ABSTRACT: The NOTCH signaling cascade, which is deregulated in T-cell acute lymphoblastic leukemia (T-ALL) and many other human cancers, offers an attractive target for molecular therapy. One approach employs gamma-secretase inhibitors (GSIs) to suppress production of the intracellular form of NOTCH (NICD), leading to cell growth arrest and apoptosis. Here we show that missense mutations or homozygous deletion of FBW7, which encodes a ubiquitin ligase that targets the NICD for destruction, mediate constitutive NICD expression. FBW7 mutations target key arginine residues needed for binding to the NICD. Although the mutant forms of Fbw7 still bind to MYC, they do not target MYC for degradation, suggesting that stabilization of both NICD and MYC may contribute to transformation in leukemias with mutations in FBW7. Leukemias with FBW7 mutations are resistant to the growth suppressive and apoptotic effects of the MRK-003 GSI. In some resistant lines that express the NICD, this resistance is accompanied by sustained mRNA levels of the NOTCH target DELTEX1 as well as MYC mRNA and protein, implying that residual NICD activity due to the mutant FBW7 can contribute to GSI resistance. Keywords: T-ALL, gamma-secretase, GSI, cell line comparison, FBW7 mutation Total RNA isolated from cultured cells was used to make fluorescently labeled cRNA that was hybridized to DNA oligonucleotide. Briefly, 4 µg of total RNA was used to synthesize dsDNA through reverse transcription. cRNA was produced by in vitro transcription and labeled postsynthetically with Cy3 or Cy5. Two populations of labeled cRNA, a reference population and an experimental population, were compared with each other by competitive hybridization to microarrays. Two hybridizations were done with each cRNA sample pair using a fluorescent dye reversal strategy. Human microarrays contained oligonucleotide probes corresponding to approximately 21,000 genes. All oligonucleotide probes on the microarrays were synthesized in situ with inkjet technology (Agilent Technologies, Palo Alto, CA). After hybridization, arrays were scanned and fluorescence intensities for each probe were recorded. Ratios of transcript abundance (experimental to control) were obtained following normalization and correction of the array intensity data. Gene expression data analysis was done with the Rosetta Resolver gene expression analysis software (version 6.0, Rosetta Biosoftware, Seattle, WA).
