In this study, we extended the application of MotifModeler to simultaneously identify putative cis-acting elements that represent both transcription factor and miRNA binding sites from array-derived gene expression data. Using the microarray data of alcohol-induced gene expression in FAS mouse embryos with distinct biological consequence, our model predicted many TFBS and MBS whose functions differ as a result of alcohol treatment.
Remarkably, in this particular model, it is predicted that most of the 5' motifs showed down-regulatory effects, and all of the 3' miRNA motifs showed up-regulatory effects on gene expression after alcohol treatment. This is clearly shown in Fig. 1B, where most of predicted 6-bp motifs (27 out of top 30) in the 5'-end demonstrated inhibitory effects (Xm < 0) and all the top 30 predicted 7-bp motifs in the 3'-end demonstrated stimulatory effects (Xm > 0). This unbalance observation is unlikely to be random. There was a developmental delay caused by alcohol treatment, as shown by morphological analyses . Therefore, one possible interpretation is that most genes that were at higher levels after alcohol treatment were highly expressed in a previous developmental stage and were not appropriately down-regulated. This could be due to delayed expression of miRNAs that would normally reduce expression.
A number of negative-TFBS are found closely related to the down-regulation of identified gene groups or function of the gene clusters related to developmental deficit in FAS. For example, the Egr-2/Krox-20 early growth response gene product, and early growth response 3 gene product (Egr-2, Egr-3) related to down regulation of Growth GO set related to growth deficit. The Krox, Pax3, and Winged-helix factor nude (Whn) homeodomain are fate-determinants for early development of neural axis and neural patterning. The Neuron-restrictive silencer factor (NRSF), Rat Olf-1/EBF-associated zinc finger protein (Roaz), and Runt-related transcription factor 2 (Osf2, a.k.a. Runx2) are mediating neural specification; these two TF groups together are plausible regulators for down-regulated neurodevelopmental genes seen in embryos with neurodevelopmental deficit [19, 20]. Another outstanding set of predicted TFs, AML, Hogness BOX, and a murine myeloid leukaemia-associated transcription factor (Evi-1) are related to hemopoiesis. They may mediate the global decrease of hemopoiesis genes observed in the microarray data. The group of TFs, nuclear respiratory factor 1 (Nrf-1), tumor suppressor P53 (p53), max, and c-Myc, are predicted which are known to be closely related to mediating apoptosis. The up-regulation of Nrf-1 and P53, and down-regulation of Max-c-Myc may mediate to the downstream gene expression and apoptosis in the alcohol treated embryos. In summary, these predicted TFs / TFBS are closely related to the alteration of core gene expressions related to growth, neural specification, hemopoiesis, and apoptosis which are major developmental deficits in the FAS.
Based on several recent reports that regulatory targets of miRNAs can be identified by searching conserved complementarily motifs in the 3'-UTR, we selected 7-bp motifs as putative miRNA binding sites. Several recent studies suggested that the complementarity of 5'-extremity of miRNA and 3'-UTR of the target gene is critical for the miRNA function [16, 23–25]. Further analysis indicated that starting from whole-genome alignments of five vertebrates, regulatory targets of miRNAs can be identified by searching conserved complementarity, on the 3'-UTR, to the 6-bp seed (nucleotides 2-7) of miRNA. An additional requirement of either conserved matches at nucleotide 8 of miRNA or conserved adenosines at position 1 of target mRNA greatly increases signal-to-noise ratios of the prediction, and therefore is desired to anchor the miRNA binding .
MiRNA target prediction is a challenging and unsettled area. Most bioinformatic procedures to predict miRNA targets take advantage of cross-species comparison, based on the assumption that target sites of miRNAs are evolutionarily conserved [10, 26, 27]. A more recent study demonstrated that putative miRNA binding sites that are not conserved across evolution also mediate repression . This observation not only allows but also requires us to identify potential miRNA binding sites from genomic sequences of single organism, as implemented here. Other approaches have predicted post-transcriptional mechanisms using sequence information within one species [29, 30]. Our method, however, is the first attempt to integrate the effects of TFBS and MBS into array-derived gene expression data analysis in a mammalian system.
As many RNA-binding proteins bind in the 3'-UTR, it is important to acknowledge that post-transcriptional regulation of gene expression is a multi-factorial phenomenon that includes miRNA and other molecules. In addition to facilitating translation, other factors can destabilize mRNA and thus serve similar functions as the RNA Induced Silencing Complex (RISC) recruited by miRNA . An example of such an RNA-binding protein includes AU-rich elements (ARE) , a family of PUF proteins . Our method identifies cis-acting elements in the 3'-untranslated region that could represent either miRNA binding sites or protein binding sites. This application in the field of fetal alcohol syndrome is in its infancy. Experimental evaluation is necessary to better understand the transcriptional and post-transcriptional mechanisms that regulate the global gene expression pattern.