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Technical Briefs |
1
Laboratoire de Biochimie A, Hôpital Bichat, 75877 Paris Cédex 18, France
2
Service de Génétique Médicale, INSERM U393 Hôpital Necker, 75015 Paris, France
3
Faculté de Pharmacie, Université Paris XI, 92296 Châtenay-Malabry Cédex, France
4
Faculté de Pharmacie, Université Paris V, 75006 Paris, France
a address correspondence to this author at: Laboratoire de Biochimie A, Hôpital Bichat-Claude Bernard, 46, rue Henri Huchard, 75877 Paris Cédex 18, France; fax 33-1-40-25-88-21,
nathalie.seta{at}bch.ap-hop-paris.fr
Congenital disorders of glycosylation [CDG; previously carbohydrate-deficient glycoprotein syndrome (1)] represent a newly delineated group of inherited diseases (2). The CDG are now clearly classified in two groups including subgroups. CDG I, by far the most common type with >300 patients described in the literature, is characterized by defects in the assembly of dolichol pyrophosphate oligosaccharide and/or in the transfer of oligosaccharide from dolichol pyrophosphate to an Asn residue on the nascent proteins. The other group, CDG II, reflects defects in the processing of protein-bound glycans. Only a few cases have been described (1).
The diagnosis of CDG I is based on biochemical changes involving a unique carbohydrate deficiency observed in serum transferrin (TRF). In healthy subjects, serum TRF is fully glycosylated, containing two N-glycan chains, whereas in CDG I patients, it is partially (one chain) or totally deglycosylated (3). This structural abnormality is associated with different enzyme deficiencies (4). The most common, subtype Ia, is a deficiency of phosphomannomutase (PMM; EC 5.4.2.8) (5) and is present in 70% of CDG I patients. The disease is linked to chromosome 16p13, and numerous missense mutations have been identified in the PMM2 gene (6)(7). The condition is an autosomal recessive multisystemic disorder affecting the nervous system and numerous organs, including the liver, kidney, heart, adipose tissue, bone, and genitalia (4).
The characteristic biochemical abnormalities of CDG can be demonstrated
by various methods, including microanion-exchange chromatography or
isoelectric focusing of TRF (8), based on sialic acid
content, and Western-blot analysis of plasma glycoproteins
(9), based on variations of protein molecular weight. Fig. 1A
shows typical isoelectric focusing patterns for serum from a
healthy subject and a CDG I patient; Fig. 1B
shows typical Western-blot
patterns for serum TRF, 
1-antitrypsin,
haptoglobin, and 1-acid glycoprotein from a
healthy subject and a CDG I patient. The detection limit of the
Western-blot method, tested by serial dilution, was <1 ng on the gel
regardless of the glycoprotein tested. No discordance was observed
between the TRF Western-blot assays and isoelectric focusing when >20
CDG I patient patterns were compared (data not shown).
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We report here two cases of CDG Ia for which the condition could not be detected as easily as usual.
In the first family (F1), the sibling pair was composed of a
16-year-old girl (F1J) and a 6-year-old boy (F1D). Both have classical
clinical features of CDG I, including psychomotor retardation,
cerebellar ataxia, strabismus, and cerebellar hypoplasia; the girl also
has hypogonadism. When Western blotting of the four different
glycoproteins was performed on sera from both children, the results
were puzzling. The boy’s results showed a characteristic CDG I pattern
(Fig. 1B
, lane 4), consistent with the clinical features. By contrast,
the pattern of serum glycoproteins of the girl (Fig. 1B
, lane 3) was
identical to the one of healthy subjects. The serum
carbohydrate-deficient transferrin (CDT) was measured for both
siblings (reference interval, 10–30 units/L CDT; F1J, 38
units/L CDT; F1D, 148 units/L CDT) and was consistent with the results
of the Western-blot analysis. These results were also confirmed by the
isoelectric focusing pattern (Fig. 1A
). Three months later, the results
were confirmed on new serum samples. During the intervening period, PMM
activity was measured according to the method of Van Schaftingen and
Jaeken (5) on mononucleated leukocytes and on cultured skin
fibroblasts from both children. The results obtained from the two cell
types demonstrated undetectable PMM activity for both children.
Identical mutations of the PMM2 gene, R141H and T226S as
determined by complete sequencing of cDNA, were found in both children.
In the other family (F2), the sibling pair was composed of two adult
men (F2L and F2T) with nonprogressive cerebellar ataxia. The
Western-blot pattern of the serum glycoproteins from F2T (Fig. 1B
, lane
6) was typical for CDG I. In contrast, fewer bands or paler lower bands
were found for F2L (Fig. 1B
, lane 5). Similarly, the isoelectric
focusing patterns showed a characteristic CDG I profile for F2T but
only a partially abnormal one for F2L (Fig. 1A
). PMM activity measured
in the leukocytes of both patients was undetectable, corresponding to
an identical double mutation on the PMM2 gene, R141H and C9Y
as determined by complete sequencing of cDNA.
Until now, the diagnosis of CDG I has been based on clinical features and confirmed by the presence of abnormally glycosylated serum glycoproteins. Considering our results, we are facing a new situation: patients who have clinical CDG I features and belong to families in which other relatives are clinically and biologically CDG I patients, but who have either intermediate electrophoretic patterns corresponding to glycoproteins lacking fewer glycan chains, or even non-CDG patterns corresponding to normally glycosylated serum glycoproteins.
In the first family, the patient F1J with the normal pattern is almost an adult, and the results can be related to those observed in adult patients (10). Stibler et al. (10) reported that concentrations of CDT are profoundly increased in all patients but tend to be lower in adults than in patients younger than 15 years, with a loss of correlation with age in older patients. The normalization of the glycoprotein glycan content could reflect an adaptation to the metabolic abnormalities. In the case of the 16-year-old patient, the adaptation could be complete although the PMM activity was deficient. In the other family, age does not explain the findings of a typical CDG I pattern in one sibling and, in the other, a pattern with fewer or paler lower bands for all glycoproteins tested, despite similar clinical presentations for both subjects.
In conclusion, we have seen at least one clinically confirmed CDG Ia patient with normal serum glycoproteins. The diagnosis of CDG Ia presently based on the evidence of abnormal glycosylation of serum glycoproteins, whatever the method used, might lack sensitivity when applied to teenagers or adults. Biologists who are involved in the diagnosis of CDG should be aware of the possibility of false-negative results.
References
The following articles in journals at HighWire Press have cited this article:
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  C. Perez-Cerda, D. Quelhas, A. I. Vega, J. Ecay, L. Vilarinho, and M. Ugarte Screening Using Serum Percentage of Carbohydrate-Deficient Transferrin for Congenital Disorders of Glycosylation in Children with Suspected Metabolic Disease Clin. Chem., January 1, 2008; 54(1): 93 - 100. [Abstract] [Full Text] [PDF]
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 C. Flahaut, J.C. Michalski, T. Danel, M.H. Humbert, and A. Klein The effects of ethanol on the glycosylation of human transferrin Glycobiology, March 1, 2003; 13(3): 191 - 198. [Abstract] [Full Text] [PDF]
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H. H. Freeze Update and perspectives on congenital disorders of glycosylation Glycobiology, December 1, 2001; 11(12): 129R - 143R. [Abstract] [Full Text] [PDF]
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