The oncogenic role of histone chaperone ASF1 proteins in solid tumors

Abstract

Chromatin is the essential medium connecting regulatory signals such as transcription factors and signaling pathways to the alteration of gene activity and cellular phenotypes. Aberrant chromatin (epigenetic) environment plays an important role in carcinogenesis. The fundamental unit of chromatin is the nucleosome which is composed of a histone core wrapped with 145-147 base pairs of DNA around. In the last decades, great efforts have been made to delineate the role of aberrant DNA methylation and chromatin/histone-remodeling factors in oncogenesis. However, recent evidence has merged that the dysregulation of histone chaperones also acts as a cancer-driver. Anti-silencing function 1 (ASF1) is the most conserved histone H3-H4 chaperone, regulating histone metabolism. ASF1 proteins include two paralogs ASF1A and ASF1B in mammals. ASF1A and ASF1B have been reported as oncogenes in human cancers. Data from the Cancer Genome Atlas (TCGA) and the Genotype-Tissue Expression (GTEx) databases show that ASF1A and ASF1B are overexpressed in 20 and 24 different types of cancers, respectively. Thus, in this thesis, I explored the oncogenic role of histone chaperone ASF1 and underlying molecular mechanisms in several solid tumors.

In Paper I, the role for ASF1A in gastrointestinal cancer (GIC) was investigated. We discovered that ASF1A interacted with the oncogenic transcription factor β-catenin and promoted the transcription of β-catenin target genes (c-MYC, cyclin D1, ZEB1 and LGR5). The increased expression of these genes stimulated proliferation, stemness and migration/invasion of GIC cells. Over-expression and knockdown of ASF1A boosts and inhibits in vivo tumor growth and/or metastasis in mouse models, respectively. Higher levels of ASF1A expression predict significantly shorter patient survival in colorectal cancer (CRC). Further analyses of the Gene Expression Omnibus dataset validate higher ASF1A expression predicting a poor prognosis in CRC patients. Taken together, this study reveals the novel function of ASF1A as a transcription co-factor independent of its canonical role and the potential value of ASF1A for outcome prediction and targeted treatment in GIC. In Paper II, we show that ASF1A overexpression is widespread in human malignancies and is required for the infinite proliferation of cancer cells. When ASF1A was knocked-down in wild-type (wt) p53 carrying cells derived from hepatocellular carcinoma (HCC) and prostate cancer (PCa), DNA damage response was activated and up-regulation of p53-p21cip1 expression consequently occurred. These cells eventually underwent cellular senescence. Higher ASF1A expression and/or lower p21cip1 expression predicts a poor outcome in HCC patients. Thus, ASF1A may be a therapeutic target and a prognostic factor in HCC and other cancers. In Paper III, we evaluated whether ASF1B has diagnostic and prognostic values in adrenocortical carcinoma (ACC) and regulates invasion and metastasis. We first analyzed TCGA and GTEx data and found that the ASF1B gene was amplified in two thirds of ACC tumors and associated with its overexpression. ASF1B expression correlates with the ACC diagnostic criteria of the Weiss scoring system. Higher ASF1B expression and ASF1B copy number predict a poor outcome in the TCGA cohort of ACC patients. Knockdown of ASF1B in ACC cells impairs migration and invasion ability by inhibiting expression of the transcription factor FOXM1; whereas ASF1B over-expression exhibits opposing effects. These findings suggest that ASF1B may be a useful factor for ACC diagnostics and prognostication, and potentially a novel target for ACC therapy as well.

Collectively, the results presented in this thesis gain profound insights into the oncogenic role of ASF1 in several solid tumors and demonstrated novel activities of ASF1 proteins beyond their conserved histone chaperone function. These findings will inspire further exploration of both the clinical and biological roles of ASF1 in precision oncology.