Diseases of glycosylation beyond classical congenital disorders of glycosylation

Abstract

Background

Diseases of glycosylation are rare inherited disorders, which are often referred to as congenital disorders of glycosylation (CDG). Several types of CDG have been described in the last decades, encompassing defects of nucleotide-sugar biosynthesis, nucleotide-sugar transporters, glycosyltransferases and vesicular transport. Although clinically heterogeneous, most types of CDG are associated with neurological impairments ranging from severe psychomotor retardation to moderate intellectual disabilities. CDG are mainly caused by defects of N-glycosylation, owing to the simple detection of under-glycosylated serum transferrin by isoelectric focusing.

Scope of review

In the last years, several disorders of O-glycosylation, glycolipid and glycosaminoglycan biosynthesis have been described, which are known by trivial names not directly associated with the family of CDG. The present review outlines 64 gene defects affecting glycan biosynthesis and modifications, thereby underlining the complexity of glycosylation pathways and pointing to unexpected phenotypes and functional redundancies in the control of glycoconjugate biosynthesis.

Major conclusions

The increasing application of whole-genome sequencing techniques unravels new defects of glycosylation, which are associated to moderate forms of mental disabilities.

General significance

The knowledge gathered through the investigation of CDG increases the understanding of the functions associated to protein glycosylation in humans. This article is part of a Special Issue entitled Glycoproteomics.

Introduction

Diseases of glycosylation are rather recent additions to the growing list of known inherited diseases. In fact, most disorders of glycosylation have been described in the last 20 years. These diseases were first called carbohydrate-deficient glycoprotein syndromes (CDGS) [1], but were renamed congenital disorders of glycosylation (CDG) in 1999 [2] to encompass all types of glycoconjugates. In fact, some gene defects affect multiple glycosylation pathways, hence leading to structural alteration in multiple classes of glycoconjugates. Originally, CDG have been divided into two groups. CDG-I included all disorders of N-glycosylation site occupancy and CDG-II all other disorders of N-glycosylation, O-glycosylation and glycolipid biosynthesis. As the number of glycosylation disorders approached the mark of 50, nomenclature has been simplified by focusing on the name of the mutated gene followed by the abbreviation CDG [3]. Accordingly, the disorder caused by mutations in the phosphomannomutase-2 gene is referred to as PMM2-CDG.

The initial impetus for the discovery of several diseases of glycosylation was given by the serendipitous identification of CDG cases while applying a blood test aimed at detecting alcohol abuse. In the late seventies, the neurologist Helena Stibler observed the loss of negatively charged serum transferrin in situations of chronic alcohol abuse [4]. Serum transferrin normally carries two N-glycans that are terminated by negatively-charged sialic acid (Sia). The loss of N-glycans on transferrin can easily be monitored by isoelectric focusing using few microliters of blood serum. The pediatricians Jaak Jaeken [5] and Helena Stibler [6] were the first to identify CDG patients using serum transferrin isoelectric focusing. The broad application of this simple test has paved the way to the identification of several defects of N-glycosylation. Unfortunately, similar tests unraveling defects of O-glycosylation or glycolipid glycosylation are not available, mainly because of the structural heterogeneity of O-glycans and of their tissue-specific expression. Accordingly, only few defects of O-glycosylation and glycolipid biosynthesis have been characterized so far. In fact, most of these defects were identified by genetic linkage analysis in large families.

In the last years, advanced sequencing techniques have revealed new gene defects linked to glycosylation disorders. These disorders broaden the range of symptoms and organ involvements associated with CDG. Originally, the study of severe clinical phenotypes led to the discovery of most types of CDG. However, recent developments have shown that medical teams should also consider milder symptoms and even additional clinical findings linked to mutations in known genes as the variability of clinical phenotypes will undoubtedly expand. In spite of the rapid progress achieved, the establishment of relationships between CDG phenotypes and specific glycoprotein and glycolipid functions remains a major challenge.