Lysosomal enzyme activities in serum and leukocytes from patients with carbohydrate-deficient glycoprotein syndrome type IA (phosphomannomutase deficiency)

In vitro treatment of congenital disorder of glycosylation type Ia using PLGA nanoparticles loaded with GDP‑Man

Congenital disorder of glycosylation (CDG) type Ia is a multisystem disorder that occurs due to mutations in the phosphomannomutase 2 (PMM2) gene, which encodes for an enzyme involved in the N‑glycosylation pathway. Mutated PMM2 leads to the reduced conversion of mannose‑6‑P to mannose‑1‑P, which results in low concentration levels of guanosine 5'‑diphospho‑D‑mannose (GDP‑Man), a nucleotide‑activated sugar essential for the construction of protein oligosaccharide chains. In the present study, an in vitro therapeutic approach was used, based on GDP‑Man‑loaded poly (D,L‑lactide‑co‑glycolide) (PLGA) nanoparticles (NPs), which were used to treat CDG‑Ia fibroblast cultures, thus bypassing the glycosylation pathway reaction catalysed by PMM2. To assess the degree of hypoglycosylation in vitro, the present study examined the activities of α‑mannosidase, β‑glucoronidase and β‑galactosidase in defective and normal fibroblasts. GDP‑Man (30 µg/ml GDP‑Man PLGA NPs) was incubated for 48 h with the cells and the specific activities of α‑mannosidase and β‑galactosidase were estimated at 69 and 92% compared with healthy controls. The residual activity of β‑glucoronidase increased from 6.5 to 32.5% and was significantly higher compared with that noted in the untreated CDG‑Ia fibroblasts. The glycosylation process of fibroblasts was also analysed by two‑dimensional electrophoresis. The results demonstrated that treatment caused the reappearance of several glycosylated proteins. The data in vitro showed that GDP‑Man PLGA NPs have desirable efficacy and warrant further evaluation in a preclinical validation animal model.

Hereditary fructose intolerance mimicking a biochemical phenotype of mucolipidosis: A review of the literature of secondary causes of lysosomal enzyme activity elevation in serum

We describe a patient with failure to thrive, hepatomegaly, liver dysfunction, and elevation of multiple plasma lysosomal enzyme activities mimicking mucolipidosis II or III, in whom a diagnosis of hereditary fructose intolerance (HFI) was ultimately obtained. She presented before introduction of solid foods, given her consumption of a fructose-containing infant formula. We present the most extensive panel of lysosomal enzyme activities reported to date in a patient with HFI, and propose that multiple enzyme elevations in plasma, especially when in conjunction with a normal plasma α-mannosidase activity, should elicit a differential diagnosis of HFI. We also performed a review of the literature on the different etiologies of elevated lysosomal enzyme activities in serum or plasma. © 2016 Wiley Periodicals, Inc.

Prospective phenotyping of NGLY1-CDDG, the first congenital disorder of deglycosylation

PURPOSE

The cytosolic enzyme N-glycanase 1, encoded by NGLY1, catalyzes cleavage of the β-aspartyl glycosylamine bond of N-linked glycoproteins, releasing intact N-glycans from proteins bound for degradation. In this study, we describe the clinical spectrum of NGLY1 deficiency (NGLY1-CDDG).

METHODS

Prospective natural history protocol.

RESULTS

In 12 individuals ages 2 to 21 years with confirmed, biallelic, pathogenic NGLY1 mutations, we identified previously unreported clinical features, including optic atrophy and retinal pigmentary changes/cone dystrophy, delayed bone age, joint hypermobility, and lower than predicted resting energy expenditure. Novel laboratory findings include low cerebral spinal fluid (CSF) total protein and albumin and unusually high antibody titers toward rubella and/or rubeola following vaccination. We also confirmed and further quantified previously reported findings noting that decreased tear production, transient transaminitis, small feet, a complex hyperkinetic movement disorder, and varying degrees of global developmental delay with relatively preserved socialization are the most consistent features.

CONCLUSION

Our prospective phenotyping expands the clinical spectrum of NGLY1-CDDG, offers prognostic information, and provides baseline data for evaluating therapeutic interventions.Genet Med 19 2, 160-168.

Cholesteatoma-associated pathogenicity: potential role of lysosomal exoglycosidases

The enzymatic profile of lysosomal exoglycosidases in middle ear cholesteatoma has not been well known. The assessment of glycoconjugate catabolism may contribute to a better understanding of cholesteatoma pathogenesis.

OBJECTIVE

The study aim was to evaluate catabolic processes of glycoproteins, glycolipids, and proteoglycans in cholesteatoma through outlining the concentration of N-acetyl-β-hexosaminidase (HEX), β-glucuronidase (GLUC), and β-galactosidase (GAL) activity as well as in serum of cholesteatoma patients and healthy volunteers.

STUDY DESIGN

Acquired cholesteatomas (n = 25) and normal retroauricular skin specimens (n = 25) were taken during surgery as well as serum from cholesteatoma patients and healthy volunteers. HEX, GAL, and GLUC activity was assessed on basis of p-nitrophenol release from derivatives of the substrate (HEX: N-acetylglucosamine i N-acetylgalactosamine, GAL from galactose, and GLUC from glucuronide).

RESULTS

The mean concentration of activity of HEX 1142.39 pKat/ml, GAL 8.90 pKat/ml, and GLUC 14.06 pKat/ml was significantly higher compared with the concentration of enzyme activity in normal tissue: HEX 267.65 pKat/ml, GAL 3.44 pKat/ml, and GLUC 3.90 pKat/ml. In the serum of cholesteatoma patients, the mean concentration of enzyme activities were as follows: HEX 641.62 pKat/ml, GAL 4.55 pKat/ml, and GLUC 12.80 pKat/ml and were significantly higher compared with the concentration of HEX activity (215.75 pKat/ml), GAL (1.89 pKat/ml), and GLUC (5.51 pKat/ml) in the serum of the healthy control group. In cholesteatoma compared with the normal tissue, there is an increase of the glycoconjugate catabolism due to significantly higher concentration of HEX, GAL, and GLUC activity in cholesteatoma. Cholesteatoma causes systemic reaction due to the increase of HEX, GAL, and GLUC activity in patient serum.

Distinct features of congenital disorder of glycosylation type IIx in Kuwait: a case report

Congenital disorders of glycosylation are a rare but complex group of inborn metabolic diseases that result in carbohydrate-deficient glycoproteins. Biochemical and clinical features of both types I and II of this group of disorders have been well characterized and reported from various parts of the world except the Middle East. The authors describe a patient with congenital disorders of glycosylation type IIx who presented with psychomotor retardation, development delay, hypotonia, and hepatomegaly. Computed tomography scan of the liver showed multiple hepatic focal lesions, and magnetic resonance imaging revealed mild brain and corpus callosum atrophy. This is the first report of congenital disorders of glycosylation type IIx from Kuwait that shows its prevalence and distinct features in the Middle East.

The clinical spectrum of phosphomannomutase 2 deficiency (CDG-Ia)

Congenital disorders of glycosylation are a clinically and genetically heterogeneous group of disorders resulting from abnormal glycosylation of various glycoconjugates. The first description of congenital disorders of glycosylation was published in the early 80s and once screening tests for glycosylation disorders (CDGs) became readily available, CDG-Ia became the most frequently diagnosed CDG subtype. CDG-Ia is pan-ethnic and the spectrum of the clinical manifestations is still evolving: it spans from severe hydrops fetalis and fetal loss to a (nearly) normal phenotype. However, the most common presentation in infancy is of a multisystem disorder with central nervous system involvement.

Serum N-acetyl-beta-D-glucosaminidase profiles in type 1 diabetes secondary complications: causes of changes and significance of determination

The connection between changes in the activity of serum N-acetyl-beta-D-glucosaminidase (NAG, E.C.3.2.1.30) and iso-enzymes and degree of secondary complications was analyzed in four groups of type 1 diabetic patients (n=69): without complications (n=22); with retinopathy (n=16); with retinopathy and polyneuropathy (n=13), and with retinopathy, neuropathy, and nephropathy (n=18). In all groups statistically significant higher (P<0.001) percent fraction of A form (83.84+/-6.09, 84.37+/-5.74, 81.76+/-6.02, 76.37+/-7.38%, resp.) and lower (P<0.001, P<0.01) fraction of B form (15.87+/-5.65, 15.66+/-5.74, 18.33+/-5.98, 23.63+/-7.38, resp.) in total NAG compared with the control (A=69.38+/-4.79%, B=30.61+/-4.78%) were found. The differences in A as well as B forms between diabetic groups were not statistically significant. Significant strong positive correlations between total NAG and glycemia (0.494-0.623), total NAG and A form (0.934-0.966), and A form and glycemia (0.512-0.638) were found in all groups. No correlation was found between the fractions of B and A forms, except in the fourth group. The A form of diabetic patients in the fourth group was more acidic compared with the control and other diabetic groups. It was concluded that the changes in serum NAG and iso-enzymic profiles in diabetes are the consequence of its increased exocytose, especially of the A form, in hyperglycemia and posttranslational modifications of iso-enzymes. The total activity of serum NAG and iso-enzymic profiles cannot be used for monitoring the development and distinction of type 1 diabetes secondary complications.

The skeletal manifestations of the congenital disorders of glycosylation

The congenital disorders of glycosylation (CDG) are a rapidly expanding disease group with protean presentations. Specific end-organ involvement leads to significant morbidity and mortality, and the skeletal manifestations are often not appreciated, apart from the common association of osteopaenia with CDG-Ia. We performed a literature review of all documented skeletal manifestations in reported CDG patients, revealing a diverse range of skeletal phenotypes. We discuss the possible underlying mechanisms of these skeletal manifestations observed in CDG that are important and frequently under-recognized.

Clinical and biochemical presentation of siblings with COG-7 deficiency, a lethal multiple O- and N-glycosylation disorder

Congenital disorders of glycosylation (CDG) represent a group of inherited multiorgan diseases caused by defects in the biosynthesis of glycoproteins. We report on two dysmorphic siblings with severe liver disease who died at the age of a few weeks. Increased activities of lysosomal enzymes in plasma were found, though total sialic acid in plasma was strongly decreased. Isoelectric focusing of serum sialotransferrins showed a type 2-like CDG pattern. Some of the known CDG subtypes were excluded. O-Glycosylation was investigated by isoelectric focusing of apolipoprotein C-III, which showed increased fractions of hyposialylated isoforms. In a consecutive study a defect in the conserved oligomeric Golgi complex was established at the level of subunit COG-7, leading to disruption of multiple glycosylation functions of the Golgi. This report on patients with a new variant of CDG, due to a multiple Golgi defect, emphasizes in addition to sialotransferrins the importance of analysis of a serum O-linked glycoprotein, e.g. apolipoprotein C-III, in unclassified CDG-X cases.

Low levels of occupational exposure to arsenic and antimony: effects on lysosomal glycohydrolase levels in plasma of exposed workers and in lymphocyte cultures

BACKGROUND

Heavy metals have been shown to alter the mechanism and release of lysosomal enzymes. In the present study, the activities of lysosomal glycohydrolases were determined in order to evaluate the asymptomatic toxic effects of low levels of exposure to arsenic (As) and antimony (Sb) in art glass workers.

METHODS

N-acetyl-beta-D-glucosaminidase (NAG), beta-D-glucuronidase (GCR), alpha- and beta-D-galactosidase, alpha-D-glucosidase, and alpha-D-mannosidase were determined by a fluorimetric assay in the plasma of 26 art glass workers. Lymphocytes cultured in the presence of different species of As and Sb served as an in vitro model for the study of the protective action of selenium and zinc.

RESULTS

No significant difference in the plasma levels of the various enzymes was detected in art glass workers or control subjects. The in vitro experiments demonstrated that secretion of lysosomal glycohydrolases was increased by Sb (225%) and decreased by As (57%) at the same concentration of elements (200 microg/L). The addition of bivalent selenium to the culture neutralized the effects of both metals, while zinc chloride did not show any protective effect.

CONCLUSIONS

As for the plasma glycohydrolases, no praecox signs of toxicity related to a low concentration of As and Sb was evident in art glass workers. This may be due to the antagonistic effects demonstrated by these two metals in vitro. Their different mechanism of action on release of glycohydrolases is being discussed.

Abnormal lysosomal inclusions in liver hepatocytes but not in fibroblasts in congenital disorders of glycosylation (CDG)

We report a retrospective electron-microscopical study of liver biopsies and fibroblast cultures of 19 patients with congenital disorders of glycosylation (CDG) of different subtypes. A constant finding in liver biopsies of all CDG-I cases was that of abnormal lysosomal lamellar inclusions in the hepatocytes, which were not found in CDG-II. None of the patients showed significant abnormalities in their fibroblasts.

Increased biosynthesis of glycosphingolipids in congenital disorder of glycosylation Ia (CDG-Ia) fibroblasts

Congenital disorder of glycosylation Ia (CDG-Ia) is an autosomal recessive disease, characterized by the impaired biosynthesis of the N-linked oligosaccharide chains of proteins due to a deficiency of phosphomannomutase (PMM), the enzyme converting mannose-6-phosphate into mannose-1-phosphate. We investigated the consequences of the altered N-linked glycoprotein (GP) biosynthesis on the quantity and quality of glycosphingolipids (GSLs) in fibroblasts of CDG-Ia patients. First, we found that CDG-Ia fibroblasts contain an increased amount of total GSLs when compared with normal fibroblasts. Further, we assessed by metabolic labeling of CDG-Ia fibroblasts with radioactive sugar precursors, including galactose and N-acetylmannosamine, that a diminished biosynthesis of cellular GPs is antagonized by an increased biosynthesis of GSLs. An increased GSL biosynthesis was also observed by means of radiolabeled lipid precursors including sphingosine and lactosylceramide. Notably, also the degradation of GLSs is slowed down in CDG-Ia fibroblasts. Finally, when we labeled normal human fibroblasts and CHO cells with radioactive galactose in the presence and absence of deoxymannojirimycin (dMM), an inhibitor of N-glycan processing, we found that this cellular model mimics what occurs in CDG-Ia fibroblasts. Since an inverse relationship between GP expression and GSL content does exist, we assume that increased glycosphingolipid biosynthesis is secondary to protein hypoglycosylation. Altogether, our data suggest that the cell metabolic machinery may be able to partially re-equilibrate protein hypoglycosylation with increased biosynthesis of glycosphingolipids, possibly to preserve the overall physico-chemical equilibrium of the outer layer of the plasma membrane.

Congenital disorders of glycosylation

Congenital disorders of glycosylation (CDG) are a rapidly growing group of genetic diseases that are due to defects in the synthesis of glycans and in the attachment of glycans to other compounds. Most CDG are multisystem diseases that include severe brain involvement. The CDG causing sialic acid deficiency of N-glycans can be diagnosed by isoelectrofocusing of serum sialotransferrins. An efficient treatment, namely oral D-mannose, is available for only one CDG (CDG-Ib). In many patients with CDG, the basic defect is unknown (CDG-x). Glycan structural analysis, yeast genetics, and knockout animal models are essential tools in the elucidation of novel CDG. Eleven primary genetic glycosylation diseases have been discovered and their basic defects identified: six in the N-glycan assembly, three in the N-glycan processing, and two in the O-glycan (glycosaminoglycan) assembly. This review summarizes their clinical, biochemical, and genetic characteristics and speculates on further developments in this field.

beta-hexosaminidase, alpha-D-mannosidase, and beta-mannosidase expression in serum from patients with carbohydrate-deficient glycoprotein syndrome type I

The activity of beta-hexosaminidase, determined with 4-methylumbelliferyl-beta-N-acetylglucopyranoside substrate, and of beta-D-mannosidase was significantly higher in the serum of patients with carbohydrate-deficient glycoprotein (CDG) syndrome type IA (phosphomannomutase deficiency) than in controls. No significant differences were observed in the activity of beta-hexosaminidase, determined using 4-methylumbelliferyl-beta-N-acetylglucopyranoside-6-sulphate as substrate, and the activity of alpha-D-mannosidase. Using DEAE-cellulose chromatography, a greater amount of hexosaminidase B than hexosaminidase A was detected in CDG serum. In CDG serum, hexosaminidase A was eluted in a more basic position in the salt gradient. An isoenzyme of alpha-D-mannosidase and beta-D-mannosidase was identified in control and CDG sera. alpha-D-Mannosidase isoenzyme was eluted in a slightly more basic position in CDG serum than in control serum, whereas beta-D-mannosidase isoenzyme was eluted in the same position.

Carbohydrate-deficient glycoprotein syndromes

Four types of carbohydrate-deficient glycoprotein syndrome have been described, and the cause of two of them has been found. The symptoms and signs of these syndromes are described, with variations that occur at different ages. The commonest is type Ia with an autosomal recessive form of inheritance, and the gene responsible has been mapped to 16p. The typical pathology is atrophy of the cerebellum and brainstem, sometimes also involving the cortex, although both the pathology and the biochemical deficiencies vary between different types of syndrome. The diagnosis depends firstly on recognising the clinical features, including the presence of complications such as thyroid disorders. Then biochemical tests can be carried out, especially chromatographic carbohydrate-deficient transferrin assay and isoelectric focusing of serum transferrin. The prognosis depends on the complications, renal, hepatic, and cardiac, but affected children will be severely handicapped. Therefore treatment consists mainly of coping with the complications, and supporting the child and the family. Oral infusion of mannose can be effective in type Ib disease.

[Carbohydrate-deficient blood glycoprotein syndrome]

Carbohydrate-deficient glycoprotein syndrome (CDGS) is a newly delineated group of inherited multisystemic disorders associated with abnormal glycosylation of a number of serum glycoproteins. Several types have been described on the basis of clinical presentation and biochemical changes of the glycosylation of serum transferrin and attributed to different enzymatic defects; their clinical presentations are fully different and a clinical heterogeneity is observed within a same type of CDGS. Patients with CDGS type la usually present with neurologic (hypotonia, strabismus and cerebellar hypoplasia) and cutaneous (inverted nipples, abnormal distribution of adipose tissue) abnormalities, together with multivisceral involvement (digestive, hepatic, cardiac, renal). However, neurologic and cutaneous symptoms may be absent, so that CDGS must be looked for in case of unexplained organ failure such as isolated liver insufficiency, cardiomyopathy, pericarditis, tubulopathy, nephrotic syndrome, vascular accident or retinitis pigmentosa. Patients with CDGS type Ib present with liver disease, enteropathy and hypoglycemia without neurologic involvement. These patients are successfully treated with oral mannose administration emphasizing the importance of making the diagnosis. Patients with CDGS type Ic present with mild psychomotor retardation and seizures. Patients with CDGS type II have psychomotor retardation association with severe gastrointestinal disorder, dysmorphic features and abnormal electroretinogram. Other types (III, IV) are less clearly defined and the clinical presentation includes convulsive encephalopathy. Biological abnormalities such as mild hepatic cytolysis, hematologic and hormonal abnormalities are consistently observed in CDGS type I, as well as renal hyperechogeneity, leading one to look for this syndrome when they are associated. Until now, only four enzymatic deficiencies have been identified (types Ia, Ib, Ic, II).

Screening for "prelysosomal disorders": carbohydrate-deficient glycoprotein syndromes

Physicians have become accustomed to thinking of certain inborn errors of metabolism (e.g., lysosomal, peroxisomal, and mitochondrial diseases) as being associated with specific subcellular organelles. In recent years, a family of disorders of N-glycosylation has been recognized, in which the metabolic defect is expressed in the cytosol, endoplasmic reticulum, and Golgi apparatus. These could be conveniently thought of as "prelysosomal" disorders. At least six of these entities are characterized by hypoglycosylation of many glycoconjugates, and have been designated as the carbohydrate-deficient glycoprotein syndromes. Given the ubiquity of the products of N-glycosylation in the cellular economy, it is not surprising that these defects in metabolism have protean clinical manifestations. Delayed development and other neurologic symptoms are wedded to variable dysfunctions of the heart, liver, and endocrine and coagulation systems. Patients can have dysmorphic features or cerebellar hypoplasia, attesting to the antenatal expression of these disorders. The most frequently recognized phenotype (several hundred cases worldwide) has been designated carbohydrate-deficient glycoprotein syndrome type la, and results from mutations in phosphomannomutase, a cytosolic enzyme involved in the synthesis of the lipid-linked oligosaccharide that is eventually attached to nascent glycoproteins through the amide group of asparagine residues. All forms of carbohydrate-deficient glycoprotein syndrome express an excess of hypoglycosylated isoforms of circulating transferrin, which serves as a useful screening tool. Physicians should consider screening for carbohydrate-deficient glycoprotein syndrome in individuals with delayed development, seizures, strokelike episodes, cerebellar hypoplasia, and demyelinating neuropathy with or without other signs of multisystem disease.

Carbohydrate-deficient glycoprotein syndrome type IA (phosphomannomutase-deficiency)

The carbohydrate-deficient glycoprotein or CDG syndromes (OMIM 212065) are a recently delineated group of genetic, multisystem diseases with variable dysmorphic features. The known CDG syndromes are characterized by a partial deficiency of the N-linked glycans of secretory glycoproteins, lysosomal enzymes, and probably also membranous glycoproteins. Due to the deficiency of terminal N-acetylneuraminic acid or sialic acid, the glycan changes can be observed in serum transferrin or other glycoproteins using isoelectrofocusing with immunofixation as the most widely used diagnostic technique. Most patients show a serum sialotransferrin pattern characterized by increased di- and asialotransferrin bands (type I pattern). The majority of patients with type I are phosphomannomutase deficient (type IA), while in a few other patients, deficiencies of phosphomannose isomerase (type IB) or endoplasmic reticulum glucosyltransferase (type IC) have been demonstrated. This review is an update on CDG syndrome type IA.

Carbohydrate-deficient glycoprotein syndrome type II

The carbohydrate-deficient glycoprotein syndromes (CDGS) are a group of autosomal recessive multisystemic diseases characterized by defective glycosylation of N-glycans. This review describes recent findings on two patients with CDGS type II. In contrast to CDGS type I, the type II patients show a more severe psychomotor retardation, no peripheral neuropathy and a normal cerebellum. The CDGS type II serum transferrin isoelectric focusing pattern shows a large amount (95%) of disialotransferrin in which each of the two glycosylation sites is occupied by a truncated monosialo-monoantennary N-glycan. Fine structure analysis of this glycan suggested a defect in the Golgi enzyme UDP-GlcNAc:alpha-6-D-mannoside beta-1,2-N-acetylglucosaminyltransferase II (GnT II; EC 2.4.1.143) which catalyzes an essential step in the biosynthetic pathway leading from hybrid to complex N-glycans. GnT II activity is reduced by over 98% in fibroblast and mononuclear cell extracts from the CDGS type II patients. Direct sequencing of the GnT II coding region from the two patients identified two point mutations in the catalytic domain of GnT II, S290F (TCC to TTC) and H262R (CAC to CGC). Either of these mutations inactivates the enzyme and probably also causes reduced expression. The CDG syndromes and other congenital defects in glycan synthesis as well as studies of null mutations in the mouse provide strong evidence that the glycan moieties of glycoproteins play essential roles in the normal development and physiology of mammals and probably of all multicellular organisms.