6mer seed toxicity in tumor suppressive microRNAs

Area of Science:

• Molecular Biology
• Cancer Biology
• Genetics

Background:

• Small-interfering RNAs (siRNAs) can exhibit toxicity to cancer cells.
• This toxicity is often mediated by a specific 6-nucleotide seed sequence on the guide strand.
• Understanding the sequence determinants of siRNA toxicity is crucial for therapeutic development.

Purpose of the Study:

• To systematically screen all possible 6mer seed sequences for siRNA toxicity.
• To identify sequence features associated with toxic siRNAs.
• To investigate the targeting mechanisms and evolutionary implications of toxic siRNA seeds.

Main Methods:

• Conducted a comprehensive siRNA screen using all 4096 possible 6mer seed sequences.
• Analyzed the sequence composition (G/C content, specific positions) of toxic siRNAs.
• Investigated the 3'UTRs of targeted genes and the role of microRNAs (miRNAs).

Main Results:

• Identified a preference for guanine at positions 1 and 2, and high G/C content in toxic siRNA 6mer seeds.
• Demonstrated that toxic siRNAs target survival genes with C-rich 3'UTRs.
• Showed that the tumor-suppressive miRNA miR-34a-5p utilizes a G-rich toxic seed and is upregulated under genotoxic stress.
• Observed an evolutionary trend for most miRNAs to avoid 5' guanine in their 6mer seed, contrasting with some tumor-suppressive miRNAs.

Conclusions:

• siRNA toxicity is strongly influenced by the 6mer seed sequence, particularly its G/C content.
• Toxic siRNAs likely function by targeting essential survival genes in cancer cells.
• Certain tumor-suppressive miRNAs may have evolved G-rich toxic seeds to eliminate cancer cells, despite a general evolutionary avoidance of this feature.