The telomere biology disorder (TBD) dyskeratosis congenita (DC) and its severe variants are childhood onset, multisystem disorders caused by impaired maintenance of the ends of chromosomes known as telomeres. Bone marrow failure (BMF) is a major threat to life, occurring in 50-80% of affected individuals by age 30. The long- term objectives of this project are to elucidate the mechanisms by which mutations in the genes associated with the TBDs lead to telomere shortening and to discover how this shortening can be slowed or, better yet, reversed. With this knowledge, we will meet our long-term goal of identifying novel treatment avenues that target telomeres for these life-threatening disorders. To date, 15 genes have been associated with the TBDs, a number that poses a challenge to developing general therapeutic targets. Mutation of the TINF2 gene, which encodes the TIN2 protein, is the second most common cause of DC in children. How mutations in TINF2, which cluster, lead to marked telomere shortening is ill-defined. Our preliminary data suggest the TIN2-DC mutant protein has a new or more robust function than the normal protein, however, many critical gaps in knowledge remain as to the molecular aspects of this gain-of-function. In addition to the complex genetics, a second major challenge in the field is the lack of preclinical model systems to develop and test interventions. Through a collaboration established in 2018, the Bertuch and Hockemeyer labs have developed pluripotent and hematopoietic stem cell culture disease models as well as a humanized mouse model that reproduce the telomere shortening associated with TINF2-DC mutations. Here we propose to use these unique stem cell-based systems to examine the impact of TINF2-DC mutations on the maintenance of telomere length and hematopoiesis, and test therapeutic approaches to treat the associated-BMF. Overall, we will undertake two complementary Aims: Aim 1 will investigate the molecular determinants by which TIN2-DC mutant protein leads to short telomeres, such as how it interacts with the telomere shelterin complex proteins TRF1 and TRF2, and telomeres. We will determine if TIN2-DC mutant protein’s toxic effect on telomere length requires interaction with TRF1 and the role of TIN2- TRF2 interaction in the telomere shortening it induces. Lastly, we will determine if critical binding events are altered by mutation of DC cluster region. In Aim 2, we will determine pharmacologic and genetic mechanisms to restore telomere length and rescue the impaired fitness of TINF2-DC mutant cells. We will establish the gene expression signature of telomere shortening in our stem cell models to derive insight into the pathways impairing fitness. We will test the effect of danazol, which is a treatment for DC BMF, to assess the relationships between impacts on telomere length, TERT, and hematopoietic potential. Lastly, we will develop a genetic strategy to elongate telomeres in TINF2-DC mutant patient hemato...