IMPLICON - bisulfite amplicon data for imprinted loci

This is a processing guide for IMPLICON data, with detailed intructions of how to get from raw data to methylation consistancy plots at imprinted loci.

Publication at NAR: IMPLICON: an ultra-deep sequencing method to uncover DNA methylation at imprinted regions

Last update: 28/03/2021

Table of Contents

• Quick Start
• Detailed Processing Guide
  1. UMI-handling
  2. Adapter-/Quality Trimming
  3. Genome Alignments
     • Allele-specific alignments (optional)
  4. UMI-aware deduplication
  5. Allele-specific sorting (optional)
  6. Methylation extraction
• Read-level methylation consistency analysis
  1. Filtering Implicon CpG context
  2. Plotting read-level methylation consistency

QUICK START

The following commands are designed to work with a hypothetical example paired-end IMPLICON dataset consisting of reads from a C57BL/6 mouse:

Read 1: test_R1.fastq.gz Read 2: test_R2.fastq.gz

Step I: UMI-handling

trim_galore --paired --implicon *fastq.gz

Step II: Adapter-/quality trimming

trim_galore --paired *UMI*fastq.gz

Step III: Genome alignments

bismark --genome /Genomes/Mouse/GRCm38/ -1 test_8bp_UMI_R1_val_1.fq.gz -2 test_8bp_UMI_R2_val_2.fq.gz

* for allele-specific alignments see below

Step IV: UMI-aware deduplication

deduplicate_bismark --barcode test_8bp_UMI_R1_val_1_bismark_bt2_pe.bam

Step V: Allele-specific sorting

* for allele-specific sorting please see below

Step VI: Methylation extraction

bismark_methylation_extractor --bedGraph --gzip test_8bp_UMI_R1_val_1_bismark_bt2_pe.deduplicated.bam

For filtering and visualising CpG level results, please see Read-level methylation consistency analysis.

DETAILED DATA PROCESSING GUIDE

Step I: UMI handling

At its 5’ end, Read 2 carries 8 bp of randomised nucleotides that serve as unique molecular identifiers (UMI) for the amplification reaction. The UMI sequence needs to be transferred from the start of Read 2 to the readID of both reads to allow UMI-aware deduplication later, a step that can be accomplished using the Trim Galore with the option --implicon (for more information type trim_galore --help). In this step, the UMI of Read 2 is added to the readID of both reads as the last element separated by a “:”, e.g.:

@HWI-D00436:407:CCAETANXX:1:1101:4105:1905 1:N:0: CGATGTTT:CAATTTTG

To run this specialised UMI-transfer trimming on all files of a MiSeq run you can run this command:

trim_galore --paired --implicon *fastq.gz

The FastQC per base sequence content plot would look something like this:

Raw FastQ file:

UMI trimmed file:

As an example, we are using the following set of test files to demonstrate subsequent steps that need to be taken:

Input files:

test_R1.fastq.gz
test_R2.fastq.gz

Output files:

test_8bp_UMI_R1.fastq.gz
test_8bp_UMI_R2.fastq.gz

Step II: Adapter-/quality trimming

Following UMI-handling, UMI-treated IMPLICON reads require adapter and quality trimming. A standard Trim Galore run should identify and remove read-through adapter contamination as well as poor quality base calls, like so:

trim_galore --paired *UMI*fastq.gz

Output files:

test_8bp_UMI_R1_val_1.fq.gz
test_8bp_UMI_R2_val_2.fq.gz

Step III: Genome alignments

Alignments to the mouse or human genome can then be obtained with a standard Bismark paired-end run, e.g.:

bismark --genome /Genomes/Mouse/GRCm38/ -1 test_8bp_UMI_R1_val_1.fq.gz -2 test_8bp_UMI_R2_val_2.fq.gz

Allele-specific alignments

Note for allele-specific mouse hybrid data, the alignments need to be carried out against a genome that has SNPs between the parental strains masked out with Ns (N-masking). To prepare such a genome, please follow the instructions over at the SNPsplit project page. The alignments as well as the output file will behave and look exactly the same, but an additional allele-sorting step needs to be carried out for allele-specific data (see Step V below.

Just briefly, if we assume a hybrid strain of Black6 (C57BL/6) and Castaneus (CAST_EiJ), the genome can be prepared using a command like this:

Hybrid genome preparation

SNPsplit_genome_preparation -vcf mgp.v5.merged.snps_all.dbSNP142.vcf.gzSNP142.vcf.gz  --strain CAST_EiJ --reference Genomes/Mouse/GRCm38/

Hybrid genome indexing

The N-masked genome then requires indexing (as a one-off process), with a command like this:

bismark_genome_preparation --verbose /Genomes/Mouse/CAST_EiJ_N-masked/

Allele-specific alignments

bismark --genome /Genomes/Mouse/CAST_EiJ_N-masked/ -1 test_8bp_UMI_R1_val_1.fq.gz -2 test_8bp_UMI_R2_val_2.fq.gz

Relevant output files:

test_8bp_UMI_R1_val_1_bismark_bt2_pe.bam

The output BAM file is then ready for a UMI-aware deduplication step.

Step IV: UMI-aware deduplication

In this step, paired-end read alignments are deduplicated based on:

• chromosome

• start position

• end position

• alignment orientation

• UMI from the read header (see Step I)

deduplicate_bismark --barcode test_8bp_UMI_R1_val_1_bismark_bt2_pe.bam

Relevant output files:

test_8bp_UMI_R1_val_1_bismark_bt2_pe.deduplicated.bam

Step V: Allele-specific sorting

This step is optional, and only needs to be carried out for alignments against N-masked hybrid genomes processed with the SNPsplit package. The command below needs the SNP annotation file for the mouse strains in question (this file is generated by the SNPsplit genome preparation):

SNPsplit --snp_file all_SNPs_CAST_EiJ_GRCm38.txt.gz test_8bp_UMI_R1_val_1_bismark_bt2_pe.deduplicated.bam

Relevant output files

test_8bp_UMI_R1_val_1_bismark_bt2_pe.deduplicated.genome1.bam (C57BL/6)
test_8bp_UMI_R1_val_1_bismark_bt2_pe.deduplicated.genome2.bam (CAST_EiJ)

Step VI: Methylation extraction

The methylation extraction process can be carried out with a default parameters:

bismark_methylation_extractor --bedGraph --gzip test_8bp_UMI_R1_val_1_bismark_bt2_pe.deduplicated.bam

For allele-specific alignments, the files ...genome1.bam and ...genome2.bam should be used instead.

Relevant output files

General methylation analysis (coverage file):

test_8bp_UMI_R1_val_1_bismark_bt2_pe.deduplicated.bismark.cov.gz

CpG context files for bisulfite consistency analysis:

CpG_OT_test_8bp_UMI_R1_val_1_bismark_bt2_pe.deduplicated.txt.gz (top strand)
CpG_OB_test_8bp_UMI_R1_val_1_bismark_bt2_pe.deduplicated.txt.gz (bottom strand)

READ-LEVEL METHYLATION CONSISTENCY

filtering CpG context files

This filtering step checks all CpG context result files in the current working directory for information about annotated cytosine positions of imprinted loci. The imprinted CpG annotation file (e.g. Imprinted CpG positions Mouse (GRCm38) or Imprinted CpG positions Human (GRCh38)) is provided as command line argument to the script below, and needs to contain all relevant CpG positions as a single line in this format (tab-delimited) (Start and End are the same position (= a single cytosine)):

Probe   Chromosome  Start   End Feature
Chr2:152686786-152686786    2   152686786   152686786   H13
Chr2:152686810-152686810    2   152686810   152686810   H13
...
Chr2:174295708-174295708    2   174295708   174295708   Gnas
Chr2:174295728-174295728    2   174295728   174295728   Gnas
...
Chr3:34649370-34649370  3   34649370    34649370    Sox2
Chr3:34649383-34649383  3   34649383    34649383    Sox2
...

If a read pair is found to overlap a known imprinted locus, all CpG positions of the entire read pair are written to a file called: methylation_state_consistency.txt. All other reads are skipped.

In contrast to standard methylation result files, such as the Bismark coverage file, each line of the methylation consistency file contains the CpG methylation data of an entire read-pair, and thus preserves the methylation state on a per-read level. The methylation consistency file looks like this:

readID  sample  implicon    1   2   3   4   5   6   7   8   9   10  11  12  13  14  15  16  17  18  19  20  21  22
    23  24  25  26  27  28  29  30  31
2   test    Klf4    0   0   0   0   0   0   0   0   0   0   0   0   0   0   0
3   test    Klf4    0   0   0   0   0   0   0   0   0   0   0   0   0   0   0
4   test    Klf4    0   0   0   0   0   0   0   0   0   0   0   0   0   0   0
5   test    Sox2    NA  1   1   1   1   0   1   1   1   1   1   1   1   1
...

readID: incremental number reads processed. sample: extracted from the filename via regex (might have to be adapted to different types of filenames). implicon: name of implicon in question (as provided by the annotation file above, last column Feature). 1..31: methylation call for subsequent positions of cytosines (in CpG context) within that implicon. 0 means unmethylated, 1 means methylated. Annotated positions without a methylation call recieve 'NA'.

The filtering scripts in the following section require Python 3 (3.6 and above).

Command for mouse data (non allele-specific):

./filter_coordinates_mouse_not_allele_specific.py CpG_imprinted_positions_mouse.txt

Command for mouse data (allele-specific):

./filter_coordinates_mouse_allele_specific.py CpG_imprinted_positions_mouse.txt 

As outlined above, allele-specific processing requires the data to have been processed with SNPsplit. In addition to the output for non-allelic processing above, using the script filter_coordinates_mouse_allele_specific.py also requires the filenames to contain genome1 (the reference) or genome2 (the alternative strain) which is extracted and added to the methylation consistency file as 'allele', like so:

readID  sample  allele  implicon    1   2   3   4   5   6   7   8   9   10  11  12  13  14  15  16  17  18  19  20  21
    22  23  24  25  26  27  28  29  30  31
1   2C_B6_miPSC B6  Gnas    0   0   0   0   0   0   0   0   0   0
2   2C_B6_miPSC B6  Commd1  NA  0   0   0   0   0   0   0   0   0   0   0   0
3   2C_B6_miPSC B6  Commd1  NA  0   0   0   0   0   0   0   0   0   0   0   0
4   2C_B6_miPSC B6  Gnas    0   1   1   1   1   0   1   1   1   1
5   2C_B6_miPSC B6  Dlk1    NA  0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   1   0NA NA

Whereby allele is one of 'B6' (genome1), or 'CAST' (genome2).

Command for human data:

./filter_coordinates_human_amplicons.py CpG_imprinted_positions_human.txt

Relevant output files

methylation_state_consistency.txt

Plotting read-level methylation consistency

In this last section, you really only need to fire up an R-Studio session and open one of the provided Rmd scripts, e.g. visualise_implicon_methylation_consistency_mouse_allele_specific.Rmd.

Simply add the methylation_state_consistency.txt file to the same folder as the Rmd script and potentially change the filename in this line of the script:

read_tsv("methylation_state_consistency.txt") -> input

The visualisation Rmd script processes the methylation consistency file, which may contain one or several samples (and alleles for allele-specific data), etracts Sample name as well as Implicon name, and applies the following procedures:

• filters the comprehensive data table for a single Implicon

• filters for a single Sample

• transform the data to long format

• group the data by annotated CpG position, and exclude positions that were not covered at all (NA call)

• extract up to 5000 reads,

• sort reads from highly to lowly methylated, and finally

• plot reads per gene, per sample

Either running or knitting this file to HTML format should produce various output plots which look like this:

For any questions about this data processing guide, please get in touch.