RAPID DETECTION OF THE CISAB ALLELE CONSISTING OF A CHIMERA OF NORMALA-ALLELE AND B-ALLELE BY PCR-RFLPS

DNA samples were analysed from Japanese individuals with the very rare ABO variant phenotype, cisAB (A(2)B(3)), which is characterized by th e apparent inheritance of both A and B genes on one chromosome. The na ture of the bases present at nucleotide positions (nps) 261, 526, 703, 796 and 803 is important for the specificity of the alleles at the AB O locus and the DNA from the cis AB donors was analysed by polymerase chain reaction - restriction fragment length polymorphism (PCR-RFLP) t o determine which nucleotides are present at these positions. The resu lts indicated that the cisAB allele had the AAAB-structure, which was a chimera of normal A and B alleles, when the expression 'AAAA' and 'B BBB' indicated the nucleotides of normal A (C, G, C and G) and B (G, A , A and C) genes at nps 526, 703, 796 and 803, respectively. The AAAB allele was found in all 27 individuals (17 families) with the cisAB in cluding three phenotypes A(2)B(3), A(1)B(3) and A(2)B and no other chi meric gene was found. The causative gene of cisAB was the AAAB allele, and the A and B alleles were not on one chromosome. The cisAB allele appeared to be a product of the normal A allele due to a point mutatio n at nucleotide position 803, from G to C. The AAAB allele is thought to be normally transcribed and translated to produce an unusual transf erase polypeptide, which has weak A- and weaker B-specific activity. P CR-RFLP is a rapid and useful means of detecting the cisAB allele (the AAAB allele) without a family study, even when they have A(1)B(3) and A(2)B phenotypes, because trans-type A(1)B(3) and A(2)B samples have obviously different PCR-RFLP profiles.