JAK signaling globally counteracts heterochromatic gene silencing

Abstract

The JAK/STAT pathway has pleiotropic roles in animal development, and its aberrant activation is implicated in multiple human cancers [1][2][3] . JAK/STAT signaling effects have been attributed largely to direct transcriptional regulation by STAT of specific target genes that promote tumor cell proliferation or survival. We show here in a Drosophila melanogaster hematopoietic tumor model, however, that JAK overactivation globally disrupts heterochromatic gene silencing, an epigenetic tumor suppressive mechanism 4 . This disruption allows derepression of genes that are not direct targets of STAT, as evidenced by suppression of heterochromatin-mediated position effect variegation. Moreover, mutations in the genes encoding heterochromatin components heterochromatin protein 1 (HP1) and Su(var)3-9 enhance tumorigenesis induced by an oncogenic JAK kinase without affecting JAK/STAT signaling. Consistently, JAK loss of function enhances heterochromatic gene silencing, whereas overexpressing HP1 suppresses oncogenic JAK-induced tumors. These results demonstrate that the JAK/STAT pathway regulates cellular epigenetic status and that globally disrupting heterochromatin-mediated tumor suppression is essential for tumorigenesis induced by JAK overactivation.The D. melanogaster genome contains a single JAK, named Hopscotch (Hop), and a single STAT, STAT92E, that are most similar to JAK2 and STAT5, respectively [5][6][7] . Hop and STAT92E function in a canonical JAK/STAT pathway, mediating cell proliferation, differentiation and migration in a variety of developmental processes [7][8][9] . Tumorous-lethal (Tum-l) is an oncogenic allele of hop and encodes a hyperactive JAK kinase due to a G341E substitution [10][11][12] . hop Tum-l is associated with high incidence of hematopoietic tumors in heterozygous animals, a leukemia-like phenotype. These tumors manifest as melanotic masses of blood cell aggregates in the larval or adult body cavity, resulting from overproliferation and differentiation of particular blood cell types 10,11 . It has been shown that Tum-l induces hematopoietic tumors by overactivating STAT92E, as STAT92E is hyperphosphorylated by Hop Tum-l , and that reducing the gene dosage of STAT92E suppresses Tum-l tumorigenicity 5,6,13 . Classical genetic screens and genome-wide RNA interference (RNAi) analyses in D. melanogaster have led to the identification of many components involved in the JAK/STAT pathway or in its modulation 5,6,[13][14][15] . However, as Author Contributions: The initial Df screen was performed by H.C.C., J.L. and L.L.; identification of modifier genes and phenotype assessment were performed by S.S.; F.X. contributed to the protein blot analysis. W.X.L. designed the study and wrote the paper. Competing Interests Statement:The authors declare that they have no competing financial interests. In order to understand how overactivation of the JAK/STAT pathway causes hematopoietic tumors, we undertook a genetic screen to identify genes important for the tumorigenicity of hop Tum-l . We first...

Drosophila STAT is required for directly maintaining HP1 localization and heterochromatin stability

“…2c). Notably, the effects of STAT92E overexpression were opposite to those of JAK overactivation on heterochromatin shown previously 7 , consistent with the opposite effects we observed on PEV. Thus, STAT92E levels seem to be important for heterochromatin formation or stability.…”

“…We have demonstrated that oncogenic JAK disrupts heterochromatin formation globally, allowing transcriptional activation of genes that are not necessarily direct targets of STAT 7,9 . The molecular mechanism underlying the effects of Hopscotch (Hop, Drosophila JAK) on heterochromatin remains unclear.…”

“…1e with 1k), suggesting that increasing HP1 levels also antagonizes the effects of reducing stat92E + dosage. Thus, STAT92E and HP1 are interdependent and both required for heterochromatic gene silencing.We next investigated the requirement for STAT92E in heterochromatin formation by genetic mosaic analyses (see Methods) using HP1 and dimethylated histone H3mK9 as markers in larval salivary glands 7,13 . We found that clonal overexpression of stat92E + resulted in higher levels of heterochromatin (H3mK9 and HP1 accumulation), compared with neighbouring wild-type cells (Fig.…”

“…1b, c) or the white + gene in its original locus (data not shown; also see ref. 7), suggesting that P[white + ] or white + is not simply a transcriptional target of STAT92E. Thus, in contrast to Hop, which counteracts heterochromatic gene silencing 7 , STAT92E seems to promote this silencing process.…”

“…We next investigated the requirement for STAT92E in heterochromatin formation by genetic mosaic analyses (see Methods) using HP1 and dimethylated histone H3mK9 as markers in larval salivary glands 7,13 . We found that clonal overexpression of stat92E + resulted in higher levels of heterochromatin (H3mK9 and HP1 accumulation), compared with neighbouring wild-type cells (Fig.…”