Investigating novel mechanisms of genetic stability at the replication fork

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The human body is made up of trillions of cells. To replenish old or dying cells, billions of new cells have to be generated each day by duplication and differentiation of progenitor cells. In order to do this, our cells have to carefully replicate their genetic material, the DNA, and correctly transmit it to their daughter cells. This challenging task is performed by several proteins, including helicases, that unwind DNA and a series of proteins called DNA polymerases that copy DNA from the parental strands. At the heart of this machinery is DNA Polymerase Epsilon (PolE) which is a component of the unwinding DNA helicase and also synthesizes part of the DNA. Dysfunction of DNA replication can lead to the accumulation of errors in the DNA and cause genetic diseases and cancer. Thus, understanding how PolE works and discovering the mechanism responsible for efficient and accurate DNA replication is essential for human health as well as for cancer prevention and therapy. Our body harbours more than 200 different cell types. The process that is responsible for the production of such a different repertoire of cells is called cellular differentiation and depends on the expression of specific sets of genes. Thus, despite all the cells of an individual share the same genetic material, only a subset of those genes are active in a cell. Our genes are endowed into a protein structure called chromatin, mainly composed of histone proteins, which can be modified to allow the expression of specific genes. Altogether, the repertoire of specific histones and DNA modifications that determine the functionality of our genes is known as the epigenome. Importantly, when cells duplicate their genetic material, the epigenetic information that is contained in histones and their modifications has also to be duplicated. Therefore, these two processes are irreversibly intertwined. How the machinery that replicates DNA couples these two processes remains poorly defined.During my post-doctoral studies, I discovered that two components of PolE, named POLE3 and POLE4, are important to promote the stability of the whole PolE protein complex and its functions during DNA replication. Strikingly, I also found that these proteins bind to histones during DNA replication, which suggests they might participate in the process of duplication of our epigenetic information. This exciting finding opened a series of research questions that I now aim to address in my laboratory. In particular, I want to understand how the whole PolE complex perform these two processes and their relative contributions to cellular function. In addition to this, recent studies have discovered that loss of POLE3 and POLE4 increases the response of cancer cells to some specific anti-cancer drugs. The reason for this increased sensitivity remains to be unveiled and its identification represents the other fundamental aim of my proposal. Indeed, these results while providing important clues in the mechanism of action of these drugs, will also determine the main functions of POLE3 and POLE4 and identify potential new therapeutic targets and markers predicting sensitivity to such drugs. In summary, answering these exciting new questions will significantly increase our understanding of essential processes required for genetic and epigenetic stability and will help identify effective cancer therapies.