Origin of the vasculature supporting growth of primary patient tumor xenografts.

BACKGROUND: Studies of primary patient tumor xenografts grown in immunodeficient mice have shown that these tumors histologically and genetically closely resemble the original tumors. These patient xenograft models are becoming widely used for therapeutic efficacy studies. Because many therapies are directed at tumor stromal components and because the tumor microenvironment also is known to influence the response of a tumor to therapy, it is important to understand the nature of the stroma and, in particular, the vascular supply of patient xenografts. METHODS: Patient tumor xenografts were established by implanting undisrupted pieces of patient tumors in SCID mice. For this study, formalin fixed, paraffin embedded specimens from several types of solid tumors were selected and, using species-specific antibodies which react with formalin fixed antigens, we analyzed the species origin of the stroma and blood vessels that supported tumor growth in these models. Additionally, we investigated the kinetics of the vascularization process in a colon tumor and a mesothelioma xenograft. In mice bearing a head and neck xenograft, a perfusion study was performed to compare the functionality of the human and mouse tumor vessels. RESULTS: In patient tumors which successfully engrafted, the human stroma and vessels which were engrafted as part of the original tumor did not survive and were no longer detectable at the time of first passage (15-25 weeks). Uniformly, the stroma and vessels supporting the growth of these tumors were of murine origin. The results of the kinetic studies showed that the loss of the human vessels and vascularization by host vessels occurred more rapidly in a colon tumor (by 3 weeks) than in a mesothelioma (by 9 weeks). Finally, the perfusion studies revealed that while mouse vessels in the periphery of the tumor were perfused, those in the central regions were rarely perfused. No vessels of human origin were detected in this model. CONCLUSIONS: In the tumors we investigated, we found no evidence that the human stromal cells and vessels contained in the original implant either survived or contributed in any substantive way to the growth of these xenografts.

mTOR complex 1 plays critical roles in hematopoiesis and Pten-loss-evoked leukemogenesis.

The mechanistic target of rapamycin (mTOR) pathway serves as a key sensor of cellular-energetic state and functions to maintain tissue homeostasis. Hyperactivation of the mTOR pathway impairs hematopoietic stem cell (HSC) function and is associated with leukemogenesis. However, the roles of the unique mTOR complexes (mTORCs) in hematopoiesis and leukemogenesis have not been adequately elucidated. We deleted the mTORC1 component, regulatory-associated protein of mTOR (Raptor), in mouse HSCs and its loss causes a nonlethal phenotype characterized by pancytopenia, splenomegaly, and the accumulation of monocytoid cells. Furthermore, Raptor is required for HSC regeneration, and plays largely nonredundant roles with rapamycin-insensitive companion of mTOR (Rictor) in these processes. Ablation of Raptor also significantly extends survival of mice in models of leukemogenesis evoked by Pten deficiency. These data delineate critical roles for mTORC1 in hematopoietic function and leukemogenesis and inform clinical strategies based on chronic mTORC1 inhibition.

MLL-rearranged leukemia is dependent on aberrant H3K79 methylation by DOT1L.

The histone 3 lysine 79 (H3K79) methyltransferase Dot1l has been implicated in the development of leukemias bearing translocations of the Mixed Lineage Leukemia (MLL) gene. We identified the MLL-fusion targets in an MLL-AF9 leukemia model, and conducted epigenetic profiling for H3K79me2, H3K4me3, H3K27me3, and H3K36me3 in hematopoietic progenitor and leukemia stem cells (LSCs). We found abnormal profiles only for H3K79me2 on MLL-AF9 fusion target loci in LSCs. Inactivation of Dot1l led to downregulation of direct MLL-AF9 targets and an MLL translocation-associated gene expression signature, whereas global gene expression remained largely unaffected. Suppression of MLL translocation-associated gene expression corresponded with dependence of MLL-AF9 leukemia on Dot1l in vivo. These data point to DOT1L as a potential therapeutic target in MLL-rearranged leukemia.