Chromatin insulators are responsible for orchestrating long-range interactions between enhancers and promoters throughout the genome and align with the boundaries of Topologically Associating Domains (TADs). Here, we demonstrate an association between gypsy insulator proteins and the phosphorylated histone variant H2Av (γH2Av), normally a marker of DNA double strand breaks. Gypsy insulator components colocalize with γH2Av throughout the genome, in polytene chromosomes and in diploid cells in which Chromatin IP data shows it is enriched at TAD boundaries. Mutation of insulator components su(Hw) and Cp190 results in a significant reduction in γH2Av levels in chromatin and phosphatase inhibition strengthens the association between insulator components and γH2Av and rescues γH2Av localization in insulator mutants. We also show that γH2Av, but not H2Av, is a component of insulator bodies, which are protein condensates that form during osmotic stress. Phosphatase activity is required for insulator body dissolution after stress recovery. Together, our results implicate the H2A variant with a novel mechanism of insulator function and boundary formation.

Chromatin insulators are responsible for mediating long-range interactions between enhancers and promoters throughout the genome and align with the boundaries of topologically associating domains (TADs). Here, we demonstrate an interaction between proteins that associate with the gypsy insulator and the phosphorylated histone variant H2Av (γH2Av), a marker of DNA double strand breaks. Gypsy insulator components colocalize with γH2Av throughout the genome. Mutation of insulator components prevents stable H2Av phosphorylation in polytene chromatin. Phosphatase inhibition strengthens the association between insulator components and γH2Av and rescues γH2Av localization in insulator mutants. We also show that γH2Av is a component of insulator bodies, and that phosphatase activity is required for insulator body dissolution after recovery from osmotic stress. We further demonstrate a tight association between γH2Av and TAD boundaries. Together, our results indicate a novel mechanism linking insulator function with a histone H2A variant and with genome stability.

Genome architecture is regulated by chromatin insulator proteins, and misregulation of insulator function is associated with genome instability and transcriptional regulatory defects in both vertebrates and Drosophila. Indeed, mutations of the sole insulator protein in humans, CTCF, are carcinogenic and mutations in the Drosophila insulator protein Suppressor of Hairy wing [Su(Hw)] lead to chromosomal rearrangements. However, the mechanism that links the DNA damage response and the regulation of transcription with insulator function is not yet understood. Here we show that enrichment of Su(Hw) insulator proteins at insulator sites increases after DNA damage. Additionally, Su(Hw) is necessary for phosphorylation of the histone variant H2Av in response to both UV treatment and X-ray irradiation. The requirement of Su(Hw) for H2Av phosphorylation appears to be tissue-specific, since H2Av is phosphorylated in response to DNA damage also in neurons, where Su(Hw) is not normally expressed. Similarly, we provide evidence that Su(Hw) and H2Av work together to ensure proper development of the lymph gland in Drosophila larvae. We show that H2Av regulates formation of the larval lymph gland, and mutation of H2Av causes formation of large melanotic tumors that are rescued by mutation of su(Hw) in the His2Av810 mutant background. Double mutants of su(Hw)- and His2Av810 also form supernumerary lymph gland lobes that are larger than in wild type. Our findings support a role for the Su(Hw) in the DNA damage response through the regulation of H2Av phosphorylation and suggest that Su(Hw) and H2Av also work together to ensure proper development of the lymph gland.