Seurat - Guided Clustering Tutorial
Compiled: March 04, 2020
Setup the Seurat Object
For this tutorial, we will be analyzing the a dataset of Peripheral Blood Mononuclear Cells (PBMC) freely available from 10X Genomics. There are 2,700 single cells that were sequenced on the Illumina NextSeq 500. The raw data can be found here.
We start by reading in the data. The
Read10X function reads in the output of the cellranger pipeline from 10X, returning a unique molecular identified (UMI) count matrix. The values in this matrix represent the number of molecules for each feature (i.e. gene; row) that are detected in each cell (column).
We next use the count matrix to create a
Seurat object. The object serves as a container that contains both data (like the count matrix) and analysis (like PCA, or clustering results) for a single-cell dataset. For a technical discussion of the
Seurat object structure, check out our GitHub Wiki. For example, the count matrix is stored in
library(dplyr) library(Seurat) library(patchwork) # Load the PBMC dataset pbmc.data <- Read10X(data.dir = "../data/pbmc3k/filtered_gene_bc_matrices/hg19/") # Initialize the Seurat object with the raw (non-normalized data). pbmc <- CreateSeuratObject(counts = pbmc.data, project = "pbmc3k", min.cells = 3, min.features = 200) pbmc
## An object of class Seurat ## 13714 features across 2700 samples within 1 assay ## Active assay: RNA (13714 features)
What does data in a count matrix look like?
# Lets examine a few genes in the first thirty cells pbmc.data[c("CD3D", "TCL1A", "MS4A1"), 1:30]
## 3 x 30 sparse Matrix of class "dgCMatrix" ## ## CD3D 4 . 10 . . 1 2 3 1 . . 2 7 1 . . 1 3 . 2 3 . . . . . 3 4 1 5 ## TCL1A . . . . . . . . 1 . . . . . . . . . . . . 1 . . . . . . . . ## MS4A1 . 6 . . . . . . 1 1 1 . . . . . . . . . 36 1 2 . . 2 . . . .
. values in the matrix represent 0s (no molecules detected). Since most values in an scRNA-seq matrix are 0, Seurat uses a sparse-matrix representation whenever possible. This results in significant memory and speed savings for Drop-seq/inDrop/10x data.
dense.size <- object.size(as.matrix(pbmc.data)) dense.size
## 709548272 bytes
sparse.size <- object.size(pbmc.data) sparse.size
## 29861992 bytes
## 23.8 bytes
Standard pre-processing workflow
The steps below encompass the standard pre-processing workflow for scRNA-seq data in Seurat. These represent the selection and filtration of cells based on QC metrics, data normalization and scaling, and the detection of highly variable features.
QC and selecting cells for further analysis
Seurat allows you to easily explore QC metrics and filter cells based on any user-defined criteria. A few QC metrics commonly used by the community include
- The number of unique genes detected in each cell.
- Low-quality cells or empty droplets will often have very few genes
- Cell doublets or multiplets may exhibit an aberrantly high gene count
- Similarly, the total number of molecules detected within a cell (correlates strongly with unique genes)
- The percentage of reads that map to the mitochondrial genome
- Low-quality / dying cells often exhibit extensive mitochondrial contamination
- We calculate mitochondrial QC metrics with the
PercentageFeatureSetfunction, which calculates the percentage of counts originating from a set of features
- We use the set of all genes starting with
MT-as a set of mitochondrial genes
# The [[ operator can add columns to object metadata. This is a great place to stash QC stats pbmc[["percent.mt"]] <- PercentageFeatureSet(pbmc, pattern = "^MT-")
Where are QC metrics stored in Seurat?
- The number of unique genes and total molecules are automatically calculated during
- You can find them stored in the object meta data
# Show QC metrics for the first 5 cells head(firstname.lastname@example.org, 5)
In the example below, we visualize QC metrics, and use these to filter cells.
- We filter cells that have unique feature counts over 2,500 or less than 200
- We filter cells that have >5% mitochondrial counts
# Visualize QC metrics as a violin plot VlnPlot(pbmc, features = c("nFeature_RNA", "nCount_RNA", "percent.mt"), ncol = 3)