The Kidd blood group antigen system on human erythrocytes is defined by two alternate codominant antithetical specificities, Jka and Jkb (60). The Kidd antigens are localized on a 43-kDa red blood cell integral membrane protein that functions as a urea transporter (63-67). The JKA and JKB allele frequencies in Caucasians are 0.51 and 0.49, respectively, giving rise to the three common phenotypes Jk(a+b-), Jk(a-b+), and Jk(a+b+). The Jk(a-b-) phenotype, which is very rare in Caucasians, is observed in 0.1-1.4% of Polynesian populations (60). The KIDD/urea transporter gene (HUT11) is located on chromosome 18 (q12-q21) and encodes a peptide of 391-amino-acid residues (64-67). The Jka and Jkb epitopes arise through a single G^A transition at nucleotide position 838, which results in the incorporation of aspartic acid or asparagine at amino acid residue 280, respectively (67). ASPCR has been used by our laboratory to define JKA and JKB by detecting the G^A transition within codon 280 (Fig. 3) (10). We have also observed that Polynesian individuals possessing the recessive silent Jk(a-b-) phenotype genotype as JKB homozygotes, indicating that the null allele responsible for this phenotype was derived from JKB, and this allele would generate false JKB-positive results in our genotyping assay. The molecular mechanisms responsible for the Jk(a-b-) phenotype have been described (63,68). In Polynesian Jk(a-b-) individuals, a G^A 3' splice acceptor site mutation exists within intron 5, which results in skipping of exon 6, whereas in Finnish Jk(a-b-) individuals, another point mutation within exon 9 (T871C) predicts the loss of a potential glycosylation site (63). To determine the potential impact of the more common Polynesian null JK allele on prenatal genotyping, 753 unrelated subjects of different racial/ethnic groups were screened using ASPCR to detect the intron 5 G^A 3' splice acceptor site mutation (69). The intron 5 G^A 3' splice acceptor mutation responsible for the Polynesian Kidd (a-b-) phenotype was not observed in African-Americans, Caucasians, Hispanics, Native Americans, or Koreans screened, whereas one heterozygous carrier was observed among 91 Asian Indians (69). Genotyping the heterozygous individual for the Jka/Jkb dimorphism at nt838 revealed only the presence of "G," indicating a single normal JKA allele and a single null JKA allele. Unfortunately, fresh blood for serological analysis of this Asian Indian individual was
Normal i Null i
154 bp 154 bp
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