Experimental RNAi

RNA interference uses a multi-step mechanism to suppress genes. The enzyme Dicer cleaves short hairpin RNA into siRNA molecules of approximately 22 base pairs. The RNA-induced silencing complex (RISC) converts siRNA into single-stranded RNA, which hybridizes with complementary mRNA. The Argonaute enzyme within RISC then cleaves the target mRNA, degrading it and preventing translation, resulting in gene silencing.

RISC, the RNA-induced silencing complex, is the central effector enzyme in RNAi. It picks up siRNA molecules and converts them into single-stranded RNA during assembly. RISC then binds to complementary sequences on target mRNA transcripts. The Argonaute protein within RISC catalyzes mRNA cleavage, leading to transcript degradation and gene silencing.

A vector coding for short hairpin RNA targeting a specific mRNA is introduced into embryonic stem cells of mice. The vector sequence integrates into the host genome, producing transgenic mice that express the hairpin RNA. In the cytoplasm, RNAi machinery recognizes these hairpin structures and cleaves them into double-stranded RNA, which is then incorporated into RISC for gene silencing.

RNAi is reversible, whereas gene knockout is permanent, allowing researchers to control when genes are silenced. RNAi enables large-scale silencing of protein-coding genes and can precisely target genes with single nucleotide variations. Additionally, RNAi effector molecules function at low concentrations compared to older methods like oligonucleotides or ribozymes, making it more potent and efficient.

RNAi helps identify gene functions across diverse organisms. Researchers used RNAi to screen C. elegans chromosomes and identify genes involved in cell division and embryonic development. In coffee plants, RNAi knocked out the theobromine synthase gene to produce decaffeinated varieties. Studies show siRNA can inhibit infections from HIV, hepatitis B, and poliovirus in cultured human cells, demonstrating therapeutic potential.

Researchers constructed a vector coding for short hairpin RNA targeting the APC mRNA transcript and introduced it into cells. Transgenic mice with silenced APC genes developed colon tumors more frequently than control mice with functional APC genes. This comparison demonstrated that APC likely functions as a tumor suppressor, revealing the gene's role in preventing cancer development.

RNAi has been applied across nearly all eukaryotes since its discovery in 1998. Researchers used RNAi in C. elegans to identify genes controlling cell division and embryonic development. In Drosophila melanogaster, RNAi identified genes essential for embryonic development and biochemical signaling cascades. RNAi has also been used in coffee plants and mammals, demonstrating its broad applicability across diverse species.