From time to time, you might get a list of variants in map format, or as a list with only chromosome and position information. Reference and alternative alleles are not specified because the variants are from GWA studies with well-defined reference and alternative alleles. In this case, you can use format map, which automatically retrieve reference and alternative alleles from a specified dbSNP database.
vtools show format format/map.fmt
Format: map
Description: This input format imports variants from files in MAP format (with
columns chr, name gen_dist, pos), or any delimiter-separated format
with columns chr and pos. Because these input files do not contain
reference and alternative alleles of variants, this format queries
such information from the dbSNP database using chr and pos. Records
that does not exist in dbSNP will be discarded. Records with
multiple alternative alleles will lead to multiple records.
Columns:
None defined, cannot export to this format
variant:
chr Chromosome
pos 1-based position
ref Reference allele, '-' for insertion.
alt Alternative allele obtained from another database
Format parameters:
db_file (default: dbSNP.DB)
pos_idx Index of column for pyhysical location in the map file, should be 4
for a standard map file with chr, pos, gen_dist, pos.
(default: 4)
ref_field Name of ref field from the annotation database, used to retrieve
reference allele at specified location. (default:
refNCBI)
alt_field Name of alt field from the annotation database, used to retrieve
alternative allele at specified location. (default:
alt)
chr_field Name of chr field from the annotation database, used to locate
variants from the dbSNP database. (default: chr)
pos_field Name of pos field from the annotation database, used to locate
variants from the dbSNP database. (default: start)
separator Separator of the input file, default to space or tab. (default: None)
You are given a list of variants with chromosome as the first column, and physical position as the third column:
CHR SNP BP A1
12 rs11054701 12180423 C
12 rs2075241 12182746 C
12 rs2160521 12184905 T
12 rs10590349 12223570 G
12 rs10492120 12224619 C
12 rs3825258 12170662 C
12 rs11054697 12163740 C
12 rs16907786 12205869 A
12 rs11054665 12113442 G
dbSNPThe first step is to get the right version of dbSNP. Because the variants are in hg18, you cannot use the default version of dbSNP, which uses hg19. Instead, you should run
vtools show annotations
vtools use dbSNP-hg18_130
The first command lists all available annotation databases, and the second command will download dbSNP and put a database such as dbSNP-hg18_130.DB under your project directory.
The default column for the position column is 4, but the input file has it at the third column, so you will need to use parameter --pos_idx 3. You do not have to set other parameters (fields) if you are using dbSNP database provided by variant tools.
To import the data, you can run command
vtools import gwas_result.txt --format map --db_file dbSNP-hg18_130.DB \
--pos_idx 3 --build hg18
This format will import multiple variants if there are multiple variants (usually SNV and indels) at the same location at dbSNP. For example, you can get one SNV and two deletions at position 12223570 of chromosome 12 in the following example.
vtools output variant chr pos dbSNP.name ref alt
12 12180423 rs11054701 T C
12 12182746 rs2075241 G C
12 12184905 rs2160521 C T
12 12223570 rs7974059 G T
12 12223570 rs10590349 GATA -
12 12223570 rs56097732 GA -
12 12224619 rs10492120 C T
12 12170662 rs3825258 T C
12 12163740 rs11054697 T C
12 12205869 rs16907786 G A
12 12113442 rs11054665 T G
If you believe that all your variants are SNVs, you can run
vtools select variant 'ref != "-"' 'alt != "-"' -t SNV
to put all SNVs into a separate table, or run
vtools select variant 'ref = "-" OR alt = "-"' -t indels
vtools remove variants indels
to remove all indels.
This format will ignore all variants that are not in dbSNP. If you would like to keep them, you can modify map.fmt, and add parameter default='-' to DatabaseQuerier.