INK4d-deficient mice are fertile despite testicular atrophy

The INK4 family of cyclin-dependent kinase (CDK) inhibitors includes four 1 5- to 19-kDa polypeptides (p16(INK4a), p15(INK4b), p18(INK4c), and p19(INK4 d)) that bind to CDK4 and CDK6. By disrupting cyclin D dependent holoenzyme s, INK4 proteins prevent phosphorylation of the retinoblastoma protein and block entry into the DNA-synthetic phase of the cell division cycle. The fo unding family member, p16(INK4a), is a potent tumor suppressor in humans, w hereas involvement, if any, of other INK4 proteins in tumor surveillance is less well documented. INK4c and INK4d are expressed during mouse embryogen esis in stereotypic tissue-specific patterns and are also detected, togethe r with INK4b, in tissues of young mice. INK4a is expressed neither before b irth nor at readily appreciable levels in young animals, but its increased expression later in life suggests that it plays some checkpoint function in response to cell stress, genotoxic damage, or aging per se. We used target ed gene disruption to generate mice lacking INK4d. These animals developed into adulthood, had a normal life span, and did not spontaneously develop t umors. Tumors did not arise at increased frequency in animals neonatally ex posed to ionizing radiation or the carcinogen dimethylbenzanthrene. Mouse e mbryo fibroblasts, bone marrow-derived macrophages, and lymphoid T and B ce lls isolated from these animals proliferated normally and displayed typical lineage-specific differentiation markers. Males exhibited marked testicula r atrophy associated with increased apoptosis of germ cells, although they remained fertile. The absence of tumors in INK4d-deficient animals demonstr ates that, unlike INK4a, INK4d is not a tumor sup presser but is instead in volved in spermatogenesis.