In mammals, the so-called “germinal” genes are only expressed in the cells brought about to become gametes, ovules or spermatozoa. In all other cells, a chemical modification of DNA – the methylation – keeps those genes quiet. A team from Strasbourg has looked into the details of this phenomenon to discover the proteins involved. Ultimately, this work could benefit cancer research because germinal genes are abnormally reactivated in certain tumours.
How is germline gene expression repressed in somatic cells of the organism, that is to say in all the cells which are not destined to become gametes? To answer this question, the team of Michaël Weber, researcher Inserm at the University of Strasbourg, dissects the course of an epigenetic mechanism involved in this phenomenon: DNA methylation. This process is fundamental since the proteins coded by the germinal genes lead to the formation of cells amputated from half of their genetic material. And if this is what is expected during gametogenesis, the result can be disastrous in other circumstances: ” The inadvertent expression of germline genes in cells that are not destined to form gametes can lead to genetic imbalance. This is also what is observed in a certain number of cancers where the reactivation of germinal genes in tumor cells can promote the progression of the disease. By studying this mechanism, we want to understand it at the molecular level. Ultimately, we hope to contribute to solutions to combat this dysfunction. », clarifies Michaël Weber.
DNA methylation consists of adding “methyl” groups (CH3: one carbon atom and three hydrogen) to target regions of the genome. This chemical modification blocks the expression of the genes concerned. The role of this mechanism in silencing germline genes in somatic cells is well documented, but scientists don’t know exactly how it all works and don’t know the identity of the proteins involved. To fill this gap, the Strasbourg team inactivated each somatic cell gene one by one: “ This sieve genomics was performed using the CRISPR-Cas9 technique. It allowed us to test the involvement of each of the 20,000 genes in the genome that code for a protein. Previously, the cells had been modified to emit a fluorescent signal in the event of reactivation of a germinal gene “, explains Michaël Weber. Thus, by selecting the fluorescent cells at the end of the experiment, the researchers were able to identify the proteins whose presence is essential for the repression of germline genes. And the stronger the fluorescent signal, the more the missing protein a priori played a major role in this mechanism.
Finally, about thirty proteins were identified. ” This first stage of our work has made it possible to list them, but we must now study them to understand how they operate. “, continues the researcher. Some were already known for their role in DNA methylation. But others have appeared, like the proteins encoded by the genes Usp7, Shfm1 or Erh. In particular, the team took a closer look at Usp7. The analysis of its functions, co-directed by Sylvain Daujat, also an Inserm researcher in Michaël Weber’s team, reveals that it stabilizes a protein complex (PRC1.6) necessary for the repression of germinal genes, and promotes methylation of these. The study of the other identified proteins is continuing. ” Our goal is to understand the cascade of epigenetic events that ensure the initiation and permanent maintenance of germinal gene methylation, from early embryonic development until the end of life. », sums up Michaël Weber.
Michaël Weber leads the Epigenetic regulation of cellular identity teamin the Biotechnology and cell signaling (CNRS/University of Strasbourg), in Illkirch.
Source : H. Al Adhami H et coll. Systematic identification of factors involved in the silencing of germline genes in mouse embryonic stem cells. Nucleic Acids Res, 11 février 2023 ; doi : 10.1093/nar/gkad071