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Disrupting cellular memory to overcome drug resistance

Guillaume Harmange, Raúl A. Reyes Hueros, Dylan Schaff, Benjamin Emert, Michael Saint-Antoine, Shivani Nellore, Mitchell E. Fane, Gretchen M. Alicea, Ashani T. Weeraratna, Abhyudai Singh, Sydney M. Shaffer

Preprint posted on 5 August 2022 https://www.biorxiv.org/content/10.1101/2022.06.16.496161v3

Predicting the future of drug naïve tumor cells by looking at their past.

Selected by Nate Mullin

Background

Is there a way to understand, or even predict, the seemingly random behavior of certain cells? If so, how could such an understanding be leveraged to combat insidious cellular behaviors such as chemotherapy resistance?

In normal development and in disease states like cancer, the character of individual cells is malleable. Two descriptions of what a cell may be able to do at any particular time are cell state and cell type. While the exact distinction between these categories remains open to debate, cell type is generally regarded as a permanent description based on developmental lineage and function. Cell state exists on the opposite end of the permanence spectrum and can be thought of as a transient set of characteristics taken up by a cell, perhaps in response to an external stimulus or perhaps due only to random chance.

The preprint by Harmange et al. focuses on a type of cellular identity that sits in between these two extremes: cell memory. Shaffer and colleagues previously described cell memory as a heritable (across cell division), yet impermanent feature of a cell. For example, gene expression memory would describe a state of gene expression present in a cell for a certain amount of time that might even be passed into daughter cells yet is not part of the core program that defines the cell’s identity (i.e., once the transient time period ends, the cell would not lose its identity nor cease to be that type).

In the context of cancer, gene expression memory states have important clinical implications. Cell memory may confer drug resistance to certain lineages of cells, leading to growth of drug-resistant tumors after chemotherapy. Because of the transient nature of gene expression memory, these states are not genetically encoded, and may be gained or lost in particular lineages across time. Harmange et al. used the well-described melanoma model of drug resistance to BRAF and MEK inhibitors to define memory states of cancer cells primed to resist or succumb to drug treatment. They then went on to experimentally identify the pathway that allows state switching in these cells prior to drug treatment.

 

Key findings

Single-cell memory sequencing (scMemorySeq) combines cellular lineage tracing with gene expression profiling through transient cell state transitions.

In order to understand the transcriptional landscape of cells in different states (i.e. primed for drug resistance or susceptible to treatment) and to track lineages of cells that transition between states, the authors developed scMemorySeq (Figure 1). This technique combines lineage tracing via lentiviral barcoding with phenotyping and single-cell gene expression analysis. ScMemorySeq allows the past behavior of a cell to be linked to its current cell state.

Figure 1 scMemorySeq overview.
Figure 1 scMemorySeq overview. Cells were permanently tagged with a heritable barcode that could be read via polyA-enriched RNA sequencing. Cell state (i.e., primed for resistance) was measured and cells were allowed to expand. Rare state-switching events that occur during expansion are shown in the schematic. Finally, the transcriptome of individual cells was read out, and lineages were reconstructed based on embedded barcodes. Adapted from Harmange et al. 2022.

 

Lineages that switch between susceptible and resistance-primed states are transcriptomically distinct from those that do not.

Related cells (i.e. with a common progenitor) can change state to either gain or lose the transcriptional profile associated with drug susceptibility. By comparing lineages of cells that make this switch to lineages that do not, key genes involved in state conversion can be identified (Figure 2). These can be aggregated into a gene expression module (“crossing lineage score”) that individual cells can be scored on. The authors identified epithelial mesenchymal transition (EMT)-related genes as enriched in state-switching lineages.

Figure 2 State-switching characterization by scMemorySeq. Reconstructed lineages identified cells that had switched from primed to susceptible or vice versa during the pooled expansion. Comparing the gene expression profiles of switched lineages allowed for the identification of state switching-associated genes. Adapted from Harmange et al. 2022.

 

Modulation of drug-response behavior through targeting of transition-mediating pathways.

The authors targeted the pathways involved in state switching identified by scMemorySeq using agonists and antagonists of the TGFB and PI3K pathways. Inhibition of PI3K in a mixed population of primed and susceptible cells led to a decrease in the proportion of primed cells, whereas activation of TGFB signaling increased the proportion of primed cells (Figure 3). Transcriptionally, newly primed or susceptible cells resembled those without treatment (Figure 3F).

Figure 3 Targeting pathways involved in state-switching can modulate the proportion of cells in different states. Primed and drug-susceptible states could be identified based on gene expression profile. TGFB1 activation increased the proportion of cells in the primed state, while PI3K inhibition pushed cells toward drug susceptibility. Notably, the cells in either state following treatment with TGFB or PI3K-targetting compounds resembled untreated cells of the same state transcriptionally. Adapted from Harmange et al. 2022.

 

PI3K modulation increases the proportion of drug-susceptible cells through state-switching.

State-switching behavior was modeled using a simulation taking into account three parameters: the rate of state switching, the rate of cell death, and the rate of cell division. The authors found that an increased rate of state switching best explained their experimental data. This finding supports the hypothesis that drug treatment alters population dynamics through increased switching, not by killing off certain populations. The authors could finally show that pre-treatment with state-switching drugs does in fact increase the effectiveness of BRAFi/MEKi treatment against drug-naïve melanoma cells in vitro.

 

What I like about this work

In recent years, single-cell genomic and transcriptomic techniques have put the spotlight on cellular heterogeneity in the context of disease and normal physiology. However, such are mostly restricted to a single point in time. This means that while we can observe sub-populations of cells within a tissue, we cannot know how they have in the past or would in the future react to particular insults or unique environments. Such knowledge is essential for understanding how disease develops (e.g. why are some, but not all cells of a certain type affected by a genetic variant) and how transient subpopulations could best be targeted in the treatment of disease

In this preprint, the authors have demonstrated how scMemorySeq can be used to overcome this temporal hurdle. They showed that even within validated clusters, pre-fated trajectories are coded into individual cells. As a reader and researcher interested in genetic disease, the concept that these fate trajectories are knowable and detectable with current technology shifts how I think about cellular heterogeneity.

 

Questions for authors

  1. Is state-switching itself a lineage-specific phenomenon? Do certain lineages switch states more frequently than others?
  2. Similarly, what do you think changes a lineage’s ability to switch states? Could this be genetically encoded, or are all cells genetically identical?
  3. Are transient gene expression memory states important outside of cancer? Do you think that these underly cell fate decisions in normal development?

 

Posted on: 18 September 2022

doi: https://doi.org/10.1242/prelights.32390

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Author's response

Guillaume Harmange shared

  1. Is state-switching itself a lineage-specific phenomenon? Do certain lineages switch states more frequently than others?

We have not specifically tested whether all lineages undergo state-switching, or if some lineages switch states more frequently than others, however, our observations in the lab suggest that all lineages have roughly the same state-switching rate. One piece of evidence for consistent state-switching rates between lineages is that when we grow single cell clones from these cell lines, the percentage of primed cells in each clone is very consistent, suggesting similar rates of state-switching in each clonal population.

  1. Similarly, what do you think changes a lineage’s ability to switch states? Could this be genetically encoded, or are all cells genetically identical?

The cells used in this study are clonal cell lines, therefore the cells in the population should be very similar to each other genetically. We have done experiments in which we isolate individual clones and found both states regardless of the clone. These experiments show that it is not a single genetic clone that is capable of state-switching. Therefore, we believe genetics is not the main determinant for the ability of these cells to switch states. The root cause of state-switching is still an open question. Based on the findings of this paper, what we can say is that cells seem to switch into the primed state if the TGFB signaling pathway is activated, and can switch back to the drug-susceptible state by stopping PI3K signaling. However, the endogenous triggers (or the rate-limiting factors) for these signaling pathways remain to be fully characterized.

  1. Are transient gene expression memory states important outside of cancer? Do you think that these underlie cell fate decisions in normal development?

This is an interesting question that we do not think has been fully explored yet. Developmental systems could be an interesting application for scMemory-seq and can help determine if there are memory states important in the development process. To our knowledge, there are no studies specifically using these techniques to quantify memory of gene expression in development. However, Weinreb et al. is an example where they used cellular barcoding in the context of development to demonstrate that cell lineage is an important factor in determining cell fate (https://www.science.org/doi/10.1126/science.aaw3381).

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