matchEnhancers: GRanges method to match enhancers with putative targets sites
In compEpiTools: Tools for computational epigenomics

GRanges method to match enhancers with putative targets sites (either TSS or TF-bound TSS)

To be used in this form:

matchEnhancers(enhGR, minD=2e4, maxD=2e5, txdb, EG2GS, TFGR=NULL)

where:

enhGR: a GRanges of enhancer sites, such as those provided by the enhancers method
minD: a positive numeric; the minimum distance between an enhancer and a target genomic region
maxD: a positive numeric; the maximum distance between an enhancer and a target genomic region
txdb: an object of class TxDb
EG2GS: an object of class OrgDb; like org.Mm.eg.db, org.Hs.eg.db (use the exact name of object)
TFGR: an optional GRanges collecting genomic regions bound from a transcription factor which also binds promoters

This methods relies on a previous identification of enhancer sites, as the one performed by the enhancers compEpiTools method based on H3K4me1 peaks (or alternatively H3K27ac, mediator, ..). Genomic regions which are evaluated as putative target sites of these enhancers are either transcription start sites (TSS) or TSS which are bound at least by a transcription factor (TF) whose binding sites are provided wuth TFGR.

Putative target sites of the provided enhancers are here defined based on the maximum and minimum distance from an enhancer. In addition, no additional TSS (belonging to differe genes, isoforms of the same gene do not count) have to be in between the enhancer and the reference target region.

If a set of TF bound regions is also provided, which is supposed to contain binding sites at the level of promoters, this method returns a list with 5 items:

XmP: a GRanges with location of TSS with no enhancers, based on the distance contraints
EmP.E: a GRanges of TF-unbound enhancers putatively associated to the TF-bound promoters EmP.mP
EmP.mP: a GRanges of TF-bound promoters putatively associated to the TF-unbound enhancer sites EmP.E
mEmP.mE: a GRanges of TF-bound enhancers putatively associated to the TF-bound promoter EmP.mP
mEmP.mP: a GRanges of TF-bound promoters putatively associated to the TF-bound enhancer sites EmP.E

otherwise this method returns a list with 3 items

XP: a GRanges with location of TSS with no enhancers, based on the distance contraints
EP.E: a GRanges of enhancers putatively associated to the promoters EP.P
EP.P: a GRanges of promoters putatively associated to the enhancer sites EP.E

	require(org.Mm.eg.db)
	require(TxDb.Mmusculus.UCSC.mm9.knownGene)
	txdb <- TxDb.Mmusculus.UCSC.mm9.knownGene
	# loading H3K4me1 peaks as a GRanges object
  	# built based on the BED file from the GEO GSM1234488 sample
  	# limited to chr19:3200000-4000000
  	H3K4me1GR <- system.file("extdata", "H3K4me1GR.Rda", package="compEpiTools")
  	load(H3K4me1GR)
	enh <- enhancers(H3K4me1GR, txdb)
	m.enh <- matchEnhancers(enhGR=enh, minD=2e4, maxD=2e5, 
		txdb=txdb, EG2GS=org.Mm.eg.db)