Identification of the crossing-over point of a hybrid gene encoding human glycophorin variant Sta. Similarity to the crossing-over point in haptoglobin-related genes

One of the human glycophorin variants, Stones (Sta), has been shown to be the product of a hybrid gene of which the 5'-half derived from the glycophorin B (GPB) gene whereas the 3'-half derived from the glycophorin A (GPA) gene. The present study reveals the crossing-over point of this hybrid gene from the analysis of polymerase chain reaction products. The genomic sequences encompassing the region corresponding to exon 3 to exon 4 of GPA were amplified by polymerase chain reaction with oligonucleotide primers synthesized according to GPA and GPB genomic sequences (Kudo, S., and Fukuda, M. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 4619-4623). After subcloning the products, the nucleotide sequences derived from GPA, GPB, and putative Sta genes were determined. Comparison of the nucleotide sequences of GPA, GPB, and Sta genes indicate that the crossing-over took place 200 base pairs upstream from the first nucleotide of exon 4. Intriguingly, the nucleotide sequence surrounding the putative crossing-over point is homologous to the crossing-over point proposed for haptoglobin genes (Maeda, N., McEvoy, S.M., Harris, H.F., Huisman, T.H.J., and Smithies, O. (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 7395-7399). These results suggest strongly that homologous recombination through unequal crossing-over can be facilitated by specific genomic elements, such as those in common in these two crossing-over events. The present study also revealed that this Sta individual has a variant GPA gene; substitution of adenine for guanine at the nucleotide for codon 39 results in substitution of lysine for arginine at amino acid 39, and loss of an SstI restriction site.