The Cellular Function of USP22 and its Role in Tissue Maintenance and Tumor Formation

Abstract

Colorectal cancer (CRC) is the most frequent malignancy of the gastrointestinal tract and therefore reflects a global health issue. Despite improved detection and treatment options the high mortality and morbidity rates of this disease emphasize the urgent need to unravel underlying mechanisms. Over the last decade extensive investigation allowed the identification of numerous key players involved in the onset and progression of CRC. Importantly, epigenetics is an emerging research area which has already contributed significantly to understanding this disease. Generally, epigenetic mechanisms can affect the gene expression profile without altering the underlying DNA sequences. One mediator involved in these processes is the Ubiquitin-Specific Protease 22 (USP22) which is able to deubiquitinate the core histones H2A and H2B as well as other target proteins. Interestingly, the overexpression of USP22 in CRC patients was revealed by several studies and it was identified as a member of the so-called 11-gene “death-from-cancer” signature. This signature was correlated with poor prognosis and distant metastasis. However, the physiological function of USP22 in organ maintenance as well as its role in intestinal tumorigenesis remain to be elucidated. In the current project we investigated the effect of a global reduction and intestinal deletion of Usp22 in vivo. By generating a global Usp22 hypomorphic mouse line we could overcome the embryonic lethal effect of a complete loss of this gene. Using a lacZ reporter gene expressed under the control of the endogenous Usp22 locus, we detected ubiquitous Usp22 expression during embryonic development. Accordingly, a global reduction of Usp22 expression resulted in systemic effects in adult mice, i.e. reduced body size and weight. Moreover, while the gross organ morphology was only marginally affected in these animals, cell differentiation and lineage specification were influenced in the brain and, as we have observed earlier, the small intestine. The involvement of Usp22 in developmental and differentiation processes was confirmed in mouse embryonic fibroblasts by microarray-based gene expression analysis. Moreover, the function of Usp22 in tumorigenesis was explored in animals possessing an intestinal Usp22 loss and a truncation mutation in the tumor suppressor Adenomatous Polyposis Coli (APC). Surprisingly, Usp22 deletion combined with an APC mutation resulted in decreased survival rates, increased intestinal inflammation and tumor burden and importantly, led to the formation of invasive carcinomas with a mucinous phenotype. Our findings, that not only overexpression, as stated in the literature, but also downregulated USP22 expression can increase intestinal tumor burden were supported by in vitro and in silico analyses which revealed highly heterogeneous USP22 expression among CRC patients. Using global expression analyses we detected the involvement of USP22 in several cancer-related processes and identified the heat shock protein HSP90AB1 as an important USP22 target. Notably, we discovered that USP22 directly binds to the HSP90AB1 protein and prevents its proteasomal degradation. In addition, USP22 knockdown led to reduced HSP90AB1 expression levels. As a consequence USP22 knockdown cells were more sensitive towards elevated temperatures. Intriguingly, we were able to induce synthetic lethality in CRC cells with low USP22 levels by treating them with an HSP90 inhibitor or the Bromo- and Extra-Terminal (BET) domain inhibitor JQ1. In summary, in this study we significantly contributed to the current knowledge about USP22 by demonstrating its relevance in organ maintenance and intestinal tumor formation in vivo as well as by describing how CRC cells with low USP22 levels can be therapeutically targeted in vitro.