Differential Expression Talk

Goals

Three overall:

Primarily, it's about:

• Identify differentially expressed genes in two or more conditions (e.g. normal v cancer)

Generally, it's about:

• Gain biological insight into which genes cause / respond to a condition

And with an eye towards future project: looking for more promising places to look:

• Identify biomarkers for a condition

Three principal themes

1. Data normalisation
2. Data quality control
3. Differential expression analysis

Data filtering

• Due to random noise / sampling errors, genes with low read counts across all samples cannot be found to be differentially expressed
• Removing these:

- reduces amount of data

- improves speed of analysis

- reduces number of genes to be counted in multiple test correction

Data normalisation

What affects read count? Read count not only affected by:

• level of transcription

but also by:

• Between genes

- length of gene

- GC content

• Between libraries

- sequencing depth (library size)

- RNA composition

RNA composition

• A few extremely highly expressed genes may contribute a very large part of the sequenced reads
• Changes in the expression of these change the relative abundance of all other genes

Normalisation methods

• Total Count (TC)

- TC = reads mapping to gene / total reads in library

• Other methods of normalising counts:

- Reads per Kilobase per Million mapped reads (RPKM)

- Upper Quartile (UQ)

- Median (Med)

- DESeq

- Trimmed Mean of M-values (TMM) (used by edgeR)

- Quantile (Q)

Normalisation Example

• Consider two samples
• Almost all genes have identical read counts in library 1 and library 2
• A few genes are highly expressed in library 2
• How should library 2 be normalised to make it comparable to library 1?
• Correct normalisation factor would be 1 (no change)

Normalisation Example

Trimmed Mean of M-values (TMM)

Normalisation conclusion

• Dillies et al. conclude that only TMM and DESeq can cope with large changes in highly expressed genes.
• These lean on the assumption that:

- the majority of genes are not differentially expressed

- for those differentially expressed, there is an approxmiately balanced proportion of over- and under-expression.

Data quality control

• Do (technical and biological) replicates cluster together?
• we can see on an MDS plot:

- Shows the level of similarity of individual cases of a dataset

- Distances represent fold-changes

• Dataset: 10 patients

- Cancerous samples

- Non-cancerous samples

Plotting the samples 1

• A Multidimensional scaling plot is in fact a PCA Principle Component plot with the first two dimension.
• These are the dimensions internal to the data where most variation in values is seen.
• The distances here represent fold-changes.
• Ten patients

- Cancerous samples in red

- Non-cancerous samples in black

Plotting the samples 2

• Multiple samples from the same patient cluster together

Plotting the samples 3

• Cancerous samples cluster together
• Non-cancerous samples cluster together

- though not a very tight separation between the two

Plotting the samples 4

• Removing two patients improves the separation
• Two out of ten patients: maybe not justified.

Differential expression methods

• For each gene, two measures of expression level will show up:

- between the two groups of samples

- within groups of samples

• Might the difference within groups of samples be big enough to explain the difference between groups of samples?

Differential expression methods

• Cancer samples in red

- Mean logcount is 116

• Non cancer samples in black

- Mean logcount is 132

Differential expression methods

• Count based:

– most tools

• Coverage based:

– Cuffdiff

• Methods may be parametric or non-parametric

- non-parametric build up their own parameters from the data.

• Some tools allow a variety of experimental designs

Differential expression methods

• Parametric methods

– e.g. edgeR, DESeq

– assume a negative binomial distribution to account for biological variation

– have problems when the data don’t fit this distribution

• Non-parametric methods

– e.g. SAMseq and NOISeq

– need to learn the distribution from the data

– may require more replicates

edgeR

• assumes that normalised counts for each gene across biological replicates follows a negative binomial distribution with the dispersion representing the biological variation
• calculates a dispersion factor for each gene
• calculates a dispersion factor that fits the data as a whole

Diversity of low count

• Genes with fewer counts can appear to be highly variable due to sampling errors

Two types of comparsions

Grouped comparisons

Matched-pair comparison

edgeR output

P-values

Testing 100 genes for DE ...

Add FDR to P-values

Testing 100 genes for DE ...

MA Plot comparison

Summary

• Before differential expression analysis is done there are multiple initial steps
• Data must be filtered, normalised and outliers removed
• A variety of techniques to both normalise data and call differentially expressed genes are used
• Understanding of the experimental design is important
• Different techniques can give different results, especially for low numbers of replicates, noisy data and lowly expressed genes
• No standard way of doing any of this, best practices are still evolving.

Further reading

• Dillies et al "A comprehensive evaluation of normalization methods for Illumina high-throughput RNA sequencing data analysis” Brief Bioinform. 2013 Nov;14(6):671-83.
• Soneson and Delorenzi "A comparison of methods for differential expression analysis of RNA-seq data.” BMC Bioinformatics. 2013 Mar 9;14:91.
• Rapaport et al "Comprehensive evaluation of differential gene expression analysis methods for RNA-seq data.” Genome Biol. 2013;14(9):R95.
• Huang et al "RNA-Seq analyses generate comprehensive transcriptomic landscape and reveal complex transcript paLerns in hepatocellular carcinoma.” PLoS One 2011 17;6(10):e26168.