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The genetics of common diseases: the implications of population variability
The results of recent work on the inheritance of susceptibility to malaria suggest that, over what may have been a relatively short evolutionary period, a remarkably diverse series of gene families have been modified in response to the selective drive of this infectious disease. The phenotypic consequences are not confined to the erythrocyte, they also involve the immune system, cytokines and many other systems. It seems likely that the mechanisms of genetic susceptibility to current environmental agents will reflect at least a similar degree of complexity and, if the selective pressures have been present for longer periods of our evolutionary history, may be even more diverse. These issues are discussed in the light of current efforts to identify some of the major genes involved in variation in susceptibility to the common non-infectious diseases of the developed world.
Interaction of hemoglobin E and pyrimidine 5' nucleotidase deficiency
A Bangladeshi family is described in which the genes for both hemoglobin E (Hb E) and pyrimidine 5' nucleotidase deficiency are segregating. An individual homozygous for both these conditions has a severe hemolytic anemia, whereas family members who are homozygous for Hb E are asymptomatic and those homozygous for pyrimidine 5' nucleotidase deficiency have the mild hemolytic anemia that is characteristic of this disorder. Globin-chain synthesis experiments have shown that the mechanism underlying the interaction between these two genotypes is a marked decrease in the stability of Hb E in pyrimidine 5' nucleotidase-deficient red blood cells (RBCs). It has also been found that in the enzyme-deficient RBCs in which Hb E is highly unstable, free alpha-chains, though not beta E-chains, acoumulate on the membrane. In view of the increasing evidence that the hemolysis associated with pyrimidine 5' nucleotidase deficiency results not only from an increase in the level of erythrocyte pyrimidines, but also from inhibition of the hexose monophosphate shunt activity in young erythrocytes, it is likely that the marked instability of Hb E in the enzyme-deficient cells results from oxidant damage acting on a mildly unstable Hb variant. These observations may have important implications for the better understanding of the pathophysiology of Hb E/beta-thalassemia, globally the commonest important form of thalassemia.
Thalassaemia and malaria, revisited
Haldane's attractive hypothesis that the high gene frequencies for thalassaemia in the Mediterranean population may have resulted from heterozygote advantage in regions where Plasmodium falciparum malaria was common in the past has been extremely difficult to verify at the population or experimental level. However, the molecular era has provided some powerful new tools to attack this old problem. It is now clear that the thalassaemias are the commonest monogenic diseases in man, with a broad distribution throughout the Mediterranean, Middle East, Indian sub-continent and South-east Asia. All these populations have specific types of thalassaemia mutations which, presumably, have arisen locally and been expanded by selection together with drift and founder effect. Recent work indicates that alpha thalassaemia provides protection against severe malaria. Quite unexpectedly at least some of this protection may be mediated by rendering very young children more susceptible to both P. vivax and P. falciparum malaria; such early immunization may provide some protection against the disease in later life.
Thalassemia in the next millennium. Keynote address
Over the next decade it will be essential to make the thalassemia problem more visible to governments and international health agencies that are involved in health care in the emerging countries. This will require detailed population surveys to determine the gene frequencies of the important forms of thalassemia, together with a better understanding of their natural history and of the factors that modify their clinical phenotypes. In particular, more needs to be learnt about the natural history and ways of managing the intermediate forms of beta thalassemia. While research should continue towards definitive forms of treatment it is important, in the meantime, to pursue the development of cheap and safe oral chelating agents and to carry out clinical trials of drugs that may interact one with another to elevate the level of fetal hemoglobin, particularly in patients with different types of beta thalassemia intermedia. The partial control of the disease by carrier detection and prenatal diagnosis will only be feasible in emerging countries if it is possible to obtain the financial support of the major international agencies and the cooperation of their governments and communities; the remarkable success of this approach in some of the Mediterranean islands is a good example of what can be achieved.
Elimination of transfusions through induction of fetal hemoglobin synthesis in Cooley's anaemia
Pharmacological stimulation of fetal hemoglobin production is of considerable interest as an alternative approach to therapy for Cooley's anemia. While intravenous compounds have been effective in inducing short-term increases in fetal hemoglobin in a few patients, long-term elimination of transfusion requirement has not been reported. In patients with Cooley's anemia, treatment with oral sodium phenylbutyrate alone, sodium phenylbytyrate combined with hydroxyurea, and hydroxyurea alone, has augmented fetal hemoglobin production and increased total hemoglobin concentration as much as 5 g/dl over baseline eliminating transfusion requirement in two patients. Parallel declines in circulating nucleated red cell count, and concentrations of serum transform receptor and erythropoietin, are consistent with more effective erythropoiesis. Over extended periods of treatment, no induction of other fetal proteins and no adverse effects were observed. Particular disease mutations and other genetic factors may be of prime importance in determining the response to agents that induce production of fetal hemoglobin.
Is hemoglobin instability important in the interaction between hemoglobin E and beta thalassemia?
Hemoglobin E (HbE; alpha2beta226glu-lys), globally the commonest hemoglobin variant, is synthesized at a slightly reduced rate and has a homozygous phenotype similar to heterozygous beta thalassemia. Yet, when it is inherited together with a beta thalassemia allele, the resulting condition, HbE/beta thalassemia, is sometimes characterized by a severe, transfusion-dependent thalassemia major. The severity of this interaction has not been explained. We have explored the possibility that it may reflect the instability of HbE consequent upon globin chain imbalance imposed by the beta thalassemia allele. Time-course and pulse-chase globin chain synthesis studies at 37 degrees C on peripheral blood and bone marrow suggest that hemoglobin instability is not significant in steady-state HbE/beta thalassemia; this is confirmed by density-gradient centrifugation studies that show no decrease in HbE levels relative to HbA as HbE/beta+ thalassemia red blood cells age. Globin binding to membranes was assessed and only alpha globin chains were found, in contrast to other unstable hemoglobins in which both alpha and beta chains were present. However, in experiments performed on blood from HbE/beta thalassemics in the temperature range 39 degrees C to 41 degrees C, there was evidence of instability of HbE, a finding that was also observed in homozygous HbE. These findings suggest that the phenotype of HbE/beta thalassemia is primarily the result of the interaction of two beta thalassemia alleles; however, hemoglobin instability may be important during febrile episodes, contributing to worsening anemia.
Hemoglobinopathies worldwide: present and future.
The genetic disorders of hemoglobin, the commonest monogenic diseases, occur at some of their highest frequencies in the developing countries, particularly those of Sub-Saharan Africa and Asia. Although progress towards their control and management continues to be made, the prospects for curing them, apart from marrow transplantation, remain uncertain. In many countries expertise and facilities for their control are extremely limited. Although a great deal can be done to help the situation by developing further North/South and South/South partnerships for disseminating better practice, the major problem for the future lies in the unwillingness of governments and international health agencies to accept that the hemoglobinopathies represent a health burden comparative to that of communicable and other major diseases. However, preliminary analyses suggest that, at least in the case of Asia, this may not be true. Further work of this type, together with more detailed frequency and economic data, is required to provide solid evidence for the health burden posed by the hemoglobin disorders, particularly in the developing world. Unless this is done, the increasingly large populations of patients with these diseases will continue to be neglected.