RNF128 Regulation of TP53 in Barrett's Progression

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Abstract: Esophageal adenocarcinoma (EAC) is a deadly cancer that arises within a premalignant field called Barrett’s metaplasia (BE). EAC is increasing in incidence at a rate greater than any other cancer and is associated with gastroesophageal reflux disease (GERD) and obesity. Understanding which properties of Barrett’s metaplastic epithelium are associated with progression to EAC may potentially identify novel therapeutic strategies to prevent this cancer. We utilized the BE obtained from patients who progressed to high-grade dysplasia and EAC and performed RNAseq analysis for gene expression as well as isoform-specific mRNA analysis in non-dysplastic Barrett’s, low-grade dysplasia (LGD), high-grade dysplasia (HGD) to EAC. Importantly, we observe that transition from Barrett’s/LGD to HGD/EAC is associated with significant isoform changes within individual genes. This “isoform switching” is consistent with pathway analysis indicating splicing as the top pathway altered in BE progression to EAC. We observe that “isoform switching” coincides with the loss of protective mucins, increased inflammation, activation of ATM/DNA damage response pathway and increased H2AX staining in HGD and EAC. ATM activation was recently reported to regulate splicing. One of our top genes that demonstrate dramatic isoform switching is RNF128 or Grail, an ubiquitin ligase (E3) that functionally interacts with the N-terminus of wild type (WT) TP53 to target its degradation and modulate its transactivation activity. Mutations in the TP53 gene are the most frequent event in EAC and increase in LGD and HGD. There are two Grail isoforms: RNF128-Iso1 and RNF128-Iso2 that have separate promoters. Iso1 is abundant in BE/LGD, dramatically reduced in HGD and EAC and decreased in EAC cells following treatment with either inflammatory (-IFN) or DNA damaging (H2O2) agents. Additionally, we found that the reduction in Iso1 causes a compensatory upregulation of the glycosylated form of Iso2 protein, which, in contrast to WT TP53, stabilizes mutant TP53* protein to increase clonogenic survival and resistance to anti-inflammatory drugs (statins) in immortalized BE cells. Thus, understanding the role of Grail isoforms in the regulation of wild type and TP53* function/stability in BE/EAC may lead to effective methods to modulate this important gene and prevent cancer development in patients with Barrett’s metaplasia.