Corinne Augé-Gouillou

Background/Objectives: Glioblastoma remains the most aggressive and lethal form of brain cancer, with no effective cure to date. The molecular mechanisms sustaining its development and relentless proliferation are still not fully understood. SETMAR, a protein lysine methyltransferase involved in various DNA repair and chromatin processes, has been reported as dysregulated in several cancers, including glioblastoma. Interestingly, S-SETMAR, a shorter isoform of SETMAR, has been suggested to antagonize the oncogenic properties of the full-length protein. Here, we explored the cellular and molecular consequences of S-SETMAR overexpression in glioblastoma cells. Methods: We compared native glioblastoma cells (8MGBA) with a recombinant 8MGBA line stably over-expressing αS-SETMAR, a stable form of S-SETMAR, using complementary cellular and molecular approaches. Results: Overexpression of αS-SETMAR markedly prolonged the cell cycle duration (from 27 to 37 h), leading to a significant decrease in cell proliferation. Unexpectedly, αS-SETMAR triggered genomic alterations characterized by an increased DNA content and extensive chromosomal instability, including aneuploidy, chromoanasynthesis-like rearrangements, and tripolar mitoses. Moreover, αS-SETMAR-expressing cells displayed heightened sensitivity to stress conditions mimicking chemotherapy and radiotherapy, resulting in increased apoptosis. Conclusions: Our findings identify αS-SETMAR as a dual modulator of glioblastoma cell fate—simultaneously slowing proliferation and promoting chromosomal instability while enhancing vulnerability to genotoxic stress. These results suggest that αS-SETMAR could serve as both a prognostic marker and a potential therapeutic tool in glioblastoma management.

Background/Objectives: Glioblastoma remains the most aggressive and lethal form of brain cancer, with no effective cure to date. The molecular mechanisms sustaining its development and relentless proliferation are still not fully understood. SETMAR, a protein lysine methyltransferase involved in various DNA repair and chromatin processes, has been reported as dysregulated in several cancers, including glioblastoma. Interestingly, S-SETMAR, a shorter isoform of SETMAR, has been suggested to antagonize the oncogenic properties of the full-length protein. Here, we explored the cellular and molecular consequences of S SETMAR overexpression in glioblastoma cells. Methods: We compared native glioblastoma cells (8MGBA) with a recombinant 8MGBA line stably overexpressing αS SETMAR, a stable form of S-SETMAR, using complementary cellular and molecular approaches. Results: Overexpression of αS-SETMAR markedly prolonged the cell cycle duration (from 27 to 37 hours), leading to a significant decrease in cell proliferation. Unexpectedly, αS-SETMAR triggered genomic alterations characterized by an increased DNA content and extensive chromosomal instability, including aneuploidy, chromoanasynthesis-like rearrangements, and tripolar mitoses. Moreover, αS-SETMAR-expressing cells displayed heightened sensitivity to stress conditions mimicking chemotherapy and radiotherapy, resulting in increased apoptosis. Conclusions: Our findings identify αS-SETMAR as a dual modulator of glioblastoma cell fate - simultaneously slowing proliferation and promoting chromosomal instability while enhancing vulnerability to genotoxic stress. These results suggest that αS SETMAR could serve as both a prognostic marker and a potential therapeutic tool in glioblastoma management.

Glioblastomas (GBs) are incurable brain tumors. The persistence of aggressive stem-like tumor cells after cytotoxic treatments compromises therapeutic efficacy, leading to GBM recurrence. Forcing the GBM cells to irreversibly abandon their aggressive stem-like phenotype may offer an alternative to conventional cytotoxic treatments. Here, we show that the RNA binding protein CELF2 is strongly expressed in mitotic and OLIG2-positive GBM cells, while it is downregulated in differentiated and non-mitotic cells by miR-199a-3p, exemplifying GBM intra-tumor heterogeneity. Using patient-derived cells and human GBM samples, we demonstrate that CELF2 plays a key role in maintaining the proliferative/OLIG2 cell phenotype with clonal and tumorigenic properties. Indeed, we show that CELF2 deficiency in patient-derived GSCs drastically reduced tumor growth in the brains of nude mice. We further show that CELF2 promotes TRIM28 and G9a expression, which drive a H3K9me3 epigenetic profile responsible for the silencing of the SOX3 gene. Thus, CELF2, which is positively correlated with OLIG2 and Ki67 expression in human GBM samples, is inversely correlated with SOX3 and miR-199a-3p. Accordingly, the invalidation of SOX3 in CELF2-deficient patient-derived cells rescued proliferation and OLIG2 expression. Finally, patients expressing SOX3 above the median level of expression tend to have a longer life expectancy. CELF2 is therefore a crucial target for the malignant potential of GBM and warrants attention when developing novel anticancer strategies.

Background: We carry out a review of the history and biological activities of one domesticated gene in higher primates, SETMAR, by discussing current controversies. Our purpose is to open a new outlook that will serve as a framework for future work about SETMAR, possibly in the field of cognition development. Main body: What is newly important about SETMAR can be summarized as follows: (1) the whole protein sequence is under strong purifying pressure; (2) its role is to strengthen existing biological functions rather than to provide new ones; (3) it displays a tissue-specific pattern of expression, at least for the alternative-splicing it undergoes. Studies reported here demonstrate that SETMAR protein(s) may be involved in essential networks regulating replication, transcription and translation. Moreover, during embryogenesis, SETMAR appears to contribute to brain development. Short conclusion: Our review underlines for the first time that SETMAR directly interacts with genes involved in brain functions related to vocalization and vocal learning. These findings pave the way for future works regarding SETMAR and the development of cognitive abilities in higher primates.

Recent evidence suggests that the chimeric protein SETMAR is a factor of interest in cancer, especially in glioblastoma. However, little is known about the expression of this protein in glioblastoma tissues, and no study has been done to assess if SETMAR could be a prognostic and/or diagnostic marker of glioblastoma. We analyzed protein extracts of 47 glioblastoma samples coming from a local and a national cohort of patients. From the local cohort, we obtained localized biopsies from the central necrosis area, the tumor, and the perilesional brain. From the French Glioblastoma Biobank (FGB), we obtained three types of samples: from the same tumors before and after treatment, from long survivors, and from very short survivors. We studied the correlations between SETMAR amounts, clinical profiles of patients and other associated proteins (PTN, snRNP70 and OLIG2). In glioblastoma tissues, the shorter isoform of SETMAR (S-SETMAR) was predominant over the full-length isoform (FL-SETMAR), and the expression of both SETMAR variants was higher in the tumor compared to the perilesional tissues. Data from the FGB showed that SETMAR amounts were not different between the initial tumors and tumor relapses after treatment. These data also showed a trend toward higher amounts of S-SETMAR in long survivors. In localized biopsies, we found a positive correlation between good prognosis and large amounts of S-SETMAR in the perilesional area. This is the main result presented here: survival in Glioblastoma is correlated with amounts of S-SETMAR in perilesional brain, which should be considered as a new relevant prognosis marker.

Both classes of transposable elements (DNA and RNA) are tightly regulated at the transcriptional level leading to the inactivation of transposition via epigenetic mechanisms. Due to the high copies number of these elements, the hypothesis has emerged that their regulation can coordinate a regulatory network of genes. Herein, we investigated whether transposition regulation of HsMar1, a human DNA transposon, differs in presence or absence of endogenous HsMar1 copies. In the case where HsMar1 transposition is regulated, the number of repetitive DNA sequences issued by HsMar1 and distributed in the human genome makes HsMar1 a good candidate to regulate neighboring gene expression by epigenetic mechanisms. A recombinant active HsMar1 copy was inserted in HeLa (human) and CHO (hamster) cells and its genomic excision monitored. We show that HsMar1 excision is blocked in HeLa cells, whereas CHO cells are competent to promote HsMar1 excision. We demonstrate that de novo HsMar1 insertions in HeLa cells (human) undergo rapid silencing by cytosine methylation and apposition of H3K9me3 marks, whereas de novo HsMar1 insertions in CHO cells (hamster) are not repressed and enriched in H3K4me3 modifications. The overall analysis of HsMar1 endogenous copies in HeLa cells indicates that neither full-length endogenous inactive copies nor their Inverted Terminal Repeats seem to be specifically silenced, and are, in contrast, devoid of epigenetic marks. Finally, the setmar gene, derived from HsMar1, presents H3K4me3 modifications as expected for a human housekeeping gene. Our work highlights that de novo and old HsMar1 are not similarly regulated by epigenetic mechanisms. Old HsMar1 are generally detected as lacking epigenetic marks, irrespective their localisation relative to the genes. Considering the putative existence of a network associating HsMar1 old copies and SETMAR, two non-mutually exclusive hypotheses are proposed: active and inactive HsMar1 copies are not similarly regulated or/and regulations concern only few loci (and few genes) that cannot be detected at the whole genome level.

SETMAR isoforms in glioblastoma: A matter of protein stability

Mutations in ATRX, a chromatin remodeling protein, are associated with the telomerase-independent ALT pathway of telomere maintenance in several types of cancer, including human gliomas. In telomerase-positive glioma cells, we found that by immunofluorescence ATRX localized not far from the chromosome ends but not exactly at the telomere termini. Chromatin immunoprecipitation (ChIP) experiments confirmed a subtelomeric localization for ATRX. Yet, shRNA-mediated genetic inactivation of ATRX failed to trigger the ALT pathway. Cohesin has been recently shown to be part of telomeric chromatin. Here, using ChIP, we showed that genetic inactivation of ATRX provoked a diminution in the amount of cohesin in subtelomeric regions of telomerase-positive glioma cells. Inactivation of ATRX also led to a diminution in the amount of TERRAs, non-coding RNAs resulting from transcription of telomeric DNA, as well as to a decrease in RNAPII levels at the telomeres. Our data suggest that ATRX might establish functional interactions with cohesin on telomeric chromatin in order to control TERRA levels and that one or the other or both of these events might be relevant to the triggering of the ALT pathway in cancer cells that exhibit genetic inactivation of ATRX.

Transposases are specific DNA-binding proteins that promote the mobility of discrete DNA segments. We used a combination of physicochemical approaches to describe the association of MOS1 (an eukaryotic transposase) with its specific target DNA, an event corresponding to the first steps of the transposition cycle. Because the kinetic constants of the reaction are still unknown, we aimed to determine them by using quartz crystal microbalance on two sources of recombinant MOS1: one produced in insect cells and the other produced in bacteria. The prokaryotic-expressed MOS1 showed no cooperativity and displayed a K_d of about 300 nM. In contrast, the eukaryotic-expressed MOS1 generated a cooperative system, with a lower K_d (∼2 nm). The origins of these differences were investigated by IR spectroscopy and AFM imaging. Both support the conclusion that prokaryotic- and eukaryotic-expressed MOS1 are not similarly folded, thereby resulting in differences in the early steps of transposition.