While the genetic contributions to the predisposition of Bernese mountain dogs (BMDs) to histiocytic sarcoma (HS) remains unclear, some insights into key genetic drivers have been gained. Our group recently reported a mutation in the PTPN11 gene (E76K). We have now identified a second missense mutation in PTPN11 (G503V), and a mutation in KRAS (Q61H) present in HS cell lines. These mutations are associated with malignancies in humans, and known to be gain-of-function mutations that result in activation of the mitogen-activated protein kinase (MAPK) pathway. The goal of the present study was to evaluate the prevalence of these mutations in a large sample of HS cases from BMDs and golden retrievers, and in lymphoma cases, from a cohort of BMDs. Mutations in PTPN11 were present in HS in 41/96 (43%) BMDs, and in 3/13 (23%) golden retrievers. PTPN11 mutations E76K and G503V did not coexist in the same neoplasm. The KRAS mutation was much less frequent, with a prevalence of 3.1% (3/96). We did not identify either PTPN11 nor KRAS mutations in any of the lymphoma samples. These results point out the potential relevance of PTPN11 and KRAS mutations as activators of the oncogenic MAPK pathway for canine HS, particularly in BMDs.

Canine histiocytic sarcoma (HS), historically called malignant histiocytosis, is a highly aggressive hematopoietic neoplasm of malignant cells of dendritic cell lineage. Although rare (fewer than 1% of all cancers), 5 several dog breeds are highly predisposed to HS, including Bernese mountain dogs (more than 25% of the population), 1,10 flat-coated retrievers, 11,12 golden retrievers, and Rottweilers. 2,7,30 Canine HS most frequently develops in the spleen, lymph nodes, liver, lung, and bone marrow, 2,23 where it progresses rapidly, disseminating to multiple organs in 70% to 91% of cases. 7,12,23,30 Due to the lack of effective options for treatment, patients respond poorly to available protocols, resulting in survival times that range from a few weeks to 3 mo. 14,28,30 Studies by our group using HS cell lines in culture have indicated dasatinib as a potential treatment option for dogs with HS. 34 Other research groups shared similar results after testing dasatinib in HS cell lines 17 and in a subcutaneous xenograft mouse model of HS. 18 Dasatinib is a tyrosine kinase inhibitor of multiple targets including SRC family kinases (SRC, LCK, YES, and FYN). This drug is approved for people with Philadelphia chromosome-positive chronic myeloid leukemia or acute lymphoblastic leukemia. 31 The efficacy of dasatinib in dogs and humans with HS is unknown.Human HS is a similar and equally aggressive malignancy to canine HS and is extremely rare, accounting for less than 1% of all hematopoietic neoplasms. 19,20 In humans, HS is associated with a poor prognosis and a survival time of less than 1 y. 24,38 Human HS may present as a localized disease that is treatable with surgical resection or radiation therapy; however, it typically presents at an advanced stage that involves multiple organs, including lymph nodes, the gastrointestinal tract, spleen, and lungs. 16,29,35 Patients with disseminated disease have a poor prognosis due to limited response to available treatments. 13,16 Studies to identify more effective therapeutic approaches are warranted, but given the low incidence of HS in humans, research progress is slow. Dogs are the only species that spontaneously develops a similar form of HS at an appreciable frequency, thus representing an important translational model for this rare disease in humans.Xenograft mouse models are important tools to evaluate the in vivo response to novel cancer therapeutic interventions. Although no human HS xenograft model has been reported in the literature to date, xenograft models of subcutaneous canine HS were successfully established in 2 studies, but neither of these models developed metastatic disease. 3,39 Other authors described a xenograft model that presented as disseminated

Histiocytic sarcoma (HS) is an orphan hematological malignancy with an incidence of less than 1% of all non-Hodgkin's lymphoma, affecting both children and adults. It is a highly metastatic disease, and can affect any organ in the body including spleen, lungs, bone marrow, liver and lymph nodes. It is associated with a poor prognosis as treatment options are scarce and responses are short lived. Dogs spontaneously develop a similar form of HS with an appreciable frequency, and therefore present a unique translational model to investigate better treatments. Preliminary data from in vitro studies identified dasatinib, a multi tyrosine kinase inhibitor, as a highly potent drug against HS cell lines derived from tumors in dogs, but non-toxic to normal normal tissue surrogate fibroblasts. The current study was undertaken to evaluate if the efficacy of dasatinib could be re-capitulated in an in vivo orthotopic xenograft model of HS. The mouse model was generated in NOD-SCID mice by surgical injection of HS cells, transfected with a luciferase reporter construct, into their spleen. Tumor growth was monitored using In Vivo Imaging System. Mice were treated with 30 mg/kg of dasatinib intraperitoneally once a day or vehicle control, with five mice in each group. Dasatinib treated mice had a significantly decreased tumor growth rate in comparison to untreated mice. Most importantly, treatment with dasatinib significantly increased their survival time. All mice from the vehicle control group reached humane endpoints for euthanasia by day 27, while treated mice were clinically stable at day 53, with the difference in survival being highly significant (p<0.0001). In conclusion, here we report the first orthotopic xenograft mouse model of HS, that offers the possibility to test therapeutic response to novel agents, such as dasatinib. Our findings indicate that dasatinib is a promising treatment option for HS, and provide a rational for the initiation of clinical trials. Citation Format: Marilia Takada, Lauren Smyth, Vilma Yuzbasiyan-Gurkan. Dasatinib displays antitumor efficacy in an orthotopic xenograft mouse model of histiocytic sarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2166.