By Deepak Mathur & team
It is common knowledge that human DNA gets damaged when exposed to high-energy radiation like gamma rays or high-energy electrons. Natural radiation, like cosmic rays, damage our DNA and its genetic material. But DNA also has self-healing properties. The remarkable thing is that in normal circumstances, human DNA is able to repair itself at a rate that is almost equivalent to the extent of damage routinely caused by natural radiation and chemical agents.
When the rate of damage exceeds the ability to repair, like when a human being is exposed to radiation from a leak in a nuclear reactor, genetic disorders can occur. While the impact of high-energy radiation on DNA has been studied extensively, we do not very well understand the effect of low-energy particles or radiation on our DNA. We do not know, for example, how human DNA would be impacted during space travel to Mars, when it might be exposed to sustained low-energy radiation. Our group has been exploring this particular question.
The difficulty in a study like this is in generating low-energy radiation, and then make it react with DNA that is ‘active’ or ‘alive’. DNA is always in aqueous form, in liquid state. And to get the best results, we needed to expose this aqueous DNA to low-energy radiation. We used a laser beam that produces large densities of photons, light particles, in a very short period of time, of the order of 10-15 seconds (femto-seconds). Individually, these photons have very low energy but since trillions and trillions of them are produced in a matter of a few femto-seconds, they lead to the creation of a strong, though momentary, electric field.
We then exposed the DNA kept in water to this electric field. The plasma that is produced ionises water molecules, producing very low energy electrons. The water molecules also break up, forming OH radicals. These radicals also have very low energy. All these interacted with the DNA dispersed in the water, and we were now ready to study the impact.
Using a technique called gel-electrophoresis, we were able to analyse the changes induced in the DNA as a result of its interaction with low-energy particles. We noticed that there was damage caused to the DNA. In some instances, one strand of the double-helix structure of DNA was found broken, at others, both. Now, single-strand breakages are easily repairable, by the DNA itself, but double-strand damages are not so easily fixed. It is the breakage of both the strands of DNA that can lead to cancer and formation of tumours.
By refining our experiment further, we were also able to infer that the OH- ion had a four times higher probability of causing damage to DNA than the electron. It was also seen that the OH radical was more likely to cause double-strand breakages, making it much more “dangerous” to the DNA. We have also been able to show that by controlling how the photons in the laser beam are focused, thereby affecting the strength of the electric field, we can control the extent of damage to the DNA. Our work has been published in two articles in the Physical Review Letters and one in Scientific Reports, as well as in the recent book Ultrafast Biophotonics, authored by P Vasa and Deepak Mathur.
Our experiments have shown a new way to study the impact of low-energy radiation on bio-molecules, not just DNA, in the physiologically relevant aqueous environment. There may be larger implications. It is possible to focus the laser beam to an extremely small area, say, a few microns. In that case, we can control the damage to the DNA only within that particular volume. One of the ways cancer is treated these days is by irradiating the tumour cells by energetic gamma-rays. But in the process of such irradiation, lots of neighbouring tissues also suffer collateral damage. Our optical method may provide an effective way of targeting only the tumour cells for destruction by a high-density photon beam. In that way, it may have the potential to improve cancer treatment.