thumb|250px|right|Chemical structure of an LNA monomer an additional bridge bonds the 2' oxygen and the 4' carbon of the pentose
A locked nucleic acid (LNA), often referred to as inaccessible RNA, is a modified RNA nucleotide in which the ribose moiety is modified with an extra bridge connecting the 2' oxygen and 4' carbon. The bridge "locks" the ribose in the 3'-endo (North) conformation, which is often found in the A-form duplexes. This structure provides for increased stability against enzymatic degradation. LNA also offers improved specificity and affinity in base-pairing as a monomer or a constituent of an oligonucleotide. LNA nucleotides can be mixed with DNA or RNA residues in a oligonucleotide. LNA is one type of bridged nucleic acid.
Synthesis
Obika et al. were the first to chemically synthesize LNA in 1997, independently followed by Jesper Wengel's group in 1998. To date, two different approaches, referred to as linear and convergent strategies respectively, have been shown to produce high yield and efficient LNAs. The linear strategy of synthesis was first detailed in the works of Obika et al.
The addition of different moieties has remained a possibility with the maintenance of key physicochemical properties like the high affinity and specificity evident in the originally synthesized LNA. Such oligomers are synthesized chemically and are commercially available.
Incorporation into DNA/RNA
LNA can be incorporated into DNA and RNA using the promiscuity of certain DNA and RNA polymerases. Phusion DNA polymerase, a commercially designed enzyme based on a Pfu DNA polymerase, efficiently incorporates LNA into DNA.
Properties
LNA offers enhanced biostability compared to biological nucleic acids. LNA modified oligonucleotides have demonstrated improved thermodynamics in hybridization to RNA, ssDNA, and dsDNA. However, they demonstrate more efficient cleavage of phosphodiester bonds compared to their unmodified counterparts. Modification of the substrate recognition arms of DNAzymes with LNA monomers yields a LNAzyme which recognizes coxsackievirus A21 (CAV-21) and cleaves its RNA target sequence similar to one in the 5' untranslated region (5' UTR) of the human rhinovirus-14 (HRV-14); a sequence unrecognized by unmodified DNAzymes.
Therapeutics
Using LNA based oligonucleotides therapeutically is an emerging field in biotechnology. A variety of LNA oligonucleotides have been assessed for their pharmacokinetic and toxicity profiles. Studies concluded that LNA toxicity is generally independent of oligonucleotide sequence, and displays a preferential safety profile for translatable therapeutic applications.
LNA has also been applied to Miravirsen, an experimental therapeutic intended for the treatment of Hepatitis C, constituting a 15-nucleotide phosphorothioate sequence with binding specificity for MiR-122 (a miRNA expressed in hepatocytes).
Detection and diagnosis
Allele-specific PCR using LNA allows for the design of shorter primers, without compromising binding specificity.
LNA has been incorporated in fluorescence in situ hybridization (FISH). FISH is a common technique used to visualize genetic material in a variety of cells, but studies noted that this technique has been limited by low probe hybridization efficiency. Conversely, LNA-incorporated probes demonstrated increased hybridization efficiency in both DNA and RNA. The improved efficiency of LNA-incorporated FISH has resulted in FISH analysis of the human chromosome, several types of non-human cells, and microarrays.