UC San Diego

The ends of linear chromosomes are composed of distinctive structures that maintain chromosome stability and cellular viability. Both sequence and structure of telomeres are important to their function. They are composed of TG-rich, repetitive, non-coding sequences and present unique challenges for genomes. A balance of both shortening and lengthening pathways maintains telomere length homeostasis. Telomere length dysregulation has been linked to human aging, cancer, and disease (telomeropathies), so studying the details of how this regulation occurs is vital to our understanding of telomere biology and human health.

Telomeres have been proposed to serve two distinct functions; end protection and end replication. As end protection factors, they help distinguish the natural ends of linear chromosomes from double stranded breaks of DNA that need to be repaired by DNA repair mechanisms and prevent end-to-end fusions. Telomeric DNA is lost at every round of replication due to incomplete replication. This loss is compensated by de novo synthesis of telomeric repeats by the enzyme telomerase. A lot has been learned about the factors implicated in both these functions and how they contribute to telomere length regulation.

In the budding yeast, Saccharomyces cerevisiae, a trimeric complex composed of Cdc13-Stn1-Ten was initially proposed to be the main players in telomere “capping” and serving an end-protection function. More recently, this complex has been shown to have homology to the RPA complex and function in the efficient replication of duplex telomeric DNA, and so was aptly named t-RPA. This discovery led to multiple questions about the role of replication at telomeres and how that contributes to telomere length regulation.

This dissertation describes my efforts in identifying a new model for telomere length regulation, which revolves around the significant effects of efficient replication to telomere length homeostasis. Errors that lead to replication fork collapse within the genome can have detrimental and even lethal results for the cell. At chromosome ends, that are recognizably difficult to replicate regions of the genome, replication fork collapse has been exploited by the cell to regulate telomere length. By following the events that occur immediately after a replication fork collapse, I found that collapsed forks at telomeres are recognizes by telomerase with high efficiency and subject to extensive elongation. Moreover, this process is under genetic control. Strikingly, when I studied the effects of known telomere length regulators, I found that they were modulating telomere length through their effects on replication fork collapse. This illustrated the previously unappreciated role of replication, and supported a new model for telomere length homeostasis that is driven by replication fork collapse and the subsequent response by telomerase.