Maciej Adamowicz

SRD5A3 Is Required for Converting Polyprenol to Dolichol and Is Mutated in a Congenital Glycosylation Disorder

N-linked glycosylation is the most frequent modification of secreted and membrane-bound proteins in eukaryotic cells, disruption of which is the basis of the congenital disorders of glycosylation (CDGs). We describe a new type of CDG caused by mutations in the steroid 5alpha-reductase type 3 (SRD5A3) gene. Patients have mental retardation and ophthalmologic and cerebellar defects. We found that SRD5A3 is necessary for the reduction of the alpha-isoprene unit of polyprenols to form dolichols, required for synthesis of dolichol-linked monosaccharides, and the oligosaccharide precursor used for N-glycosylation. The presence of residual dolichol in cells depleted for this enzyme suggests the existence of an unexpected alternative pathway for dolichol de novo biosynthesis. Our results thus suggest that SRD5A3 is likely to be the long-sought polyprenol reductase and reveal the genetic basis of one of the earliest steps in protein N-linked glycosylation.

Multiple phenotypes in phosphoglucomutase 1 deficiency

BACKGROUND Congenital disorders of glycosylation are genetic syndromes that result in impaired glycoprotein production. We evaluated patients who had a novel recessive disorder of glycosylation, with a range of clinical manifestations that included hepatopathy, bifid uvula, malignant hyperthermia, hypogonadotropic hypogonadism, growth retardation, hypoglycemia, myopathy, dilated cardiomyopathy, and cardiac arrest. METHODS Homozygosity mapping followed by whole-exome sequencing was used to identify a mutation in the gene for phosphoglucomutase 1 (PGM1) in two siblings. Sequencing identified additional mutations in 15 other families. Phosphoglucomutase 1 enzyme activity was assayed on cell extracts. Analyses of glycosylation efficiency and quantitative studies of sugar metabolites were performed. Galactose supplementation in fibroblast cultures and dietary supplementation in the patients were studied to determine the effect on glycosylation. RESULTS Phosphoglucomutase 1 enzyme activity was markedly diminished in all patients. Mass spectrometry of transferrin showed a loss of complete N-glycans and the presence of truncated glycans lacking galactose. Fibroblasts supplemented with galactose showed restoration of protein glycosylation and no evidence of glycogen accumulation. Dietary supplementation with galactose in six patients resulted in changes suggestive of clinical improvement. A new screening test showed good discrimination between patients and controls. CONCLUSIONS Phosphoglucomutase 1 deficiency, previously identified as a glycogenosis, is also a congenital disorder of glycosylation. Supplementation with galactose leads to biochemical improvement in indexes of glycosylation in cells and patients, and supplementation with complex carbohydrates stabilizes blood glucose. A new screening test has been developed but has not yet been validated. (Funded by the Netherlands Organization for Scientific Research and others.).

Gene identification in the congenital disorders of glycosylation type I by whole-exome sequencing

Congenital disorders of glycosylation type I (CDG-I) form a growing group of recessive neurometabolic diseases. Identification of disease genes is compromised by the enormous heterogeneity in clinical symptoms and the large number of potential genes involved. Until now, gene identification included the sequential application of biochemical methods in blood samples and fibroblasts. In genetically unsolved cases, homozygosity mapping has been applied in consanguineous families. Altogether, this time-consuming diagnostic strategy led to the identification of defects in 17 different CDG-I genes. Here, we applied whole-exome sequencing (WES) in combination with the knowledge of the protein N-glycosylation pathway for gene identification in our remaining group of six unsolved CDG-I patients from unrelated non-consanguineous families. Exome variants were prioritized based on a list of 76 potential CDG-I candidate genes, leading to the rapid identification of one known and two novel CDG-I gene defects. These included the first X-linked CDG-I due to a de novo mutation in ALG13, and compound heterozygous mutations in DPAGT1, together the first two steps in dolichol-PP-glycan assembly, and mutations in PGM1 in two cases, involved in nucleotide sugar biosynthesis. The pathogenicity of the mutations was confirmed by showing the deficient activity of the corresponding enzymes in patient fibroblasts. Combined with these results, the gene defect has been identified in 98% of our CDG-I patients. Our results implicate the potential of WES to unravel disease genes in the CDG-I in newly diagnosed singleton families.

Loss-of-function mutations in ATP6V0A2 impair vesicular trafficking, tropoelastin secretion and cell survival

Autosomal recessive cutis laxa type 2 (ARCL2), a syndrome of growth and developmental delay and redundant, inelastic skin, is caused by mutations in the a2 subunit of the vesicular ATPase H+-pump (ATP6V0A2). The goal of this study was to define the disease mechanisms that lead to connective tissue lesions in ARCL2. In a new cohort of 17 patients, DNA sequencing of ATP6V0A2 detected either homozygous or compound heterozygous mutations. Considerable allelic and phenotypic heterogeneity was observed, with a missense mutation of a moderately conserved residue p.P87L leading to unusually mild disease. Abnormal N- and/or mucin type O-glycosylation was observed in all patients tested. Premature stop codon mutations led to decreased ATP6V0A2 mRNA levels by destabilizing the mutant mRNA via the nonsense-mediated decay pathway. Loss of ATP6V0A2 either by siRNA knockdown or in ARCL2 cells resulted in distended Golgi cisternae, accumulation of abnormal lysosomes and multivesicular bodies. Immunostaining of ARCL2 cells showed the accumulation of tropoelastin (TE) in the Golgi and in large, abnormal intracellular and extracellular aggregates. Pulse-chase studies confirmed impaired secretion and increased intracellular retention of TE, and insoluble elastin assays showed significantly reduced extracellular deposition of mature elastin. Fibrillin-1 microfibril assembly and secreted lysyl oxidase activity were normal in ARCL2 cells. TUNEL staining demonstrated increased rates of apoptosis in ARCL2 cell cultures. We conclude that loss-of-function mutations in ATP6V0A2 lead to TE aggregation in the Golgi, impaired clearance of TE aggregates and increased apoptosis of elastogenic cells.

A novel cerebello-ocular syndrome with abnormal glycosylation due to abnormalities in dolichol metabolism

Cerebellar hypoplasia and slowly progressive ophthalmological symptoms are common features in patients with congenital disorders of glycosylation type I. In a group of patients with congenital disorders of glycosylation type I with unknown aetiology, we have previously described a distinct phenotype with severe, early visual impairment and variable eye malformations, including optic nerve hypoplasia, retinal coloboma, congenital cataract and glaucoma. Some of the symptoms overlapped with the phenotype in other congenital disorders of glycosylation type I subtypes, such as vermis hypoplasia, anaemia, ichtyosiform dermatitis, liver dysfunction and coagulation abnormalities. We recently identified pathogenic mutations in the SRD5A3 gene, encoding steroid 5α-reductase type 3, in a group of patients who presented with this particular phenotype and a common metabolic pattern. Here, we report on the clinical, genetic and metabolic features of 12 patients from nine families with cerebellar ataxia and congenital eye malformations diagnosed with SRD5A3-congenital disorders of glycosylation due to steroid 5α-reductase type 3 defect. This enzyme is necessary for the reduction of polyprenol to dolichol, the lipid anchor for N-glycosylation in the endoplasmic reticulum. Dolichol synthesis is an essential metabolic step in protein glycosylation. The current defect leads to a severely abnormal glycosylation state already in the early phase of the N-glycan biosynthesis pathway in the endoplasmic reticulum. We detected high expression of SRD5A3 in foetal brain tissue, especially in the cerebellum, consistent with the finding of the congenital cerebellar malformations. Based on the overlapping clinical, biochemical and genetic data in this large group of patients with congenital disorders of glycosylation, we define a novel syndrome of cerebellar ataxia associated with congenital eye malformations due to a defect in dolichol metabolism.

Inhibition of phosphomannose isomerase by fructose 1-phosphate: an explanation for defective N-glycosylation in hereditary fructose intolerance.

Isoelectrofocusing of serum sialotransferrins from patients with untreated hereditary fructose intolerance (HFI) shows a cathodal shift similar to that in carbohydrate-deficient glycoprotein (CDG) syndrome type I and in untreated galactosemia. This report is on serum lysosomal enzyme abnormalities in untreated HFI that are identical to those found in CDG syndrome type I but different from those in untreated galactosemia. CDG syndrome type I is due to phosphomannomutase deficiency, a defect in the early glycosylation pathway. It was found that fructose 1-phosphate is a potent competitive inhibitor(Ki ≃ 40 μM) of phosphomannose isomerase (EC 5.3.1.8), the first enzyme of the N-glycosylation pathway thus explaining the N-glycosylation disturbances in HFI.

Defining the phenotype in an autosomal recessive cutis laxa syndrome with a combined congenital defect of glycosylation

Autosomal recessive cutis laxa is a genetically heterogeneous condition. Its molecular basis is largely unknown. Recently, a combined disorder of N- and O-linked glycosylation was described in children with congenital cutis laxa in association with severe central nervous system involvement, brain migration defects, seizures and hearing loss. We report on seven additional patients with similar clinical features in combination with congenital disorder of glycosylation type IIx. On the basis of phenotype in 10 patients, we define an autosomal recessive cutis laxa syndrome. The patients have a complex phenotype of neonatal cutis laxa, transient feeding intolerance, late closure of the fontanel, characteristic facial features including down-slanting palpebral fissures, short nose and small mouth, and developmental delay. There is a variable degree of the central nervous system involvement and variable systemic presentation. The biochemical analysis using transferrin isoelectric focusing gives false negative results in some of the youngest patients. Analysis of the apolipoprotein C-III isoelectric focusing, however, is diagnostic in all cases.

Plasma N-Glycan Profiling by Mass Spectrometry for Congenital Disorders of Glycosylation Type II

BACKGROUND Determination of the genetic defect in patients with a congenital disorder of glycosylation (CDG) is challenging because of the wide clinical presentation, the large number of gene products involved, and the occurrence of secondary causes of underglycosylation. Transferrin isoelectric focusing has been the method of choice for CDG screening; however, improved methods are required for the molecular diagnosis of patients with CDG type II. METHODS Plasma samples with a typical transferrin isofocusing profile were analyzed. N-glycans were released from these samples by PNGase F [peptide-N4-(acetyl-β-glucosaminyl)-asparagine amidase] digestion, permethylated and purified, and measured on a MALDI linear ion trap mass spectrometer. A set of 38 glycans was used for quantitative comparison and to establish reference intervals for such glycan features as the number of antennae, the level of truncation, and fucosylation. Plasma N-glycans from control individuals, patients with known CDG type II defects, and patients with a secondary cause of underglycosylation were analyzed. RESULTS CDGs due to mannosyl (α-1,6-)-glycoprotein β-1,2-N-acetylglucosaminyltransferase (MGAT2), β-1,4-galactosyltransferase 1 (B4GALT1), and SLC35C1 (a GDP-fucose transporter) defects could be diagnosed directly from the N-glycan profile. CDGs due to defects in proteins involved in Golgi trafficking, such as subunit 7 of the conserved oligomeric Golgi complex (COG7) and subunit V0 a2 of the lysosomal H(+)-transporting ATPase (ATP6V0A2) caused a loss of triantennary N-glycans and an increase of truncated structures. Secondary causes with liver involvement were characterized by increased fucosylation, whereas the presence of plasma sialidase produced isolated undersialylation. CONCLUSIONS MALDI ion trap analysis of plasma N-glycans documents features that discriminate between primary and secondary causes of underglycosylation and should be applied as the first step in the diagnostic track of all patients with an unsolved CDG type II.

Multiplexed glycoproteomic analysis of glycosylation disorders by sequential yolk immunoglobulins immunoseparation and MALDI-TOF MS.

This study applied yolk immunoglobulins immunoaffinity separation and MALDI‐TOF MS for clinical proteomics of congenital disorders of glycosylation (CDG) and secondary glycosylation disorders [galactosemia and hereditary fructose intolerance (HFI)]. Serum transferrin (Tf) and α1‐antitrypsin (AAT) that are markers for CDG, were purified sequentially to obtain high‐quality MALDI mass spectra to differentiate single glycoforms of the native intact glycoproteins. The procedure was found feasible for the investigation of protein macroheterogeneity due to glycosylation site underoccupancy then ensuing the characterization of patients with CDG group I (N‐glycan assembly disorders). Following PNGase F digestion of the purified glycoprotein, the characterization of protein microheterogeneity by N‐glycan MS analysis was performed in a patient with CDG group II (processing disorders). CDG‐Ia patients showed a typical profile of underglycosylation where the fully glycosylated glycoforms are always the most abundant present in plasma with lesser amounts of partially and unglycosylated glycoforms in this order. Galactosemia and HFI are potentially fatal diseases, which benefit from early diagnosis and prompt therapeutic intervention. In symptomatic patients with galactosemia and in those with HFI, MALDI MS of Tf and AAT depicts a hypoglycosylation profile with a significant increase of underglycosylated glycoforms that reverses by dietary treatment, representing a clue for diagnosis and treatment monitoring.

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. Hum Mutat 16:247–252, 2000.