Quality control of glycoproteins bearing truncated glycans in an ALG9-defective (CDG-IL) patient

Deficient glycan extension and endoplasmic reticulum stresses in ALG3-CDG

ALG3-CDG is a rare congenital disorder of glycosylation (CDG) with a clinical phenotype that includes neurological manifestations, transaminitis, and frequent infections. The ALG3 enzyme catalyzes the first step of endoplasmic reticulum (ER) luminal glycan extension by adding mannose from Dol-P-Man to Dol-PP-Man5GlcNAc2 (Man5) forming Dol-PP-Man6. Such glycan extension is the first and fastest cellular response to ER stress, which is deficient in ALG3-CDG. In this study, we provide evidence that the unfolded protein response (UPR) and ER-associated degradation activities are increased in ALG3-CDG patient-derived cultured skin fibroblasts and there is constitutive activation of UPR mediated by the IRE1-α pathway. In addition, we show that N-linked Man3-4 glycans are increased in cellular glycoproteins and secreted plasma glycoproteins with hepatic or non-hepatic origin. We found that like other CDGs such as ALG1- or PMM2-CDG, in transferrin, the assembling intermediate Man5 in ALG3-CDG, are likely further processed into a distinct glycan, NeuAc1Gal1GlcNAc1Man3GlcNAc2, probably by Golgi mannosidases and glycosyltransferases. We predict it to be a mono-antennary glycan with the same molecular weight as the truncated glycan described in MGAT2-CDG. In summary, this study elucidates multiple previously unrecognized biochemical consequences of the glycan extension deficiency in ALG3-CDG which will have important implications in the pathogenesis of CDG.

Heterozygosity of ALG9 in Association with Autosomal Dominant Polycystic Liver Disease

α-1,2-mannosyltransferase (ALG9) germline variants are linked to autosomal dominant polycystic kidney disease (ADPKD). Many individuals affected with ADPKD possess polycystic livers as a common extrarenal manifestation. We performed whole exome sequencing in a female with autosomal dominant polycystic liver disease (ADPLD) without kidney cysts and established the presence of a heterozygous missense variant (c.677G>C p.(Gly226Ala)) in ALG9. In silico pathogenicity prediction and 3D protein modeling determined this variant as pathogenic. Loss of heterozygosity is regularly seen in liver cyst walls. Immunohistochemistry indicated the absence of ALG9 in liver tissue from this patient. ALG9 expression was absent in cyst wall lining from ALG9- and PRKCSH-caused ADPLD patients but present in the liver cyst lining derived from an ADPKD patient with a PKD2 variant. Thus, heterozygous pathogenic variants in ALG9 are also associated with ADPLD. Somatic loss of heterozygosity of the ALG9 enzyme was seen in the ALG9 patient but also in ADPLD patients with a different genetic background. This expanded the phenotypic spectrum of ADPLD to ALG9.

Metabolic Cardiomyopathies and Cardiac Defects in Inherited Disorders of Carbohydrate Metabolism: A Systematic Review

Heart failure (HF) is a progressive chronic disease that remains a primary cause of death worldwide, affecting over 64 million patients. HF can be caused by cardiomyopathies and congenital cardiac defects with monogenic etiology. The number of genes and monogenic disorders linked to development of cardiac defects is constantly growing and includes inherited metabolic disorders (IMDs). Several IMDs affecting various metabolic pathways have been reported presenting cardiomyopathies and cardiac defects. Considering the pivotal role of sugar metabolism in cardiac tissue, including energy production, nucleic acid synthesis and glycosylation, it is not surprising that an increasing number of IMDs linked to carbohydrate metabolism are described with cardiac manifestations. In this systematic review, we offer a comprehensive overview of IMDs linked to carbohydrate metabolism presenting that present with cardiomyopathies, arrhythmogenic disorders and/or structural cardiac defects. We identified 58 IMDs presenting with cardiac complications: 3 defects of sugar/sugar-linked transporters (GLUT3, GLUT10, THTR1); 2 disorders of the pentose phosphate pathway (G6PDH, TALDO); 9 diseases of glycogen metabolism (GAA, GBE1, GDE, GYG1, GYS1, LAMP2, RBCK1, PRKAG2, G6PT1); 29 congenital disorders of glycosylation (ALG3, ALG6, ALG9, ALG12, ATP6V1A, ATP6V1E1, B3GALTL, B3GAT3, COG1, COG7, DOLK, DPM3, FKRP, FKTN, GMPPB, MPDU1, NPL, PGM1, PIGA, PIGL, PIGN, PIGO, PIGT, PIGV, PMM2, POMT1, POMT2, SRD5A3, XYLT2); 15 carbohydrate-linked lysosomal storage diseases (CTSA, GBA1, GLA, GLB1, HEXB, IDUA, IDS, SGSH, NAGLU, HGSNAT, GNS, GALNS, ARSB, GUSB, ARSK). With this systematic review we aim to raise awareness about the cardiac presentations in carbohydrate-linked IMDs and draw attention to carbohydrate-linked pathogenic mechanisms that may underlie cardiac complications.

Gillessen-Kaesbach-Nishimura syndrome in two fetuses from Turkey

Gillessen-Kaesbach-Nishimura syndrome (GIKANIS) is a congenital disease of glycosylation (CDG) linked to the ALG9 gene. GIKANIS is a lethal disorder characterized by atypical facial features, generalized skeletal changes with shortening of the long bones with broad, round metaphyses, round ilia, and deficient ossification of the skull, cervical spine and pubic bones, and visceral abnormalities including polycystic kidneys and congenital cardiac defects. GIKANIS is caused by a homozygous splicing variant (c.1173 + 2 T > A) leading to skipping of exon 10, frameshift, and premature termination codon of the ALG9 gene. To our best knowledge, only two affected families with confirmed molecular analyses have been reported. We present an additional report on two siblings with the same mutation, emphasizing the prenatal ultrasonographic features. Their facial and skeletal manifestations recapitulated those previously reported. Ultrasonography revealed polycystic kidneys and unbalanced atrioventricular septal defect (AVSD) with transposition of the great arteries.

Missense variant c.1460 T > C (p.L487P) enhances protein degradation of ER mannosyltransferase ALG9 in two new ALG9-CDG patients presenting with West syndrome and review of the literature

ALG9-CDG is a CDG-I defect within the group of Congenital Disorders of Glycosylation (CDG). We here describe the clinical symptoms of two new and unrelated ALG9-CDG patients, both carrying the novel homozygous missense variant c.1460 T > C (p.L487P) in the ALG9 gene which led to global developmental delay, psychomotor disability, facial dysmorphisms, brain and heart defects, hearing loss, hypotonia, as well as feeding problems. New clinical symptoms comprised West syndrome with hypsarrhythmia. Quantitative RT-PCR analysis revealed a significantly enhanced ALG9 mRNA transcript level, whereas the protein amount in fibroblasts was significantly reduced. This could be ascribed to a stronger degradation of the mutated ALG9 protein in patient fibroblasts. Lipid-linked oligosaccharide analysis showed an ALG9-CDG characteristic accumulation of Man₆GlcNAc₂-PP-dolichol and Man₈GlcNAc₂-PP-dolichol in patient cells. The clinical findings of our patients and of all previously published ALG9-CDG patients are brought together to further expand the knowledge about this rare N-glycosylation disorder. SYNOPSIS: Homozygosity for p.L487P in ALG9 causes protein degradation and leads to West syndrome.

Recognizable phenotypes in CDG

Pattern recognition, using a group of characteristic, or discriminating features, is a powerful tool in metabolic diagnostic. A classic example of this approach is used in biochemical analysis of urine organic acid analysis, where the reporting depends more on the correlation of pertinent positive and negative findings, rather than on the absolute values of specific markers. Similar uses of pattern recognition in the field of biochemical genetics include the interpretation of data obtained by metabolomics, like glycomics, where a recognizable pattern or the presence of a specific glycan sub-fraction can lead to the direct diagnosis of certain types of congenital disorders of glycosylation. Another indispensable tool is the use of clinical pattern recognition-or syndromology-relying on careful phenotyping. While genomics might uncover variants not essential in the final clinical expression of disease, and metabolomics could point to a mixture of primary but also secondary changes in biochemical pathways, phenomics describes the clinically relevant manifestations and the full expression of the disease. In the current review we apply phenomics to the field of congenital disorders of glycosylation, focusing on recognizable differentiating findings in glycosylation disorders, characteristic dysmorphic features and malformations in PMM2-CDG, and overlapping patterns among the currently known glycosylation disorders based on their pathophysiological basis.

ALG9-CDG: New clinical case and review of the literature

Congenital disorders of glycosylation (CDG) are a group of metabolic diseases resulting from defects in glycan synthesis or processing. The number of subgroups and their phenotypic spectrums continue to expand with most related to deficiencies of N-glycosylation. ALG9-CDG (previously CDG-IL) is the result of a mutation in ALG9. This gene encodes the enzyme alpha-1,2-mannosyltransferase. To date, a total of 10 patients from 6 different families have been reported with one of four ALG9 mutations. Seven of these patients had a similar phenotype with failure to thrive, dysmorphic features, seizures, hepatic and/or renal cysts; the other three patients died in utero from a lethal skeletal dysplasia. This report describes an additional patient with ALG9-CDG who has a milder phenotype. This patient is a term female born to Caucasian, Canadian, non-consanguineous parents of Scottish decent. Prenatally, dysmorphic features, numerous renal cysts and minor cardiac malformations were detected. Post-natally, dysmorphic features included shallow orbits, micrognathia, hypoplastic nipples, talipes equinovarus, lipodystrophy and cutis marmorata. She developed failure to thrive and seizures. The metabolic work-up included analysis of a transferrin isoelectric focusing, which showed a type 1 pattern. This was confirmed by glycan profiling, which identified ahomozygous mutation in ALG9, c.860A > G (p.Tyr287Cys) (NM_1234567890). This had been previously published as a pathogenic mutation in two Canadian patients. Our goal is to contribute to the growing body of knowledge for this disorder by describing the phenotypic spectrum and providing further insight on prognosis.

Cardiac complications of congenital disorders of glycosylation (CDG): a systematic review of the literature

Congenital disorders of glycosylation (CDG) are inborn errors of metabolism due to protein and lipid hypoglycosylation. This rapidly growing family of genetic diseases comprises 103 CDG types, with a broad phenotypic diversity ranging from mild to severe poly-organ -system dysfunction. This literature review summarizes cardiac involvement, reported in 20% of CDG. CDG with cardiac involvement were divided according to the associated type of glycosylation: N-glycosylation, O-glycosylation, dolichol synthesis, glycosylphosphatidylinositol (GPI)-anchor biosynthesis, COG complex, V-ATPase complex, and other glycosylation pathways. The aim of this review was to document and interpret the incidence of heart disease in CDG patients. Heart disorders were grouped into cardiomyopathies, structural defects, and arrhythmogenic disorders. This work may contribute to improved early management of cardiac complications in CDG.

Phenotypic and genotypic spectrum of congenital disorders of glycosylation type I and type II

BACKGROUND

Congenital disorders of glycosylation (CDG) are inborn defects of glycan metabolism. They are multisystem disorders. Analysis of transferrin isoforms is applied as a screening test for CDG type I (CDG-I) and type II (CDG-II). We performed a retrospective cohort study to determine spectrum of phenotype and genotype and prevalence of the different subtypes of CDG-I and CDG-II.

MATERIAL AND METHODS

All patients with CDG-I and CDG-II evaluated in our institution's Metabolic Genetics Clinics were included. Electronic and paper patient charts were reviewed. We set-up a high performance liquid chromatography transferrin isoelectric focusing (TIEF) method to measure transferrin isoforms in our Institution. We reviewed the literature for the rare CDG-I and CDG-II subtypes seen in our Institution.

RESULTS

Fifteen patients were included: 9 with PMM2-CDG and 6 with non-PMM2-CDG (one ALG3-CDG, one ALG9-CDG, two ALG11-CDG, one MPDU1-CDG and one ATP6V0A2-CDG). All patients with PMM2-CDG and 5 patients with non-PMM2-CDG showed abnormal TIEF suggestive of CDG-I or CDG-II pattern. In all patients, molecular diagnosis was confirmed either by single gene testing, targeted next generation sequencing for CDG genes, or by whole exome sequencing.

CONCLUSION

We report 15 new patients with CDG-I and CDG-II. Whole exome sequencing will likely identify more patients with normal TIEF and expand the phenotypic spectrum of CDG-I and CDG-II.

Liver involvement in congenital disorders of glycosylation (CDG). A systematic review of the literature

Congenital disorders of glycosylation (CDG) are a rapidly growing family of genetic diseases caused by defects in glycosylation. Nearly 100 CDG types are known so far. Patients present a great phenotypic diversity ranging from poly- to mono-organ/system involvement and from very mild to extremely severe presentation. In this literature review, we summarize the liver involvement reported in CDG patients. Although liver involvement is present in only a minority of the reported CDG types (22 %), it can be debilitating or even life-threatening. Sixteen of the patients we collated here developed cirrhosis, 10 had liver failure. We distinguish two main groups: on the one hand, the CDG types with predominant or isolated liver involvement including MPI-CDG, TMEM199-CDG, CCDC115-CDG, and ATP6AP1-CDG, and on the other hand, the CDG types associated with liver disease but not as a striking, unique or predominant feature, including PMM2-CDG, ALG1-CDG, ALG3-CDG, ALG6-CDG, ALG8-CDG, ALG9-CDG, PGM1-CDG, and COG-CDG. This review aims to facilitate CDG patient identification and to understand CDG liver involvement, hopefully leading to earlier diagnosis, and better management and treatment.

Serum transferrin carrying the xeno-tetrasaccharide NeuAc-Gal-GlcNAc2 is a biomarker of ALG1-CDG

ALG1-CDG (formerly CDG-Ik) is a subtype of congenital disorders of glycosylation (CDG) where the genetic defect disrupts the synthesis of the lipid-linked oligosaccharide precursor required for N-glycosylation. The initial step in the investigation for these disorders involves the demonstration of hypoglycosylated serum transferrin (TF). There are no specific biomarkers of this CDG subtype known to date. An LC/MS approach was used to analyze sera from patients with ALG1-CDG, PMM2-CDG, suspected CDG, and individuals with alcohol abuse. We show mass spectrometric data combined with data from enzymatic digestions that suggest the presence of a tetrasaccharide consisting of two N-acetylglucosamines, one galactose, and one sialic acid, appearing on serum TF, is a biomarker of this particular CDG subtype. This is the first time analysis of serum TF can suggest a specific CDG type I subtype and we suggest this tetrasaccharide be used in the clinic to guide the ALG1-CDG diagnostic process.

Further Delineation of the ALG9-CDG Phenotype

ALG9-CDG is one of the less frequently reported types of CDG. Here, we summarize the features of six patients with ALG9-CDG reported in the literature and report the features of four additional patients. The patients presented with drug-resistant infantile epilepsy, hypotonia, dysmorphic features, failure to thrive, global developmental disability, and skeletal dysplasia. One patient presented with nonimmune hydrops fetalis. A brain MRI revealed global atrophy with delayed myelination. Exome sequencing identified a novel homozygous mutation c.1075G>A, p.E359K of the ALG9 gene. The results of our analysis of these patients expand the knowledge of ALG9-CDG phenotype.

A novel phenotype in N-glycosylation disorders: Gillessen-Kaesbach-Nishimura skeletal dysplasia due to pathogenic variants in ALG9

A rare lethal autosomal recessive syndrome with skeletal dysplasia, polycystic kidneys and multiple malformations was first described by Gillessen-Kaesbach et al and subsequently by Nishimura et al. The skeletal features uniformly comprise a round pelvis, mesomelic shortening of the upper limbs and defective ossification of the cervical spine. We studied two unrelated families including three affected fetuses with Gillessen-Kaesbach-Nishimura syndrome using whole-exome and Sanger sequencing, comparative genome hybridization and homozygosity mapping. All affected patients were shown to have a novel homozygous splice variant NM_024740.2: c.1173+2T>A in the ALG9 gene, encoding alpha-1,2-mannosyltransferase, involved in the formation of the lipid-linked oligosaccharide precursor of N-glycosylation. RNA analysis demonstrated skipping of exon 10, leading to shorter RNA. Mass spectrometric analysis showed an increase in monoglycosylated transferrin as compared with control tissues, confirming that this is a congenital disorder of glycosylation (CDG). Only three liveborn children with ALG9-CDG have been previously reported, all with missense variants. All three suffered from intellectual disability, muscular hypotonia, microcephaly and renal cysts, but none had skeletal dysplasia. Our study shows that some pathogenic variants in ALG9 can present as a lethal skeletal dysplasia with visceral malformations as the most severe phenotype. The skeletal features overlap with that previously reported for ALG3- and ALG12-CDG, suggesting that this subset of glycosylation disorders constitutes a new diagnostic group of skeletal dysplasias.

N-linked glycan profiling of GGTA1/CMAH knockout pigs identifies new potential carbohydrate xenoantigens

BACKGROUND

The temporary or long-term xenotransplantation of pig organs into people would save thousands of lives each year if not for the robust human antibody response to pig carbohydrates. Genetically engineered pigs deficient in galactose α1,3 galactose (gene modified: GGTA1) and N-glycolylneuraminic acid (gene modified: CMAH) have significantly improved cell survival when challenged by human antibody and complement in vitro. There remains, however, a significant portion of human antibody binding.

METHODS

To uncover additional xenoantigens, we compared the asparagine-linked (N-linked) glycome from serum proteins of humans, domestic pigs, GGTA1 knockout pigs, and GGTA1/CMAH knockout pigs using mass spectrometry. Carbohydrate structures were determined with assistance from GlycoWorkbench, Cartoonist, and SimGlycan software by comparison to existing database entries and collision-induced dissociation fragmentation data.

RESULTS

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) analysis of reduced and solid-phase permethylated glycans resulted in the detection of high-mannose, hybrid, and complex type N-linked glycans in the 1000-4500 m/z ion range. GGTA1/CMAH knockout pig samples had increased relative amounts of high-mannose, incomplete, and xylosylated N-linked glycans. All pig samples had significantly higher amounts of core and possibly antennae fucosylation.

CONCLUSIONS

We provide for the first time a comparison of the serum protein glycomes of the human, domestic pig, and genetically modified pigs important to xenotransplantation.

Congenital protein hypoglycosylation diseases

Glycosylation is an essential process by which sugars are attached to proteins and lipids. Complete lack of glycosylation is not compatible with life. Because of the widespread function of glycosylation, inherited disorders of glycosylation are multisystemic. Since the identification of the first defect on N-linked glycosylation in the 1980s, there are over 40 different congenital protein hypoglycosylation diseases. This review will include defects of N-linked glycosylation, O-linked glycosylation and disorders of combined N- and O-linked glycosylation.

Overexpression of Man2C1 leads to protein underglycosylation and upregulation of endoplasmic reticulum-associated degradation pathway

Unfolded glycoproteins retained in the endoplasmic reticulum (ER) are degraded via the ER-associated degradation (ERAD) pathway. These proteins are subsequently transported to the cytosol and degraded by the proteasomal complex. Although the sequential events of ERAD are well described, its regulation remains poorly understood. The cytosolic mannosidase, Man2C1, plays an essential role in the catabolism of cytosolic free oligomannosides, which are released from the degraded proteins. We have investigated the impact of Man2C1 overexpression on protein glycosylation and the ERAD process. We demonstrated that overexpression of Man2C1 led to modifications of the cytosolic pool of free oligomannosides and resulted in accumulation of small Man(2-4)GlcNAc(1) glycans in the cytosol. We further correlated this accumulation with incomplete protein glycosylation and truncated lipid-linked glycosylation precursors, which yields an increase in N-glycoprotein en route to the ERAD. We propose a model in which high mannose levels in the cytosol interfere with glucose metabolism and compromise N-glycan synthesis in the ER. Our results show a clear link between the intracellular mannose-6-phosphate level and synthesis of the lipid-linked precursors for protein glycosylation. Disturbance in these pathways interferes with protein glycosylation and upregulated ERAD. Our findings support a new concept that regulation of Man2C1 expression is essential for maintaining efficient protein N-glycosylation.

Congenital disorders of glycosylation in hepatology: the example of polycystic liver disease

Autosomal dominant polycystic liver disease (PCLD) is a rare progressive disorder characterized by an increased liver volume due to many (>20) fluid-filled cysts of biliary origin. Disease causing mutations in PRKCSH or SEC63 are found in approximately 25% of the PCLD patients. Both gene products function in the endoplasmic reticulum, however, the molecular mechanism behind cyst formation remains to be elucidated. As part of the translocon complex, SEC63 plays a role in protein import into the ER and is implicated in the export of unfolded proteins to the cytoplasm during ER-associated degradation (ERAD). PRKCSH codes for the beta-subunit of glucosidase II (hepatocystin), which cleaves two glucose residues of Glc(3)Man(9)GlcNAc(2) N-glycans on proteins. Hepatocystin is thereby directly involved in the protein folding process by regulating protein binding to calnexin/calreticulin in the ER. A separate group of genetic diseases affecting protein N-glycosylation in the ER is formed by the congenital disorders of glycosylation (CDG). In distinct subtypes of this autosomal recessive multisystem disease specific liver symptoms have been reported that overlap with PCLD. Recent research revealed novel insights in PCLD disease pathology such as the absence of hepatocystin from cyst epithelia indicating a two-hit model for PCLD cystogenesis. This opens the way to speculate about a recessive mechanism for PCLD pathophysiology and shared molecular pathways between CDG and PCLD. In this review we will discuss the clinical-genetic features of PCLD and CDG as well as their biochemical pathways with the aim to identify novel directions of research into cystogenesis.

Congenital disorders of glycosylation: an update on defects affecting the biosynthesis of dolichol-linked oligosaccharides

Defects in the biosynthesis of the oligosaccharide precursor for N-glycosylation lead to decreased occupancy of glycosylation sites and thereby to diseases known as congenital disorders of glycosylation (CDG). In the last 20 years, approximately 1,000 CDG patients have been identified presenting with multiple organ dysfunctions. This review sets the state of the art by listing all mutations identified in the 15 genes (PMM2, MPI, DPAGT1, ALG1, ALG2, ALG3, ALG9, ALG12, ALG6, ALG8, DOLK, DPM1, DPM3, MPDU1, and RFT1) that yield a deficiency of dolichol-linked oligosaccharide biosynthesis. The present analysis shows that most mutations lead to substitutions of strongly conserved amino acid residues across eukaryotes. Furthermore, the comparison between the different forms of CDG affecting dolichol-linked oligosaccharide biosynthesis shows that the severity of the disease does not relate to the position of the mutated gene along this biosynthetic pathway.