Small RNAs such as microRNAs and piRNAs help control how genes are turned on or off. They do this by binding to messenger RNAs, which carry the instructions for making proteins. However, figuring out exactly where these small RNAs bind, and which binding events actually matter for gene regulation, has been challenging.

Researchers at National Cheng Kung University, Taiwan have developed a new computational tool called MUTACLASH to better interpret data from CLASH experiments. CLASH, which stands for crosslinking, ligation, and sequencing of hybrids, is a specialized form of RNA sequencing that captures direct interactions between small RNAs and their target mRNAs.

Identification of crosslinking-induced mutations (CIMs) in CLASH data at piRNA and miRNA targeting sites

A model illustrating the CLASH experimental procedure of PIWI and Argonaute complexes and the production of CIMs during cDNA synthesis. 

One powerful feature of CLASH data is the presence of crosslinking-induced mutations, or CIMs. These small changes in the RNA sequence occur at the exact position where a protein, such as Argonaute or PIWI, contacts the RNA. Because of this, CIMs can serve as precise molecular footprints that mark where RNA binding proteins interact with their targets.

The MUTACLASH tool was designed to systematically analyze these CIMs. By mapping where mutations occur in CLASH datasets from the model organism Caenorhabditis elegans, the researchers found that CIMs are strongly enriched at the center of small RNA binding sites. They also observed enrichment at positions where piRNAs bind with mismatches, suggesting that even imperfect pairing can be biologically meaningful.

Importantly, the team showed that mRNAs with CIM-marked binding sites often exhibit stronger regulatory effects. This includes noncanonical binding sites that might be missed by standard prediction tools. In other words, looking for CIMs can help identify which small RNA interactions are truly functional, rather than simply predicted by sequence rules.

Overall, MUTACLASH provides a more precise way to interpret CLASH-based RNA sequencing data. By using mutation footprints as evidence of real binding events, the tool helps researchers uncover functional small RNA target sites that may otherwise remain hidden.

Availability – The MUTACLASH pipeline and all custom scripts used in this study are available at GitHub deposit link: https://github.com/lu1215/MutaCLASH.