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Inactivation of p15INK4b, an inhibitor of cyclin-dependent kinases, through DNA methylation is one of the most common epigenetic abnormalities in myeloid leukemia. Although this suggests a key role for this protein in myeloid disease suppression, experimental evidence to support this has not been reported. To address whether this event is critical for premalignant myeloid disorders and leukemia development, mice were generated that have loss of p15Ink4b specifically in myeloid cells. The p15Ink4b fl/fl -LysMcre mice develop nonreactive monocytosis in the peripheral blood accompanied by increased numbers of myeloid and monocytic cells in the bone marrow resembling the myeloproliferative form of chronic myelomonocytic leukemia. Spontaneous progression from chronic disease to acute leukemia was not observed. Nevertheless, MOL4070LTR retrovirus integrations provided cooperative genetic mutations resulting in a high frequency of myeloid leukemia in knockout mice. Two common retrovirus insertion sites near c-myb and Sox4 genes were identified, and their transcript up-regulated in leukemia, suggesting a collaborative role of their protein products with p15Ink4b-deficiency in promoting malignant disease. This new animal model demonstrates experimentally that p15Ink4b is a tumor suppressor for myeloid leukemia, and its loss may play an active role in the establishment of preleukemic conditions. (Blood. 2010;116(6):979-987) IntroductionThe INK4a/ARF/INK4b locus covers a 40-kb region located on the human chromosome 9. This locus encodes for 3 genes (p15INK4b, p14ARF, and p16INK4a) with essential roles in regulation of the cell cycle. Whereas ARF regulates the p53 pathway, 2 other proteins (p16INK4a and p15INK4b) are important inhibitors of cyclin-dependent kinases (CDKs) CDK4 and CDK6, which play a critical role in regulation of the G 1 -to-S transition of the cell cycle. 1 The entire locus is very frequently deleted in a variety of human tumors, including leukemias. 1 In the hematopoietic system, deletions of the entire locus are mainly associated with malignancies of immature lymphocytes, most frequently in T-cell acute lymphocytic leukemia. 2 Interestingly, specific inactivation of p15INK4b, via increased methylation of the CpG island in the 5Ј end of the gene, is frequently observed in leukemias of myeloid origin. Indeed, the hypermethylation-associated repression of the p15INK4b expression has been reported in almost 80% of all patients with acute forms of myeloid leukemia. 3,4 Specific inactivation of the gene has also been frequently detected in 50% of patients diagnosed with myelodysplastic syndrome and almost 60% of patients with myeloproliferative disorders, respectively. 5,6 More importantly, hypermethylation of p15INK4b has been linked with poor prognosis in patients. [7][8][9] These clinical results established the repression of p15Ink4b expression as the most common epigenetic abnormality in myeloid leukemia and suggest that p15Ink4b is a potent suppressor of myeloid disease. Thus far, however, only minimal e...