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Euclides
Thursday, March 18th, 2004, 10:46 PM
Sequence variation at the human ABO locus

S. P. YIP1

The ABO blood group is the most important blood group system in transfusion medicine. Since the ABO gene was cloned and the molecular basis of the three major alleles delineated about 10 years ago, the gene has increasingly been examined by a variety of DNA-based genotyping methods and analysed in detail by DNA sequencing. A few coherent observations emerge from these studies. First, there is extensive sequence heterogeneity underlying the major ABO alleles that produce normal blood groups A, B, AB and O when in correct combination with other alleles. Second, there is also extensive heterogeneity underlying the molecular basis of various alleles producing ABO subgroups such as A2, Ax and B3. There are over 70 ABO alleles reported to date and these alleles highlight the extensive sequence variation in the coding region of the gene. A unifying system of nomenclature is proposed to name these alleles. Third, extensive sequence variation is also found in the non-coding region of the gene, including variation in minisatellite repeats in the 5' untranslated region (UTR), 21 single nucleotide polymorphisms (SNPs) in intron 6 and one SNP in the 3' UTR. The haplotypes of these variations reveal a specific relationship with the major ABO alleles. Fourth, 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. In particular, 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. Finally, a large number of ABO genotyping methods are available and are based on restriction analysis, allele specific amplification, mutation screening techniques or their combinations.

Euclides
Thursday, June 3rd, 2004, 04:01 PM
Transfusion. 2004 May;44(5):707-15. Related Articles, Links


Evolution of the O alleles of the human ABO blood group gene.

Roubinet F, Despiau S, Calafell F, Jin F, Bertanpetit J, Saitou N, Blancher A.

Laboratory of Molecular Immunology, Paul Sabatier University, Rangueil Hospital, 31403 Toulouse Cedex 4, France.

BACKGROUND: To date, at least 40 different alleles O have been characterized on the basis of exon 6 and exon 7 sequences but not always for intron 6. STUDY DESIGN AND METHODS: Among 415 individuals, from four continents (Africa, Europe, South America, and Asia), studied for exon 6 and exon 7 sequences, we selected 46 individuals (of respective phenotypes O [39], AB [3], B [3], or A [1]) for sequencing 1800-bp amplicons spanning exon 6, intron 6, and exon 7. The amplicons were characterized either by direct sequencing or after cloning when required. RESULTS: We defined 14 new intron 6 O allele sequences, including four recombinant alleles. Based on sequence comparison, a phylogenetic network was constructed. It confirmed recombinant allele origins and that most O alleles are derived by point mutations from the two worldwide distributed alleles O01 and O02. CONCLUSION: Allele O phylogenetic analysis suggests that the most frequent silencing mutation (deletion of a G in exon 6) appeared once in human evolution in the ancient O02 allele lineage and that allele O01 resulted from an interallele exchange between O02 and A101. Assuming constancy of evolutionary rate, diversification of the representative alleles of the three human ABO lineages (A101, B101, and O02) was estimated at 4.5 to 6 million years ago.