First Person is a series of interviews with the first authors of a selection of papers published in Journal of Cell Science, helping early-career researchers promote themselves alongside their papers. Yuichiro Saito is first author on ‘RIF1 controls replication initiation and homologous recombination repair in a radiation dose-dependent manner’, published in JCS. Yuichiro conducted the research described in this article while a PhD student in Kenshi Komatsu's lab at Department of Genome Repair Dynamics, at Kyoto University, Kyoto, Japan. He is now a postdoc in the lab of Masato T. Kanemaki at Department of Chromosome Science, National Institute of Genetics, Mishima, Shizuoka, Japan, investigating maintenance of the genome integrity in human cells.
How would you explain the main findings of your paper in lay terms?
Genomic DNA is challenged by various types of stress. Ionizing radiation (IR) cuts both DNA strands at the same region, generating a so-called DNA double-strand break (DSB). Because one unrepaired DSB is sufficient to elicit permanent growth arrest or cell death, cells must repair DSBs. DSBs are repaired via two pathways; an error-prone, non-homologous end-joining (NHEJ) pathway or a precise, homologous recombination repair (HRR) pathway. Mammalian cells predominantly use error-prone NHEJ to repair DSBs. However, this idea came from studies using a high dose of IR. In our paper, we used low and high doses of IR, and show that HRR has a prominent role to repair DSBs after a low dose of IR, and that HRR is suppressed after a high dose of IR. We identified RIF1 as a crucial factor that controls HRR activity in an IR-dose-dependent manner. RIF1 was known to be involved in controlling DNA replication timing and DSB repair. But there had been no work to simultaneously investigate these functions. We revealed that HRR at DSB sites is controlled by RIF1-dependent DNA replication timing, demonstrating an intimate link between DNA replication and DSB repair after exposure to IR.
Were there any specific challenges associated with this project? If so, how did you overcome them?
In the1950s, researchers found that IR sensitivity of mammalian cells showed as a non-linear curve, with a resistant region at low IR doses. In 1985, Prof. Goodhead proposed ‘saturable repair models’ that could nicely explain the resistance at low IR doses. However, no work has demonstrated this in living cells. Reporter assays, which quantitatively measure DNA-repair activity at an endonuclease-induced DSB, were developed, and it was good timing to test the idea. By combining these reporter assays and classic IR methods, we successfully demonstrated that the IR-dose dependent suppression of HRR.
Why did you choose Journal of Cell Science for your paper?
Journal of Cell Science publishes high-quality papers in cell biology fields, including DNA repair and DNA replication. Especially, I was inspired by a Commentary written by John F. X. Diffley and colleagues (Yekezare et al., 2013). In this review, they suggested a local checkpoint, in which DNA replication activity is inhibited globally but activated locally near the stalled replication forks. I could understand that cells can isolate a region from others to control its replication activity. This hinted to me the possibility of a RIF1-dependent local checkpoint at DSB sites.
What motivated you to pursue a career in science, and what have been the most interesting moments on the path that led you to where you are now?
I studied chemistry during my undergraduate years. At the time of choosing my future lab, I watched a documentary TV program about a genetic disease called Hutchinson-Gilford progeria syndrome. I was really struck by the fact that a single nucleotide mutation in a gene can cause such a severe syndrome. Thereafter, I was very interested and wanted to engage in studying genome integrity. I initially decided to study DNA repair mechanisms in yeast. It was the first time for me to study biology, so I needed to learn everything from zero. At the beginning, I failed many experiments but kept learning. The most interesting moment came suddenly, when I purified a protein complex from yeast cell extracts. The gel showed many bands of binding proteins after a pull-down purification! I published this work in Scientific Reports (Saito et al., 2013). As a PhD student, I moved to Kyoto University to work with Prof. Komatsu, who originally had cloned the Nijmegen breakage syndrome gene (NBS1) and was working on DNA repair in human cells. I have been fascinated by molecular biology since I looked at my gel.
Tell us something interesting about yourself that wouldn't be on your CV
Before starting my scientific career, I was a cook in a Japanese restaurant. I had trained as a cook for 3 years. I was actually offered my own restaurant but I decided to take a scientific career when I finished my undergraduate program. Now I cook a meal with my wife every week, and it is a good time to refresh my mind and focus myself on science.
Yuichiro Saito's contact details: Department of Chromosome Science, National Institute of Genetics, Mishima, Shizuoka 411-8540, Japan.