Using reduced representation bisulfite sequencing (RRBS) to characterize genomic DNA methylation

Background

• Reduced-representation bisulfite sequencing (RRBS) is an experimental ​protocol to measure and compare genomic methylation patterns. These experiments use one or multiple restriction enzymes on genomic DNA to produce sequence specific fragments that are subsequently treated with bisulfite and sequenced. The workflow below illustrates how to use ​bismark to analyze RRBS data on the resources at the Whitehead Institute.

Step by step analysis

• Quality control
  • Start by using ​Trim Galore or ​another read trimmer to apply quality filters and remove adapters.
  • See our ​QC and preprocessing guidelines for details on running Trim Galore.
  • A command for running Trim Galore on paired end reads from RRBS experiments can look like:

bsub trim_galore --paired --rrbs --fastqc -o trimmedReads /path/to/raw/data/reads_1.fq.gz /path/to/raw/data/reads_2.fq.gz

• Quantification of methylation calls
  • Bismark can (since version 0.6.beta1) use ​bowtie2 to map short reads to reference genome(s).
  • In the command below, bismark will expect to find C-to-T and a G-to-A versions of the reference genome. From the following the steps above, these will be located within /nfs/genomes/myOrganism/bowtie/ (for Institute-wide reference genomes) or /path/to/custom/genome/bowtie for other reference genomes.
  • ​Bismark produces BAM file(s) of aligned reads and methylation calls.

bsub bismark --genome /nfs/genomes/myOrganism/bowtie/ -1 trimmedReads_1.fq.gz -2 trimmedReads_2.fq.gz

• Bismark can (since version 0.6.beta1) use ​bowtie2 to map short reads to reference genome(s).
• In the command above, bismark will expect to find C-to-T and a G-to-A versions of the reference genome.

• Extract methylation percentages
  • After running bismark, the methylation percentages can be extracted from the BAM output using bismark_methylation_extractor.
  • For paired ended data, use the "-p" flag:

bsub bismark_methylation_extractor -p --gzip --bedGraph trimmedReads_bismark_bt2_pe.bam 

• The resulting file trimmedReads_bismark bt2_pe.bismark.cov.gz summarizes coverage in the following ​format:

<chromosome> <start position> <end position> <methylation percentage> <count methylated> <count unmethylated>