The JR blood group system: genetic and molecular investigations

Abstract

Background Jra is a high‐incidence erythrocyte antigen previously designated ISBT 901.005. The rare Jr(a−) phenotype appeared to be inherited as an autosomal recessive trait, but the gene controlling the expression of Jra was not known. We conducted genetic and molecular investigations in an attempt to resolve this issue.Aim This lecture and accompanying manuscript will detail the homozygosity by descent (HBD) gene mapping strategy we used to establish that ABCG2 null alleles define the Jr(a−) phenotype (Zelinski et al., Nat Genet 2012;44:131–132).Materials and Methods Genomic DNA was genotyped for single nucleotide polymorphisms (SNPs) using a Human GeneChip (Affymetrix, 250K NspI) Mapping array. The six Jr (a−) study subjects were two Caucasian sisters, two Asian siblings and two unrelated Asian individuals referred for serologic testing. Anti‐Jra was identified in the plasma from four of the six. Intronic primers flanking exons 2–16 of ABCG2 were designed and used for PCR amplification. Both the forward and reverse strands of each purified product were Sanger sequenced at The Centre for Applied Genomics (Toronto, Canada).Results Analysis of the SNP genotype data revealed that the Caucasian sisters were identically homozygous for large tracts of consecutive SNPs in seven different chromosomal regions; chromosome 1q (3·3 Mb), 3q (8·8 Mb), 4q (28·9 Mb), 7q (10·5 and 7·7 Mb), 9q (3·1 Mb) and 10q (2·0 Mb). DNA from the four Asian individuals did not consistently contain homozygous SNP haplotypes in any of these regions except for chromosome 4q22.1. In this region, the siblings and one unrelated Asian had blocks of homozygosity that overlapped for approximately 400 000 bp and were identical to each other, but, different from the Caucasians. The remaining unrelated Asian was homozygous (approximately 520 000 bp) for a third SNP haplotype. The 4q22.1 region of homozygosity identified in all six study subjects contains four validated genes, only one of which, ABCG2, is known to be highly expressed on erythrocytes. DNA sequencing revealed that homozygosity for each of the three SNP haplotypes corresponded with homozygosity for a different nonsense (c.736C>T or c.376C>T) or missense (c.34G>A) mutation in ABCG2.Conclusion The genetic and molecular evidence revealed by this study proves that ABCG2 null alleles define the Jr(a−) blood group phenotype. Independently, our French colleagues established that Jra was carried on the ABCG2 protein (Saison et al., Nat Genet 2012;44:174–177). Based on these results the JR blood group system (ISBT 032) has been established.