SIGLEC12 (sialic acid binding Ig-like lectin 12)

Siglec-XII Is a molecule found to be expressed on a few epithelial surfaces and on some macrophages in the chimpanzee. The gene underwent a human specific mutation of the critical arginine residue required for sialic acid binding. The roman numeral XII is used in place of -12 to designated this loss of sialic acid binding in the protein. This R122C mutation of the Siglec-XII protein is fixed in the human population, i.e. it occurred prior to the origin of modern humans. Additional mutations have since completely inactivated the SIGLEC12 gene in some but not all humans. The most common inactivating mutation is a single nucleotide frameshift that markedly shortens the open reading frame with a global allele frequency of 58%. Unlike other CD33-related Siglecs that are primarily found on immune cells, the Siglec-XII protein is expressed not only on some macrophages but also on various epithelial cell surfaces in humans and chimpanzees. Expression was also noted on certain human prostate epithelial carcinomas and carcinoma cell lines. The biological significance of this event is unknown at this time.