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 alpha 2- and alpha 1-mRNA in several patients with heterozygosities for alpha 2- or alpha 1-globin gene mutations, in subjects with two forms of alpha-thalassemia-2 (-3.7 kb; -4.2 kb), and in two children with an alpha-globin gene triplication. Mutations in either one of the two genes do not affect the mRNA production, and the alpha 2- to alpha 1-mRNA ratios in our heterozygotes are the same (approximately 2.7) as in normal persons with four alpha-globin genes, while the alpha/alpha X ratios of approximately 1.7 for alpha 2 variants and of approximately 6.2 for alpha 1 variants agree with the theoretic values. The deletion of 3.7 kb (leading to the formation of the alpha 2 alpha 1 hybrid gene) and of 4.2 kb (resulting in the presence of only the alpha 1 gene) causes the alpha 2/alpha 1 ratio to decrease to approximately 1.7, indicating that both are expressed as an alpha 1 gene. Data obtained for an Hb G-Philadelphia heterozygote (alpha alpha/-alpha G) show that the alpha 2 alpha 1 hybrid gene produces approximately 30% less mRNA than an alpha 1-globin gene on a normal chromosome, which may be caused by loss of some sequences 3' to the alpha 2 gene. The same may be the case for the alpha 1-globin gene on the chromosome with the 4.2 kb alpha-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 alpha-globin gene triplication; the high alpha 2/alpha 1-mRNA ratio can be explained by assuming that the alpha 1 alpha 2 hybrid gene of the alpha 2(alpha 1 alpha 2)alpha 1 triplication expresses as an alpha 2 gene.