Loss of p16Ink4a with retention of p19Arf predisposes mice to tumorigenesis

The cyclin-dependent kinase inhibitor p16(INK4a) can induce senescence of human cells, and its loss by deletion, mutation or epigenetic silencing is among the most frequently observed molecular lesions in human cancer(1,2). Overlapping reading frames in the INK4A/ARF gene encode p16(INK4a) and a distinct tumour-suppressor protein, p19(ARF) (ref. 3). Here we describe the generation and characterization of a p16(Ink4a)-specific knockout mouse that retains normal p19(Arf) function. Mice lacking p16(Ink4a) were born with the expected mendelian distribution and exhibited normal development except for thymic hyperplasia. T cells deficient in p16(Ink4a) exhibited enhanced mitogenic responsiveness, consistent with the established role of p16(Ink4a) in constraining cellular proliferation. In contrast to mouse embryo fibroblasts (MEFs) deficient in p19(Arf) (ref. 4), p16(Ink4a)-null MEFs possessed normal growth characteristics and remained susceptible to Ras-induced senescence. Compared with wild-type MEFs, p16(Ink4a) null MEFs exhibited an increased rate of immortalization, although this rate was less than that observed previously for cells null for Ink4a/Arf, p19(Arf) or p53 (refs 4, 5). Furthermore, p16(Ink4a) deficiency was associated with an increased incidence of spontaneous and carcinogen-induced cancers. These data establish that p16(Ink4a), along with p19(Arf), functions as a tumour suppressor in mice.