Natural radiation poses no major risk
doi:10.1038/nindia.2009.71 Published online 27 February 2009
Genetic risk due to high natural background radiation is "a lot less than one has imagined earlier," according to a study just reported by researchers at the National Institute of Immunology NII in New Delhi1.
While natural background radiation may be responsible for varying alterations, it leaves the DNA of the vital germ line intact, they say. "Our study shows that the germ line is protected because the human genome has some unknown but powerful mechanism of protecting it," Sher Ali, a molecular geneticist who led the team told Nature India.
The findings should put at rest fears that people living in coastal areas of the south Indian state of Kerala are at risk of becoming infertile because they are exposed to natural background radiation that is ten-fold higher than the worldwide average. The radiation is the result of thorium bearing monazite minerals in the beach sands.
In their study the NII team looked for possible structural variations in the Y chromosome in blood and semen samples of 390 males of two generations at Chavara and Thevara — two high radiation regions of Kerala. They compared the results with those obtained from similar samples from 390 healthy unexposed males of matched age groups from Kochi (Kerala) and 400 normal males from different parts of India.
"We chose the Y chromosome as it is an ideal candidate to be assessed for possible genetic alterations induced by ionizing radiation due to its haploid status (having only one member of chromosome pair)," Ali said.
Containing fewer than 50 genes or gene families coding for proteins, Y-chromosome is not essential for life but harbors several testis-specific genes necessary for sperm production, and hence continuation of the species. At least three non-overlapping regions of the Y chromosome — (Azoospermia factors a, b, and c) — are essential for sperm production. Micro-deletions in these regions affecting one or more of the candidate genes (DAZ, RBMY, DBY, and USP9Y) cause male infertility. The SRY gene in the Y chromosome plays a predominant role in male sex determination.
The scientists found that barring two per cent, all the 790 unexposed (control) males from Kerala and other parts of India were free from micro-deletions. In contrast more than 90 per cent of males exposed to background radiation showed random micro-deletions in the Azoospermia factor. And, in about 80 per cent of them tandem duplication and multiple copies of 11 different Y-linked genes were found. For instance, more than 85 per cent of exposed males showed 2-8 copies of the SRY gene.
But remarkably all these genetic alterations in the exposed population were detected only in blood samples but not in the germ line (sperm) samples suggesting that sperm cells are somehow protected from background radiation. "Absence of micro-deletions in the germline explains the normal fertility status of the exposed males," Sanjay Premi, one of the authors told Nature India.
Where do the germ cells derive the protection from? Ali says that germ cells, originating from the gonad's somatic cells, most likely activate some still unknown genes that sense the defective ones and triggers apoptosis (programmed cell death). "The defective sperms are eliminated and quickly replaced by fresh ones and thus what we see is always a clean picture in case of germ line," Ali said. However that is not true in case of blood cells meaning blood cells are not treated in the same way as germ cells in the body. "I would love to call it nature's marvel," Ali said adding that "this is the hypothesis; we are yet to prove this."
According to the NII scientists the tandem duplication and multiple copies of genes detected in blood samples of exposed population "is a possible genome strategy to absorb the mutational loads caused by the radiation."
Previous studies on the high radiation areas of Kerala were focused on the cases of Down's syndrome, congenital malformations, chromosomal aberrations in humans and phenotypic changes in the rats. Some groups also focused on the frequencies of the repeated abortions in human females. "In other parts of world, people have studied rates of minisattellite mutations, mitochondrial mutations in the populations exposed to radiations due to industrial accidents or nuclear bombings," said Premi. "On the contrary, we have analysed the single chromosome Y in a much greater detail both at somatic and germ line levels. To our information no one in the world has analysed germ line and somatic cells in the manner we have done."
Premi said that on the basis of the study it has been concluded that the study populations have adapted themselves to the high background radiation. "The (genetic) alterations we recorded were random and did not represent any major Y chromosome-related recombination losses or duplications reported in the literature which has been held responsible for the infertility."
- Premi, S. et al. Unique signatures of natural background radiation on human Y chromosomes from Kerala, India. PLoS ONE 4, e4541. 2009)