Varki is senior author of two new papers describing a genetic mutation at the root of a structural difference between an important cell surface molecule common in humans and chimpanzees. In a paper published in the September 28 issue of the Proceedings of the National Academy of Sciences, Varki and a team of researchers from UCSD, the San Diego Veterans Affairs Medical Center, the Living Links Center of the Yerkes Primate Center, the Howard Hughes Medical Institute at Emory University, and Baylor College of Medicine describe a missing enzymatic action resulting from a mutated gene in humans that is the basis for the lack of a single oxygen atom in an otherwise identical sialic acid molecule. Thus, the obvious differences in physical appearance and functional abilities are likely to be explained by relatively few differences in the underlying genetic code. We have found what appears to be the first known major biochemical and genetic difference between humans and chimpanzees. For various reasons, the finding is an intriguing one, but further work is needed to determine its precise significance.
Sialic acids are a type of sugar molecule found on cell surfaces.
Because of the missing enzyme, humans only express Neu5Ac, which is different from Neu5Gc only by a single oxygen atom. Traces of Neu5Gc can be found in human fetal tissue and in certain human tumors, and humans with certain malignancies or certain inflammatory or infectious diseases develop antibodies against Neu5Gc. Apes, on the other hand, are not known to develop certain cancers and also appear immune to some of the infectious diseases that plague humans. Varki and colleagues also note the intriguing fact that while Neu5Gc is found throughout the mammalian body, it appears to be suppressed in the mammalian brain, including the ape brain. Further studies are being conducted to explore the significance, if any, of the complete lack of this sialic acid in the brain.