SETDB1 modulates the differentiation of both the crystal cells and the lamellocytes in Drosophila

Proper genetic and epigenetic regulation is necessary to maintain the identity and integrity of cells. Enzymes involved in post-transcriptional modifications of histones are key factors in epigenetic mechanisms. Such modifications are also gaining importance for their role in growth and development of cancer. SETDB1 catalyzes the epigenetic mark of lysine-9 methylation of histone-3. In this study, we explored the role of SETDB1 in Drosophila hematopoiesis. We show that SETDB1 controls the differentiation of matured blood cells in wandering third instar larvae. There are three matured blood cells in wild type Drosophila melanogaster: plasmatocytes, crystal cells and lamellocytes. We found that loss-of-function mutants of SETDB1 show hematopoietic defects; increased blood cell proliferation, decreased number of crystal cells, greater differentiation of blood cells into lamellocytes, dysplasia of the anterior lobes of lymph gland and presence of hematopoietic tumors. Cell type specific knockdown of SETDB1 provided similar phenotype i.e., decreased number of crystal cells and an increase in lamellocyte differentiation. In animals with loss of function of SETDB1, Notch pathway was downregulated. Further, over-expression of SETDB1 in blood cells resulted in an increase in the number of crystal cells. This increase is accompanied with an increase in the number of Notch^ICD expressing cells. We therefore performed genetic rescue using UAS-GAL4 system to rescue loss of function SETDB1 mutants. Our data show that the rescued larvae carrying a wild type copy of SETDB1 in mutant background are devoid of blood tumors. We have identified a novel dual function of SETDB1 methylatransferase as a critical regulator of two of the matured hemocytes, crystal cells and lamellocytes. We propose a novel role of SETDB1 in modulating the differentiation of crystal cells and lamellocytes from a common progenitor and underscore the importance of SETDB1 in Drosophila blood tumor suppression.