(Deltabeta)°-thalassemia is relatively common in the populations of the Mediterranean area where the so-called Sicilian type is found. In addition, nine other types have been detected in other parts of the world. In all instances, the deletion involves (part of) the delta- globin gene and the beta-globin gene. The sizes of the deletions are shown in Fig. 14; for several types the 3' end of the deletion is found in the L1 repeat 3' to the beta-globin gene.
FIG. 14. The sizes and locations of 10 deletions resulting in a (deltabeta)°-thalassemia.
Heterozygous individuals generally have a mild anemia (Hb 10-12 g/dl) with a distinct microcytosis and hypochromia (MCV 60-65 fl; MCH 18-24 pg). The level of Hb A2 is (low) normal (1.8-2.9%) which is the result of an increased activity of the delta-globin gene in trans. Hb F is elevated (5-15%) with Ggamma values which are mainly determined by the presence or absence of the C->T mutation at position -158 (Ggamma). Homozygotes produce no Hb A and no Hb A2 but Hb F only. Their clinical course is one of thalassemia intermedia with a Hb level of 8-10 g/dl. Interactions of (deltabeta)°-thal with beta-thal determinants and with Hb S have been reported. A beta°/(deltabeta)°-thal compound heterozygote usually has severe disease resembling beta-thalassemia major; exact diagnosis often requires family studies. A Hb S/ (deltabeta)°-thal compound heterozygote is less severely affected, perhaps because high levels of Hb F (20-35%) are present in such individuals. Identification of known (deltabeta)°-thal alleles is greatly facilitated with the use of PCR and appropriate primers, and the identification of abnormal fragments which include the deletion (1).
|1.||Craig, J.E., Barnetson, R.A., Prior, J., Raven, J.L., and Thein, S.L.: Blood, 83:1673, 1994.|