The rapid progress of protocols for sequencing single-cell transcriptomes over the past decade has been accompanied by equally impressive advances in the computational methods for analysis of such data. As capacity and accuracy of the experimental techniques grew, the emerging algorithm developments revealed increasingly complex facets of the underlying biology, from cell type composition to gene regulation to developmental dynamics. At the same time, rapid growth has forced continuous reevaluation of the underlying statistical models, experimental aims, and sheer volumes of data processing that are handled by these computational tools. Here, I review key computational steps of single-cell RNA sequencing (scRNA-seq) analysis, examine assumptions made by different approaches, and highlight successes, remaining ambiguities, and limitations that are important to keep in mind as scRNA-seq becomes a mainstream technique for studying biology.
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The following scRNA-seq datasets were used in creating example figures:
• 10x Genomics PBMC 10k (https://support.10xgenomics.com/single-cell-gene-expression/datasets/3.0.0/pbmc_10k_v3).
• 10x Genomics PBMC 66k (https://support.10xgenomics.com/single-cell-gene-expression/datasets/3.1.0/5k_pbmc_NGSC3_aggr).
• Metadata on the single-cell RNA-seq experiments were taken from http://www.nxn.se/single-cell-studies/.
The notebooks and scripts for the figures presented in the paper can be found on the author’s website: http://pklab.med.harvard.edu/peterk/review2020/.
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P.V.K was supported by the NHLBI R01HL131768 award from NIH and CAREER (NSF-14-532) award from NSF.
P.V.K. serves on the scientific advisory boards of Celsius Therapeutics and Biomage Inc.
Peer review information Nature Methods thanks Martin Hemberg, Michael Morgan and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Lei Tang was the primary editor on this article and managed its editorial process and peer review in collaboration with the rest of the editorial team.
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a. Dependency between cost per cell (x axis) and the expected depth (UMIs per cell, y axis) is shown for a number of popular methods, largely based on the assessment by Ding. et al.1. b–d. Systematic transcript-specific bias of different scRNA-seq protocols. b. The scatter plot shows average log10(CPM+1) values for different genes (each dot represents a gene), as assessed using 10x Chromium (x axis) or dropseq (y axis) platforms. Genes showing higher (red) or lower (green) expression (above 10-fold threshold) are highlighted. c, d. Similar scatter plots shown for other two cell lines: H2228 (b) and HCC827 (c) cells. The set of differential genes determined from analysis of the H1975 cell line (a) is shown. Most of the genes that showed large discrepancy in the detection rate in H1975 results also show same discrepancy in the other two cell lines, illustrating stable detection bias between the two platforms. e. The ability to distinguish nearest neighbors decreases as the dimensionality of the space increases. The difference between closest (mind) and furthest (maxd) points from the origin, normalized by mind (y axis) is shown for different distance measures as a function of increasing number of dimensions (x axis). For each dimensionality n, a set of 100 random points are drawn from the n-dimensional uniform distribution, and a median of 1000 draws is shown. The distinction between closest and furthest points approaches 0 at high dimensions. In other words, relative to the origin, in high-dimensional space the points appear to be distributed on the surface of a high-dimensional sphere. f. Principal tree fit to the PBMC10k dataset. The tree shows computationally optimal spanning of the PBMC populations, yet the interpreting it as a dynamic process is incorrect.
a. A t-SNE embedding of the PBMC10k dataset (left); projection of cells onto the first two principal components (middle); projection of cells onto first two basis of the non-negative matrix factorization (right); b. Projection of cells onto the first two principal components, based on re-analysis of a subset of the PBMC10k dataset that contains only T lymphocytes. Given this restricted cellular context, the first two components are much better at capturing separation between different subsets of T cells, compared to the PCA on the full dataset shown in the previous panel. c. Visualization of the PBMC10k dataset in the 2D latent space determined by an autoencoder structure shown in (d). d. The architecture of an autoencoder used to reduce dimensions of the PBMC10k dataset in the previous panel. The autoencoder starts with a vector of top 3000 most variable genes, and then for each cell transforms this expression profile through a series of non-linear transformations, first into increasingly narrow dimensions, culminating in a two-dimensional middle layer, and then back into a full 3000-dimensional vector. The values of the two-dimensional middle layer are shown in (d). The parameters of the transformations connecting each layer are optimized so that they minimize the discrepancy between the original expression vector (leftmost layer) and the reconstructed vector (rightmost layer). e, f. Using neural networks to learn non-linear mapping from high-dimensional expression state to the coordinates of a t-SNE embedding. As t-SNE embeddings are based on empirical optimization of the relative positions of neighboring cells, there is no obvious analytical function connecting the expression state with the resulting t-SNE coordinates. Neural networks, however, can be used to approximate highly nonlinear and noisy functions. Here, a neural network with an architecture shown in (f) was used to approximate such a function. The parameters of the transformations connecting the layers were optimized based on a training set of 3000 cells, and then an additional set of 3000 test cells was used to illustrate the resulting fit. The left panel in (e) shows the actual positions of the 3000 test cells, and the right plot shows the positions predicted by the trained network.
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Kharchenko, P.V. The triumphs and limitations of computational methods for scRNA-seq. Nat Methods 18, 723–732 (2021). https://doi.org/10.1038/s41592-021-01171-x