Exposure to low-dose X-rays poses no harm to stem cells, a find that could be important for assessing patients undergoing stem cell therapy and diagnostic X-ray exams simultaneously.

That’s according to Russian biophysicists from the Moscow Institute of Physics and Technology, who claim that stem cells remain healthy, proliferate and do not accumulate damage to human DNA that could be passed down to their progeny when exposed to low-dose X-rays. Their findings were published as a paper in the journal, “Aging”.

"Computed tomography scans, delivering doses between 5 and 80 milligrays (mGy) to patients, depending on the type of scan and the portion of the body involved, are the main means of follow-up examination for patients receiving stem cell transplants to monitor the results of the transplantation (proper homing to the targeted tissue) or potential complications and side effects," Andreyan Osipov, professor at the Russian Academy of Sciences and the head of the Experimental Radiobiology and Radiation Medicine Department at the Federal Medical Biophysical Center, and one of the authors of the paper, told HCB News. "With the growing number of stem cell therapies and the diseases they target (hundreds of clinical trials in progress), the number of patients receiving both stem cell transplants and low doses of X-rays will increase."

Ionizing radiation from medical diagnostic procedures, such as CT scans and mammograms, supposedly causes damage in stem cells that can be passed on to their progeny and lead to cell death, accelerated cellular aging and malignant transformations. This, however, has only been observed extensively in larger-dose exposure, compared to low-dose, which is defined as 100 mGy.

One crucial example of damage that worries researchers is double-strand breaks in DNA which can take long to repair, with uncorrected breaks possibly leading to cytogenetic abnormalities, tumor suppressor gene inactivation, oncogene activation and cell death.

Researchers conducted a series of experiments on the delayed effects of low-dose radiation exposure. Following 24 hours of exposure to X-ray radiation, stem cells were found to consist of more yH2AX foci, a protein used as a damage marker in DNA, compared to ones exposed to an intermediate dose.

The increased accumulation of yH2AX foci, however, was only found in cells undergoing division, rather than quiescent cells, making double strand breaks natural rather than the product of low-dose radiation exposure. Such breaks can be repaired through homologous recombination, a slow but virtually error-free way of restoring lost genetic information in damaged DNA sequences.

Upon examining the progeny of cells eleven cellular generations following low-dose exposure, researchers found no differences between them and a control group of cells not treated with X-rays. The progeny of the cells also did not show any signs of genome instability, proliferation abnormalities or accelerated senescence.

The authors say their results challenge the linear no-threshold model, which estimates ionizing radiation to be harmful to all living cells regardless of the dosage, thereby providing insight for better treatment of patients.

"Our findings provide evidence that the LNT model may not account for compensatory mechanisms inherently present in cells and may not predict the outcomes accurately. The truth is, when the LNT model was proposed, scientists not only did not know the double helix structure of the DNA molecule, they did not even know that DNA repair existed. Lastly, the radiation protection regulatory documents acknowledge that the LNT is a hypothesis; and like any hypothesis, it can be proved or disproved as new scientific knowledge accumulates, a detail that many forget."

"Our research suggests that the presence of γH2AX foci in cultured human stem cells 24 hours after low-dose X-ray irradiation is associated with cell division processes and does not lead to delayed effects related to aging,” Osipov said in a statement. “This is an important conclusion to draw, because γH2AX foci are now actively used in biodosimetry. Misunderstanding the biological significance of residual foci might lead to a severe overestimation of the risks associated with low-dose exposure."

Researchers caution against jumping to the conclusion that medical imaging does not increase health risks, explaining that stem cells are only a small fraction of cells within the body, and that the study did not directly measure transformation of irradiated cells into cancerous cells, meaning the results found only speak for the type of cells used, mesenchymal stem cells, which are indirectly related to cancerous transformation.

Research was financed partially through a grant from the Russian Foundation for Basic Research, and was conducted with the financial support of the Ministry of Education and Science of the Russian Federation.