Abstract 3855: Modeling MLL-PTD related Myelodysplastic Syndromes (MDS) in mouse.
Authors: Zhang Yue , Yan Xiao-mei , Sashida Goro , Chen Ai-Li , Zhao Xing-Hui , Whitman Susan P. , Caligiuri Michael A. , Xiao Zhi-Jian , Grimes Harry Leighton , Huang Gang 
Published: August 2014

Abstract
MLL partial tandem duplication (MLL-PTD) is found in 5-8% of human MDS, secondary acute myeloid leukemia (s-AML) and de novo AML. Mll-PTD knock-in mouse model shows that Mll-PTD HSPCs exhibited a proliferative advantage over wild-type HSPCs, and The ST-HSCs/MPPs and even GMPs have self-renewal capabilities. However, Mll-PTD HSPCs never develop MDS or s-AML in primary or transplanted recipients, suggesting that additional genetic and/or epigenetic defects are necessary for transformation. Recently, high frequent co-existences of both MLL-PTD and RUNX1 mutations have been reported in MDS/AML patients, which suggest a potential cooperation for transformation between these two mutations. Thus, we hypothesize that reducing RUNX1 dosage may facilitate the MLL-PTD mediated transformation toward MDS and/or s-AML.
We first generated the mice containing one allele of Mll-PTD in a Runx1+/- background and assessed HSPCs of Mll-PTD/Runx1+/− double heterozygous (DH) mice. The DH newborns are runty; they frequently die in early postnatal stage and barely survive to adulthood, compared to the normal life span of wild type (WT) or single heterozygous (Mllwt/wt/Runx1+/− and Mll-PTD/Runx1+/+) mice. We studied DH embryos fetal liver hematopoiesis and found reduced LK and LSK cells, partly because of increased apoptosis. Enhanced proliferation was found in DH fetal liver cells (FLCs) in vitro CFU replating assays over WT controls. FLCs of DH also showed dominant expansion in both serial competitive and non-competitive BMT assays compared to WT controls. The DH derived ST-HSCs/MPPs and GMPs also have enhanced self-renewal capabilities, rescuing hematopoiesis by giving rise to long-term repopulating cells in recipient mice better than cells derived from Mll-PTD mice. However, DH HSPCs do not develop MDS or s-AML in primary or in BMT recipient mice. We further generated Mll-PTD/Runx1Δ/Δ mice using Mx1-Cre mediated deletion. These mice showed thrombocytopenia one month after pI-pC injection, and developed pancytopenia 2-4 months later. The complete blood count exhibited increased MCV, RDW and severe anemia. All these Mll-PTD/Runx1Δ/Δ mice died of MDS induced complications within 7-8 months, and tri-lineages dysplasias (TLD) were found in bone marrow aspirate. However, there are no spontaneous s-AML found in Mll-PTD/Runx1Δ/Δ mice, which suggests that RUNX1 mutants found in MLL-PTD patients may not be simply loss-of-function mutations and present gain-of-function activities which cooperate with MLL-PTD in human diseases onsets. In conclusion, our study demonstrates that: 1) RUNX1 gene dosage reverse-correlates with HSPCs self-renewal activity; 2) Runx1 complete deletion causes MDS in Mll-PTD background. Future studies are needed to fully understand the collaboration between MLL-PTD and RUNX1 mutations for MDS development and leukemic transformation, which should facilitate improved therapies and patient outcomes.
Citation Format: Yue Zhang, Xiao-mei Yan, Goro Sashida, Ai-Li Chen, Xing-Hui Zhao, Susan P. Whitman, Michael A. Caligiuri, Zhi-Jian Xiao, Harry Leighton Grimes, Gang Huang. Modeling MLL-PTD related Myelodysplastic Syndromes (MDS) in mouse. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3855. doi:10.1158/1538-7445.AM2013-3855