ChIP-seq: Quality Control | Workshop ChIPATAC 2020

7. ChIP-seq: Quality control

We can run a number of quality controls for the ChIP-seq dataset; these different quality controls measure several properties of the datasets

• presence of PCR duplicates?
• proportion of the reads contained inside the peaks
• footprinting: enrichment of signal using a Lorez curve

We have previously seen other quality controls related to the quality of the sequencing: read / mapping quality, proportion of duplicates, etc...

We will check some of the metrics which have been developed by the ENCODE consortium; you can check here some of the ENCODE quality standards for transcription factor ChIP-seq

Presence of PCR duplicates

The PCR bottleneck coefficient (PBC) measures the proportion of the genomic position in which a unique read starts, compared to the number of positions on which at least one read aligns.

We will use the R library encodeChIPqc, which implements some of these metrics; this library is available on this GitHub repository

cdmkdir-panalysis/QC/ChIP/usr/bin/R# You are now in R console !!library(encodeChIPqc)PBC("data/processed/CTCF/Bowtie2/CTCF_Rep1_ENCFF001HLV_trimmed_aligned_nofilt.bam")

What does the value mean? Is the data of good quality?

Now run, on the filtered .bam

PBC("data/processed/CTCF/Bowtie2/CTCF_Rep1_ENCFF001HLV_trimmed_aligned_filt_sort_nodup.bam")

Do you understand the result?

Fraction of reads in peaks

The Fraction of reads in peaks (FRiP) determines, as the name says, the proportion of reads that fall within the peak regions. The higher this proportion, the more the signal-to-noise ratio of high, indicating that the signal is concentrated in specific enriched regions. However, this quality metrics depends on the called peaks, hence it might be as well a quality metrics for how well the peaks were called!

We first read the peaks called using MACS2:

peaks = read.table('analysis/MACS2/CTCF/CTCF_peaks.narrowPeak') colnames(peaks) = c('chr','start','end','name','score','strand','signal','pval','qval','peak') peaks.gr = makeGRangesFromDataFrame(peaks,keep.extra.columns=TRUE) peaks.gr

Now we compute the FRiP value:

bam.file = "data/processed/CTCF/Bowtie2/CTCF_Rep1_ENCFF001HLV_trimmed_aligned_filt_sort_nodup.bam" frip(bam.file,peaks.gr)

What does this mean? What would be the maximum value?

Fingerprint

The fingerprint plot displays how unevenly the signal is distributed over the genome. A good IP should concentrate a lot of signal in a few enriched regions, a little signal in the background. This distribution is displayed using a Lorenz curve. We will use the functions of the package DeepTools. *Beware! this tool is not an R package! So we need to go back to the standard console!

If you are still in the R console (check if you see a > as the prompt), you first need to quit R:

quit() # and then type "n"

Make sure that you see a $ sign as a prompt: you are now back in the bash console!

We now run the plotFingerprint function from the DeepTools package; to limit computational time, we run it only on Chromosome 10:

cd plotFingerprint \ --bamfiles data/processed/CTCF/Bowtie2/CTCF_Rep1_ENCFF001HLV_trimmed_aligned_filt_sort_nodup.bam \ data/processed/CTCF/Bowtie2/CTCF_Control_ENCFF001HME_trimmed_aligned_filt_sort_nodup.bam \ --labels CTCF Control \ --region chr10 \ --numberOfProcessors 3 \ --plotFile analysis/QC/ChIP/CTCF_fingerprint.png

Number of reads and non-redundant reads

The simplest quality control is to check the number of reads, in the unfiltered and the filtered bam files.

## Number of reads in the unfiltered bam file samtools view -c -@ 3 data/processed/CTCF/Bowtie2/CTCF_Rep1_ENCFF001HLV_trimmed_aligned_nofilt.bam ## Number of reads in the filtered bam file (remember how we filtered the file? [check again here](./03_Alignment.md)) samtools view -c -@ 3 data/processed/CTCF/Bowtie2/CTCF_Rep2_ENCFF001HLW_trimmed_aligned_filt_sort_nodup.bam calc 12308756/18238437

If you take the ratio of these 2 numbers, you will get something like the non-redundant fraction (NFR).

We have a second replicate of CTCF in the folder data/processed/CTCF/Bowtie2/; determine the different quality metrics for this second replicate; which replicate seems to be better?