Barry Grant < http://thegrantlab.org/bggn213/ >
2018-12-07 (15:56:16 PST on Fri, Dec 07)

Overview

In this hands-on session, we will examine some real life multivariate data in order to explain, in simple terms what PCA achieves. We will perform a principal component analysis of several different data sets and examine the results.

1. PCA of UK food data

Suppose that we are examining the following data, from the UK’s ‘Department for Environment, Food and Rural Affairs’ (DEFRA), showing the consumption in grams (per person, per week) of 17 different types of foodstuff measured and averaged in the four countries of the United Kingdom in 1997.

England Wales Scotland N.Ireland
Cheese 105 103 103 66
Carcass_meat 245 227 242 267
Other_meat 685 803 750 586
Fish 147 160 122 93
Fats_and_oils 193 235 184 209
Sugars 156 175 147 139
Fresh_potatoes 720 874 566 1033
Fresh_Veg 253 265 171 143
Other_Veg 488 570 418 355
Processed_potatoes 198 203 220 187
Processed_Veg 360 365 337 334
Fresh_fruit 1102 1137 957 674
Cereals 1472 1582 1462 1494
Beverages 57 73 53 47
Soft_drinks 1374 1256 1572 1506
Alcoholic_drinks 375 475 458 135
Confectionery 54 64 62 41

We shall say that the 17 food types are the variables and the 4 countries are the observations. This would be equivalent to our samples and genes respectively from the lecture example (and indeed the second main example further below).

Lets read this data from the provided UK_foods.csv input file (note we placed this file in a data sub-directory within or working R studio project directory.

x <- read.csv("data/UK_foods.csv")

Q1. How many rows and columns are in your new data frame named x? What R functions could you use to answer this questions?

## Complete the following code to find out how many rows and columns are in x?
___(x)
## [1] 17  5

HINT: You can use the dim() function, which returns the number of rows and columns or the nrow() and ncol() functions to return each separately, i.e. dim(x); ncol(x); nrow(x)

Checking your data

It is always a good idea to examine your imported data to make sure it meets your expectations. At this stage we want to make sure that no odd things have happened during the importing phase that will come back to haunt us later.

For this task we can use the View() function to display all the data (in a new tab in RStudio) or the head() and tail() functions to print only a portion of the data (by default 6 rows from either the top or bottom of the dataset respectively).

Side-Note: Never leave a View() function call uncommented in your Rmarkdown document as this is intended for interactive use and will stop the Knit rendering process when you go to Knit and generate HTML, PDF, MD etc. reports.


## Preview the first 6 rows
___(x)


Hmm, it looks like the row-names here were not set properly as we were expecting 4 columns (one for each of the 4 countries of the UK - not 5 as reported from the dim() function).

Here it appears that the row-names are incorrectly set as the first column of our x data frame (rather than set as proper row-names). This is very common error. Lets try to fix this up with the following code, which sets the rownames() to the first column and then removes the troublesome first column (with the -1 column index):

# Note how the minus indexing works
rownames(x) <- x[,1]
x <- x[,-1]
head(x)

This looks much better, now lets check the dimensions again:

dim(x)
## [1] 17  4

Side-note: An alternative approach to setting the correct row-names in this case would be to read the data filie again and this time set the row.names argument of read.csv() to be the first column (i.e. use argument setting row.names=1), see below:

x <- read.csv("data/UK_foods.csv", row.names=1)
head(x)

Q2. Which approach to solving the ‘row-names problem’ mentioned above do you prefer and why? Is one approach more robust than another under certain circumstances?

HINT: What happens if you run the first approach code block (i.e. the one with x <- x[,-1]), multiple times?

PCA to the rescue

We need some way of making sense of the above data. Are there any trends present which are not obvious from glancing at the array of numbers?

Traditionally, we would use a series of pairwise plots (i.e. bivariate scatter plots) and analyse these to try and determine any relationships between variables, however the number of such plots required for such a task is clearly too large even for this small dataset. Therefore, for large data sets, this is not feasible or fun.

PCA generalizes this idea and allows us to perform such an analysis simultaneously, for many variables. In our example here, we have 17 dimensional data for 4 countries. We can thus ‘imagine’ plotting the 4 coordinates representing the 4 countries in 17 dimensional space. If there is any correlation between the observations (the countries), this will be observed in the 17 dimensional space by the correlated points being clustered close together, though of course since we cannot visualize such a space, we are not able to see such clustering directly (see the lecture slides for a clear description and example of this).

To perform PCA in R there are actually lots of functions to chose from and many packages offer slick PCA implementations and useful graphing approaches. However here we will stick to the base R prcomp() function.

As we noted in the lecture portion of class, prcomp() expects the observations to be rows and the variables to be columns therefore we need to first transpose our data.frame matrix with the t() transpose function.

# Use the prcomp() PCA function 
pca <- ___( t(x) )
summary(pca)
## Importance of components:
##                             PC1      PC2      PC3       PC4
## Standard deviation     324.1502 212.7478 73.87622 4.189e-14
## Proportion of Variance   0.6744   0.2905  0.03503 0.000e+00
## Cumulative Proportion    0.6744   0.9650  1.00000 1.000e+00

The first task of PCA is to identify a new set of principal axes through the data. This is achieved by finding the directions of maximal variance through the coordinates in the 17 dimensional space. This is equivalent to obtaining the (least-squares) line of best fit through the plotted data where it has the largest spread. We call this new axis the first principal component (or PC1) of the data. The second best axis PC2, the third best PC3 etc.

The summary print-out above indicates that PC1 accounts for more than 67% of the sample variance, PC2 29% and PC3 3%. Collectively PC1 and PC2 together capture 96% of the original 17 dimensional variance. Thus these first two new principal axis (PC1 and PC2) represent useful ways to view and further investigate our data set. Lets start with a simple plot of PC1 vs PC2.

Q7. Complete the code below to generate a plot of PC1 vs PC2. The second line adds text labels over the data points.

# Plot PC1 vs PC2
plot(pca$x[___,___], pca$x[___,___], xlab="PC1", ylab="PC2", xlim=c(-270,500))
text(pca$x[,1], pca$x[,2], colnames(x))

Q8. Customize your plot so that the colors of the country names match the colors in our UK and Ireland map and table at start of this document.

HINT: You can provide a color vector as input to the text() function just like you can with the plot() function itself.

Once the principal components have been obtained, we can use them to map the relationship between variables (i.e. countries) in therms of these major PCs (i.e. new axis that maximally describe the original data variance).

In our food example here, the four 17 dimensional coordinates are projected down onto the two principal components to obtain the graph above.

As part of the PCA method, we automatically obtain information about the contributions of each principal component to the total variance of the coordinates. This is typically contained in the Eigenvectors returned from such calculations. In the prcomp() function we can use the summary() command above or examine the returned pca$sdev (see below).

In this case approximately 67% of the variance in the data is accounted for by the first principal component, and approximately 97% is accounted for in total by the first two principal components. In this case, we have therefore accounted for the vast majority of the variation in the data using only a two dimensional plot - a dramatic reduction in dimensionality from seventeen dimensions to two.

In practice, it is usually sufficient to include enough principal components so that somewhere in the region of 70% of the variation in the data is accounted for. Looking at the so-called scree plot can help in this regard. Ask Barry about this if you are unsure what we mean here.

Below we can use the square of pca$sdev , which stands for “standard deviation”, to calculate how much variation in the original data each PC accounts for.

v <- round( pca$sdev^2/sum(pca$sdev^2) * 100 )
v
## [1] 67 29  4  0
## or the second row here...
z <- summary(pca)
z$importance
##                              PC1       PC2      PC3          PC4
## Standard deviation     324.15019 212.74780 73.87622 4.188568e-14
## Proportion of Variance   0.67444   0.29052  0.03503 0.000000e+00
## Cumulative Proportion    0.67444   0.96497  1.00000 1.000000e+00

This information can be summarized in a plot of the variances (eigenvalues) with respect to the principal component number (eigenvector number), which is given below.

barplot(v, xlab="Principal Component", ylab="Percent Variation")

Digging deeper (variable loadings)

You can include R code in the document as follows:

We can also consider the influence of each of the original variables upon the principal components (typically known as loading scores). This information can be obtained from the prcomp() returned $rotation component. It can also be summarized with a call to biplot(), see below:

## Lets focus on PC1 as it accounts for > 90% of variance 
par(mar=c(10, 3, 0.35, 0))
barplot( pca$rotation[,1], las=2 )

Here we see observations (foods) with the largest positive loading scores that effectively “push” N. Ireland to right positive side of the plot (including Fresh_potatoes and Soft_drinks).

We can also see the observations/foods with high negative scores that push the other countries to the left side of the plot (including Fresh_fruit and Alcoholic_drinks).

Q9: Generate a similar ‘loadings plot’ for PC2. What two food groups feature prominantely and what does PC2 maninly tell us about?

HINT: Ask Barry to discuss this with you if he has not already ;-) PCA and related techniques are often tricky to get your head around at the start but we will build more intuition about these techniques as we go through the course - because they are tremendously useful for helping us to analyze all sorts of multidimensional data.

Biplots

Another way to see this information together with the main PCA plot is in a so-called biplot:

## The inbuilt biplot() can be useful for small datasets 
biplot(pca)

Observe here that there is a central group of foods (red arrows) around the middle of each principal component, with four on the periphery that do not seem to be part of the group. Recall the 2D score plot (Figure above), on which England, Wales and Scotland were clustered together, whilst Northern Ireland was the country that was away from the cluster. Perhaps there is some association to be made between the four variables that are away from the cluster in the main PCA plot and the country that is located away from the rest of the countries i.e. Northern Ireland. A look at the original data in Table 1 reveals that for the three variables, Fresh potatoes, Alcoholic drinks and Fresh fruit, there is a noticeable difference between the values for England, Wales and Scotland, which are roughly similar, and Northern Ireland, which is usually significantly higher or lower.

Note: PCA has the awesome ability to be able to make these associations for us. It has also successfully managed to reduce the dimensionality of our data set down from 17 to 2, allowing us to assert (using our figures above) that countries England, Wales and Scotland are ‘similar’ with Northern Ireland being different in some way. Furthermore, digging deeper into the loadings we were able to associate certain food types with each cluster of countries.

2. PCA of RNA-seq data

RNA-seq results often contain a PCA (or related MDS plot). Usually we use these graphs to verify that the control samples cluster together. However, there’s a lot more going on, and if you are willing to dive in, you can extract a lot more information from these plots. The good news is that PCA only sounds complicated. Conceptually, as we have hopefully demonstrated here and in the lecture, it is readily accessible and understandable.

In this example, a small RNA-seq count data set (available from the class website (expression.csv) is read into a data frame called rna.data where the columns are individual samples (i.e. cells) and rows are measurements taken for all the samples (i.e. genes).

rna.data <- read.csv("data/expression.csv", row.names=1)
head(rna.data)

NOTE: The samples are columns, and the genes are rows!

Q10: How many genes and samples are in this data set?

Generating barplots etc. to make sense of this data is really not an exciting or worthwhile option to consider. So lets do PCA and plot the results:

## Again we have to take the transpose of our data 
pca <- prcomp(t(rna.data), scale=TRUE)
 
## Simple un ploished plot of pc1 and pc2
plot(pca$x[,1], pca$x[,2])

This quick plot looks interesting with a nice separation of samples into two groups of 5 samples each. Now we can use the square of pca$sdev, which stands for “standard deviation”, to calculate how much variation in the original data each PC accounts for:

## Variance captured per PC 
pca.var <- pca$sdev^2

## Precent variance is often more informative to look at 
pca.var.per <- round(pca.var/sum(pca.var)*100, 1)
pca.var.per
##  [1] 92.6  2.3  1.1  1.1  0.8  0.7  0.6  0.4  0.4  0.0

We can use this to generate our standard scree-plot like this

barplot(pca.var.per, main="Scree Plot", 
        xlab="Principal Component", ylab="Percent Variation")

We can see from these results that PC1 is were all the action is. This indicates that we have sucesfully reduced a 100 diminesional data set down to only one dimension that retains the main essential (or principal) features of the origional data. This is quite amazing!

Now lets make our main PCA plot a bit more attractive and useful…

## A vector of colors for wt and ko samples
colvec <- colnames(rna.data)
colvec[grep("wt", colvec)] <- "red"
colvec[grep("ko", colvec)] <- "blue"

plot(pca$x[,1], pca$x[,2], col=colvec, pch=16,
     xlab=paste0("PC1 (", pca.var.per[1], "%)"),
     ylab=paste0("PC2 (", pca.var.per[2], "%)"))

text(pca$x[,1], pca$x[,2], labels = colnames(rna.data), pos=c(rep(4,5), rep(2,5)))

Maybe this is an easier approach to understand for our coloring task:

## Another way to color by sample type
## Extract the first 2 characters of the sample name
sample.type <- substr(colnames(rna.data),1,2)
sample.type
##  [1] "wt" "wt" "wt" "wt" "wt" "ko" "ko" "ko" "ko" "ko"
## now use this as a factor input to color our plot
plot(pca$x[,1], pca$x[,2], col=as.factor(sample.type), pch=16)

Find the top 10 measurements (genes) that contribute most to pc1 in either direction (+ or -).

loading_scores <- pca$rotation[,1]

## Find the top 10 measurements (genes) that contribute
## most to PC1 in either direction (+ or -)
gene_scores <- abs(loading_scores) 
gene_score_ranked <- sort(gene_scores, decreasing=TRUE)

## show the names of the top 10 genes
top_10_genes <- names(gene_score_ranked[1:10])
top_10_genes 
##  [1] "gene100" "gene66"  "gene45"  "gene68"  "gene98"  "gene60"  "gene21" 
##  [8] "gene56"  "gene10"  "gene90"

These may be the genes that we would like to focus on for further analysis (if there expression changes are significant - we will deal with this and further RNA-Seq analysis in Lectures 13-15).

Muddy Point Assessment

Link to today’s muddy point assesment.

Session Information

sessionInfo()
## R version 3.5.1 (2018-07-02)
## Platform: x86_64-apple-darwin15.6.0 (64-bit)
## Running under: macOS High Sierra 10.13.6
## 
## Matrix products: default
## BLAS: /Library/Frameworks/R.framework/Versions/3.5/Resources/lib/libRblas.0.dylib
## LAPACK: /Library/Frameworks/R.framework/Versions/3.5/Resources/lib/libRlapack.dylib
## 
## locale:
## [1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
## 
## attached base packages:
## [1] stats     graphics  grDevices utils     datasets  methods   base     
## 
## other attached packages:
## [1] formattable_0.2.0.2
## 
## loaded via a namespace (and not attached):
##  [1] htmlwidgets_1.3 compiler_3.5.1  backports_1.1.2 magrittr_1.5   
##  [5] rprojroot_1.3-2 tools_3.5.1     htmltools_0.3.6 yaml_2.2.0     
##  [9] Rcpp_0.12.19    stringi_1.2.4   rmarkdown_1.10  knitr_1.20     
## [13] jsonlite_1.5    stringr_1.3.1   digest_0.6.18   evaluate_0.12