VI-d.   The Deletional Alpha-Thalassemia Alleles Involving Both Alpha-Globin Genes (alpha-thal-1)

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.

[Figure not yet available.]

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.

This material is from the book A Syllabus of Thalassemia Mutations (1997) by Titus H.J. Huisman, Marianne F.H. Carver, and Erol Baysal, published by The Sickle Cell Anemia Foundation in Augusta, GA, USA. Copyright © 1997 by Titus H.J. Huisman. All rights reserved. Neither this work nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, microfilming and recording, or by any information storage and retrieval systems, without permission in writing from the Author.