callpeak

Overview

This is the main function in MACS3. It will take alignment files in various format (please check the detail below) and call the significantly enriched regions in the genome as ‘peaks’. It can be invoked by macs3 callpeak . If you type this command with -h, you will see a full description of command-line options. Here we only list the essentials.

Essential Commandline Options

Input and Output

• -t/--treatment

  This is the only REQUIRED parameter for MACS3. The file can be in any supported format – see detail in the --format option. If you have more than one alignment file, you can specify them as -t A B C. MACS3 will pool up all these files together.

• -c/--control

  The control, genomic input or mock IP data file. Please follow the same direction as for -t/--treatment.

• -n/--name

  The name string of the experiment. MACS3 will use this string NAME to create output files like NAME_peaks.xls, NAME_negative_peaks.xls, NAME_peaks.bed , NAME_summits.bed, NAME_model.r and so on. So please avoid any confliction between these filenames and your existing files.

• -f/--format FORMAT

  Format of tag file can be ELAND, BED, ELANDMULTI, ELANDEXPORT, SAM, BAM, BOWTIE, BAMPE, or BEDPE. Default is AUTO which will allow MACS3 to decide the format automatically. AUTO is also useful when you combine different formats of files. Note that MACS3 can’t detect BAMPE or BEDPE format with AUTO, and you have to implicitly specify the format for BAMPE and BEDPE.

  Nowadays, the most common formats are BED or BAM (including BEDPE and BAMPE). Our recommendation is to convert your data to BED or BAM first.

  Also, MACS3 can detect and read gzipped file. For example, .bed.gz file can be directly used without being uncompressed with --format BED.

  Here are detailed explanation of the recommended formats:

  • BED

    The BED format can be found at UCSC genome browser website.

    The essential columns in BED format input are the 1st column chromosome name, the 2nd start position, the 3rd end position, and the 6th, strand.

    Note that, for BED format, the 6th column of strand information is required by MACS3. And please pay attention that the coordinates in BED format are zero-based and half-open. See more detail at UCSC site.

  • BAM/SAM

    If the format is BAM/SAM, please check the definition in samtools. If the BAM file is generated for paired-end data, MACS3 will only keep the left mate(5’ end) tag. However, when format BAMPE is specified, MACS3 will use the real fragments inferred from alignment results for reads pileup.

  • BEDPE or BAMPE

    A special mode will be triggered while the format is specified as BAMPE or BEDPE. In this way, MACS3 will process the BAM or BED files as paired-end data. Instead of building a bimodal distribution of plus and minus strand reads to predict fragment size, MACS3 will use actual insert sizes of pairs of reads to build fragment pileup.

    The BAMPE format is just a BAM format containing paired-end alignment information, such as those from BWA or BOWTIE.

    The BEDPE format is a simplified and more flexible BED format, which only contains the first three columns defining the chromosome name, left and right position of the fragment from Paired-end sequencing. Please note, this is NOT the same format used by bedtools, and the bedtools version of BEDPE is actually not in a standard BED format. You can use MACS3 subcommand randsample or filterdup to convert a BAMPE file containing paired-end information to a BEDPE format file:

    macs3 randsample -i the_BAMPE_file.bam -f BAMPE -p 100 -o the_BEDPE_file.bed
    

    or

    macs3 filterdup -i the_BAMPE_file.bam -f BAMPE --keep-dup all -o the_BEDPE_file.bed
    

• --outdir

  MACS3 will save all output files into the specified folder for this option. A new folder will be created if necessary.

• -B/--bdg

  If this flag is on, MACS3 will store the fragment pileup, control lambda in bedGraph files. The bedGraph files will be stored in the current directory named NAME_treat_pileup.bdg for treatment data, NAME_control_lambda.bdg for local lambda values from control.

Options controling peak calling behaviors

• -g/--gsize

  It’s the mappable genome size or effective genome size which is defined as the genome size which can be sequenced. Because of the repetitive features on the chromosomes, the actual mappable genome size will be smaller than the original size, about 90% or 70% of the genome size. The default hs ~2.9e9 is recommended for human genome. Here are all precompiled parameters for effective genome size from deeptools:

  • hs: 2,913,022,398 for GRCh38

  • mm: 2,652,783,500 for GRCm38

  • ce: 100,286,401 for WBcel235

  • dm: 142,573,017 for dm6

  Please check deeptools webpage to find the appropriate effective genome size if you want a more accurate estimation regarding specific assembly and read length.

  Users may want to use k-mer tools to simulate mapping of Xbps long reads to target genome, and to find the ideal effective genome size. However, usually by taking away the simple repeats and Ns from the total genome, one can get an approximate number of effective genome size. A slight difference in the number won’t cause a big difference of peak calls, because this number is used to estimate a genome-wide noise level which is usually the least significant one compared with the local biases modeled by MACS3.

• -s/--tsize

  The size of sequencing tags. If you don’t specify it, MACS3 will try to use the first 10 sequences from your input treatment file to determine the tag size. Specifying it will override the automatically determined tag size.

• -q/--qvalue

  The q-value (minimum FDR) cutoff to call significant regions. Default is 0.05. For broad marks, you can try 0.01 as the cutoff. The q-values are calculated from p-values using the Benjamini-Hochberg procedure.

• -p/--pvalue

  The p-value cutoff. If -p is specified, MACS3 will use p-value instead of q-value.

• --min-length, --max-gap

  These two options can be used to fine-tune the peak calling behavior by specifying the minimum length of a called peak and the maximum allowed a gap between two nearby regions to be merged. In other words, a called peak has to be longer than min-length, and if the distance between two nearby peaks is smaller than max-gap then they will be merged as one. If they are not set, MACS3 will set the DEFAULT value for min-length as the predicted fragment size d, and the DEFAULT value for max-gap as the detected read length. Note, if you set a min-length value smaller than the fragment size, it may have NO effect on the result. For broad peak calling with --broad option set, the DEFAULT max-gap for merging nearby stronger peaks will be the same as narrow peak calling, and 4 times of the max-gap will be used to merge nearby weaker (broad) peaks. You can also use --cutoff-analysis option with the default setting, and check the column avelpeak under different cutoff values to decide a reasonable min-length value.

• --nolambda

  With this flag on, MACS3 will use the background lambda as local lambda. This means MACS3 will not consider the local bias at peak candidate regions. It is particularly recommended while calling peaks without control sample.

• --slocal, --llocal

  These two parameters control which two levels of regions will be checked around the peak regions to calculate the maximum lambda as local lambda. By default, MACS3 considers 1000bp for small local region(--slocal), and 10000bps for large local region(--llocal) which captures the bias from a long-range effect like an open chromatin domain. You can tweak these according to your project. Remember that if the region is set too small, a sharp spike in the input data may kill a significant peak.

• --nomodel

  While on, MACS3 will bypass building the shifting model. Please combine the usage of --extsize and --shift to achieve the effect you expect.

• --extsize

  While --nomodel is set, MACS3 uses this parameter to extend reads in 5’->3’ direction to fix-sized fragments. For example, if the size of the binding region for your transcription factor is 200 bp, and you want to bypass the model building by MACS3, this parameter can be set as 200. This option is only valid when --nomodel is set or when MACS3 fails to build model and --fix-bimodal is on.

• --shift

  Note, this is NOT the legacy --shiftsize option which is replaced by --extsize from MACS version 2! You can set an arbitrary shift in bp here to adjust the alignment positions of reads in the whole library. Please use discretion while setting it other than the default value (0). When --nomodel is set, MACS3 will use this value to move cutting ends (5’) then apply --extsize from 5’ to 3’ direction to extend them to fragments. When this value is negative, the cutting ends (5’) will be moved toward 3’->5’ direction, otherwise 5’->3’ direction. Recommended to keep it as default 0 for ChIP-Seq datasets, or -1 * half of EXTSIZE together with --extsize option for detecting enriched cutting loci such as certain DNAseI-Seq datasets. Note, you can’t set values other than 0 if the format is BAMPE or BEDPE for paired-end data. The default is 0.

  Here are some examples for combining --shift and --extsize:

  1. To find enriched cutting sites such as some DNAse-Seq datasets. In this case, all 5’ ends of sequenced reads should be extended in both directions to smooth the pileup signals. If the wanted smoothing window is 200bps, then use --nomodel --shift -100 --extsize 200.

  2. For certain nucleosome-seq data, we need to pile up the centers of nucleosomes using a half-nucleosome size for wavelet analysis (e.g. NPS algorithm). Since the DNA wrapped on nucleosome is about 147bps, this option can be used: --nomodel --shift 37 --extsize 73.

• --keep-dup

  It controls the MACS3 behavior towards duplicate tags at the exact same location – the same coordination and the same strand. You can set this as auto, all, or an integer value. The auto option makes MACS3 calculate the maximum tags at the exact same location based on binomial distribution using 1e-5 as p-value cutoff; and the all option keeps every tag. If an integer is given, at most this number of tags will be kept at the same location. The default is to keep one tag at the same location. Default: 1

• --broad

  This option, along with the bdgbroadcall command, facilitates broad peak calling, producing results in the UCSC gappedPeak format which encapsulates a nested structure of peaks. To conceptualize ‘nested’ peaks, picture a gene structure housing regions analogous to exons (strong peaks) and introns coupled with UTRs (weak peaks). The broad peak calling process utilizes two distinct cutoffs to discern broader, weaker peaks (--broad-cutoff) and narrower, stronger peaks (-p or -q), which are subsequently nested to provide a detailed peak landscape. Please note that, the max-gap value for merging nearby weaker/broad peaks is 4 times of max-gap for merging nearby stronger peaks. The later one can be controlled by --max-gap option, and by default it is the average fragment/insertion length in the PE data. DEFAULT: False

  Please note that, if you only want to call ‘broader’ peak and not interested in the nested peak structure, please simply use -p or -q with weaker cutoff instead of using --broad option.

• --broad-cutoff

  Cutoff for the broad region. This option is not available unless --broad is set. Please note that if -p is set, this is a p-value cutoff, otherwise, it’s a q-value cutoff. DEFAULT: 0.1

• --scale-to <large|small>

  When set to large, linearly scale the smaller dataset to the same depth as the larger dataset. By default or being set as small, the larger dataset will be scaled towards the smaller dataset. Beware, to scale up small data would cause more false positives. So the default behavior small is recommended.

• --call-summits

  MACS3 will now reanalyze the shape of signal profile (p or q-score depending on the cutoff setting) to deconvolve subpeaks within each peak called from the general procedure. It’s highly recommended to detect adjacent binding events. While used, the output subpeaks of a big peak region will have the same peak boundaries, and different scores and peak summit positions.

Other options

• --buffer-size

  MACS3 uses a buffer size for incrementally increasing internal array size to store reads alignment information for each chromosome or contig. To increase the buffer size, MACS3 can run faster but will waste more memory if certain chromosome/contig only has very few reads. In most cases, the default value 100000 works fine. However, if there are a large number of chromosomes/contigs in your alignment and reads per chromosome/contigs are few, it’s recommended to specify a smaller buffer size in order to decrease memory usage (but it will take longer time to read alignment files). Minimum memory requested for reading an alignment file is about # of CHROMOSOME * BUFFER_SIZE * 8 Bytes. DEFAULT: 100000

• --cutoff-analysis

  While set, MACS3 will analyze the number or total length of peaks that can be called by different cutoff then output a summary table to help the user decide a better cutoff. Note, minlen and maxgap may affect the results. DEFAULT: False

  Different with the option in bdgpeakcall, callpeak will perform both tasks to call peaks and to generate a report for cutoff analysis. Please check the section Cutoff Analysis for more detail.

Output files

1. NAME_peaks.xls is a tabular file which contains information about called peaks. You can open it in excel and sort/filter using excel functions. Information include:

   • chromosome name

   • start position of peak

   • end position of peak

   • length of peak region

   • absolute peak summit position

   • pileup height at peak summit

   • -log10(pvalue) for the peak summit (e.g. pvalue =1e-10, then this value should be 10)

   • fold enrichment for this peak summit against random Poisson distribution with local lambda,

   • -log10(qvalue) at peak summit

   Coordinates in XLS is 1-based which is different from BED format. When --broad is enabled for broad peak calling, the pileup, p-value, q-value, and fold change in the XLS file will be the mean value across the entire peak region, since peak summit won’t be called in broad peak calling mode.

2. NAME_peaks.narrowPeak is BED6+4 format file which contains the peak locations together with peak summit, p-value, and q-value. You can load it to the UCSC genome browser. Definition of some specific columns are:

   • 5th: integer score for display. It’s calculated as int(-10*log10pvalue) or int(-10*log10qvalue) depending on whether -p (pvalue) or -q (qvalue) is used as score cutoff. Please note that currently this value might be out of the [0-1000] range defined in UCSC ENCODE narrowPeak format. You can let the value saturated at 1000 (i.e. p/q-value = 10^-100) by using the following 1-liner awk: awk -v OFS="\t" '{$5=$5>1000?1000:$5} {print}' NAME_peaks.narrowPeak

   • 7th: fold-change at peak summit

   • 8th: -log10pvalue at peak summit

   • 9th: -log10qvalue at peak summit

   • 10th: relative summit position to peak start

   The file can be loaded directly to the UCSC genome browser. Remove the beginning track line if you want to analyze it by other tools.

3. NAME_summits.bed is in BED format, which contains the peak summits locations for every peak. The 5th column in this file is the same as what is in the narrowPeak file. If you want to find the motifs at the binding sites, this file is recommended. The file can be loaded directly to the UCSC genome browser. Remove the beginning track line if you want to analyze it by other tools.

4. NAME_peaks.broadPeak is in BED6+3 format which is similar to narrowPeak file, except for missing the 10th column for annotating peak summits. This file and the gappedPeak file will only be available when --broad is enabled. Since in the broad peak calling mode, the peak summit won’t be called, the values in the 5th, and 7-9th columns are the mean value across all positions in the peak region. Refer to narrowPeak if you want to fix the value issue in the 5th column.

5. NAME_peaks.gappedPeak is in BED12+3 format which contains both the broad region and narrow peaks. The 5th column is the score for showing grey levels on the UCSC browser as in narrowPeak. The 7th is the start of the first narrow peak in the region, and the 8th column is the end. The 9th column should be RGB color key, however, we keep 0 here to use the default color, so change it if you want. The 10th column tells how many blocks including the starting 1bp and ending 1bp of broad regions. The 11th column shows the length of each block and 12th for the start of each block. 13th: fold-change, 14th: -log10pvalue, 15th: -log10qvalue. The file can be loaded directly to the UCSC genome browser. Refer to narrowPeak if you want to fix the value issue in the 5th column.

6. NAME_model.r is an R script which you can use to produce a PDF image of the model based on your data. Load it to R by:

   $ Rscript NAME_model.r

   Then a pdf file NAME_model.pdf will be generated in your current directory. Note, R is required to draw this figure.

7. The NAME_treat_pileup.bdg and NAME_control_lambda.bdg files are in bedGraph format which can be imported to the UCSC genome browser or be converted into even smaller bigWig files. The NAME_treat_pielup.bdg contains the pileup signals (normalized according to --scale-to option) from ChIP/treatment sample. The NAME_control_lambda.bdg contains local biases estimated for each genomic location from the control sample, or from treatment sample when the control sample is absent. The subcommand bdgcmp can be used to compare these two files and make a bedGraph file of scores such as p-value, q-value, log-likelihood, and log fold changes.

Cutoff Analysis

Since cutoff can be an arbitrary value during peak calling, there are many approaches proposed in the community to guide the cutoff selection such as the IDR approach. In MACS3, we provide a simple way to do the cutoff analysis. The cutoff analysis function is provided by --cutoff-analysis option in callpeak, bdgpeakcall, and hmmratac. Among them, the function in bdgpeakcall is more flexible and can be applied on any scoring scheme. We will sperate this function into a dedicated subcommand in the future.

Please note that if this --cutoff-anlaysis option is on, the report will be written into a file named NAME_cutoff_analysis.txt.

When the option is on, we will generate a list of possible pvalue cutoffs to check from pscore cutoff from 0.3 to 10, with a step of 0.3. When -log10(pvalue) is 0.3, it represents an extremely loose cutoff pvalue 0.5; and when it’s 10, it represents an extremely strigent cutoff pvalue 1e-10. Please note that the is different with bdgpeakcall where users can control how the cutoff should be calculated.

Then for each cutoff we plan to investigate, we will check the number of peaks that can be called, their average peak length, and their total length.

The report consists of four columns:

1. score: the possible fold change cutoff value.

2. npeaks: the number of peaks under this cutoff.

3. lpeaks: the total length of all peaks.

4. avelpeak: the average length of peaks.

While there’s no universal rule to suggest the best cutoff, here are a few suggestions:

• You can use elbow analysis to find the cutoff that dramatically change the trend of npeaks, lpeaks, or avelpeak. But you need to think about how to define ‘dramatical change’.

• You can use some common expectation to decide the cutoff. For example, the number of peaks should be thousands/ or the avelpeak should be around 500bps. Of course, it’s arbitrary but the table will give you some insight.