scclusteval

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The goal of scclusteval(Single Cell Cluster Evaluation) is to evaluate the single cell clustering stability by subsampling the cells and provide many visualization methods for comparing clusters.

For Theory behind the method, see Christian Henning, “Cluster-wise assessment of cluster stability,” Research Report 271, Dept. of Statistical Science, University College London, December 2006)

Citation

Ming Tang, Yasin Kaymaz,Brandon L. Logeman, Stephen Eichhorn, Zhengzheng S. Liang, Catherine Dulac and Timothy B. Sackton. Evaluating single-cell cluster stability using the Jaccard similarity index (2020) Bioinformatics btaa956, https://doi.org/10.1093/bioinformatics/btaa956.

Parameters that affect the clustering

The most popular clustering method for single cell RNAseq is shared nearest neighbor (SNN) followed by Louvain community detection algorithm which is implemented in {Seurat::FindClusters}. See a paper by Mark Robinson group for comparing single cell RNAseq clustering methods:A systematic performance evaluation of clustering methods for single-cell RNA-seq data SNN in Seurat is the most accurate and fast one.

The parameter k.param which specifies the number of nearest neighbors has a great effect on the number of clusters. Other Parameters such as the number of PCs and the resolution can affect the number of clusters as well.

The process is as follows.

To assess which k is best to use by subsampling the original data:

1. Performing the clustering at many different K values on the full data set.

2. We then sample without replacement a subset of the data set (e.g. 80% of the cells in the full data set), and then repeat the clustering procedure on just this subset of data (so repeating all aspects of clustering, including calling variable genes, calculating PCs, building the neighbor graph, etc), and we do this n times.

3. So for each K value, we have 1 clustering outcome for the full data set, and 20 clustering outcomes for subsampled portions of the data set. From this we identify the cluster in the first subsample clustering that is most similar to the full cluster 1 cells (the one that gives the maximum Jaccard coefficient) and record that value. If this maximum Jaccard coefficient is less than 0.6 (this is quite subjective), the original cluster is considered to be dissolved-it didn’t show up in the new clustering. A cluster that’s dissolved too often is probably not a “real” cluster.

As a rule of thumb, clusters with a stability value less than 0.6 should be considered unstable. Values between 0.6 and 0.75 indicate that the cluster is measuring a pattern in the data, but there isn’t high certainty about which points should be clustered together. Clusters with stability values above about 0.85 can be considered highly stable (they’re likely to be real clusters).

4. Repeat this for all subsample clustering outcomes, and then the stability value of a cluster is the median or mean Jaccard coefficient. If it’s greater than 0.6 (or a cutoff you set) we say it’s stable, otherwise it’s unstable. So for a given K value this gives you a stable/unstable assignment for each cluster. We choose the k value to select for clustering the data by looking at which k value yielded the largest number of stable clusters while still having most of the cells from the data set in a stable cluster.

We can repeat the 1-4 for different resolution and number of PCs and the combination of all different parameters.

The workflow is:

The subsampling process is implemented in a Snakemake workflow

Because for each subsampling, one has to re-run the whole process of FindVariableGenes, ScaleData, RunPCA, JackStraw and FindClusters and for large data set, it can take very long time to run.

E.g. if you test 5 different K, and for each K you subsample the full dataset 100 times. that’s 500 runs.

Snakemake will take advantage of the HPC cluster with large number of CPUs avaiable.

The R package works with the output from the Snakemake workflow: pyflow_seuratv3_parameter.

The scclusteval R package is for downstream analysis

Installation

You can install the scclusteval from github:

devtools::install_github("crazyhottommy/scclusteval")

Useful functions

library(scclusteval)
#> Loading required package: Seurat
?RandomSubsetData
?MergeMultipleSeuratObjects
?PreprocessSubsetData
?PairWiseJaccardSets

## in Rstudio type below and tab to see all avaiable functions
## scclusteval::

Examples

Examples to use the scclusteval package can be found at https://crazyhottommy.github.io/EvaluateSingleCellClustering/

Acknowledgements

Thanks to Tim Sackton and Catherine Dulac for their supervision and support.
Thanks to Yasin Kaymaz in Sackton group for fruitful discussion.
Thanks to Stephen Eichhorn in Xiaowei Zhuang lab for the idea and sharing the python code working on Scanpy object.
Thanks to Sophia(Zhengzheng) Liang and Brandon Logeman in Dulac lab for sharing data and giving feedbacks.
Thanks David Robinson’s geomflatviolin function which was used in the raincloudplot.

Why this package?

I saw {fpc} package has a function clusterboot. However, this function does not support SNN clustering. Although one can write a customer clustering function to feed into clusterboot, I need to build things upon Seurat package and those two can not be easilily integrated. In addition, clusterboot is not parallelized, I have to implement the snakemake workflow for faster processing.

read this blog post http://www.win-vector.com/blog/2015/09/bootstrap-evaluation-of-clusters/ and https://www.czasopisma.uni.lodz.pl/foe/article/view/983

To do list

• implement more visualization functions.
• plot number of cells subsampled for each cluster in each iteration in raincloudplot.
• impurity metric for assessing cluster stability.
• read this post from Jean Fan from Xiaowei Zhuang’s lab https://jef.works/blog/2018/02/28/stability-testing/ getComMembership function works on raw data matrix. It can be used independent of Seurat’s FindClusters. chat with Jean for more details.
• gene sets enrichment for each cluster.