by A team of scientists led by Dr. Kei-ichi TAKATA of the Center for Genomic Integrity (CGI) within the Institute for Basic Science (IBS), has discovered a new type of DNA repair mechanism that cancer cells use to recover from next-generation cancer radiation therapy.
Ionizing radiation (IR) therapy is commonly used in cancer treatment and is believed to destroy cancer cells by inducing DNA breaks. The latest type of radiation therapy uses radiation produced by a particle accelerator, which consists of charged heavy particles such as carbon ions. The particle accelerator accelerates the carbon ions to about 70% of the speed of light, which collide with and destroy the DNA of cancer cells.
These ions have a high linear energy transfer (LET) and release most of their energy within a short range called the Bragg peak. The next-generation cancer radiotherapy works by targeting the Bragg peak to the tumor, which has the added benefit of minimizing damage to surrounding normal tissues compared to commonly used low-LET radiation such as gamma or X-rays .
Only a handful of medical facilities in the world currently have the capacity to deliver this next-generation radiation therapy, although more are expected to be deployed in the future.
DNA lesions caused by heavy ion bombardment (high LET radiation) are “more complex” than those caused by traditional radiation therapy (low LET radiation). The former involves additional DNA damage, such as the apurine/apyrimidinic (AP) site and thymine glycol (Tg) in the vicinity of the double-strand break (DSB) sites, which are much more difficult to repair than normal DNA damage. As a result, the advanced therapy is more cytotoxic per unit dose than low LET radiation.
This makes next-generation radiation therapy a powerful weapon against cancer cells. However, it has not been fully investigated how these high LET-induced lesions are processed in mammalian cells, as DNA damage from heavy ion bombardment is a process that rarely occurs in nature (e.g., more likely in space). Figuring out the complex DSB repair mechanism is an attractive research interest, as blocking the repair mechanism of cancer cells could make the new radiation therapy even more effective.
To conduct research, the IBS team visited QST Hospital in Japan to use the synchrotron called HIMAC (Heavy Ion Medical Accelerator in Chiba), which has the ability to produce high LET 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. therapy in cancer patients.
The research team of Dr. Takata found that DNA polymerase θ (POLQ) is an important factor in repairing complex DSBs, such as those caused by heavy ion bombardment. POLQ is a unique DNA polymerase that can perform both microhomology-mediated end-joining and translesion synthesis (TLS) across an abasic (AP) site and thymine glycol (Tg). This TLS activity turned out to be the biologically significant factor enabling complex DSB repair.
Ms. SUNG Yubin, one of the joint first authors, explains, “We provided evidence that POLQ’s TLS activity plays a critical role in repairing hiLET DSBs. We found that POLQ efficiently anneals and elongates substrates mimicking complex DSBs”.
The researchers also found that preventing the expression of POLQ in cancer cells significantly increased their vulnerability to the new radiation treatment.
“We have shown that genetic disruption of POLQ results in an increase in chromatid breaks and improved cellular sensitivity after treatment with high LET radiation,” explained Mr. YI Geunil, another joint first author.
The research team used biochemical techniques and Fluorescence Resonance Energy Transfer (FRET) to find out that POLQ protein can effectively repair synthetic DNA molecules that mimic complex DSB. This means that POLQ could be a potential new drug target to increase the vulnerability of cancer cells to complex radiation damage.
The single-molecule FRET assay system to monitor POLQ-mediated annealing and DNA elongation was developed in collaboration with Prof. KIM Hajin and Mr. KIM Chanwoo of UNIST. Ms. RA Jae Sun of IBS-CGI analyzed chromatid breaks caused by high LET radiation. prof. FUJIMORI Akira and Mr. HIRAKAWA Hirokazu of QST, and Prof. KATO Takamitsu of Colorado State University helped conduct the experiments with HIMAC.
prof. Takata comments, “We are proud to announce the publication of our paper, which was only possible through the great teamwork of all involved. Our findings provide new insights into the mechanisms of how hiLET-DSB is repaired in mammalian cells and further suggest that the inhibition of POLQ may increase the efficacy of heavy ion irradiation.”
This work was published in Research on nucleic acids on February 20, 2023.
