SiRNA

Small interfering RNA (siRNA) refers to short double-stranded RNA molecules, typically 21 to 23 nucleotides in length, with 2-nucleotide 3' overhangs. In cells, siRNA is a key effector of RNA interference, mediating sequence-specific silencing of gene expression by directing the destruction of target mRNA. SiRNA can be produced endogenously or introduced from outside for research or therapeutic purposes.

In the RNA interference pathway, long double-stranded RNA or hairpin precursors are processed by the enzyme Dicer into siRNA duplexes. The duplex is incorporated into the RNA-induced silencing complex (RISC), where one strand (the guide strand) is retained and the other (the passenger strand) is discarded. The guide strand directs Argonaute-containing RISC to complementary mRNA sequences, and Argonaute 2 usually cleaves the mRNA between nucleotides 10 and 11 relative to the guide strand. Perfect or near-perfect complementarity typically leads to mRNA cleavage and degradation, while partial complementarity can suppress translation.

siRNA design emphasizes high target specificity and minimal off-target effects. Off-target silencing, immune activation, and saturation of RNAi machinery are potential concerns. In experiments, siRNA can be delivered as synthetic duplexes, used in transfection, or expressed from vectors as short hairpin RNA (shRNA) to achieve longer-term knockdown. Therapeutic development faces additional challenges in delivery to specific tissues, stability in vivo, and avoiding unwanted immune responses.

Historically, the concept of RNA interference was demonstrated in 1998 by Fire and Mello, leading to a Nobel Prize in 2006. SiRNA is distinguished from microRNA, which typically regulates gene expression through imperfect base pairing and translational repression rather than mRNA cleavage.

SiRNA remains a fundamental tool in functional genomics and a growing area of therapeutic research.