Hiroshi Iwasaki
Professor
Cell Biology Unit
Institute of Innovative Research
Tokyo Institute of Technology
Japan
AsianScientist (Apr. 5, 2017) – Siblings share the same genetic source material (their parents) but can sometimes grow up to be as different as chalk and cheese. One of the reasons is homologous recombination, a genetic shuffle that occurs during the formation of sperm and egg cells and produces new combinations of DNA.
However, homologous recombination also plays an important role in maintaining the stability of the genome by repairing double stranded breaks that sometimes occur in DNA, a role that Professor Hiroshi Iwasaki calls “uniquely paradoxical but absolutely essential.”
Iwasaki, who in 2016 was awarded the Kihara Prize by the Genetics Society of Japan, is recognized for his discovery of the key enzymes involved in homologous recombination, particularly Rad51 and the Swi5-Sfr1 complex. In this interview for Asia’s Scientific Trailblazers, Iwasaki tells Asian Scientist Magazine of his own uniquely paradoxical but absolutely essential journey to molecular biology, a path that first took him though a phase of Kendo and dreams of pear farming.
- What first attracted you to the field of molecular biology?
- How would you describe homologous recombination to a layperson?
- You recently received the 2016 Kihara Prize. Could you briefly describe the notable work that won you the award?
- How has your research contributed to our understanding of DNA strand exchange?
- In your opinion, what sort of impact on society will research into this topic have?
- What are you working on currently that most excites you?
- Where do you see the field of molecular genetics in the next ten years?
I developed an aspiration to study molecular biology when I read a Japanese translation of The Double Helix by James D. Watson in my high school days (late 1970’s). At that time, I was just a boy crazy about Kendo; and like a typical high school sports enthusiast, I neglected my studies. Thus, a career in molecular biology seemed like an unrealistic dream that was far beyond my reach.
But I had a vague feeling of anxiety about the future that made me feel uneasy; I didn’t know which career path to pursue. I thought that it would be a good idea to become an agricultural engineer to take care of Japanese pears, since my hometown was famous for an Asian pear whose brand name is Twentieth Century pear. I thought the 20th century was ending soon, so we should develop a new and improved type of pear to serve as a local delicacy.
I had some ideas with shallow connections to genetics, such as breeding of different pears to combine desirable traits, with superficial notions about DNA and inheritance. Therefore, I enrolled at the Faculty of Agriculture at Kyushu University. However, none of the topics that I was exposed to interested me; I found them all to be rather boring.
This all changed when I attended a lecture on molecular biology, in which an associate professor in his 50’s taught us by using the textbook Molecular Biology of the Gene by Watson as a guide. I was enthralled by the topic and realized, “Oh, this is it, this is what I want to study!”
However, I couldn’t find a laboratory within the Faculty of Agriculture at Kyushu University that studied molecular biology; even the associate professor who gave the molecular biology lecture was not researching molecular biology. At that time, molecular biology was a cutting-edge science, and it was my impression that it could only be studied at certain universities.
I made the decision to attend the Graduate School of Medicine at Osaka University, where I started by studying bacterial molecular genetics. My PhD project was titled “Molecular mechanisms of SOS mutagenesis,” and Professors Atsuo Nakata and Hideo Shinagawa were my mentors. I was so excited to do experiments everyday because I was immersed in the same world as Molecular Biology of the Gene.
One day, Professor Nakata asked me, “Why did you come to Osaka University?” I replied “To study molecular biology, of course.” With a puzzled expression on his face, he said, “Did you not know Professor Mutsuo Sekiguchi? He is my best friend and a very famous molecular biologist.” He had a laboratory in the Faculty of Science at Kyushu University. I was such an ignorant and naive student at the time!
The continuity of life is strictly dependent on accurate genetic inheritance. This means that when a single cell divides to produce two cells, both daughter cells will, in theory, contain the same genetic material (or DNA sequence) as the mother cell. In practice, however, no two biological entities contain exactly the same genetic information. With the exception of identical (monozygotic) twins, even offspring of the same parents, who inherit genetic material from the same source, are not genetically identical.
When considering the continuity of life, the colossal difference in the genetic makeup of siblings is explained by the process of homologous recombination. The human genome is comprised of 23 chromosomes. However, within most DNA-containing cells, there are actually two sets of 23 chromosomes; one set of chromosomes is inherited from the mother and the other set is inherited from the father.
Through a specialized cell division, humans produce egg and sperm cells that contain only one set of chromosomes. The 23 chromosomes within this set are the reshuffled combination of the maternal and paternal chromosome sets; this dynamic process of DNA sequence reorganization occurs via homologous recombination and leads to the formation of a unique germline cell.
The subsequent process of fertilization, whereby a new organism is formed through the fusion of a unique egg cell with a unique sperm cell, therefore generates offspring that are genetically different to their parents as well as their siblings.
Furthermore, homologous recombination is employed by somatic cells to repair DNA damage during cell division in non-sex cells. As an example, when a double-stranded DNA molecule is broken, the homologous chromosome (or in many cases the newly replicated sister chromosome) is used to copy relevant genetic material and repair the broken DNA strand (recombinational repair).
Consequently, homologous recombination has the uniquely paradoxical but absolutely essential role of ensuring intra-species genetic variation at the same time as maintaining the genetic stability of cells within an individual organism. Intriguingly, as a fundamental maintenance process, homologous recombination is also conserved throughout all forms of life on Earth, like other notable intranuclear processes such as DNA replication and the transcription of genetic information.
Many questions concerning the molecular mechanisms of homologous recombination and recombinational repair remain unanswered, and of the many events occurring simultaneously within the nucleus, it may be said that these processes represent some of the final frontiers in molecular genetics.
The central step in homologous recombination is the DNA strand exchange reaction, in which two homologous DNA molecules exchange genetic information that is inscribed in their strands. This reaction is promoted by the action of proteins called recombinases. A Holliday junction, one of the most important intermediates in this reaction, is a transient four-way crossover formed between DNA strands of the two chromosomes by DNA strand exchange. This intermediate was first proposed by Robin Holliday in 1964.
Only the RecA protein, which is the recombinase from bacteria, had been shown to promote the formation of Holliday junctions. I used fission yeast as a model system to analyze the molecular mechanism in eukaryotic cells by which the Rad51 recombinase mediates DNA strand exchange. In 2008, our laboratory demonstrated for the first time that the Rad51 recombinases from fission yeast and humans promote Holliday junction formation.
In addition, we identified the highly conserved Swi5-Sfr1 hetrodimeric protein complex in fission yeast as a Rad51-interacting partner that promotes the DNA strand exchange reaction mediated by Rad51. Since this discovery in 2003, we have been working to elucidate the molecular mechanism whereby this protein complex stimulates the DNA strand exchange activity of Rad51.
My peers deemed that these studies focusing on Rad51 was enough for me to enter the upper echelons of molecular biology research in Japan and I am honored to say that I was awarded the prestigious Kihara Prize.
As I mentioned above, eukaryotic recombinases require Swi5-Sfr1 to efficiently perform homologous recombination. This is because the intrinsic strand exchange activity of Rad51 is very weak compared to bacterial RecA, and thus it requires appropriate activation within the overcrowded eukaryotic nucleus. In other words, the activity of Rad51 is strictly regulated by cells because homologous recombination is a dicey acrobatic DNA rearrangement that may lead to malignant mutations, chromosome loss and/or cell death. Therefore, understanding how the Swi5-Sfr1 accessary protein up-regulates Rad51 is important.
In addition to the Swi5-Sfr1 complex, there are many other accessory proteins that regulate Rad51 activity. One of them is BRCA2, which is a breast cancer susceptibility gene found in vertebrates. The study of Swi5-Sfr1 is intimately linked to understanding the molecular function of this tumor suppressor gene.
The importance of basic science or discovery science is tough to digest in ordinary society. Who among the general public could recognize the link between HR and cancer in 1964 when Robin Holliday proposed the general model for homologous recombination?
I would like to quote Dr. Yoshinori Ohsumi, a close colleague of mine and the recipient of the 2016 Nobel prize in Physiology or Medicine: “Do not ask science for immediate benefit. It will eventually be ascribed to human happiness”. I agree whole-heartedly with this sentiment.
Some of our achievements may seem trivial to society now, but I believe knowledge about the molecular mechanisms of homologous recombination can have not only medical and agricultural applications but also make unexpected contributions to seemingly unrelated fields in the future.
One of my graduate students has established a real-time assay to monitor the DNA strand exchange reaction. This is very exciting and extremely fun. In addition to performing research on homologous recombination, we are always trying to develop new tools and techniques that we can apply to our studies.
I don’t know, and I doubt I’m the only one in the dark about this! This unpredictability is precisely why science is so exciting. But one thing is definitely clear; the technical revolution for biological analyses has started. I’m referring to the so-called big data, single-molecule analysis, super resolution microscopy, cryo-electron microscope, and so on. Scientists who choose to ignore these ground-breaking innovations do so at their own peril, as they will struggle to find new horizons in molecular genetics and also in many other fields of biology.
This article is from a monthly series called Asia’s Scientific Trailblazers. Click here to read other articles in the series.
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Copyright: Asian Scientist Magazine; Photo: Hiroshi Iwasaki/Tokyo Institute of Technology.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.
