Experimental RNAi

RNA interference, or RNAi, is a process cells use to silence gene expression. 

In this mechanism, an enzyme, Dicer, cleaves short hairpin RNA into short interfering RNA, or siRNA, approximately twenty-two base pairs in length.

This siRNA is picked up by an enzyme complex called the RNA induced silencing complex, or RISC, and converted into single-stranded RNA during risk assembly on its target mRNA.

The single-stranded RNA can hybridize with complementary mRNA. An enzyme in the RISC complex called Argonaute cleaves the target mRNA. The degradation of mRNA inhibits the translation process, resulting in gene silencing.

RNA interference has been used by researchers to study the function of various genes, such as the APC, a gene thought to be involved in cancer.  A vector is constructed that codes for a short hairpin RNA that targets a specific mRNA transcript.

This vector can be added to cells or an organism to silence a gene.  If the vector is introduced into the embryonic stem cells of mice, its sequence will be integrated into the host genome, producing transgenic mice.

These mice produce RNA from the inserted sequence that fold into hairpin structures. These structures are recognized by the RNAi machinery in the cytoplasm and are cleaved into double stranded RNA.

The RNA is then incorporated into RISC as a single-stranded RNA.  RISC binds to the complementary sequence on a target mRNA transcript resulting in its degradation.

Mice with an inactivated APC gene develop tumors in their colon more often than control mice with a functional APC gene.  This indicates that it is likely that APC is responsible for suppressing tumor growth.