Step 1: Download the data

There are two major public repositories of microarray datasets: GEO (NCBI) and ArrayExpress (EMBL). Today we’ll be pulling data from GEO.

Using the search features on the GEO website, find the DeRisi 1997 yeast diauxic shift dataset.

If you’re having trouble getting the tables to load on GEO today, here are local links:

(Hint: if the series is not GSE28, you’re on the wrong record)

Download microarray data corresponding to the pre-diauxic shift samples (0’ time point) and at least one other time point of your choosing.

Step 2: Look at the format of the data table

The two files you just downloaded hold the results of a microarray image analysis, along with the results of the data pre-processing done for the published paper.

Answer the following questions for yourself about the structure of the data:

Use R to load the data in these tables into data.frame structures. Think carefully about how you are naming your variables and keeping track of your work. Unlike the genome annotation lab, I won’t be providing templates for you to follow here, so its up to you to partition your code into a logical and organized set of functions!

Step 3: Calculating a ratio

In this data file the authors have done some of the data pre-processing work for us – namely, normalizing the ratios – but let’s pretend they haven’t!

Using ONLY the data in the CH1D_MEAN and CH2D_MEAN columns, add a column to your data structure that calculates the ratio of the Cy5 (CH1) to Cy3 (CH2) samples for each spot on the array.

(Hint: you may have to convert the numbers read from the file from strings to integers.)

Add another column that converts these plain ratios to the Log2(ratio). Think carefully about why a Log2 transformation of the raw image ratios makes sense before going on to the next section.

Step 4: Distribution of ratio values

At this point, it would make sense for us to look at the distribution of the ratio values on our array. One common problem with competitive hybridization microarrays (two-color arrays) is that a sample with one of the dies has often been loaded at a slightly higher concentration than the other. This is usually do to subtle pipetting or cDNA concentration calculation errors. Examining a distribution of ratio values on our array should help us identify any bias towards the green or red end of the color spectrum.

Functions which might prove useful to you are the ggplot2 geom_density, geom_histogram, or geom_boxplot functions. Use R to produce at least two different plots that allow us to visualize the distribution of ratio values.

For this excercise will you want to be working with the raw ratio values or the Log2 transformed values? Why?

Save these plots in thoughtfully named files. Do you see a ratio bias in these data?

How do the ratios compare if you use _MEDIAN pixel intensities instead of _MEAN intensities?

Step 5: Distribution of ratio values, Part 2

To help us to better understand the distributions we see above, it might be useful to consider a couple additional strategies for visualizing these data.

First, it would be interesting to examine a scatter plot comparing the red channel pixel intensity to the green channel pixel intensity for each individual spot on the array. Produce this scatter plot (in ggplot2 geom_points). Make sure your plot includes clearly labeled axes (labs, xlab, and ylab).

What can we learn about the overall distribution of ratios on the array from this plot? On average are the changes large or small? Are many genes affected or only a small number?

Second, we should also examine whether or not the total intensity of a spot is related to the ratio bias observed above. Make a scatter plot aimed at addressing this question (compare the color ratio of each spot to its total intensity). You’ll have to decide how you want to calculate the total intensity for spot.

Was there an intensity bias? Why might this be of a concern to us? What might this tell us about the behavior of the dies used to label the two samples on this array?

Step 6: Background intensity

So far we’ve been ignoring background pixel intensities. These values are a measure of the brightness in the regions surround each spot (the “background”). Background is obviously noise and doesn’t correspond to cDNA’s binding specifically to probes on the array.

Produce a visualization to help us asses how variable the background intensities are across the surface of the array. One great way to do this would be to produce a heat map, where each box corresponds to a spot position on our array and the color corresponds to the intensity of the background at that spot.

This can be done with the image and heat.colors functions. Explore the way these functions work using some sample data. Think about the structure of the data you want to pass into image and how you can make a matrix conforming to this architecture using your existing data.frames.

Are the background intensities uniform or are some much higher than others? Where are the problems the greatest?

How should we use background intensity information?

Step 8: Global median normalization

Knowing what you know now about your data, design a way to add a normalized ratio column to your data structure. Implement it, run it, and check to make sure it worked!

Step 9: Collecting pre-processed data and integrating annotations

Up to this point we’ve been looking at our data one microarray table at a time. As we start exploring the data it would be more convenient to have a new table which contains just the set of final normalized ratio values for each array. Let’s list arrays across the columns and genes down the rows.

You also may have noticed that the array data files we downloaded from GEO don’t tell us which row is associated with which yeast gene – instead it gives us an ID_REF column. The IDs in this column match up with the IDs in the platform table (see step #1). Since we’re building a new table at this point it makes sense to pull in the gene annotations at this point as well from the platform record.

Implement a function to do it! Your function will probably need to take multiple source array data tables, know which column to pull data from and take a platform table.

Step 10: Exploring the data set

Once you have a data.frame will all of the ratio data from each microarray loaded and annotated, it’s time to start exploring the data set.

Here are some challenges to try: