alpha-thalassaemia

alpha-Thalassaemias are genetic defects extremely frequent in some populati ons and are characterized by the decrease or complete suppression of alpha- globin polypeptide chains. The gene cluster, which codes for and controls t he production of these polypeptides, maps near the telomere of the short ar m of chromosome 16, within a G+C rich and early-replicating DNA region. The genes expressed during the embryonic (zeta) or fetal and adult stage (alph a(2) and alpha(1)) can be modified by point mutations which affect either t he processing-translation of mRNA or make the polypeptide chains extremely unstable. Much more frequent are the deletions of variable size (from appro ximate to 3 to more than 100 kb) which remove one or both alpha genes in ci s or even the whole gene cluster. Deletions of a single gene are the result of unequal pairing during meiosis, followed by reciprocal recombination. T hese unequal cross-overs, which produce also alpha gene triplications and q uadruplications, are made possible by the high degree of homology of the tw o alpha genes and of their flanking sequences. Other deletions involving on e or more genes are due to recombinations which have taken place within non -homologous regions (illegitimate recombinations) or in DNA segments whose homology is limited to very short sequences. Particularly interesting are t he deletions which eliminate large DNA areas 5' of zeta or of both alpha ge nes. These deletions do not include the structural genes but, nevertheless, suppress completely their expression. Larger deletions involving the tip o f the short arm of chromosome 16 by truncation, interstitial deletions or t ranslocations result in the contiguous gene syndrome ATR-16. In this comple x syndrome alpha-thalassaemia is accompanied by mental retardation and vari able dismorphic features. The study of mutations of the 5' upstream. Rankin g region has led to the discovery of a DNA sequence, localized 40 kb upstre am of the zeta-globin gene, which controls the expression of the alpha gene s (alpha major regulatory element or HS-40). In the acquired variant of hae molglobin H (WbH) disease found in rare individuals with myelodysplastic di sorders and in the X-linked mental retardation associated with alpha-thalas saemia, a profound reduction or absence of alpha gene expression has been o bserved, which is not accompanied by structural alterations of the coding o r controlling regions of the alpha gene complex. Most probably the acquired alpha-thalassaemia is due to the lack of soluble activators (or presence o f repressors) which act in trans and affect the expression of the homologou s clusters and are coded by genes not (closely) linked to the alpha genes. The ATR-X syndrome results from mutations of the XH2 gene, located on the X chromosome (Xq13.3) and coding for a transacting factor which regulates ge ne expression. The interaction of the different alpha-thalassaemia determin ants results in three phenotypes: the alpha-thalassaemic trait, clinically silent and presenting only limited alterations of haematological parameters , WbH disease, characterized by the development of a haemolytic anaemia of variable degree, and the (lethal) Hb Bart's hydrops fetalis syndrome. The d iagnosis of alpha-thalassaemia due to deletions is implemented by the elect rophoretic analysis of genomic DNA digested with restriction enzymes and hy bridized with specific molecular probes. Recently polymerase chain reaction (PCR) based strategies have replaced the Southern blotting methodology The straightforward identification of point mutations is carried out by the specific amplification of the alpha(2) or a lpha(1) gene by PCR followed by the localization and identification of the mutation with a variety of screening systems (denaturing gradient gel elect rophoresis (DGGE), single strand conformation polymorphisms (SSCP)) and dir ect sequencing.