There is increasing evidence to suggest that multiple miRNAs can work together either cooperatively or competitively to alter the expression of their mRNA targets (Grimson et al., 2007; Hua et al., 2006; Nachmani et al., 2010; Sathyan et al., 2007). Supporting this idea, we have shown that combinatorial miRNA targeting events are over-represented among down-regulated mRNAs. To identify the down-regulated mRNAs that may be targeted by multiple miRNAs in the frontal cortex of alcoholics we performed an over-representation analysis of miRNA targeting events for each individual down-regulated mRNA. Out of the 102 downregulated mRNAs that are putative targets of multiple miRNAs, 17 were predicted to be targeted by a significantly higher number of up-regulated miRNAs than expected by chance (hypergeometric p< 0.05 and FDR < 25%, Table 3). This proportion is also significantly different (z-test P = 0.00086) from the equivalent one obtained in the random control group, where only 1 out of 54 relevant mRNAs displayed such behaviour. These results suggest that, similarly to the upregulated miRNA core group, this downregulated “over-targeted” mRNA group may be important in mediating the effects of miRNA regulation due to chronic alcohol consumption.
