Whole genome and transcriptome sequencing of a cohort of 67 leiomyosarcomas has been revealed ATRX to be one of the most frequently mutated genes in leiomyosarcomas after TP53 and RB1. While its function is well described in the alternative lengthening of telomeres mechanism, we wondered whether its alteration could have complementary effects on sarcoma oncogenesis. ATRX alteration is associated with the down-expression of genes linked to differentiation in leiomyosarcomas, and to immunity in an additional cohort of 60 poorly differentiated pleomorphic sarcomas. In vitro and in vivo models showed that ATRX down-expression increases tumor growth rate and immune escape by decreasing the immunity load of active mast cells in sarcoma tumors. These data indicate that an alternative to unsuccessful targeting of the adaptive immune system in sarcoma could target the innate system. This might lead to a better outcome for sarcoma patients in terms of ATRX status.

Mutations in the epigenetic regulator Additional Sex Combs-Like 1 (ASXL1) are frequently observed in chronic neutrophilic leukemia (CNL). CNL is a myeloproliferative neoplasm (MPN) driven by activating mutations in the Colony Stimulating Factor 3 Receptor (CSF3R), which cause excessive neutrophil production. Despite the high rates of co-occurrence, the interplay between ASXL1 and CSF3R mutations in hematopoiesis and leukemia remains poorly understood. Here, we present a new mouse model with both Asxl1Y588X and Csf3rT621I mutations, which recapitulates features of human MPNs. Csf3r-mutant mice exhibit an age-associated depletion of hematopoietic stem cells, which is tempered by adding Asxl1Y588X. This combination of mutations causes an expansion of myeloid-biased long-term hematopoietic stem cells. As the mice age, they develop neutrophilia, but leukemia is rare, suggesting additional mutations may be required for transformation. Using models of myeloid differentiation, we find that Asxl1 truncation enhances CSF3RT618I-driven neutrophil differentiation, activating inflammatory pathways associated with mature myeloid cell production. Moreover, cells with both mutations have increased H3K4me1 at neutrophil-associated enhancers. Mutant ASXL1 is known to decrease the genome-wide abundance of the repressive histone mark H2AK119ub. Although we see the expected decrease in H2AK119ub in Asxl1-mutant cells, this effect is reversed when CSF3R is also mutated, suggesting a complex interplay between these mutations in regulating chromatin dynamics during hematopoiesis. Our findings highlight context-dependent effects of ASXL1 mutation in myeloid disorders and provide insights into the mechanisms underlying neutrophil differentiation in ASXL1 and CSF3R dual-mutant MPN.

Whole genome and transcriptome sequencing of a cohort of 67 leiomyosarcomas revealed ATRX to be one of the most frequently mutated genes in leiomyosarcomas after TP53 and RB1 . While its function is well described in the alternative lengthening of telomeres mechanism, we wondered whether its alteration could have complementary effects on sarcoma oncogenesis. ATRX alteration is associated with the down-expression of genes linked to differentiation in leiomyosarcomas, and to immunity in an additional cohort of 60 poorly differentiated sarcomas. In vitro and in vivo models showed that ATRX loss increases tumor growth rate and immune escape by decreasing the immunity load of active mast cells in sarcoma tumors. These data indicate that an alternative to unsuccessful targeting of the adaptive immune system in sarcoma could be to target the innate system. This might lead to a better outcome for sarcoma patients in terms of ATRX status.

Les sarcomes à génomique complexe et plus particulièrement les léiomyosarcomes (LMS), qui ont une différenciation musculaire lisse, sont des tumeurs rares et agressives dont l'oncogenèse reste aujourd'hui incomprise. Afin de mieux appréhender cette oncogenèse et de mettre en évidence ces drivers, une intégration de plusieurs OMICS et des données cliniques d'une cohorte de 120 LMS a été réalisée. Cela permet de séparer les LMS en deux grands groupes cliniquement différents : hLMS et oLMS. Dans ce dernier groupe ainsi que dans les LMS utérins, un des gènes les plus altérés est ATRX. Par l'étude de l'impact de cette altération dans des modèles murins, je montre qu'elle entraine une croissance plus rapide des tumeurs et un échappement au système immunitaire. A l'opposé des oLMS, les hLMS ont un transcriptome homogène, ils sont aussi plus agressifs, se situent au niveau du tronc et du rétropéritoine, ont des marques d'histones des muscles lisses et des gènes régulés par des facteurs de transcription spécifiques des muscles lisses comme SRF. De plus, les hLMS ont des altérations spécifiques notamment la surexpression de MYOCD, le cofacteur de SRF dans les muscles lisses, et la perte de PTEN, capable de limiter la fixation SRF à ses gènes cibles de la différenciation des muscles lisses. J'ai alors démontré que l'inhibition de l'interaction entre SRF et MYOCD entraine la mort par apoptose des cellules de hLMS plus fortement que celles de oLMS, démontrant ainsi que l'activation de cet axe est indispensable à la survie des hLMS, cela en fait une cible thérapeutique prometteuse. Pour mieux comprendre comment l'activation pathologique de ce complexe dirige l'oncogenèse et comment ces cellules en deviennent dépendantes, j'ai mis au point une analyse par ChIPseq pour étudier, dans le contexte des LMS, les cibles de la fixation du complexe SRF/MYOCD dont l'analyse est en cours. L'ensemble de ces travaux apporte de nouvelles voies thérapeutiques dont l'étude préclinique doit maintenant se poursuivre in-vivo.