Suppressor Screens

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Science  01 Jul 2005:
Vol. 309, Issue 5731, pp. 23
DOI: 10.1126/science.309.5731.23b

Two groups have used RNA interference (RNAi)-based screening of human cell lines to identify tumor suppressor genes. Westbrook et al.infected immortalized mammary epithelial cells with a retroviral RNAi library in which each RNA was tagged with a DNA barcode and looked for clones that showed anchorage-independent growth (indicative of malignant transformation). Array-based comparative genomic hybridization indicated that two genes associated with the formation of anchorage-independent colonies, TGFBR2 (which encodes the known tumor suppressor transforming growth factor-β receptor II) and REST (RE1-silencing transcription factor), were frequently deleted in colorectal cancers. Knockdown of REST promoted signaling through the PI3K (phosphoinositide 3-kinase) pathway, and expression of a dominant negative form of the PI3K regulatory subunit inhibited transformation, consistent with REST acting by suppressing PI3K signaling.

Using a similar approach on immortalized fibroblasts (which can be transformed by activated RAS), Kolfschoten et al.identified the homeodomain pituitary transcription factor PITX1. Knockdown of PITX1 enhanced RAS signaling and produced a phenotype similar to that seen with overexpression of activated RAS. PITX1 expression was reduced in colon cancers that expressed wild-type RAS. The promoter of the GTPase-activating protein RASAL1 contained a PITX1 binding site; transfection with PITX1 enhanced RASAL1 mRNA abundance, whereas PITX1 knockdown reduced RASAL1 mRNA. Thus, PITX1 appears to function as a tumor suppressor that acts through RASAL1 to repress RAS signaling. — EMA

Cell 121, 837; 849 (2005).

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