(From the May 2016 Issue of Research Now)
Although there are many types of cancer, all cancers have one thing in common: they develop when normal processes in the body break down. Unlike normal cells, cancer cells ignore signals that normally tell cells to stop dividing, causing them to continue to grow and spread into surrounding tissues. The changes that contribute to cancer are genetic, meaning that they can be inherited from parents. They also might arise during a person’s lifetime because of errors or mutations in DNA that occur while the cells divide.
A team of researchers from the Center for Childhood Cancer and Blood Diseases in The Research Institute at Nationwide Children’s Hospital recently demonstrated how one of these normal processes goes awry in chromosomes, which are made of DNA and protein. The study, published in Cell Reports in April, provides insight on how a protein in the centromere region of chromosomes may contribute to cancer development.
“The presence of chromatin containing the histone H3 variant, CENP-A, dictates the location of the centromere in a DNA sequence-independent manner,” says Katsumi Kitagawa, PhD, principal investigator in the Center for Childhood Cancer and Blood Diseases, and senior author of the study. “But the mechanism by which centromere inheritance occurs is largely unknown.”
CENP-A is a protein that is required for locating the centromere region of the chromosome, but the activity of CENP-A is not related to the sequence of the DNA within the chromosome. One of the goals of this study was to understand how CENP-A may work, and to expand upon previous research that the team had conducted.
In earlier studies, researchers demonstrated that CENP-A goes through a process called ubiquitylation, in which another small protein called ubiquitin attaches to a lysine group on CENP-A. Ubiquitin is a protein found in almost all tissues of the body and can be specific to almost any protein, and lysine is an amino acid that makes up proteins. In the case of CENP-A, ubiquitin attached to lysine 124, abbreviated as K124.
“We previously reported that CENP-A K124 ubiquitylation, mediated by CUL4A-RBX1 COPS8 E3 ligase activity, is required for CENP-A deposition at the centromere,” explains Dr. Kitagawa, whose lab members co-authored the study. “Thus, we wanted to investigate the function of K124 ubiquitylation in the centromere inheritance mechanism.”
When even a single ubiquitin attaches to another protein, the function of that target protein can be altered, resulting in different activity within the cell. However, the process of ubiquitylation is still not fully understood and is therefore a significant focus of cancer research.
Researchers used various methods to model how CENP-A ubiquitylation occurs and to demonstrate how CENP-A determines the location of centromeres within the chromosomes. The methods ranged from immunofluorescence, a common laboratory technique that uses fluorescent labels to view processes of interest that are occurring within cells, to in vitro ubiquitylation assays, which allow detection of protein ubiquitination or ubiquitin modification on a protein of interest, such as CENP-A.
“We propose models in which CENP-A ubiquitylation is inherited, and, through dimerization, determines centromere location,” says Dr. Kitagawa of the study’s results. “Our models of epigenetic inheritance of CENP-A ubiquitylation suggest that errors in CENP-A targeting, heterodimerization, and/or ubiquitylation induce abnormal accumulation of heterotypic nucleosomes.”
Among the researchers’ findings was that the ubiquitin protein attaches to CENP-A, and this ubiquitylation causes CENP-A to determine centromere location when two CENP-A proteins come together to form a dimer, or two-part structure. Errors or problems in the process lead to a CENP-A “mutant” that does not function correctly, resulting in an abnormal buildup of nucleosomes, which are the basic units of DNA packaging. The changes are inheritable, meaning that they can be passed on, and they are epigenetic, meaning that they change the function of DNA or proteins without changing the original genetic sequence of the DNA.
“Our findings may provide a basis for potential insights into understanding the mechanisms of cancer development,” continues Dr. Kitagawa, who is also an associate professor in the Department of Pediatrics at The Ohio State University College of Medicine and in the College of Public Health at Ohio State. “We would like to investigate further the epigenetic mechanism of centromere inheritance.”
Niikura Y, Kitagawa R, Kitagawa K. CENP-A Ubiquitylation is Inherited Through Dimerization Between Cell Divisions. Cell Reports. 2016 Apr 5; 15(1):61-76. Epub 2016 Mar 24.
Niikura Y, Kitagawa R, Ogi H, Abdulle R, Pagala V, Kitagawa K. CENP-A K124 Ubiquitylation is Required for CENP-A Deposition at the Centromere. Developmental Cell. 2015 Mar 9; 32(5):589-603. Epub 2015 Feb 26.