5.7 Oligonucleotides as Reagents and Therapeutics
The ability to synthesise DNA and RNA oligonucleotides of defined sequence rapidly (Sections 4.1 and 4.2),
including a range of nucleotide analogues (Section 4.4), has led to a large number of applications as thera-
peutic and diagnostic agents.^26 Many applications involve the principle of recognition of a linear sequence of
RNA or DNA by the oligonucleotide. For example, antisense, steric block and short interfering RNA
(siRNA) all involve targeting of RNA within cells to form duplexesas a means of control of gene expres-
sion^27 (Sections 5.7.1 and 5.7.2). Synthetic oligonucleotides have been used also to form triplexes(Sections
2.3.6 and 9.10.1) with double-stranded DNA to block gene expression,^28 but this principle has so far not led
to therapeutic products. Other types of application include in vitroselectionand design of oligonucleotides
that recognise and bind to nucleic acids structures, to proteins or to small molecule ligands (Section 5.7.3).
5.7.1 Antisense and Steric Block Oligonucleotides
In 1979, Zamecnik and Stephenson^29 were the first to show that a synthetic oligonucleotide could be used
to block specific gene expression in Rous Sarcoma Virus. This pioneering work led to the study of
oligonucleotides and their analogues as therapeutic agents. This field is commonly known as ‘antisense’,
since the principle of biological activity usually involves either degradation or steric blocking of the sense
Nucleic Acids in Biotechnology 193
Figure 5.20 Creation of transgenic animals by use of retroviral vectors