17.5: De novo motif discovery

As discussed in beginning of this chapter, the core problem for motif finding is to define the criteria for what is a valid motif and where they are located. Since most motifs are linked to important biological functions, one could subject the organism to a variety of conditions in hope of triggering these biological functions. One could then search for differentially expressed genes, and then use those genes as a basis for which genes are functionally related and thus likely to be controlled by the same motif instance. However, this technique not only relies on prior knowledge of interesting biological functions to probe for, but is also subject to biases in the experimental procedure. Alternatively, one could use ChIP-seq to search for motifs, but this method relies on not only having a known Transcription Factor of interest, but also requires developing antibodies to recognize said Transcription Factor, which can be costly and time consuming.

Ideally one would be able to discover motifs de novo, or without relying on an already known gene set or Transcription Factor. While this seems like a difficult problem, it can in fact be accomplished by taking advantage of genome-wide conservation. Because biological functions are usually conserved across species and have distinct evolutionary signatures, one can align sequences from close species and search specifically in conserved regions (also known as Island of Conservation) in order to increase the rate of finding functional motifs.

Motif discovery using genome-wide conservation

Conservation islands often overlap known motifs, so doing genome-wide scans through evolutionary conserved regions can help us discover motifs, de novo. However, not all conserved regions will be motifs; for instance, nucleotides surrounding motifs may also be conserved even though they are not themselves part of a motif. Distinguishing motifs from background conserved regions can be done by looking for enrichments which will select more specifically for kmers involved in regulatory motifs. For instance, one can find regulatory motifs by searching for conserved sequences enriched in intergenic regions upstream of genes as compared to control regions such as coding sequences, since one would expect motifs to be enriched in or around promoters of genes. One can also expand this model to find degenerate motifs: we can look for conservation of smaller, non-degenerate motifs separated by a gap of variable length, as shown in the figure below. We can also extend this motif through a greedy search in order to get closer to find the local maximum likelihood motif. Finally, evolution of motifs can also reveal which motifs are degenerate; since a particular motif is more likely to be degenerate if it is often replaced by another motif throughout evolution, motif clustering can reveal which kmers are likely to correspond to the same motif.

In fact, the strategy has its biological relevance. In 2003, Professor Kellis argued that there must be some selective pressure to cause a particular sequence to be occur on specific places. His PhD. thesis on the topic can be found at the following location:

Figure 17.7: Using motif seeds to find degenerate motifs

Validation of discovered motifs with functional datasets

These predicted motifs can then be validated with functional datasets. Predicted motifs with at least one of the following features are more likely to be real motifs: -enrichment in co-regulated genes. One can extend this further to larger gene groups; for instance, motifs have been found to be enriched in genes expressed in specific tissues -overlap with TF binding experiments -enrichment in genes from the same complex -positional biases with respect to the transcription start site (TSS): motifs are enriched in gene TSS’s -upstream vs. downstream of genes, inter- vs. intra-genic positonal biases: motifs are generally depleted in coding sequences -similarity to known transcription factor motifs: some, but not all, discovered motifs may match known motifs (however, not all motifs are conserved and known motifs may not be exactly correct)