Abstract
BackgroundMYC family members are among the most frequently deregulated oncogenes in human cancers, yet direct therapeutic targeting of MYC in cancer has been challenging thus far. Synthetic lethality provides an opportunity for therapeutic intervention of MYC-driven cancers.MethodsA pooled kinase shRNA library screen was performed and next-generation deep sequencing efforts identified that PRKDC was synthetically lethal in cells overexpressing MYC. Genes and proteins of interest were knocked down or inhibited using RNAi technology and small molecule inhibitors, respectively. Quantitative RT-PCR using TaqMan probes examined mRNA expression levels and cell viability was assessed using CellTiter-Glo (Promega). Western blotting was performed to monitor different protein levels in the presence or absence of RNAi or compound treatment. Statistical significance of differences among data sets were determined using unpaired t test (Mann–Whitney test) or ANOVA.ResultsInhibition of PRKDC using RNAi (RNA interference) or small molecular inhibitors preferentially killed MYC-overexpressing human lung fibroblasts. Moreover, inducible PRKDC knockdown decreased cell viability selectively in high MYC-expressing human small cell lung cancer cell lines. At the molecular level, we found that inhibition of PRKDC downregulated MYC mRNA and protein expression in multiple cancer cell lines. In addition, we confirmed that overexpression of MYC family proteins induced DNA double-strand breaks; our results also revealed that PRKDC inhibition in these cells led to an increase in DNA damage levels.ConclusionsOur data suggest that the synthetic lethality between PRKDC and MYC may in part be due to PRKDC dependent modulation of MYC expression, as well as MYC-induced DNA damage where PRKDC plays a key role in DNA damage repair.Electronic supplementary materialThe online version of this article (doi:10.1186/1471-2407-14-944) contains supplementary material, which is available to authorized users.
Highlights
MYC family members are among the most frequently deregulated oncogenes in human cancers, yet direct therapeutic targeting of MYC in cancer has been challenging far
We identified and confirmed PRKDC (Protein Kinase, DNAactivated, Catalytic polypeptide), a protein kinase with a major role in non-homologous end joining (NHEJ) DNA repair [24,25]), as a novel synthetic lethal target in MYCoverexpressing lung cancer cells
We were specially intrigued by the reduction in PRKDC shRNA levels in LMYC1-overexpressing cells as compared to the isogenic vector control and L-MYC isoform 2 (L-MYC2)-overexpressing cell lines (Figure 1 and Additional file 1: Table S1)
Summary
MYC family members are among the most frequently deregulated oncogenes in human cancers, yet direct therapeutic targeting of MYC in cancer has been challenging far. MYC expression in normal cells including regenerative tissues such as the gastrointestinal tract, skin and bone marrow, raises concern for achieving an acceptable therapeutic index in MYC-targeted therapies. To this end, a dominant-negative MYC mutant (OmoMyc) was developed to inhibit MYC’s interaction with its key binding partner, MAX [8,9,10]. OmoMyc-mediated MYC inhibition led to a dramatic decrease in tumorburden in a murine Kras lung cancer model, with only mild side effects [14,15], suggesting differential MYC dependency between tumor and normal tissues
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