|, CMAH, CSAH, cytidine monophospho-N-acetylneuraminic acid hydroxylase, pseudogene|
Sialic acids are terminal components of the carbohydrate chains of glycoconjugates involved in ligand–receptor, cell–cell, and cell–pathogen interactions. The two most common forms of sialic acid found in mammalian cells are N-acetylneuraminic acid (Neu5Ac) and its hydroxylated derivative, N-glycolylneuraminic acid (Neu5Gc). Studies of sialic acid distribution show that Neu5Gc is not detectable in normal human tissues although it was an abundant sialic acid in other mammals. Neu5Gc is, in actuality, immunogenic in humans.
The absence of Neu5Gc in humans is due to a deletion within the human gene CMAH encoding cytidine monophosphate-N-acetylneuraminic acid hydroxylase, an enzyme responsible for Neu5Gc biosynthesis. Sequences encoding the mouse, pig, and chimpanzee hydroxylase enzymes were obtained by cDNA cloning and found to be highly homologous. However, the homologous human cDNA differs from these cDNAs by a 92-bp deletion in the 5' region. This deletion, corresponding to exon 5 of the mouse hydroxylase gene, causes a frameshift mutation and premature termination of the polypeptide chain in human. It seems unlikely that the truncated human hydroxylase mRNA encodes for an active enzyme explaining why Neu5Gc is undetectable in normal human tissues.
The deletion that deactivated this gene occurred approximately 3.2 mya, after the divergence of humans from the African great apes, and quickly swept to fixation in the human population. The lineage of this pseudogene in humans indicates another deep split in Africa dating to 2.9 Mya, with a complex subsequent history. Causes of the selection against the CMAH gene could include a severe infectious disease that specifically binds to Neu5Gc, a change in binding preference of a sialic acid binding protein favoring the loss of Neu5Gc or accumulation of Neu5Ac, or protection from viruses originating in individuals with Neu5Gc due to anti-Neu5Gc antibodies in CMAH-negative individuals 
Effects of loss of functioning human CMAH
The functional loss of this gene after the divergence of humans from the great apes leads to several possible implications for its role in human development, the most notable of which being less constrained brain growth. In most mammals, CMAH expression is down-regulated in the brain and in fact, when higher expression of cytidine monophosphate-N-acetylneuraminic acid hydroxylase is forced in mouse brains, it proves lethal. Human brains are different from most primate brains in that they continue to grow for some time postnatally, unlike primate brains that stop growing soon after birth. It is therefore possible that even the small amounts of Neu5Gc present in most mammalian brains could be inhibiting their brain growth; losing the CMAH gene may have released the human brain from this constraint.
The loss of CMAH has also played a role in human viral history. On one side, it has made humans more susceptible to some viruses by decreasing sialic acid diversity. Viruses that bind to Neu5Ac before entering the cell will see their binding enhanced via the cluster glycoside effect, which wouldn't be seen as strongly if other sialic acids like Neu5Gc were also present. A potential example of this is the most serious form of malaria in humans, P. falciparum, which initially targets Neu5Ac on red blood cells for binding. Oppositely though, losing CMAH should protect humans against any virus that targets Neu5Gc, such as those that cause certain diarrheal diseases in livestock, E. coli K99, transmissible gastroenteritis coronavirus, and simian virus 40 (SV40). Interestingly, another form of malaria, P. reichenowii, may have been the original selecting agent against Neu5Gc. Thus only organisms negative for Neu5Gc would survive, with the outcome being humans who are completely resistant. Further support from this idea comes from the fact that P. falciparum malaria appears to have evolved in the last tens of thousands of years.
Even though humans do not have a functioning CMAH gene, Neu5Gc has been found present in normal human tissue, with larger amounts found in both fetal and cancerous  tissues. In fact, studies suggest that it could be an excellent cancer cell marker. Since Neu5Gc can only be made by functioning cytidine monophosphate-N-acetylneuraminic acid hydroxylase, another explanation of how it comes to be found in human tissue is needed. Current research indicates it is incorporated into human tissues through food sources, most notably from red meats (beef, pork, lamb) and to a lesser extent, dairy. This incorporation process involves macropinocytosis, delivery to the lysosome, and export of free Neu5Gc to the cytosol via the sialin transporter. Because Neu5Gc differs from Neu5Ac by only one oxygen, it is handled like a native sialic acid by human biochemical pathways. The immune system does not work the same way, however; all humans have varying, though still significant, amounts of a diverse spectrum of anti-Neu5Gc antibodies, with the commonest being from the IgG class; these, in combination with constant incorporation of Neu5Gc into tissue, can be a source of chronic inflammation, especially in blood vessels and the linings of hollow organs. These sites are also common places for atherosclerosis and epithelial carcinomas, both of which are associated with red meat and dairy consumption and are aggravated by chronic inflammation. Red meat ingestion and chronic inflammation have also been associated with diseases like type-2 diabetes and age-dependent macular degeneration so Neu5Gc may be linked to the development of these disorders as well.
As for accounting for the larger amounts of Neu5Gc found in tumors, recent data suggests that the hypoxic conditions in carcinomas can up-regulate the expression of the lysosomal sialic acid transporter necessary for Neu5Gc incorporation. In addition, growth factors may activate enhanced macropinocytosis, which can also augment Neu5Gc incorporation in tissues. Studies have shown that fetal tissues are also capable of taking up Neu5Gc from maternal dietary sources, which may explain elevated levels there.
The presence of Neu5Gc in various biotherapeutics derived from mammals or animal products may also be an issue, and indeed, short- and long-term effects of these are still being studied. Some complications could include immune hypersensitivity reactions, reduced half-life of the biotherapeutic in circulation, immune-complex formation, boosting of Neu5Gc antibody levels, enhancing immune reactivity against the underlying biotherapeutic polypeptide and directly loading more Neu5Gc into tissues.
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