulum (ER) and Golgi apparatus, where they are synthesized, folded and subjected to covalent modifications, most particularly glycosylation. N-glycosylation begins in the ER with synthesis and transfer of glycan onto nascent protein and proceeds in Golgi apparatus where maturation occurs. This process not only requires the precise localization of glycosyltransferases, glycosidases and substrates but also an efficient, finely regulated and bidirectional vesicular trafficking among membrane-enclosed organelles. Basically, it is no surprise that alterations in membrane transport or related pathways can lead to glycosylation abnormalities. During the last few years, this has particularly been highlighted in genetic diseases called CDG (Congenital Disorders of Glycosylation). Alterations in mechanisms of vesicle formation due to COPII coat component SEC23B deficiency, or in vesicles tethering, caused by defects of the COG complex, but also impaired Golgi pH homeostasis due to ATP6V0A2 defects have been discovered in CDG patients. This mini review will summarize these fascinating discoveries.

Differential effects of lobe A and lobe B of the Conserved Oligomeric Golgi complex on the stability of {beta}1,4-galactosyltransferase 1 and {alpha}2,6-sialyltransferase 1.

Initially described by Jaeken et al. in 1980, congenital disorders of glycosylation (CDG) is a rapidly expanding group of human multisystemic disorders. To date, many CDG patients have been identified with deficiencies in the conserved oligomeric Golgi (COG) complex which is a complex involved in the vesicular intra-Golgi retrograde trafficking. Composed of eight subunits that are organized in two lobes, COG subunit deficiencies have been associated with Golgi glycosylation abnormalities. Analysis of the total serum N-glycans of COG-deficient CDG patients demonstrated an overall decrease in terminal sialylation and galactosylation. According to the mutated COG subunits, differences in late Golgi glycosylation were observed and led us to address the question of an independent role and requirement for each of the two lobes of the COG complex in the stability and localization of late terminal Golgi glycosylation enzymes. For this, we used a small-interfering RNAs strategy in HeLa cells stably expressing green fluorescent protein (GFP)-tagged ß1,4-galactosyltransferase 1 (B4GALT1) and α2,6-sialyltransferase 1 (ST6GAL1), two major Golgi glycosyltransferases involved in late Golgi N-glycosylation. Using fluorescent lectins and flow cytometry analysis, we clearly demonstrated that depletion of both lobes was associated with deficiencies in terminal Golgi N-glycosylation. Lobe A depletion resulted in dramatic changes in the Golgi structure, whereas lobe B depletion severely altered the stability of B4GALT1 and ST6GAL1. Only MG132 was able to rescue their steady-state levels, suggesting that B4GALT1- and ST6GAL1-induced degradation are likely the consequence of an accumulation in the endoplasmic reticulum (ER), followed by a retrotranslocation into the cytosol and proteasomal degradation. All together, our results suggest differential effects of lobe A and lobe B for the localization/stability of B4GALT1 and ST6GAL1. Lobe B would be crucial in preventing these two Golgi glycosyltransferases from inappropriate retrograde trafficking to the ER, whereas lobe A appears to be essential for maintaining the overall Golgi structure.

Congenital disorders of glycosylation (CDG): Quo vadis?

The survey summarizes in its first part the current status of knowledge on the Congenital Disorders of Glycosylation (CDG) with regard to their phenotypic spectrum, diagnostic and therapeutic strategies, and pathophysiology. It documents the clinical and basic research activities, and efforts to involve patients and their families. In the second part, it tries to look into the future of CDG. More specific biomarkers are needed for fast CDG diagnosis and treatment monitoring. Whole genome sequencing will play an increasingly important role in the molecular diagnosis of unsolved CDG. Epigenetic defects are expected to join the rapidly expanding genetic and allelic heterogeneity of the CDG family. Novel treatments are urgently needed particularly for PMM2-CDG, the most prevalent CDG. Patient services such as apps should be developed e.g. to document the natural history and monitor treatment. Networking (EURO-CDG, the European Reference Networks (MetabERN)) is an efficient tool to disseminate knowledge and boost collaboration at all levels. The final goal is of course to improve the quality of life of the patients and their families.

Mutations in the X-linked ATP6AP2 cause a glycosylation disorder with autophagic defects.

The biogenesis of the multi-subunit vacuolar-type H+-ATPase (V-ATPase) is initiated in the endoplasmic reticulum with the assembly of the proton pore V0, which is controlled by a group of assembly factors. Here, we identify two hemizygous missense mutations in the extracellular domain of the accessory V-ATPase subunit ATP6AP2 (also known as the [pro]renin receptor) responsible for a glycosylation disorder with liver disease, immunodeficiency, cutis laxa, and psychomotor impairment. We show that ATP6AP2 deficiency in the mouse liver caused hypoglycosylation of serum proteins and autophagy defects. The introduction of one of the missense mutations into Drosophila led to reduced survival and altered lipid metabolism. We further demonstrate that in the liver-like fat body, the autophagic dysregulation was associated with defects in lysosomal acidification and mammalian target of rapamycin (mTOR) signaling. Finally, both ATP6AP2 mutations impaired protein stability and the interaction with ATP6AP1, a member of the V0 assembly complex. Collectively, our data suggest that the missense mutations in ATP6AP2 lead to impaired V-ATPase assembly and subsequent defects in glycosylation and autophagy.

Assessing ER and Golgi N-glycosylation process using metabolic labeling in mammalian cultured cells.

Modifications of N-glycosylation in disease states are common and illustrate the crucial requirement of glycosylation in human biology. Mainly based on glycan permethylation and the use of mass spectrometry analysis, we can easily understand that many different methods to analyze the N-glycome have seen the day. While extremely powerful, these methods are mainly used to analyze qualitative variations of N-glycosylation of human serum proteins and do not necessarily reflect the glycosylation status of derived mammalian cultured cells. This chapter summarizes two methods that we are routinely using in our laboratory to assess the ER and Golgi N-glycosylation process. The proposed methodology allows pinpointing ER as well as Golgi glycosylation deficiencies in mammalian cultured cells. The first approach is based on direct metabolic labeling of cultured mammalian cells with [2-(3)H] mannose followed by sequential extraction and HPLC analysis of the purified oligosaccharides. The second one is based on the copper-catalyzed azide alkyne cycloaddition (CuAAC) strategy. We propose the use of alkyne-tagged sialic acid (SialNAl) to visualize the Golgi glycosylation efficiency. Their metabolic incorporation into newly synthesized glycoproteins can then be chemoselectively coupled to complementary azide-functionalized fluorophores, and visualized by using confocal laser scanning microscopy. To summarize, we present here a detailed description of our know-how in the field of ER and Golgi N-glycosylation.

Identification of phosphorylated oligosaccharides in cells of patients with a congenital disorders of glycosylation (CDG-I).

Protein N-glycosylation is initiated by the dolichol cycle in which the oligosaccharide precursor Glc(3)Man(9)GlcNAc(2)-PP-dolichol is assembled in the endoplasmic reticulum (ER). One critical step in the dolichol cycle concerns the availability of Dol-P at the cytosolic face of the ER membrane. In RFT1 cells, the lipid-linked oligosaccharide (LLO) intermediate Man(5)GlcNAc(2)-PP-Dol accumulates at the cytosolic face of the ER membrane. Since Dol-P is a rate-limiting intermediate during protein N-glycosylation, continuous accumulation of Man(5)GlcNAc(2)-PP-Dol would block the dolichol cycle. Hence, we investigated the molecular mechanisms by which accumulating Man(5)GlcNAc(2)-PP-Dol could be catabolized in RFT1 cells. On the basis of metabolic labeling experiments and in comparison to human control cells, we identified phosphorylated oligosaccharides (POS), not found in human control cells and present evidence that they originate from the accumulating LLO intermediates. In addition, POS were also detected in other CDG patients' cells accumulating specific LLO intermediates at different cellular locations. Moreover, the enzymatic activity that hydrolyses oligosaccharide-PP-Dol into POS was identified in human microsomal membranes and required Mn(2+) for optimal activity. In CDG patients' cells, we thus identified and characterized POS that could result from the catabolism of accumulating LLO intermediates.