The impact of telomere and telomerase dysfunction on human cancer
The upregulation of telomerase reverse transcriptase (TERT), the protein component of the enzyme that replenishes telomeres, is a key driver mutation in melanoma, glioblastoma and many other types of human malignancies. Indeed, promoter mutations in Tert are more prevalent than p53 mutations. Work from our lab and others established that telomerase function is necessary for the long-term survival of human cancer cells (Zhang et al., Genes Dev 1999; Taboski et al., Cell Rep 2012), and there is intense interest in the development of telomerase inhibitors as a treatment for cancer. At present, there are no telomerase inhibitors approved for clinical use, and there is a dearth of information about the potential emergence of drug resistance to telomerase inhibitors. We have used biochemical, genetic and chemical means to investigate the structural and functional mechanisms by which telomerase promotes survival of human cancers, and to study the possibility of resistance to telomerase inhibition.
Our lab has used yeast and human cell models to biochemically characterize small molecule telomerase inhibitors (Wong et al., Chem Biol 2013). A current and complementary thrust in the lab is to understand how drug resistance to telomerase may develop in human cancer. The telomerase reverse transcriptase (RT) has structural similarity to HIV RT and thus may acquire mutations that permit drug resistance. We undertook systematic mutagenesis of TERT, and a CRISPR/Cas9 base- editing approach to introduce mutations throughout the TERT gene in a blood cancer line. In follow-up biochemical analysis of purified, in vitro-produced human telomerase we have identified possible drug-resistant alleles of TERT and are testing for their ability to resist inhibition in human cell-based models (Borges and Harrington, unpublished). We are also carrying out structural and biochemical characterization of these resistant TERT variants, to understand how they affect the catalytic properties of TERT.
Another important aspect of this research problem is the need to understand the genetic landscape of cancer cells that permit the emergence of drug resistance. Most cancer treatment strategies involving telomerase inhibition are hindered by a “therapeutic lag”, i.e. a delay in the onset of cell death until telomeres are critically eroded (Taboski et al., Cell Rep 2012). Using CRISPR/Cas9 screening technology, we characterized the genetic landscape of genes that are essential for survival in the presence of the telomerase inhibitor BIBR1532. These screens identified several new candidate genes whose deletion sensitizes cancer cells to telomerase inhibition, including TAPR1 (telomere attrition and p53 response protein 1) (Benslimane et al., Aging Cell 2021). Our data suggests that TAPR1 may act through the E3 ubiquitin ligase HUWE1 to attentuate p53 activation via increased proteolysis in response to stressors included eroded telomeres. We are further characterizing the biochemical activities of TAPR1 and other new targets identified in this screen. It is possible that inhibition of these gene targets might reduce the therapeutic lag period incurred upon telomerase inhibition (particularly in cells with longer average telomere lengths), which could have therapeutic benefit in the treatment of cancer.