Alternative titles; symbols
Other entities represented in this entry:
HGNC Approved Gene Symbol: ABO
Cytogenetic location: 9q34.2 Genomic coordinates (GRCh38): 9:133,250,401-133,275,201 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 9q34.2 | [Blood group, ABO system] | 616093 | 3 |
The ABO gene encodes a glycosyltransferase that catalyzes the transfer of carbohydrates to the H antigen (FUT1; 211100), forming the antigenic structures of the ABO blood group (616093). The proteins encoded by the A and B alleles of ABO differ minimally in amino acid sequence but catalyze the transfer of different carbohydrates (N-acetylgalactosamine or galactose) onto the H antigen to form the A or B antigens. Individuals with the O blood group do not produce either the A or B antigens because of a single-base deletion (summary by Amundadottir et al., 2009).
Yamamoto et al. (1990) cloned and sequenced cDNA encoding the specific primary gene product that they referred to as the histo-blood group A gene (A transferase). Nucleotide sequence shows a coding region of 1,062 bp encoding a protein of 41 kD. No RFLP was found to correlate with ABO blood group type.
Yamamoto et al. (1995) isolated genomic DNA clones encompassing 30 kb of the ABO locus. The locations of the exons were mapped and the nucleotide sequences of the exon/intron boundaries determined. The human ABO genes consist of at least 7 exons, and the coding sequence in the 7 coding exons spans over 18 kb of genomic DNA. The exons range in size from 28 to 688 bp, with most of the coding sequence lying in exon 7.
In an early instance of deletion mapping, Ferguson-Smith et al. (1976) localized the ABO-NPS1 (161200)-AK1 (103000) linkage group to chromosome 9q34 by regional assignment of AK1 in studies of a chromosome deletion.
Cook et al. (1978) collated evidence that ABO and AK1 lie in band 9q34. They could exclude MNSs (111300), GPT (138200), and Gc from chromosome 9. Possible linkage of DBH (609312) to ABO was indicated by a maximum lod score of 1.82 at 0% and 10% recombination fractions for males and females, respectively (Goldin et al., 1982). Elston et al. (1979) found a lod score of 2.32 at 0 recombination, to give a combined score of 2.32. Narahara et al. (1986) assigned the ABO and AK1 loci to 9q31.3-qter by studies in a family with a complex chromosomal rearrangement. The order of loci on the distal portion of 9q appears to be: cen--AK1--ABL (189980)--ASS (603470)--ABO--qter.
Gross (2014) mapped the ABO gene to chromosome 9q34.2 based on an alignment of the ABO sequence (GenBank BC069595) with the genomic sequence (GRCh38).
ABO Blood Group System
Yamamoto et al. (1990) detected bands in Northern hybridization of mRNAs from cell lines expressing A, B, AB, or H antigens, suggesting that sequences of ABO genes have only minimal differences and that the inability of the O gene to encode A or B transferases is probably due to a structural difference rather than to failure of expression of the A or B transferases. Yamamoto et al. (1990) showed that cells of the histo-blood group phenotype O express a message similar to that of A and B alleles. Indeed, they found that the O allele is identical in DNA sequence to the A allele, except for a single-base deletion, 258G, in the coding region close to the N terminus of the protein (110300.0001). The deletion shifts the reading frame, resulting in translation of an entirely different protein. It is therefore unlikely that O individuals express a protein immunologically related to the A and B transferases, which agrees with the absence of crossreacting protein in O cells when specific monoclonal antibody directed toward soluble A transferase is used. Yamamoto et al. (1990) also reported the single-base substitutions responsible for the 4 amino acid substitutions that distinguish the A and B glycosyltransferases (see 110300.0002). Thus, the ABO polymorphism, discovered by Landsteiner (1900), was finally elucidated 90 years later.
As a result of further investigations of a puzzling paternity case where exclusion of the putative father was observed only in the ABO blood group, Suzuki et al. (1997) found that the child had 1 ABO allele of hybrid nature, comprising exon 6 of the B allele and exon 7 of the O1 allele. They found other evidence that recombination events are involved in the genesis of sequence diversity of the ABO gene.
Studies by Olsson et al. (2001) of the ABO gene in persons with serologically atypical findings, including sequencing of the full coding region (exons 1-7) and 2 proposed regulatory regions of the gene, revealed 15 novel A and B subgroup alleles. These included 2 which were the first examples of mutations outside exon 7 associated with weak subgroups. Each allele was characterized by a missense or nonsense mutation for which screening by allele-specific primer PCR was performed. As a result of this study, the number of definable alleles associated with weak ABO subgroups was increased from the 14 previously published to a total of 29.
Yip (2002) reviewed the extensive studies of the ABO blood groups by DNA-based genotyping methods and DNA sequencing. Extensive sequence heterogeneity underlying the major ABO alleles that produce normal blood groups A, B, AB, and O had been described. There was also extensive heterogeneity underlying the molecular basis of various alleles producing ABO subgroups such as A(2), A(x), and B(3). Over 70 ABO alleles had been reported to date. Yip (2002) proposed a unifying system of nomenclature to name these alleles. Furthermore, extensive sequence variation is found in the noncoding region of the gene, including 21 SNPs in intron 6. Excluding the common alleles, about half of the remaining alleles are due to new mutations and the other half can better be explained by intragenic recombination (both crossover and gene conversion) between common alleles. The recombination sites in hybrid alleles can be quite precisely defined through haplotype analysis of the SNPs in intron 6. This indicates that recombination is equally as important as point mutations in generating the genetic diversity of the ABO locus.
Seltsam et al. (2003) stated that 83 ABO alleles discriminated at 52 polymorphic sites within the coding region of ABO had been reported up to that time. In most cases, investigators analyzed only exons 6 and 7. The study by Seltsam et al. (2003) involved complete sequencing of the ABO gene (with the exception of intron 1) and 2 regulatory regions in 55 individuals. They documented 3 new mutations with coding regions and described many new mutations within introns.
Blumenfeld (2003), on the basis of her blood group antigen mutation database, stated that 88 allelic variants had been documented.
Data on gene frequencies of allelic variants were tabulated by Roychoudhury and Nei (1988).
ABO Blood Groups and Infectious Disease Susceptibility
The relationship between ABO blood groups and susceptibility to a specific infectious disease is a topic of long interest to those seeking an explanation for the variations in gene frequencies around the world. Several studies (reviewed by Glass et al., 1985) indicated that individuals with blood group O are at higher risk of contracting cholera (due to Vibrio cholerae 01) than those with other blood groups and that individuals with blood group AB are relatively resistant to cholera. Clemens et al. (1989) demonstrated that individuals with blood group O are at higher risk for cholera due to only the E1 Tor biotype of V. cholerae 01. During a large field trial in Bangladesh of killed oral cholera vaccine, persons with blood group O were significantly less protected against severe cholera than were persons with AB blood group. Faruque et al. (1994) found that patients in Bangladesh who had diarrhea due to V. cholerae 0139 were nearly twice as likely as controls to be of blood group O (64 vs 34%) and that individuals with blood group AB were at no risk for diarrhea due to V. cholerae 0139. Individuals with blood group O were the most susceptible to diarrhea due to V. cholerae 0139, followed in order by groups B, A, and AB.
Rowe et al. (2007) noted that Plasmodium falciparum-induced rosetting (i.e., the spontaneous binding of infected erythrocytes to uninfected erythrocytes) is thought to contribute to the pathogenesis of severe malaria (see 611162) by obstructing microvascular blood flow. Rosetting is reduced in blood group O erythrocytes compared with non-O blood groups, presumably due to group O individuals having disaccharide H antigens resulting from a lack of the terminal glycosyltransferases necessary to produce the trisaccharides found with A and B antigens. Rosettes that do form in group O red cells are smaller and more easily disrupted than those in group A, B, or AB red cells. Rowe et al. (2007) confirmed that rosetting was reduced in individuals with blood group O, intermediate in blood groups A and B, and highest in group AB. A matched case control study of 567 Malian children found that group O was present in only 21% of severe malaria cases compared with approximately 44% of uncomplicated malaria control cases and healthy controls. Rowe et al. (2007) concluded that group O is associated with a 66% reduction in the odds of developing severe malaria compared with non-O blood groups, and they reported preliminary evidence that similar protection is found in Kenyan children. The authors also proposed that group O does not occur at higher frequency in some malaria endemic regions due to increased susceptibility to cholera and other diarrheal diseases, resulting in balanced polymorphism.
To examine association of ABO blood groups with susceptibility to severe malaria, Fry et al. (2008) performed an association study involving more than 9,000 individuals from 3 African populations. They genotyped populations from The Gambia (West Africa), Malawi (South Central Africa), and Kenya (East Africa) for 4 key functional polymorphisms in ABO: the O allele frameshift mutation affecting amino acid 176 (rs8176719) and the 3 key A/B nonsynonymous SNPs, G235S (rs8176743), L266M (rs8176746), and G268R (rs8176747). Fry et al. (2008) determined that there is substantial linkage disequilibrium between these SNPs; that the frameshift mutation underlying blood group O is associated with protection from severe P. falciparum malaria, particularly anemia; and that the haplotypes producing A and B antigens are associated with susceptibility to severe malaria. Family studies showed that full-length alleles of rs8176719, which mark the A and B or non-O haplotypes, are associated with a greater risk of severe malaria if transmitted from a mother rather than from a father. Fry et al. (2008) noted that half the peoples of sub-Saharan Africa, as well as other populations, are homozygotes for the null mutation resulting in blood group O and are thus protected from life-threatening malaria.
Associations Pending Confirmation
Amundadottir et al. (2009) conducted a 2-stage genomewide association study of pancreatic cancer, genotyping over 500,000 SNPs in 1,896 individuals with pancreatic cancer and 1,939 controls drawn from 12 prospective cohorts plus 1 hospital-based case-control study. The authors conducted a combined analysis of these groups plus an additional 2,457 affected individuals and 2,654 controls from 8 case-control studies, adjusting for study, sex, ancestry, and 5 principal components. Amundadottir et al. (2009) identified an association between a locus on 9q34 and pancreatic cancer marked by the SNP rs505922 (combined P = 5.37 x 10(8); multiplicative per-allele odds ratio 1.20; 95% confidence interval 1.12-1.28). This SNP maps to the first intron of the ABO gene. The protective allele T of rs505922 is in complete linkage disequilibrium with the O allele of the ABO locus, consistent with earlier epidemiologic evidence suggesting that people with blood group O may have a lower risk of pancreatic cancer than those with groups A or B.
Blood soluble E-selectin (sE-selectin; SELE, 131210) levels have been related to various conditions, such as type-2 diabetes (see 125853). Qi et al. (2010) performed a genomewide association study among 1,005 women of European ancestry from the Nurses' Health Study and identified genomewide-significant associations between a cluster of markers at the ABO locus on chromosome 9q34 and plasma sE-selectin concentration. The strongest association was with rs651007, which explained approximately 9.71% of the variation in sE-selectin concentrations. rs651007 was also nominally associated with soluble intracellular cell adhesion molecule-1 (sICAM1; 147840) and TNFR2 (TNFRSF1B; 191191) levels, independent of sE-selectin. The genetic-inferred ABO blood group genotypes were also associated with sE-selectin concentrations. The genetic-inferred blood group B was associated with a decreased risk of type 2 diabetes compared with blood group O, adjusting for sE-selectin, sICAM1, TNFR2, and other covariates. The authors concluded that the genetic variants at the ABO locus affect plasma sE-selectin levels and diabetes risk, and that the genetic associations with diabetes risk were independent of sE-selectin levels.
Yamamoto et al. (1990) demonstrated that the blood group O allele of the ABO system (616093) differs from the blood group A allele by deletion of guanine at nucleotide 258 of ABO. The deletion, occurring in the portion of the gene encoding the part near the N terminus of the protein, causes a frameshift and results in translation of an almost entirely different protein incapable of modifying the H antigen.
Whereas ABO blood groups (616093) A, B, and AB in individuals express glycosyltransferase activities that convert the H antigen into A or B antigen, O(H) persons lack such activities. Yamamoto et al. (1990) found 7 nucleotide differences between the alleles that code for the A and B glycosyltransferase enzymes; 4 of the nucleotide differences were accompanied by change in amino acid residue in the transferase. The A gene had A, C, C, G, C, G, and G as nucleotides 294, 523, 654, 700, 793, 800, and 927; the B gene was found to have G, G, T, A, A, C, and A at these positions.
Yamamoto et al. (1992) demonstrated that the A2 allele, which encodes a minor subtype of A in the ABO blood group system (616093), has a single base deletion in a region of the ABO gene that encodes the C terminus of the protein. As a result of frameshifting, the A2 transferase possesses an extra domain. Introduction of this single base deletion into the A1 transferase cDNA expression construct drastically decreased the A transferase activity in DNA-transfected HeLa cells. The protein encoded by the A1 allele had 21 additional amino acids. The same nucleotide deletion was found in a total of 8 individuals with A2 blood type. (The single base deletion in O alleles is located close to the N terminus (see 110300.0001), whereas that of the A2 allele is close to the C terminus.) All 8 A1 alleles studied also showed a single base substitution (T in A2 and C in A1 at nucleotide position 467 counting from the A residue of the initiation codon) resulting in an amino acid difference (leucine in A2 transferase and proline in A1 transferase at amino acid position 156). Based on the observed expression of chimeric cDNAs in transfected HeLa cells, the amino acid substitution was shown to be incapable of drastically altering enzymatic activity or sugar-nucleotide donor specificity. The single nucleotide deletion occurred in a stretch of 3 Cs in nucleotide positions 1059-1061 of the A1 allele.
Seyfried et al. (1964) and Yamaguchi et al. (1965, 1966) described instances in which blood group O of the ABO system (616093) was inherited from 1 parent and both blood group A and blood group B from the other parent. This was referred to as cis-AB to discriminate this rare phenotype from ordinary trans-AB. Yoshida et al. (1980) reported 2 possible genetic mechanisms: unequal chromosomal crossing over and structural mutation in the blood group glycosyltransferase. In the latter instance, mutation in either the A or the B gene had produced a single abnormal enzyme with bifunctional activity. Yamamoto et al. (1993) determined the nucleotide sequence of the coding region in the last 2 exons of the ABO genes from 2 unrelated cis-AB individuals of the genotype cis-AB/O. They found that the cis-AB alleles were identical to one another while different from the A1 allele by 2 nucleotide substitutions. Both of these substitutions resulted in amino acid replacements. The first substitution was identical to the one previously found in the A2 allele, i.e., a C-to-T transition at nucleotide 467 resulting in the amino acid substitution pro156-to-leu. The other substitution was found at the fourth position of the 4 amino acid substitutions that discriminate A1 and B transferases, i.e., a G-to-C transversion at nucleotide 803 resulting in a gly268-to-ala amino acid substitution. The 2 patients in the study were Japanese; judging from the report, the cis-AB phenotype may be more common in Japanese than in others.
In 3 members of a family with the B(A) blood group phenotype of the ABO system (616093), Yu et al. (1999) identified a 700C-G transversion in the gene encoding the B transferase, resulting in a pro234-to-ala (P234A) substitution. The substitution occurs just ahead of the second of 4 amino acid residues that differentiate the specificities of the A and B transferases. Functional expression studies showed that the P234A substitution resulted in lower transferase B activity than wildtype, which correlated with the observation of a smaller amount of B antigen on the individuals' B(A) red cells. The substitution also resulted in higher transferase A activity compared to the minute amount of transferase A activity present in wildtype group B sera. Yu et al. (1999) concluded that the B(A) phenotype in this family was due to shifting of the specificity of transferase B rather than to enhanced transferase B activity. The findings showed that not only the 4 critical residues but also neighboring areas may influence the specificity of the A and B transferases.
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