A series of studies by Greg Hannon at Cold Spring Harbor Laboratory (CSHL) have revealed a great deal of information about the mechanism of RNAi, as well as how RNAi can be adapted for use in medical research. These and other studies led Science to name discoveries concerning RNAi the "Breakthrough of the Year" for 2002 among all of the sciences.

Now, researchers at CSHL have shown that RNAi can be used to set the level of gene activity in stem cells on "low," "medium," or "high."

The new study indicates that stable suppression of deleterious genes by RNAi--in which adult stem cells are isolated, modified ex vivo, and then re-introduced into the affected individual--might be an effective strategy for treating human disease.

The study, published in the February issue of Nature Genetics, focussed on the role of a tumor suppressor gene called p53 in a mouse model of lymphoma.

In the mouse model, forced expression of the Myc oncogene in B-cells causes the mice to develop B-cell lymphomas by 4 to 6 months of age. The scientists, led by Greg Hannon and his CSHL colleague, Scott Lowe, knew that completely deleting the p53 gene causes lymphomas to develop much sooner, and in a more aggressive, highly-invasive form, than lymphomas that develop when the p53 gene is present.

To test the effect of decreasing p53 to particular levels via RNA interference, the scientists reconstituted the blood cells of mice by first irradiating the animals to destroy their endogenous, bone marrow supply of hematopoietic stem cells, and then injected the mice with a fresh supply of hematopoietic stem cells that had been engineered through RNAi to produce low, medium, or high levels of p53.

The study showed that establishing different levels of p53 in B-cells by RNAi produces distinct forms of lymphoma. Similar to lymphomas that form in the absence of p53, lymphomas that formed in mice with low p53 levels developed rapidly (reaching terminal stage after 66 days, on average), infiltrated lung, liver, and spleen tissues, and showed little apoptosis or "programmed cell death."

In contrast, lymphomas that formed in mice with intermediate p53 levels developed less rapidly (reaching terminal stage after 95 days, on average), did not infiltrate lung, liver, or spleen tissues, and showed high levels of apoptosis. In mice with high B-cell p53 levels, lymphomas did not develop at an accelerated rate, and these mice did not experience decreased survival rates compared to control mice.

The study illustrates the ease with which RNAi "gene knockdowns" can be used to create a full range of mild to severe phenotypes (something that geneticists dream about), as well as the potential of RNAi in developing stem cell-based and other therapeutic strategies.

Along with a recent study by Hannon and his colleagues that demonstrated germline transmission of RNAi, the current study establishes RNAi as a convenient alternative to traditional, laborious, and less flexible homologous recombination-based gene knockout strategies for studying the effects of reduced gene expression in a wide variety of settings.

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