Zombie Cells Survive and Divide Amidst Heavy Mutations

    Scientists at the University of Southern California say they have developed a yeast model to study a gene mutation that disrupts the duplication of DNA, causing massive damage to a cell’s chromosomes, while somehow allowing the cell to continue dividing.

    The result is a group of cells that by all rights shouldn’t be able to survive, let alone divide, with their chromosomes shattered and strung out between tiny micronuclei. Sometimes they’re connected to each other by ultrafine DNA bridges. Frequently, the micronuclei, which are thought to retain the most damaged portions of the DNA, rejoin the parent nuclei and incorporate mutations into the survivors.

    The mutation has been associated with cancer in mice, and micronuclei are often found in human cancer cells. With their new yeast model, researchers hope to learn more about each.

    “Using a simple yeast system, we have developed a powerful genetic model to investigate a recently identified characteristic of human cancer cells,” said Susan Forsburg, Ph.D., senior author of an article (“Replication stress in early S phase generates apparent micronuclei and chromosome rearrangement in fission yeast”) about the research published in  Molecular Biology of the Cell. “This will enable us to rapidly identify genes responsible for this abnormal division.”

    Since the genes that regulate division in human and yeast cells are the same, this simple organism provides a tool for human cell discovery, said Dr. Forsburg.

    DNA is vulnerable to damage when it’s unzipped into two single strands for replication by a cell’s MCM helicase. Typically, the single stranded DNA triggers repair of damage by special enzymes or in extreme cases, drives the damaged cell to suicide. Either way, mitosis is halted while the issue is dealt with.

    But in cancer cells, despite the damaged DNA, the cells continue to divide, creating tumors full of genetic mutations. In this study, a mutation in the yeast’s MCM helicase triggered responses similar to those in mammals where mutations in this gene are associated with cancer and the formation of micronuclei.

    To study the phenomenon, Dr. Forsburg and lead author Sarah Sabatinos, Ph.D., collected videos of the damaged cells dividing so that they could maintain continuous monitoring of individual cells, and record cell division from beginning to end. They watched what happened in the mutant in real time. Then, they used a super-resolution microscope at USC that generates 3D images of objects at the nanometer scale, to examine the damage structures in crisp detail.

    “The devil’s in the division. In real time, we’re able to see that these mutant cells ignore the damage caused during DNA replication, which results in the creation of unusual structures like micronuclei,” explained Dr. Sabatinos, who conducted the research as a post-doc at USC, and is now an assistant professor at Ryerson University in Toronto.

    The researchers believe their work will inform future studies into how a cancer cell evades biological checkpoints that should halt its division and spread.