In the early years of molecular biology research, scientists studying chromosomal structure and composition noticed that the terminal ends of chromosomes, called telomeres, would gradually become shorter with each successive round of cellular replication. This process would continue until the chromosome reached a certain length, ultimately becoming unstable and causing the cell to die. Conversely, the scientists noticed that for certain genetic disorders, such as cancer, an abnormally long telomere length led to genome anomalies that were closely associated with the cancer phenotype.
In 1984, researchers Elizabeth Blackburn, Ph.D., and Carol Greider, Ph.D., who was at the time a graduate student in Dr. Blackburn’s laboratory, discovered the telomerase enzyme, which was responsible for maintaining the appropriate length of telomerase after chromosomal replication. Drs. Blackburn and Greider would go on to be awarded the 2009 Nobel Prize in Physiology and Medicine, along with Jack Szostak, Ph.D. for their work on molecular mechanisms of the telomerase enzyme.
Yet, even during their seminal work, the investigators quickly realized that other molecules besides telomerase must be involved in maintaining the protective caps at the end of chromosomes. Now, researchers at Johns Hopkins report uncovering the role of an additional enzyme crucial to telomere length and say the novel method they could be used to speed discovery of other proteins and processes that are involved in telomere stability.
“We've known for a long time that telomerase doesn't tell the whole story of why chromosomes' telomeres are a given length, but with the tools we had, it was difficult to figure out which proteins were responsible for getting telomerase to do its work,” explained Dr. Greider, professor, and director of molecular biology and genetics in the Johns Hopkins Institute for Basic Biomedical Sciences.
The findings from this study were published recently in Cell Reports through an article entitled “ATM Kinase Is Required for Telomere Elongation in Mouse and Human Cells.”
Understanding the mechanisms that are needed to lengthen telomeres has broad health implications, since shortened telomeres have been implicated in aging and diseases as diverse as lung and bone marrow disorders, while overly long telomeres are linked to cancer. Cells need a well-tuned process to keep adding the right number of building blocks back onto telomeres over an organism's lifetime.
Unfortunately, until recently, the methods researchers used to study telomere length were extremely time-consuming, often taking months of work to study cells grown in vitro, searching for detectable differences in telomere length. However, Dr. Greider’s team developed a new tool for measuring telomere length in yeast. The idea was to artificially cut mammalian cells' telomeres and then detect elongation by telomerase—a test that would take less than a day, and could be performed even if the blocked proteins were needed for cells to divide.
The new test, dubbed addition of de novo initiated telomeres (ADDIT) was used to observe an enzyme long suspected to be involved in telomere maintenance, ATM kinase. “ATM kinase was known to be involved in DNA repair, but there were conflicting reports about whether it had a role in telomere lengthening,” noted Dr. Greider.
The Hopkins researchers blocked the enzyme in lab-grown mouse cells and used ADDIT to find that it was indeed needed to lengthen telomeres. They confirmed their result by using the old, three-month-long telomere test, which lead to the same outcome.
Additionally, the team also found that in normal mouse cells, a drug that blocks an enzyme called PARP1 would activate ATM kinase and spur telomere lengthening. This finding has the potential to impact drug-based telomere elongation for treating short-telomere diseases, such as bone marrow failure.
Dr. Greider and her team were excited by their findings and plan to use ADDIT to find out more about the telomere-lengthening biochemical pathway that ATM kinase participates.
“The potential applications are very exciting,” stated lead author Stella Lee, Ph.D., postdoctoral fellow in Dr. Greider’s laboratory. “Ultimately ADDIT can help us understand how cells strike a balance between aging and the uncontrolled cell growth of cancer, which is very intriguing.”