MIT in new approaches to gene silencing

A short form of RNA designed by researchers at the Massachusetts Institute of Technology (MIT) can turn off a targeted gene in much the same way as naturally occurring microRNAs. RNA was long thought to be only DNA's messenger, but MIT researchers have now found that tiny segments of double-stranded RNA, dubbed short interfering RNAs (siRNAs), can be designed to shut down any given gene.

MicroRNAs are small, naturally occurring RNA molecules that are similar to siRNAs but function differently to silence genes. Small RNAs, 21-25 nucleotides long, work in animals in at least two distinct ways: siRNA pair exactly to mRNA and destroy it, while microRNAs partially pair with mRNAs and repress their translation without destroying the mRNA.

The researchers at MIT hope to use their new-found information about siRNAs to understand how naturally occurring microRNAs work to stop the translation of certain genes in mammals. Last June the researchers created siRNAs that inhibited the growth of HIV through gene silencing.

Meanwhile, another team at MIT has developed a way to exploit RNA interference for the first time to silence genes in a wide variety of mammalian cells, including embryonic cells. This new approach allows genes to be switched off by inserting short pieces of ribonucleic acid (RNA) into developing cells. It is currently being used to help researchers uncover the function of the more than 30,000 genes found in humans, as well as in animal models of important diseases.

This technology may be used to shut down disease-causing genes such as those involved in cancer, high cholesterol or autoimmune disease; to create easily transplantable organs and tissues; or to convey immunity to viruses such as HIV.

RNA interference is a potentially powerful tool, but important cell types have been resistant to the introduction of short interfering RNAs (siRNAs) and short hairpin RNAs (shRNAs) required to trigger this process. The researchers have created a system based on a disarmed lentivirus – a retrovirus that can introduce genetic material into almost every cell or tissue, including stem cells and neurons – and have used this virus to deliver shRNAs into mammalian cells and even induce RNA interference (RNAi) in transgenic animals.

Using this new technology, they have created transgenic mice in which important immune genes or cancer genes, such as p53, have been silenced. These mice are now being studied to understand more about how these genes contribute to autoimmune disease and cancer.

'We show that lentivirus-delivered shRNAs are capable of specific, highly stable and functional silencing of gene expression in a variety of cell types and also in transgenic mice,' said MIT biology graduate student Douglas A. Rubinson, lead author of the study. This method 'should permit rapid and efficient analysis of gene function in primary human and animal cells and tissues and generation of animals that show reduced expression of specific genes. It may also provide new approaches for gene therapy.'