The tumour suppressor p53 as a supporter of DNA replication

by Ina Klusmann
Doctoral thesis
Date of Examination:2018-08-30
Date of issue:2018-11-21
Advisor:Prof. Dr. Matthias Dobbelstein
Referee:PD Dr. Halyna Shcherbata
Referee:PD Dr. Halyna Shcherbata
Persistent Address: http://dx.doi.org/10.53846/goediss-7143

 

 

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Abstract

English

The tumour suppressor protein p53 is well known for its role in maintaining genetic stability by preventing the propagation of damage onto successive generations of cells. Depending on the extent of damage experienced by a cell, p53 promotes cell cycle arrest to allow for DNA repair or induces apoptosis in cases of severe damage. The canonical functions of p53, commonly referred to as the “guardian of the genome”, imply action only after the damage has occurred within a cell. Our work challenges this view by providing evidence pointing to a more direct and pro-active role for p53 in protecting a cellular genome from damage. p53 activation enhances the processivity of DNA replication and reduces replicative stress, whereas the removal of p53 reduces fork progression. This was observed in tumour-derived cells as well as non-transformed murine embryonic fibroblasts with heterozygous or homozygous p53 deletion, and in freshly isolated thymocytes from mice with differential p53 status. These results expand the tumour-suppressive functions of p53 with an ex-ante activity that prevents DNA damage during replication. MDM2, target gene and main negative regulator of p53, acts to support DNA replication downstream of p53. In particular, its RING finger domain with intrinsic E3 ubiquitin ligase activity is required for this non-canonical role of MDM2. We demonstrated a mechanism by which MDM2 and its interaction partners of the Polycomb repressor complexes which mediate changes in chromatin compaction, prevent the formation of R-loops on the DNA template that impair replication progression. The p53-MDM2 axis helps to protect the genome during S phase, by preventing R-loops and hence enhancing replication. Clinically, our study highlights the importance of small molecule inhibitors against MDM2 that inhibit its RING domain on top of inhibitors targeting its interaction with p53 currently evaluated in clinical trials. We propose that the dual inhibition of both these domains of MDM2 using combination treatments would present a more promising chemotherapeutic strategy.
Keywords: p53; Mdm2; Polycomb repressor complex; ubiquitination; histone; RNF2; DNA replication; R-loops
 
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