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.

ALG13-Congenital Disorder of Glycosylation (ALG13-CDG): Updated clinical and molecular review and clinical management guidelines.

ALG13-Congenital Disorder of Glycosylation (CDG), is a rare X-linked CDG caused by pathogenic variants in ALG13 (OMIM 300776) that affects the N-linked glycosylation pathway. Affected individuals present with a predominantly neurological manifestation during infancy. Epileptic spasms are a common presenting symptom of ALG13-CDG. Other common phenotypes include developmental delay, seizures, intellectual disability, microcephaly, and hypotonia. Current management of ALG13-CDG is targeted to address patients' symptoms. To date, less than 100 individuals have been reported with ALG13-CDG. In this article, an international group of experts in CDG reviewed all reported individuals affected with ALG13-CDG and suggested diagnostic and management guidelines for ALG13-CDG. The guidelines are based on the best available data and expert opinion. Neurological symptoms dominate the phenotype of ALG13-CDG where epileptic spasm is confirmed to be the most common presenting symptom of ALG13-CDG in association with hypotonia and developmental delay. We propose that ACTH/prednisolone treatment should be trialed first, followed by vigabatrin, however ketogenic diet has been shown to have promising results in ALG13-CDG. In order to optimize medical management, we also suggest early cardiac, gastrointestinal, skeletal, and behavioral assessments in affected patients.

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.

A complement C4-derived glycopeptide is a biomarker for PMM2-CDG.

BACKGROUNDDiagnosis of PMM2-CDG, the most common congenital disorder of glycosylation (CDG), relies on measuring carbohydrate-deficient transferrin (CDT) and genetic testing. CDT tests have false negatives and may normalize with age. Site-specific changes in protein N-glycosylation have not been reported in sera in PMM2-CDG.METHODSUsing multistep mass spectrometry-based N-glycoproteomics, we analyzed sera from 72 individuals to discover and validate glycopeptide alterations. We performed comprehensive tandem mass tag-based discovery experiments in well-characterized patients and controls. Next, we developed a method for rapid profiling of additional samples. Finally, targeted mass spectrometry was used for validation in an independent set of samples in a blinded fashion.RESULTSOf the 3,342 N-glycopeptides identified, patients exhibited decrease in complex-type N-glycans and increase in truncated, mannose-rich, and hybrid species. We identified a glycopeptide from complement C4 carrying the glycan Man5GlcNAc2, which was not detected in controls, in 5 patients with normal CDT results, including 1 after liver transplant and 2 with a known genetic variant associated with mild disease, indicating greater sensitivity than CDT. It was detected by targeted analysis in 2 individuals with variants of uncertain significance in PMM2.CONCLUSIONComplement C4-derived Man5GlcNAc2 glycopeptide could be a biomarker for accurate diagnosis and therapeutic monitoring of patients with PMM2-CDG and other CDGs.FUNDINGU54NS115198 (Frontiers in Congenital Disorders of Glycosylation: NINDS; NCATS; Eunice Kennedy Shriver NICHD; Rare Disorders Consortium Disease Network); K08NS118119 (NINDS); Minnesota Partnership for Biotechnology and Medical Genomics; Rocket Fund; R01DK099551 (NIDDK); Mayo Clinic DERIVE Office; Mayo Clinic Center for Biomedical Discovery; IA/CRC/20/1/600002 (Center for Rare Disease Diagnosis, Research and Training; DBT/Wellcome Trust India Alliance).