Frontiers in congenital disorders of glycosylation consortium, a cross-sectional study report at year 5 of 280 individuals in the natural history cohort.

OBJECTIVE: Our report describes clinical, genetic, and biochemical features of participants with a molecularly confirmed congenital disorder of glycosylation (CDG) enrolled in the Frontiers in Congenital Disorders of Glycosylation (FCDGC) Natural History cohort at year 5 of the study. METHODS: We enrolled individuals with a known or suspected CDG into the FCDGC Natural History Study, a multicenter prospective and retrospective natural history study of all genetic causes of CDG. We conducted a cross-sectional analysis of baseline study visit data from participants with confirmed CDG who were consented into the FCDGC Natural History Study (5U54NS115198) from October 2019 to November 2023. RESULTS: Three hundred thirty-three subjects consented to the FCDGC Natural History Study. Of these, 280 unique individuals had genetic data available that was consistent with a diagnosis of CDG. These 280 individuals were enrolled into the study between October 8, 2019 and November 29, 2023. One hundred forty-one (50.4%) were female, and 139 (49.6%) were male. Mean and median age at enrollment was 10.1 and 6.5 years, respectively, with a range of 0.22 to 71.4 years. The cohort encompassed individuals with disorders of N-linked protein glycosylation (57%), glycosylphosphatidylinositol anchor disorder (GPI anchor) (15%), disorders of Golgi homeostasis, trafficking and transport (12%), dolichol metabolism disorders (5%), disorders of multiple pathways (6%), and other (5%). The most frequent presenting symptom(s) leading to diagnosis were developmental delay/disability (77%), followed by hypotonia (56%) and feeding difficulties (42%). Mean and median time between first related symptom and diagnosis was 2.7 and 0.8 years, respectively. One hundred percent of individuals in our cohort had developmental differences/disabilities at the time of their baseline visit, followed by 97% with neurologic involvement, 91% with gastrointestinal (GI)/liver involvement, and 88% with musculoskeletal involvement. Severity of disease in individuals was scored on the Nijmegen Progression CDG Rating Scale (NPCRS) with 27% of scores categorized as mild, 44% moderate, and 29% severe. Of the individuals with N-linked protein glycosylation defects, 83% of those with data showed a type 1 pattern on carbohydrate deficient transferrin (CDT) analysis including 82/84 individuals with PMM2-CDG, 6% a type 2 pattern, 1% both type 1 and type 2 pattern and 10% a normal or nonspecific pattern. One hundred percent of individuals with Golgi homeostasis and trafficking defects with data showed a type 2 pattern on CDT analysis, while Golgi transport defect showed a type II pattern 73% of the time, a type 1 pattern for 7%, and 20% had a normal or nonspecific pattern. Most of the variants documented were classified as pathogenic or likely pathogenic using ACMG criteria. For the majority of the variants, the predicted molecular consequence was missense followed by nonsense and splice site, and the majority of the diagnoses are inherited in an autosomal recessive pattern but with disorders of all major nuclear inheritance included. DISCUSSION: The FCDGC Natural History Study serves as an important resource to build future research studies, improve clinical care, and prepare for clinical trial readiness. Herein is the first overview of CDG participants of the FCDGC Natural History Study.

Coagulation abnormalities and vascular complications are common in PGM1-CDG.

Phosphoglucomutase-1-congenital disorder of glycosylation (PGM1-CDG) is a rare genetic disorder caused by biallelic variants in the PGM1 gene, leading to the deficiency of the PGM1 enzyme. The most common clinical presentations include muscle involvement, failure to thrive, cleft palate, and cardiac involvement. Abnormal serum N-glycosylation, hypoglycemia, and liver function abnormalities including coagulation abnormalities are the most common laboratory abnormalities. While PGM1-CDG has been extensively studied, little is known about the extent of the coagulation abnormalities in individuals with PGM1-CDG. Unlike most CDG, some symptoms of PGM1-CDG are treatable with D-galactose (D-gal) supplementation, though reliable clinical endpoints are necessary to appropriately evaluate the potential improvement with D-gal in PGM1-CDG. Here, we aimed to describe the incidence of coagulation abnormalities in PGM1-CDG and their evolution, their relation to clinical events, and the ability of D-gal treatment to improve them. A retrospective analysis was conducted on 73 reported individuals. All individuals had a molecularly confirmed PGM1-CDG diagnosis. All incidences of antithrombin (AT), aPTT, PT, factor (F) XI, FX, FIX, FVII, protein C and protein S data and major clinical events related to coagulation abnormalities, were collected. Coagulation information was available for only 58.9 % of the reported individuals, out of which 67.4 % of PGM1-CDG individuals were reported to have abnormalities. The most frequently observed abnormality was AT (mean: 30.8% R:80-120 %) deficiency. Four individuals had major thrombotic events. Coagulation status on D-gal treatment, were reported in 19 individuals. Several factors showed improvement including AT (mean: 64.5 %), indicating galactose is beneficial in treating coagulation abnormalities in PGM1-CDG. Due to the scarcity of the reported data on coagulation parameters, we also evaluated data collected in sixteen PGM1-CDG individuals enrolled in the FCDGC Natural History Study. Longitudinal data showed improvements in several coagulant parameters and disease severity improved for almost all patients of whom we had multiple datapoints on D-gal. AT showed significant improvement on D-gal. We conclude that coagulation abnormalities are frequently present in PGM1-CDG and show improvement on D-gal. We recommend coagulation parameters should be routinely checked in individuals with PGM1-CDG or suspected of having PGM1-CDG. Finally, AT may be used as a primary or secondary clinical endpoint for upcoming clinical trials in PGM1-CDG individuals.

Cardiomyopathy, an uncommon phenotype of congenital disorders of glycosylation: Recommendations for baseline screening and follow-up evaluation.

INTRODUCTION: Congenital disorders of glycosylation (CDG) are a continuously expanding group of monogenic disorders that disrupt glycoprotein and glycolipid biosynthesis, leading to multi-systemic manifestations. These disorders are categorized into various groups depending on which part of the glycosylation process is impaired. The cardiac manifestations in CDG can significantly differ, not only across different types but also among individuals with the same genetic cause of CDG. Cardiomyopathy is an important phenotype in CDG. The clinical manifestations and progression of cardiomyopathy in CDG patients have not been well characterized. This study aims to delineate common patterns of cardiomyopathy across a range of genetic causes of CDG and to propose baseline screening and follow-up evaluation for this patient population. METHODS: Patients with molecular confirmation of CDG who were enrolled in the prospective or memorial arms of the Frontiers in Congenital Disorders of Glycosylation Consortium (FCDGC) natural history study were ascertained for the presence of cardiomyopathy based on a retrospective review of their medical records. All patients were evaluated by clinical geneticists who are members of FCDGC at their respective academic centers. Patients were screened for cardiomyopathy, and detailed data were retrospectively collected. We analyzed their clinical and molecular history, imaging characteristics of cardiac involvement, type of cardiomyopathy, age at initial presentation of cardiomyopathy, additional cardiac features, the treatments administered, and their clinical outcomes. RESULTS: Of the 305 patients with molecularly confirmed CDG participating in the FCDGC natural history study as of June 2023, 17 individuals, nine females and eight males, were identified with concurrent diagnoses of cardiomyopathy. Most of these patients were diagnosed with PMM2-CDG (n = 10). However, cardiomyopathy was also observed in other diagnoses, including PGM1-CDG (n = 3), ALG3-CDG (n = 1), DPM1-CDG (n = 1), DPAGT1-CDG (n = 1), and SSR4-CDG (n = 1). All PMM2-CDG patients were reported to have hypertrophic cardiomyopathy. Dilated cardiomyopathy was observed in three patients, two with PGM1-CDG and one with ALG3-CDG; left ventricular non-compaction cardiomyopathy was diagnosed in two patients, one with PGM1-CDG and one with DPAGT1-CDG; two patients, one with DPM1-CDG and one with SSR4-CDG, were diagnosed with non-ischemic cardiomyopathy. The estimated median age of diagnosis for cardiomyopathy was 5 months (range: prenatal-27 years). Cardiac improvement was observed in three patients with PMM2-CDG. Five patients showed a progressive course of cardiomyopathy, while the condition remained unchanged in eight individuals. Six patients demonstrated pericardial effusion, with three patients exhibiting cardiac tamponade. One patient with SSR4-CDG has been recently diagnosed with cardiomyopathy; thus, the progression of the disease is yet to be determined. One patient with PGM1-CDG underwent cardiac transplantation. Seven patients were deceased, including five with PMM2-CDG, one with DPAGT1-CDG, and one with ALG3-CDG. Two patients died of cardiac tamponade from pericardial effusion; for the remaining patients, cardiomyopathy was not necessarily the primary cause of death. CONCLUSIONS: In this retrospective study, cardiomyopathy was identified in ∼6% of patients with CDG. Notably, the majority, including all those with PMM2-CDG, exhibited hypertrophic cardiomyopathy. Some cases did not show progression, yet pericardial effusions were commonly observed, especially in PMM2-CDG patients, occasionally escalating to life-threatening cardiac tamponade. It is recommended that clinicians managing CDG patients, particularly those with PMM2-CDG and PGM1-CDG, be vigilant of the cardiomyopathy risk and risk for potentially life-threatening pericardial effusions. Cardiac surveillance, including an echocardiogram and EKG, should be conducted at the time of diagnosis, annually throughout the first 5 years, followed by check-ups every 2-3 years if no concerns arise until adulthood. Subsequently, routine cardiac examinations every five years are advisable. Additionally, patients with diagnosed cardiomyopathy should receive ongoing cardiac care to ensure the effective management and monitoring of their condition. A prospective study will be required to determine the true prevalence of cardiomyopathy in CDG.

Genome and RNA sequencing were essential to reveal cryptic intronic variants associated to defective ATP6AP1 mRNA processing.

The diagnosis of Mendelian disorders has notably advanced with integration of whole exome and genome sequencing (WES and WGS) in clinical practice. However, challenges in variant interpretation and uncovered variants by WES still leave a substantial percentage of patients undiagnosed. In this context, integrating RNA sequencing (RNA-seq) improves diagnostic workflows, particularly for WES inconclusive cases. Additionally, functional studies are often necessary to elucidate the impact of prioritized variants on gene expression and protein function. Our study focused on three unrelated male patients (P1-P3) with ATP6AP1-CDG (congenital disorder of glycosylation), presenting with intellectual disability and varying degrees of hepatopathy, glycosylation defects, and an initially inconclusive diagnosis through WES. Subsequent RNA-seq was pivotal in identifying the underlying genetic causes in P1 and P2, detecting ATP6AP1 underexpression and aberrant splicing. Molecular studies in fibroblasts confirmed these findings and identified the rare intronic variants c.289-233C > T and c.289-289G > A in P1 and P2, respectively. Trio-WGS also revealed the variant c.289-289G > A in P3, which was a de novo change in both patients. Functional assays expressing the mutant alleles in HAP1 cells demonstrated the pathogenic impact of these variants by reproducing the splicing alterations observed in patients. Our study underscores the role of RNA-seq and WGS in enhancing diagnostic rates for genetic diseases such as CDG, providing new insights into ATP6AP1-CDG molecular bases by identifying the first two deep intronic variants in this X-linked gene. Additionally, our study highlights the need to integrate RNA-seq and WGS, followed by functional validation, in routine diagnostics for a comprehensive evaluation of patients with an unidentified molecular etiology.

SSR4-CDG, an ultra-rare X-linked congenital disorder of glycosylation affecting the TRAP complex: Review of 22 affected individuals including the first adult patient.

Congenital disorders of glycosylation (CDG) are a group of rare, often multi-systemic genetic disorders that result from disturbed protein and lipid glycosylation. SSR4-CDG is an ultra-rare, comparably mild subtype of CDG, presenting mostly in males. It is caused by pathogenic variants in the SSR4 gene, which is located on the X chromosome. SSR4 (signal sequence receptor protein 4) is a subunit of the translocon-associated protein (TRAP) complex, a structure that is needed for the translocation of proteins across the ER membrane. A deficiency of SSR4 leads to disturbed N-linked glycosylation of proteins in the endoplasmic reticulum. Here, we review the most common clinical, biochemical and genetic features of 18 previously published individuals and report four new cases diagnosed with SSR4-CDG, including the first adult affected by this disorder. Based on our review, developmental delay, speech delay, intellectual disability, muscular hypotonia, microcephaly and distinct facial features are key symptoms of SSR4-CDG that are present in all affected individuals. Although these symptoms overlap with many other neurodevelopmental disorders, their combination with additional clinical features, and a quite distinguishable facial appearance of affected individuals make this disorder a potentially recognizable type of CDG. Additional signs and symptoms include failure to thrive, feeding difficulties, connective tissue involvement, gastrointestinal problems, skeletal abnormalities, seizures and, in some cases, significant behavioral abnormalities. Due to lack of awareness of this rare disorder, and since biochemical testing can be normal in affected individuals, most are diagnosed through genetic studies, such as whole exome sequencing. With this article, we expand the phenotype of SSR4-CDG to include cardiac symptoms, laryngeal abnormalities, and teleangiectasia. We also provide insights into the prognosis into early adulthood and offer recommendations for adequate management and care. We emphasize the great need for causal therapies, as well as effective symptomatic therapies addressing the multitude of symptoms in this disease. In particular, behavioral problems can severely affect quality of life in individuals diagnosed with SSR4-CDG and need special attention. Finally, we aim to improve guidance and education for affected families and treating physicians and create a basis for future research in this disorder.

D-mannose as a new therapy for fucokinase deficiency-related congenital disorder of glycosylation (FCSK-CDG).

INTRODUCTION: Fucokinase deficiency-related congenital disorder of glycosylation (FCSK-CDG) is a rare autosomal recessive inborn error of metabolism characterized by a decreased flux through the salvage pathway of GDP-fucose biosynthesis due to a block in the recycling of L-fucose that exits the lysosome. FCSK-CDG has been described in 5 individuals to date in the medical literature, with a phenotype comprising global developmental delays/intellectual disability, hypotonia, abnormal myelination, posterior ocular disease, growth and feeding failure, immune deficiency, and chronic diarrhea, without clear therapeutic recommendations. PATIENT AND METHODS: In a so far unreported FCSK-CDG patient, we studied proteomics and glycoproteomics in vitro in patient-derived fibroblasts and also performed in vivo glycomics, before and after treatment with either D-Mannose or L-Fucose. RESULTS: We observed a marked increase in fucosylation after D-mannose supplementation in fibroblasts compared to treatment with L-Fucose. The patient was then treated with D-mannose at 850 mg/kg/d, with resolution of the chronic diarrhea, resolution of oral aversion, improved weight gain, and observed developmental gains. Serum N-glycan profiles showed an improvement in the abundance of fucosylated glycans after treatment. No treatment-attributed adverse effects were observed. CONCLUSION: D-mannose is a promising new treatment for FCSK-CDG.

Liposome-encapsulated mannose-1-phosphate therapy improves global N-glycosylation in different congenital disorders of glycosylation.

Phosphomannomutase 2 (PMM2) converts mannose-6-phospahate to mannose-1-phosphate; the substrate for GDP-mannose, a building block of the glycosylation biosynthetic pathway. Pathogenic variants in the PMM2 gene have been shown to be associated with protein hypoglycosylation causing PMM2-congenital disorder of glycosylation (PMM2-CDG). While mannose supplementation improves glycosylation in vitro, but not in vivo, we hypothesized that liposomal delivery of mannose-1-phosphate could increase the stability and delivery of the activated sugar to enter the targeted compartments of cells. Thus, we studied the effect of liposome-encapsulated mannose-1-P (GLM101) on global protein glycosylation and on the cellular proteome in skin fibroblasts from individuals with PMM2-CDG, as well as in individuals with two N-glycosylation defects early in the pathway, namely ALG2-CDG and ALG11-CDG. We leveraged multiplexed proteomics and N-glycoproteomics in fibroblasts derived from different individuals with various pathogenic variants in PMM2, ALG2 and ALG11 genes. Proteomics data revealed a moderate but significant change in the abundance of some of the proteins in all CDG fibroblasts upon GLM101 treatment. On the other hand, N-glycoproteomics revealed the GLM101 treatment enhanced the expression levels of several high-mannose and complex/hybrid glycopeptides from numerous cellular proteins in individuals with defects in PMM2 and ALG2 genes. Both PMM2-CDG and ALG2-CDG exhibited several-fold increase in glycopeptides bearing Man6 and higher glycans and a decrease in Man5 and smaller glycan moieties, suggesting that GLM101 helps in the formation of mature glycoforms. These changes in protein glycosylation were observed in all individuals irrespective of their genetic variants. ALG11-CDG fibroblasts also showed increase in high mannose glycopeptides upon treatment; however, the improvement was not as dramatic as the other two CDG. Overall, our findings suggest that treatment with GLM101 overcomes the genetic block in the glycosylation pathway and can be used as a potential therapy for CDG with enzymatic defects in early steps in protein N-glycosylation.

Expanded prenatal phenotype of ALG12-associated congenital disorder of glycosylation including bilateral multicystic kidneys.

Congenital disorders of glycosylation (CDG) are a group of rare autosomal recessive genetic disorders caused by pathogenic variants in genes coding for N-glycosylated glycoproteins, which play a role in folding, degrading, and transport of glycoproteins in their pathway. ALG12-CDG specifically is caused by biallelic pathogenic variants in ALG12. Currently reported features of ALG12-CDG include: developmental delay, hypotonia, failure to thrive and/or short stature, brain anomalies, recurrent infections, hypogammaglobulinemia, coagulation abnormalities, and genitourinary abnormalities. In addition, skeletal abnormalities resembling a skeletal dysplasia including shortened long bones and talipes equinovarus have been seen in more severe neonatal presentation of this disorder. We report on a case expanding the phenotype of ALG12-CDG to include bilateral, multicystic kidneys in a neonatal demise identified with homozygous pathogenic variants in the ALG12 gene at c.1001del (p.N334Tfs*15) through clinical trio exome sequencing.

Novel insight into FCSK-congenital disorder of glycosylation through a CRISPR-generated cell model.

BACKGROUND: FCSK-congenital disorder of glycosylation (FCSK-CDG) is a recently discovered rare autosomal recessive genetic disorder with defective fucosylation due to mutations in the fucokinase encoding gene, FCSK. Despite the essential role of fucokinase in the fucose salvage pathway and severe multisystem manifestations of FCSK-CDG patients, it is not elucidated which cells or which types of fucosylation are affected by its deficiency. METHODS: In this study, CRISPR/Cas9 was employed to construct an FCSK-CDG cell model and explore the molecular mechanisms of the disease by lectin flow cytometry and real-time PCR analyses. RESULTS: Comparison of cellular fucosylation by lectin flow cytometry in the created CRISPR/Cas9 FCSK knockout and the same unedited cell lines showed no significant change in the amount of cell surface fucosylated glycans, which is consistent with the only documented previous study on different cell types. It suggests a probable effect of this disease on secretory glycoproteins. Investigating O-fucosylation by analysis of the NOTCH3 gene expression as a potential target revealed a significant decrease in the FCSK knockout cells compared with the same unedited ones, proving the effect of fucokinase deficiency on EGF-like repeats O-fucosylation. CONCLUSION: This study expands insight into the FCSK-CDG molecular mechanism; to the best of our knowledge, it is the first research conducted to reveal a gene whose expression level alters due to this disease.

Neuroectoderm phenotypes in a human stem cell model of O-GlcNAc transferase associated with intellectual disability.

Pathogenic variants in the O-GlcNAc transferase gene (OGT) have been associated with a congenital disorder of glycosylation (OGT-CDG), presenting with intellectual disability which may be of neuroectodermal origin. To test the hypothesis that pathology is linked to defects in differentiation during early embryogenesis, we developed an OGT-CDG induced pluripotent stem cell line together with isogenic control generated by CRISPR/Cas9 gene-editing. Although the OGT-CDG variant leads to a significant decrease in OGT and O-GlcNAcase protein levels, there were no changes in differentiation potential or stemness. However, differentiation into ectoderm resulted in significant differences in O-GlcNAc homeostasis. Further differentiation to neuronal stem cells revealed differences in morphology between patient and control lines, accompanied by disruption of the O-GlcNAc pathway. This suggests a critical role for O-GlcNAcylation in early neuroectoderm architecture, with robust compensatory mechanisms in the earliest stages of stem cell differentiation.

Clinical and biochemical footprints of congenital disorders of glycosylation: Proposed nosology.

We have identified 200 congenital disorders of glycosylation (CDG) caused by 189 different gene defects and have proposed a classification system for CDG based on the mode of action. This classification includes 8 categories: 1. Disorders of monosaccharide synthesis and interconversion, 2. Disorders of nucleotide sugar synthesis and transport, 3. Disorders of N-linked protein glycosylation, 4. Disorders of O-linked protein glycosylation, 5. Disorders of lipid glycosylation, 6. Disorders of vesicular trafficking, 7. Disorders of multiple glycosylation pathways and 8. Disorders of glycoprotein/glycan degradation. Additionally, using information from IEMbase, we have described the clinical involvement of 19 organs and systems, as well as essential laboratory investigations for each type of CDG. Neurological, dysmorphic, skeletal, and ocular manifestations were the most prevalent, occurring in 81%, 56%, 53%, and 46% of CDG, respectively. This was followed by digestive, cardiovascular, dermatological, endocrine, and hematological symptoms (17-34%). Immunological, genitourinary, respiratory, psychiatric, and renal symptoms were less frequently reported (8-12%), with hair and dental abnormalities present in only 4-7% of CDG. The information provided in this study, including our proposed classification system for CDG, may be beneficial for healthcare providers caring for individuals with metabolic conditions associated with CDG.

Transcriptomic analysis identifies dysregulated pathways and therapeutic targets in PMM2-CDG.

PMM2-CDG (MIM # 212065), the most common congenital disorder of glycosylation, is caused by the deficiency of phosphomannomutase 2 (PMM2). It is a multisystemic disease of variable severity that particularly affects the nervous system; however, its molecular pathophysiology remains poorly understood. Currently, there is no effective treatment. We performed an RNA-seq based transcriptomic study using patient-derived fibroblasts to gain insight into the mechanisms underlying the clinical symptomatology and to identify druggable targets. Systems biology methods were used to identify cellular pathways potentially affected by PMM2 deficiency, including Senescence, Bone regulation, Cell adhesion and Extracellular Matrix (ECM) and Response to cytokines. Functional validation assays using patients' fibroblasts revealed defects related to cell proliferation, cell cycle, the composition of the ECM and cell migration, and showed a potential role of the inflammatory response in the pathophysiology of the disease. Furthermore, treatment with a previously described pharmacological chaperone reverted the differential expression of some of the dysregulated genes. The results presented from transcriptomic data might serve as a platform for identifying therapeutic targets for PMM2-CDG, as well as for monitoring the effectiveness of therapeutic strategies, including pharmacological candidates and mannose-1-P, drug repurposing.

Dysregulated proteome and N-glycoproteome in ALG1-deficient fibroblasts.

Asparagine-linked glycosylation 1 protein is a ß-1,4-mannosyltransferase, is encoded by the ALG1 gene, which catalyzes the first step of mannosylation in N-glycosylation. Pathogenic variants in ALG1 cause a rare autosomal recessive disorder termed as ALG1-CDG. We performed a quantitative proteomics and N-glycoproteomics study in fibroblasts derived from patients with one homozygous and two compound heterozygous pathogenic variants in ALG1. Several proteins that exhibited significant upregulation included insulin-like growth factor II and pleckstrin, whereas hyaluronan and proteoglycan link protein 1 was downregulated. These proteins are crucial for cell growth, survival and differentiation. Additionally, we observed a decrease in the expression of mitochondrial proteins and an increase in autophagy-related proteins, suggesting mitochondrial and cellular stress. N-glycoproteomics revealed the reduction in high-mannose and complex/hybrid glycopeptides derived from numerous proteins in patients explaining that defect in ALG1 has broad effects on glycosylation. Further, we detected an increase in several short oligosaccharides, including chitobiose (HexNAc2) trisaccharides (Hex-HexNAc2) and novel tetrasaccharides (NeuAc-Hex-HexNAc2) derived from essential proteins including LAMP1, CD44 and integrin. These changes in glycosylation were observed in all patients irrespective of their gene variants. Overall, our findings not only provide novel molecular insights into understanding ALG1-CDG but also offer short oligosaccharide-bearing peptides as potential biomarkers.

Establishment of a human induced pluripotent stem cell line（SDQLCHi059-A）from a patient with congenital disorder of glycosylation carrying heterozygous mutation in MPI gene.

Congenital disorder of glycosylation (CDG) is inherited metabolicdiseasecaused by defects in the genes important for the process of protein and lipidglycosylation. We established an induced pluripotent stem cell (iPSC) line from peripheral blood mononuclear cells of a 6-month-old boy with congenital disorder of glycosylation carrying heterozygous mutations c.1193 T > C (p.I398T) and c.376_384dup CCGCAGCAC (p.P126_H128 dupPQH) in MPI gene. This iPSC line was free of exogenous gene, expressed pluripotency markers, has normal karyotype, exhibited differentiation potential and harbored the same mutations found in the patient. This cell line will provide a reliable cell model for further studies on the potential therapeutic targets of CDG.

Revisiting the immunopathology of congenital disorders of glycosylation: an updated review.

Glycosylation is a critical post-translational modification that plays a pivotal role in several biological processes, such as the immune response. Alterations in glycosylation can modulate the course of various pathologies, such as the case of congenital disorders of glycosylation (CDG), a group of more than 160 rare and complex genetic diseases. Although the link between glycosylation and immune dysfunction has already been recognized, the immune involvement in most CDG remains largely unexplored and poorly understood. In this study, we provide an update on the immune dysfunction and clinical manifestations of the 12 CDG with major immune involvement, organized into 6 categories of inborn errors of immunity according to the International Union of Immunological Societies (IUIS). The immune involvement in phosphomannomutase 2 (PMM2)-CDG - the most frequent CDG - was comprehensively reviewed, highlighting a higher prevalence of immune issues during infancy and childhood and in R141H-bearing genotypes. Finally, using PMM2-CDG as a model, we point to links between abnormal glycosylation patterns in host cells and possibly favored interactions with microorganisms that may explain the higher susceptibility to infection. Further characterizing immunopathology and unusual host-pathogen adhesion in CDG can not only improve immunological standards of care but also pave the way for innovative preventive measures and targeted glycan-based therapies that may improve quality of life for people living with CDG.

Glycosphingolipids in congenital disorders of glycosylation (CDG).

Congenital disorders of glycosylation (CDG) are a large family of rare disorders affecting the different glycosylation pathways. Defective glycosylation can affect any organ, with varying symptoms among the different CDG. Even between individuals with the same CDG there is quite variable severity. Associating specific symptoms to deficiencies of certain glycoproteins or glycolipids is thus a challenging task. In this review, we focus on the glycosphingolipid (GSL) synthesis pathway, which is still rather unexplored in the context of CDG, and outline the functions of the main GSLs, including gangliosides, and their role in the central nervous system. We provide an overview of GSL studies that have been performed in CDG and show that abnormal GSL levels are not only observed in CDG directly affecting GSL synthesis, but also in better known CDG, such as PMM2-CDG. We highlight the importance of studying GSLs in CDG in order to better understand the pathophysiology of these disorders.

Association between acute complications in PMM2-CDG patients and haemostasis anomalies: Data from a multicentric study and suggestions for acute management.

OBJECTIVES: Patients with PMM2-CDG develop acute events (stroke-like episodes (SLEs), thromboses, haemorrhages, seizures, migraines) associated with both clotting factors (factor XI) and coagulation inhibitors (antithrombin, protein C and protein S) deficiencies. The aim of the study was to correlate acute events to haemostasis and propose practical guidelines. METHODS: In this multicentric retrospective study, we evaluated clinical, radiological, haemostasis and electroencephalography data for PMM2-CDG patients hospitalized for acute events. Cerebral events were classified as thrombosis, haemorrhage, SLE, or "stroke mimic" (SM: normal brain imaging or evoking a migraine). RESULTS: Thirteen patients had a total of 31 acute episodes: 27 cerebral events with 7 SLEs, 4 venous thromboses, 4 haemorrhages (3 associated with thrombosis), 15 SMs at a mean age of 7.7 years; 4 non-cerebral thromboses, one of which included bleeding. A trigger was frequently involved (infection, head trauma). Although sometimes normal at baseline state, factor XI, antithrombin and protein C levels decreased during these episodes. No correlation between haemostasis anomalies and type of acute event was found. DISCUSSION: Acute events in PMM2-CDG are not negligible and are associated with haemostasis anomalies. An emergency protocol is proposed for their prevention and treatment (https://www.filiere-g2m.fr/urgences). For cerebral events, brain Magnetic Resonance Imaging with perfusion weight imaging and diffusion sequences, electroencephalogram and haemostasis protein levels guide the treatment: anticoagulation, antithrombin or fresh frozen plasma supplementation, antiepileptic therapy. Preventing bleeding and thrombosis is required in cases of surgery, prolonged immobilization, hormone replacement therapy. CONCLUSION: Acute events in PMM2-CDG are associated with abnormal haemostasis, requiring practical guidance.

Metabolic adaptation of human skin fibroblasts to ER stress caused by glycosylation defect in PMM2-CDG.

PMM2-CDG is the most prevalent type of congenital disorders of glycosylation (CDG). It is caused by pathogenic variants in the gene encoding phosphomannomutase 2 (PMM2), which converts mannose-6-phosphate to mannose-1-phosphate and thus activates this saccharide for further glycosylation processes. Defective glycosylation can lead to an abnormal accumulation of unfolded proteins in endoplasmic reticulum (ER) and cause its stress. The ER is a key compartment for glycosylation, and its connection and communication with mitochondria has been described extensively in literature. Their crosstalk is important for cell proliferation, calcium homeostasis, apoptosis, mitochondrial fission regulation, bioenergetics, autophagy, lipid metabolism, inflammasome formation and unfolded protein response. Therefore, in the present study we posed a question, whether defective glycosylation leads to bioenergetic disruption. Our data reveal possible chronic stress in ER and activated unfolded protein response via PERK pathway in PMM2-CDG fibroblasts. Presumably, it leads to bioenergetic reorganization and increased assembly of respiratory chain complexes into supercomplexes together with suppressed glycolysis in PMM2-CDG patient cells. These changes cause alterations in Krebs cycle, which is tightly connected to electron transport system in mitochondria. In summary, we present data showing metabolic adaptation of cells to glycosylation defect caused by various pathogenic variants in PMM2.

Cysteine Pathogenic Variants of PMM2 Are Sensitive to Environmental Stress with Loss of Structural Stability.

Congenital disorders of glycosylation (CDG) are severe metabolic disorders caused by an imbalance in the glycosylation pathway. Phosphomannomutase2 (PMM2-CDG), the most prevalent CDG, is mainly due to the disorder of PMM2. Pathogenic variants in cysteine have been found in various diseases, and cysteine residues have a potential as therapeutic targets. PMM2 harbor six cysteines; the variants Cys9Tyr (C9Y) and Cys241Ser (C241S) of PMM2 have been identified to associate with CDG, but the underlying molecular mechanisms remain uncharacterized. Here, we purified PMM2 wild type (WT), C9Y, and C241S to investigate their structural characteristics and biophysical properties by spectroscopic experiments under physiological temperature and environmental stress. Notably, the variants led to drastic changes in the protein properties and were prone to aggregate at physiological temperature. Meanwhile, PMM2 was sensitive to oxidative stress, and the cysteine pathogenic variants led to obvious aggregate formation and a higher cellular apoptosis ratio under oxidative stress. Molecular dynamic simulations indicated that the pathogenic variants changed the core domain of homomeric PMM2 and subunit binding free energy. Moreover, we tested the potential drug targeting PMM2-celastrol in cell level and explained the result by molecular docking simulation. In this study, we delineated the pathological mechanism of the cysteine substitution in PMM2, which addressed the vital role of cysteine in PMM2 and provided novel insights into prevention and treatment strategies for PMM2-CDG.

Higher frequency of TMEM199-CDG in the southern mediterranean area is associated with c.92G>C (p.Arg31Pro) mutation.

Congenital disorders of glycosylation (CDG) are genetic multisystem diseases, characterized by defective glycoconjugate synthesis. A small number of CDG with isolated liver damage have been described, such as TMEM199-CDG, a non-encephalopathic liver disorder with Wilson disease-like phenotype. Only eight patients with TMEM199-CDG have been described including seven Europeans (originating from Greece and Italy) and one Chinese. Three patients from southern Italy (Campania) shared the same known missense mutation pathogenetic variant NM_152464.3:c. 92G > C (p.Arg31Pro), also found in a Greek patient. Here we report a new patient from southern Italy (Sicily), with a homozygous c.92G > C p.(Arg31Pro) variant in TMEM199. The patient's phenotype is characterized by mild non-progressive hepatopathy with a normal hepatic echo structure. A persistent increase in serum transaminases, total and low-density lipoprotein cholesterol and low serum ceruloplasmin and copper levels and normal urinary copper excretion were observed. Matrix-assisted laser desorption/ionization mass spectrometry analyses showed abnormal N- and O- protein glycosylation, indicative of a Golgi processing defect and supporting the function of TMEM199 in maintaining Golgi homeostasis. TMEM199-CDG is an ultra-rare CDG relatively frequent in the southern Mediterranean area (7 in 9 patients, 77%). It is mainly associated with the c.92G > C (p.Arg31Pro) pathogenetic allele globally reported in 4 out of 7 families (57%), including one from Greece and three unrelated families from southern Italy. The almost uniform clinical phenotype described in patients with TMEM199-CDG appears to reflect a higher prevalence of the same variant in patients from the southern Mediterranean area.