BRAF-mutant hematopoietic malignancies

Oncogene-induced senescence in hematopoietic progenitors features myeloid restricted hematopoiesis, chronic inflammation and histiocytosis

Activating mutations in the BRAF-MAPK pathway have been reported in histiocytoses, hematological inflammatory neoplasms characterized by multi-organ dissemination of pro-inflammatory myeloid cells. Here, we generate a humanized mouse model of transplantation of human hematopoietic stem and progenitor cells (HSPCs) expressing the activated form of BRAF (BRAF^V600E). All mice transplanted with BRAF^V600E-expressing HSPCs succumb to bone marrow failure, displaying myeloid-restricted hematopoiesis and multi-organ dissemination of aberrant mononuclear phagocytes. At the basis of this aggressive phenotype, we uncover the engagement of a senescence program, characterized by DNA damage response activation and a senescence-associated secretory phenotype, which affects also non-mutated bystander cells. Mechanistically, we identify TNFα as a key determinant of paracrine senescence and myeloid-restricted hematopoiesis and show that its inhibition dampens inflammation, delays disease onset and rescues hematopoietic defects in bystander cells. Our work establishes that senescence in the human hematopoietic system links oncogene-activation to the systemic inflammation observed in histiocytic neoplasms.

P21-activated kinase 1 regulates resistance to BRAF inhibition in human cancer cells

BRAF is a commonly mutated oncogene in various human malignancies and a target of a new class of anti-cancer agents, BRAF-inhibitors (BRAFi). The initial enthusiasm for these agents, based on the early successes in the management of metastatic melanoma, is now challenged by the mounting evidence of intrinsic BRAFi-insensitivity in many BRAF-mutated tumors, by the scarcity of complete responses, and by the inevitable emergence of drug resistance in initially responsive cases. These setbacks put an emphasis on discovering the means to increase the efficacy of BRAFi and to prevent or overcome BRAFi-resistance. We explored the role of p21-activated kinases (PAKs), in particular PAK1, in BRAFi response. BRAFi lowered the levels of active PAK1 in treated cells. An activated form of PAK1 conferred BRAFi-resistance on otherwise sensitive cells, while genetic or pharmacologic suppression of PAK1 had a sensitizing effect. While activation of AKT1 and RAC1 proto-oncogenes increased BRAFi-tolerance, the protective effect was negated in the presence of PAK inhibitors. Furthermore, combining otherwise ineffective doses of PAK- and BRAF-inhibitors synergistically affected intrinsically BRAFi-resistant cells. Considering the high incidence of PAK1 activation in cancers, our findings suggests PAK inhibition as a strategy to augment BRAFi therapy and overcome some of the well-known resistance mechanisms.

Biological and clinical significance of somatic mutations in Langerhans cell histiocytosis and related histiocytic neoplastic disorders

Langerhans cell histiocytosis (LCH), juvenile xanthogranuloma (JXG), and Erdheim-Chester disease (ECD) represent histiocytic disorders with a wide range of clinical manifestations. Until recently, mechanisms of pathogenesis have been speculative and debate has focused on classification of these conditions as reactive versus neoplastic. Genomic studies have been challenged by scarce tissue specimens, as well as heterogeneous nature of the lesions with variable infiltration of pathologic histiocytes. Whole-exome sequencing recently revealed a very low frequency of somatic mutations in LCH, JXG, and ECD compared to other neoplastic disorders. However, at least in the cases of LCH and ECD, there is a very high frequency of activating mutations in MAPK pathway genes, most notably BRAF-V600E, as well as MAP2K1, in LCH and NRAS in ECD. In ECD, recurrent mutations in the PI3K pathway gene PIK3CA have also been described. The heterogeneous clinical manifestations of these disorders may therefore be the cumulative result of activation of MAPK mutations (along with modifying signals from other pathways) at distinct stages of myeloid differentiation. Implications of this model include redefinition of LCH, JXG, and ECD as a group of clinically diverse myeloid neoplastic disorders with a common mechanism of pathogenesis. This model supports refocusing therapeutic strategies for these diseases on a personalized approach based on specific mutations and the cell(s) of origin.