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ATAC-seq problem with STAR - Multimapping reads

Hi,

I have a doubt with STAR in ATAC-seq analysis. When I have performed the alignment, i have obtained I big amount of multi-mapping reads. Now I need this data for ATAC-seq, and I do not know if I have to remove all the multimapping reads, an keep only the Uniquely mapped reads, or allow STAR to put the default option of MAX 10 locus that a read can align to be considered good mapping and stay in the SAM file.

                            UNIQUE READS:
           Uniquely mapped reads number |   15689415
                Uniquely mapped reads % |   51.99%
                  Average mapped length |   98.45
               Number of splices: Total |   9626
    Number of splices: Annotated (sjdb) |   0
               Number of splices: GT/AG |   4180
               Number of splices: GC/AG |   159
               Number of splices: AT/AC |   34
       Number of splices: Non-canonical |   5253
              Mismatch rate per base, % |   0.22%
                 Deletion rate per base |   0.01%
                Deletion average length |   1.57
                Insertion rate per base |   0.01%
               Insertion average length |   1.38
                     MULTI-MAPPING READS:
Number of reads mapped to multiple loci |   12343702
     % of reads mapped to multiple loci |   40.90%
Number of reads mapped to too many loci |   82088
     % of reads mapped to too many loci |   0.27%
atac-seq chip-seq star

You don't have to realign your data, just take what you have and filter by MAPQ. And please add comments via ADD REPLY rather than the answer field to keep things structured.

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1 answer

There is to my knowledge no established and accepted way to deal with multimappers beyond simply discarding them. This is typically done by applying a MAPQ threshold since multimappers usually get low MAPQs. I personally filter for MAPQ > 20 (bowtie2), check the STAR manual on MAPQs since there is no standard on how aligners calculate them. I guess 20 would work for STAR as well, e.g. samtools view -o filtered.bam -q 20 in.bam

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