The proto-oncogene KRAS is mutated in a wide array of human cancers, most of which are aggressive and respond poorly to standard therapies. Although the identification of specific oncogenes has led to the development of clinically effective, molecularly targeted therapies in some cases, KRAS has remained refractory to this approach. A complementary strategy for targeting KRAS is to identify gene products that, when inhibited, result in cell death only in the presence of an oncogenic allele1,2. Here we have used systematic RNA interference (RNAi) to detect synthetic lethal partners of oncogenic KRAS and found that the non-canonical IκB kinase, TBK1, was selectively essential in cells that harbor mutant KRAS. Suppression of TBK1 induced apoptosis specifically in human cancer cell lines that depend on oncogenic KRAS expression. In these cells, TBK1 activated NF-κB anti-apoptotic signals involving cREL and BCL-XL that were essential for survival, providing mechanistic insights into this synthetic lethal interaction. These observations identify TBK1 and NF-κB signaling as essential in KRAS mutant tumors and establish a general approach for the rational identification of co-dependent pathways in cancer.

BRCA1 immunostaining reveals discrete, nuclear foci during S phase of the cell cycle. Human Rad51, a homolog of bacterial RecA, behaves similarly. The two proteins were found to colocalize in vivo and to coimmunoprecipitate. BRCA1 residues 758-1064 alone formed Rad51-containing complexes in vitro. Rad51 is also specifically associated with developing synaptonemal complexes in meiotic cells, and BRCA1 and Rad51 were both detected on asynapsed (axial) elements of human synaptonemal complexes. These findings suggest a functional interaction between BRCA1 and Rad51 in the meiotic and mitotic cell cycles, which, in turn, suggests a role for BRCA1 in the control of recombination and of genome integrity.

Dicer is the enzyme that cleaves double-stranded RNA (dsRNA) into 21-25-nt-long species responsible for sequence-specific RNA-induced gene silencing at the transcriptional, post-transcriptional, or translational level. We disrupted the dicer-1 (dcr-1) gene in mouse embryonic stem (ES) cells by conditional gene targeting and generated Dicer-null ES cells. These cells were viable, despite being completely defective in RNA interference (RNAi) and the generation of microRNAs (miRNAs). However, the mutant ES cells displayed severe defects in differentiation both in vitro and in vivo. Epigenetic silencing of centromeric repeat sequences and the expression of homologous small dsRNAs were markedly reduced. Re-expression of Dicer in the knockout cells rescued these phenotypes. Our data suggest that Dicer participates in multiple, fundamental biological processes in a mammalian organism, ranging from stem cell differentiation to the maintenance of centromeric heterochromatin structure and centromeric silencing.[Keywords: RNA interference; microRNA; heterochromatin silencing; DNA methylation] Supplemental material is available at http://www.genesdev.org.

BRCA2 mutations predispose carriers to breast and ovarian cancer and can also cause other cancers and Fanconi anemia. BRCA2 acts as a "caretaker" of genome integrity by enabling homologous recombination (HR)-based, error-free DNA double-strand break repair (DSBR) and intra-S phase DNA damage checkpoint control. Described here is the identification of PALB2, a BRCA2 binding protein. PALB2 colocalizes with BRCA2 in nuclear foci, promotes its localization and stability in key nuclear structures (e.g., chromatin and nuclear matrix), and enables its recombinational repair and checkpoint functions. In addition, multiple, germline BRCA2 missense mutations identified in breast cancer patients but of heretofore unknown biological/clinical consequence appear to disrupt PALB2 binding and disable BRCA2 HR/DSBR function. Thus, PALB2 licenses key cellular biochemical properties of BRCA2 and ensures its tumor suppression function.