Sands of cancer

Thorium in the sands of Kerala's Chavara-Neendakara area alters DNA sequence of the local population

Published: Thursday 31 October 2002

monazite sands containing highly radioactive thorium are altering the dna sequences of local population of Kerala's Chavara-Neendakara coastal belt -- an area having the world's highest levels of natural radiation. This is the finding of a latest international study. However, the study is silent on the medical implications of the findings. Even medical experts working among the inhabitants of the area describe the study as scientifically interesting but medically insignificant. "The study does not seem to have any immediate medical significance, even if it is an excellent scientific work," said Raghuram K Nair of the Thiruvananthapuram-based Regional Cancer Centre. While ionisation radiation is known to cause cancer and other chromosome breakages, experts have so far failed to report unusually high rate of genetic aberrations among residents of the area.

What is interesting about the new study is that, unlike the investigations in the past, it studied the alterations in the dna of the mitochondria -- the tiny energy factories that power the cells. Mitochondrial dna are small and contain just a few genes as compared to the long strands of dna making up the chromosomes in the cell nucleus. While 23 pairs of chromosomes found in cells of a human being are inherited from both parents, mitochondrial dna is derived only from mother's egg. Hence the study of mitochondrial dna across two generations can clearly establish whether a mutation has been passed on from mother to her offsprings.

The researchers found 22 partial mutations -- in which the mitochondria contained both mutant and original dna -- in high-radiation exposed families. By comparison, only one mutation was found in low-radiation exposed families. Moreover, the mutation was not inherited by the immediate descendants.

Interestingly, these radiation associated mutations were located at nucleotide positions long considered as evolutionary 'hot spots' that undergo mutations much more frequently than other positions. Based on these findings the authors conclude that naturally occurring radiation accelerates the evolutionary dna mutation. The results also indicate that dna mutations in Kollam residents are comparable with those affected by the Chernobyl accident. These results will be of interest to evolutionary geneticists, who rely on a constant mutation rate for dating human prehistory.

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