HOMER

Software for motif discovery and ChIP-Seq analysis

Gene Based Analysis (Microarray/RNA-Seq etc.)

There are 3 basic ways to run HOMER - with FASTA files, with Gene Identifiers, or from Genomic Positions. This section will outline gene-based analysis. Gene-based analysis is handled by the program findMotifs.pl. This program does more than just finding motifs, including gene ontology analysis. By default, this is a promoter-based motif finding analysis, but can also be used to look for RNA motifs in mRNAs.

Input Files:

findMotifs.pl will analyze the promoters of genes and look for motifs that are enriched in your target gene promoters relative to other promoters. The idea is to provide a list of genes that you believe should contain the same elements, such as genes that are co-regulated. For example, you may want to analyze the genes that are up-regulated by a stimulus, or genes that are specific to a certain cell type, or genes that appear in the same gene-expression cluster when doing clustering analysis. Alternatively, the gene IDs could come from a promoter ChIP-Chip experiment where each of the promoters are bound by the same transcription factor.

The primary input data is a list of gene identifiers, placed in single text file where each line contains a gene ID. This can be a gene expression spreadsheet, but findMotifs.pl will expect the first column to contain a gene ID. Below are examples of acceptable input file formats:

If you're having trouble with the program, 9 out of 10 times it is due to an incorrectly formatted input file. If using EXCEL (especially on the Mac), make sure to save input files as "Text (Windows)". HOMER will choke on binary XLS files. HOMER accepts a broad range of different types of gene identifiers:

NCBI Entrez Gene IDs
NCBI Unigene IDs
NCBI Refseq IDs (mRNA, protein)
Ensembl Gene IDs
Gene Symbols (i.e. Official Gene names, like "Nfkb1" )
popular affymetrix probe IDs (MOE430, U133plus, U95, U75A)

If your favorite ID isn't listed above, then you will have to covert to one of these before using HOMER. Your input file can have a mix of different IDs too. HOMER will let you know how many of the IDs it was able to "understand", so you can give it a try.

Running findMotifs.pl

findMotifs.pl takes 3 mandatory arguments: A gene ID input file, the name of the promoter set (which is tied to an organism), and an output directory for all of the output files. For now, HOMER only supports 7 organisms, although you can contact me (cbenner@ucsd.edu) if you think it would be good to add more. It is also possible to add support for an organism and/or a custom promoter set yourself. For each of these organisms, a default "Promoter Set" was constructed based on RefSeq Genes:

human (Homo sapiens)
mouse (Mus musculus)
rat (Rattus norvegicus)
fly (Drosophila melanogaster)
worm (Caenorhabditis elegans)
zebrafish (Danio rerio)
yeast (Saccharomyces cerevisiae)

To run findMotifs.pl, type the following:

findMotifs.pl <inputfile.txt> <promoter set> <output directory> [options]

i.e. findMotifs.pl lpsInducedGenes.pl mouse LPSMotifResults/ -start -400 -end 100 -len 8,10 -p 4
This will search for motifs of length 8 and 10 from -400 to +100 relative to the TSS, using 4 threads (i.e. 4 CPUs)

findMotifs.pl will produce a number of output files in the "output directory". The primary output will be in HTML files that should be opened with you favorite web browser.

What does findMotifs.pl do?

This program performs a number of operations en route to providing a basic analysis of motif and functional enrichment. The various steps are outlined below:

Convert Gene IDs to consistent gene identifier (usually Entrez Gene ID)
Select appropriate background IDs (usually all confident genes, i.e. not olfactory genes), or take user supplied list (see below)
Perform Gene Ontology enrichment calculation (for details, see here).
Assign weights to background promoters based on the distribution of CpG content in the target gene promoters such that comparable numbers of low and high-CpG promoters are analyzed.
Perform de novo motif analysis
Create output HTML pages for de novo analysis.
Perform known motif enrichment analysis and corresponding output pages.

Output files

The first output page created is for de novo results (beside the GO analysis). This page contains a sorted list of non-redundant motifs ranked by their enrichment p-values. Below is an example generate using genes up-regulated after 1 hour of treatment with LPS in murine macrophages (sample gene list):

findMotifs.pl upLPS.mouse.txt mouse outputDirectory/ -len 8,10,12

This should produce:

Information is provided for each motif. One thing I've been debating is whether or not to include my best guess for the motif identity ("Best Match/Details"). The problem is that people take this result too literally and it can be a huge problem and a source of misunderstanding. In many cases the best match doesn't not look very convincing, but few people seem to pay attention to that (see below).

More information about HOMER motif finding output

Important motif finding parameters

Repeat Masked vs. Unmasked Sequences

Actually, this usually doesn't matter that much. Since HOMER is a differential motif discovery algorithm, common repeats are usually in both the target and background sequences. However, it is not uncommon that a transcription factor binds to a certain class of repeats, which may cause several large stretches of similar sequence to be processed, biasing the results. Usually it's safer to go with the masked version. To use the unmasked version, use "-nomask".

Promoter Region ("-start <#>" and "-end <#>", default: -300, 50)

Different parts of the promoter can be used for motif finding. In the "old days", everyone would search 1kb upstream and look for motifs there. As it turns out, most of the action is within 200 bp of the promoter, with the motif density dropping of considerably after that. The maximum sizes handled by HOMER are -2000 and 2000.

Motif length ("-len <#>" or "-len <#>,<#>,...", default 8,10,12)

Specifies the length of motifs to be found. HOMER will find motifs of each size separately and then combine the results at the end. The length of time it takes to find motifs increases greatly with increasing size. In general, it's best to try out enrichment with shorter lengths (i.e. less than 15) before trying longer lengths. Much longer motifs can be found with HOMER, but it's best to use smaller sets of sequence when trying to find long motifs (i.e. use "-len 20 -start -150 -end 50"), otherwise it may take way too long (or take too much memory). The other trick to reduce the total resource consumption is to reduce the number of background sequences.

Mismatches allowed in global optimization phase ("-mis <#>", default: 2)

HOMER looks for promising candidates by initially checking ordinary oligos for enrichment, allowing mismatches. The more mismatches you allow, the more sensitive the algorithm, particularly for longer motifs. However, this also slows down the algorithm a bit. If searching for motifs longer than 12-15 bp, it's best to increase this value to at least 3 or even 4.

Number of CPUs to use ("-p <#>", default 1)

HOMER is now multicore compliant. It's not perfectly parallelized, however, certain types of analysis can benefit. In general, the longer the length of the motif, the better the speed-up you'll see.

Number of motifs to find ("-S <#>", default 25)

Specifies the number of motifs of each length to find. 25 is already quite a bit. If anything, I'd recommend reducing this number, particularly for long motifs to reduce the total execution time.

Normalize CpG% content instead of GC% content ("-cpg")

Consider tying if HOMER is stuck finding "CGCGCGCG"-like motifs. You can also play around with disabling GC/CpG normalization ("-noweight").

Region level autonormalization ("-nlen <#>", default 3, "-nlen 0" to disable)
Motif level autonormalization (-olen <#>, default 0 i.e. disabled)

Autonormalization attempts to remove sequence bias from lower order oligos (1-mers, 2-mers ... up to <#>). Region level autonormalization, which is for 1/2/3 mers by default, attempts to normalize background regions by adjusting their weights. If this isn't getting the job done (autonormalization is not guaranteed to remove all sequence bias), you can try the more aggressive motif level autonormalization (-olen <#>). This performs the autonormalization routine on the oligo table during de novo motif discovery. (see here for more info)

User defined background genes ("-bg <file of Gene IDs to use as background>")

By default HOMER uses all other promoters as the background set. You can choose a specific set of background promoters by placing the gene identifiers in a file (just like the target genes) and using the "-bg <file>" option. These will still be normalized for CpG% or GC% content just like normal and autonormalized unless these options are turned off (i.e. "-nlen 0 -noweight"). This can be very useful since HOMER is a differential motif discovery algorithm.

Binomial enrichment scoring ("-b")

By default, findMotifs.pl uses the hypergeometric distribution to score motifs. If the set of sequences you are analyzing is very large, you may want to use the binomial to speed things up. In general, it is recommended to use the hypergeometric since it does a better job of describing biological enrichment.

Find enrichment of individual oligos ("-oligo").

This creates output files in the output directory named oligo.length.txt.

Only search for motifs on + strand ("-norevopp")

By default, HOMER looks for transcription factor-like motifs on both strands. This will force it to only look at the + strand (relative to the TSS, so - strand if the TSS is on the - strand).

Mask motifs ("-mask <motif file>")

Mask the motif(s) in the supplied motif file before starting motif finding. Multiple motifs can be in the motif file.

Optimize motifs ("-opt <motif file>")

Instead of looking for novel de novo motifs, HOMER will instead try to optimize the motif supplied. This is cool when trying to change the length of a motif, or find a very long version of a given motif. For example, if you specify "-opt <file>" and "-len 50", it will try to expand the motif to 50bp and optimize it.

Dump FASTA files ("-dumpFasta")

Like the fact that HOMER organizes and extracts your sequence files, but don't care for HOMER as a motif finding algorithm? That's cool, just specify "-dumpFasta" and the files "target.fa" and "background.fa" will show up in your output directory. You can then use them with MEME or whatever. Just remember, Chuck knows where you live...

Removing redundant promoters ("-noredun")

By default, HOMER only keeps one promoter if it is shared by two genes (i.e. bidirectional promoter) so that the sequence isn't duplicated. If the duplicated promoter is found in both the target promoter group and the background group, the background instance is removed.

Convert IDs to Human for GO analysis ("-humanGO")

Finding Instances of Specific Motifs

By default, HOMER does not return the locations of each motif found in the motif discovery process. To recover the motif locations, you must first select the motifs you're interested in by getting the "motif file" output by HOMER. You can combine multiple motifs in single file if you like to form a "motif library". To identify motif locations, you have two options:

1. Run findMotifs.pl with the "-find <motif file>" option. This will output a tab-delimited text file with each line containing an instance of the motif in the target peaks. The output is sent to stdout.

For example: findMotifs.pl lpsGenes.txt mouse MotifOutputDirectory/ -find motif1.motif > outputfile.txt

The output file will contain the columns:

Peak/Region ID
Offset from the TSS
Sequence of the site
Name of the Motif
Strand
Motif Score (log odds score of the motif matrix, higher scores are better matches)

2. Run annotatePeaks.pl with the "-m <motif file>" option in tss mode (see the here for more info). To use this option, you must install the proper genome. Chuck prefers doing it this way. This will output a tab-delimited text file with each line containing a peak/region and a column containing instance of each motif separated by commas to stdout

For example: annotatePeaks.pl tss mm9 -size -300,50 -m motif1.motif > outputfile.txt

The output file will contain columns:

Peak/Region ID
Chromosome
Start of TSS region
End of TSS region
Strand of Peaks

    6-18: annotation information
    19. CpG%
    20. GC%
    21. Motif Instances
    ...

Motif Instances have the following format:
<distance from TSS>(<sequence>,<strand>,<conservation>)
i.e -29(TAAATCAACA,+,0.00)

To limit the search to only the target set of genes (or any subset of genes), use the option "-list <gene id file>".

This can also be used to find histograms of motif density relative to the TSS - just add the "-hist <#>" option.

For example: annotatePeaks.pl tss mm9 -size -500,250 -hist 10 -m yy1.motif > outputfile.txt

Graphing the output file with EXCEL, we can see the distribution of TSS-associated motif YY1:

Add "-list <gene id list>" to make a histogram on a specific subset of genes.

Practical Tips for Motif Finding

Can't figure something out? Questions, comments, concerns, or other feedback:
cbenner@ucsd.edu