Genomic organization of glycophorin A gene family revealed by yeast artificial chromosomes containing human genomic DNA.

Human glycophorins A, B, and E are members of the glycophorin gene family located at chromosome 4, band q31. These genes were apparently generated by two successive gene duplications, and the following evolutionary pathway was proposed based on the genomic sequences. First, the ancestral gene was duplicated, and one of the duplicated genes evolved directly into the GPA gene. Another duplicated gene acquired a 3' sequence from the precursor genomic segment and was then duplicated to yield glycophorin B and E genes (Onda, M., Kudo, S., Rearden, A., Mattei, M.-G. and Fukuda, M. (1993) Proc. Natl. Acad. Sci. USA 90, 7220-7224). Although the above hypothesis was proposed, it has been difficult to provide solid evidence for this, because no genomic clones studied encompass two members of the gene family. In this study we isolated several yeast artificial chromosome (YAC) clones encoding the entire region or a portion of this gene locus. The physical gene mapping was carried out by testing the presence of specific sequences using a polymerase chain reaction. Moreover, the YAC clones were digested by rare cutting restriction enzymes, and the resultant large DNA fragments were separated by pulse-field electrophoresis. Southern blot analyses of those DNA fragments were carried out using various probes encoding specific portions of glycophorin genes. The results obtained revealed that the distance between glycophorin A and B genes is almost the same as that between glycophorin B and E genes. Moreover, it was shown that no precursor genomic sequence was present in the sequences downstream from the GPB or GPE gene, suggesting that the duplication of glycophorin B/E progenitor gene was made through unequal crossing over homologous recombination. These combined results suggest that two duplication events took place tandemly to produce these three members of the glycophorin gene family.