ALBERT

Description: BACKGROUND: Activating mutations of the FLT3 gene are common in acute myeloid leukemia (AML), with approximately 25-30% of cases containing an internal tandem duplication (ITD) in the juxtamembrane domain. The presence of a FLT3-ITD mutation is associated with a poor prognosis, the severity of which, can be modulated by the combination of co-occuring mutations. In animal models, expression of Flt3-ITD by transgenesis, bone marrow transplantation or gene knock-in does not lead to an acute leukemia but a myeloproliferative disease, resembling CMML, suggesting a requirement for additional co-operating mutations (Lee et al, Cancer Cell. 2007, 12: 367). This is supported by in vivo models which demonstrate that the combination of the Flt3-ITD mutation with other genetic lesions leads to the development of an acute leukemia in mice (Chen et al, Genes Dev. 2013, 27: 1974). AIMS: To identify and characterise novel genes that alter Flt3-ITD induced MPN by using an N-ethyl-N-Nitroso-urea (ENU) mutagenesis strategy in mice with an Flt3-ITD homozygous knock-in background. METHODS: An autosomal dominant screen for Flt3-ENU co-operating mutations was carried out at the Australian Phenomics Facility by mating ENU-mutagenised male mice homozygous for the Flt3-ITD knock-in allele to homozygous Flt3-ITD females. G1 mice were screened for changes in blood cell parameters indicative of an altered disease state (compared to that induced by Flt3-ITD alone). Mice with blood cell parameters outside two standard deviations of the relevant G1 mean were identified as potential mutation carriers and bred to Flt3-ITD homozygous knock-in mice to test for heritability of the phenotype. Where pedigrees were generated demonstrating heritable phenotypes, multiple affected and unaffected littermates were subject to exome sequencing and analysis to identify a list of candidate gene mutations segregating with the disease phenotype. RESULTS: 150 G1 mice were screened, leading to the identification of four pedigrees with heritable phenotypes marked by an exacerbated MPN. Exome sequencing has identified a short list of 3 genes for one pedigree (pedigree 37) that includes a mutation in Neurofibromatosis 1 (Nf1), a gene known to be involved in the induction of juvenile myelomonocytic leukemia and frequently lost in AML (Parkin et al,Clin Cancer Res 2010, 16:4135). In another pedigree (pedigree 24) we identified a mutation in Ndufa10 as the single candidate segregating with the phenotype (Figure 1A-B). Ndufa10 encodes a subunit of the mitochondrial respiratory complex I, which is the first and largest complex in the mitochondrial electron transport chain. Importantly, germline mutations in this gene lead to a complex I deficiency syndrome in humans, indicating that it is a critical subunit of this complex. We hypothesise that mutation of Ndufa10 leads to altered cellular metabolism in hematopoietic stem and progenitor cells which contributes to exacerbation of the MPN, possibly through an alteration in production of reactive oxygen species and a shift in the balance between glycolysis and oxidative phosphorylation. Breeding of the Ndufa10 mutation onto a non Flt3-ITD background shows that action of the mutation is not dependent on the presence of Flt3-ITD, as these mice also have altered blood counts, including increased WBC (Figure 1C). CONCLUSIONS: It is possible to identify mutations that exacerbate Flt3-ITD induced MPN through mutagenesis and an efficient blood screening strategy. In addition, using this strategy, we have identified novel mutations that act independently of Flt3-ITD to induce changes in the haematological compartment. Translation of these findings to human AML may indicate pathways that will be targets for new and complementary treatments in AML. Figure 1. A. Flt3-ITD Pedigree 24 indicating affected mice and genotyping for the Ndufa10 mutation. +/+=Ndufa10 wt, m/+=Ndufa10 heterozygous mutant. B. WBC counts for male mice from Pedigree 24 at 15-17 weeks (+/+, n=3; m/+, n=8). WBC from ENU G1 mice are shown as a comparison (G1, n=75). C. WBC counts for male mice on a wildtype C57 background at 16-18 weeks. +/+=Ndufa10 wt (n=7), m/+=Ndufa10 heterozygous mutant (n=8), m/m=Ndufa10 homozygous mutant (7). Figure 1. A. Flt3-ITD Pedigree 24 indicating affected mice and genotyping for the Ndufa10 mutation. +/+=Ndufa10 wt, m/+=Ndufa10 heterozygous mutant. B. WBC counts for male mice from Pedigree 24 at 15-17 weeks (+/+, n=3; m/+, n=8). WBC from ENU G1 mice are shown as a comparison (G1, n=75). C. WBC counts for male mice on a wildtype C57 background at 16-18 weeks. +/+=Ndufa10 wt (n=7), m/+=Ndufa10 heterozygous mutant (n=8), m/m=Ndufa10 homozygous mutant (7). Disclosures No relevant conflicts of interest to declare.

Description: Abstract 2527 Poster Board II-504 Identifying the genes that regulate the development, self-renewal and differentiation of stem cells is of vital importance for understanding normal organogenesis, tailoring tissue engineering for regenerative medicine, cellular reprogramming and cancer. A forward genetic screen for aberrant long-term hematopoietic stem cells and progenitors provides an unbiased and tractable approach to finding genes responsible for stem cell homeostasis and differentiation. Here we demonstrate that chemical mutagenesis of mice combined with advances in hematopoietic stem cell reagents and genome/mapping resources can identify genes essential for mammalian stem cells and blood development. A pilot flow cytometry-based recessive screen comprehensively analyzed nine subsets of hematopoietic stem, progenitor, and red cells in over one thousand mouse embryos at embryonic day (E) 14.5 from 34 pedigrees and recovered five strains with defects in early hematopoiesis. One mutant strain (Booreana - an Australian Aboriginal name meaning white) which has excess long-term hematopoietic stem cells and platelets but reduced myelo-erythroid progenitors was outcrossed and the genetic mutation mapped and identified as a novel mis-sense mutation in the transcription factor c-Myb. The mutation in the trans-activation domain (TA) completely ablates transcriptional activation in a reporter assay which contrasts with other TA domain mutants which are partly dysfunctional[TJG1] . Moreover, the Booreana (Boo) mutation completely abrogates interaction with the transcriptional co-activator, CBP. Boo/Boo homozygous mutant mice survive into adulthood, albeit with severe anemia and massively increased platelet counts, whereas c-Myb−/− mice die by E15.5 of development, suggesting c-Myb has essential functions in vivo which are independent of transcriptional activation. This ENU-generated mutation provides another allele of c-Myb with a phenotype in between the complete loss-of-function allele and previously identified mutant alleles from other ENU screens. ENU-generated point mutants such as the Booreana mutation can provide novel informative insights into key functional domains of proteins, and protein interactions and networks, which are missed in gene knockout mice. Other phenodeviants generated in our screen are currently being mapped and will be presented. [TJG1]Not so in our hands, at least -– the Sandberg M303V is just as dead. Disclosures: No relevant conflicts of interest to declare.

Description: Abstract 1400 BACKGROUND: Activating mutations of the FLT3 gene are common in AML, with approximately 25–30% of cases containing an internal tandem duplication (ITD) in the juxtamembrane domain. The presence of a FLT3-ITD mutation is associated with a poor prognosis which can be modulated by the combination of co-operating mutations. In animal models, expression of FLT3-ITD by transgenesis, bone marrow transplantation or gene knock-in does not lead to an acute leukemia but a myeloproliferative disease, resembling CMML, suggesting a requirement for additional co-operating mutations (Lee et al, Cancer Cell. 12: 367). This is supported by in vivo models which demonstrate that the combination of the FLT3-ITD mutation with other genetic lesions leads to an acute leukemia in mice. AIMS: To identify novel genes co-operating with FLT3-ITD to induce acute leukemia by using an N-ethyl-N-Nitroso-urea (ENU) mutagenesis strategy in mice with a FLT3-ITD background. METHODS: An autosomal dominant screen for FLT3-ENU co-operating mutations was carried out at the Australian Phenomics Facility by mating ENU mutagenised male mice homozygous for the FLT3-ITD knock-in allele to females homozygous for the FLT3-ITD knock-in allele. G1 mice were screened for changes in blood cell parameters indicative of an altered disease state. Peripheral blood differential analysis was performed at 8, 12 and 16 weeks of age as well as immunophenotyping for the myeloid markers Mac1 and Gr1 and the progenitor marker c-kit. Analysis of a non-mutagenised cohort of mice indicated a sex-specific differential effect of FLT3-ITD on multiple blood cell parameters, so male and female data were analysed separately. Mice with blood cell parameters outside two standard deviations of the relevant G1 mean were identified as potential mutation carriers (Figure 1) and bred to FLT3-ITD homozygous knock-in mice to test for heritability of the phenotype (Figure 2). RESULTS: To date 150 G1 mice have been screened, leading to the identification of four heritable phenotypes (Designated pedigrees 12, 21, 24 and 37, Figure 1). All pedigrees were characterised by increased WBC counts compared to ‘unaffected’ mice (FLT3-ITD homozygous background) and by further expansion of the myeloid compartment. Preliminary analysis of the bone marrow from affected mice identified an increase in colony replating ability in response to GM-CSF, suggestive of a block in differentiation. Three out of the four pedigrees showed disease penetrance only in males, with several mice from pedigree 24 succumbing to disease (Figure 2). Whole exome sequencing is being undertaken to identify the sequence variants that segregate with the phenotype in each of the pedigrees. CONCLUSIONS: It is possible through mutagenesis to identify co-operating mutations with FLT3-ITD. Further characterisation of these pedigrees, the associated phenotype and identification of the associated mutation in each case, will provide important information regarding pathways that co-operate with FLT3-ITD in leukemogenesis. Translation of these findings to human AML will have potential implications for predicting the course of disease in AML patients and may indicate pathways that will be targets for new and complementary treatments in AML. Disclosures: No relevant conflicts of interest to declare.

Description: Identification of genes that regulate the development, self-renewal, and differentiation of stem cells is of vital importance for understanding normal organogenesis and cancer; such knowledge also underpins regenerative medicine. Here we demonstrate that chemical mutagenesis of mice combined with advances in hematopoietic stem cell reagents and genome resources can efficiently recover recessive mutations and identify genes essential for generation and proliferation of definitive hematopoietic stem cells and/or their progeny. We used high-throughput fluorescence-activated cell sorter to analyze 9 subsets of blood stem cells, progenitor cells, circulating red cells, and platelets in more than 1300 mouse embryos at embryonic day (E) 14.5. From 45 pedigrees, we recovered 6 strains with defects in definitive hematopoiesis. We demonstrate rapid identification of a novel mutation in the c-Myb transcription factor that results in thrombocythemia and myelofibrosis as proof of principal of the utility of our fluorescence-activated cell sorter–based screen. Such phenotype-driven approaches will provide new knowledge of the genes, protein interactions, and regulatory networks that underpin stem cell biology.