Beta-thalassemia with exceptionally high hemoglobin A2. Differential expression of the delta-globin gene in the presence of beta-thalassemia.
Hemoglobin A2 levels in normal adults are rarely greater than 3.5%. In patients heterozygous for beta-thalassemia, they average about 5% but do not usually exceed 7%. We studied a family in which four patients with heterozygous beta-thalassemia had HbA2 levels of 8.4% to 11.2%. Globin biosynthesis studies and restriction endonuclease mapping of the alpha-globin loci showed homozygous or heterozygous alpha-thalassemia-2 as well as beta-thalassemia in some family members. The delta- and beta-globin genes were examined by using the restriction enzymes Eco RI, Pvu II, and Xba I, which cut both within and outside the coding portions of the delta- and beta-loci. Only the expected delta- and beta-globin gene containing fragments were present, excluding a crossover event producing a fusion gene that would code for delta-globin but possibly be under the regulatory influence of nucleotide sequences that control the expression of the beta-gene. This kindred provides evidence that in the presence of beta-thalassemia, expression of the delta-gene, beyond that commonly seen, is possible. This could be a direct result of the gene defect producing beta-thalassemia or be due to differences in the delta-globin gene linked to this beta-thalassemia gene. The interactions of alpha- and beta-thalassemia may alter tetramer assembly and increase HbA2 levels; however, this possibility seems less likely.