Post-transcriptional regulation of MRE11 expression in muscle-invasive bladder tumours

Rebecca M Martin,Selina Bhattarai,Bradly G Wouters,Anne E Kiltie,Martin Kerr,Sarah J Jevons,Marianne Koritzinsky,Mark T.W Teo

doi:10.18632/oncotarget.1627

Abstract

Predictive assays are needed to help optimise treatment in muscle-invasive bladder cancer, where patients can be treated by either cystectomy or radical radiotherapy. Our finding that low tumour MRE11 expression is predictive of poor response to radiotherapy but not cystectomy was recently independently validated. Here we investigated further the mechanism underlying low MRE11 expression seen in poorly-responding patients. MRE11 RNA and protein levels were measured in 88 bladder tumour patient samples, by real-time PCR and immunohistochemistry respectively, and a panel of eight bladder cancer cell lines was screened for MRE11, RAD50 and NBS1 mRNA and protein expression. There was no correlation between bladder tumour MRE11 protein and RNA scores (Spearman's rho 0.064, p=0.65), suggesting MRE11 is controlled post-transcriptionally, a pattern confirmed in eight bladder cancer cell lines. In contrast, NBS1 and RAD50 mRNA and protein levels were correlated (p=0.01 and p=0.03, respectively), suggesting primary regulation at the level of transcription. MRE11 protein levels were correlated with NBS1 and RAD50 mRNA and protein levels, implicating MRN complex formation as an important determinant of MRE11 expression, driven by RAD50 and NBS1 expression. Our findings of the post-transcriptional nature of the control of MRE11 imply that any predictive assays used in patients need to be performed at the protein level rather than the mRNA level.

Highlights

Muscle invasive bladder cancer can be treated by surgical removal of the bladder or radical radiotherapy, with similar cure rates [1]
MRE11 protein expression is controlled at the post-transcriptional level and correlates with expression of RAD50 and NBS1
To determine the range of MRE11 protein expression levels in radiotherapy patients’ tumour samples and to determine whether control of expression occurs pre- or post-transcriptionally, formalin-fixed paraffin-embedded (FFPE) patient tumour sections were stained by IHC for MRE11 and 0.6 mm cores taken from a homogeneous tumour area for RNA extraction

Summary

Introduction

Muscle invasive bladder cancer can be treated by surgical removal of the bladder (cystectomy) or radical radiotherapy, with similar cure rates [1]. In two cohorts of radiotherapy patients, we found that patients whose tumours expressed low levels of the DNA damage signalling protein, MRE11, as measured by immunohistochemistry (IHC), had a significantly worse survival rate following radiotherapy than those expressing high levels of MRE11 (43% versus 70% 3-year cause-specific survival) [2]. Low MRE11 expression was predictive of poor response to radiotherapy, rather than being a prognostic marker in bladder cancer, as expression was not correlated with outcome in our surgical cohort. We observed lower MRE11 expression in tumour cells than normal urothelium [2], as seen previously in breast tumours [4, 5]; low MRE11 was associated with poorer radiotherapy outcomes in breast www.impactjournals.com/oncotarget cancer [5].

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