Reduced mRNA expression levels of NFE2L2 are associated with poor outcome in breast cancer patients: Beyond the Abstract

The gene nuclear factor, erythroid 2-like 2 (NFE2L2; previously also known as NRF2) encodes a basic leucine zipper transcription factor which regulates the expression of antioxidant enzymes that protect against oxidative damage triggered by injury and inflammation. NFE2L2 is located under physiologic conditions in the cytoplasm where it is bound by its redox-sensitive adapter protein kelch-like erythroid cell-derived protein with CNC homology-associated protein 1 (KEAP1) and cullin 3 (CUL3).

In response to NFE2L2 inducers such as excess of reactive oxygen species leading to oxidative stress or chemopreventive compounds, KEAP1 undergoes conformational changes that partially disrupt the interaction with NFE2L2. Thus, NFE2L2 is stabilized, accumulates and translocates to the nucleus, where it dimerizes with members of the small musculoaponeurotic fibrosarcoma (MAF) protein family and binds to antioxidant response elements (ARE) or MAF recognition elements (MARE) in the promoter sequence of its target genes to initiate their transcription.  

Several research groups reported increased susceptibility to chemically induced carcinogenesis and decreased protection from metastasis in Nfe2l2-deficient mice. Therefore, NFE2L2 has long been considered a cytoprotective transcription factor which is essential for the defence against oxidative stress and activation of the NFE2L2 pathway has been proposed as potential preventive strategy against carcinogenesis due to its function as a master regulator of the expression of antioxidant and detoxifying enzymes. Interestingly, an increasing number of contrasting findings is emerging, uncovering the ’dark side of NFE2L2’. Also associations between high NFE2L2 expression and aggressive tumour behaviour were reported. Therefore it seems that NFE2L2 plays a dual role in cancer which is extensively discussed.

In the present study we investigated the role of NFE2L2 mRNA expression levels in breast cancer patients in several independent cohorts. First, we used the publicly available transcriptomic dataset of the Molecular Taxonomy of Breast Cancer International Consortium (METABRIC) with overall survival (OS) and disease-specific survival (DSS) data of 1942 patients as training set, second, a cohort derived from our own biobank consisting of 176 breast cancer patients including OS and relapse-free survival (RFS) data as test set and third, the publicly available The Cancer Genome Atlas (TCGA) dataset from 108 paired normal and cancerous breast tissues. 

We identified a predictive potential of NFE2L2 mRNA expression levels in breast cancer, especially in ER positive breast cancer, since high NFE2L2 expression was associated with better survival. A comparison of paired normal and cancerous breast tissues identified a higher NFE2L2 mRNA expression in normal tissues, what underscores its role as a tumour suppressor.

Thus, determination of the NFE2L2 mRNA expression level might be clinically useful to improve the characterization of breast cancer, eventually leading to more efficient and personalized treatment of breast cancer patients. However, high mRNA levels do not always have to result in increased protein levels and thus functional effects. Therefore we cannot exclude, whether upregulation of NFE2L2 expression is only a transcriptional (side-) effect not being further translated into protein or not exerting any cellular functions. 

Written by: Barbara P. Wolf1, Georg Goebel2, Hubert Hackl3, Heidi Fiegl*
1Department of Obstetrics and Gynaecology, 2Department of Medical Statistics, Informatics and Health Economics, 3Division of Bioinformatics, Biocenter, Medical University of Innsbruck, 6020 Innsbruck, Austria.