The Translational Switch of MYC Protein Aliases in Myeloma Tumor Cells

Our systematic investigation on MYC (C-Myc) - a crucial pro-regeneration transcription factor - has determined that genetic aberrations (chromosomal rearrangements) and overexpression of the gene are highly prevalent in multiple myeloma (MM). At diagnosis, the rearrangements of MYC locus (8q24) were detected in 51% of MM patients using karyotyping and interphase FISH with DNA probe sets to MYC and the flank genomic sequences. The abnormalities of 8q24 in MM include monosomy (5%), amplification (≥3 copies; 13%), break-apart (14%), jumping (11%), and others (8%). Such aberrations were present in 49% of low-risk MM (183/376) and in 62% of high-risk MM (51/82) defined by the gene expression profiling cancer-risk model (GEP-70). The GEP also indicated that 74% of MM patients had excessively high levels of MYC overexpression. Nonetheless, there was no significant correlation between the MYC cytogenetic abnormalities and the levels of MYC expression in myeloma tumor cells - unlike the primary IGH translocations that drive transcriptional spikes of its partner genes. MYC expression by GEP was unable to precisely predict the presence of MYC chromosomal rearrangements in MM. In fact, the clinical outcomes of MM patients with MYC aberrations vs. the patients without detectable 8q24 events showed that neither the numerical variations nor the structural rearrangements of 8q24 had significant impact on the overall survival of patients who were treated on Total Therapy protocols. Further, we have shown that chromosomal rearrangements and the levels of MYC overexpression were not intrinsically relevant to the translation of MYC protein in myeloma. Importantly, the fate of myeloma cells was associated with the presence of two endogenous polypeptide-isoforms of MYC - the canonical MYC protein alias (P01106; 48.8kD) and an alternative MYC protein alias (alt-MYC; > 50kD). In newly diagnosed MM, P01106 was absent in most of the patients; whereas, it was a constitutive alias well maintained across the myeloma tumor cells from the patients with relapsed and progressive diseases and in all of the self-renewable myeloma cell lines. In contrast, alt-MYC alias was present as a stand-alone alias or co-existing with P01106 alias in newly diagnosed myeloma. Most importantly, both MYC aliases were absent in some cell lines and primary tumor cells suggesting cancer cell growth may not be dependent on any of MYC aliases despite of high levels of MYC transcription. Since the MYC gene originated from a viral integration, MYC mRNA has dual-translation features due to an internal ribosome entry segment (IRES) at 5'UTR. Previous studies have determined that the translations can be alternatively initiated from an in-frame CUG (Leucine) codon to generate NP_002458 alias or at a downstream canonical cap-dependent AUG (Methionine) codon to generate P01106 alias. Although these two aliases share identical C-termini, the translational switch can extend the N-terminal in NP_002458 alias. In vitro, the switch of P01106 to alt-MYC has resulted in cell growth arrest. Such alternation was induced by a transient gene transfection of P53 tumor suppressor (TP53); seemingly, the alternative MYC alias acts as an antagonist to P01106 in cell proliferation.

Overall, it was generally unpredictable which of the MYC aliases were the dominant driver; therefore, MYC cytogenetic rearrangements and overexpression may have unknown indirect impacts on the outcomes. We are currently examining the precise amino acid sequences on both N-terminal and C-terminal ends of MYC aliases to study the regulatory mechanisms of the translational switching. This may indicate a novel anti-cancer strategy to cease cancer proliferation by deliberately inducing alternative translation of the antagonistic alias of an oncogene, and eventually overcome cooperation of the chromosomal rearrangements and dysregulated oncogene transcriptions in progressive cancer cells.