Synergismus genetischer und epigenetischer Dysregulation als Mechanismus in der Leukämogenese
Zusammenfassung der Projektergebnisse
Aim of our project "Synergism of genetic and epigentic dysregulation as a mechanism in leukemogenesis" was to study the impact of reduced histone 3, lysine 9 trimethylation (H3K9me3) on oncogene induced leukemogenesis. To answer this question we developed two oncogene-induced mouse models by retroviral overexpression of NRas or c-Myc in murine BM cells and transplantation into wildtype recipient mice. By the transduction of Suv39h1-deficient donor cells we were able to combine genetic and epigenetic defects in these models. Suv39h1 encodes a histone methyl transferase that mainly mediates H3K9me3. In both models, the transplanted mice developed myeloid leukemias with different phenotypical presentations. The c-Myc leukemias had a myeloid-granulocytic phenotype similar to human AML M2-M4. Expression of c-Myc in Suv39h1-deficient cells did not alter the phenotype or latency of the disease, however, it prevented c-Myc induced chromosomal instability. This was mediated by improved maintenance of telomere lengths and upregulation of important proteins of the DNA repair pathway. Chromosomal aberrations in c-Myc wildtype leukemias were mainly numerical in primary recipients and structural in the secondary and tertiary recipients, namely end-to-end fusion and translocations. In respect to the end-to-end fusions, the number of critical short telomeres correlated with the occurrence of the aberrations. Similar to our observations in the mouse model, we found the expression of Suv39h1 to be reduced in human AML with normal karyotype in published gene expression studies, supporting our finding that reduced H3K9me3 prevents chromosomal instability. In the NRas-induced leukemia model, the combination with the Suv39h1-deficiency accelerated the disease progression and altered the phenotype. In contrast to NRas-induced myeloid leukemias with mainly myelo-monocytic phenotype similar to human AML M4, the expression of NRas in Suv39h1-deficient cells induced maligne histiocytosis (mHC). As histiocytes and macrophages/monocytes develop from a common precursor we speculate that the leukemic transformation initiated in the same cell type but that reduced H3K9me3 altered differentiation. We confirmed this hypothesis by gene expression analysis in NRas transduced granulocytic-monocytic progenitors (GMP) sorted from Suv39h1-/- or wildtype leukemias. The expression signature in NRas/Suv39h1-/- GMP strongly correlated with data sets of differentiated, mature blood cells by gene set enrichment analysis. Secondary transplantations of Suv39h1-deficient mHC supported leukemia progression to an AML with an immature phenotype similar to the myeloid leukemias induced by NRas in wildtype cells. NRas-induced leukemias in wildtype mice selected for clones with activating insertions in Evi1 and Prdm16. Similar to our mouse model, in 24-28% of human AML with EVI1 overexpression due to translocations or inversion in 3q26 also co-occuring mutations in NRAS were found, indicating that these two mutations collaborate in human leukemogenesis. We further developed an in vitro model to study collaborations of Evi1 and NRas that may in the future be explored in mouse models in vivo.
