A team of researchers led by Dr. Kei-ichi TAKATA from the Center for Genomic Integrity (CGI) at 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 therapy (IR) is often used in the treatment of cancer and is believed to destroy cancer cells by inducing DNA breaks. The newest 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, colliding with and destroying the cancer cells’ DNA.
These ions have a high linear energy transfer (LET) and release most of their energy within 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 benefit of minimizing damage to surrounding normal tissue compared to the commonly used low-LET radiation such as gamma or X-rays.
Only a handful of medical facilities in the world currently have the capability to deliver this next-generation radiation therapy, although more are hoped to be deployed in the future.
DNA lesions generated by powerful ion bombardment (high LET radiation) are more “complex” than those induced by traditional radiation therapy (low LET radiation). The former carries additional DNA damage such as apurine/apyrimidine (AP) site and thymine glycol (Tg) in close proximity to double-strand breaks (DSB), which are far more difficult to repair than normal DNA damage. As a result, the advanced treatment is more chemotherapy per unit dose than low LET radiation.
This makes next-generation radiotherapy a potent weapon against cancer cells. However, how these high LET-induced lesions are processed in mammalian cells has not been fully investigated, as DNA damage from heavy ion bombardment is a process that rarely occurs in nature (e.g., greater chance in space). Figuring out the complex DSB repair mechanism is an attractive research interest since blocking the cancer cells’ repair mechanism may allow the new radiotherapy to be even more effective.
To conduct research, the IBS team visited QST Hospital in Japan to use the HIMAC (Heavy Ion Medical Accelerator in Chiba) synchrotron, 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. Dr. Takata’s research team intends to help establish a basic research program using these synchrotrons in South Korea to improve heavy ion therapy in cancer patients.
Dr. Takata’s research team discovered 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 capable of performing microhomology-mediated end joining as well as translesion synthesis (TLS) across an abasic (AP) site and thymine glycol (Tg). This TLS activity was found to be the biologically significant factor enabling complex DSB repair.
SUNG Yubin, one of the joint first authors, explains: “We provided evidence that the TLS activity of POLQ plays a critical role in the repair of hiLET DSBs. We found that POLQ effectively anneals and extends substrates that mimic complex DSBs is”.
The researchers also discovered that preventing the expression of POLQ in cancer cells increased their vulnerability to the new radiation therapy.
“We demonstrated that genetic disruption of POLQ results in an increase of chromatid breaks and increased cellular sensitivity after treatment with high LET radiation,” explains YI Geunil, another joint first author.
The research team used biochemical techniques and Fluorescence Resonance Energy Transfer (FRET) to find that POLQ protein can efficiently repair synthetic DNA molecules that mimic complex DSBs. This means that POLQ may be a possible new drug target to increase cancer cells’ vulnerability 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 at UNIST. Mrs. RA Jae Sun at IBS-CGI analyzed chromatid breaks induced by high LET radiation. Prof. FUJIMORI Akira and Mr. HIRAKAWA Hirokazu at QST, and Prof. KATO Takamitsu at Colorado State University helped conduct the HIMAC experiments.
Prof. Takata notes, “We are proud to announce the publication of our paper which was only possible through the great teamwork of everyone involved. Our findings provide new insights into the mechanisms of how hiLET-DSBs are repaired in mammalian cells and further suggest that inhibition of POLQ may increase the effect of heavy ion radiation therapy.”
This work was published in Nucleic acid research on 20 February 2023.