Non-deletion haemoglobin H disease in Papua New Guinea

The molecular basis of Hb H disease in Turkey

A total of 25 unrelated Hb H patients were studied at the DNA level. Ten different genotypes were found to be responsible for the disease. The most prevalent alpha-thalassemia-2 determinant was the alpha alpha/-alpha (3.7) kb deletion (56%) which was followed by a nondeletional type of alpha-thalassemia, namely the pentanucleotide deletion in the 5' first intervening sequence splice junction [alpha(-5nt) alpha] (16%). The two most frequent alpha-thalassemia-1 determinants were alpha alpha/-20.5 kb and alpha alpha/-17.5 kb (MED-I) deletions. In two patients, homozygosity for the polyadenylation signal mutation [alpha (PA-2)alpha] was found to be responsible for Hb H disease. Clinical and hematological expression seems more severe in patients with the alpha (-5nt) alpha deletion at the donor site of the first intervening sequence and the alpha(PA-2) alpha mutation in trans to an alpha-thalassemia-1 determinant. Homozygosity for the alpha (PA-2)alpha mutation was also found to be associated with severe phenotype.

Alpha-thalassaemia and globin gene rearrangements in French Polynesia

The prevalence of alpha-thalassaemia and various globin gene rearrangements was determined in 1992 individuals living on 11 islands in French Polynesia. The gene frequencies for alpha(+)-thalassaemia (almost exclusively the -alpha 3.7III deletion form) range from 5.3% to 19.2%. Haematological indices on 177 heterozygotes and 27 homozygotes for the -alpha 3.7III variant showed considerable overlap with indices of normal individuals; although there was a broad correlation of average indices with alpha-globin genotype, individual values were a poor indication of genotype. A non-deletion form of alpha(+)-thalassaemia (alpha alpha Th), triplicated alpha genes (alpha alpha alpha) and single zeta gene (-zeta) chromosomes were present at low frequencies (< 1%), whereas triplicated gamma gene (gamma gamma gamma) and triplicated zeta (zeta zeta zeta) arrangements were more common (1.1-16.3%). alpha 0-thalassaemia, probably introduced from Southeast Asia in the early part of this century, was observed in a number of individuals of Chinese and Chinese/Polynesian ancestry. Because of the high frequency of alpha(+)-Thalassaemia on some islands, it therefore seems likely that haemoglobin H disease (resulting from the interaction between alpha 0 and alpha(+)-thalassaemia) must occur in parts of French Polynesia.

Thalassaemia in Vanuatu, south-west Pacific: frequency and haematological phenotypes of young children

The archipelago of Vanuatu situated in the South-West Pacific has a high frequency of alpha + thalassaemia and additionally on some of the islands there is a high frequency of beta thalassaemia. As part of a large cohort study to investigate the clinical effect of thalassaemia on malaria on the islands of Espiritu Santo and Maewo in Vanuatu, the gene frequencies of the thalassaemias were determined and blood counts were performed on a cohort of infants from birth to 3 years. The haematological phenotypes of the different thalassaemic genotypes are compared, providing a detailed description of the clinical manifestations of alpha + thalassaemia during early development. In addition, cross-sectional surveys of the population of the two islands were performed to establish the frequency of thalassaemia and other red cell polymorphisms and their geographical distribution.

Molecular epidemiology of the thalassaemias (including haemoglobin E)

The thalassaemias are the most common genetic disorders of man, and over the last decade the molecular epidemiology of these defects has been studied in detail. After briefly reviewing the great diversity of mutations giving rise to these conditions, four global regions are discussed in more detail. The thalassaemias, of which haemoglobin E is one, are most frequent in Asia, where recent work has defined the molecular basis of the beta thalassaemias and the frequencies of the various types of alpha + and alpha 0 thalassaemia. Oceanic populations have a range of globin gene variants remarkably different to those of south-east Asia. Most is known about the nature and frequencies of thalassaemia mutations in Mediterranean countries, where prenatal diagnosis programmes have been very successful in reducing the frequency of new cases of thalassaemia major. alpha + Thalassaemia is the most common haemoglobinopathy in sub-Saharan Africa, and molecular studies of American Blacks with beta thalassaemia have elucidated the probable molecular basis of the mild form of this disorder in Africans. Although each geographical region has its own group of common beta thalassaemia mutations, with little overlap, most of these appear to have had a single origin. The question of single or multiple origins for HbE in south-east Asia is unresolved. Recombination events producing alpha + thalassaemia deletions are frequent, whereas alpha 0 thalassaemia is produced by a variety of large deletions, each of which has had a single origin. The evidence favouring natural selection by P. falciparum malaria as the primary cause of high frequencies of the thalassaemias throughout the tropics and subtropics is reviewed. While the mechanism of protection remains unclear, epidemiological evidence supporting the hypothesis is strong, but more information is required from case-control studies on the amount of protection provided by the various thalassaemia genotypes.

Unusual molecular basis of Hb H disease in the Azores Islands, Portugal

An Azorean family with Hb H disease (10% Hb H) was studied in order to elucidate its molecular basis. DNA studies on the patient only revealed a 4.2 kb "leftward" deletion of paternal origin which implies the co-inheritance of a nondeletional alpha-thalassemia determinant. Restriction endonuclease and oligonucleotide analysis allowed the exclusion of five point mutations: initiation codon (at both alpha 1- and alpha 2-globin genes), IVS-I donor splice junction pentanucleotide deletion, codon 125 CTG----CCG substitution, and Saudi Arabian polyadenylation signal mutation. These findings suggest that the molecular basis of this form of Hb H disease is probably different from those described previously.