Non-genetic factors can cause individual cells to fluctuate substantially in gene expression levels over time. Yet it remains unclear whether these fluctuations can persist for much longer than the time for a single cell division. Current methods for measuring gene expression in single cells mostly rely on single time point measurements, making the time of a fluctuation of gene expression or cellular memory difficult to measure. Here, we report a method combining Luria and Delbrück's fluctuation analysis with population-based RNA sequencing (MemorySeq) for identifying genes transcriptome-wide whose fluctuations persist for several cell divisions. MemorySeq revealed multiple gene modules that express together in rare cells within otherwise homogeneous clonal populations. Further, we found that these rare cell subpopulations are associated with biologically distinct behaviors in multiple different cancer cell lines, for example, the ability to proliferate in the face of anti-cancer therapeutics. The identification of non-genetic, multigenerational fluctuations has the potential to reveal new forms of biological memory at the level of single cells and suggests that non-genetic heritability of cellular state may be a quantitative property. Main text:Cellular memory in biology, meaning the persistence of a cellular or organismal state over time, occurs over a wide range of timescales and can be induced by a variety of mechanisms. Genetic differences are one form of memory, encoding variation between organisms on multi-generational timescales. Within an organism, epigenetic mechanisms encode the differences between cell types in different tissues, with cells retaining memory of their state over a large number of cell divisions ( 1 ) . In contrast, recent measurements suggest that expression of many genes in single cells may have very little memory, displaying highly transient .

Single-cell transcriptomic methods classify new and existing cell types very effectively, but alternative approaches are needed to quantify the individual regulatory states of cells in their native tissue context. We combined the tissue preservation and single-cell resolution of laser capture with an improved preamplification procedure enabling RNA sequencing of 10 microdissected cells. This in situ 10-cell RNA sequencing (10cRNA-seq) can exploit fluorescent reporters of cell type in genetically engineered mice and is compatible with freshly cryoembedded clinical biopsies from patients.Through recombinant RNA spike-ins, we estimate dropout-free technical reliability as low as ~250 copies and a 50% detection sensitivity of ~45 copies per 10-cell reaction. By using small pools of microdissected cells, 10cRNA-seq improves per-cell reliability and sensitivity beyond existing approaches for single-cell RNA sequencing (scRNA-seq). Accordingly, in multiple tissue and tumor settings, we observe 1.5-2-fold increases in genes detected and overall alignment rates compared to scRNA-seq. Combined with existing approaches to deconvolve small pools of cells, 10cRNA-seq offers a reliable, unbiased, and sensitive way to measure cell-state heterogeneity in tissues and tumors.

Cellular memory describes the length of time a particular transcriptional state exists at the single-cell level. Transcriptional states with memory can underlie important processes in biology, including therapy resistance in cancer. Here we present a new experimental and computational approach for identifying gene expression states with memory at single-cell resolution by combining single-cell RNA sequencing (scRNA-seq) with cellular barcoding. With this technique, we can systematically quantify the full expression profile of these memory states as well as their population dynamics including the relative growth rates of cells within different states and the rates of switching between states. We applied this approach to human melanoma cells and uncovered memory gene expression states that are predictive of which cells will be resistant to combination BRAF and MEK inhibition. While these cells have not been treated with targeted therapies, they already express markers of resistance, and thus are referred to here as “primed” for resistance. From the scRNA-seq data alone, the drug-susceptible and primed cells appear as two distinct and unrelated cell populations. From the lineage barcodes, we found that most cells remain within the same state, demonstrating memory of gene expression. However, we also directly observe state switching as we find that about 18% of lineages contain cells that have switched states. While the molecular drivers of state switching are not immediately apparent from scRNA-seq data alone, we specifically analyzed lineages that undergo state switching and identified TGF-β and PI3K as drivers of state switching at the single-cell level. Through functional validation and single-cell RNA-sequencing, we show mechanistic single-cell manipulation of state switching that ultimately yields changes in cellular phenotype. We leverage these mechanisms of state switching to delay resistance by demonstrating that reducing the number of primed cells with a PI3K inhibitor prior to BRAF and MEK inhibition ultimately leads to fewer resistant colonies. Our results show that modulation of cellular signaling can globally regulate gene expression programs to overcome therapy resistance.

Here, we developed an in situ RNA detection assay for RNA generated by the SARS-CoV-2 virus. We found viral RNA in lung, lymph node, and placenta samples from pathology specimens from COVID patients. Using high-magnification microscopy, we can visualize the subcellular distribution of these RNA in single cells.

Single-cell transcriptomic methods classify new and existing cell types very effectively, but alternative approaches are needed to quantify the individual regulatory states of cells in their native tissue context. We combined the tissue preservation and single-cell resolution of laser capture with an improved preamplification procedure enabling RNA sequencing of 10 microdissected cells. This in situ 10-cell RNA sequencing (10cRNA-seq) can exploit fluorescent reporters of cell type in genetically engineered mice and is compatible with freshly cryoembedded clinical biopsies from patients. Through recombinant RNA spike-ins, we estimate dropout-free technical reliability as low as ~250 copies and a 50% detection sensitivity of ~45 copies per 10-cell reaction. By using small pools of microdissected cells, 10cRNA-seq improves technical per-cell reliability and sensitivity beyond existing approaches for single-cell RNA sequencing (scRNA-seq). Detection of low-abundance transcripts by 10cRNA-seq is comparable to random 10-cell groups of scRNA-seq data, suggesting no loss of gene recovery when cells are isolated in situ. Combined with existing approaches to deconvolve small pools of cells, 10cRNA-seq offers a reliable, unbiased, and sensitive way to measure cell-state heterogeneity in tissues and tumors.