The 21 known abnormalities concern the deletion of the alpha2- and (part of) the alpha1-globin genes, while in nine of these conditions the deletion also includes the zeta-globin gene. Heterozygotes have a mild anemia with a distinct microcytosis and hypochromia (MCV 60-70 fl; MCH 20-25 pg) and an imbalanced in vitro chain synthesis (alpha/beta 0.6-0.8). Homozygotes have the hydrops fetalis syndrome with Hb Bart's (gamma4) as its major Hb type together with some embryonic Hbs (zeta2gamma2; zeta2beta2; zeta2epsilon2). Fetuses with a homozygosity for the larger deletions which include the zeta-globin gene are usually aborted at 20 weeks of gestation. Fig. 24 illustrates most of the deletions which are discussed individually on the next several pages. The three most commonly occurring alpha-thal-1 alleles are - -(SEA) in the populations of Southeast Asia, and the -(alpha)20.5 kb and - -(MED-I) alleles in the populations of the Mediterranean area.
FIG. 24. The locations of the alpha-thal-1 deletions (modified from Ref. 2).
|1.||Higgs, D.R.: in The Haemoglobinopathies, edited by D.R. Higgs, and D.J. Weatherall, Bailliere's Clinical Haematology, Vol. 6, page 117, W.B. Saunders Company, London, 1993.|
|2.||Kattamis, A.C., Camaschella, C., Sivera, P., Surrey, S., and Fortina, P.: Am. J. Hematol., 53:81, 1996.|