Biowulf High Performance Computing at the NIH
Impute, qctool, gtool and snptest on Biowulf

IMPUTE version 2 (also known as IMPUTE2) is a genotype imputation and haplotype phasing program based on ideas from Howie et al. 2009:

QCTOOL is a command-line utility program for basic quality control of GWAS datasets. It supports the same file formats used by the WTCCC studies, as well as the binary file format described on the qctool webpage and the Variant Call Format, and is designed to work seamlessly with SNPTEST and related tools

GTOOL is a program for transforming sets of genotype data for use with the programs SNPTEST and IMPUTE. GTOOL can be used to:

SNPTEST is a program for the analysis of single SNP association in genome-wide studies. The tests implemented include

Impute, qctool, gtool and snptest were all developed at the University of Oxford.

Important Notes

Interactive job
Interactive jobs should be used for debugging, graphics, or applications that cannot be run as batch jobs.

Allocate an interactive session and run the program. Sample session:

[user@biowulf]$ sinteractive
salloc.exe: Pending job allocation 46116226
salloc.exe: job 46116226 queued and waiting for resources
salloc.exe: job 46116226 has been allocated resources
salloc.exe: Granted job allocation 46116226
salloc.exe: Waiting for resource configuration
salloc.exe: Nodes cn3144 are ready for job

[user@cn3144 ~]$ module load impute

[user@cn3144 ~]$ impute2 -ref_samp_out -m ./ -h ./chr16.haps \
  -l ./chr16.legend -g ./chr16.reference.gtypes -s ./chr16.reference.strand \
  -Ne 11418 -int 5000000 5500000 -buffer 250 -k 10 -iter 10 -burnin 3 \
  -o ./Results/chr16.multi_panel.ref_gtypes.impute2 \
  -i ./Results/ \
  -r ./Results/chr16.multi_panel.ref_gtypes.impute2.summary

The seed for the random number generator is 1115038504.

Command-line input: impute2 -ref_samp_out -m ./ -h ./chr16.haps -l ./chr16.legend -g ./chr16.reference.gtypes -s ./chr16.reference.strand -Ne 11418 -int 5000000 5500000 -buffer 250 -k 10 -iter 10 -burnin 3 -o ./Results/chr16.multi_panel.ref_gtypes.impute2 -i ./Results/ -r ./Results/chr16.multi_panel.ref_gtypes.impute2.summary

 IMPUTE version 2.0.3 

Copyright 2008 Bryan Howie, Peter Donnelly, and Jonathan Marchini
Please see the LICENCE file included with this program for conditions of use.

    haplotypes file : ./chr16.haps
        legend file : ./chr16.legend
 ref genotypes file : NULL
ref gen strand file : NULL
     genotypes file : ./chr16.reference.gtypes
        strand file : ./chr16.reference.strand
           map file : ./
 excluded SNPs file : NULL
 included SNPs file : NULL
    ref samp infile : NULL
        output file : ./Results/chr16.multi_panel.ref_gtypes.impute2
          info file : ./Results/
       summary file : ./Results/chr16.multi_panel.ref_gtypes.impute2.summary
  Accuracy assessment for imputation of type 0 SNPs (those with data in the haploid reference panel only) .The maximum imputed genotype calls are distributed as follows:
  Interval  #Genotypes %Concordance         Interval  %Called %Concordance
  [0.0-0.1]          0          0.0         [ >= 0.0]   100.0         96.7
  [0.1-0.2]          0          0.0         [ >= 0.1]   100.0         96.7
  [0.2-0.3]          0          0.0         [ >= 0.2]   100.0         96.7
  [0.3-0.4]          0          0.0         [ >= 0.3]   100.0         96.7
  [0.4-0.5]         10         40.0         [ >= 0.4]   100.0         96.7

[user@cn3144 ~]$  ls Results

[user@cn3144 ~]$ exit
salloc.exe: Relinquishing job allocation 46116226
[user@biowulf ~]$

Batch job
Most jobs should be run as batch jobs.

Create a batch input file (e.g. For example:

set -e
module load impute

impute2 -ref_samp_out -m -h chr16.haps  -l chr16.legend \
	-g gtypes -s refstrand1  -Ne 11418 -int 5000000 5500000 -buffer 250 \
	-k 10 -iter 10 -burnin 3  -o out1  -i info1  -r summary1

Submit this job using the Slurm sbatch command.

sbatch  [--mem=#]
Swarm of Jobs
A swarm of jobs is an easy way to submit a set of independent commands requiring identical resources.

Create a swarmfile (e.g. impute.swarm). For example:

# this file is impute_swarm
cd /data/user/dir1; impute2 -ref_samp_out -m -h chr16.haps  \
      -l chr16.legend -g gtypes -s refstrand1  -Ne 11418 -int 5000000 5500000 \
      -buffer 250 -k 10 -iter 10 -burnin 3  -o out1  -i info1  -r summary1
cd /data/user/dir2; impute2 -ref_samp_out -m -h chr26.haps \
      -l chr26.legend -g gtypes -s refstrand2  -Ne 22428 -int 5000000 5500000 \
      -buffer 250 -k 20 -iter 20 -burnin 3  -o out2  -i info2  -r summary2
cd /data/user/dir3; impute2 -ref_samp_out -m -h chr36.haps  \
      -l chr36.legend -g gtypes -s refstrand3  -Ne 33438 -int 5000000 5500000 \
      -buffer 250 -k 30 -iter 30 -burnin 3  -o out3  -i info3  -r summary3

Submit this job using the swarm command.

swarm -f TEMPLATE.swarm [-g #]  --module impute,qctool,gtool
-g # Number of Gigabytes of memory required for each process (1 line in the swarm command file)
--module TEMPLATE Loads the TEMPLATE module for each subjob in the swarm

Note about parallelization

In principle, it is possible to impute genotypes across an entire chromosome in a single run of IMPUTE2. However, we prefer to split each chromosome into smaller chunks for analysis, both because the program produces higher accuracy over short genomic regions and because imputing a chromosome in chunks is a good computational strategy: the chunks can be imputed in parallel on multiple computer processors, thereby decreasing the real computing time and limiting the amount of memory needed for each run.

We therefore recommend using the program on regions of ~5 Mb or shorter, and versions from v2.1.2 onward will throw an error if the analysis interval plus buffer region is longer than 7 Mb. People who have good reasons to impute a longer region in a single run can override this behavior with the -allow_large_regions flag.

See this informative snippet from the Impute website for more details about dealing with whole chromosomes.