The Molecular Basis for the Absence ofN-Glycolylneuraminic Acid in Humans

Abstract

N-Glycolylneuraminic acid (NeuGc) is abundantly expressed in most mammals, but it is not detectable in humans. The expression of NeuGc is controlled by cytidine monophospho-N-acetylneuraminic acid (CMPNeuAc) hydroxylase activity. We previously cloned a cDNA for mouse CMP-NeuAc hydroxylase and found that the human genome contains a homologue. We report here the molecular basis for the absence of NeuGc in humans. We cloned a cDNA for human CMP-NeuAc hydroxylase from a HeLa cell cDNA library. The cDNA encodes a 486-amino acid protein, and its deduced amino acid sequence lacks a domain corresponding to the N-terminal 104 amino acids of the mouse CMPNeuAc hydroxylase protein, although the human protein is highly identical (93%) to the rest of the mouse hydroxylase protein. The N-terminal truncation of the human hydroxylase is caused by deletion of a 92-base pair-long exon in human genomic DNA. The human hydroxylase expressed in COS-7 cells exhibited no enzymatic activity, and a mouse hydroxylase mutant, which lacks the N-terminal domain, was also inactive. A chimera composed of the human hydroxylase and the Nterminal domain of the mouse hydroxylase displayed the enzyme activity. These results indicate that the human homologue of CMP-NeuAc hydroxylase is inactive because it lacks an N-terminal domain that is essential for enzyme activity. The absence of NeuGc in human glycoconjugates is due to a partial deletion in the gene that encodes CMP-NeuAc hydroxylase.Sialic acids are components of the carbohydrate chains of glycoconjugates and are involved in cell-cell recognition (1, 2) and cell-pathogen interactions (3, 4). Sialic acid is a generic designation used for N-acylneuraminic acids and their derivatives (5, 6). N-Acetylneuraminic acid (NeuAc) 1 and N-glycolylneuraminic acid (NeuGc) are two of the most abundant derivatives. Previous studies suggest that some adhesion molecules recognize glycoconjugates containing NeuGc and NeuAc with different affinities. Sialoadhesin, a cell adhesion molecule of marginal zone macrophages, recognizes NeuAc␣2-3Gal but not NeuGc␣2-3Gal (7). On the other hand, mouse CD22, a B cellrestricted adhesion molecule, binds more strongly to NeuGc␣2-6Gal than to NeuAc␣2-6Gal (7). Influenza hemagglutinins recognize glycoconjugates that contain NeuAc and NeuGc with different affinities (8). These studies suggest that the diversity of sialic acids is biologically important in recognition events mediated by glycoconjugates. Although NeuGc-containing glycoconjugates are found in most mammals (5, 6), NeuGc is not detectable in normal human tissues. Glycoconjugates containing NeuGc are immunogenic in human, and an antibody against NeuGc, which is known as Hanganutziu-Deicher antibody, is produced by patients who receive therapeutic injections of animal antisera (9, 10).NeuGc is assumed to be produced from NeuAc through enzymatic hydroxylation of the N-acetyl residue of free NeuAc, CMP-NeuAc, or glycoconjugate-linked NeuAc (11,12). Previous studies showed that the major mechanism for biosynthesis of N...

Human B-lymphocytes Express α2-6-Sialylated 6-Sulfo-N-acetyllactosamine Serving as a Preferred Ligand for CD22/Siglec-2

“…Recently, however, it was shown that N-glycolylation of sialic acid by CMP-NeuAc hydroxylase significantly affects CD22 ligand activity in murine system, as glycans carrying NeuGc␣2-6 termini are much preferred ligands for murine CD22 compared with glycans carrying NeuAc␣2-6 termini (16). Because humans have no NeuGc due to deletion of exons in the CMP-NeuAc hydroxylase gene (37,38), this regulatory mechanism is not applicable to humans. Instead, our results suggest a role for 6-O-sulfotransferases, which are known to be affected by various stimuli and depend on cellular activation status (34, 39 -41), in the regulation of CD22 ligand activity, as well as ␣2-6-sialyltransferases.…”

Human uptake and incorporation of an immunogenic nonhuman dietary sialic acid

“…Indeed, humans generate immune responses against i.v.-administered molecules carrying Neu5Gc, e.g., the ''serum sickness'' reaction to equine antithymocyte globulin therapy (9,10). These findings are explained by a human-specific inactivating mutation in the CMAH gene (11)(12)(13)) that occurred Ϸ2.5 million to 3 million years ago (14).…”

Fixation of the Human-Specific CMP-N-Acetylneuraminic Acid Hydroxylase Pseudogene and Implications of Haplotype Diversity for Human Evolution

“…The suppression of Neu5Gc is most conspicuous in humans in that there is no detectable level of Neu5Gc in almost all tissues (Muchmore et al 1998). This was explained by the finding that the human CMAH locus is genetically inactivated despite the fact that it is a single-copy gene located on 6p21 (Chou et al 1998;Irie et al 1998), and it has been suggested that this defective mutation might be responsible for some biochemical or physiological characteristics specific to humans (Varki 2002). …”