In a new study, researchers from the Johns Hopkins University School of Medicine used human breast and lung cells to map a molecular pathway that tricks the cells into taking a dangerous path. That is, making too many copies of their genome, which is a characteristic of cancer cells. The discovery sheds light on what can go wrong when a group of molecules and enzymes trigger and regulate what's called the cell cycle. They believe their findings could be used to develop therapies that interrupt cell cycle disorders and potentially halt cancer growth. Relevant research results were recently published in the journal Science. The paper is titled "CDK4/6 activity is required during G2 arrest to prevent stress-induced endoreplication".
To replicate, a cell follows an orderly process, first copying the entire genome, then separating the genome copies, and finally dividing the copied DNA equally between the two "daughter" cells.
There are 23 pairs of chromosomes in human cells - of each pair, half comes from the mother and half comes from the father, including the sex chromosomes X and Y. But cancer cells are known to go through an intermediate state in which they have twice the number of chromosomes, 92. How this happened remains a mystery.
"A perennial question for scientists in the cancer field is: How did cancer cell genomes become so bad? Our study challenges fundamental knowledge about the cell cycle and makes us reevaluate our ideas about how the cell cycle is regulated." Dr. Sergi Regot, corresponding author of the paper and associate professor of molecular biology and genetics at Johns Hopkins University School of Medicine, said, "Cells that are stressed after copying their genome will enter a dormant or senescent stage and mistakenly risk copying their genome again."
Typically, these dormant cells are eventually cleared by the immune system after being "recognized" as problematic. But sometimes, especially as we age, the immune system is unable to clear these cells. If these abnormal cells are allowed to wander around the body, they copy the genome again, rearranging the chromosomes the next time they divide, and cancer begins to grow.
Figure 1. Cellular stress drives G2 cell cycle exit and WGD.
To determine the details of the molecular pathways that go wrong during the cell cycle, Regot, graduate research assistant Connor McKenney, and their team focused on human cells in the lining of breast ducts and lung tissue. The reason is that these cells often divide faster than other cells in the body, increasing the opportunity to visualize their cell cycle. Regot's lab specializes in imaging single cells, so it is particularly well-suited to discovering the very few cells that do not enter a dormant period and continue to replicate their genomes.
In the new study, the authors looked closely at thousands of images of single cells during cell division. They developed luminescent biosensors to tag cellular enzymes called cyclin-dependent kinases (CDKs).
They found that multiple CDKs are activated at different stages of the cell cycle. They found that the activity of CDK4 and CDK6 was reduced after the cells were exposed to environmental stressors, such as drugs that interfere with protein production, ultraviolet radiation, or so-called osmotic stress. Five or six hours later, when the cells began to prepare to divide, CDK2 was also inhibited. At this point, a protein complex called the anaphase-promoting complex (APC) is activated at a stage before the cell separates and divides (a step called mitosis).
"In the stress environment of the study, APC was activated before mitosis, whereas normally it is only known to be activated during mitosis," Regot said.
When exposed to any environmental stressor, approximately 90% of breast and lung cells exit the cell cycle and enter a quiescent state. In their experimental cells, not all cells entered a quiescent state. They found that about 5% to 10% of breast and lung cells returned to the cell cycle and divided chromosomes again.
Through another series of experiments, the authors found that an increase in the activity of so-called stress-activated protein kinases is associated with a small proportion of cells emerging from the quiescent phase and continuing to double their genomes.
Researchers are currently conducting clinical trials testing DNA-damaging agents and drugs that block CDK. The combination of drugs may prompt some cancer cells to copy their genome twice, creating heterogeneity and eventually drug resistance. In addition, there may be drugs that can prevent APC from being activated before mitosis, thus preventing cancer cells from making two genome copies and preventing tumor stage progression.
Reference
McKenney C, et al. CDK4/6 activity is required during G2 arrest to prevent stress-induced endoreplication. Science, 2024, 384(6695): eadi2421.