For the first time, researchers in India have found that mice and rats exposed to endosulphan suffer from DNA damage and genomic instability, and impaired DNA damage response.
The results published on August 4 in the journal, Carcinogenesis, by a team of researchers led by Prof. Sathees Raghavan from the Department of Biochemistry, IISc, Bengaluru show that endosulfan — an organochlorine pesticide — induces breaks in DNA strands and disturbs the damage response mechanism found in cells thus leading to compromised DNA strand repair.
The team found mice and rats exposed to endosulfan generated reactive oxygen species, a potent DNA damaging agent. The reactive oxygen species, in turn, caused DNA damage in the form of breaks in DNA strands. The broken DNA strands generally tend to repair themselves by rejoining. But endosulfan treatment was found to cause “extensive processing of broken DNA” leading to increased and long deletion in the strands.
Endosulfan also increased the damage by promoting erroneous repair of the broken DNA strands. Erroneous repair would lead to undesirable genome level changes leading to genomic instability, which may cause cancer and genetic abnormalities.
One of the major mechanisms by which broken DNA strands are repaired is through non-homologous end joining (NHEJ). There was no difference in the rate at which both control animals and endosulfan-treated mice repaired broken strands using this mechanism. However, in endosulfan-treated mice and rats, there was enhanced activity of an error-prone and harmful repair mechanism (microhomology mediated end joining — MMEJ).
“The MMEJ repair mechanism is responsible for long deletions in the DNA strands, chromosomal translocations and genomic instability leading to cancer,” said Robin Sebastian, the first author of the paper and a Ph.D scholar at the Department of Biochemistry, IISc.
Though only sub-lethal dose of endosulfan was used, it was more than sufficient to cause genomic instability.
“MMEJ occurs at extremely low frequency in normal cells. But in endosulfan-treated animals, MMEJ became predominant,” said Prof. Raghavan.
Sub-lethal doses used
Five animals per group were studied and the experiment was repeated many times. Three different concentrations of endosulfan were tested on animals and four doses of each concentration were given at 24 hours interval.
Even the highest dosage of 3 mg per kg of body weight used in the experiments is well below the lethal concentration (capable of killing 50 per cent of treated animals) of 12 mg/kg body weight.
“Though we used only sub-lethal dose of endosulfan, it was more than sufficient to cause genomic instability,” said Mr. Sebastian. “We propose that by causing genomic instability, endosulfan might be causing genetic abnormality. Definite studies have not been done to prove this but we have provided a strong molecular evidence to suggest this.”
Lungs and testes of the animals were used for the study. These two organs show very proficient DNA repair mechanism. “So it is easy to test any perturbation in the DNA repair activity in these organs upon endosulfan exposure,” Mr. Sebastian said.
“Genetic abnormality and increased cancer occurrences have been seen in people who have had occupational exposure of 2-70 microgram/litre of endosulfan in blood. But we didn’t see this in animals that had 20 microgram/litre of endosulfan in blood. It’s probably because people are exposed continuously and for longer duration than the animals used in our study,” said Prof. Raghavan.