By Alison Aiken This post highlights the work of Ryan Draker from Dr. Peter Cheung’s lab, published earlier this year in Nucleic Acids Research (PMID: 21245042) [Link to Paper].

In order for DNA to fit into cells, it gets coiled around octamers of small, positively charged proteins: the histones. Two each of histones H2A, H2B, H3 and H4 form the core of the nucleosome, around which DNA is coiled. Variant histones, which can differ significantly from core histones in their amino acid composition, can replace core histones in the nucleosome and are normally deposited in specific regions. A variant of H2A, H2A.Z, has been implicated in several cellular processes, including the regulation of transcription. Interestingly, H2A.Z can have either a positive or negative influence on gene transcription. This histone variant can be post-translationally modified by either acetylation or mono-ubiquitylation. Previous work by the Cheung lab has shown that H2A.Z is ubiquitylated by Ring1b, an E3 ligase that is part of the Polycomb Repressive Complex 1, and that mono-ubiquitylated H2A.Z is associated with transcriptionally inactive chromatin. The authors hypothesize, therefore, that the deubiquitylase(s) for H2A.Z would play an important role in the activation of transcription.

In this paper, three deubiquitylases for H2A.Z are identified for the first time: ubiquitin-specific protease 10 (USP10), USP16 and 2A-deubiquitylating enzyme (2A-DUB). Of these, USP10 and DUB-2 seem to have preferential activity for H2A.Z over H2A in in vitro deubiquitylation assay. While USP16 activity has been linked with mitosis, USP10 and 2A-DUB have been associated with activating gene expression following androgen receptor (AR) stimulation.

Focusing on USP10, the authors establish that it exists in largely separate cytoplasmic and nuclear pools, suggesting specific functions for USP10 depending on localization. Subsequently, the authors use an androgen-responsive luciferase construct to confirm that expression of USP10 does in fact increase gene expression in response to the AR ligand dihydrotestosterone (DHT). Conversely, knockdown of USP10 decreases expression of the androgen-responsive luciferase after DHT treatment and also results in an increase in mono-ubiquitylated H2A.Z. Next, Ryan and his colleagues looked at endogenous AR responsive genes: KLK3, better known as Prostate Specific Antigen (PSA), and KLK2. Both USP10 and H2A.Z are shown to be necessary for KLK2 and PSA expression. Finally, a series of elegant chromatin immunoprecipitation (ChIP) experiments provide proof of direct regulation of PSA and KLK2 by mono-ubiquitylated H2A.Z following DHT treatment.

In addition to identifying three H2A.Z deubiquitylases, this paper provides the first direct evidence that the mono-ubiquitylated form of H2A.Z is physically associated with the promoters and enhancers of repressed PSA, and that removal of the ubiquitin moiety increases gene expression. PSA levels are currently measured as part of prostate cancer screening protocols, and are monitored during and following treatment as indicators of therapeutic success. Thus, a better understanding of how androgen-responsive genes are regulated could one day be of help to men diagnosed with prostate cancer.