Thesis defense Ana Bolhaqueiro: On the origins of chromosomal instability and karyotype heterogeneity in colorectal cancer

Mistakes in cell division
Healthy human cells contain 23 pairs of chromosomes, thus 46 chromosomes in total, that collectively form the human genome. During cell division these chromosomes need to be duplicated and segregated correctly to the daughter cells. When mistakes occur during the process of chromosome segregation this can result in aneuploidy, an incorrect chromosome content in the cell, where one of the daughter cells will have too many chromosomes and the other will have too few. Chromosomal instability is the frequency by which cells make mistakes in chromosome segregation.

Heterogeneity
Aneuploidy is a hallmark of cancer, including colorectal cancer. However, it was unclear until now whether human tumors are chromosomally unstable as well, and therefore continue to make mistakes. Chromosomal instability increases the tumor karyotype heterogeneity: the karyotype, or chromosome content, differs between daughter cells after cell division. These differences affect treatment response of the tumor: some cells may respond to a certain treatment, while other, different, cells may not. This increases the risk of the cancer coming back after treatment. Therefore, learning more about chromosomal instability and its relationship with heterogeneity in tumors is of value for the development of cancer treatments in the future.

Looking at cancer cells
During her PhD, Bolhaqueiro developed a protocol to measure chromosomal instability by looking at the process of chromosome segregation during cell division. “If you only look at chromosome numbers at a certain time point, which is what has been done until now, you miss valuable information that tells you whether the chromosome segregation mistakes are still ongoing in the tumor,” says Bolhaqueiro. Because imaging cells in a living human tumor is not possible, she used organoids, mini-organs grown from a piece of tissue from a patient that resembles the original tissue very closely. By placing the organoids under a microscope, live segregation of chromosomes during cell division can be visualized, and evaluated to determine whether cell division happened correctly or incorrectly (Figure).

Colorectal tumor organoids
By studying chromosome segregation in tumor organoids of colorectal cancer, Bolhaqueiro found that the vast majority of colorectal tumor organoids makes many more mistakes in chromosome segregation than organoids from a healthy part of the colon. Many different cellular defects turn out to contribute to chromosomal instability in colorectal tumor organoids. She also found that tumors with higher chromosomal instability levels usually had more cell-to-cell genomic variability and that not all tumors tolerate mistakes in chromosome segregation equally well. These characteristics may influence the tumors response to treatment. During her PhD,Bolhaqueiro has however not found a relationship between chromosomal instability levels and the response of the tumor to certain treatments and this will require further investigation.