Dr. de Lange's lab studies telomeres, protective elements at the ends of chromosomes critical for the stability and maintenance of the genetic information. Deficiency in telomere function can cause genomic alterations found in cancer, and the gradual loss of telomeres contributes to aging of human cells. Dr. de Lange seeks to understand how telomere protection is established and what happens when telomere function is lost during the early stages of tumor formation.
Dr. de Lange’s lab focuses on mammalian telomeres, which are made up of long arrays of double-stranded TTAGGG repeats that end in a single-stranded 3′ overhang. These telomeric repeats wither away in a shortening process that is associated with cell proliferation. Telomerase can counteract this attrition and stabilizes telomeres by adding back telomeric repeats. However, telomerase is absent from most human somatic cells, and, as a result, cells eventually die due to depletion of their telomere reserve. Cancer cells, on the other hand, usually reactivate telomerase, thereby achieving unlimited proliferative potential. The goal of Dr. de Lange’s research is to understand how telomeres protect chromosome ends, and what happens when telomere function is lost during the early stages of tumorigenesis before telomerase is activated.
The de Lange lab identified a six-subunit protein complex, which they named shelterin, that specifically binds to telomeres. Using Cre-mediated conditional deletion in mouse embryo fibroblasts, Dr. de Lange and her colleagues determined the fate of telomeres lacking one or more of the six shelterin subunits. This work showed that cells lacking shelterin perceive their natural chromosome ends as sites of DNA damage.
Six distinct DNA damage response pathways are repressed by shelterin. Two DNA damage signaling pathways, initiated by the ATM and ATR checkpoint kinases, inappropriately respond to chromosome ends that lack shelterin. In addition, shelterin represses three DNA double-strand break repair pathways at telomeres, two types of nonhomologous end joining and homology-directed repair, which have detrimental outcomes at telomeres. Finally, shelterin protects telomeres against a sixth threat: the inappropriate resection of the telomeric DNA by nucleases.
Dr. de Lange’s group is now working to determine the mechanism by which each shelterin protein inhibits its designated pathway, and how loss of telomere protection contributes to genome instability in human cancer. A major mechanistic insight arose from the identification of the t-loop structure of telomeres in which the single-stranded overhang is inserted in the double-stranded repeat array of the telomere, thereby hiding the telomere end from the DNA damage response. Recent data showed that the TRF2 component of shelterin is required to establish and/or maintain this structure. Since TRF2 is responsible for the repression of the ATM kinase pathway and non-homologous end joining, it is likely that the t-loop structure is critical to prevent these two pathways from acting inappropriately on chromosome ends.