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Vargas

Updated: March 9th, 2020

Vargas computes optimal alignments of short reads to a directed acyclic graph (DAG). After building a graph, reads are aligned using a SIMD-vectorized version of the Smith-Waterman dynamic programming algorithm, with 16 -- 64 reads per vector depending on the instruction set.

Preprint "Vargas: heuristic-free alignment for assessing linear and graph read aligners" is now posted on bioRxiv. Supplementary processing scripts available at vargas-experiments.

Building

When cloning, use the --recursive option to automatically retrieve dependencies.

git clone --recursive [email protected]:langmead-lab/vargas.git

Vargas relies on htslib to provide core file processing. Once cloned, the htslib is built with autoconf (version 2.63+).

cd htslib
autoconf && autoheader && ./configure && make -j4

Vargas is built with CMake.

With GCC compiler, SSE 4.1 (default), AVX2 (-DBUILD_AVX2_GCC=ON) or AVX512-BW (-DBUILD_AVX512BW_GCC=ON) can be targeted for SIMD support. Requires GCC version 6 or above

mkdir build && cd build
cmake -DCMAKE_BUILD_TYPE=Release -DBUILD_AVX512BW_GCC=ON -DCMAKE_CXX_COMPILER=g++ -DCMAKE_C_COMPILER=gcc .. && make -j4

With Intel compiler, AVX2 (-DBUILD_AVX2_INTEL=ON) or AVX512-BW (-DBUILD_AVX512BW_INTEL=ON) can be targeted for SIMD support.

mkdir build && cd build
cmake -DCMAKE_BUILD_TYPE=Release -DBUILD_AVX512BW_INTEL=ON -DCMAKE_CXX_COMPILER=icpc -DCMAKE_C_COMPILER=icc .. && make -j4

Modes of Operation

vargas -h

Vargas version [date]
by Ravi Gaddipati, Charlotte Darby, Daniel Baker, Ben Langmead ([email protected], www.langmead-lab.org)

        define          Define a set of graphs for use with sim and align.
        sim             Simulate reads from a set of graphs.
        align           Align reads to a set of graphs.
        convert         Convert a SAM file to a CSV file.
        query           Convert a graph to DOT format.
        test            Run unit tests.

define

NOTE! If your VCF file contains any overlapping variants, it must be pre-processed using the Python script vargas_preprocess_VCF.py, otherwise Vargas will not build the correct graph. This script identifies clusters of overlapping variants and merges these records into one new record with several ALT alleles enumerating all possible combinations of variants in the cluster. These new records remove any INFO fields that may have been present in the original file. See vargas_preprocess_VCF.py for usage information and requirements for your input VCF.

vargas define -h

Define subgraphs deriving from a reference and VCF file.
Usage:
  vargas define [OPTION...]

  -h, --help  Display this message.

 Input options:
  -f, --fasta arg  <str> *Reference FASTA filename.

 Optional options:
  -v, --vcf arg       <str> Variant file (vcf, vcf.gz, or bcf).
  -t, --out arg       <str> Output filename. (default: stdout)
  -g, --region arg    <CHR[:MIN-MAX];...> list of regions. (default: all)
  -s, --subgraph arg  <str> Subgraph definitions, see below.
  -p, --filter arg    <str> Filter by sample names in file.
  -n, --limvar arg    <N> Limit to the first N variant records
  -c, --notcontig     VCF records for a given contig are not contiguous.


Subgraphs are defined using the format "label=N[%]",
where 'N' is the number of samples or percentage of samples to select.
The samples are selected from the parent graph, scoped with ':'.
The BASE graph is implied as the root for all labels. Example:
        a=50;a:b=10%;a:c=5

See Define documentation.

align

vargas align -h

Align reads to a graph.
Usage:
  vargas align [OPTION...]

  -h, --help  Display this message.

 Input options:
  -g, --gdef arg   <str> *Graph definition file.
  -U, --reads arg  <str> *Unpaired reads in SAM, FASTQ, or FASTA format.

 Optional options:
  -S, --sam arg            <str> Output file.
      --msonly             Only report max score.
      --maxonly            Only report max score, position, and count.
      --phred64            Qualities are Phred+64, not Phred+33.
  -p, --subsample arg      <N> Sample N random reads, 0 for all. (default: 0)
  -a, --alignto arg        <str> Target graph, or SAM Read Group -> graph
                           mapping."(RG:ID:<group>,<target_graph>;)+|<graph>"
  -s, --assess [=arg(=.)]  [ID] Use score profile from a previous alignment.
  -c, --tolerance arg      <N> Correct if within readlen/N. (default: 4)
  -f, --forward            Only align to forward strand.

 Scoring options:
      --ete      End to end alignment.
      --ma arg   <N> Match bonus. (default: 2)
      --mp arg   <MX,MN> Mismatch penalty. Lower qual=lower penalty.
                 (default: 6,2)
      --np arg   <N> Penalty for non-A/C/G/T. (default: 1)
      --rdg arg  <N1,N2> Read gap open/extension penalty. (default: 1,3)
      --rfg arg  <N1,N2> Ref gap open/extension penalty. (default: 1,3)

 Threading options:
  -j, --threads arg  <N> Number of threads. (default: 1)
  -u, --chunk arg    <N> Partition into tasks of max size N. (default: 64)

Reads are aligned to graphs specified in the GDEF file. --ete will preform end to end alignment and is generally faster than full local alignment. The memory usage increase is marginal for high numbers of threads. As a result, as many threads as available should be used (271 on Xeon Phi KNL).

For example:

vargas align  -g test.gdef -r reads.fa -t reads.sam --ete

See the Alignment documentation for more information.

convert

vargas convert -h

Export a SAM file as a CSV file.
Usage:
  vargas convert [OPTION...] positional parameters

  -f, --format arg  <str> Output format.
  -h, --help        Display this message.


Required column names:
        QNAME, FLAG, RNAME, POS, MAPQ, CIGAR, RNEXT, PNEXT, TLEN, SEQ, QUAL
Prefix with "RG:" to obtain a value from the associated read group.

Convert a SAM file into a CSV file, outputting the specified fields. Any of the SAM required fields or any aux tags can be output. For example,

Ex. vargas convert -f "RG:ID,mp,ms" a.sam b.sam

will report the corresponding read group ID, max score position, and max score for each alignment. If multiple SAM files are provided, field 1 will be the file name. See vargas align for tag information.

sim

vargas sim -h

Simulate reads from genome graphs.
Usage:
  vargas sim [OPTION...]

 Required options:
  -g, --graph arg  <str> *Graph definition file.

 Optional options:
  -t, --out arg       <str> Output file. (default: stdout)
  -s, --sub arg       <S1,S2..> Subgraphs to simulate from. (default: base)
  -f, --file          -s specifies a filename.
  -l, --rlen arg      <N> Read length. (default: 50)
  -n, --numreads arg  <N> Number of reads to generate. (default: 1000)
  -j, --threads arg   <N> Number of threads. (default: 1)

 Stratum options:
  -d, --vnodes arg  <N1,N2...> Number of variant nodes. '*' for any. (default: *)
  -b, --vbases arg  <N1,N2...> Number of variant bases. '*' for any. (default: *)
  -m, --mut arg     <N1,N2...> Number of mutations. '*' for any. (default: 0)
  -i, --indel arg   <N1,N2...> Number of insertions/deletions. '*' for any. (default: 0)
  -a, --rate        Interpret -m, -i as error rates.

  -h, --help  Display this message.

sim generates -n reads of each combination of -m, -i, -v, and -b. -m Introduces mutation errors, substituting N bases with an alternate base. Likewise, -i will delete a base or insert a random base. With -a, -m and -i are interpreted as rates (0.0 to 1.0). -s controls which subgraphs are used to generate reads.

For example:

vargas sim -g test.gdef -t reads -n 1000 -m 0,1,2 -v 0,1,2 -j 12

will generate 1000 reads for each combination of -m, -v, for each graph in test.gdef.

Provided SAM tags:

  • ro Unmutated read
  • nd VCF sample simulated from
  • se Number of substitution errors
  • vd Number of variant nodes traversed
  • vb Number of variant bases traversed
  • ni Number of indel errors
  • gd Read Group tag. Graph simulated from.
  • rt Read Group tag. Rates were used.
  • ph Read Group tag. GDEF file.

Reads are randomly sampled by weighting graph nodes by their length. As a result this isn't feasible for large graphs or specific conditions that are rare.

query

vargas query -h

Query a graph and export a DOT graph.
Usage:
  vargas query [OPTION...]

  -g, --graph arg           *<str> Graph file to query.
  -d, --dot arg             <str> Subgraph to export as a DOT graph.
  -t, --out arg             <str> DOT output file. (default: stdout)
  -a, --stat [=arg(=base)]  <str> Print statistics about a subgraph.
  -h, --help                Display this message.

Export a subgraph to a DOT graph, or get graph statistics.

Other

vargas test executes unit tests using the doctest framework (included as a dependency of this repository). The unit tests are included at the end of the relevant .cpp source files. These tests verify the core vectorized graph dynamic programming algorithm with 16-bit and 8-bit lanes, graph building and processing, file input/output, and simulation.

vargas profile generates a summary of performance.

Run profiles. 
Usage:
  vargas profile [OPTION...]

  -f, --fasta arg    <str> *Reference FASTA.
  -v, --vcf arg      <str> *Variant file (vcf, vcf.gz, or bcf)
  -g, --region arg   <str> *Region of format "CHR:MIN-MAX". "CHR:0-0" for all.
  -i, --ingroup arg  <N> Ingroup percentage. (default: 100)
  -n, --nreads arg   <N> Number of reads. (default: 32)
  -l, --len arg      <N> Number of reads. (default: 50)
  -h, --help         Display this message.

License

The MIT License (MIT)

Copyright 2019 Ravi Gaddipati, Charlotte Darby, Daniel Baker, Ben Langmead.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.