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Andrea Miyahira: Hi, I’m Andrea Miyahira at the Prostate Cancer Foundation. Today with me is Dr. Roni Haas of UCLA. She will discuss her paper, “The Proteogenomics of Prostate Cancer Radioresistance,” that was recently published in Cancer Research Communications. Dr. Haas, thanks for joining us.

Roni Haas: It is a pleasure. Thank you very much for having me. And thank you for the introduction. So I will walk you through the highlights from our recent publication. Our study centered around radiotherapy. It is standard of care in prostate cancer. And traditionally, radiation therapy involves many small radiation doses spread over weeks of treatment. But now with advancement in technology, there is a tendency toward using hypofractionation radiotherapy. And that means less treatment with a higher radiation dose. And unfortunately, radioresistance can emerge following both hyper- and hypofractionation radiotherapy, but we don’t really understand the reasons for that.

So the main goal of the study was to characterize the molecular underpinning of radioresistance, and specifically to test the differences between the radioresistance generated following the conventional hypofractionation versus hyperfractionation radiotherapy. So to study this, we first created two types of radioresistant cell lines, following the conventional treatment and following the hypofractionation radiotherapy. We comprehensively characterized these cell lines from a biomolecular perspective. And then we selected targets that were involved in radioresistance and tested their clinical relevance using patient data.

So just the highlights. We wanted to have the biomolecular phenotype for each cell line. And we started with DNA. And we saw that the number of mutations induced by radiation is much larger following the conventional treatment compared to the hypofractionation. And then at the RNA level, we tested dysregulation of driver genes, which are genes that are known to be directly involved in cancer progression and development. And strikingly, there was much more dysregulation of driver genes following the conventional treatment compared to the hypofractionation, because we can see a very modest dysregulation overall. And we saw the same also at the protein level, especially when we focus on whole cells and the nuclear pellet. Again, more dysregulation of drivers after the conventional treatment. And in the bottom line, the conclusion was that following the conventional treatment, there is a more aggressive radioresistance biomolecular phenotype.

And then in the second step, we took targets from the preclinical models that we identified are involved in radioresistance, and we tested their clinical relevance using patient data. And one gene that we found particularly interesting was POLQ. It is DNA polymerase theta. And we chose it because it was associated with aggressive disease in so many different ways. And here, for example, POLQ amplification is associated with biochemical recurrence of the disease.

And then we thought it would be interesting to see if POLQ inhibition can cause radiosensitization in radioresistant cells. And indeed, after a genetic inhibition, both the conventional and hypofractionation radioresistant cells showed radiosensitization compared to the corresponding control. And the same was true after a pharmacological inhibition.

So to summarize this, we showed that splitting radiation dose into more fractions resulted in a dramatically more aggressive radioresistant phenotype, with more mutations induced by radiation and more dysregulation of driver genes. And then we identified the DNA repair gene POLQ as a modulator of radioresistance in preclinical models. And the take-home message is that prostate cancer radioresistance is fractionation-specific and that POLQ is a new radioresistance modulator. Thank you very much for listening. And just to quickly thank all people that were involved in this work, and especially Paul Boutros, my mentor, and Dr. Stanley Liu and Dr. Thomas Kislinger from Toronto University, our collaborators in this project. Thank you very much.

Andrea Miyahira: Thank you, Dr. Haas, for sharing this study. So what are the mechanisms that you think underlie the differences in the alterations that are driving resistance to conventional versus hypofractionation radiation?

Roni Haas: Yes, so we show that there is a more aggressive radioresistant phenotype following the conventional method. And we think that there is a logical explanation for that. So when we use a conventional radiotherapy treatment, there is the involvement of small radiation doses daily over weeks of treatment. And since the radiation doses are relatively small, they are less toxic to cancer cells. So cancer cells have more chances to survive. And when they survive, they can also gain mutations between treatments. And these mutations can confer evolutionary advantages that can help the cells become more aggressive. And compellingly, if we use hypofractionation methods, the radiation doses are larger. So the cancer cells are more likely to be killed and gain less mutations between these single treatments. So I think that, overall, our study supports the tendency in the field to go toward using hypofractionation methods. It stands the idea of increasing the radiation dose. And reducing the number of treatments seems to be biologically better. And it’s also in line with some clinical evidence showing better disease control in some situations with hypofractionation radiotherapy.

Andrea Miyahira: OK, thank you. And are any of the genes identified, such as POLQ, possible targets for therapy in combination with radiation therapy?

Roni Haas: Yes. So we tested several targets that were involved in radioresistance preclinically using patient data. And that convinced us that they can also be clinically relevant. We moved on only with POLQ for technical reasons. But testing the other targets will be also very interesting in the future. In terms of POLQ, we chose it because we thought it can be a promising target, first, because its expression in tumor cells is much higher compared to normal cells. So targeting POLQ would minimize damage to normal cells. And then the other thing is that POLQ is a DNA repair gene. It is involved in one of three main pathways for double-strand break DNA repair. And it has been investigated as a potential therapeutic target with a clinical trial, especially in the context of patients with BRCA deficiency or HR DNA repair deficiency. But it was also shown that POLQ together with RT can cause radiosensitization in treatment-naive cells. But now, in our study, we also show that POLQ can cause radiosensitization in radioresistant cells—cells that were already after the treatment and gained radioresistance. We showed it preclinically. And the steps in the next step will be, of course, to show this in vivo.

Andrea Miyahira: Thank you. So should patients be pre-screened for any of these alterations to determine if they would more likely benefit from surgery or drive treatment decisions between conventional and hypofractionation?

Roni Haas: Yeah, I think it’s a great question and exactly where our thoughts are going. We have started testing that, to see if information about these targets can predict treatment responses. It is very initial, but definitely something that we are doing. And in terms of driving decisions between CF and HF, I think it’s a great idea. We would probably need to wait a little bit until we have more data because currently, most of the patients that we have went through a conventional therapy and not a hypofractionation, because it’s relatively new. But I believe that in the near future, we will have more data of patients that went through hypofractionation. And that’s definitely something that we would like to test as well.

Andrea Miyahira: OK, well, thank you so much for sharing this study with us today, Dr. Haas.

Roni Haas: Thank you. Thank you for having me.