ABO exon and intron analysis in individuals with the AweakB phenotype reveals a novel O1v-A2 hybrid allele that causes four missense mutations in the A transferase.
BACKGROUND
Since the cloning in 1990 of cDNA corresponding to mRNA transcribed at the blood-group ABO locus, polymorphisms due to ethnic and/or phenotypic variations have been reported. Some subgroups have been explained at the molecular level, but unresolved samples are frequently encountered in the reference laboratory.


RESULTS
ABO blood grouping discrepancies were investigated serologically and by ABO genotyping [duplex polymerase-chain-reaction (PCR)--restriction-fragment-length-polymorphism (RFLP) and PCR--allele-specific-primer (ASP) across intron 6] and DNA sequencing of the ABO gene and its proposed regulatory elements. Blood samples from five individuals living in Portugal, Switzerland, Sweden and the USA were analysed. These individuals were confirmed to be of Black ethnic origin and had the unusual AweakB phenotype but appeared to have the A2B genotype without previously reported mutations associated with weak A or B expression. Sequencing of this A allele (having 467C>T and 1061delC associated with the common A2 [A201] allele) revealed three mutations regularly encountered in the O1v [O02] allele: 106C>T (Val36Phe), 188G>A (Arg63His), 220C>T (Pro74Ser) in exons 3, 4 and 5, respectively. The additional presence of 46G>A (Ala16Thr) was noted, whilst 189C>T that normally accompanies 188G>A in O1v was missing, as were all O1v-related mutations in exons 6 and 7 (261delG, 297A>G, 646T>A, 681G>A, 771C>T and 829G>A). On screening other samples, 46G>A was absent, but two new O alleles were found, a Jordanian O1 and an African O1v allele having 188G>A but lacking 189C>T. Sequencing of introns 2, 3, 4 and 5 in common alleles (A1 [A101], A2, B [B101], O1, O1vand O2 [O03]) revealed 7, 12, 17 and 8 polymorphic positions, respectively, suggesting that alleles could be defined by intronic sequences. These polymorphic sites allowed definition of a breakpoint in intron 5 where the O1v-related sequence was fused with A2 to form the new hybrid. Intron 6 has previously been sequenced. Four new mutations were detected in the hybrid allele and these were subsequently also found in intron 6 of A2 alleles in other Black African samples.


CONCLUSIONS
A novel O1v-A2 hybrid was defined by ABO exon/intron analysis in five unrelated individuals of African descent with the AweakB blood group phenotype.
Hosseini-Maaf
Bahram
B
Dept of Transfusion Medicine, Institute of Laboratory Medicine, Lund University & Blood Centre, University Hospital, SE-221 85 Lund, Sweden. bahram.hosseini_maaf@transfumed.lu.se
Hellberg
Asa
A
Rodrigues
Maria J
MJ
Chester
M Alan
MA
Olsson
Martin L
ML
D000017:ABO Blood-Group System / Q000235:genetics*; D000483:Alleles; D001483:Base Sequence; D001788:Blood Grouping and Crossmatching; D004252:DNA Mutational Analysis; D005091:Exons; D005646:Fucosyl Galactose alpha-N-Acetylgalactosaminyltransferase / Q000235:genetics*; D005838:Genotype; D006801:Humans; D007438:Introns; D020125:Mutation, Missense; D010641:Phenotype; D014166:Transferases
D000017:ABO Blood-Group System; D014166:Transferases; D005646:Fucosyl Galactose alpha-N-Acetylgalactosaminyltransferase