Hyoung Soo Choi

The MYCC and MYCN loci are each associated with two upstream open reading frames (uORFs) potentially encoding small proteins (9–21 kDa). We previously demonstrated that uORFs mrtl and MYCHEX1 of MYCC are translated, and their protein products may function to regulate the expression of the “parent” oncogene. We hypothesized that a similar relationship might exist between MYCN and its two uORFs: MYCNOT and MNOP, and investigated the uORF‐encoded proteins associated with MYCN to confirm their expression and intracellular location in neuroblastoma and medulloblastoma cells and tissues. MNOP, MYCNOT, mrtl, and MYCHEX1 were readily detected via reverse transcription polymerase chain reaction and Western blot analysis in tumor cell lines. In tumor tissue, MNOP protein expression was confirmed; however, MCYNOT generated from alternative splicing MYCNΔ1b mRNA was not detected. Immunofluorescence staining of MYCNOT displayed multiple bright foci in the nucleus and diffuse staining in the cytoplasm, suggesting that this small protein may function in both the nucleus and cytoplasm. Upon JQ1 treatment, MYCN, MYCNOT, and mrtl decreased substantially or disappeared completely in three different tumor cell lines. Significant levels of apoptosis were observed in each pediatric embryonal tumor cell line but not T47D breast carcinoma cells, suggesting that response to JQ1 transcriptional inhibition is greatest in tumor cells, which depend on MYC to maintain an undifferentiated phenotype. In conclusion, both MYCN uORF‐encoded proteins MNOP and MYCNOT, together with the two MYCC uORF‐encoded proteins mrtl and MYCHEX1 were detected simultaneously in tumor cell lines and tumor tissues. These four distinct proteins are translated from the “5′‐untranslated region” of MYCN or MYCC mRNA and display consistent distribution patterns within the cell. Additional studies to further elucidate the physiological and pathological roles of these uORF‐encoded proteins are warranted, as insights gained could inform new strategies for modulating MYC‐family oncogenes by targeting their uORFs.