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The oncogene MYC is deregulated and overexpressed in a high variety of human
cancers and is considered an important driver in tumorigenesis. The MYC protein
binds to virtually all active promoters of genes which are also bound by the RNA
Polymerase II (RNAPII). This results in the assumption that MYC is a transcription
factor regulating gene expression. The effects of gene expression are weak and often
differ depending on the tumor entities or MYC levels. These observations could
argue that the oncogene MYC has additional functions independent of altering gene
expression. In relation to this, the high diversity of interaction partners might be
important. One of them is the RNAPII associated Factor I complex (PAF1c).
In this study, direct interaction between PAF1c and MYC was confirmed in an
in-vitro pulldown assay. ChIP sequencing analyses revealed that knockdown of PAF1c
components resulted in reduced MYC occupancy at active promoters. Depletion
or activation as well as overexpression of MYC led to reduced or enhanced global
occupancy of PAF1c in the body of active genes, arguing that MYC and PAF1c
bind cooperatively to chromatin. Upon PAF1c knockdown cell proliferation was
reduced and additionally resulted in an attenuation of activation or repression of
MYC-regulated genes. Interestingly, knockdown of PAF1c components caused an
accumulation in S-phase of cells bearing oncogenic MYC levels. Remarkably, enhanced
DNA damage, measured by elevated gH2AX and pKAP1 protein levels, was observed
in those cells and this DNA damage occurs specifically during DNA synthesis.
Strikingly, MYC is involved in double strand break repair in a PAF1c-dependent
manner at oncogenic MYC levels.
Collectively the data show that the transfer of PAF1c from MYC onto the RNAPII
couples the transcriptional elongation with double strand break repair to maintain
the genomic integrity in MYC-driven tumor cells.
The MYC oncoprotein binds to promoter-proximal regions of virtually all transcribed genes and enhances RNA polymerase II (Pol II) function, but its precise mode of action is poorly understood. Using mass spectrometry of both MYC and Pol II complexes, we show here that MYC controls the assembly of Pol II with a small set of transcription elongation factors that includes SPT5, a subunit of the elongation factor DSIF. MYC directly binds SPT5, recruits SPT5 to promoters, and enables the CDK7-dependent transfer of SPT5 onto Pol II. Consistent with known functions of SPT5, MYC is required for fast and processive transcription elongation. Intriguingly, the high levels of MYC that are expressed in tumors sequester SPT5 into non-functional complexes, thereby decreasing the expression of growth-suppressive genes. Altogether, these results argue that MYC controls the productive assembly of processive Pol II elongation complexes and provide insight into how oncogenic levels of MYC permit uncontrolled cellular growth.