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LiftOver

5,845 bytes added, 18:33, 15 July 2015
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LiftOver is a necesary step to bring all genetical analysis to the same reference build.
Particularly, our current data are mainly in either NCBI build 36 (UCSC hg 18) or NCBI build 37 (UCSC hg19).Although lift over LiftOver can be from higher build to lower build, we always recommend lift lower build to higher/current build.have three use cases:
LiftOver is not hard. The easier way is (1) [[#Lift genome positions | Convert genome position from one genome assembly to use UCSC liftOver tool to lift [http://another genome.ucsc.edu/FAQ/FAQformat.html#format1 BED formatassembly]] file to BED format file. With additional steps, we can also lift Merlin and PLINK data files.
Besides introducing In most scenarios, we have known genome positions in NCBI build 36 (UCSC hg 18) and hope to lift them over genomic positions, lifting SNPs is also introducedto NCBI build 37 (UCSC hg19).
== (2) [[#Lift dbSNP rs numbers | Convert dbSNP rs number from one build to another]] (3) [[#Lift Merlin/PLINK format | Convert both genome position and dbSNP rs number over using different versions]] It is likely to see such type of data in Merlin/PLINK format. We will explain the work flow for the above three cases. In the rest of this article, our example is to lift over from lower/older build to newer/higher build, as it is the common practice. Using different tools, liftOver can be easy.For example, UCSC liftOver tool is able to lift [http://genome.ucsc.edu/FAQ/FAQformat.html#format1 BED format] file between builds.With our customized scripts, we can also lift rsNumber and Merlin/PLINK data files .  ==Lift genome positions ==Genome positions are best represented in [http://genome.ucsc.edu/FAQ/FAQformat.html#format1 BED format]. UCSC provides tools to convert BED file from one genome assembly to another.
=== Binary liftOver tool ===
Download the We need [http://hgdownload.cse.ucsc.edu/admin/exe/linux.x86_64/liftOver liftOver binary] from UCSC and [http://hgdownload.cse.ucsc.edu/goldenPath/hg18/liftOver/hg18ToHg19.over.chain.gz hg18 to hg 19 chain file].
Provide BED format file (e.g. input.bed)
NOTE: Use the 'chr' before each chromosome name
liftOver input.bed hg18ToHg19.over.chain.gz output.bed unlifted.bed
unlifted .bed file will contain all genomic genome positions that cannot be lifted. The reason for that varies. See [[#Various reasons that lift over could fail | Various reasons that lift over could fail]]
=== Web interface ===
Alternatively, you can lift over BED file in web interface
at: [http://genome.ucsc.edu/cgi-bin/hgLiftOver Link]
Web interface can tell you why some genomic genome position cannot
be lifted if you click "Explain failure messages"
== Lift dbSNP rs numbers ==rs number is release by dbSNP. UCSC also make their own copy from each dbSNP version. Be aware that the same version of dbSNP from these two centers are not the same.When we convert rs number from lower version to higher version, there are practically two ways. === Use RsMergeArch and SNPHistory ===It is necessary to quickly summarize how dbSNP merge/re-activate rs number: # when different rs number are found to refer to the same SNP, then higher rs number will be merged to lower rs number, and the merging will be recorded in RsMergeArch.bcp.gz.# when rs number have to be retracted, rs number will be recorded in SNPHistory.bcp.gz# a retracted SNP can be [http://www.ncbi.nlm.nih.gov/books/NBK44496/#Schema.rs4823903_which_has_merged_into re-activated] in SNPHistory.bcp.gz by adding comment With the above in mind, we are able to combine these two tables to obtain the relationship between older rs number and new rs number.We have developed a script (for internal use), named [http://genome.sph.umich.edu/wiki/LiftRsNumber.py liftRsNumber.py] for lift rs numbers between builds.This scripts require RsMergeArch.bcp.gz and SNPHistory.bcp.gz, those can be found in [[#Resources | Resources]]. Example input:<pre>300030013002</pre> Command:<pre>python liftRsNumber.py input.rs > output.rs</pre> Example output:<pre>unchanged 3000lifted 2032unchanged 3002</pre> == Lift Merlin /PLINK format ==In Merlin/PLINK .map files, each line contains both genome position and dbSNP rs number. Our goal here is to use both information to liftOver as many position as possible.There are 3 methods to liftOver and we recommend the first 2 method. The first method is common and applicable in most cases, and in our observations it lifts the most genome positions, however, it does not reflect the rs number change between different dbSNP builds. The second method is more robust in the sense that each lifted rs number has valid genome position, as it lift over old rs number as the first step by using dbSNP data. The third method is not straigtforward, and we just briefly mention it. === Lift Merlin format ===
PLINK format and [http://www.sph.umich.edu/csg/abecasis/Merlin/tour/input_files.html Merlin format are nearly identical].
to obtain Merlin .map file.
=== Lift PLINK format ===
[http://pngu.mgh.harvard.edu/~purcell/plink/data.shtml PLINK] format usually referrs to .ped and .map files.
 ==== Method 1 ====We recommend split mainly use UCSC LiftOver binary tools to help lift over. We have a script [[#Resources | liftMap.py]], however, it is recommended to understand the jobs in several stepsjob step by step: (1) convert Convert .map to .bed file 
By rearrange columns of .map file, we obtain a standard BED format file.
(2) liftOver LiftOver .bed file Use method mentioned [[#Lift genome positions | above ]] to convert .bed file from one build to another. (3) Convert lifted .bed file back to .map file
(3) convert lifted .bed file back to .map file
Rearrange column of .map file to obtain .bed file in the new build.
(4) modify Modify .ped file 
.ped file have many column files. By convention, the first six columns are family_id, person_id, father_id, mother_id, sex, and phenotype.
From the 7th column, there are two letters/digits representing a genotype at the certain marker. In step (2), as some genomic genome positions cannot
be lifted to the new version, we need to drop their corresponding columns from .ped file to keep consistency. You can use PLINK --exclude those snps,
see [http://pngu.mgh.harvard.edu/~purcell/plink/dataman.shtml#exclude Remove a subset of SNPs].
(5) (optionally) change the rs number in the .map file
 
Similar to the human reference build, dbSNP also have different versions. You may consider change rs number from the old dbSNP version to new dbSNP version
depending on your needs . Such steps are described in [[#Lift dbSNP rs numbers | Lift dbSNP rs numbers]].
(6) (optionally) additional method ==== Method 2 ====The idea is to use [[#Resources |LiftRsNumber.py]] to convert old rs number to lift dbSNP postionNCBI dbSNP team has provided a provisional map for converting new rs number, use the genomic position of data file b132_SNPChrPosOnRef_37_1.bcp.gz (a larget set data file containing each dbSNP from NCBI build 36 to and its positions in NCBI build 37. In the second step), we have obtained unlifted genomic positions, so we can try to use the table to convert those unlfted dbSNPsand adjust .After this step, there are still some SNPs that cannot be lifted, map and they are mostly located on non-reference chromosome.ped files accordingly.
== Lift (1) Extract and lift rs numbers Use the tools [[#Use RsMergeArch and SNPHistory | LiftRsNumber.py]] to lift the rs number in the map file from old build to new build. (2) Lookup SNP positions from rs number dbSNP provides a file [[#Resources | b132_SNPChrPosOnRef_37_1.bcp.gz]] which contains rsNumber, chromosome and its position.Use this file along with the new rsNumber obtained in the first step.In practice, some rs numbers ==do not exist in build 132, or not suitable to be considered ( e.g. they do not reside on human reference, or they are mapped to multiple locations, these scenarios are noted by the chromosome column with values like "AltOnly", "Multi", "NotOn", "PAR", "Un"), we can drop them in the liftover procedure.We will obtain the rs number is release and its position in the new build after this step. (3) Lift .map file and .ped file To lift over .map files, we can scan its content line by dbSNPline, and skip those not lifted rs number. UCSC also make their own copy Accordingly, it is necessary to drop the un-lifted SNP genotypes from each .ped file. ==== Method 3 ====NCBI dbSNP version. Be aware that team has provided a [[ #Resources | provisional map ]] for converting the same version genome position of a larget set dbSNP from these two centers NCBI build 36 to NCBI build 37. In the second step, we have obtained unlifted genome positions, so we can try to use the table to convert those unlfted dbSNPs.After this step, there are still some SNPs that cannot be lifted, as they are not mostly located on non-reference chromosome.Note: due to the limitation of the sameprovisional map, some SNP can have multiple locations.When For example, we cannot convert rs10000199 to chromosome 4, 7, 12.<pre>10000199 A/G 4 166142415 166142415 2 3 G + 4 165922965 165922965 2 3 G +10000199 A/G 7 4589694 4589694 2 3 C - 7 4623168 4623168 2 3 C -10000199 A/G 12 57008620 57008620 2 3 C - 12 58722353 58722353 2 3 C -10000199 A/G 5 156018406 156018406 2 3 C - 5 156085828 156085828 2 3 C -</pre> We can dissect this method into steps: (1) Remove invalid record in dbSNP provisional map. Provisional map have duplicated rs number from lower version or the chromsome in the new build can be "Unable to higher versionmap"(UN), there are practically two wayswe need to clean this table. (2) Use provisional map to update .map file
=== Use RsMergeArch By joining .map file and SNPHistory ===this provisional map, we can obtain the new genome position in the new build.Note: provisional map uses 1-based chromosomal index. Things will get tricker if we want to lift non-single site SNP e.g. AA/GGSince provisional map provides a range in this case, it is necessary to know the genome position of that single base provided in the .map file, In shortand then we can look up the table, so it is not straigtforward.
# when different rs number are found to refer to the same SNP, then higher rs number will be merged to lower rs number, (3) Adjust .map and the merging will be recorded in RsMergedArch.bcp.gz.# when rs number have to be retracted, rs number will be recorded in SNPHistory.bcp.gzped file
So For those lifted dbSNP, we need to combine these two tables keep them in the .map files, otherwise, we need to obtain the relationship between older rs number and new rs numberdelete them.LuckilyAccordingly, we have a script need to deleted SNP genotypes for internal use. See liftRsNumberthose cannot be lifted.py
== Various reasons that lift over could can fail ==
=== Genomic Genome position cannot be lifted ===When a SNP resides in a contig that only exists in older reference build, liftOver cannot give it new genomicgenome.
You can try the following SNP (in BED format) in UCSC online liftOver site:
=== SNP in higher build are located in non-referernce assembly ===
Some SNP are not in autosomes or sex chromosomes in NCBI build 37. dbSNP does not include them.
You cannot use dbSNP database to lookup its genomic genome position by rs number.
Take rs1006094 as an example:
Thus it is probably not very useful to lift this SNP.
=== Cannot find rs number changed in newer dbSNP build ===
It is possible that new dbSNP build does not have certain rs numbers.
When dbSNp release new build, higher rs number may be merged to lower rs number because of those rs numbers are actually the same SNP.
This merge process can be complicate. For short description, see [[#Use RsMergeArch and SNPHistory | Use RsMergeArch and SNPHistory]].
For detail, see:
* SNPs that are not mapped on the reference genome (GRCh37)
For UCSC release, see [[#Resources | UCSC dbSNP track note]]
Use rs1054140 as an example:
</pre>
== Resouces Resources ==* liftRsNumber.py [[liftRsNumber.py]] and its interal location: /net/fantasia/home/zhanxw/amd/analyze/verifyBamID/liftRsNumber.py* liftMap.py [[liftMap.py]]
* NCBI provisional map [ftp://ftp.ncbi.nih.gov:/snp/organisms/human_9606/misc/exchange/Remap_36_3_37_1.txt.gz file] and [ftp://ftp.ncbi.nih.gov:/snp/organisms/human_9606/misc/exchange/Remap_36_3_37_1.info info]
* NCBI RgMergeArch [ftp://ftp.ncbi.nih.gov:/snp/organisms/human_9606/database/organism_data/RsMergeArch.bcp.gz file ] and [http://www.ncbi.nlm.nih.gov/SNP/snp_db_table_description.cgi?t=RsMergeArch schema]* NCBI SNPHistory [ftp://ftp.ncbi.nih.gov:/snp/organisms/human_9606/database/organism_data/SNPHistory.bcp.gz file ] and [http://www.ncbi.nlm.nih.gov/SNP/snp_db_table_description.cgi?t=SNPHistory schema]* NCBI SNPChrPosOnRef build 132 [http://qbrc.swmed.edu/zhanxw/software/liftOver/b132_SNPChrPosOnRef_37_1.bcp.gz file] and [http://www.ncbi.nlm.nih.gov/SNP/snp_db_table_description.cgi?t=SNPChrPosOnRef schema]. If this link becomes unavailable, please consider using this updated file ([ftp://ftp.ncbi.nih.gov/snp/organisms/human_9606/database/organism_data/b144_SNPChrPosOnRef_107.bcp.gz link]). 
* How UCSC dbSNP differs from NCBI dbSNP [http://genomewiki.ucsc.edu/index.php/DbSNP_Track_Notes UCSC dbSNP track note]
* The dbSNP mapping process [http://www.ncbi.nlm.nih.gov/books/NBK44455/ link]
* NCBI dbSNP release 132 [ftp://ftp.ncbi.nih.gov:/snp/organisms/human_9606/VCF/v4.0/ByChromosomeNoGeno/00-All.vcf.gz 00-All.vcf.gz]
* UCSC dbSNP release 132 [http://hgdownload.cse.ucsc.edu/goldenPath/hg19/database/snp132.txt.gz snp132.txt.gz]
 
== Third party contribution ==
 
* liftOver for BEDPE format by [mailto://doug.phanstiel@gmail.com Doug] - [http://www.sharedproteomics.com/forum/showthread.php?2930-liftOverBedpe.py-Convert-genome-coordinates-for-bedpe-files&p=4854#post4854 liftOverBedpe.py].
== Acknowledge ==
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