Spliced exon9 ADRM1 promotes liver oncogenicity via selective degradation of tumor suppressor FBXW7

Abstract

Background & Aims

The ubiquitin receptor ADRM1/Rpn13 governs the specificity of eukaryotic protein degradation. We first discovered a novel spliced variant of ADRM1 with a skipped exon 9, termed ADRM1-ΔEx9, in human hepatocellular carcinoma (HCC) by SMRT sequencing. The aim of this study was to elucidate this novel ubiquitin receptor's underlying biology and clinical implications in HCC.

Methods

The role of ADRM1-ΔEx9 in early liver carcinogenesis was studied using human liver-derived non-tumoral organoids and a murine model with hydrodynamic in vivo transfection. ADRM1-ΔEx9 biology in HCC and its potential as a biomarker for predicting olaparib response were investigated using patient-derived tumor organoids and xenograft models. The underlying mechanism was delineated using the Proteome Profiler Human Ubiquitin Array.

Results

ADRM1-ΔEx9, not its full-length counterpart, conferred human liver organoids with pro-survival advantages and led to more profound tumor formation in a hydrodynamic transfected murine model. Functional knockdown resulted in spontaneous apoptosis in cell lines and patient-derived organoids, highlighting a pivotal role for ADRM1-ΔEx9 in HCC oncogenicity. Mechanistically, the shortened C-terminus of ADRM1-ΔEx9 interacted with a different deubiquitinase partner (BAP1) to alter proteasome specificity. The new exon 8-10 fusion in ADRM1-ΔEx9 creates a de novo binding site for the tumor suppressor protein FBXW7, resulting in its selective proteasome-mediated degradation. The loss of FBXW7 protein in ADRM1-ΔEx9-expressing tumors underscores their sensitivity to the PARP inhibitor olaparib. Notably, findings on ADRM1-ΔEx9 in primary HCC tumors denote its overexpression in a subgroup of patients with inferior survival and a window of therapeutic opportunity through a synthetic lethal association with olaparib.

Conclusion

ADRM1-ΔEx9 redirects ubiquitin proteasome specificity to selectively degrade the tumor suppressor protein FBXW7. This promotes HCC tumor formation and provides a synthetic lethal link for PARP inhibitor therapy.

Impact and implications

Reduced tumor suppressor protein FBXW7 expression is pivotal in hepatocellular carcinoma (HCC) pathogenesis and other liver diseases. However, the regulatory mechanism governing FBXW7 protein expression remains elusive. Herein, we unveil a non-canonical spliced isoform of the ubiquitin receptor ADRM1 that selectively degrades FBXW7 protein, thereby promoting the premalignant transformation of hepatic cells and conferring growth advantages to HCC tumors. Furthermore, our results demonstrate that ADRM1-ΔEx9-expressing HCC tumors exhibited sensitivity to olaparib in a dose-dependent manner, implicating the potential use of olaparib in targeting ADRM1-ΔEx9-driven HCC growth.