The Importance of the 3′ Untranslated Region for the Expression of the α-Globin Genes

Abstract

With a reverse transcription-polymerase chain reaction procedure, we have determined the relative quantities of α2- and α1-mRNA in several patients with heterozygosities for α2- or α1-globin gene mutations, in subjects with two forms of α-thalassemia-2 (-3.7 kb; -4.2 kb), and in two children with an α-globin gene triplication. Mutations in either one of the two genes do not affect the mRNA production, and the α2- to α1-mRNA ratios in our heterozygotes are the same (∼2.7) as in normal persons with four α-globin genes, while the α/α^X ratios of ∼1.7 for α2 variants and of ∼6.2 for α1 variants agree with the theoretic values. The deletion of 3.7 kb (leading to the formation of the α2α1 hybrid gene) and of 4.2 kb (resulting in the presence of only the α1 gene) causes the α2/α1 ratio to decrease to ∼1.7, indicating that both are expressed as an α1 gene. Data obtained for an Hb G-Philadelphia heterozygotes (αα/-α^G) show that the α2α1 hybrid gene produces ∼30% less mRNA than an α1-globin gene on a normal chromosome, which may be caused by loss of some sequences 3′ to the α2 gene. The same may be the case for the α1-globin gene on the chromosome with the 4.2 kb α-thal-2 deletion. These results suggest an important role for sequences located 3′ to the terminating codon in regulating transcription. Support for this hypothesis was obtained from data for the two children with an α-globin gene triplication; the high α2/α1-mRNA ratio can be explained by assuming that the α1α2 hybrid gene of the α(α1α2)α1 triplication expresses as an α2 gene.