A deeper picture of meoitic recombination The BRCA2 gene is known to suppress cancer and also plays a major role in meiotic recombination. We spoke to Professor Hiroki Shibuya about his work in investigating the fundamental mechanisms involved in meiotic recombination and the role of BRCA2 in repairing double strand breaks of DNA through homologous recombination. Our DNA is composed of two strands of genetic material which coil around each other in the double helix identified by Crick and Watson in 1953. When one helix is broken the damage can be repaired relatively easily, as the other strand remains intact, but a double strand break (DSB) is more difficult to repair. “With a DSB, both helices of DNA break simultaneously. Homologous DNA is required to repair the break precisely - repair takes place through homologous recombination,” explains Hiroki Shibuya, an Assistant Professor at the University of Gothenburg. There are two forms of recombination, mitotic and meiotic; Professor Shibuya is investigating the latter in a research project part-funded by the European Research Council and the Swedish research council. “A DNA break is intentionally induced in meiosis by an activity of endonuclease. In meiotic recombination homologous chromosomes, with one from the mother and one from the father, are used as a template to repair the break,” he continues. DNA break The DNA break induced in meiosis is repaired using BRCA2, a cancer suppressor gene first identified in 1995 which has since attracted a lot of attention in research. This process is not just about repairing the DNA, but also connecting the homologous chromosomes through homologous recombination into what is called a crossover structure, also known as a Chiasma. “A crossover is a structure which connects homologous chromosomes together, and it’s essential for the generation of genetic diversity by mixing maternal and paternal chromosomes, and also for chromosome segregation in meiosis,” explains Professor Shibuya. Aberrations in this step lead to a chromosome mis-segregation in meiosis called aneuploidy, that can cause down’s syndrome or certain birth defects, underlining the wider relevance of Professor Shibuya’s research. “My research is focused on investigating the basic mechanisms involved in meiotic recombination. We are sure that our research will provide fundamental knowledge, which will help us to understand the causes of genetic abnormalities caused by meiotic
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Shibuya. Researchers are also carrying out in vivo analysis of protein function in mice. “We can produce genetically modified knockout mice using CRISPR-Cas9, which is a genome editing tool,” explains Professor Shibuya. “We can then analyse any defects by carefully observing the individual germ cells or even individual chromosomes in the knockout mice, which will tell us the function of each proteins.”
Novel proteins Meiotic chromosomes (green) decorated with the DSB repair proteins (red and blue).
errors, and will be useful for diagnosis or even treatment in the long run,” he outlines. Indeed, this research holds wider relevance in terms of understanding the root causes of certain other health problems. For example, a specific mutation of BRCA2 is known to leave an individual more susceptible to breast cancer or ovarian cancer. “This is because BRCA2 helps to repair DNA breaks through homologous recombination in somatic cells. So, if BRCA2 is mutated, then accidental DNA breaks are not repaired, predisposing individuals to cancer development,” explains Professor Shibuya.
By combining these different approaches, researchers have been able to gain new insights into both the process of meiosis and also meiotic recombination. One important finding arising from Professor Shibuya’s research is that BRCA2 forms a previously unknown ternary protein complex, BRCA2-MEILB2-BRME1, in meiosis. “We have identified these two novel proteins that function with BRCA2 in meiosis. We have looked at its function in germ cells by generating the gene knockout mice,” he outlines. Without MEILB2 or BRME1, the BRCA2 function is attenuated and meiotic DSBs are not repaired properly, leading to defects in recombination and crossover formation, which then cause male sterility. Certain links have also been identified
We will look deeper into the mechanism of those proteins and how they are involved in meiotic recombination. How do they impair mitotic recombination? How do they contribute to cancer development? Researchers have identified the key proteins involved in regulating BRCA2 during meiotic recombination, specifically MEILB2 (Meiotic localizer of BRCA2) and BRME1 (BRCA2 and MEILB2-associating protein 1), now Professor Shibuya and his colleagues are looking to probe deeper into the molecular mechanisms involved. A number of different techniques are being used in this research, including in vitro biochemistry, to build a deeper picture of how these proteins function. “We purify the protein, and then we analyse its activity and function in vitro,” says Professor
between aberrant expression of those genes and cancer development, and Professor Shibuya plans to continue his research in this area in future. “We will look deeper into the mechanism of those proteins and how they are involved in meiotic recombination. How do they impair mitotic recombination, when aberrantly expressed in somatic cells? How do they contribute to cancer development?” he says. A significant degree of progress has already been made in this respect. Researchers have found that MEILB2 and
EU Research
Q A D
r Hiroki Shibuya is a researcher in the Department of Chemistry and Molecular Biology at the University of Gothenburg. We spoke to him about what inspired him to pursue a career in science, his academic experiences, and his hopes for the future.
EU Researcher: When did you become interested in molecular biology? Was it at school?
HS: I’m from Japan originally, and I spent some time at Harvard before I moved to Sweden. The culture is of course a bit different, but the good points about the research environment in Sweden are that it’s very relaxed, and there is a lot of academic freedom for researchers. It’s easy to collaborate with researchers from neighbouring European countries, and that is an advantage.
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Dr Hiroki Shibuya: Right from an early age I
was always very interested in the natural world. I remember collecting fossils and observing insects, I was fascinated by the variety and beauty of the organisms that are around us. When I went to university I was keen to pursue these interests. Molecular biology is a highly innovative part of the biology field, it’s very exciting to dissect living things at DNA and protein levels, and look at the underlying molecular interactions.
EUR: Would you like to stay in academia in future and continue your research?
HS: Yes, my ultimate hope is to continue research in whatever direction I feel is interesting. I want to study things that I find interesting in nature, I’m motivated by curiosity.
EUR: Have you been able to form
networks and relationships with other researchers in different parts of Europe?
HS: I’ve met other PIs at various international conferences and that is the main way I have built research networks. For instance conferences are organised through EMBO, which is a good way to interact and form research networks.
Meiotic telomere Study of telomere function in germ cells, relevant to the regulations of homologous recombination and telomere length maintenance across generations Project Objectives
Breast cancer susceptibility gene 2 (BRCA2) was identified in 1995 as a potent cancer suppressor gene and has been the subject of intensive research over the past 25 years. Recently, Hiroki’s group identified novel BRCA2 partner proteins, MEILB2 and BRME1, which regulate BRCA2 function in normal germ cells and functions as a potential oncogene when aberrantly expressed in somatic cells.
Project Funding
European Research Council (ERC) Starting grant : € 1 500 000
Contact Details
Project Coordinator, Hiroki Shibuya Assistant Professor, Department of Chemistry and Molecular Biology, University of Gothenburg, Medicinaregatan 9E, SE-41390, GOTHENBURG, Sweden E: hiroki.shibuya@gu.se W: https://shibuyahiroki.com/research/ W: https://cmb.gu.se/english/about_us/staf f?languageId=100001&userId=xshibh
EUR: Is your university a good
environment to do that? Do you have lots of opportunities to collaborate with researchers at other institutes?
Professor Hiroki Shibuya
Dr. Shibuya fossil hunting in Gotland, Sweden, July 2020, and a Trilobite fossil he found.
BRME1, which normally function only in germ cells, are commonly upregulated in certain human cancers. Experimentally, researchers found that when MEILB2 or BRME1 was overexpressed in somatic culture cells, the function of mitotic homologous recombination was disturbed. “A number of sporadic cancers showed similar phenotypes seen in familial BRCA2 mutated cancers even without BRCA2 mutation. In such sporadic cancer cases, the overexpression of meiotic BRCA2 partner proteins can be a driving force for cancer development. In future this could be interesting as a route towards helping to diagnose these cancers at an earlier stage, or potentially as a way of identifying a new target of cancer therapy,” says Professor Shibuya. There are also several other avenues of research that Professor Shibuya plans to explore in future. “In order to faithfully transmit paternal and maternal chromosomes to the next generation, plenty of unique and
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sophisticated events happen specifically in meiosis, such as reorganization of telomerebinding proteins, formation of chromosome axis structure, synapsis/recombination of homologous chromosomes, and then reductional segregation of homologous chromosomes. The molecular mechanisms underlying these processes are not fully understood, especially in mammalian model systems. It is very exciting to try and clear up these mysteries by discovering the key regulatory genes,” he continues. Researchers can gain a more complete picture of the molecular function of BRCA2, MEILB2 and BRME1 through locally constructing the steps involved, yet there is no clear path towards identifying new regulators of mammalian meiosis. This depends not just on scientific expertise, but also to a degree on serendipity. “It’s a bit of a fishing expedition,” acknowledges Professor Shibuya. This will form an important part of Professor Shibuya’s agenda in future.
Hiroki Shibuya is an Assistant Professor in the Department of Chemistry and Molecular Biology, University of Gothenburg, Sweden. He obtained PhD at the University of Tokyo, Japan, in 2014. After PhD, he worked at Harvard Medical School, USA, as a Human Frontier Scientific Program Long-term Fellowship Postdoc
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