Sequence variations in the 5′ flanking and IVS-II regions of the Gγ- and Aγ-globin genes of βS chromosomes with five different haplotypes

Abstract

We have amplified and sequenced the 5′ flanking and the second intervening sequence (IVS-II) regions of both the Gγ-and Aγ-globin genes of the βS chromosomes from sickle cell anemia (SS) patients with homozygosities for five different haplotypes. The sequencing data, compared with previously published sequences for the normal chromosomes A and B, show many similarities to chromosome B for haplotypes 19, 20, and 17, while haplotypes 3 and 31 are remarkably similar to chromosome A and also similar to each other. Several unique mutations were found in the 5′ flanking regions (Gγ and Aγ) of haplotypes 19 and 20 and in the IVS-II segments of the same genes of haplotypes 19, 20, and 17; the IVS-II of haplotypes 3 and 31 were identical to those of chromosome A. Dot-blot analyses of amplified DNA from additional SS patients with specific probes have confirmed that these mutations are unique for each haplotype. The two general patterns that have been observed among the five haplotypes have most probably arisen by gene conversion events between the A and B type chromosomes in the African population. These patterns correlate with high and low fetal hemoglobin expression, and it is speculated that these and other yet unknown gene conversions may contribute to the variations in hemoglobin F and Gγ levels observed among SS patients. In vitro expression experiments involving the ∼ 1.3-kb 5′ flanking regions of the Gγ- and Aγ-globin genes of the βS chromosomes with the five different haplotypes failed to detect differences between the levels of expression, suggesting that the sequence variations observed between these segments of DNA are not the primary cause of the differences in hemoglobin F levels among the SS patients. © 1991 by The American Society of Hematology.