Large Scale Gene Expression Data Analysis I

Yan Cui (ycui2@uthsc.edu)

The address of this webpage:

http://compbio.uthsc.edu/microarray/lecture1.htm

 

Algorithm

o   Hierarchical clustering

o   K-Means / K-Medians clustering

Software

o   MultiExperiment Viewer (MeV)

 

Currently, there are two types of methods for large-scale gene expression profiling

o   Microarray

o   RNA-sequencing

 

What is Microarray?

A microarray is a collection of small DNA spots attached to a solid surface. In microarray experiments, the signal collected from each spot is used to estimate the expression level of a gene. A microarray contains thousands of DNA spots, covering almost every gene in a genome.

o   The first high-throughput technology for gene expression analysis

o   Emerged in late 90s

 

What is RNA-Sequencing (RNA-Seq)?

o   A new method for large scale gene expression analysis

o   Occurred a few years ago

o   Uses deep-sequencing technologies to measure the numerical frequencies of RNA sequences in a sample

o   More accurate, more expensive

 

Microarray and RNA-Seq use very different technologies. Both of them can monitor expression levels of thousands of genes simultaneously.

 

The number of published papers referring to microarray or RNA-seq (in their titles or abstracts)

 

Expression Data Matrix

Gene expression data are usually presented in an expression matrix. Each column represents all the gene expression levels from a single experiment, and each row represents the expression of a gene across all experiments. Each element is a log ratio. The log ratio is defined as log2 (T/R), where T is the gene expression level in the testing sample, R is the gene expression level in the reference sample.

 

Description: Description: Description: Description: Data

 

Why log transformation? It is easier to link log ratio to fold change.

Original ratio

Log2-tranformed ratio

Up

Down

Up

Down

2

0.5

1

-1

4

0.25

2

-2

8

0.125

3

-3

16

0.0625

4

-4

 

The expression matrix can be presented as a matrix of colored rectangles. Each rectangle represents an element of the expression matrix.

(Adapted from MeV document)

 
Hierarchical Clustering

Hierarchical Clustering is the most popular method for gene expression data analysis. In hierarchical clustering, genes with similar expression patterns are grouped together and are connected by a series of branches (clustering tree or dendrogram). Experiments with similar expression profiles can also be grouped together using the same method.

 

 

Description: Description: Description: Description: HCL1

Description: Description: Description: Description: HCL2

Description: Description: Description: Description: HCL3

(Adapted from the documentation of MeV)

 

The two questions:

1.    How to determine the similarity between two genes?

2.    How to determine the similarity between two clusters?

To answer the first question, we calculate the distance between two expression vectors. A Gene Expression Vector consists of the expression of a gene over a set of experimental conditions.

 

 

(From the documentation of MeV)

 

The second question is: How to determine the similarity between clusters? The method for determining cluster-to-cluster distance is called linkage method.

 

Three linkage methods:

 

 

 

(Adapted from the documentation of MeV)

 

There is no theoretical guideline for selecting the best linkage method. In practice, people almost always use the average linkage method.

 

(Shannon W. et al. (2003) Analyzing microarray data using cluster analysis. Pharmacogenomics 4:41-51.)

Gene A and B are merged first at the level of 1.58. The position of the splitting point shows the distance between two genes (or clusters). A low splitting point means short distance and high similarity.

 

K-Means / K-Medians Clustering

What is Mean? What is Median?

Mean is the average.

Median is the middle number, i.e. the middle of the distribution. For an odd number of numbers, the median is simply the middle number. For example, the median of 2, 4 and 7 is 4.

For an even number of numbers, the median is the average of the two middle numbers. Thus, the median of the numbers 2, 4, 7, 12 is (4+7)/2 = 5.5.

Median is more robust against outliers

For example, the mean of 5, 6, 7, 8 and 9 is 7. The median of 5, 6, 7, 8 and 9 is also 7.

But, the mean of 5, 6, 7, 8 and 99 is 25, while the median of 5, 6, 7, 8 and 99 is still 7.

Description: Description: Description: Description: KMC1

Description: Description: Description: Description: KMC2

(Adapted from MeV documentation)

 

MultiExperiment Viewer (MeV)

MeV is an open source software for large scale gene expression data analysis. It is distributed under The Artistic License, which means you can freely download the software or get a copy from another user.

 

Run MeV

Double click the batch file (TMEV.bat) to start the program. Use the File menu to open a new Multiple Array Viewer.

Load Microarray Data

Select Load data from the File menu to launch the file-loading dialog. At the top of this dialog, use the drop-down menu to select the type of expression files to load. Use the file browser to locate the files to be loaded.

HCL: Hierarchical clustering

Type of the Input file: Stanford Files (*.txt)

Name of the Input file: Stanford_Large.txt

Select Options:

1.    Average Linkage;

2.    Cluster both genes and experiments.

Define, store and annotate clusters

Clusters of interest can be stored:

1.    Click the dendrogram to select the cluster;

2.    Open a menu by right clicking in the viewer and selecting the store cluster option;

3.    Input the name of the cluster and select a color to label the cluster.

A color bar is displayed along the right side of cluster.

 

KMC: K-Means / K-Medians Clustering

Options:

1.    Cluster genes;

2.    Use mean;

3.    Number of clusters = 5;

4.    Number of iterations = 50;

 

Further Reading

The Nature Genetics special issues on microarray analysis: The Chipping Forecast I, II, III

The Nature Reviews Genetics article series of Applications of next–generation sequencing

 

Homework

Due Date: Wednesday, February 19. Submit to Dr. Yan Cui via email (ycui2@uthsc.edu). The solution will be posted here on February 20.

Background: Microarray has shown great promise in studying complex diseases such as cancer. The genome-wide gene expression profiles of tumor tissues are considered as the molecular portraits of various cancers. For example, Clustering of breast and ovarian carcinoma cases is shown in the figure below, 68 breast and 57 ovarian cases were co-clustered to discern both similarities and disparities between the two sample sets. The common reference control consisting of equal amounts of mRNA from 11 human cancer cell lines. (Schaner, M et al., Gene Expression Patterns in Ovarian Carcinomas, Mol Biol Cell. 2003 Nov;14:4376-86).

Description: Description: Description: Description: figure4

Data: Download the dataset from http://compbio.uthsc.edu/MSCI814/Homework1.txt (Right click on the link and select Save Target As). The dataset contains gene expression profiles of 16 tumor samples. Each of the 16 samples is associated with one of the two cancers.

1. Analyze the data with hierarchical clustering (HCL)

You should use average linkage method, Euclidean distance metric and only cluster experiments.

Please infer from the dendrogram (clustering tree) the two groups of the samples (each associated with a type of cancer).

For example,

Cancer 1: Sample 1, 3,5,7,9,11,13,15

Cancer 2: Sample 2,4,6,8,10,12,14,16

2. Analyze the data with K-means clustering (KMC)

Use K-means clustering method to group the 16 samples into two clusters.

For example,

Cluster 1: Sample 1, 3,5,7,9,11,13,15

Cluster 2: Sample 2,4,6,8,10,12,14,16

What should be included in the email:

1. HCL

Cancer 1: Sample…

Cancer 2: Sample…

2. KMC

Cluster 1: Sample…

Cluster 2: Sample…