Advanced visualizations

Author

Meeta Mistry, Mary Piper, Radhika Khetani

Published

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Approximate time: 75 minutes

Learning Objectives

Setup results data for application of visualization techniques
Describe different data visualization useful for exploring results from a DGE analysis
Create a volcano plot to evaluate relationship amongst DGE statistics
Create a heatmap to illustrate expression changes of differentially expressed genes

Visualizing the results

When we are working with large amounts of data, it can be useful to display that information graphically to gain more insight. During this lesson, we will get you started with some basic and more advanced plots commonly used when exploring differential gene expression data. Many of these plots can be helpful in visualizing other types of data as well.

We will be working with three different data objects we have already created in earlier lessons:

Metadata for our samples (a dataframe): meta
Normalized expression data for every gene in each of our samples (a matrix): normalized_counts
Tibble versions of the DESeq2 results we generated in the last lesson: res_tableOE_tb and res_tableKD_tb

First, let’s create a metadata tibble from the data frame (don’t lose the row names!)

# Make metadata tibble
mov10_meta <- meta %>% 
  rownames_to_column(var="samplename") %>% 
  as_tibble()

Next, let’s bring in a column with gene symbols to the normalized_counts object, so we can use them to label our plots. Ensembl IDs are great for many things, but the gene symbols are much more recognizable to us, as biologists.

# DESeq2 creates a matrix when you use the counts() function
# First convert normalized_counts to a data frame and transfer the row names to a new column called "gene"
normalized_counts <- counts(dds, normalized=TRUE) %>% 
  data.frame() %>%
  rownames_to_column(var="gene") 
  
# Next, merge together (ensembl IDs) the normalized counts data frame
# with a subset of the annotations in the tx2gene data frame (only the columns for ensembl gene IDs and gene symbols)
grch38annot <- tx2gene %>% 
  dplyr::select(ensgene, symbol) %>% 
  dplyr::distinct()

# This will bring in a column of gene symbols
normalized_counts <- merge(normalized_counts, grch38annot, by.x="gene", by.y="ensgene")

# Now create a tibble for the normalized counts
normalized_counts <- normalized_counts %>%
  as_tibble()

# View at the tibble to make sure it looks correct
normalized_counts %>% head()

# A tibble: 6 × 10
  gene         Irrel_kd_1 Irrel_kd_2 Irrel_kd_3 Mov10_kd_2 Mov10_kd_3 Mov10_oe_1
  <chr>             <dbl>      <dbl>      <dbl>      <dbl>      <dbl>      <dbl>
1 ENSG0000000…   3925.       3794.       3961.     3952.       3941.      2727. 
2 ENSG0000000…     24.2        30.2        30.7      23.7        13.9       20.4
3 ENSG0000000…   1326.       1341.       1180.     1515.       1432.      1542. 
4 ENSG0000000…    456.        422.        476.      597.        610.       527. 
5 ENSG0000000…   1250.       1212.       1134.     1389.       1300.       965. 
6 ENSG0000000…      0.897       1.04        0         1.28        0          0  
# ℹ 3 more variables: Mov10_oe_2 <dbl>, Mov10_oe_3 <dbl>, symbol <chr>

A possible alternative to the above:

normalized_counts <- counts(dds, normalized=TRUE) %>% 
  data.frame() %>%
  rownames_to_column(var="gene") %>%
  as_tibble() %>%
  left_join(grch38annot, by=c("gene" = "ensgene"))

Plotting signicant DE genes

One way to visualize results would be to simply plot the expression data for a handful of genes. We could do that by picking out specific genes of interest or selecting a range of genes.

Using DESeq2 `plotCounts()` to plot expression of a single gene

To pick out a specific gene of interest to plot, for example MOV10, we can use the plotCounts() from DESeq2. plotCounts() requires that the gene specified matches the original input to DESeq2, which in our case was Ensembl IDs.

# Find the Ensembl ID of MOV10
grch38annot[grch38annot$symbol == "MOV10", "ensgene"]

[1] "ENSG00000155363"

# Plot expression for single gene
plotCounts(dds, gene="ENSG00000155363", intgroup="sampletype")

Note

This DESeq2 function only allows for plotting the counts of a single gene at a time, and is not flexible regarding the appearance.

Using ggplot2 to plot expression of a single gene

If you wish to change the appearance of this plot, we can save the output of plotCounts() to a variable specifying the returnData=TRUE argument, then use ggplot():

# Save plotcounts to a data frame object
d <- plotCounts(dds, gene="ENSG00000155363", intgroup="sampletype", returnData=TRUE)

# What is the data output of plotCounts()?
d %>% head()

                count           sampletype
Irrel_kd_1   1911.647              control
Irrel_kd_2   1974.191              control
Irrel_kd_3   2072.948              control
Mov10_kd_2   1419.191      MOV10_knockdown
Mov10_kd_3   1486.757      MOV10_knockdown
Mov10_oe_1 274747.523 MOV10_overexpression

# Plot the MOV10 normalized counts, using the samplenames (rownames(d)) as labels
ggplot(d, aes(x = sampletype, y = count, color = sampletype)) + 
  # Scatterplot but allow points to "jitter" so they are not on top of each other
  geom_point(position=position_jitter(w = 0.1,h = 0)) +
  # Label points
  geom_text_repel(aes(label = rownames(d))) +
  # Add title and plot tweaks
  ggtitle("MOV10") +
  theme_bw() +
  theme(plot.title = element_text(hjust = 0.5))

Label points

Note that in the plot above, we are using geom_text_repel() from the ggrepel package to label our individual points on the plot.

Plot more than one gene

If you are interested in plotting the expression of multiple genes all together, please refer to the short lesson linked here where we demo this for the top 20 most significantly expressed genes.

Heatmap

In addition to plotting subsets, we could also extract the normalized values of all the significant genes and plot a heatmap of their expression using pheatmap().

# Extract normalized expression for significant genes from the OE and control samples (2:4 and 7:9)
norm_OEsig <- normalized_counts[,c(1:4,7:9)] %>% 
  dplyr::filter(gene %in% sigOE$gene)

Now let’s draw the heatmap using pheatmap:

# Set a color palette
heat_colors <- RColorBrewer::brewer.pal(6, "YlOrRd")

# Run pheatmap using the metadata dataframe for the annotation
pheatmap(norm_OEsig[2:7], 
         color = heat_colors, 
         cluster_rows = TRUE, 
         show_rownames = FALSE,
         annotation = meta, 
         border_color = NA, 
         fontsize = 10, 
         scale = "row", 
         fontsize_row = 10, 
         height = 20)

Note

There are several additional arguments we have included in the function for aesthetics. One important one is scale="row", in which Z-scores are plotted, rather than the actual normalized count value.

Z-scores are computed on a gene-by-gene basis by subtracting the mean and then dividing by the standard deviation. The Z-scores are computed after the clustering, so that it only affects the graphical aesthetics, and the color visualization is improved.

Volcano plot

The above plot would be great to look at the expression levels of a good number of genes, but for more of a global view there are other plots we can draw. A commonly used one is a volcano plot; in which you have the log-transformed adjusted p-values plotted on the y-axis and log2 fold change values on the x-axis.

To generate a volcano plot, we first need to have a column in our results data indicating whether or not the gene is considered differentially expressed based on p-adjusted values and, for the purposes of this visualization, a log2fold change cutoff.

# Obtain logical vector where TRUE values denote padj values < 0.05 and fold change > 1.5 in either direction
res_tableOE_tb <- res_tableOE_tb %>% 
  dplyr::mutate(threshold_OE = padj < 0.05 & abs(log2FoldChange) >= log2(1.5))

Now we can start plotting. The geom_point object is most applicable, as this is essentially a scatter plot:

# Volcano plot
ggplot(res_tableOE_tb) +
  geom_point(aes(x = log2FoldChange, y = -log10(padj),
                 # Color by whether or not gene is DE by padj and log2FC
                 colour = threshold_OE)) +
  # You could choose to limit the y-axis to look at the shape of the data without outliers
  # scale_y_continuous(limits = c(0,50)) +
  # Add title and plot tweaks
  ggtitle("Mov10 overexpression") +
  xlab("log2 fold change") + 
  ylab("-log10 adjusted p-value") +
  theme(legend.position = "none",
        plot.title = element_text(size = rel(1.5), hjust = 0.5),
        axis.title = element_text(size = rel(1.25)))

This is a great way to get an overall picture of what is going on, but what if we also wanted to know where the top 10 genes (lowest padj) in our DE list are located on this plot? We could label those dots with the gene name on the volcano plot using geom_text_repel().

First, we need to order the res_tableOE tibble by padj, and add an additional column to it, to include on those gene names we want to use to label the plot.

# Add all the gene symbols as a column from the grch38 table using bind_cols()
res_tableOE_tb <- bind_cols(res_tableOE_tb,
                            symbol = grch38annot$symbol[match(res_tableOE_tb$gene, grch38annot$ensgene)])

# Create an empty column to indicate which genes to label
res_tableOE_tb <- res_tableOE_tb %>% dplyr::mutate(genelabels = "")

# Sort by padj values 
res_tableOE_tb <- res_tableOE_tb %>% dplyr::arrange(padj)

# Populate the genelabels column with contents of the gene symbols column for the first 10 rows, i.e., the top 10 most significantly expressed genes
res_tableOE_tb$genelabels[1:10] <- as.character(res_tableOE_tb$symbol[1:10])

# Look at the new table
head(res_tableOE_tb)

# A tibble: 6 × 9
  gene    baseMean log2FoldChange  lfcSE    pvalue      padj threshold_OE symbol
  <chr>      <dbl>          <dbl>  <dbl>     <dbl>     <dbl> <lgl>        <chr> 
1 ENSG00…   95772.           6.99 0.122  0         0         TRUE         MOV10 
2 ENSG00…    8090.           1.51 0.0666 3.72e-115 3.41e-111 TRUE         H1F0  
3 ENSG00…     239.           6.50 0.363  9.45e- 72 5.76e- 68 TRUE         HSPA6 
4 ENSG00…    5285.           1.37 0.0788 1.64e- 68 7.48e- 65 TRUE         TXNIP 
5 ENSG00…    1747.           1.48 0.0890 1.75e- 63 6.41e- 60 TRUE         HIST1…
6 ENSG00…    2595.           1.47 0.0956 1.49e- 54 4.54e- 51 TRUE         HIST2…
# ℹ 1 more variable: genelabels <chr>

Next, we plot it as before with an additional layer for geom_text_repel() wherein we can specify the column of gene labels we just created.

# Volcano plot with labels on some points
ggplot(res_tableOE_tb, aes(x = log2FoldChange, y = -log10(padj))) +
  # Color by whether or not gene is DE by padj and log2FC
  geom_point(aes(colour = threshold_OE)) +
  # Only label the top 10 DEG we identified above
  geom_text_repel(aes(label = genelabels)) +
  # Add title and plot tweaks
  ggtitle("Mov10 overexpression") +
  xlab("log2 fold change") + 
  ylab("-log10 adjusted p-value") +
  theme(legend.position = "none",
        plot.title = element_text(size = rel(1.5), hjust = 0.5),
        axis.title = element_text(size = rel(1.25)))

An R package for visualization of DGE results

The Bioconductor package DEGreport can use the DESeq2 results output to make the top 20 genes and the volcano plots generated above by writing much fewer lines of code. The caveat of these functions is you lose the ability to customize plots as we have demonstrated above.

If you are interested, the example code below shows how you can use DEGreport to create similar plots. Note that this is example code, do not run.

## DO NOT RUN THIS CODE

# Plot normalized expression of top n (20) genes
DEGreport::degPlot(dds = dds, # dds object is output from DESeq2
                   res = res, n = 20, xs = "type", group = "condition")

# Plot labeled volcano plot
DEGreport::degVolcano(
  data.frame(res[,c("log2FoldChange","padj")]), # table - 2 columns
  plot_text = data.frame(res[1:10,c("log2FoldChange","padj","id")])) # table to add names

# Available in the newer version for R 3.4
DEGreport::degPlotWide(dds = dds, genes = row.names(res)[1:5], group = "condition")

Materials and hands-on activities were adapted from RNA-seq workflow on the Bioconductor website

--- title: "Advanced visualizations" author: "Meeta Mistry, Mary Piper, Radhika Khetani" date: "December 4th, 2019" --- Approximate time: 75 minutes ```{r data_setup} #| echo: false # load libraries needed to render this lesson library(tidyverse) library(DESeq2) library(ggrepel) library(pheatmap) # load objects needed to render this lesson meta <- readRDS("../data/metadata.RDS") dds <- readRDS("../data/intermediate_dds_2_ran_deseq.RDS") tx2gene <- read.delim("../data/tx2gene_grch38_ens94.txt") res_tableOE_tb <- readRDS("../data/intermediate_res_tableOE_tb.RDS") padj.cutoff <- 0.05 sigOE <- res_tableOE_tb %>% dplyr::filter(padj < padj.cutoff) ``` ## Learning Objectives * Setup results data for application of visualization techniques * Describe different data visualization useful for exploring results from a DGE analysis * Create a volcano plot to evaluate relationship amongst DGE statistics * Create a heatmap to illustrate expression changes of differentially expressed genes ## Visualizing the results When we are working with large amounts of data, it can be useful to display that information graphically to gain more insight. During this lesson, we will get you started with some basic and more advanced plots commonly used when exploring differential gene expression data. Many of these plots can be helpful in visualizing other types of data as well. We will be working with three different data objects we have already created in earlier lessons: - Metadata for our samples (a dataframe): `meta` - Normalized expression data for every gene in each of our samples (a matrix): `normalized_counts` - Tibble versions of the DESeq2 results we generated in the last lesson: `res_tableOE_tb` and `res_tableKD_tb` First, let's create a metadata tibble from the data frame (don't lose the row names!) ```{r metadata} # Make metadata tibble mov10_meta <- meta %>% rownames_to_column(var="samplename") %>% as_tibble() ``` Next, let's bring in a column with gene symbols to the `normalized_counts` object, so we can use them to label our plots. Ensembl IDs are great for many things, but the gene symbols are much more recognizable to us, as biologists. ```{r add_gene_names} # DESeq2 creates a matrix when you use the counts() function # First convert normalized_counts to a data frame and transfer the row names to a new column called "gene" normalized_counts <- counts(dds, normalized=TRUE) %>% data.frame() %>% rownames_to_column(var="gene") # Next, merge together (ensembl IDs) the normalized counts data frame # with a subset of the annotations in the tx2gene data frame (only the columns for ensembl gene IDs and gene symbols) grch38annot <- tx2gene %>% dplyr::select(ensgene, symbol) %>% dplyr::distinct() # This will bring in a column of gene symbols normalized_counts <- merge(normalized_counts, grch38annot, by.x="gene", by.y="ensgene") # Now create a tibble for the normalized counts normalized_counts <- normalized_counts %>% as_tibble() # View at the tibble to make sure it looks correct normalized_counts %>% head() ``` ::: callout-note # A possible alternative to the above: ```{r add_gene_names_alternative} normalized_counts <- counts(dds, normalized=TRUE) %>% data.frame() %>% rownames_to_column(var="gene") %>% as_tibble() %>% left_join(grch38annot, by=c("gene" = "ensgene")) ``` ::: ### Plotting signicant DE genes One way to visualize results would be to simply plot the expression data for a handful of genes. We could do that by picking out specific genes of interest or selecting a range of genes. #### **Using DESeq2 `plotCounts()` to plot expression of a single gene** To pick out a specific gene of interest to plot, for example MOV10, we can use the `plotCounts()` from DESeq2. `plotCounts()` requires that the gene specified matches the original input to DESeq2, which in our case was Ensembl IDs. ```{r plot_one_gene} #| fig.width: 8 # Find the Ensembl ID of MOV10 grch38annot[grch38annot$symbol == "MOV10", "ensgene"] # Plot expression for single gene plotCounts(dds, gene="ENSG00000155363", intgroup="sampletype") ``` ::: callout-note This DESeq2 function only allows for plotting the counts of a single gene at a time, and is not flexible regarding the appearance. ::: #### **Using ggplot2 to plot expression of a single gene** If you wish to change the appearance of this plot, we can save the output of `plotCounts()` to a variable specifying the `returnData=TRUE` argument, then use `ggplot()`: ```{r plot_one_gene_ggplot} # Save plotcounts to a data frame object d <- plotCounts(dds, gene="ENSG00000155363", intgroup="sampletype", returnData=TRUE) # What is the data output of plotCounts()? d %>% head() # Plot the MOV10 normalized counts, using the samplenames (rownames(d)) as labels ggplot(d, aes(x = sampletype, y = count, color = sampletype)) + # Scatterplot but allow points to "jitter" so they are not on top of each other geom_point(position=position_jitter(w = 0.1,h = 0)) + # Label points geom_text_repel(aes(label = rownames(d))) + # Add title and plot tweaks ggtitle("MOV10") + theme_bw() + theme(plot.title = element_text(hjust = 0.5)) ``` ::: callout-note # Label points Note that in the plot above, we are using `geom_text_repel()` from the `ggrepel` package to label our individual points on the plot. ::: ::: callout-note # Plot more than one gene If you are interested in plotting the expression of multiple genes all together, please refer to [the short lesson linked here](top20_genes-expression_plotting.html) where we demo this for the top 20 most significantly expressed genes. ::: ### Heatmap In addition to plotting subsets, we could also extract the normalized values of *all* the significant genes and plot a heatmap of their expression using `pheatmap()`. ```{r extract_norm_exp} # Extract normalized expression for significant genes from the OE and control samples (2:4 and 7:9) norm_OEsig <- normalized_counts[,c(1:4,7:9)] %>% dplyr::filter(gene %in% sigOE$gene) ``` Now let's draw the heatmap using `pheatmap`: ```{r plot_heatmap} # Set a color palette heat_colors <- RColorBrewer::brewer.pal(6, "YlOrRd") # Run pheatmap using the metadata dataframe for the annotation pheatmap(norm_OEsig[2:7], color = heat_colors, cluster_rows = TRUE, show_rownames = FALSE, annotation = meta, border_color = NA, fontsize = 10, scale = "row", fontsize_row = 10, height = 20) ``` ::: callout-note There are several additional arguments we have included in the function for aesthetics. One important one is `scale="row"`, in which Z-scores are plotted, rather than the actual normalized count value. Z-scores are computed on a gene-by-gene basis by subtracting the mean and then dividing by the standard deviation. The Z-scores are computed **after the clustering**, so that it only affects the graphical aesthetics, and the color visualization is improved. ::: ### Volcano plot The above plot would be great to look at the expression levels of a good number of genes, but for more of a global view there are other plots we can draw. A commonly used one is a volcano plot; in which you have the log-transformed adjusted p-values plotted on the y-axis and log2 fold change values on the x-axis. To generate a volcano plot, we first need to have a column in our results data indicating whether or not the gene is considered differentially expressed based on p-adjusted values and, for the purposes of this visualization, a log2fold change cutoff. ```{r determine_DE} # Obtain logical vector where TRUE values denote padj values < 0.05 and fold change > 1.5 in either direction res_tableOE_tb <- res_tableOE_tb %>% dplyr::mutate(threshold_OE = padj < 0.05 & abs(log2FoldChange) >= log2(1.5)) ``` Now we can start plotting. The `geom_point` object is most applicable, as this is essentially a scatter plot: ```{r plot_volcano} # Volcano plot ggplot(res_tableOE_tb) + geom_point(aes(x = log2FoldChange, y = -log10(padj), # Color by whether or not gene is DE by padj and log2FC colour = threshold_OE)) + # You could choose to limit the y-axis to look at the shape of the data without outliers # scale_y_continuous(limits = c(0,50)) + # Add title and plot tweaks ggtitle("Mov10 overexpression") + xlab("log2 fold change") + ylab("-log10 adjusted p-value") + theme(legend.position = "none", plot.title = element_text(size = rel(1.5), hjust = 0.5), axis.title = element_text(size = rel(1.25))) ``` This is a great way to get an overall picture of what is going on, but what if we also wanted to know where the top 10 genes (lowest padj) in our DE list are located on this plot? We could label those dots with the gene name on the volcano plot using `geom_text_repel()`. First, we need to order the res_tableOE tibble by `padj`, and add an additional column to it, to include on those gene names we want to use to label the plot. ```{r label_top_DEG} # Add all the gene symbols as a column from the grch38 table using bind_cols() res_tableOE_tb <- bind_cols(res_tableOE_tb, symbol = grch38annot$symbol[match(res_tableOE_tb$gene, grch38annot$ensgene)]) # Create an empty column to indicate which genes to label res_tableOE_tb <- res_tableOE_tb %>% dplyr::mutate(genelabels = "") # Sort by padj values res_tableOE_tb <- res_tableOE_tb %>% dplyr::arrange(padj) # Populate the genelabels column with contents of the gene symbols column for the first 10 rows, i.e., the top 10 most significantly expressed genes res_tableOE_tb$genelabels[1:10] <- as.character(res_tableOE_tb$symbol[1:10]) # Look at the new table head(res_tableOE_tb) ``` Next, we plot it as before with an additional layer for `geom_text_repel()` wherein we can specify the column of gene labels we just created. ```{r plot_volcano_labeled} # Volcano plot with labels on some points ggplot(res_tableOE_tb, aes(x = log2FoldChange, y = -log10(padj))) + # Color by whether or not gene is DE by padj and log2FC geom_point(aes(colour = threshold_OE)) + # Only label the top 10 DEG we identified above geom_text_repel(aes(label = genelabels)) + # Add title and plot tweaks ggtitle("Mov10 overexpression") + xlab("log2 fold change") + ylab("-log10 adjusted p-value") + theme(legend.position = "none", plot.title = element_text(size = rel(1.5), hjust = 0.5), axis.title = element_text(size = rel(1.25))) ``` ::: callout-note # An R package for visualization of DGE results The Bioconductor package [`DEGreport`](https://bioconductor.org/packages/release/bioc/html/DEGreport.html) can use the DESeq2 results output to make the top 20 genes and the volcano plots generated above by writing much fewer lines of code. The caveat of these functions is you lose the ability to customize plots as we have demonstrated above. If you are interested, the example code below shows how you can use DEGreport to create similar plots. **Note that this is example code, do not run.** ```{r DEGreport} #| eval: false ## DO NOT RUN THIS CODE # Plot normalized expression of top n (20) genes DEGreport::degPlot(dds = dds, # dds object is output from DESeq2 res = res, n = 20, xs = "type", group = "condition") # Plot labeled volcano plot DEGreport::degVolcano( data.frame(res[,c("log2FoldChange","padj")]), # table - 2 columns plot_text = data.frame(res[1:10,c("log2FoldChange","padj","id")])) # table to add names # Available in the newer version for R 3.4 DEGreport::degPlotWide(dds = dds, genes = row.names(res)[1:5], group = "condition") ``` ::: ```{r data_save} #| echo: false # save objects needed to render future lessons saveRDS(normalized_counts, "../data/intermediate_normalized_counts_with_gene_names.RDS") ``` --- *Materials and hands-on activities were adapted from [RNA-seq workflow](http://www.bioconductor.org/help/workflows/rnaseqGene/#de) on the Bioconductor website*