The DKC1 gene encodes a pseudouridine synthase that modifies ribosomal RNA (rRNA). DKC1 is mutated in people with X-linked dyskeratosis congenita (X-DC), a disease characterized by bone marrow failure, skin abnormalities, and increased susceptibility to cancer. How alterations in ribosome modification might lead to cancer and other features of the disease remains unknown. Using an unbiased proteomics strategy, we discovered a specific defect in IRES (internal ribosome entry site)-dependent translation in Dkc1(m) mice and in cells from X-DC patients. This defect results in impaired translation of messenger RNAs containing IRES elements, including those encoding the tumor suppressor p27(Kip1) and the antiapoptotic factors Bcl-xL and XIAP (X-linked Inhibitor of Apoptosis Protein). Moreover, Dkc1(m) ribosomes were unable to direct translation from IRES elements present in viral messenger RNAs. These findings reveal a potential mechanism by which defective ribosome activity leads to disease and cancer.

Point mutations in ras genes have been found in a large number and wide variety of human tumors. These oncogenic Ras mutants are locked in an active GTP-bound state that leads to a constitutive and deregulated activation of Ras function. The dogma that ras oncogenes are dominant, whereby the mutation of a single allele in a cell will predispose the host cell to transformation regardless of the presence of the normal allele, is being challenged. We have seen that increasing amounts of Ras protooncogenes are able to inhibit the activity of the N-Ras oncogene in the activation of Elk in NIH 3T3 cells and in the formation of foci. We have been able to determine that the inhibitory effect is by competition between Ras protooncogenes and the N-Ras oncogene that occurs first at the effector level at the membranes, then at the processing level and lastly at the effector level in the cytosol. In addition, coexpression of the N-Ras protooncogene in thymic lymphomas induced by the N-Ras oncogene is associated with increased levels of p107, p130 and cyclin A and decreased levels of Rb. In the present report, we have shown that the N-Ras oncogene is not truly dominant over Ras protooncogenes and their competing activities might be depending on cellular context. Key words: oncogene inhibition; N-Ras; protooncogene; Ras processing; Ras effectors ras genes play an important role in a variety of differentiation processes and signal transduction, including the regulation of cell proliferation, vesicle movement, cell survival, T-cell activation, apoptosis and cytoskeleton. 1,2 The 3 ras genes encode 4 highly related proteins of 21 kd in size that are ubiquitously expressed: N-Ras, H-Ras, K-RasA and K-RasB. 2,3 Ras is synthesized on free ribosomes as a soluble inactive protein that requires several posttranslational modifications in order to reach the cell membranes where it exhibits its biologic activity. These modifications include prenylation, proteolysis, carboxymethylation and palmitoylation. 4 -7 Once activated by ligand-mediated extracellular stimuli, Ras induces the activation of effector molecules to propagate downstream signals in the cytoplasm and nucleus of the cell. Ras effectors preferentially bind to Ras-GTP through the effector domain loop found in amino acids 32-40. There are several different mammalian proteins that have been described as Ras effectors: Raf, phosphatidylinositol 3-kinase (PI3K), Ral GDP dissociation stimulator (RalGDS), members of the Rho family, mitogen-activating protein kinase 1 (MEKK1), Nore1, among others. 2,3,8 Specific point mutations in ras genes at codons 12, 13 and 61 have been found in a large number and wide variety of human tumors. 9 These mutations render Ras as an oncogene since it is constitutively active (in the GTP-bound form), leading to a deregulated activation of Ras function. 2,3,5,9 -11 In order to dissect the effects of the different downstream pathways of Ras, oncogenic mutants in the effector domain of Ras have been characterized. Thus, Ras/V12/E38 binds Raf and not PI3...