Impaired Histone H3 Lysine 9 Methylation Accelerates Ras-Driven Lymphomagenesis by Disabling Cellular Senescence.

As a barrier against malignant transformation, checkpoint-mediated failsafe mechanisms such as apoptosis or cellular senescence may be activated in response to mitogenic oncogenes. Acute induction of oncogenic Ras has been shown to provoke a senescence-like cell-cycle arrest that involves the retinoblastoma (Rb) pathway. Recently, Rb-mediated formation of heterochromatin was identified as a critical feature of cellular senescence. Since methylation of histone H3 lysine 9 by the Rb-bound histone methyltransferase Suv39h1 contributes to the transcriptional repression of growth promoting genes, we asked whether inactivation of Suv39h1 may disable cellular senescence as a suppressor mechanism against malignant transformation by oncogenic Ras.

To investigate the role of Suv39h1 in Ras-driven lymphomagenesis, Eμ-N-ras-transgenic mice were crossbred to mice harboring targeted deletions in the Suv39h1 locus, as well as to p53 knockout mice. Ras-transgenic mice with no additional defined genetic lesion - hereafter referred to as controls - developed a terminal condition mostly due to a neoplastic histiocytic infiltration of the liver with a median latency of about 250 days. In stark contrast, ras-transgenic mice lacking Suv39h1, or carrying heterozygous defects in the Suv39h1 or p53 locus, respectively, entered a final disease stage significantly earlier, i.e. at a median age of 50 to 100 days, due to aggressive T cell lymphomas that were virtually absent or only sporadically found in the control group. Importantly, similar to p53+/− mice which typically lost the remaining wild-type allele in lymphoma cells, Suv39h1 heterozygous mice produced tumors that invariably lacked Suv39h1 expression, rendering these lymphomas in fact Suv39h1 null. Suv39h1 null cells propagated as primary cultures grew readily in an exponential fashion, surpassed only by the growth potential of p53 null cells. In short-term cytotoxicity assays, the anticancer agent adriamycin efficiently killed both control and Suv39h1 null lymphoma cells, whereas p53 null cells were resistant, indicating no overt apoptotic defect in Suv39h1 null cells. While non-neoplastic ras-transgenic Suv39h1+/+ cells exhibited heterochromatin foci reminiscent of cellular senescence, this phenotype was completely lost in Suv39h1 null lymphoma cells. In the presence of Bcl2 - introduced to block apoptotic cell death - adriamycin-treated control cells entered cellular senescence, whereas Suv39h1 null cells failed to display a senescent phenotype in response to therapy, demonstrating that stress-inducible senescence is a Suv39h1-dependent program.

The data unveil the fundamental role of cellular senescence as a tumor suppressor principle by establishing a novel link between Ras-driven transformation and deregulated histone modification in vivo. Impaired cellular senescence due to inactivation of the histone H3 lysine 9 methyltransferase Suv39h1 promotes malignant transformation by oncogenic Ras, leading to a dramatic acceleration of Ras-driven lymphoma formation. Moreover, the observation that a full-blown malignancy may arise as a consequence of disabled senescence but without an apparent apoptotic defect has important therapeutic implications.