Whole-genome sequencing and targeted amplicon capture¶
CNVkit is primarily designed for use on hybrid capture sequencing data, where off-target reads are present and can be used improve copy number estimates. However, CNVkit can also be used on whole-genome sequencing (WGS) and targeted amplicon sequencing (TAS) datasets by using alternative command-line options.
The batch command supports these workflows through the
Whole-Genome Sequencing (WGS)¶
CNVkit treats WGS data as a capture of all of the genome’s sequencing-accessible regions, with no off-target regions.
batch --method wgs option uses the given reference genome’s
sequencing-accessible regions (“access” BED) as the “targets” – these will be
calculated on the fly if not provided. No “antitarget” regions are used.
Since the input does not contain useful per-target gene labels, a gene
annotation database is required and used to label genes in the outputs:
cnvkit.py batch -m wgs -g data/access-5kb-mappable.hg19.bed --annotate refFlat.txt *.bam
cnvkit.py target data/access-5kb-mappable.hg19.bed --split --short-names --annotate refFlat.txt -o targets.bed # For each sample cnvkit.py coverage Sample.bam targets.bed -p 0 -o Sample.targetcoverage.cnn # Create an empty antitarget coverage file, header only head -n1 Sample.targetcoverage.cnn -o Sample.antitargetcoverage.cnn cnvkit.py reference *.targetcoverage.cnn *.antitargetcoverage.cnn -o ref-wgs.cnn cnvkit.py fix Sample.targetcoverage.cnn Sample.antitargetcoverage.cnn ref-wgs.cnn --no-edge
To speed up and/or improve the accuracy of WGS analyses, try any or all of the following:
- Instead of analyzing the whole genome, use the “target” BED file to limit the analysis to just the genic regions. You can get such a BED file from the [UCSC Genome Browser](https://genome.ucsc.edu/cgi-bin/hgTables), for example.
- Increase the “target” average bin size, e.g. to at least 1000 bases for 30x coverage, or proportionally more for lower-coverage sequencing.
- Specify a smaller p-value threshold (
segment -t). For the CBS method,
1e-6may work well. Or, try the
- Use the
-p/--processesoption in the batch, coverage and segment commands to ensure all available CPUs are used.
- Ensure you are using the most recent version of CNVkit. Each release includes some performance improvements.
- Turn off the “edge” bias correction in the reference and fix commands (–no-edge).
batch -m wgs option does all of these except the first automatically.
Targeted Amplicon Sequencing (TAS)¶
When amplicon sequencing is used as a targeted capture method, no off-target reads are sequenced. While this limits the copy number information available in the sequencing data versus hybrid capture, CNVkit can analyze TAS data using only on-target coverages and excluding all off-target regions from the analysis.
batch -m amplicon option uses the given targets to infer coverage, and
leaves the antitarget coverage file empty:
cnvkit.py batch -m amplicon -t targets.bed *.bam
cnvkit.py target targets.bed --split -o targets.split.bed # For each sample cnvkit.py coverage Sample.bam targets.split.bed -p 0 -o Sample.targetcoverage.cnn # Create an empty antitarget coverage file, header only head -n1 Sample.targetcoverage.cnn -o Sample.antitargetcoverage.cnn cnvkit.py reference *.targetcoverage.cnn *.antitargetcoverage.cnn --no-edge -o ref-tas.cnn cnvkit.py fix Sample.targetcoverage.cnn Sample.antitargetcoverage.cnn ref-tas.cnn --no-edge
This approach does not collect any copy number information between targeted regions, so it should only be used if you have in fact prepared your samples with a targeted amplicon sequencing protocol. It also does not attempt to normalize each amplicon at the gene level, though this may be addressed in a future version of CNVkit.