A team of scientists led by Dr. Keiichi TAKATA of the Center for Genome Integrity (CGI) of the Institute of Basic Sciences (IBS) has discovered a new type of DNA repair mechanism that cancer cells use to repair the next generation. radiation therapy for cancer.
Ionizing radiation (IR) therapy is often used in the treatment of cancer and is believed to kill cancer cells by inducing DNA breaks. The newest type of radiation therapy uses radiation produced by a particle accelerator made up of charged heavy particles such as carbon ions. The particle accelerator accelerates carbon ions to about 70% the speed of light, which collide with the DNA of cancer cells and destroy them.
These ions have a high linear energy transfer (LET) and emit most of their energy in a short range called the Bragg peak. Next-generation cancer radiation therapy works by focusing the Bragg peak on the tumor, which has the added advantage of minimizing damage to surrounding normal tissue compared to commonly used low-LET radiation, such as gamma or X-rays.
Currently, only a handful of medical facilities in the world have the capability to deliver next-generation radiation therapy, although more are expected to be deployed in the future.
DNA damage caused by intense ion bombardment (high LPE radiation) is more “complex” than damage caused by conventional radiation therapy (low LPE radiation). The former carries additional DNA lesions, such as an apurinic/apyrimidinic (AP) site and a thymine glycol (Tg) in the vicinity of double-strand breaks (DSBs), which are much more difficult to repair than normal DNA damage. As a result, advanced therapy is more cytotoxic per unit dose than low-LEP radiation.
This makes next-generation radiation therapy a powerful weapon against cancer cells. However, how these high LET-induced damages are processed in mammalian cells has not been fully investigated, since DNA damage by intense ion bombardment is a process that is rarely seen in nature (e.g., more likely in outer space). Elucidating the complex mechanism of DSB repair is an attractive scientific interest, as blocking the repair mechanism of cancer cells may allow new radiation therapy to become even more effective.
To carry out the research, the IBS team visited QST Hospital in Japan to use a synchrotron called HIMAC (Chiba Medical Heavy Ion Accelerator), which has the ability to produce high-LEP radiation. A similar synchrotron has been installed at Yonsei University, and another is planned to be installed at Seoul National University Hospital in Kijang in 2027. Dr. Takata’s research group intends to help establish a basic research program using these synchrotrons in South Korea to improve heavy ion therapy for cancer patients.
Dr. Takata’s research group found that DNA polymerase θ (POLQ) is an important factor in the repair of complex DSBs, such as those caused by heavy ion bombardment. POLQ is a unique DNA polymerase that is able to perform microhomology-mediated end joining as well as translational synthesis (TLS) through a lingual (AP) site and thyming glycol (Tg). This TLS activity has been shown to be a biologically relevant factor that enables comprehensive DSB repair.
Ms. SUN Yubin, one of the first authors, explains: “We have provided evidence that the TLS activity of POLQ plays an important role in hiLET-DSB repair. We found that POLQ efficiently anneals and extends substrates by mimicking complex DSBs.”
The researchers also found that preventing the expression of POLQ in cancer cells significantly increased their vulnerability to the new radiation therapy.
“We have demonstrated that a genetic disorder POLQ leads to increased chromatid breaks and increased sensitivity of cells after high-LET radiation treatment,” explains Mr. YI Geunil, another joint first author.
The research team used biochemical techniques and fluorescence resonance energy transfer (FRET) to find that the POLQ protein can effectively repair synthetic DNA molecules that mimic a complex DSB. This means that POLQ may be a possible new drug target to increase the vulnerability of cancer cells to complex radiation damage.
A single-molecule FRET assay system for monitoring POLQ-mediated DNA annealing and extension was developed in collaboration with Prof. Kim Ha-jin and Mr. Kim Chang-woo from UNIST. Ms. RA Jae Sun of IBS-CGI analyzed chromatid breaks caused by radiation from high LPE. Prof. FUHIMORI Akira and Mr. HIRAKAWA Hirakazu from QST and Prof. KATO Takamitsu from Colorado State University helped to conduct experiments with HIMAC.
Professor Takata comments: “We are proud to announce the publication of our work, which was only possible thanks to the excellent collaboration of all involved. Our findings provide new insights into the mechanisms of hiLET-DSB repair in mammalian cells and suggest that POLQ inhibition can increase the effectiveness of heavy ion radiation therapy”.
This work was published in Nucleic acid research February 20, 2023
