Here,3 we have taken a novel approach to target ERG at the gene level rather than at the protein level and to our knowledge, this is the first example demonstrating the promise of using splice-switching oligonucleotides (SSOs) to target ERG in prostate cancer. SSOs are an antisense technology that work by interfering with pre-mRNA splicing and can be used to cause skipping of specific disease-associated exons from the pre-mRNA. The fusion between ERG and TMPRSS2 most often occurs between TMPRSS2 exons 1 or 2 and exon 4 of ERG. We decided to cause skipping of ERG exon 4 (218bp in size) with SSOs resulting in a frame-shift and premature stop codon, leading to nonsense-mediated decay (NMD) of the transcript and thus reduced ERG protein levels (Fig.1).
Figure 1. Overview of the study
We designed a panel of SSOs and tested these in two ERG-positive cancer cell lines – VCaPs (prostate cancer) and MG63s (osteosarcoma). In both lines we found that our SSOs caused skipping of exon 4 leading to reduced ERG protein levels (Fig.2, left panel). This in turn resulted in decreased cell proliferation, cell migration and significantly increased apoptosis in both VCaPs and MG63s (Fig.2, left panel). Furthermore, since ERG has been shown to drive Wnt/β-catenin signalling in the context of prostate cancer, we analysed the levels of several Wnt pathway components as well as pathway activity with TopFlash assays and found that pathway activity was significantly reduced (Fig.2, left panel).
We next investigated the potential of our SSOs to affect tumour growth using in vivo xenograft models of prostate cancer. Here we found that our most potent SSO based on our in vitro work could reduce tumour growth when delivered systemically (Fig.2, middle panel) and when we analysed excised tumours, we found significant exon 4 skipping and reduced ERG protein levels (Fig.2, middle panel). Importantly our SSO had no apparent toxic effects and did not affect endogenous ERG levels in the mice, demonstrating strong specificity to human ERG. Finally, we tested our most-promising SSO in an ex vivo system using patient-derived tumour explants (Fig.2, right panel). In these assays we demonstrated that SSO treatment reduced ERG protein levels in cultured tumour explants resulting in increased expression of PTEN protein (Fig.2, right panel), which we have previously shown to be repressed by ERG in prostate cancer cells.4
Figure 2. Main findings of the study
Our promising study3 paves the way for larger future studies aimed at testing similarly designed SSOs with improved delivery chemistry in larger cohorts of mouse models of prostate cancer as well as additional ex vivo patient tumour-derived samples to demonstrate the translational potential of this therapeutic approach.
Written by: Sean Porazinski, Faculty of Medicine, St Vincent's Clinical School, University of NSW; The Kinghorn Cancer Centre, 384 Victoria St.Darlinghurst, Sydney, NSW, 2010, Australia and Michael Ladomery, Faculty of Health and Applied Sciences, University of the West of England, Bristol, UK.
- Adamo, P. et al. 2016. The oncogene ERG: a key factor in prostate cancer. Oncogene 35, 403-414.
- Hagen, R.M. et al. 2014. Quantitative analysis of ERG expression and its splice isoforms in formalin fixed, paraffin-embedded prostate cancer samples: association with seminal vesicle invasion and biochemical recurrence. Am. J .Clin. Pathol. 142, 533-540.
- Li, L. et al. 2020. Targeting the ERG oncogene with splice-switching oligonucleotides as a novel therapeutic strategy in prostate cancer. British Journal of Cancer, Online ahead of print.
- Adamo, P. et al. 2017. The oncogenic transcription factor ERG represses the transcription of the tumour suppressor gene PTEN in prostate cancer cells. Oncol. Lett. 14, 5605-5610.