Annotation of SNPs as breaking or conserving TF motifs

Q:

Congrats with a very nice paper in Science (Khurana et al., 2013). I am particularly interested in how you are able to score variants in transcription factor binding sites. According to the supplementary methods you say that: "An SNV that breaks a motif is defined as a mutation that decreases the motif-matching score of the TF-binding site to the position weight matrix (PWM) of the motif (relative to the ancestral allele) (8). Conversely, an SNV that conserves a motif is defined as a mutation that increases the motif-matching score of the TF-binding site to the PWM of the motif."

This makes perfectly sense to me. But how do you define the TF-binding site in the first place? I would guess that you are applying a threshold on the motif-matching score here (to reduce the fraction of false positives), and that you then define disruption/conservation of the variant relative to this score. I cannot see any details with respect to this aspect in the paper (as far as I can see).

You refer to Mu et al. (NAR, 2011), I cannot however see any further details there.

I would very much appreciate an explanation of how you find the TF binding sites and if you use any PWM-score thresholds in this respect.

A:

The set of motifs we used in the two papers are the set of TF motifs officially released by the ENCODE project and was used in the ENCODE main publication in 2012 too. The algorithm to detect the motifs is developed by Pouya at MIT. Here is more detail about it.
http://compbio.mit.edu/encode-motifs/

In our paper, we take these motif coordinates and categorized SNVs based on their functional effects you described.

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