README.md

Introduction

This is a simple script to generate fasta formatted sequences that include IUPAC symbols for the different SNPs in the region.

What happens under the hood is that the script will fetch all the SNPs in the given region using Ensembl's REST API. Then, different filters can be used to restrict the list to variants with an available MAF (available, in this case, means that it can be found using the Ensembl API) or to variants that have a MAF higher than a given threshold. This can be used to keep only fairly common variants.

Installation

This script has a hard dependency on gepyto. After installing this package, the script can be used like any other unix executable. It is compatible with both Python 2.7+ and Python 3.

To install gepyto, simply clone or download the package and use:

python setup.py install

Usage

The usage is fairly straightforward:

./variant_aware_fasta.py --help

# usage: variant_aware_fasta.py [-h] [--maf-over MAF_OVER] [--maf-exists] region
# 
# Generate a FASTA where the SNPs are represented as their IUPAC code.
# 
# positional arguments:
#   region               The region for sequence extraction (e.g.
#                        chr3:12345-12567)
# 
# optional arguments:
#   -h, --help           show this help message and exit
#   --maf-over MAF_OVER  Filter on the minor allele frequency.
#   --maf-exists         If this flag is used, only variants with available
#                        Ensembl MAFs will be considered.

You just call the script with a region using the chrXX:START-END format. Optional arguments are --maf-over to filter for common variants and --maf-exists to keep only variants that have a MAF defined by the Ensembl database.

The reverse complement of the sequence is also output.

Example

./variant_aware_fasta.py chr11:2549137-2549192
# > chr11:2549137-2549192 with IUPAC coded variants (maf > 0)
# TGCYRTGTCCCTGTYTTGCAGCTTCCTCMTCRTCCYGGTCTKCYTCATCTTYRGYR
# > reversed_compl_chr11:2549137-2549192 with IUPAC coded variants (maf > 0)
# YRCYRAAGATGARGMAGACCRGGAYGAKGAGGAAGCTGCAARACAGGGACAYRGCA

The previous call also generated a tab separated file named variants_in_11_2549137-2549192.txt containing the following information:

chrom	pos	rs	ref	alt	evidence	maf	most_severe_consequence
11	2549140	rs377350869	C	T	ESP		Intron variant
11	2549141	rs28730661	G	A	Multiple_observations,Frequency,1000Genomes,ESP	0.00826446	Intron variant
11	2549151	rs201682200	C	T	Multiple_observations,Frequency,ESP		Splice region variant
11	2549165	rs199472684	A	C	Cited		Missense variant
11	2549168	rs199473449	G	A	Cited		Missense variant
11	2549172	rs199472685	T	C	Cited		Missense variant
11	2549178	rs199472686	G	T	Cited		Missense variant
11	2549180	rs199473450	C	T	Cited		Missense variant
11	2549188	rs149143353	C	T	Frequency,ESP		Synonymous variant
11	2549189	rs120074192	A	G	Multiple_observations,Cited		Missense variant
11	2549191	rs377520734	C	T	ESP		Synonymous variant
11	2549192	rs199472687	G	A	Cited		Missense variant

Testing

Very basic testing is also available:

python -m unittest -q variant_aware_fasta