Identification of a novel B variant allele at the ABO locus in Chinese Han individuals with B subgroup.

We studied the molecular genetic background of the B subgroup in the Chinese Han population and identified a novel allele at the ABO locus. Ten control samples from randomly selected blood donors of normal B phenotype and 6 samples from individuals diagnosed as B subgroup by serological tests were genotyped by PCR-SSP and direct DNA sequencing at exons 6 and 7 of the ABO gene. Exons 6 and 7 and the intervening intron 6 of B alleles from the 6 B subgroup samples were analyzed by cloning and haplotype-sequencing. A novel B variant allele was identified in 2 individuals who were serologically-determined as members of the B(x) and B(w) subgroups, respectively. The novel B allele differs from allele B101 by a single 695T>C missense mutation in exon 7. The family of the individual with B(x) subgroup was studied; among 8 family members tested, 4 had the novel B variant allele. No mutation at exon 6 or 7 of the ABO gene was detected in the 10 control samples or in the other 4 B subgroup samples. Mutation at position 695 where T is replaced by C results in an amino acid change from Leu to Pro, which is predicted to diminish B transferase activity. This indicates that alteration of the amino acid at position 232 is critical to the activity of glycosyltransferases.