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Poverlap

Simple, flexible, parallized significance testing of a pair of BED files.

BEDTools offers the basic machinery to perform repeated randomization and overlap testing.

This script automatically parallelizes that process. It has these additional features:

1. allows shuffling each interval to a location within N bases of its current location. This is a very basic start toward something like Bickels work for ENCODE that can preserve local structure

   a) allows shuffing intervals in a to a new location inside the interval that contains them in b.

2. given the a and b BED files specified in order, the default is to shuffle only the b BED file. However, both files may be shuffled with shuffle_both argument to poverlap.

3. This implements the sampling schema defined in Haiminen et al in BMC Bioinformatics 9: 336, whereby we give a list of possible locations, like the locations of a set of transcriptions factors, and we wish to see how two of them, CTCF and Pol2 for example, are related. To do this, we fix the locations of CTCF (from within the BED file) and we randomize the location of Pol2 to any location occupied by a TF that is not CTCF.

This is implemented as reservoir sampling. The command-line would look like:

./poverlap.py fixle wgEncodeRegTfbsClusteredV2.bed.gz CTCF Pol2 --n 10

For wgEncodeRegTfbsClusteredV2.bed.gz, which contains putative TFBS for 50+ transcription factors. The output looks like this;

> observed number of overlaps: 31572
> shuffle command: bedtools intersect -u -a atype.bed -b <(./poverlap.py bed-sample otypes.bed --n 76623) | wc -l
> simulated overlap mean: 29895.4
> simulated p-value: 0
[30147, 29782, 29835, 30023, 29943, 29844, 29567, 30286, 29868, 29659]

where, in this case, the shuffle command is repeated 10 times. We see that the observed overlap (31752) is higher than any of the simulated. So, the simulated p-value is 0.

Examples

shuffle b to within X bases of original location

./poverlap.py poverlap --a a.bed --b b.bed \
        --shuffle_loc 10000 \
        > res.shuffle_loc-10kb.txt

shuffle both a and b to within X bases of original location

./poverlap.py poverlap --a a.bed --b b.bed \
        --shuffle_both --shuffle_loc 10000 \
        > res.both.shuffle_loc-10kb.txt

we can also shuffle intervals in b to a random location within the interval that contains it in 'other.bed'. For example, b.bed may be transcription start sites and we wish to see what things look like if we randomize the location of the TSS to anywhere within the gene-body.

./poverlap.py poverlap --a a.bed --b b.bed \
        --shuffle_loc gene-bodies.bed \
        > res.gene-bodies.txt

It is possible to use Haiminen's method to shuffle to known sites. Here, a and b are a pair of TFs out of the 50+ in the wgEncodeRegTfbsClusteredV2.bed.gz

./poverlap.py fixle \
    wgEncodeRegTfbsClusteredV2.bed.gz \
    a.bed b.bed \
    > res.fixle.haiminen.txt

simple bedtools shuffle

./poverlap.py poverlap --a a.bed --b b.bed \
    -g hg19.genome \
    > res.shuffle-bt.txt

bedtools exclude:

./poverlap.py poverlap --a a.bed --b b.bed \
    -g hg19.genome --exclude repressed.encode.bed \
    > res.shuffle_bt.exclude-repressed.txt

bedtools include:

./poverlap.py poverlap --a a.bed --b b.bed \
    -g hg19.genome --include hg19.promoters.bed \
    > res.shuffle_bt.include-promoters.txt

bedtools include and exclude:

./poverlap.py poverlap --a a.bed --b b.bed \
    -g hg19.genome --include hg19.promoters.bed \
    --exclude repressed.encode.bed \
    > res.shuffle_bt.include-promoters.exclude-repressed.txt

Call intervals within 40kb of another as "overlapping". Note, this is different from shuffle_distance which determines where the interval can go.

./poverlap.py poverlap --a a.bed --b b.bed \
    --overlap_distance 40000 -g hg19.genome \
    > res.shuffle-bt-within-40kb.txt

Installation

The script is stand-alone, dependencies can be installed with

    pip install toolshed commandr

Usage

poverlap is the main function that parallelizes testing overlap between a and b. It performs n shufflings and compares the observed number of lines in the intersection to the simulated intersections to generate a p-value.

The best way to understand is to use the script. E.g start with:

./poverlap.py poverlap

When using shuffle_loc, exclude, include and chrom are ignored. Args that are not explicitly part of BEDTools are explained below, e.g. to find intervals that are within a given distance, rather than fully overlapping, one can set overlap_distance to > 0. To shuffle intervals within a certain distance of their current location, or to keep them inside a set of intervals, use shuffle_loc to retain the local structure.

Arguments:

a - first bed file
b - second bed file
genome - genome file
metric - a string that indicates a program that consumes BED intervals
         from STDIN and outputs a single, numerical value upon
         completion. Default is 'wc -l'
n - number of shuffles
chrom - shuffle within chromosomes
exclude - optional bed file of regions to exclude
include - optional bed file of regions to include
shuffle_both - if set, both A and B are shuffled. Default is B only.
overlap_distance - intervals within this distance are overlapping
shuffle_loc - shuffle each interval to a random location within this
              distance of its current location. If not an integer,
              then this should be a BED file containing regions such
              that each interval in `bed` is shuffled within its
              containing interval in `shuffle_loc`.
ncpus - number cpus to use -- if a callable does the parallelization
        use, e.g. Pool(5).map or Ipython Client[:].map