Jaak Jaeken

Phosphomannomutase deficiency is a cause of carbohydrate-deficient glycoprotein syndrome type I

Carbohydrate‐deficient glycoprotein (CDG) syndromes are genetic multisystemic disorders characterized by defective N‐glycosylation of serum and cellular proteins. The activity of phosphomannomutase was markedly deficient (⩽ 10% of the control activity) in fibroblasts, liver and/or leucocytes of 6 patients with CDG syndrome type I. Other enzymes involved in the conversion of glucose to mannose 1‐phosphate, as well as phosphoglucomutase, had normal activities. Phosphomannomutase activity was normal in fibroblasts of 2 patients with CDG syndrome type II. Since this enzyme provides the mannose 1‐phosphate required for the initial steps of protein glycosylation, it is concluded that phosphomannomutase deficiency, which is first reported here for higher organisms, is a cause, and most likely the major one, of CDG syndrome type I.

Congenital Disorders of Glycosylation

Congenital (genetic) disorders of glycosylation (CDG) are a rapidly growing disease family, with some 45 members reported since its first clinical description in 1980. Most of these are protein hypoglycosylation diseases, but recently three defects in lipid glycosylation have been identified. Most protein hypoglycosylation diseases are due to defects in the N‐glycosylation pathway (16 diseases). The remaining ones affect the O‐glycosylation pathway (8 diseases), both the N‐ and the O‐glycosylation pathways, or other pathways (17 diseases). CDG can affect nearly all organs and systems, but there is often an important neurological component. The first‐line screening for the N‐glycosylation diseases is serum transferrin isoelectrofocusing (IEF), and for the O‐glycosylation disorders apo CIII IEF. It has to be stressed that a normal test result does by no means exclude a CDG. In case of an abnormal result and as long as the basic defect has not been elucidated, the disease is labeled CDG‐x (CDG‐Ix when the transferrin IEF shows a type 1 pattern, and CDG‐IIx when it shows a type 2 pattern).

Familial psychomotor retardation with markedly fluctuating serum prolactin, FSH and GH levels, partial TBG-deficiency, increased serum arylsulphatase A and increased CSF protein: a new syndrome?: 90

Identical twin-sisters (born at 36 wks; birthweight 2.2 and 3.0 kg) presented at 2 years of age with marked psychomotor retardation and bone-age of 1 year. Physical growth and phenotype were normal. Repeated investigations revealed: markedly fluctuating basal serum prolactin (778-5652 μU/ml; nl < 800), FSH (17-55 mIU/ml; nl < 10) and GH (2-144 ng/ml; nl < 10), but normal LH; low TBG (1.1 and 1.2 mg/dl; nl 1.6-2.4) also present in the father, with otherwise normal thyroid function including TRH test, arylsulphatase A moderately increased in serum (mean 293 and 272 nmol/ml; nl 30-130) but not in leukocytes, without increase of other lysosomal enzymes, and increasing CSF protein. Normal results were found for GH response to i.m. glucagon, urinary excretion of 17-keto and 17-hydroxysteroids, at funduscopy and for lymphocyte karyotype (Giemsa banding), buffy coat of blood leukocytes and electronmicroscopy of conjunctiva. Sella tursica was normal on x-ray. Cortical and cerebellar hypotrophy was evident on CAT-scan. Electromyography was normal but nerve conduction velocity was delayed (30-31 m/sec; nl 50 ± 1). A nerve and muscle biopsy is planned. At this stage we have no satisfactory explanation for these unusual findings.

Sialic acid-deficient serum and cerebrospinal fluid transferrin in a newly recognized genetic syndrome

Identical twin-sisters with evidence of a demyelinating disease showed multiple serum glycoprotein abnormalities. The association of a low serum iron level and a normal blood haemoglobin suggested an abnormality of transferrin too. This was confirmed by finding a sialic acid-deficiency of this glycoprotein in serum as well as in cerebrospinal fluid.

Carbohydrate deficient glycoprotein syndrome type II: a deficiency in Golgi localised N-acetyl-glucosaminyltransferase II.

The carbohydrate deficient glycoprotein (CDG) syndromes are a family of genetic multisystemic disorders with severe nervous system involvement. This report is on a child with a CDG syndrome that differs from the classical picture but is very similar to a patient reported in 1991. Both these patients are therefore designated CDG syndrome type II. Compared with type I patients they have a more severe psychomotor retardation but no peripheral neuropathy nor cerebellar hypoplasia. The serum transferrin isoform pattern obtained by isoelectric focusing showed disialotransferrin as the major fraction. The serum disialotransferrin, studied in the present patient, contained two moles of truncated monoantennary Sialyl-Gal-GlcNAc-Man(alpha 1-->3)[Man(alpha 1-->6)]Man(beta 1-->4)GlcNAc (beta 1-->4)GlcNAc-Asn per mole of transferrin. A profoundly deficient activity of the Golgi enzyme N-acetylglucosaminyltransferase II (EC 2.4.1.143) was demonstrated in fibroblasts.

A broad spectrum of clinical presentations in congenital disorders of glycosylation I: a series of 26 cases

INTRODUCTION Congenital disorders of glycosylation (CDG), or carbohydrate deficient glycoprotein syndromes, form a new group of multisystem disorders characterised by defective glycoprotein biosynthesis, ascribed to various biochemical mechanisms. METHODS We report the clinical, biological, and molecular analysis of 26 CDG I patients, including 20 CDG Ia, two CDG Ib, one CDG Ic, and three CDG Ix, detected by western blotting and isoelectric focusing of serum transferrin. RESULTS Based on the clinical features, CDG Ia could be split into two subtypes: a neurological form with psychomotor retardation, strabismus, cerebellar hypoplasia, and retinitis pigmentosa (n=11), and a multivisceral form with neurological and extraneurological manifestations including liver, cardiac, renal, or gastrointestinal involvement (n=9). Interestingly, dysmorphic features, inverted nipples, cerebellar hypoplasia, and abnormal subcutaneous fat distribution were not consistently observed in CDG Ia. By contrast, the two CDG Ib patients had severe liver disease, enteropathy, and hyperinsulinaemic hypoglycaemia but no neurological involvement. Finally, the CDG Ic patient and one of the CDG Ix patients had psychomotor retardation and seizures. The other CDG Ix patients had severe proximal tubulopathy, bilateral cataract, and white matter abnormalities (one patient), or multiorgan failure and multiple birth defects (one patient). CONCLUSIONS Owing to the remarkable clinical variability of CDG, this novel disease probably remains largely underdiagnosed. The successful treatment of CDG Ib patients with oral mannose emphasises the paramount importance of early diagnosis of PMI deficiency.

Deficiency of dolichol-phosphate-mannose synthase-1 causes congenital disorder of glycosylation type Ie

Congenital disorders of glycosylation (CDG), formerly known as carbohydrate-deficient glycoprotein syndromes, lead to diseases with variable clinical pictures. We report the delineation of a novel type of CDG identified in 2 children presenting with severe developmental delay, seizures, and dysmorphic features. We detected hypoglycosylation on serum transferrin and cerebrospinal fluid beta-trace protein. Lipid-linked oligosaccharides in the endoplasmic reticulum of patient fibroblasts showed an accumulation of the dolichyl pyrophosphate Man(5)GlcNAc(2) structure, compatible with the reduced dolichol-phosphate-mannose synthase (DolP-Man synthase) activity detected in these patients. Accordingly, 2 mutant alleles of the DolP-Man synthase DPM1 gene, 1 with a 274C>G transversion, the other with a 628delC deletion, were detected in both siblings. Complementation analysis using DPM1-null murine Thy1-deficient cells confirmed the detrimental effect of both mutations on the enzymatic activity. Furthermore, mannose supplementation failed to improve the glycosylation status of DPM1-deficient fibroblast cells, thus precluding a possible therapeutic application of mannose in the patients. Because DPM1 deficiency, like other subtypes of CDG-I, impairs the assembly of N-glycans, this novel glycosylation defect was named CDG-Ie.

Congenital disorders of glycosylation (CDG): It's all in it!

Summary: Congenital disorders of glycosylation (CDGs) are due to defects in the synthesis of the glycan moiety of glycoproteins or other glycoconjugates. This review is devoted mainly to the clinical aspects of protein glycosylation defects. There are two main types of protein glycosylation: N-glycosylation and O-glycosylation. N-glycosylation generally consists of an assembly pathway (in cytosol and endoplasmic reticulum) and a processing pathway (in endoplasmic reticulum and Golgi). O-glycosylation lacks a processing pathway but is otherwise more complex. Sixteen disease-causing defects are known in protein glycosylation: 12 in N-glycosylation and four in O-glycosylation. The N-glycosylation defects comprise eight assembly defects (CDG-I) designated CDG-Ia to CDG-Ih, and four processing defects (CDG-II) designated CDG-IIa to CDG-IId. By far the most frequent is CDG-Ia (phosphomannomutase-2 deficiency). It affects the nervous system and many other organs. Its clinical expression varies from extremely severe to very mild (and thus probably underdiagnosed). The most interesting disease in this group is CDG-Ib (phosphomannose isomerase deficiency) because it is so far the only efficiently treatable CDG (mannose treatment). It hasahepatic–intestinal presentation. The O-glycosylation defects comprise two O-xylosylglycan defects (a progeroid variant of Ehlers–Danlos syndrome and the multiple exostoses syndrome) and two O-mannosylglycan defects (Walker–Warburg syndrome and muscle–eye–brain disease). All known CDGs have a recessive inheritance except for multiple exostoses syndrome, which is dominantly inherited. There is a rapidly growing group of putative CDGs with a large spectrum of clinical presentations (CDG-x). Serum transferrin isoelectrofocusing remains the cornerstone of the screening for N-glycosylation defects associated with sialic acid deficiency. Abnormal patterns can be grouped in to type 1 and type 2. However, a normal pattern does not exclude these defects. Screening for the other CDGs is much more difficult, particularly when the defect is organ- or system-restricted. The latter group promises to become an important new chapter in CDG. It is concluded that CDGs will eventually cover the whole clinical spectrum of paediatric and adult disease manifestations.

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.).

Biochemical Characteristics and Diagnosis of the Carbohydrate-deficient Glycoprotein Syndrome

29 patienxts with a new inherited complex developmental deficiency syndrome—the carbohydrate‐deficient glycoprotein syndrome—were studied biochemically. The most striking biochemical abnormality in these patients is the presence of secretory glycoproteins, that are deficient in their carbohydrate moieties. Serum transferrin shows the most pronounced carbohydrate defect, both quantitatively and qualitatively. Half of this glycoprotein is apparently missing two or four of its terminal trisaccharidcs—sialic acid, galactose and N‐acetylglucosamine—while the carbohydrate core appears to be intact. The abnormal transferrin is also present in the liver. This biochemical alteration is a highly specific marker of the syndrome, which can be diagnosed either qualitatively by isoelectric focusing of serum transferrin or quantitatively by the “carbohydrate‐deficient transferrin” (CDT) assay. In the CDT assay all of these patients have values approximately ten times above the reference level. The unique carbohydrate defect in secretory glycoproteins indicates that this disorder represents a new type of inborn error of glycoprotein metabolism. Studies of eleven enzymes in glycoprotein synthesis and catabolism have not revealed any deficiency of glycosidases or glycosyltransferases. The nature of the transferrin change and the cathepsin assays performed to date may suggest an as yet unidentified degradation abnormality.