[Show abstract][Hide abstract] ABSTRACT: The 5' untranslated region of the proto-oncogene c-myc contains an internal ribosome entry segment (IRES) (Nanbru et al., 1997; Stoneley et al., 1998) and thus c-myc protein synthesis can be initiated by a cap-independent as well as a cap-dependent mechanism (Stoneley et al., 2000). In cell lines derived from patients with multiple myeloma (MM) there is aberrant translational regulation of c-myc and this correlates with a C-T mutation in the c-myc-IRES (Paulin et al., 1996). RNA derived from the mutant IRES displays enhanced binding of protein factors (Paulin et al., 1998). Here we show that the same mutation is present in 42% of bone marrow samples obtained from patients with MM, but was not present in any of 21 controls demonstrating a strong correlation between this mutation and the disease. In a tissue culture based assay, the mutant version of the c-myc-IRES was more active in all cell types tested, but showed the greatest activity in a cell line derived from a patient with MM. Our data demonstrate that a single mutation in the c-myc-IRES is sufficient to cause enhanced initiation of translation via internal ribosome entry and represents a novel mechanism of oncogenesis.
[Show abstract][Hide abstract] ABSTRACT: A 340 nucleotide section of the c- myc 5' untranslated region (UTR) contains an internal ribosome entry segment. We have described previously a mutation in this region of RNA in cell lines derived from patients with multiple myeloma (MM) which exhibit increased expression of c- myc protein by an aberrant translational mechanism. In this study we show by electrophoretic mobility shift assays (EMSA), north-western blotting and UV cross-linking that radiolabelled c- myc 5' UTR RNA transcripts which harbour the mutation cause enhanced binding of cellular proteins. In addition, we also demonstrate that an MM derived cell line possesses an altered repertoire of RNA binding proteins. Our data suggest that the deregulated expression of c -myc in MM could result both from the effect of the mutation and the additional proteins which are present in these cell types.
Preview · Article · Aug 1998 · Nucleic Acids Research
[Show abstract][Hide abstract] ABSTRACT: Translation in eukaryotic cells is generally initiated by ribosome scanning from the 5' end of the capped mRNA. However, initiation of translation may also occur by a mechanism which is independent of the cap structure and in this case ribosomes are directed to the start codon by an internal ribosome entry segment (IRES). Picornaviruses represent the paradigm for this mechanism, but only a few examples exist which show that this mechanism is used by eukaryotic cells. In this report we show data which demonstrate that the 5' UTR of the proto-oncogene c-myc contains an IRES. When a dicistronic reporter vector, with c-myc 5' UTR inserted between the two cistrons, was transfected into both HepG2 and HeLa cells, the translation of the downstream cistron was increased by 50-fold, demonstrating that translational regulation of c-myc is mediated through cap-independent mechanisms. This is the first example of a proto-oncogene regulated in this manner and suggests that aberrant translational regulation of c-myc is likely to play a role in tumorigenesis.