Modification of hemoglobin H disease by sickle trait

Alpha-2-globin gene polyadenylation (AATAAA-->AATAAG) mutation in hemoglobin H disease among Kuwaitis

OBJECTIVES

In the Arabian Gulf region, hemoglobin (Hb) H disease usually results from homozygosity or compound heterozygosity involving the alpha2-globin gene polyadenylation (poly A) signal (AATAAA-->AATAAG) mutation (alpha(T)alpha). Here we document the clinical and hemato logical characteristics of children with Hb H disease being followed in Kuwait.

SUBJECTS AND METHODS

Twenty-four patients (0.5-12 years old, mean 4.7 +/- 3.5 years) with persistent microcytic, hypochromic anemia (and normal iron status as well as normal Hb A2 levels) were referred to the pediatric hematology clinic for further investigations. They were all screened for the alpha+-thalassemia (alpha+-thal; -3.7 kb) deletion using a standard PCR method. They were also screened for the alpha2-globin gene alpha(T)alpha allele and the 5nt deletion (-alpha5nt) in the first intervening sequence, which are common alpha-thal alleles in this population. They were followed up for periods ranging from 2 to 8 years.

RESULTS

Of the 24 patients, 4 (16.7%) also had sickle cell trait (Hb-AS), while 7 (29.2%) were glucose-6-phosphate dehydrogenase deficient. Only 1 patient had significant hepatosplenomegaly and 1 developed gallstones. While none was on chronic transfusion therapy, 8 (33.3%) had been transfused at least once and, in 3 instances, this was secondary to parvovirus B19 +ve aplastic crisis. The alpha-globin genotype was successfully determined in almost all patients. The results showed that 17 (70.8%) patients were homozygous for the poly A mutation (alpha(T)alpha/alpha(T)alpha), 6 (25.0%) were compound heterozygotes for this and the alpha+-thal (-3.7 kb) deletion (-alpha/alpha(T)alpha) and 1 (4.2%) was undetermined. There were no significant differences in the phenotypes of the 2 genotypes and their hematological features were identical.

CONCLUSIONS

Hb H disease involving the poly A mutation is a mild thal intermedia phenotype among Kuwaitis. There are no serious complications and there is no need for regular blood transfusion.

Unusual phenotype of hemoglobin EE with hemoglobin H disease: a pitfall in clinical diagnosis and genetic counseling

Two unrelated individuals previously diagnosed as hemoglobin (Hb) EE were found to be, in fact, Hb EE with Hb H disease. This globin genotype normally results as Hb EF Bart disease. This unusual genotype-phenotype interaction highlights the need for molecular analysis in affected individuals with Hb E disorders before appropriate genetic counseling and genetic risk estimation in offspring can be given.

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.

Hb H disease in a Turkish family resulting from the interaction of a deletional alpha-thalassaemia-1 and a newly discovered poly A mutation

We have analysed the alpha-globin gene defects present in several members of a large family from Southern Turkey. One deletional alpha-thalassaemia-1 (type MED-II) was found in 10 subjects: this deletion is in excess of 26.5 kb and includes all zeta- and alpha-globin genes. Besides the common types of deletional alpha-thalassaemia-2 (-3.7 kb and -4.2 kb) we observed a nondeletional alpha-thalassaemia-2 that results from an A----G mutation (AATAAA----AATGAA) in the polyadenylation signal of the alpha 2-globin gene: the same A----G replacement is present in the psi alpha l gene. The mutation must cause a considerable alpha-chain deficiency as is evidenced by the haematological data for five members with Hb H disease due to a compound heterozygosity for alpha-thalassaemia-1 (MED-II) and the newly discovered poly A mutation. Several members had additional beta-chain abnormalities (Hb S, Hb D-Los Angeles, beta-thalassaemia); the 11 persons with a Hb S heterozygosity and various alpha-globin gene defects (-alpha/alpha alpha; alpha T alpha/alpha alpha, - -/alpha alpha, -alpha/-alpha and - -/alpha T alpha) showed a decrease in the level of Hb S that was directly related to the severity of the alpha-chain deficiency.

Association in cis of beta +-thalassemia and hemoglobin S

A Moroccan woman was investigated because of a typical beta-thalassemia trait associated with a low-percentage (11%) hemoglobin (Hb) variant. The beta-thalassemia trait was manifested by a microcytosis, a high HbA2 (above 6%), and an increase of the alpha/beta biosynthetic ratio (1.31). The variant was identified to HbS by amino acid analysis of the abnormal peptide (beta T1) and by DNA mapping with Sau I (Mst II) restriction endonuclease. No additional amino acid substitution was recorded in the beta s-chain. The reduction of beta-globin synthesis occurred exclusively at the expense of the beta s-chain. These results are consistent with the existence of a beta s mutation and a beta +-thalassemia in cis.

Combination of three alpha-globin gene loci deletions and hemoglobin New York results in a severe hemoglobin H syndrome

A chinese patient with hemoglobin (Hb) H and Hb New York is described. Unlike classical Hb H disease, the additional beta New York chain defect resulted in a more marked alpha/beta-globin chain imbalance in this patient with only one alpha-gene, accounting for her severe anemia (3.4-6.8 g/dl) during the steady state.

Quantitation of hemoglobin components by high-performance cation-exchange liquid chromatography: its use in diagnosis and in the assessment of cellular distribution of hemoglobin variants

Additional data are presented that were obtained with a newly developed cation-exchange high-performance liquid chromatography (HPLC) method allowing the separation of numerous normal and abnormal human hemoglobin types. The method was found to be of considerable value for the diagnosis of certain hemoglobinopathies in the adult as well as in the newborn. Definitive differentiation between, AS, SS, S-beta+-thal, SD, SC, AC, C-beta+-thal, etc in cord bloods was readily accomplished even when the samples were collected on filter paper. Analyses of the hemoglobin in cells from Hb S and Hb C heterozygotes with different numbers of alpha chain genes (alpha alpha/alpha alpha; -alpha/alpha alpha; -alpha/-alpha) after separation by a Dextran density centrifugation technique failed to show significant changes in the relative quantitation of the variant hemoglobin types. Glycosylation of hemoglobin increased greatly with an increase in red cell age. Similar analyses of red cells from a young girl with Hb S trait and Hb H disease (the --/-alpha genic arrangement) gave comparable data. Some indication was obtained that her youngest cells contained some of the beta A4 and beta S4 tetrameric hemoglobins.

Post-translational control of human hemoglobin synthesis: the role of the differential affinity between globin chains in the control of mutated globin gene expression

The interactions between beta-thalassemia and the human hemoglobin (Hb) alpha-chain variants, Hb Hasharon, Hb O Idonesia and Hb J Paris, and between alpha-thalassemia and the beta-chain variants, Hb S, Hb C and Hb G San José, which are characterized by preferential decrease of the abnormal Hb level in peripheral bloods, have been studied. Both biosynthesis studies in reticulocytes and determination of the relative affinity of abnormal chains for normal complementary chains by in vivo recombination experiments, involving globin chains previously isolated in their native form, have been carried out in order to provide insights on the molecular events following the synthesis of the mutant chains under conditions of complementary chain deficiency. Furthermore, we have measured the relative affinity for complementary chain of beta D Los Angeles- and alpha J Rovigo-chains, the level of which does not decay in thalassemic carriers, and of alpha Legnano- and beta Osu Christiansborg-chains, which have not yet been observed in association with thalassemias. Our experiments indicated that the differential affinity for beta-chains is not always the major post-translational control mechanism which regulates the level of certain alpha-chain variants in beta-thalassemic heterozygotes, and that preferential removal of abnormal chains by proteolytic enzymes is likely to play an important role in most cases. On the other hand, the low affinity of certain variant beta-chains for alpha-chains may offer an explanation for the low level of certain beta-chain variants in peripheral blood of non-thalassemic carriers, as well as to their decrease under conditions of relative alpha-chain deficiency (alpha-thalassemias).

Hematological and clinical features of sickle cell diseases: a review

The differentiation, hematologic features and clinical manifestations of patients with the various sickling disorders are reviewed. The deficiencies in our current knowledge about the spectrum of the clinical course of patients with these conditions is discussed. The interaction of alpha thalassemia with sickle cell anemia and its possible effect upon the severity of the disease is summarized. The apparent milder disease in certain groups of patients with sickle cell anemia in whom there is an associated elevation of hemoglobin F is contrasted with the controversy surrounding the effects of hemoglobin F levels in the patients of African origin.