Use Seurat to find specific markers of each cell type

find_specific_markers.R

library(tidyverse)
library(Seurat)
# Find specific markers for each cell type

celltypes <-   # list of cell types (unique ident labels of Seurat object)
obj <-         # Seurat object
specific_markers <- NULL

# First we do all pairwise comparisons and retain any markers that 
# are even somewhat higher in the cell type of interest
for (celltype1 in celltypes) {
    other_types <- setdiff(celltypes, celltype1)
    celltype_markers <- NULL
    for (celltype2 in other_types) {
        markers <- FindMarkers(obj, ident.1=celltype1, ident.2=celltype2,
            only.pos=TRUE, min.diff.pct=0.05, test.use="roc") %>%
            rownames_to_column("gene") %>%
            mutate(ident.2=celltype2)
        celltype_markers <- rbind(celltype_markers, markers)
    }
    celltype_markers$ident.1 <- celltype1
    specific_markers <- rbind(specific_markers, celltype_markers)
}

# In specific_markers data.frame, ident.1 is which cell pop the marker is tagging
# and ident.2 is which other cell pop we are contrasting with
#
# We want markers that are considerably higher in ident.1 than in any other pop.
# As a basic filter we require any marker for ident.1 to be expressed in <50% of 
# cells of each of the other cell types.
# 
# Here we will filter to get the top 10 markers for each cell type
topN <- 10
npop <- length(celltypes)
final_markers <- mutate(specific_markers, pct_diff=pct.1 - pct.2) %>%
    group_by(ident.1, gene) %>%
    filter(n() == npop - 1, all(pct.2 < 0.6)) %>%
    summarize(median_AUC=median(myAUC), pct.1=mean(pct.1), pct.2=mean(pct.2)) %>%
    mutate(cluster=ident.1) %>%
    filter(median_AUC > 0.65) %>%
    group_by(cluster) %>%
    arrange(desc(median_AUC)) %>%
    do(head(., topN))