Identification, Tissue Distribution, and Molecular Modeling of Novel Human Isoforms of the Key Enzyme in Sialic Acid Synthesis, UDP-GlcNAc 2-Epimerase/ManNAc Kinase

Abstract

UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE) catalyzes the first two committed steps in sialic acid synthesis. In addition to the previously described 3 human GNE isoforms (hGNE1- hGNE3), our database and PCR analysis yielded an additional 5 human isoforms (hGNE4- hGNE8). hGNE1 is the ubiquitously expressed major isoform, while the hGNE2-8 isoforms are differentially expressed and may act as tissue-specific regulators of sialylation. hGNE2 and hGNE7 display a 31-residue N-terminal extension compared to hGNE1. Based on similarities to kinases and helicases, this extension does not seem to hinder the epimerase enzymatic active site. hGNE3 and hGNE8 contain a 55 residue N-terminal deletion, and a 50-residue N-terminal extension compared to hGNE1. The size and secondary structures of these fragments are similar, and modeling predicted that these modifications do not affect the overall fold compared to hGNE1. However, epimerase enzymatic activity of GNE3 and GNE8 is likely absent, since the deleted fragment contains important substrate binding residues in homologous bacterial epimerases. hGNE5-hGNE8 have a 53-residue deletion, which was assigned a role in substrate (UDP-GlcNAc) binding. Deletion of this fragment likely eliminates epimerase enzymatic activity. Our findings imply that GNE is subject to evolutionary mechanisms to increase cellular functions, without increasing the number of genes. Our expression and modeling data contribute to elucidation of the complex functional and regulatory mechanisms of human GNE, and may contribute to further elucidating the pathology and treatment strategies of the human GNE-opathies sialuria and hereditary inclusion body myopathy.

Identification of an Alu element‐mediated deletion in the promoter region of GNE in siblings with GNE myopathy

“…2011): hGNE1 (NM_005746), hGNE2 (NM_001128227), hGNE3 (NM_001190388), hGNE4 (NM_001190383), and hGNE5 (NM_001190384) (Fig. 3A).…”

“…2009b; Yardeni et al. 2011). The hGNE1 transcript contains exon 2 in its noncoding 5′ UTR, and a deletion of this exon may explain decreased mRNA transcription in our patients (Fig.…”

“…Both transcripts were expressed only in selected tissues including liver and kidney, consistent with earlier findings (Yardeni et al. 2011). We also detected hGNE2 expression in heart and hGNE3 in skeletal muscle (Fig.…”

“…2009a,b; Yardeni et al. 2011). Total hGNE mRNA is highest in liver and placenta, while skeletal muscle has low expression (Lucka et al.…”

Myopathie GNE

“…Identifié en 2001 par une équipe israélienne (Eisenberg, 2001), le gène GNE est localisé sur le chromosome 9 et code une protéine ayant une double fonction enzymatique : l'une de type épimérase, l'autre de type kinase, d'où son nom particulièrement complexe : UDP-N-acétylglucosamine-2-épimerase/N-acétylman-nosamine kinase (ou GNE en abrégé) (Kayashima, 2002 ;Reinke, 2009 ;Tong, 2009 ;Yardeni, 2011). La protéine contient, dans sa version longue récemment revue et corrigée, 753 acides aminés (Huizing, 2014) (Figure 2).…”