Andrea Miyahira: Hi, everyone. I'm Andrea Miyahira at the Prostate Cancer Foundation. With me are Doctors Hansen He, Mona Teng, and Xin Xu of the Princess Margaret Cancer Center in Canada to present their just published paper, Circular RMST Cooperates with Lineage Driving Transcription Factors to Govern Neuroendocrine Transdifferentiation published in Cancer Cell. Thank you all for joining us today.

Housheng Hansen He: Thank you, Andrea.

Mona Teng: Thank you. Again, thank you so much for the introduction and the opportunity to share our recent work.

So the clinical challenge that we wanted to investigate for this study is neuroendocrine transdifferentiation, which is a phenomenon observed where adenocarcinoma cells become distended neuroendocrine tumor cells, especially seen in lung and prostate.

The questions we had are, How can we prevent and target neuroendocrine transdifferentiation? Are there any unidentified regulators that haven't been reported before? For example, circular RNA.

Circular RNA, like the name suggests, they are a class of non-coding RNA with a covalent link circular structure. And they have been shown to possess significant biological functions in some other biological processes, but it hasn't explored in the context of neuroendocrine transdifferentiation.

Using total RNA sequencing, we identified this circular RNA, called circular RMST, that is highly abundant only in neuroendocrine tumors, but not adenocarcinomas.

We've shown that the circular RNA is essential for tumor growth, as evidenced by the middle panel, that the removal of the circular RNA leads to significant cell death.

In addition to that, using genetically engineered mouse models, we identified the circular RMST is also essential for the transdifferentiation. These mice models spontaneously develop NEPC that are transdifferentiated from adenocarcinoma, but when we delete RMST, in that background, we saw no neuroendocrine tumor formation, but all the tumor cells are sustained in the adenocarcinoma state.

Using RNA protein and various other assays, we identify circular RMST interacts with two neuroendocrine transcription factors, NKX2-1 and SOX2. So together, they form a regulatory network and drive the expression of ASCL1. We and many other papers are recognizing ASCL1 as a potent oncogene in these neuroendocrine tumors.

And this presents circular RMST as a novel therapeutic target that hasn't been identified before, and targeting it can potentially disrupt this essential transcription network.

In addition to that, we also have some exciting unpublished work demonstrating that circular RMST can have potential as a liquid biopsy biomarker. One evidence is that circular RNA, due to their structure and higher stability, they're actually present more in plasma exosomes, compared to linear RNA.

And in a small pilot study, we observed high copies of circular RMST, specifically in the plasma of patients with lung and pancreatic neuroendocrine tumors. So far, we haven't seen such high copies present in healthy individuals.

And we would just like to summarize and emphasize that we identified this novel circular RNA, called RMST, that is an essential regulator of neuroendocrine transdifferentiation and can be a therapeutic target.

We don't have time to touch on this, but we think this may be, in particular, effective in response with or in combination with immunotherapy. And we propose that it has liquid biopsy biomarker as well.

And again, thank you so much for letting us share the work. Thank you Uro Oncology and also all the funding agencies that supported this study. And definitely, all the members in the He Lab supported so much for the study, as well as external collaborators listed here. Thank you.

Andrea Miyahira: Thank you so much, Dr. Teng, for presenting that. So do you know what drives circular RMST expression in NEPC and its timing of expression in NEPC transdifferentiation?

Mona Teng: Yes. So one of the factors we know that contribute to the activation of circular RMST expression is RE1-silencing transcription factor, or REST, for short. So REST is a master suppressor of neuroendocrine-related genes and non-neuronal tissues.

And the loss of REST is one of the early on key events that initiate neuroendocrine transdifferentiation. And we've shown it in cells, when you knockdown REST, we see an induction in circular RMST's expression. So we believe REST is definitely one of the factors.

And now we have animal models that allow us to study the transition from adeno to neuroendocrine tumors. And we're planning to collect serial samples to really allow us to map out the timing and to confirm the involvement of REST, as well as identify any other transcriptional activators.

Andrea Miyahira: Thank you. And does blocking circular RMST revert NEPC cells to becoming sensitive to AR inhibition?

Mona Teng: I think for a circular RMST's function, there are really two layers to it. So in cells that are terminally differentiated in the neuroendocrine stage, for example, neuroendocrine prostate cancer, small cell lung cancer that are already in the neuroendocrine lineage, deleting circular RMST is essential. We see massive cell death. We don't see that the cells have opportunity to revert back to the AR-sensitive stage.

But in tumors that are still adenocarcinoma, when you delete RMST, we really see a prevention to the neuroendocrine stage. We are planning to perform deletion of RMST in these adenocarcinoma cells and co-treating with AR inhibition, to really answer the question, are those tumors, really again, AR-sensitive?

Andrea Miyahira: Thank you. And so you mentioned that circular RMST may be a therapeutic target in NEPC. So how can it be targeted?

Housheng Hansen He: So circular RMST is a non-coding RNA. So it's challenging to develop a traditional therapeutic approaches, including small molecule and antibodies. But there are recently evolved RNA-based therapies that can be used to target circular RNA.

So specifically, we have designed siRNA against a circular RMST and delivered by lipid nanoparticle. And we have tested it in a PDX-derived xenograft models, and then shown that silencing circular RMST using the LNP-delivered RNA drug. It can significantly reduce the tumor growths. So we're further developing that into a clinical therapeutic targeting.

Andrea Miyahira: Thank you. And what do you plan to do next in these studies?

Housheng Hansen He: So in addition to what Mona mentioned, we're interested in understanding when and how circular RMST is activated during that neuroendocrine transdifferentiation, further understand the molecular mechanism. We're also actively testing whether circular RMST can be a liquid biopsy biomarker in early detection of neuroendocrine tumor.

And the second, as I mentioned, we're also actively working on developing RNA-based therapeutics targeting circular RMST.

Andrea Miyahira: Thank you all so much for coming on and sharing this study with us today. And I hope that our viewers can also view the accompanying interview with Doctor He and Doctor Xu on another paper that just came out from their lab.

Housheng Hansen He: Thank you.

Mona Teng: Thank you.