Metabolic mimics: the disorders of N-linked glycosylation

N-linked glycosylation is essential for normal cellular function. Defects have now been described in eighteen genes that participate in the process. All give rise to complex multisystem diseases which, with a few exceptions, primarily involve the nervous system. Frequent features of these disorders include developmental delay, ataxia, seizures, stroke-like episodes, recurrent infections, coagulopathy and dysmorphism. Most cases can be detected by screening carbohydrate-deficient transferrin, but definitive diagnosis requires enzymatic and molecular confirmation, frequently in collaboration with a research glycobiologist.     

The congenital disorders of glycosylation: A multifaceted group of syndromes

The congenital disorders of glycosylation (CDG) are a rapidly expanding group of metabolic syndromes with a wide symptomatology and severity. They all stem from deficient N-glycosylation of proteins. To date the group contains 18 different subtypes: 12 of Type I (disrupted synthesis of the lipid-linked oligosaccharide precursor) and 6 of Type II (malfunctioning trimming/processing of the protein-bound oligosaccharide). Main features of CDG involve psychomotor retardation; ataxia; seizures; retinopathy; liver fibrosis; coagulopathies; failure to thrive; dysmorphic features, including inverted nipples and subcutaneous fat pads; and strabismus. No treatment currently is available for the vast majority of these syndromes (CDG-Ib and CDG-IIc are exceptions), even though attempts to synthesize drugs for the most common subtype, CDG-Ia, have been made. In this review we will discuss the individual syndromes, with focus on their neuronal involvement, available and possible treatments, and future directions.     

Neurology of inherited glycosylation disorders

Congenital disorders of glycosylation comprise most of the nearly 70 genetic disorders known to be caused by impaired synthesis of glycoconjugates. The effects are expressed in most organ systems, and most involve the nervous system. Typical manifestations include structural abnormalities (eg, rapidly progressive cerebellar atrophy), myopathies (including congenital muscular dystrophies and limb-girdle dystrophies), strokes and stroke-like episodes, epileptic seizures, developmental delay, and demyelinating neuropathy. Patients can also have neurological symptoms associated with coagulopathies, immune dysfunction with or without infections, and cardiac, renal, or hepatic failure, which are common features of glycosylation disorders. The diagnosis of congenital disorder of glycosylation should be considered for any patient with multisystem disease and in those with more specific phenotypic features. Measurement of concentrations of selected glycoconjugates can be used to screen for many of these disorders, and molecular diagnosis is becoming more widely available in clinical practice. Disease-modifying treatments are available for only a few disorders, but all affected individuals benefit from early diagnosis and aggressive management.     

Congenital disorders of glycosylation: the rapidly growing tip of the iceberg.

In addition to many other organs, the brain is affected in 10 of the 11 known congenital disorders of N-linked glycosylation, mostly to a severe degree. Because a large number of enzymes, transporters and other proteins are involved in glycosylation (both N-linked and O-linked), it is expected that the great majority of congenital disorders of glycosylation (CDG) are yet to be identified. Many neurological patients with a CDG escape diagnosis for that reason, but also because existing screening methods fail to detect all patients with a known CDG. These disorders should be looked for in any patient, regardless of age, with an unexplained neurological disorder.     

Neurologic course of congenital disorders of glycosylation.

Congenital disorders of glycosylation, formerly called carbohydrate-deficient glycoprotein syndrome, may present in infancy with slowly progressive neurologic deficits including cognitive impairment, ataxia, pigmentary retinal degeneration, and neuropathy. The metabolic defect is in N-linked oligosaccharide synthesis, and diagnosis is made by a serum transferrin isoelectric focusing. We reviewed the neurologic course of 10 children with congenital disorders of glycosylation (ages 13 months to 7 years). All had severe developmental delay and ataxia; none walked unassisted, and the highest level of communication was simple sign language in one patient. Five of 10 children had seizures (absence, complex partial, tonic clonic). Only one patient has had strokelike episodes, despite reports that they are common in this population. The underlying basis of these episodes has been hypothesized to be coagulopathy due to dysfunctional, incorrectly glycosylated coagulation factors. This 5-year-old patient with congenital disorders of glycosylation type Ia had two strokelike episodes, with evolving hemiparesis over 5 to 6 days' duration, followed by focal tonic-clonic seizures. Coagulation studies were normal. Electroencephalography showed transient hemispheric polymorphous delta-range slowing and suppression. Magnetic resonance imaging revealed corresponding cortical swelling. Magnetic resonance angiography was normal. Magnetic resonance spectroscopy revealed a decrease in the N-acetylaspartate peak, suggesting neuronal loss, with normal lactate peak. The neuroradiologic data do not support a thrombotic, embolic, or hemorrhagic basis for strokelike episodes in carbohydrate-deficient glycoprotein syndrome; other mechanisms must be considered.     

Underdiagnosis of mild congenital disorders of glycosylation type Ia

Congenital disorders of glycosylation-Ia are the most frequent type of congenital disorders of glycosylation. This condition affects the nervous system as well as other organs. The estimated incidence of congenital disorders of glycosylation-Ia is higher than the number of identified cases, therefore underdiagnosis of this heterogeneous disorder is probable. Neurologic and biologic signs are hallmarks for the identification of patients with congenital disorders of glycosylation-Ia. This report describes two children with congenital disorders of glycosylation-Ia syndrome confirmed by phosphomannomutase gene mutations with normal development and absence of biologic anomalies such as elevated transaminases and altered hemostasis. In conclusion, congenital disorders of glycosylation should be considered in cases of unexplained behavioral symptoms such as hyperactivity and concentration difficulties and mild neurologic signs. Intellectual retardation is often overestimated because of dysarthria and motor difficulties. Psychomotor reeducation might improve quality of life.     

Western blotting with diaminobenzidine detection for the diagnosis of congenital disorders of glycosylation

Congenital disorders of glycosylation (CDG) are a growing group of genetic disorders caused by a deficient assembly or processing of glycoproteins. Our aim was to improve a western blotting detection procedure previously described and to assess the efficiency of this procedure for CDG screening, using isoelectric focusing (IEF) as the reference method. We analysed transferrin and haptoglobin in serum from 12 patients with CDG-Ia, 3 patients with CDG-X and 95 healthy paediatric controls. These proteins were also studied in dried blood spot samples. Reference values for our paediatric population were established. No differences (Mann-Whitney test) were observed in the percentage of low molecular weight transferrin and haptoglobin fractions according to sex and age of the controls. Densitometric analysis showed a high percentage of the less sialylated fractions of glycoproteins in all CDG-Ia patients and normal values in the CDG-X patients. In conclusion, western blotting with diaminobenzidine detection is a simple and sensitive procedure to screen for CDG, either in serum or blood spot samples. Densitometric analysis and the establishment of reference values might improve the detection of subtle changes in the glycosylation of proteins.     

Gain of glycosylation: a new pathomechanism of myelin protein zero mutations

We report the first case of a missense mutation in MPZ causing a gain of glycosylation in myelin protein zero, the main protein of peripheral nervous system myelin. The patient was affected by a severe demyelinating neuropathy caused by a missense mutation, D32N, that created a new glycosylation sequence. We confirmed that the mutant protein is hyperglycosylated, is partially retained into the Golgi apparatus in vitro, and disrupts intercellular adhesion. By sequential experiments, we demonstrated that hyperglycosylation is the main mechanism of this mutation. Gain of glycosylation is a new mechanism in Charcot-Marie-Tooth type 1B.     

The heterogeneity of category-specific semantic disorders: evidence from a new case

We report a new case of category-specific semantic impairment, affecting living entities, in a patient with traumatic brain damage. In the present investigation we attempted to replicate as closely as possible the testing procedures which have been developed by Caramazza and Shelton (1998) to evaluate EW, a patient with a selective semantic disorder for the animal category. The results in our patient indicated a different performance profile, characterised by a more extensive semantic disorder for living entities, and by a more severe loss of specific visual rather than functional knowledge. These findings concur with other evidence indicating that category-specific semantic disorders are heterogeneous, reflecting different mechanisms of impairment, most likely associated with different neurobiological underpinnings.     

CDG (congenital disorders of glycosylation). Differential hereditary ataxia in adulthood diagnosis

Congenital disorders of glycosylation (CDG) are a group of hereditary multisystem diseases due to different defects of enzymes or transport molecules involved in the synthesis of glycoproteins. CDG-la is the most common subtype, with cerebellar ataxia as the main neurological symptom. Currently there is little information about CDG-la manifestation in adulthood. Here we present two sisters in whom the diagnosis of CDG-la was made in the fourth decade of life and who to our knowledge are the oldest known patients with the disorder in Germany. The clinical course of the disease was typical, although less severe than previously described. The carbohydrate-deficient transferrin (CDT) level was increased but lower than in other CDG patients. Isoelectric focusing of transferrin revealed changes typical of CDG, whereas those of alpha 1-antitrypsin were only moderately pathologic. This might be due to the milder manifestation of the disease in our patients or it could be indicative of a stabilization of the disease after puberty. The CDG should be included in the differential diagnostic workup of hereditary cerebellar ataxia in adults.