Genomics

See Also

• Allele
• Autosomal dominant
• Genetic drift
• Genetic linkage
• Locus
• Neutral theory of molecular evolution
• 9q34
• 19q13.3

Description

Glycosphingolipids carrying A or B oligosaccharides are integral parts of the membranes of RBCs, epithelial and endothelial cells; they are also present in soluble form in plasma. Glycoproteins that carry identical oligosaccharides are responsible for the A and B activity of secreted body fluids such as saliva. A and B oligosaccharides that lack carrier protein or lipid molecules are found in milk and urine.

Genes at three separate loci (ABO, Hh, and Sese) control the occurrence and the location of the A and B antigens. Three common alleles -A, B and O- are located at the ABO locus on chromosome 9q34. The A and B genes encode Glycosyltransferases that produce the A and B antigens, respectively. The O gene is considered to be amorphic since no detectable blood group antigen results from its action. The RBCs of group O persons lack A and B, but carry an abundant amount of H antigen because this antigen is the precursor material on which A and B antigens are built.

Family studies have shown that the genes at the remaining two loci, Hh and Sese (secretor), are closely linked. The chromosome on which they are located has not yet been identified. It is suggested that one of these loci may have arisen through gene duplication of the other. Two recognized alleles reside at each locus. Of the two alleles at the H locus, one of these, H, produces an enzyme that acts at the cellular level to construct the antigen on which A or B are built. The other allele at this locus, h, is very rare. No antigenic product has been linked to h, so this gene is also considered an amorph. The possibility exists that other alleles occur at the Hh locus that differ from H in that they cause the production of only very small amounts of H antigen.

Secretor Genetics

The Sese gene is directly responsible for the expression of H (and indirectly responsible for the expression of A and B) on the glycoproteins in epithelial secretions such as saliva. Eighty percent of the population are secretors because they have inherited the Se gene and produce H in their secretions that can be converted to A and/or B (depending on the genetic background of the secretor). The se gene, having no demonstrable product, is an Amorph.

Oligosaccharide chains on which the A and B antigens are built can exist as simple structures of a few sugar molecules linked together in linear fashion. They can also exist as more complex structures that are composed of many sugar residues connected together in branching chains. It has been proposed that the differences in cellular A, B and H activity seen between specimens from infants and adults may be related to the number of branched structures carried on the cellular membranes of each group.(6) The RBCs of infants are thought to carry A, B and H antigens built predominantly on linear oligosaccharides. Linear oligosaccharides have only one terminus to which the H, then A and B, sugars can be added. In contrast, the RBCs of adults appear to carry a high proportion of branched oligosaccharides. Branching creates additional portions on the oligosaccharide that can be converted to H and then to A and B antigens.

A and B genes do not produce antigens directly but instead produce enzymes called glycosyltransferases that add specific sugars to oligosaccharide chains that have been converted to H by the action of the H gene. H antigens are constructed on precursor oligosaccharide chain endings called Type 1 and Type 2. (1) The number 1 carbon of the terminal 6-carbon sugar b-D-galactose (Gal) is linked to the number 3 carbon of subterminal N-acetyl-glucosamine (GlcNAc) in Type 1 chains and to the number 4 carbon of GlcNAc in Type 2 chains. Blood group-active glycoproteins present on cell surfaces or in body fluids carry either Type 1 or Type 2 chains. Glycosphingolipids present in the plasma and those on the membranes of most glandular and parenchymal cells also have either Type 1 or Type 2 chain endings. In contrast, the glycolipid antigens produced by the RBCs; appear to be formed exclusively of Type 2 chains. These chains are carried on a class of glycosphingolipids called paraglobosides.

At the cellular level, the H gene transferase produces a Fucosyltransferase? that adds fucose (Fuc) in alpha (1-2) linkage to the terminal Gal of Type 2 chains. The A and B gene transferases can only attach their immunodominant sugars when the Type 2 (or Type 1) chains have been substituted with Fuc (ie, changed to H) thus, the A and B antigens are constructed at the expense of H. The A gene-specified N-acetyl-galactosaminyl-transferase and the B gene-specified galactosaminyl-transferase add GalNAc and Gal respectively in alpha (1-3) linkages to the same Gal acted on by the H gene transferase.

The alleles at the ABO locus that result in subgroups (phenotypes of A and B that differ from each other with respect to the amount of A or B carried on the RBCs) produce transferases that differ from one another in their ability to convert H antigen. (2,3) The O gene is thought to produce a protein that can be detected immunologically but has no detectable transferase activity. As a consequence, the RBCs of group O persons carry readily detectable, unconverted H antigen. The secretion of Sese persons contain Type I and Type 2 chains with no H, A or B activity. It has been suggested that the H and Se genes each encode a different Fucosyltransferase?. (4,5) The enzyme produced by H acts primarily on Type 2 chains and in RBC membranes. That produced by Se prefers (but does not limit its action to) Type 1 chains and acts primarily in the secretory. Studies performed on the secretions of persons with the rare Oh phenotype support the concept that two types of H antigen exist.(6) Persons of this phenotype, who are genetically Hh and Sese, have no H and therefore, no A or B antigens on their RBCs or in their secretions. However, H, A and B antigens are found in the secretions of genetically hh persons, who, through family studies, appear to possess at least one Se gene. (7)

A and B antigens are detected in direct agglutination tests with anti-A and anti-B reagents. ABO reagents frequently produce weaker reactions with the RBCs of newborns than with RBCs from adults. Weaker reactions are encountered because A and B antigens are not fully developed at birth, even though they can be detected on the RBCs of embryos 5-6 weeks old. By the time a person is 2-4 years old, RBC A and/or B antigen expression is fully developed. Antigenic expression remains fairly constant throughout life, although decreases have been seen in old age.

Genomics

The ABO gene codes for the glycosyltransferases that transfer specific sugar residues to H substance, resulting in the formation of blood group A and B antigens. This gene maps to chromosome 9, position 9q34.1-q34.2. It consists of 7 exons, ranging in size from 28 to 688 base pairs (bp), and 6 introns with 554 to 12 982 bp (Figure 1).1-3 The last 2 exons (6 and 7), which comprise 823 of 1062 bp of the transcribed mRNA, encode for the catalytic domain of ABO glycosyltransferases.

Figure 1. Schematic representation of the genomic organization of the ABO gene. The exons (black squares) and regulatory regions (clear squares) are drawn to scale, as are the intervening introns, although the scale of the latter is 10 times smaller. The calculated numbers of nucleotides (nts) in the exons and introns are shown. The upstream regulatory region, which includes a CBF/NF-Y binding motif, is located around nt –3800 and, depending on the ABO haplotype, 215 or 344 base pairs (bp) in size; the regulatory region in the 5' untranslated region (UTR) is located from nt –118 to –1. (8)

The 6 common ABO alleles in white individuals are ABO*A101 (A1), ABO*A201 (A2), ABO*B101 (B1), ABO*O01 (O1), ABO*O02 (O1v), and ABO*O03 (O2). In exons 6 and 7 they differ by only a few base positions. ABO*A201, which is responsible for blood group A2, is identical to ABO*A101 apart from a nonsynonymous substitution at nucleotide (nt) position 467 and a single deletion (1060delC) in exon 7. This deletion results in disruption of the stop codon and an A-transferase product with an extra 21 amino acid (AA) residue at the C-terminus. ABO*B101 is distinguishable from ABO*A101 at 7 nt positions: 3 synonymous mutations at positions 297, 657, and 930; and 4 nonsynonymous mutations at positions 526, 703, 796, and 803. The nt sequence of ABO*O01 differs from that of ABO*A101 by a single base deletion at position 261 in exon 6; this deletion shifts the reading frame, thus generating a premature stop codon. ABO*O01 is thought to be either silent or translated into a truncated and catalytically inactive peptide. In contrast, the ABO*O03 allele lacks the 216delG polymorphism but possesses nonsynonymous mutations that may abolish the protein's enzyme activity by altering the nt sugar binding site.

Eighty-three ABO alleles discriminated at 52 polymorphic sites within the coding region of the ABO gene have been reported in the literature so far. (9,10,11,12,13,14,15) In most cases the investigators analyzed only exons 6 and 7. The number of described ABO alleles increases to 88 when nt differences within intron 6 are also considered. It has been shown that studies of the nt sequence of intron 6 are crucial for elucidation of the origin of some novel haplotypes.11-13 To our knowledge, there is no information available on sequence variation of the noncoding regions upstream from exon 6 and little data on mutations within the first 5 exons of the ABO gene and their relevance for the ABO phenotypes.14 In the present study, we therefore examined the complete exon/intron sequences (except for the huge intron 1 comprising 12 982 bp) and 2 regulatory regions of common and rare ABO alleles to evaluate the genetic diversity and diversification at the ABO locus. The genomic sequence data were first correlated with the associated ABO phenotypes then used for lineage definition.

The presence of a large number of recurrent mutations is characteristic for the considerable diversity of the ABO gene. Phenotype-genotype correlation revealed that an extensive heterogeneity underlies the molecular basis of various alleles that generate serologic ABO subgroups. ABO sequence variations also include phenotypically relevant replacement mutations in exons 2 to 5. Thus, ABO genotyping strategies would have to consider all variations distributed across the entire coding region to achieve safe phenotype prediction. Therefore, ABO genotyping remains mainly reserved as a complement to serology for determination of inherited ABO subgroups and exclusion of ABO*B allele markers in the acquired B phenotype. The data on highly conserved and lineage-specific intron sequence motifs provide a powerful base for elucidating the origin of variant ABO alleles and may prove valuable for anthropologic studies on the origins and movements of populations. (16)

From Matt Ridley's Genome (2006):

"It was not until the 1920s that the genetics of the ABO blood groups fell into place, and not until 1990 that the gene involved came to light. A and B are 'co-dominant' versions of the same gene, O being the recessive form of it. The gene lies on chromosome 9, near the end of the long arm. Its text is 1,062. 'letters' long, divided into six short and one long exons (`paragraphs') scattered over several 'pages' – 18,000 letters in all – of the chromosome. It is a nedium-sized gene, then, interrupted by five longish introns. The gene is the recipe for galactosyl transferase, an enzyme, i.e. a protein with the ability to catalyse a chemical reaction."

"The difference between the A gene and the B gene is seven letters mit of 1,062, of which three are synonymous or silent: that is, they nake no difference to the amino acid chosen in the protein chain. The four that matter are letters 523, 700, 793 and 800. In people with type A blood these letters read C, G, C, G. In people with type B blood they read G, A, A, C. There are other, rare differences. A few people have some of the A letters and some of the B letters, In a rare version of the A type exists in which a letter is missing near the end. But these four little differences are sufficient to make he protein sufficiently different to cause an immune reaction to the wrong blood."

"The O group has just a single spelling change compared with A, but instead of a substitution of one letter for another, it is a deletion. In people with type O blood, the 258th letter, which should read 'G', is missing altogether. The effect of this is far-reaching, because Lt causes what is known as a reading-shift or frame-shift mutation, which is far more consequential." (17)

Links

• Felix Bernstein (Eingegangen 5 Mar 1929) 'The Foundations of the chromosomal theory of Human Heredity, with special attention to blood groups'
• Cooperative effect of adenosine deaminase and ABO-secretor genetic complex on susceptibility to childhood asthma

References