Human genetic disorders caused by mutations in genes encoding biosynthetic enzymes for sulfated glycosaminoglycans

Abstract

A number of genetic disorders are caused by mutations in the genes encoding glycosyltransferases and sulfotransferases, enzymes responsible for the synthesis of sulfated glycosaminoglycan (GAG) side chains of proteoglycans, including chondroitin sulfate, dermatan sulfate, and heparan sulfate. The phenotypes of these genetic disorders reflect disturbances in crucial biological functions of GAGs in human. Recent studies have revealed that mutations in genes encoding chondroitin sulfate and dermatan sulfate biosynthetic enzymes cause various disorders of connective tissues. This minireview focuses on growing glycobiological studies of recently described genetic diseases caused by disturbances in biosynthetic enzymes for sulfated GAGs.

FIGURE 1.

Schematic presentation of the biosynthetic assembly of the GAG backbones by various glycosyltransferases. Each glycosyltransferase requires the respective UDP-sugar as a donor substrate. Following the synthesis of specific core proteins, the synthesis of the so-called GAG-protein linkage region, GlcUAβ1–3Galβ1–3Galβ1–4Xylβ1-O-, common to CS/DS and HS/heparin (Hep) chains, is initiated by XylT, which transfers a Xyl residue from UDP-Xyl to the specific Ser residue in the endoplasmic reticulum, and is completed by the consecutive addition of each sugar by GalT-I, GalT-II, and GlcAT-I, which are common to the biosynthesis of both CS and HS, in the Golgi apparatus. Following completion of the synthesis of the linkage region, the first βGalNAc residue is transferred to the naked GlcUA residue in the linkage region by GalNAcT-I, which initiates the assembly of the chondroitin backbone. Subsequently, the repeating disaccharide region, (-3GalNAcβ1–4GlcUAβ1-)_n, is elongated by alternate additions of GlcUA and GalNAc residues from UDP-GlcUA and UDP-GalNAc catalyzed by CS-GlcAT-II and GalNAcT-II activities, respectively, of a heterocomplex (CS polymerase) formed by ChSy and ChPF. On the other hand, the addition of α1–4-linked GlcNAc to the linkage region by GlcNAcT-I initiates the assembly of the HS repeating disaccharide region, (-4GlcNAcα1–4GlcUAβ1-)_n. Then, the chain polymerization of the HS chain is catalyzed by HS-GlcAT-II and GlcNAcT-II activities of HS polymerase, which is a heterocomplex of EXT1 and EXT2. The molecular mechanism of the differential biosynthetic assembly of HS and CS chains at the GAG attachment sites remains to be elucidated, as details have been discussed in the text; and therefore, the transfer reactions of the fifth sugar (first amino sugar) are shown in this figure by the dashed and dashed-dotted arrows. After the formation of the chondroitin and heparan backbones, GAG chains are matured by sulfation at various positions and epimerization at GlcUA residues. Each enzyme (glycosyltransferase and/or epimerase), its coding gene, and the corresponding inherited disorder are described under the respective sugar symbols. Sulfotransferases involved in the chain modifications are not included but are illustrated in Fig. 2 (see also Table 2 for the inherited diseases of sulfotransferases). DSE, dermatan sulfate epimerase; DSEL, dermatan sulfate epimerase-like.

FIGURE 2.

Schematic diagram of the biosynthetic modification of CS/DS chains. After formation of the chondroitin backbone, (-4GlcUAβ1–3GalNAcβ1-), each sugar residue is modified with a number of sulfate groups. Sulfation occurs mainly at positions 4 and 6 of GalNAc and position 2 of GlcUA catalyzed by various sulfotransferases. All sulfotransferases transfer a sulfate group from PAPS, a universal donor substrate, to a specific position of the GlcUA or GalNAc residue. C4ST and C6ST transfer sulfate to position 4 or 6 of GalNAc residues, resulting in the formation of A-units (GlcUA-GalNAc(4-O-sulfate)) and C-units (GlcUA-GalNAc(6-O-sulfate)), respectively. Further sulfation is catalyzed by GalNAc4S-6ST or UST, which is essential for the formation of highly sulfated disaccharide units, E-units (GlcUA-GalNAc(4/6-O-sulfate) and D-units (GlcUA(2-O-sulfate)-GalNAc(6-O-sulfate)), respectively. After the formation of the chondroitin backbone, DS-glucuronyl C5-epimerase (DSE) converts GlcUA into IdoUA by epimerizing the C-5 carboxy group, resulting in the formation of the dermatan backbone, composed of iO-units (-4IdoUAα1–3GalNAcβ1-). Position 4 of GalNAc residues is sulfated by a distinct 4-O-sulfotransferase, D4ST, forming iA-units (IdoUA-GalNAc(4-O-sulfate)). Further sulfations of DS chains are infrequently achieved by GalNAc4S-6ST or UST, common to CS chains. The abbreviation “i” in iA, iB, and iE stands for IdoUA. 2S, 4S, and 6S, 2-, 4-, and 6-O-sulfate, respectively.