ABO (gene)

Histo-blood group ABO system transferase is an is_associated_with::enzyme with is_associated_with::glycosyltransferase activity which in humans is encoded by the ABO is_associated_with::gene.

It determines the ABO blood group of an individual by modifying the oligosaccharides on cell surface glycoproteins. Variations in the sequence of the protein between individuals determine the type of modification and the blood group.

Function
This gene encodes a protein related to the first discovered blood group system, ABO. Which allele is present in an individual determines the blood group. The 'O' blood group is caused by a deletion of guanine-258 in the gene which corresponds to a region near the N-terminus of the protein. This results in a frameshift and translation of an almost entirely different protein. This mutation results in a protein unable to modify oligosaccharides which end in is_associated_with::fucose linked to is_associated_with::galactose. This sugar combination is termed the H antigen.

The A and B alleles of the ABO gene express enzymes with glycosyltransferase activities that differ, adding either is_associated_with::N-acetyl galactosamine or is_associated_with::galactose to the H antigen, converting it into the A or B antigen respectively. Individuals who are heterozygous for this gene express both proteins and so make both A and B antigens. Their blood group is AB. Remarkably, the difference between the A and B glycosyltransferase enzymes is only four amino acids. Other minor alleles have been found for this gene.

Common alleles
There are six common is_associated_with::alleles in individuals of European descent the ABO gene that produce one's blood type:


 * A
 * A101 (A1)
 * A201 (A2)
 * B
 * B101 (B1)
 * O
 * O01 (O1)
 * O02 (O1v)
 * O03 (O2)

Many rare variants of these alleles have been found in human populations around the world.

Clinical signficance
A genome-wide association study has identified variants in the ABO locus associated with susceptibility to pancreatic cancer.