Multiple cancer pathways regulate telomere protection

Telomeres are considered as universal anti-cancer targets, as telomere maintenance is essential to sustain indefinite cancer growth. Mutations in telomerase, the enzyme that maintains telomeres, are among the most frequently found in cancer. In addition, mutations in components of the telomere protective complex, or shelterin, are also found in familial and sporadic cancers. Most efforts to target telomeres have focused in telomerase inhibition; however, recent studies suggest that direct targeting of the shelterin complex could represent a more effective strategy. In particular, we recently showed that genetic deletion of the TRF1 essential shelterin protein impairs tumor growth in aggressive lung cancer and glioblastoma (GBM) mouse models by direct induction of telomere damage independently of telomere length. Here, we screen for TRF1 inhibitory drugs using a collection of FDA-approved drugs and drugs in clinical trials, which cover the majority of pathways included in the Reactome database. Among other targets, we find that inhibition of several kinases of the Ras pathway, including ERK and MEK, recapitulates the effects of Trf1 genetic deletion, including induction of telomeric DNA damage, telomere fragility, and inhibition of cancer stemness. We further show that both bRAF and ERK2 kinases phosphorylate TRF1 in vitro and that these modifications are essential for TRF1 location to telomeres in vivo Finally, we use these new TRF1 regulatory pathways as the basis to discover novel drug combinations based on TRF1 inhibition, with the goal of effectively blocking potential resistance to individual drugs in patient-derived glioblastoma xenograft models.We thank the Confocal Microscopy, Protein Engineering, Mass Spectrometry,Comparative Pathology, and Mouse Facility Units at CNIO. MAB laboratory is funded by SAF 2013-45111-R from MINECO,Fundación Botín and Banco Santander, Worldwide Cancer Research 16-1177. LB is a fellow of the La Caixa-Severo Ochoa International PhD Programme.S

Safety of Whole-Body Abrogation of the TRF1 Shelterin Protein in Wild-Type and Cancer-Prone Mouse Models

Telomeres are considered potential anti-cancer targets. Most studies have focused on telomerase inhibition, but this strategy has largely failed in clinical trials. Direct disruption of the shelterin complex through TRF1 inhibition can block tumorigenesis in cancer mouse models by a mechanism that involves DNA damage induction and reduction of proliferation and stemness. Any anti-cancer target, however, must fulfill the requisite of not showing deleterious effects in healthy tissues. Here, we show that Trf1 genetic deletion in wild-type and cancer-prone p53- and Ink4Arf-deficient mice does not affect organismal viability and only induces mild phenotypes like decreased body weight and hair graying or hair loss, the skin being the most affected tissue. Importantly, we found that Trf1 is essential for tumorigenesis in p53- and Ink4Arf-deficient mice, as we did not find a single tumor originating from Trf1-deleted cells. These findings indicate a therapeutic window for targeting Trf1 in cancer treatment.We thank R. Serrano for mice handling and the Comparative Pathology and Mouse Facility Units at CNIO. M.A.B. laboratory is funded by SAF2013-45111-R from MINECO,Fundación Botın, and Banco Santanderand Worldwide Cancer Research 16-1177. L.B. is a fellow of the La Caixa-Severo Ochoa International PhD Program.S