Study challenges assumptions over how l-oligonucleotides behave in living systems

Researchers have uncovered that l -oligonucleotides, previously thought to be non-toxic, could actually be harmful to cells. This discovery, whilst still needing further investigation, could provide a new avenue for pharmaceutical research.

Enantiomers have identical molecular formulae but are mirror images of each other. Due to the chiral centres present in naturally occurring DNA and RNA (called d -DNA and d -RNA) it follows that they have enantiomeric pairs, namely l -DNA and l -RNA.

A D-RNA structure reflacted as an L-RNA structure in a mirror

d -oligonucleotides, short strands of d -DNA or d -RNA, regularly feature in therapeutics, but researchers are also interested in l -oligonucleotide-based compounds due to their ability to resist degradation by enzymes called nucleases.

Chen-Hsu Yu and Jonathan Sczepanski, both based at Texas A&M University, US, have investigated how damaging some non-native l -RNA compounds are to a cell compared to their naturally-occurring enantiomeric partner oligonucleotides. One of the experiments they carried out involved introducing l – and d -oligonucleotides into cells and comparing the cell death rates over 96 hours. From this they found the l -RNA compound to be most toxic. Using this and further research, they have found that, despite the general understanding that l -oligonucleotides are nontoxic and nonimmunogenic, these l -RNA compounds are cytotoxic.

Of particular interest for potential medical uses are L-oligonucleotides that have a high guanine content, also called G-rich compounds. Yu says ‘G-rich d -oligonucleotides are currently being pursued as anticancer agents. Like these compounds, G-rich l -RNA also induce apoptosis of cancer cells but have the added benefit of being nuclease resistant.’ Yu and Sczepanski have previously established the mechanism for this involves G-rich L -RNAs binding tightly to an important gene regulator called PRC2. When asked about whether other nucleotide-rich oligonucleotides could be used, Yu says they found that ‘A-rich and U-rich l -RNA showed less cytotoxic effect compared to G-rich l -RNA,’ thereby making G-rich l -oligonucleotides potential therapeutic compounds of interest.

Sterghios Moschos, an expert in oligonucleotide therapeutics at Northumbria University in the UK, says the research ‘elegantly builds upon the body of evidence that l -ribose-containing oligonucleotides are not a complete safe harbour when it comes to escaping cytotoxicity and other non-specific target effects.’ The ‘therapeutic lead and candidate sequences with culpable motifs, such as those revealed by this paper, can be now revisited using refined production methods and re-evaluated clinically for potentially higher therapeutic indices,’ he adds.

Sczepanski says ‘future proteomics studies would be useful for identifying key l -RNA-protein interactions responsible for the observed cytotoxicity and immunostimulatory effects’. He also warns that caution should be taken when employing l -RNA in the development of biotechnologies intended for intracellular use.