1 2 CTR9 is the scaffold subunit in Paf1c, a multifunctional complex regulating multiple steps 3 of RNA Pol II-mediated transcription. Using inducible and stable CTR9 knockdown breast 4 cancer cell lines, we discovered that the expression of a subset of KDMs, including 5 KDM6A and Jarid2, is strictly controlled by CTR9. Global analyses of histone modifications 6 revealed a significant increase of H3K27me3 upon loss of CTR9. Loss of CTR9 results in 7 a decrease of H3K4me3 and H3K36me3 in gene bodies, and elevated levels and genome-8 wide expansion of H3K27me3. Mechanistically, CTR9 depletion triggers a PRC2 subtype 9 switching from PRC2.2 to PRC2.1. As a consequence, CTR9 depletion generates 10 vulnerability that renders breast cancer cells hypersensitive to PRC2 inhibitors. Our 11 findings that CTR9 demarcates PRC2-mediated H3K27me3 levels and genomic 12 distribution, provide a unique mechanism of transition from transcriptionally active to 13 repressive chromatin states and sheds light on the biological functions of CTR9 in 14 development and cancer. 15 16 17 18 19 20 21 22 23 4 RESULTS 1 2 CTR9 regulates a subset of histone lysine demethylases (KDMs) in ER+ breast 3 cancer cells 4 Data mining of the CTR9-regulated transcriptome in CTR9 inducible knockdown (KD) 5 MCF7 cells (MCF7-tet-on-shCtr9) (Figure 1A) (Zeng and Xu, 2016) identified a subset of 6 KDM genes that mirrors the expression of CTR9, including KDM1A, KDM2B, KDM3B, 7KDM5B, KDM6A and JARID2. We found that these KDM genes were down-regulated 8 when CTR9 was knocked down by shRNA in MCF7 cells after a 7-day doxycycline (Dox) 9 treatment ( Figure 1B) regardless of whether 17β-estradiol (E2) was present or absent.
10RNAPII binding peaks at the transcription start site (TSS) of several KDM genes also 11 decreased in response to CTR9 depletion ( Figure S1A). We validated the transcriptome 12 array results by RT-qPCR. Figure 1C shows that silencing of CTR9 after a 7-day Dox 13 treatment reduced the total mRNA levels, and reduced the expression of ribosome-14 associated RNAs, which are indicative of actively transcribed mRNAs, of six KDMs.
15However, the mRNA levels of KDM4B and KDM6B did not significantly change. CTR9's 16 regulation of KDMs was also observed at the protein level. As shown in Figure 1D, the 17 protein levels of six KDMs significantly decreased in the total cell lysates and extracted 18 chromatin fractions. Consistent with their mRNA levels in response to CTR9 depletion, 19 KDM4B and KDM6B protein levels also remained unchanged. To exclude the possibility 20 that this observation was specific to MCF7, or due to off-target effects of anti-CTR9 shRNA, 21 we employed MCF7 cells as well as T47D cells, another well-established ERα+ breast 22 34 35