Effective drug treatment identified by in vivo screening in a transplantable patient-derived xenograft model of chronic myelomonocytic leukemia

The establishment of serially transplantable mouse models of myelodysplastic syndrome (MDS)/ myloproliferative neoplasm (MPN) overlap disorders is a great challenge, since patient cells display inconsistent engraftment of recipient mice and serial transplantations are limited.

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To evaluate novel and effective treatment combinations for chronic myelomonocytic leukemia (CMML) preclinically, we hypothesized that supplementation of CMML cells with the human oncogene Meningioma 1 (MN1) promotes expansion and serial transplantability in mice, while maintaining the functional dependencies of these cells on their original genetic profile.

To study this hypotheses, we chose the MN1 oncogene as transduction of MN1 in healthy CD34+ cord blood cells promotes self-renewal in vitro and engraftment in vivo, but does not induce acute myeloid leukemia (AML) in vivo nor does it enable serial transplantation. Using lentiviral expression of MN1 for oncogenic supplementation and transplanting transduced primary mononuclear CMML cells into immunocompromised mice, we established three serially transplantable CMML-PDX models with disease-related gene mutations that recapitulate the disease in vivo. Ectopic MN1 expression was confirmed to enhance the proliferation of CMML cells, which otherwise did not engraft upon secondary transplantation. Furthermore, MN1-supplemented CMML cells were serially transplantable into recipient mice up to 5 generations (and counting).

 This robust engraftment enabled an in vivo RNA interference screening targeting CMML-related mutated genes including NRAS, confirming that their functional relevance is preserved in the presence of MN1. The novel combination treatment with azacitidine and the MEK-inhibitor trametinib additively inhibited ERK-phosphorylation and thus depleted the signal from mutated NRAS. The combination treatment significantly prolonged survival of CMML mice compared to single agent treatment.

 In this study, we developed patient-derived CMML models that recapitulate the disease in vivo and allowed us to demonstrate that (1) ectopic MN1 expression enhances the engraftment and proliferation of cells that otherwise do not engraft in mice, (2) the functional relevance of CMML-related gene mutations is preserved in the presence of MN1, (3) MEK/ERK signaling is important for in vivo proliferation of CMML cells, (4) the serial transplantability of the MN1-CMML cells enable functional RNAi in vivo and (5) the combination of azacitidine with trametinib is an effective combination treatment in CMML-MN1 cells in vivo and primary CMML cells in vitro and suggest its clinical development to eventually improve the outcome of CMML patients.

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Arnold Kloos

Postdoctoral fellow, Hannover Medical School

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