In addition to the 4 major groups of A1, B (B1), A1B, and O, there are additional ABO subgroups. The classification of these subgroups is based on differences in the degree (strength) of agglutination of RBCs with anti-A, anti-B, and anti-A,B reagents, the presence of anti-A, anti-B, and anti-A,B antibodies in sera, and the secretion of A and B antigens in saliva, among others. The weak subgroups include A2, A3, Ax, Ael, Aint, Am, Aw, Ax, B3, Bel, Bw, and Bx. By 1993, we extended our study of the molecular genetic basis of the ABO system to several of the A and B subgroups. Compared with A101, the A201 allele that specified the A2 phenotype possessed 2 differences. The first resulted in an amino acid substitution (proline to leucine) at codon 156 (P156L). The other produced a single nucleotide deletion at nucleotide 1060 (1060delC), which caused frame-shifting and resulted in a protein with an additional 20 amino acid residues at the C-terminus (Yamamoto et al., 1992). The A301 allele that specified the A3 phenotype had a single nucleotide substitution that resulted in an amino acid substitution from aspartic acid to asparagine (D291N). Additionally, the Ax01 allele that specified the Ax phenotype had a single nucleotide substitution that resulted in an amino acid substitution of phenylalanine to isoleucine (F216I) (Yamamoto et al., 1993a; Yamamoto et al., 1993c). The B301 allele was identical to B101 except for a single nucleotide substitution resulting in an amino acid substitution from arginine to tryptophan (R352W) (Yamamoto et al., 1993a).
Yamamoto, F., McNeill, P.D., and Hakomori, S. (1992). Human histo-blood group A2 transferase coded by A2 allele, one of the A subtypes, is characterized by a single base deletion in the coding sequence, which results in an additional domain at the carboxyl terminal. Biochem Biophys Res Commun 187, 366-374.
Yamamoto, F., McNeill, P.D., Yamamoto, M., Hakomori, S., Harris, T., Judd, W.J., and Davenport, R.D. (1993a). Molecular genetic analysis of the ABO blood group system: 1. Weak subgroups: A3 and B3 alleles. Vox Sang 64, 116-119.
Yamamoto, F., McNeill, P.D., Yamamoto, M., Hakomori, S., and Harris, T. (1993c). Molecular genetic analysis of the ABO blood group system: 3. A(X) and B(A) alleles. Vox Sang 64, 171-174.
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