[BioC] MEDIPS: how does MEDIPS define a methylated region / cluster?

Lukas Chavez lukas.chavez.mailings at googlemail.com
Sat Sep 6 05:09:41 CEST 2014

Dear Allen,

please see my comments below. Moreover, please consider reading section 7.7
of the MEDIPS manual.


On Fri, Sep 5, 2014 at 1:14 AM, Allen [guest] <guest at bioconductor.org>

> Hi,
> I have aligned my sequencing data for one sample (filename: Ca2_MAPQ20.bam
> --- because I filtered out everything with MAPQ20 or less) and I've put it
> through MEDIPS. At the moment, I am not making a comparison between
> samples. Therefore, the result file shows the methylation profile for only
> one sample. In this results file, I noticed that the data is organized as
> such:
> chr     start   stop    CF      Ca2_MAPQ20.bam.counts
>  Ca2_MAPQ20.bam.rpkm     Ca2_MAPQ20.bam.rms      Ca2_MAPQ20.bam.prob
>  MSets1.counts.mean      MSets1.rpkm.mean        MSets1.rms.mean
> MSets1.prob.mean
> 1) I tried reading the Down et al. (2008) paper that explains the concept
> of coupling factor and I think it is, simply put, a measure of local CpG
> density. I am not sure if my understanding of 'coupling factor' is correct?

That is correct.

> 2) This lead to question how or what MEDIPS defines as a "region or
> cluster"? That is to say, on Chromosome 1, I have reads aligning from
> position "1002501" to "1003300" but then there is a region of 1400 bp (from
> "1003301" to "1004700") where there are no reads aligned (rpkm = 0). Then,
> again, from "1004701" to "1005400", there reads aligning to this region. So
> my question is does MEDIPS consider this to be a case of 2 methylated
> regions, that is, "1002501-1003300" and "1004701-1005400" or does MEDIPS
> try to consolidate these 2 methylated regions into one methylated
> region/cluster (that is, from "1002501-1005400") since they are fairly
> close to one another.

Each window is considered as one region; CpG density, counts, rpkm, rms,
and differential coverage will be calculated for each window separately.
When windows with significant differential enrichment between groups have
been identified, the MEDIPS.mergeFrames() function can be used to merge
adjacent significant windows into extended regions.

> 3) I used "uniq=TRUE" to get rid of stacked/clonal reads and so for each
> 100 bp bin, I am usually getting rpkm=1 within each bin but occasionally,
> it may be as many rpkm=3. This lead me to wonder how the relative
> methylation score ("Ca2_MAPQ20.bam.rms") is calculated? For example, 3 bins
> have a value of 1 rpkm each, but one has an rms value of "1660.409928",
> another an rms of "7122.10254" and the third is only "679.1814523". How is
> that possible that 3 bins that have the same rpkm can have such varying rms
> values?

This is due to the different CpG densities in these windows.

> 4)Can the rms be added up for a region so as to represent the cumulative
> methylation level for that region. I am asking because I do not know if the
> rms value is a log value or what? So, in my above question (3), if the 3
> bins are adjacent to one another and I decide to cluster them together and
> consider them as one methylated region, then would the rms value for this
> new 300bp bin that I have created be 1660.409928 + 7122.10254 + 679.1814523
> = 9461.68.

Instead of merging any values across windows, I recommend to make use of
the MEDIPS.createROIse() function which allows for analyzing specific
regions of interest.

> 5)Finally, what does "Ca2_MAPQ20.bam.prob" represent? I figured that is
> some sort of a probability score but I am not sure of what.

As mentioned in the manual, please disregard the prob values which will be
removed in a future version.

> Thanks in advance for any assistance in answering my questions.
> Best regards,
> Allen
>  -- output of sessionInfo():
> error
> --
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