Francesco Maura: The Mutagenic Impact of Radiotherapy in Hematological Malignancies

“Ionizing radiotherapy (RT) is a mainstay in the local control of hem malignancies. RT is used for both palliation and eradication intent.

Importantly, RT has been shown to increase risk of therapy myeloid neoplasms.”

“The idea that similar to melphalan and platinum, RT can promote indel mutagenesis (ID8) on tumors and normal tissues has been supported by few papers in the last years.”

“A great work from Young Seok Ju has recently provided a strong evidence that RT is responsible for unique indel signature in normal tissue: Quantitative and qualitative mutational impact of ionizing radiation on normal cells.

Cell Genomics.

“However, the impact of RT on hematological tumor genomics, evolution, and relapse has never been systematically investigated. Furthermore, in the setting of TMN it is unclear if RT promotes these tumors by direct mutagenesis of indirect mechanisms. “

“Here, thanks to our great collaborators, we leveraged a large series of 580 WGS to comprehensively assess the mutagenic impact of RT in multiple myeloma (MM), B-cell lymphoma (BCL), and t-MN.”

“Using sigprofiler and modified mmsig we found the presence of ID8 only in treated, relapsed cases, with either prior RT exposure or clear evidence of chemotherapy-associated mutagenesis from platinum or melphalan in the form of SBS31, SBS35, E_SBS37, and/or SBS99.”

“Because our data suggest that ID8 is not solely linked to RT, but can also be induced by mutagenic chemo, with Jonathan Schatz & Dhanvantri CHAHAR.

We performed an experiment in which a lymphoma cell line was treated with daily doses of RT, cisplatin, or untreated control.”

“Then single cells were expanded and subjected to WGS. Ben Diamond using different approaches, from prior studies from Boxtel Lab and Serena Nik-Zainal , we demonstrated that, similarly to RT, platinum may indeed cause the ID8 signature.”

“Importantly, we identified a dose-response relationship in which only sublethal doses of cisplatin were able to generate enough indels to confidently detect the signature. This may explain why some patients with evidence of platinum mutagenesis may not have detectable ID8.”

“Next, we asked how RT affects tumor evolution. To do so we leveraged the idea of single-cell expansion developed by our group and others (eg: Oriol Pich, Nuria Lopez-Bigas) to assess if post-RT relapse is driven by a RT surviving cell or by a cell not exposed by RT.”

“In this model, chemo signatures are only measurable in bulk WGS if a single cell bearing its unique catalog of chemo mutations expands to clonal dominance. If a progression has ID8, it means that a post-RT cell led the progression by seeding another anatomical site.”

“Combining genomic, radiological, and genomic data, we show that a single cell resistant to RT may survive the initial exposure and migrate to a distant anatomic site to seed a systemic relapse (see example below).”

“Finally, we re-analyzed a dataset of 39 TMN, of which 19 had prior RT exposure to assess how much the risk of post-RT TMN is driven by direct mutagenesis or not.”

“Overall only 2 cases in the set had ID8 and one case could clearly be attributed to platinum chemotherapy. While prior studies have associated RT with an increased risk of CH and TM, tour data suggest that RT promotes t-MN with an indirect, non-mutagenic mechanism.”

“Overall, this study showed that ID8 can be caused by RT and mutagenic chemo. We also demonstrate that post-RT surviving resistant cells may seed relapse both locally and at distant sites placing further emphasis on the need for developing truly eradicative strategies.”

“This study wouldn’t have been possible without our great network of collaborators. Huge thanks to Moffitt Cancer Center, Heidelberg Myeloma, Memorial Sloan Kettering Cancer for sharing wgs and clinical data!!

Thanks to my friend Jonathan Schatz at University of Miami for helping with the super-important validation!”

“Finally, huge kudos to Ben Diamond for leading this analysis and work! It is great to see how an MD seeing patients and running trials is able to successfully analyze>600.”