Ink4‐Arf locus in cancer and aging

Three tumor suppressor genes at the small (<50 kb) INK4‐ARF ( CDKN2A/B ) locus on human chromosome 9p21 coordinate a signaling network that depends on the activities of the retinoblastoma (RB) protein and the p53 transcription factor. Disruption of this circuitry, frequently by codeletion of INK4‐ARF , is a hallmark of cancer, begging the question of why the intimate genetic linkage of these tumor suppressor genes has been maintained in mammals despite the risk of their coinactivation. The INK4‐ARF locus is not highly expressed under normal physiologic conditions in young mammals, but its induction becomes more pronounced as animals age. Notably, INK4‐ARF is actively silenced en bloc in embryonic, fetal, and adult stem cells but becomes poised to respond to oncogenic stress signals as stem cells lose their self‐renewal capacity and differentiate, thereby providing a potent barrier to tumor formation. Epigenetic remodeling of the locus as a whole provides a mechanism for coordinating the activities of RB and p53. A hypothesis is that the INK4‐ARF locus may have evolved to physiologically restrict the self‐renewal capacities and numbers of stem and progenitor cells with the attendant consequence of limiting tissue regenerative capacity, particularly as animals age. Deletion of INK4‐ARF contributes to the aberrant self‐renewal capacity of tumor cells and occurs frequently in many forms of human cancer. WIREs Dev Biol 2012 doi: 10.1002/wdev.40 This article is categorized under: Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Adult Stem Cells, Tissue Renewal, and Regeneration > Stem Cells and Aging