NTU Scientists Reveal the Essential Role of Polyamines to DNA Repair

How metabolites and cellular small chemicals contribute to genome integrity has been poorly explored. A research team led by Prof. Hung-Yuan (Peter) Chi (冀宏源) of the NTU Institute of Biochemical Sciences found that naturally occurring small cations, called polyamines (Fig. 1a for schematic), play an essential role in the DNA repair process. By combining animal study, cell-based analysis, and biochemical assays, the study revealed that polyamines specifically facilitate homologous recombination-mediated DNA double-strand break (DSB) repair. It is worth noting that many cancers harbor an elevated level of polyamines to sustain their survival. Thus, the team’s findings provide a fundamental base for further studies directed at devising strategies for cancer treatment. This work was published under the title, “Promotion of Homology-Directed DNA Repair by Polyamines,” in Nature Communications, a well-known international scientific journal, on January 8, 2019.

Homologous recombination is a predominantly error-free repair pathway in cells to fix DNA DSBs. Furthermore, it is a prerequisite for maintaining genome integrity and preventing tumorigenesis. Initiation of homologous recombination is catalyzed by RAD51 recombinase. RAD51 assembles onto single-strand DNA (ssDNA), forming a nucleoprotein filament at damage sites. The RAD51-ssDNA filament then engages duplex DNA, searches for homology in the duplex, and upon the homology location, catalyzes the invasion of the duplex DNA for subsequent DNA exchange and synthesis to repair DSBs. However, the underlying mechanism of the homology search process remains unclear. What cellular factors might be involved in homology search is a crucial issue to be addressed.

The research team provided evidence, for the first time, that it is polyamines that specifically promote homologous recombination, rather than other DSB repair pathways such as non-homologous end-joining (NHEJ) and single-strand annealing (SSA; Fig. 1b). Mechanistically, polyamines significantly stimulate the DNA strand exchange activity of RAD51 by the enhancement of duplex DNA capture in the DNA homology search process (Fig. 1c).

The first author of the study, Dr. Chih-Ying Lee (李致瑩), proposed three possible mechanistic actions to explain how polyamines stimulate RAD51-mediated DNA exchange activity (Fig. 1d). First, polyamines could act as a linker to facilitate the collision between RAD51 filament and duplex DNA (Fig. 1d, inserted box i). Second, polyamines could stabilize the pairing between the invading ssDNA and the complementary ssDNA of the duplex template (Fig. 1d, inserted box ii). Finally, polyamines could condense duplex DNA and thus allow RAD51 filament to search for homology in multiple regions simultaneously (Fig. 1d, inserted box iii). According to Dr. Lee, the study revealed how polyamines can expedite and facilitate RAD51 filament’s search for homology through the vast genome.

This fundamental work has important clinical implications. As cancer cells possess an elevated level of polyamines, their DNA repair activity is expected to be upregulated. Consistent with this premise, the research team found that the small molecule drug, 2-difluoromethylornithine (DFMO), significantly reduces the level of polyamines and leads to impaired DNA repair activity. Most excitingly, the team also confirmed that the treatment combining DFMO with either radiation or the FDA-approved drug, olaparib, can suppress cancer cell survival. It is worth noting that the DFMO drug is currently undergoing the phase II clinical trial for neuroblastoma and shows significant therapeutic benefit. “Our work thus provides a mechanistic link directed at devising the strategies for cancer treatment such as the combination of DNA damaging agents with DFMO,” said Prof. Chi.

This study was conducted collaboratively by Dr. Chih-Ying Lee (李致瑩), Dr. Guan-Chin Su (蘇綸勤), Dr. Wen-Yen Huang (黃文彥), Min-Yu Ko (柯旻佑), Dr. Hsin-Yi Yeh (葉欣怡), Prof. Geen-Dong Chang (張震東), Prof. Sung-Jan Lin (林頌然), and Prof. Hung-Yuan (Peter) Chi (冀宏源). This work was supported by NTU, Academia Sinica, and Taiwan’s Ministry of Science and Technology.

(Source: Dr. Hung-Yuan (Peter) Chi, Associate Professor from the Institute of Biochemical Sciences, NTU)