Like trying to find a lit match in a room full of burning furniture, isolating cancerous Hodgkin lymphoma cells from inside an inflamed tumor is a challenge that has hampered efforts to better understand the cancer’s genes and biology. Now, a group of researchers has isolated cancerous cells from noncancerous ones, but for the first time, they sequenced their entire genomes.
Reported in Blood Cancer Discovery, the study revealed a range of previously unknown genetic changes, including mutations in the genes that drive the cancer’s progression (1). By reconstructing a timeline of how those changes arose, researchers highlighted how the cancer goes from a seemingly innocuous lit match on the table, to dining room set alight, to room ablaze.
“Identifying these changes allows us to create a catalog of possible targets that we can use to develop new treatments,” said cancer geneticist and coauthor Francesco Maura from the University of Miami. “It also helps us understand why certain existing drugs work so well.”
Maura and his colleagues isolated elusive Hodgkin and Reed Sternberg (HRS) tumor cells from the tumors of 25 patients with Hodgkin lymphoma using optimized fluorescence-activated cell sorting. HRS cells are notoriously difficult to study because they only make up about one percent of a tumor’s mass.
Once isolated, the researchers amplified the cells’ DNA and sequenced it. When Maura’s team looked into the data, they found a host of changes to the HRS cell genome, including specific mutations, whole genome duplication in some cases, and structural changes like chromothripsis, which researchers described as the damaged chromosomes looking like they’d been hit by sledgehammers. Some of these changes were unsurprising and had been discovered before, but others were new (2).
Tomohiro Aoki, a hematologist and oncologist from the Princess Margaret Cancer Center who was not involved in the study, said in an email that “this study significantly upgraded our understanding of the pathogenesis of Hodgkin lymphoma in a sophisticated manner,” adding that the workflow the researchers described could be used to understand how other cancer types progress.
A particularly interesting finding was modifications to driver genes that help the tumor grow. One example is changes to the MSL2 gene, where a mutation contributes to impaired DNA repair. Ninety-five percent of the patients Maura’s team studied had at least one mutated driver gene; some had as many as 14.
“If we want to use the information to develop an eventual personalized treatment strategy, then we need to understand the heterogeneity of the tumor and the key drivers,” said Maura. At this stage, researchers still don't know exactly what each change to the DNA contributes to the symptoms patients experience or the severity of their cancers.
The researchers also figured out which of the genetic changes came before others. They saw that deletions of sections of DNA and some gene driver mutations happened early on in the tumor’s development, whereas doubling of the genome or big structural changes came later.
The prevailing theory of how Hodgkin lymphoma develops is that B cells mutate into cancerous HRS cells inside a germinal center (3). The timeline Maura and his team reconstructed instead suggests that the first genomic event can happen in B cells before the germinal center forms.
Knowing the order in which mutations arise could mean that, in the future, it will be possible to target treatments at those early driver events and halt the cancer’s progression. “Step by step, we will hopefully understand which drivers and in which order they need to be acquired for these cells to transform into Hodgkin lymphoma,” said Maura.
Aoki said that the timeline shows how unique the progression of Hodgkin lymphoma is compared to other blood cancers.
The study established a proof of concept including a relatively small number of patients. Next, Maura and his collaborators hope to apply similar techniques to a bigger pool of samples to help them identify biomarkers of Hodgkin lymphoma that might be found using less invasive and complicated means than sequencing the whole genome, such as through blood sampling.
References
- Maura, F., Ziccheddu, B., Xiang, J.Z., Bhinder, B., Rosiene, J., Abascal, F., Maclachlan, K.H., et al. Molecular evolution of classic Hodgkin lymphoma revealed through whole genome sequencing of Hodgkin and Reed Sternberg cells. Blood Cancer Discovery BCD-22-0128 (2023).
- Wienand K., Chapuy B., Stewart C., Dunford A.J., Wu D., Kim J., Kamburov A., et al. Genomic analyses of flow-sorted Hodgkin Reed-Sternberg cells reveal complementary mechanisms of immune evasion. Blood Advances 3 (2019).
- Piris, M.A., Medeiros, L.J., Chang, K-C. Hodgkin lymphoma: a review of pathological features and recent advances in pathogenesis. Pathology 52 (2020).