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Reduced abundance of the E3 ubiquitin ligase E6AP contributes to decreased expression of the INK4/ARF locus in non-small cell lung cancer

Abstract

The tumor suppressor p16INK4a, one protein encoded by the INK4/ARF locus, is frequently absent in multiple cancers, including non-small cell lung cancer (NSCLC). Whereas increased methylation of the encoding gene (CDKN2A) accounts for its loss in a third of patients, no molecular explanation exists for the remainder. We unraveled an alternative mechanism for the silencing of the INK4/ARF locus involving the E3 ubiquitin ligase and transcriptional cofactor E6AP (also known as UBE3A). We found that the expression of three tumor suppressor genes encoded in the INK4/ARF locus (p15INK4b, p16INK4a, and p19ARF) was decreased in E6AP-/- mouse embryo fibroblasts. E6AP induced the expression of the INK4/ARF locus at the transcriptional level by inhibiting CDC6 transcription, a gene encoding a key repressor of the locus. Luciferase assays revealed that E6AP inhibited CDC6 expression by reducing its E2F1-dependent transcription. Chromatin immunoprecipitation analysis indicated that E6AP reduced the amount of E2F1 at the CDC6 promoter. In a subset of NSCLC samples, an E6AP-low/CDC6-high/p16INK4a-low protein abundance profile correlated with low methylation of the gene encoding p16INK4a (CDKN2A) and poor patient prognosis. These findings define a previously unrecognized tumor-suppressive role for E6AP in NSCLC, reveal an alternative silencing mechanism of the INK4/ARF locus, and reveal E6AP as a potential prognostic marker in NSCLC.

Type Journal
ISBN 1937-9145 (Electronic) 1945-0877 (Linking)
Authors Gamell, C.; Gulati, T.; Levav-Cohen, Y.; Young, R. J.; Do, H.; Pilling, P.; Takano, E.; Watkins, N.; Fox, S. B.; Russell, P.; Ginsberg, D.; Monahan, B. J.; Wright, G.; Dobrovic, A.; Haupt, S.; Solomon, B.; Haupt, Y.
Responsible Garvan Author Prof Neil Watkins
Publisher Name SCIENCE SIGNALLING
Published Date 2017-01-10
Published Volume 10
Published Issue 461
Published Pages eaaf8223
Status ePublication
DOI 10.1126/scisignal.aaf8223
URL link to publisher's version https://www.ncbi.nlm.nih.gov/pubmed/28074012
OpenAccess link to author's accepted manuscript version https://publications.gimr.garvan.org.au/open-access/14242