XPO7 is a tumor suppressor regulating p21CIP1-dependent senescence.

Senescence is a key barrier to neoplastic transformation. To identify senescence regulators relevant to cancer, we screened a genome-wide shRNA library. Here, we describe exportin 7 (XPO7) as a novel regulator of senescence and validate its function in telomere-induced, replicative, and oncogene-induced senescence (OIS). XPO7 is a bidirectional transporter that regulates the nuclear-cytoplasmic shuttling of a broad range of substrates. Depletion of XPO7 results in reduced levels of TCF3 and an impaired induction of the cyclin-dependent kinase inhibitor p21CIP1 during OIS. Deletion of XPO7 correlates with poorer overall survival in several cancer types. Moreover, depletion of XPO7 alleviated OIS and increased tumor formation in a mouse model of liver cancer. Our results suggest that XPO7 is a novel tumor suppressor that regulates p21CIP1 expression to control senescence and tumorigenesis.

A preclinical mouse model of glioma with an alternative mechanism of telomere maintenance (ALT).

Glioblastoma multiforme is the most aggressive primary tumor of the central nervous system. Glioma stem cells (GSCs), a small population of tumor cells with stem-like properties, are supposedly responsible for glioblastoma multiforme relapse after current therapies. In approximately thirty percent of glioblastoma multiforme tumors, telomeres are not maintained by telomerase but through an alternative mechanism, termed alternative lengthening of telomere (ALT), suggesting potential interest in developing specific therapeutic strategies. However, no preclinical model of ALT glioma was available until the isolation of TG20 cells from a human ALT glioma. Herein, we show that TG20 cells exhibit a high level of telomeric recombination but a stable karyotype, indicating that their telomeres retain their protective function against chromosomal instability. TG20 cells possess all of the characteristic features of GSCs: the expression of neural stem cell markers, the generation of intracerebral tumors in NOD-SCID-IL2Rγ (NSG) mice as well as in nude mice, and the ability to sustain serial intracerebral transplantations without expressing telomerase, demonstrating the stability of the ALT phenotype in vivo. Furthermore, we also demonstrate that 360B, a G-quadruplex ligand of the pyridine derivative series that impairs telomere replication and mitotic progression in cancer cells, prevents the development of TG20 tumors. Together, our results show that intracerebral grafts of TG20 cells in immunodeficient mice constitute an efficient preclinical model of ALT glioblastoma multiforme and that G-quadruplex ligands are a potential therapy for this specific type of tumor.

TRF1 averts chromatin remodelling, recombination and replication dependent-break induced replication at mouse telomeres.

Telomeres are a significant challenge to DNA replication and are prone to replication stress and telomere fragility. The shelterin component TRF1 facilitates telomere replication but the molecular mechanism remains uncertain. By interrogating the proteomic composition of telomeres, we show that mouse telomeres lacking TRF1 undergo protein composition reorganisation associated with the recruitment of DNA damage response and chromatin remodellers. Surprisingly, mTRF1 suppresses the accumulation of promyelocytic leukemia (PML) protein, BRCA1 and the SMC5/6 complex at telomeres, which is associated with increased Homologous Recombination (HR) and TERRA transcription. We uncovered a previously unappreciated role for mTRF1 in the suppression of telomere recombination, dependent on SMC5 and also POLD3 dependent Break Induced Replication at telomeres. We propose that TRF1 facilitates S-phase telomeric DNA synthesis to prevent illegitimate mitotic DNA recombination and chromatin rearrangement.

Telomere regulation during ageing and tumorigenesis of the grey mouse lemur.

Telomere erosion leading to replicative senescence has been well documented in human and anthropoid primates, and provides a clue against tumorigenesis. In contrast, other mammals, such as laboratory mice, with short lifespan and low body weight mass have different telomere biology without replicative senescence. We analyzed telomere biology in the grey mouse lemur, a small prosimian model with a relative long lifespan currently used in ageing research. We report an average telomere length by telomere restriction fragment (TRF) among the longest reported so far for a primate species (25-30 kb), but without detectable overall telomere shortening with ageing on blood samples. However, we demonstrate using universal STELA (Single Telomere Length Amplification) the existence of short telomeres, the increase of which, while correlating with ageing might be related to another mechanism than replicative senescence. We also found a low stringency of telomerase restriction in tissues and an ease to immortalize fibroblasts in vitro upon spontaneous telomerase activation. Finally, we describe the first grey mouse lemur cancer cell line showing a dramatic telomere shortening and high telomerase activity associated with polyploidy. Our overall results suggest that telomere biology in grey mouse lemur is an exception among primates, with at best a physiologically limited replicative telomere ageing and closest to that observed in small rodents.