Precision Medicine Bioinformatics

Introduction to bioinformatics for DNA and RNA sequence analysis


Obtain Additional GATK resource files needed

Use Google’s gsutil to download various annotation files that will be used by GATK and other resources. gsutil will be used to download these file from Google cloud storage. They could also be downloaded using wget from the course file server from here:

cd /workspace/inputs/references/
mkdir -p gatk
cd gatk

# SNP calibration call sets - dbsnp, hapmap, omni, and 1000G
# Runtime: < 2min
gsutil cp gs://genomics-public-data/resources/broad/hg38/v0/Homo_sapiens_assembly38.dbsnp138.vcf .
# Runtime: ~ 2min
bgzip --threads 8 Homo_sapiens_assembly38.dbsnp138.vcf
gsutil cp gs://genomics-public-data/resources/broad/hg38/v0/hapmap_3.3.hg38.vcf.gz .
gsutil cp gs://genomics-public-data/resources/broad/hg38/v0/1000G_omni2.5.hg38.vcf.gz .
gsutil cp gs://genomics-public-data/resources/broad/hg38/v0/1000G_phase1.snps.high_confidence.hg38.vcf.gz .

# Indel calibration call sets - dbsnp, Mills
gsutil cp gs://genomics-public-data/resources/broad/hg38/v0/Homo_sapiens_assembly38.known_indels.vcf.gz .
gsutil cp gs://genomics-public-data/resources/broad/hg38/v0/Mills_and_1000G_gold_standard.indels.hg38.vcf.gz .

# Interval lists that can be used to parallelize certain GATK tasks
gsutil cp gs://genomics-public-data/resources/broad/hg38/v0/wgs_calling_regions.hg38.interval_list .
gsutil cp -r gs://genomics-public-data/resources/broad/hg38/v0/scattered_calling_intervals/ .

# list the files we just downloaded
ls -lh

Download coordinates describing the Exome Reagent used to generate our exome data (SeqCapEZ_Exome_v3.0)

The reagent used to produce the exome sequenced data for this course was the SeqCapEZ_Exome_v3.0 from Roche Nimblegen. Nimblegen provides a set of files describing this reagent which can be downloaded from their site here. We are most interested in the file named SeqCap_EZ_Exome_v3_hg19_capture_targets.bed which contains the chromosome, start, stop, and gene annotation for each probe used in the reagent. Let’s go ahead and downlod these files for later use.

# change directories
mkdir -p /workspace/inputs/references/exome
cd /workspace/inputs/references/exome

# download the files
wget -c

# remove the zip
rm -f

Convert SeqCapEZ_Exome_v3.0

You might have noticed that these annotation files from Nimblegen are all from the hg19 genome assembly. Obviously this presents a problem as the analysis we’re performing is using the newer hg38 assembly. This issue is actually not an uncommon situation and a variety of tools exist that are designed to make converting from one assembly to another easier. In the next section we will be using UCSC liftover to perform this task.

To start we first need to download a chain file specific to the assembly conversion we want to perform (in our case hg19 -> hg38). These files provide a mapping between the two assemblies and can be downloaded from the UCSC site. Once we have our chain file we can run liftOver which will take following positional arguments.

  1. Path to bed file to convert
  2. Path to chain file for the desired conversion (downloaded from UCSC)
  3. Path to output file that will contain the new coordinates
  4. Path to output file containing any coordinates that do not convert successfully

It will also be usefull to have a version of this file without the gene annotations lets go ahead and create that here as well.

Here we are processing two of the downloaded files, SeqCap_EZ_Exome_v3_hg19_primary_targets.bed and SeqCap_EZ_Exome_v3_hg19_capture_targets.bed. The former corresponding to the exome regions that we are interested in and the latter corresponding to the regions that probes were able to be designed on.

# change to the appropriate directory
cd /workspace/inputs/references/exome

# download the chain file
wget -c

# run liftover
liftOver SeqCapEZ_Exome_v3.0_Design_Annotation_files/SeqCap_EZ_Exome_v3_hg19_primary_targets.bed  hg19ToHg38.over.chain.gz SeqCap_EZ_Exome_v3_hg38_primary_targets.bed unMapped.bed

liftOver SeqCapEZ_Exome_v3.0_Design_Annotation_files/SeqCap_EZ_Exome_v3_hg19_capture_targets.bed  hg19ToHg38.over.chain.gz SeqCap_EZ_Exome_v3_hg38_capture_targets.bed unMapped1.bed

# create a version in standard bed format (chr, start, stop)
cut -f 1-3 SeqCap_EZ_Exome_v3_hg38_primary_targets.bed > SeqCap_EZ_Exome_v3_hg38_primary_targets.v2.bed

cut -f 1-3 SeqCap_EZ_Exome_v3_hg38_capture_targets.bed > SeqCap_EZ_Exome_v3_hg38_capture_targets.v2.bed

# take a quick look at the format of these files
head SeqCap_EZ_Exome_v3_hg38_primary_targets.v2.bed
head SeqCap_EZ_Exome_v3_hg38_capture_targets.v2.bed

# a very common question in exome (or any capture based approach) is: how big is my capture space?
# use bedtools to determine the size of the capture space represented by this bed file

# first sort the bed files and store the sorted versions
bedtools sort -i SeqCap_EZ_Exome_v3_hg38_primary_targets.v2.bed > SeqCap_EZ_Exome_v3_hg38_primary_targets.v2.sort.bed

bedtools sort -i SeqCap_EZ_Exome_v3_hg38_capture_targets.v2.bed > SeqCap_EZ_Exome_v3_hg38_capture_targets.v2.sort.bed

# now merge the bed files to collapse any overlapping regions so they are not double counted.
bedtools merge -i SeqCap_EZ_Exome_v3_hg38_primary_targets.v2.sort.bed > SeqCap_EZ_Exome_v3_hg38_primary_targets.v2.sort.merge.bed

bedtools merge -i SeqCap_EZ_Exome_v3_hg38_capture_targets.v2.sort.bed > SeqCap_EZ_Exome_v3_hg38_capture_targets.v2.sort.merge.bed

# finally use a Perl one liner to determine the size of each non-overlapping region and determine the cumulative sum
perl -ne 'chomp; @l=split("\t",$_); $size += $l[2]-$l[1]; print "$size\n"' SeqCap_EZ_Exome_v3_hg38_primary_targets.v2.sort.merge.bed
perl -ne 'chomp; @l=split("\t",$_); $size += $l[2]-$l[1]; print "$size\n"' SeqCap_EZ_Exome_v3_hg38_capture_targets.v2.sort.merge.bed

# note that the size of the space targeted by the exome reagent is ~63 Mbp. Does that sound reasonable?

# now create a subset of these bed files
grep -w -P "^chr6|^chr17" SeqCap_EZ_Exome_v3_hg38_primary_targets.v2.sort.merge.bed > exome_regions.bed

grep -w -P "^chr6|^chr17" SeqCap_EZ_Exome_v3_hg38_capture_targets.v2.sort.merge.bed > probe_regions.bed

# clean up intermediate files
#rm -fr SeqCapEZ_Exome_v3.0_Design_Annotation_files/ SeqCap_EZ_Exome_v3_hg38_primary_targets.bed SeqCap_EZ_Exome_v3_hg38_primary_targets.v2.bed SeqCap_EZ_Exome_v3_hg38_primary_targets.v2.sort.bed unMapped.bed

Obtaining an interval list for the exome bed files

NOTE: The following interval list is being created for an annotation file the corresponds to the whole genome. The ref_genome.dict we downloaded in the previous section only contains chr6 and chr17. We need to either trim the exome ROI file down to just these chrs as well. Or also download/create a full version of the dict file.

# first for the complete exome and probe bed file
cd /workspace/inputs/references/
mkdir temp
cd temp
cd /workspace/inputs/references/exome
java -jar $PICARD BedToIntervalList I=SeqCap_EZ_Exome_v3_hg38_primary_targets.v2.sort.merge.bed O=SeqCap_EZ_Exome_v3_hg38_primary_targets.v2.sort.merge.interval_list SD=/workspace/inputs/references/temp/ref_genome.dict
java -jar $PICARD BedToIntervalList I=SeqCap_EZ_Exome_v3_hg38_capture_targets.v2.sort.merge.bed O=SeqCap_EZ_Exome_v3_hg38_capture_targets.v2.sort.merge.interval_list SD=/workspace/inputs/references/temp/ref_genome.dict
rm -fr /workspace/inputs/references/temp/

# next for our subset exome and probe regions file
cd /workspace/inputs/references/exome
java -jar /usr/local/bin/picard.jar BedToIntervalList I=exome_regions.bed O=exome_regions.bed.interval_list SD=/workspace/inputs/references/genome/ref_genome.dict
java -jar /usr/local/bin/picard.jar BedToIntervalList I=probe_regions.bed O=probe_regions.bed.interval_list SD=/workspace/inputs/references/genome/ref_genome.dict

Obtaining transcriptome reference files

In this section we will download some reference transcriptome annotations in the mostly widely used formats (.fa and .gtf). As with the other annotation files, we have created a subsetted version of them to make downstream analysis more practical for a live tutorial demonstration. To create these annotation files we followed these basic steps:

Complete details of how these files were created can be found in the Developer Notes.

Now lets actually download these files and examine them…

# make sure CHRS environment variable is set.  If this command doesn't give a value, please return to the Environment section of the course
echo $CHRS

# create a directory for transcriptome input files
mkdir -p /workspace/inputs/references/transcriptome
cd /workspace/inputs/references/transcriptome

# download the files

# take a look at the contents of the gtf file. Press 'q' to exit the 'less' display.
less -p start_codon -S ref_transcriptome.gtf

# How many unique gene IDs are in the .gtf file?
# We can use a perl command-line command to find out:
perl -ne 'if ($_ =~ /(gene_id\s\"ENSG\w+\")/){print "$1\n"}' ref_transcriptome.gtf | sort | uniq | wc -l

# what are all the feature types listed in the third column of the GTF?
# how does the following command (3 commands piped together) answer that question?
cut -f 3 ref_transcriptome.gtf | sort | uniq -c

Review key definitions for this section