Standfirst
Combined genomic and functional analyses have led to the isolation of a new breast cancer oncogene, the inhibitor of kB kinase E (IKBKE), thus establishing a link between the NFkB pathway and breast tumours.
Detecting the genetic alterations that drive tumorigenesis is not an easy task. William Hahn and colleagues tackled this problem by combining genomic and functional analyses to study the Ras pathway, and have identified a novel breast cancer oncogene.
Oncogenic mutations of Ras (such as HRAS-GV12) trigger the activation of many downstream pathways, but it is likely that only a few are crucial to transformation. So, the authors assessed whether co-activation of the downstream mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K) pathways, which are often deregulated in cancer, could replace HRAS-GV12 in transformation. They created a panel of 16 immortalized human embryonic kidney cell lines (HA1E) expressing single or paired combinations of mutants activating either the MAPK (CRAF, BRAF and MEK1 mutants) or the PI3K (PIK3CA, AKT1 and PTEN mutants) pathway. The co-expression of mutants that activate AKT1 and MEK1 kinases induced anchorage-independent growth and tumour growth in immunodeficient mice, showing that the activation of specific members of the MAPK and PI3K pathways cooperate to transform human cells.
So, can kinases other than AKT1 cooperate with MEK1 to induce tumorigenesis? The authors constructed a library of 354 known and putative human kinases activated by myristoylation, introduced them into HA1E cells that expressed the MEK1 activated mutant (HA1E–M) and selected the colonies that showed anchorage-independent growth. Analysis of these cells led to the identification of five different kinases (including AKT1, a finding that validated this screening approach) that cooperated with activated MEK1 and mimicked HRAS-GV12 in inducing tumorigenesis. By analysing 179 cancer cell lines using a high-density single nucleotide polymorphism (SNP) array, they found copy-number gain and amplification of the 1q32 locus containing one of these kinase-encoding genes, IKBKE (inhibitor of 
B kinase 
), in 8 of 49 breast cancer cell lines. Moreover, comparative genomic hybridization analysis of 30 breast tumour samples showed that 1q copy-number gain occurred in 
33% of cases. Several strategies showed that IKBKE and its product IKK are overexpressed in 47.6% of breast cancer samples.
So, what is the function of IKBKE overexpression? The authors showed that short hairpin RNA-mediated downregulation of IKBKE expression impaired proliferation and viability of a breast cancer cell line harbouring the 1q32 copy-number gain. In addition, overexpression of IKK in HA1E–M cells resulted in the activation of nuclear factor B (NFB) through degradation of its inhibitor IB
. NFB activation is required for tumorigenesis because the expression of a mutant IB, which is resistant to degradation and represses NFB activity, inhibited anchorage-independent growth of both IKBKE-transformed and AKT1-transformed human embryonic kidney cell lines.
Beyond identifying a new breast cancer oncogene and linking the NFB pathway to at least a subset of breast tumours, which could have therapeutic implications, this study provides a framework for the identification and functional characterization of new cancer-associated mutations.