Genomic Health & Safety: Does Elg1 maintain genome stability by resetting chromatin factors used in DNA replication and repair?

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The preservation of correct and accurate information is critical in almost all aspects of life. The information that enables life is encoded in DNA, which is folded up with proteins inside cells to form chromosomes. The failure to maintain chromosomes leads to the loss of or rewriting of important information, which can cause cancer and other diseases. The protein called Elg1 is a cellular factor that is very important in maintaining chromosomes. Defects in this one protein cause tumors in humans and mice, but we do not know why. The aim of this work is to understand how the Elg1 protein maintains chromosome stability so that we may exploit these findings to develop new cancer therapies.Whenever a cell divides to produce two new cells, the genetic information in the chromosomes must be duplicated precisely. This duplication process is called DNA replication. In most cases, cells achieve precise DNA replication without any mistakes. Hundreds of proteins in cells contribute to such precise DNA replication. If such 'chromosome stability' proteins cannot carry out their proper functions, cell cannot duplicate genetic information precisely, resulting in loss of or rewriting of important information, and leading eventually to cancer, genetic disorders, and ageing. A protein called Elg1 is one of these chromosome stability proteins. Recently I discovered that the molecular function of Elg1 is removal of a sliding clamp called PCNA from DNA during DNA replication. PCNA is ring-shaped and encircles DNA. PCNA stabilises the machinery that copies DNA, and additionally the PCNA ring acts like a tool-belt, recruiting many other collaborating proteins that are important for chromosome maintenance and precise DNA replication. During DNA replication, the PCNA tool-belt is repeatedly loaded on DNA, and repeatedly removed from DNA by Elg1 after each section of the DNA replication task is complete. In cells lacking Elg1, the PCNA tool-belt and its tools (i.e. its collaborating proteins) accumulate on chromosomes since the PCNA tool-belt is not removed even after completion of each new DNA section. This abnormal accumulation of the PCNA tool-belt resembles a construction worker with ten tool-belts around his body, each containing several unnecessary tools. Workers carrying ten tool-belts and lots of unnecessary tools may be not able to move their bodies flexibly, respond effectively to unexpected events, or use tools efficiently-and so will be more liable to make mistakes. In chromosome stablity, even occasional mistakes may cause catastrophe. My hypothesis is that the aberrant accumulation on DNA of the PCNA tool-belt and its associated tools causes loss of or rewriting of genetic information during chromosome duplication, ultimately resulting in cancer. I aim to understand why PCNA removal by Elg1 is important for chromosome maintenance, and whether its collaborating proteins are involved. First, by manipulating the PCNA tool-belt, I will test whether unwanted accumulation of the PCNA tool-belt interferes directly with chromosome maintenance. Next I will test whether the accumulation of particular unnecessary tools (i.e. unwanted retention of the collaborating proteins) contributes to chromosome instability. These experiments will be carried out using the baker's yeast system, which allows sophisticated molecular genetic approaches to be used for careful dissection of the chromosome stability machinery. In the third part, I am keen to extend this investigation to test the role of Elg1 in human cells, since loss of Elg1 is directly implicated in mammalian tumors. Since this project studies the mechanism of action of gene that is associated with cancer, this work holds long-term potential for cancer therapy.