Background: microRNAs (miRNAs) are small noncoding segments of RNA that negatively
regulate gene expression at the post-transcriptional level and fine-tune gene functions. A global
repression in miRNA expression is a phenomenon observed in different types of cancer. In this study
we aimed to reveal a possible association of miRNAs downregulation in cancer, with the guanine (G)
content in the terminal loop (TL) sequences of their precursors.
Methods: Lists of most significantly downregulated miRNAs in different tumor types, obtained from
previously published microarray experiments, were selected for bioinformatics analysis. The complete
precursor, TL, and mature miRNA sequences, were analyzed for evaluation of nucleotide composition
and motif enrichment.
Results: Herein, we show an association of miRNAs downregulation in cancer, with G enrichment in
the TL sequences of their precursors. High G (and GG) content was mostly found in repressed miRNAs
of breast, lung and ovary cancers, predominantly in poorly differentiated tumors. The mature sequences
of repressed miRNAs had significantly low G content and were enriched with an ACA motif.
Conclusion: This study suggests a new link between G enrichment of precursor miRNAs TLs and
carcinogenesis, and the possible association of specific sequence motifs with the regulation of their