Characterization of a newly discovered alpha-thalassaemia-1 in two Spanish patients with Hb H disease

Alpha-thalassaemia

alpha-Thalassaemias are genetic defects extremely frequent in some populations and are characterized by the decrease or complete suppression of alpha-globin polypeptide chains. The gene cluster, which codes for and controls the production of these polypeptides, maps near the telomere of the short arm 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 (alpha 2 and alpha 1) can be modified by point mutations which affect either the processing-translation of mRNA or make the polypeptide chains extremely unstable. Much more frequent are the deletions of variable size (from approximately 3 to more than 100 kb) which remove one or both alpha genes in cis or even the whole gene cluster. Deletions of a single gene are the result of unequal pairing during meiosis, followed by reciprocal recombination. These unequal cross-overs, which produce also alpha gene triplications and quadruplications, are made possible by the high degree of homology of the two alpha genes and of their flanking sequences. Other deletions involving one 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 the deletions which eliminate large DNA areas 5' of zeta or of both alpha genes. These deletions do not include the structural genes but, nevertheless, suppress completely their expression. Larger deletions involving the tip of the short arm of chromosome 16 by truncation, interstitial deletions or translocations result in the contiguous gene syndrome ATR-16. In this complex syndrome alpha-thalassaemia is accompanied by mental retardation and variable dismorphic features. The study of mutations of the 5' upstream flanking region has led to the discovery of a DNA sequence, localized 40 kb upstream of the zeta-globin gene, which controls the expression of the alpha genes (alpha major regulatory element or HS-40). In the acquired variant of haemoglobin H (HbH) disease found in rare individuals with myelodysplastic disorders and in the X-linked mental retardation associated with alpha-thalassaemia, a profound reduction or absence of alpha gene expression has been observed, which is not accompanied by structural alterations of the coding or controlling regions of the alpha gene complex. Most probably the acquired alpha-thalassaemia is due to the lack of soluble activators (or presence of repressors) which act in trans and affect the expression of the homologous 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 gene expression. The interaction of the different alpha-thalassaemia determinants results in three phenotypes: the alpha-thalassaemic trait, clinically silent and presenting only limited alterations of haematological parameters, HbH disease, characterized by the development of a haemolytic anaemia of variable degree, and the (lethal) Hb Bart's hydrops fetalis syndrome. The diagnosis of alpha-thalassaemia due to deletions is implemented by the electrophoretic analysis of genomic DNA digested with restriction enzymes and hybridized 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 alpha 1 gene by PCR followed by the localization and identification of the mutation with a variety of screening systems (denaturing gradient gel electrophoresis (DGGE), single strand conformation polymorphisms (SSCP)) and direct sequencing.

Nondeletional alpha-thalassemia: first description of alpha Hph alpha and alpha Nco alpha mutations in a Spanish population

Several different deletions underlie the molecular basis of alpha-thalassemia. The most common alpha-thalassemia determinant in Spain is the rightward deletion (-alpha 3.7). To our knowledge, however, no cases of alpha-thalassemia due to nondeletional mutations have so far been described in this particular Mediterranean area. Here, we report the existence of nondeletional forms of alpha-thalassemia in ten Spanish families. The alpha 2-globin gene was characterized in ten unrelated patients and their relatives only when the presence of deletional alpha-thalassemia was ruled out. The alpha 2-globin gene analysis was performed using the polymerase chain reaction (PCR) followed by restriction enzyme analysis or by allelespecific priming. This allowed the identification of a 5-base pair (bp) deletion at the donor site of IVS I (alpha Hph alpha) in 9 cases and the alpha 2 initiation codon mutation (alpha Nco alpha) in one case. Although these alpha 2-globin gene mutations are found in other mediterranean areas, our results demonstrate their presence in the Spanish population and suggest that the alpha Hph alpha/alpha alpha genotype is probably the most common nondeletional form of alpha-thalassemia in Spain.

Molecular characterization of a new family with alpha-thalassemia-1 (--MA mutation)

A Spanish family with alpha-thalassemia-1 (alpha-Thal-1), deletion (--MA), is described. In addition to the loss of 22 kb of DNA with a deletion of the alpha 1, alpha 2, psi alpha 2, and psi zeta 1 genes, a triplication of the zeta gene cluster in "cis" is produced. The structure of this triplication is formed by the psi zeta 1 gene, the interzeta region, and, possibly, the insertion of the psi alpha 2 fragment.

A new deletional alpha-thalassemia detected in Yemenites with hemoglobin H disease

A new large deletion from the human alpha-globin gene cluster is characterized. It involves at least 39 kb and includes the two alpha-globin genes, the theta 1-gene, all the pseudogenes, and the two hypervariable regions (HVRs), interzeta-HVR and alpha-globin 3'HVR. The conserved zeta-globin gene has been identified in various restriction fragments of abnormal size. The new deletion was found in four unrelated Israeli patients with Hb H disease, all originating in Yemen, and has been designated--YEM. It is the only two-gene deletion identified in this ethnic group.

Characterization of a new alpha-thalassemia-1 mutation in a Spanish family

A novel alpha-thalassemia-1 deletion of 14-15.4 kb that removes the alpha-2, alpha-1, theta-1 genes and pseudo-alpha-1 genes, has been detected in a father and 2 of his children from northern Spain.

alpha-Thalassaemia

The large number of naturally occurring mutants of this well-characterized locus provides an excellent opportunity for elucidating the relationship between its structure and function. Comparisons of what has been learned about the alpha-globin locus with complementary observations on the beta-globin locus, provide a strategy for understanding the co-ordinate regulation of eukaryotic gene expression. From a practical point of view it is important to remember that millions of individuals throughout the world are carriers of alpha-thalassaemia and every year many thousands of pregnancies are at risk of producing children with the severe alpha-thalassaemia syndromes. The data summarized here provide the basis for accurately predicting the genotype in such cases and thus enabling appropriate prenatal testing. However, because this is a genetic disease that predominantly affects individuals from countries with limited health resources, simpler and cheaper methods of screening and diagnosis will have to be developed before this information has a significant impact on the attendant morbidity and mortality (see Chapter 9, this volume).

Hemoglobin comparisons between African American and European American males with hemoglobin values in the normal range

There are conflicting reports about the nature and magnitude of hemoglobin (Hb) differences between African Americans (AA) and European Americans (EA). Several reports suggest that AA have lower mean Hb values and that differences between AA and EA are large and are due to "racial" differences. Some have even called for separate Hb standards. In this study, Hb comparisons were made between AA and EA males in the Second National Health and Nutrition Examination Survey, 1976-1980. Comparison of AA and EA males with normal Hb values (i.e., 13.0 g/dl or greater) indicated that observed mean Hb differences exist primarily at the lower end of the Hb distribution (i.e., the distribution is skewed to the left). Several factors are likely contributors to lower hemoglobin values at the lower end of the distribution, including hemoglobinopathies, thalassemias, and/or nutritional deficiencies. If racial differences exist between these groups, controlled prospective studies are necessary to demonstrate this possibility. This study supports a single Hb standard for AA and EA males. © 1992 Wiley-Liss, Inc.

A newly-characterized alpha-thalassaemia-1 deletion removes the entire alpha-like globin gene cluster in an Italian family

We describe a new deletional form of alpha thalassaemia which encompasses the entire alpha-like globin gene cluster in a 15-year-old boy of Southern Italian descent. The deletion removes approximately 31 kb, the 5'-end point is located approximately 4 kb upstream of the xi gene, while the 3'-end point maps between the alpha 1- and theta 1-globin genes. The interaction of this deletion with the common-alpha 3.7 form gives origin to a classical form of haemoglobin (Hb) H disease in the propositus of this study. Deletional forms of xi alpha-thalassaemia are uncommon in the Mediterranean basin; as for other unusual xi alpha-thalassaemia forms, heterozygotes for this mutation may escape detection in population surveys based on zeta and alpha probes.