Incomplete penetrance and variable expressivity: is there a microRNA connection?

Incomplete penetrance and variable expressivity are non-Mendelian phenomena resulting in the lack of correlation between genotype and phenotype. Not withstanding the diversity in mechanisms, differential expression of homologous alleles within cells manifests as variations in penetrance and expressivity of mutations between individuals of the same genotype. These phenomena are seen most often in dominantly inherited diseases, implying that they are sensitive to concentration of the gene product. In this framework and the advances in understanding the role of microRNA (miRNA) in fine-tuning gene expression at translational level, we propose miRNA-mediated regulation as a mechanism for incomplete penetrance and variable expressivity. The presence of miRNA binding sites at 3' UTR, co-expression of target gene-miRNA pairs for genes showing incomplete penetrance and variable expressivity derived from available data lend support to our hypothesis. Single nucleotide polymorphisms in the miRNA target site facilitate the implied differential targeting of the transcripts from homologous alleles.