H2A.Z chaperones promote melanoma cell growth by  facilitating transcription of E2F target genes through H2A.Z deposition and H4 acetylation
High levels of the histone variant H2A.Z are characteristic for melanoma progression and correlate with poor prognosis of melanoma patients. Two chaperone complexes are reported to deposit H2A.Z isoforms into chromatin: SRCAP and P400-TIP60. YL1 is a common subunit of both complexes and directly binds to the H2A.Z-H2B dimer. Here, we showed that the knockdown of SRCAP (SRCAP-specific), P400 (P400-TIP60-specific) and YL1 (shared subunit) results in loss of H2A.Z deposition into chromatin in melanoma cells, confirming them as H2A.Z chaperone subunits in this system. Further, we demonstrated that H2A.Z, YL1 and the SRCAP-specific subunit ZNHIT1 co-localize at active promoters of cell cycle-related genes, such as the transcription factor E2F1. Knockdown of YL1, SRCAP and P400 downregulates E2F1 and its targets, resulting in a loss of proliferation and cell cycle arrest in melanoma cells as seen after knockdown of H2A.Z (Vardabasso et al. 2015). Strikingly, besides H2A.Z loss, we observed a dramatic loss of H4 acetylation in melanoma chromatin upon knockdown of H2A.Z chaperone subunits. The majority of genes whose promoters showed a reduction in H4 acetylation were previously bound by H2A.Z, YL1 and ZNHIT1. This is suggestive of a direct link between H2A.Z deposition and H4 acetylation to promote the expression of underlying genes. In agreement, the genes that lost H4 acetylation were enriched for E2F1 target genes. Interestingly, we additionally found that knockdown of YL1 did not only prohibit cell cycle progression, but also induces apoptosis, which is in contrast to H2A.Z knockdown affecting cell cycle only. In addition, YL1 (but not SRCAP or P400) was found to be overexpressed in melanoma and high YL1 levels were a predictor of poor melanoma patient outcome. These findings provide a rationale for targeting the H2A.Z-YL1 interaction as novel epigenetic strategy for melanoma treatment. Overall design: To investigate the role of the H2A.Z Chaperone Subunits YL1, SRCAP and P400 in melanoma cells, we performed shRNA-mediated knockdown of each gene (VPS72, SRCAP, EP400) and performed RNA-seq analysis 3 or 6 days after infection. A scrambled shRNA (SCR) served as negative control.