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p. 38-39 / OVERVIEW ON OLIGOS
Recent achievements in therapeutic oligonucleotide chemistry
ANDREI LAIKHTER
Chief Scientific Officer, Chemgenes Corporation

KEYWORDS: Antisense, targeting mRNA, RNAi Technology, exon-skipping therapy.

In recent times both basic and applied molecular biology studies have made extensive use of modified oligonucleotides as tools in the study of gene regulation and drug discovery. In particular, modified oligonucleotides have been used in applications such as antisense gene regulation and in different hybridization-based assays.

ANTISENSE AND EXON SKIPPING TECHNOLOGIES

Many modified oligonucleotides have been used as antisense molecules, targeting mRNA for the study of gene regulation. These DNAs or their analogues can hybridize to the complementary region of a corresponding DNA or RNA and affect gene expression. For antisense molecules that target mRNA, gene expression is suppressed by Rnase H catalyzed cleavage of the bound mRNA of the duplex. When one is designing oligonucleotide molecules for antisense studies, there are three major considerations. The affinity and stability of heteroduplexes of mRNA with DNA or its modified analog should be taken into account. Furthermore, one must consider the vulnerability of the molecule to nuclease activity. Finally, a characteristic important for in vivo study is the ability of the molecule to undergo transmembrane transport (uptake) into the cell.
The most commonly used antisense oligonucleotide composition is a gapmer made of chimeric oligonucleotide 21-24 nucleotoides long and the terminuses 3-4 nucleotides with the ribose moiety having predominant 3’-endo configuration that enhancing hybridization property of the chimeric oligonucleotide, the middle (gap) is usually phosphorotioated DNA that is from one hand more nuclease resistant compared to natural phosphate diester DNA and from other hand enhances RNA cleavage in presence of RNase H, when it hybridized to the target mRNA molecule.
The most common approach is to stabilize 3’-endo sugar conformation using 2’-O-Me, 2’-F and 2’-O-methoxyethyl (2’-MOE) chemistries (1, 2). Thus the conformationally restricted structures locked in 3’-endo structure (LNA) even more attractive and it has been evaluated in antisense study (3-6).
Recently the antisense approach resulted in successful development of hypercholesterolemia drug Kynamro (mipomersen sodium) by ISIS Pharmaceuticals that has been approved by FDA in 2015.

The exon-skipping therapy is another technology that utilizing mRNA steric blocking of the spliced exon that is resulted in the translation of the truncated proteins in the case of Duchenne muscular dystrophy (DMD). Restoring the reading frame within the gene leads to less severe Becker muscular dystrophy (BMD). That technology is the most promising therapeutical approach and recently was successfully tested in BioMarin/Prosensa clinical trials.

RNA
I TECHNOLOGY
The short interfering RNA (siRNA) are naturally accruing double stranded RNA molecules that bind RNA-induced silencing complex or RISC and subsequently recognizes and cleaves the complement mRNA strand that is resulting in gene silencing. Since chemically modified RNA molecules have been used and evaluated in anti-sense technology (1-6), the same type of modifications such as 2’-O-Me, 2’-MOE and...In order to continue reading this article please register to our website – registration is for free and no fees will be applied afterwards to download contents.

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