Thursday, February 22, 2007

Human ABO Gene

The human ABO gene encodes N-acetylgalactosaminyltransferase. This is the enzyme that determines the ABO blood types (see ABO Blood Types). This gene is found in all mammals, which makes it unfortunate that HUGO choose such a human-centered name [ABO Gene]. The gene will have a very different name in other species.

The GenBank website for this gene is GeneID=28. The ABO gene islocated at 9q34.1-q34.2 on chromosome 9. This is in approximately the same region as the HSPA5 gene but don't let that fool you. The genes are very far apart. [map]

There are many variants of this gene [OMIM 110300]. The DNA used in the human genome project came from people with different blood types so all three major variants (A, B, O) are present. Many of the other known variants have also been sequenced. You can look at the sequences in the Evidence Viewer on the Entrez Gene website [Evidence Viewer ABO Gene. Here's a bit of the sequence from the O allele and the A allele.

The nucleotide sequence of the O allele is shown at the top with the amino acid sequence. (It's hard to see at this scale. Go to the evidence viewer for a better view.) Note the presence of a single nucleotide deletion. This shifts the reading frame of the coding region so that it ends shortly after the deletion in a stop codon (*). The O allele produces a truncated defective protein.

The nucleotide sequence of the active gene (A allele) has a "G" (small red blob) at that position. The reading frame continues uninterrupted beyond the region shown and a functional enzyme is produced from this allele.

The ABO gene has seven exons, some of them are quite short. There are six introns and one of them is large so the total length of the gene is over twice as long as the length of the coding region.

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