Establishing a Dual-Reporter Mouse Model to Monitor INK4A/ARF Regulation in Vivo

The INK4B-ARF-INK4A locus on human chromosome 9p21 (and on mouse chromosome 4) encodes three linked, yet functionally distinctive, gene products that coordinate signaling events in both normal development and disease states. INK4 proteins exert their function as gate-keepers for cell cycle progression by inhibiting D-type cyclin-dependent kinases (CDKs), which are responsible for the phosphorylation of the retinoblastoma (RB) protein and consequent G1 exit. The structurally distinct ARF protein (p14ARF in human and p19Arf in mouse), however, activates another major tumor suppressor, TP53, via direct inhibition of its negative regulator, murine double minute 2 (MDM2). The identification of these separate but interacting effector pathways has highlighted the importance of this locus in the initiation and progression of tumorigenesis. Indeed, loss of expression of these gene products, either by deletion or gene silencing, has been linked to many forms of adult and pediatric cancer. To gain a better understanding of how the two gene products of the Cdkn2a locus, p16Ink4a and p19Arf, are regulated in vivo, I generated a BAC reporter mouse by replacing the coding sequence of each gene with a unique fluorescent reporter, allowing us to monitor the coordinate and independent regulation of the two genes at single-cell resolution. Here, I report the generation of the dual-reporter mouse model, functional validation in response to physiological processes previously associated with Cdkn2a expression as well as the novel discovery of Arf promoter activation in the central nervous system under normal conditions. The use of this transgenic reporter system will further allow for the identification of cues conferring Ink4a/Arf locus regulation during normal development as well as disease pathogenesis.