Mechanisms of Telomere Maintenance and Protection

Telomeres are repetitive DNA structures at the ends of eukaryotic chromosomes that prevent degradation and recognition of the ends by the DNA repair machinery. The chromosome ends resemble DNA double-strand breaks (DSBs) and, therefore, must be protected from unwanted repair that can lead to chromosome fusions and other forms of chromosome instability. To distinguish telomeres from other regions of the genome, telomeres are bound by protein complexes that shield the ends from the DNA damage response (DDR). The major telomere protection complex is called shelterin. Our preliminary data point to a novel role of another telomere-associated complex, CST (CTC1-STN1-TEN1), in telomere protection. CST is a single-stranded DNA binding protein that functions in telomere replication as well as DNA replication and repair. Mutations in CST cause the telomere biology disorders Coats plus and dyskeratosis congenita and CST dysfunction has also been associated with a variety of cancers. Yet, the cellular roles of CST and how they relate to human disease are still incompletely understood. Previous work demonstrated that loss of CST leads to hyperextension of the telomeric G-rich single- stranded DNA overhang, leading to the association of DNA repair and damage response factors. Surprisingly, despite the presence of these DDR factors, telomeres remain mostly protected in cells lacking CST, suggesting a novel mechanism of telomere protection. Preliminary data suggest that the DDR kinase ATR is necessary for this protection. The overall goal of this proposal is to delineate the consequences of CST deletion on telomere protection and genome stability and how CST deficiency leads to disease. Aim 1 will define the factors associated with telomeres in the absence of CST to identify how they remain protected. Aim 2 will identify whether CST disease mutations affect telomere protection and ATR signaling. This proposal will use a combination of well- established molecular and cellular assays that will allow us to delve into the mechanisms of telomere protection. Completion of these aims will significantly advance our understanding of chromosome end protection and how CST dysfunction promotes disease as well as provide important training experiences to undergraduate students.