The XMN I Site (-158, C→T) 5′ to ttle Gγ GENE: Correlation with the Senegalese Haplotype and Gγ Globin Expression

There are three major African haplotypes associated with the sickle mutation: Benin (#19), Senegalese (#3), and Central African Repblic (#20). Previous studies have suggested that the Xmn I site (-158 bp 5′ to the ^G_γ gene) is associated with elevated levels of ^G_γ andwiththe Senegalesehaplotype, while other investigators questioned this association. In order to clarify theissue.we have determined βhalotypes, tested fortlie presenceof theXninI site, and measured HbF and ^G_γ expressionlevels in 143 AmericanBlackpatients with sickle cell anemia. Haplotypes were determined using eight polvmorphic sites in the β-like globin gene cluster: Hinc II 5′ to ?, Hind III in IVS-II ^G_γ and ^A_γ, Hinc II within and 3′ to φ β, Ava II in IVS-II of β, and Hpa I and Bani HI 3′ to β. The ^G_γ/^A_γ ratio was analyzed by high performance liquid chromatography using a C₁₈ column. The Xmn I site was present in all 31chromosomes with the Sengalese haplotype. Of the remaining 255 chromosomes with other haplotypes, only 2 (0.8%) had the Xmn I site present. Therewas significant correlation between the presence of the Xmn I site and increased ^G_γ/^A_γ ratio in a dose-dependent manner.The Hb F level was not significantly, increased in thepresence of the Xmn I site.The data indicate that the Xmn I site maintains a ^G_γ/^A_γ ratio typical of fetal life but does not necessarily cause elevation of Hb F. The latter seems to depend on factors other than the Xmn I site.