Missegregation linked to DNA Damage

Tumor formation may require fewer steps to get started than previously thought, according to a new study that shows how chromosome instability (CIN) and DNA damage—two tumorigenesis triggers typically considered independent phenomena—can arise from a single defect in how chromosomes segregate during cell division.

“This paper really provides a link between the mechanism behind CIN and the mechanism underlying chromosome damage,” said Dartmouth University biochemist Duane Compton, who was not involved in the research. Prior to this study, most researchers were not investigating how these two phenomena might be related, he added.

The results, published today (September 29) inScience, hint at new avenues for developing anti-cancer therapeutics that target chromosome missegregation as a central event in the development of both abnormal chromosome number and structural DNA damage as tumorogenesis precursors.

Lead author Aniek Janssen, graduate student at University Medical Center (UMC) Utrecht, in The Netherlands,  got the idea to study the relationship between CIN and DNA damage after her mentors, cell and molecular biologists Rene Medema and Geert Kops, noticed that most tumor cells exhibited both aneuploidy (abnormal chromosome number) and structural DNA damage. To divide normally, cells must ensure that their chromosomes properly segregate, with each daughter cell receiving a copy of the duplicated chromosomes. Aneuploidy results from missegregation of chromosomes—when one daughter cell receives two copies of a specific chromosome, and the other receives none. If a missing chromosome contained a gene encoding a protein important for DNA repair, chromosome loss might lead indirectly to DNA damage, explained Kops. In later divisions, a cell missing this protein would not be able to repair any broken DNA, which tends to accumulate with more replication and cell division. But Kops and Medema noticed that DNA damage sometimes occurred within a single division after chromosome missegregation, before the daughter cells would need to start synthesizing new DNA repair proteins, leading them to hypothesize that missegregation was leading directly to damaged DNA.

To cause aneuploidy in dividing human retinal pigment epithelial cells, Janssen treated them with a chemical that causes spindle fibers to connect a single centromere to both poles of the cell, so that the fibers wouldn’t properly split the paired chromosomes. Within hours after division occurred, real-time videos tracking fluorescent staining for DNA damage showed a missegregation event correlated with higher levels of DNA damage.

“The strongest evidence for associating CIN with DNA damage is the live imaging,” said Compton. Janssen and her colleagues also showed that double-stranded DNA breaks occurred, suggesting that the chromosomes were breaking into smaller pieces.

Blocking cytokinesis, the physical division of cells where the membrane pinches in and daughter cells separate, reduced the occurrence of the DNA damage, suggesting that the damage occurs early in the process, Kops said. Though how this leads to DNA breaks is unclear, the damage resulted in structural aberrations including chromosomal translocation, a genetic abnormality often seen in cancer cells.

DNA damage is classically thought of as being the result of mechanisms other than missegregation, said Medema. A defect in DNA repair genes would lead to improper mending of breaks, leading to mutations that might result in cancer.

“But our study shows that cells can get translocations just by being chromosome unstable,” said Medema. The pieces of broken chromosomes could segregate into daughter cells, and be grafted onto a different chromosome via normal repair mechanisms—resulting in a translocation even with adequate DNA repair machinery.

The study’s results should prompt reevaluation of the fields understanding of how the chromosomal aberrations that underlie tumorigenesis occur, said Medema. He explained that the work, which was supported by Top Institute Pharma, and included partners at Merck Sharp & Dohme (MSD), the Utrecht University Medical Center, and The Netherlands Cancer Institute-Antoni van Leeuwenhoek Hospital, could inform strategies for designing new therapies. Enhancing genomic damage to induce cell death is one therapy currently being pursued, and this study suggests that it might be achievable by targeting the mechanism behind missegregation alone.