| 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.
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The hemoglobinopathies are probably the world's most common
genetic diseases: The World Health Organization has estimated that at least
5% of the population are carriers for one or other of the most serious forms,
the alpha- and beta-thalassemias and the structural variant hemoglobins
S, C, and E, which are found at polymorphic frequencies in many countries.
All these hemoglobinopathies are believed to provide protection against
malaria, and it is thought that, in malarial regions of the world, natural
selection has been responsible for elevating and maintaining their gene
frequencies, an idea first proposed 50 years ago by J.B.S. Haldane. Epidemiological
studies undertaken in the 1950s on hemoglobin S in Africa provided support
for the "malaria hypothesis," but until recently it has proved
extremely difficult to verify it for the thalassemias. The application of
molecular methods has, however, provided new opportunities to address this
old question. Population and molecular genetic analysis of thalassemia variants,
and microepidemiological studies of the relationship between alpha-thalassemia
and malaria in the southwest Pacific, have provided unequivocal evidence
for protection. Surprisingly, some of this protection appears to derive
from enhanced susceptibility in very young thalassemic children to both
Plasmodium falciparum and, especially, P. vivax, and this early exposure
appears to provide the basis for better protection in later life.
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