GPI-anchor and GPI-anchored protein expression in PMM2-CDG patients

Vicente Vicente,Javier Corral,Teresa Sevivas,Encarna Guillén-Navarro,Trinidad Hernández-Caselles,Soren R Kristensen,Maria L Lozano,Maria E De La Morena-Barrio,Carmen Altisent,Antonia Miñano,Irene Martínez-Martínez,Jaak Jaeken,Roberto García-López,Belen Pérez-Dueñas

doi:10.1186/1750-1172-8-170

Abstract

BackgroundMutations in PMM2 impair phosphomannomutase-2 activity and cause the most frequent congenital disorder of glycosylation, PMM2-CDG. Mannose-1-phosphate, that is deficient in this disorder, is also implicated in the biosynthesis of glycosylphosphatidyl inositol (GPI) anchors.ObjectiveTo evaluate whether GPI-anchor and GPI-anchored proteins are defective in PMM2-CDG patients.MethodsThe expression of GPI-anchor and seven GPI-anchored proteins was evaluated by flow cytometry in different cell types from twelve PMM2-CDG patients. Additionally, neutrophil CD16 and plasma hepatic proteins were studied by Western blot. Transferrin glycoforms were evaluated by HPLC.ResultsPatients and controls had similar surface expression of GPI-anchor and most GPI-anchored proteins. Nevertheless, patients displayed a significantly diminished binding of two anti-CD16 antibodies (3G8 and KD1) to neutrophils and also of anti-CD14 (61D3) to monocytes. Interestingly, CD16 immunostaining and asialotransferrin levels significantly correlated with patients’ age. Analysis by flow cytometry of CD14 with MΦP9, and CD16 expression in neutrophils by Western blot using H-80 ruled out deficiencies of these antigens.ConclusionsPMM2 mutations do not impair GPI-anchor or GPI-anchored protein expression. However, the glycosylation anomalies caused by PMM2 mutations might affect the immunoreactivity of monoclonal antibodies and lead to incorrect conclusions about the expression of different proteins, including GPI-anchored proteins. Neutrophils and monocytes are sensitive to PMM2 mutations, leading to abnormal glycosylation in immune receptors, which might potentially affect their affinity to their ligands, and contribute to infection. This study also confirms less severe hypoglycosylation defects in older PMM2-CDG patients.

Highlights

Congenital disorders of glycosylation (CDG) are caused by defective glycosylation of glycoproteins and glycolipids
The glycosylation anomalies caused by PMM2 mutations might affect the immunoreactivity of monoclonal antibodies and lead to incorrect conclusions about the expression of different proteins, including glycosylphosphatidyl inositol (GPI)-anchored proteins
Neutrophils and monocytes are sensitive to PMM2 mutations, leading to abnormal glycosylation in immune receptors, which might potentially affect their affinity to their ligands, and contribute to infection

Summary

Introduction

Congenital disorders of glycosylation (CDG) are caused by defective glycosylation of glycoproteins and glycolipids. The tendency towards thrombosis in patients with PMM2-CDG could be multifactorial, and potentially (at least in part), due to deficiency of GPIanchored complement inhibitors on the surface of circulating erythrocytes and other cells [11]. In this context, it has to be noted that two GPI-anchor synthesis defects, paroxysmal nocturnal haemoglobinuria (PNH), and PIGM-CDG (Figure 1), exhibit a high rate of occurrence of severe thrombotic events due to complement-mediated hemolysis resulting from the deficiency of complement regulatory proteins, CD55 and CD59 [12,13]. Mannose-1-phosphate, that is deficient in this disorder, is implicated in the biosynthesis of glycosylphosphatidyl inositol (GPI) anchors

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