Role of STAT in heterochromatin initiation - Project Summary Heterochromatin is a tightly packed form of DNA essential for chromosomal compaction, transcriptional silencing, genome stability, animal longevity, and tumor suppression. The mechanisms behind heterochromatin establishment remain incompletely understood. This project aims to investigate the initiation of heterochromatin, focusing on the interplay between Heterochromatin Protein 1 (HP1) and Signal Transducer and Activator of Transcription (STAT). Our previous research has shown that a fraction of STAT, not phosphorylated at the critical tyrosine residue around amino acid 700 (uSTAT), localizes in the nucleus in association with HP1. We demonstrated that STAT is essential for heterochromatin maintenance and that its activation by phosphorylation disrupts heterochromatin. Additionally, human uSTAT5A and STAT3 have been shown to promote heterochromatin formation and suppress tumor growth. Preliminary studies using chromatin immunoprecipitation followed by deep sequencing (ChIP-seq) revealed that a significant amount of chromatin- bound Drosophila STAT is localized in constitutive heterochromatin. Loss of STAT leads to a global decrease in the levels of trimethylated histone 3 at lys9 (H3K9me3), a heterochromatin marker. Furthermore, when uSTAT is forced to bind to euchromatin, it can repress nearby genes in an HP1-dependent manner. These findings suggest that uSTAT plays a crucial role not only in maintaining but also in initiating heterochromatin formation. HP1 and H3K9me3 are hallmarks of heterochromatin, with HP1 being the central component. While recruiting HP1 to euchromatin is sufficient to initiate heterochromatin formation, HP1 does not bind DNA directly and has a weak affinity for H3K9me3. This suggests that HP1 may require DNA-binding protein factors for its initial recruitment to heterochromatic loci or to strengthen its binding to H3K9me3. Indeed, protein factors and RNA molecules have been implicated in heterochromatin establishment. We hypothesize that uSTAT is among the protein factors required for the establishment of a subset of heterochromatin. We will employ a combination of genomic, genetic, and biochemical tools to investigate the initial events leading to heterochromatin establishment. Understanding the molecular mechanisms of heterochromatin establishment could lead to novel cancer therapeutics through targeted epigenetic gene silencing.
