A new study by a team of TSRI scientists has found that too much of a key protein, called cyclin E, slows down DNA replication, introducing potentially harmful mutations contributing to cancers such as leukemia and breast cancer.
Steven Reed is a professor at The Scripps Research Institute.
Cells must copy their DNA before dividing into two identical daughter cells. Each round of copying and cell division comes with the risk of DNA replication errors – such as duplicating, deleting, or being out of sequence. Some of these errors can lead to cancer.
In normal cells, cyclin E binds to and activates an enzyme, Cdk2, which begins the DNA replication process. Cells need just the right amount of cyclin E to divide properly. However, some genetic mutations can cause too much cyclin E production in cells.
"Overexpression of cyclin E is one route to cancer," said TSRI Professor Steven Reed, senior author of the new study.
Dr. Reed and his TSRI colleagues, who originally discovered cyclin E, previously found that abnormally high levels of cyclin E are associated with chromosome instability, increasing the chances that a chromosome will acquire more mutations as it divides. Other research found that cyclin E is frequently overexpressed in cancer cells and that overexpression is linked to a decreased survival rate for breast cancer patients.
Before this most recent study, however, scientists did not know exactly how cyclin E introduces chromosome instability and errors into DNA. The team investigated the role of cyclin E by comparing normal human mammary cells with human mammary cells forced to overexpress cyclin E at the same levels seen in some breast cancer cells.
The researchers found that DNA replication took significantly longer in the cyclin E-deregulated cells. In fact, the cells seemed to enter the next stage of cell division before the DNA was even done replicating. A small number (16) of very specific regions on the chromosomes frequently failed to complete replication. Within those areas, the chromosomes of daughter cells stuck together.
"You could see a tug-of-war going on that would cause either the chromosome to tear or both chromosomes to go to one side," said Dr. Reed. After division, a third of the cyclin E-deregulated cells showed DNA deletions at the specific regions that tended to not finish replication prior to cell division. The researchers investigated how the genetic instability from these deletions could contribute to cancer. Interestingly, many of the sites with deletions were areas in which DNA was already known to be fragile or difficult to replicate.
Using a database of tumor DNA sequences, they found that 6 of the 16 DNA regions identified in their cell-based studies showed damage in breast tumors that could be directly linked to cyclin E overexpression. One such region matched with an area commonly rearranged in a type of leukemia called mixed lineage leukemia – where cyclin E had already been shown to be a contributing factor.
One unanswered question posed by this work is how cells are allowed to divide before all the chromosomes are completely replicated. It had been believed that "checkpoints" exist to prevent such accidents from happening, but Dr. Reed thinks these unreplicated regions are small enough to bypass the "checkpoints," allowing the cells to continue dividing and accumulating potentially harmful mutations.
The next step for Dr. Reed and his team will be sequencing the entire genomes of cells that undergo damage from cyclin E overexpression to understand exactly how the deletions contribute to cancer.