Condensin IDC, H4K20me1, and perinuclear tethering maintain X chromosome repression in C. elegans
Dosage compensation in Caenorhabditis elegans equalizes X-linked gene expression between XX hermaphrodites and XO males. The process depends on a condensin-containing dosage compensation complex (DCC), which binds the X chromosomes in hermaphrodites to repress gene expression. Condensin IDC and an additional five DCC components must be present on the X during early embryogenesis in hermaphrodites to establish dosage compensation. However, whether the DCC's continued presence is required to maintain the repressed state once established is unknown. Beyond the role of condensin IDC in X chromosome compaction, additional mechanisms contribute to X-linked gene repression. DPY-21, a non-condensin IDC DCC component, is an H4K20me2/3 demethylase whose activity enriches the repressive histone mark, H4 lysine 20 monomethylation on the X chromosomes. In addition, CEC-4 tethers H3K9me3-rich chromosomal regions to the nuclear lamina, which also contributes to X-linked gene repression. To investigate the necessity of condensin IDC during the larval and adult stages of hermaphrodites, we used the auxin-inducible degradation system to deplete the condensin IDC subunit DPY-27. While DPY-27 depletion in the embryonic stages resulted in lethality, DPY-27 depleted larvae and adults survive. In these DPY-27 depleted strains, condensin IDC was no longer associated with the X chromosome, the X became decondensed, and the H4K20me1 mark was gradually lost, leading to X-linked gene derepression. These results suggest that the stable maintenance of dosage compensation requires the continued presence of condensin IDC. A loss-of-function mutation in cec-4, in addition to the depletion of DPY-27 or the genetic mutation of dpy-21, led to even more significant increases in X-linked gene expression, suggesting that tethering heterochromatic regions to the nuclear lamina helps stabilize repression mediated by condensin IDC and H4K20me1. Overall design: RNA-seq profiles of L3 larval stage C. elegans samples. Synced L1s were placed on HGM plates with or without auxin, and NA22. 24-hours later, L3 larvae were collected for RNA extraction. Genotype and treatment for each sample are listed separately.