Signalling DNA damage by regulating p53 co-factor activity

Regulation of Tip60-dependent p53 acetylation in cell fate decision

The acetylation of p53 plays an essential role in regulating its transcriptional activity. Nevertheless, how p53 acetylation is modulated in cell fate decision is less understood. We developed a network model to reveal how Tip60-dependent p53 acetylation is regulated to modulate cellular outcome. We proposed that p53 is progressively activated and exhibits distinct dynamics depending on the severity of DNA damage. For mild damage, p53 is primarily activated to trigger cell cycle arrest by transactivating p21, with its concentration showing pulses. For severe damage, p53 is acetylated at Lysine 120 (K120) by Tip60 to trigger apoptosis by inducing PUMA, and its concentration increases to high levels. Several p53-centered feedback loops coordinate to regulate its acetylation status, ensuring a robust decision on cell fate.

Identification of DBC1 as a transcriptional repressor for BRCA1

BACKGROUND

DBC1/KIAA1967 (deleted in breast cancer 1) is a putative tumour-suppressor gene cloned from a heterozygously deleted region in breast cancer specimens. Caspase-dependent processing of DBC1 promotes apoptosis, and depletion of endogenous DBC1 negatively regulates p53-dependent apoptosis through its specific inhibition of SIRT1. Hereditary breast and ovarian cancer susceptibility gene product BRCA1, by binding to the promoter region of SIRT1, is a positive regulator of SIRT1 expression.

METHODS

A physical interaction between DBC1 and BRCA1 was investigated both in vivo and in vitro. To determine the pathophysiological significance of DBC1, its role as a transcriptional factor was studied.

RESULTS

We found a physical interaction between the amino terminus of DBC1 and the carboxyl terminus of BRCA1, also known as the BRCT domain. Endogenous DBC1 and BRCA1 form a complex in the nucleus of intact cells, which is exported to the cytoplasm during ultraviolet-induced apoptosis. We also showed that the expression of DBC1 represses the transcriptional activation function of BRCT by a transient expression assay. The expression of DBC1 also inhibits the transactivation of the SIRT1 promoter mediated by full-length BRCA1.

CONCLUSION

These results revealed that DBC1 may modulate the cellular functions of BRCA1 and have important implications in the understanding of carcinogenesis in breast tissue.

Mice hypomorphic for Atr have increased DNA damage and abnormal checkpoint response

The ATR checkpoint pathway responds to DNA damage during the S/G2 phases of the cell cycle and is activated early in tumorigenesis. Investigation of ATR's role in development and tumorigenesis is complicated by the lethality of homozygous knockout mice and the limited effects of heterozygous deficiency. To overcome this limitation, we sought to create mice with a hypomorphic Atr mutation based on the ATR mutation in the human disease Seckel syndrome-1 (SCKL1). Homozygous SCKL1 mice were generated by targeted knock-in of the A --> G SCKL1 mutation. Western blot and RT-PCR analysis established that homozygotes have no reduction in Atr protein or increase in missplicing as is seen in humans. Thus, the A --> G substitution alone is not sufficient to reproduce in mice the effects that are seen in humans. However, homozygous SCKL1 mice that retain the neo cassette used for targeting have an estimated 66-82% reduction in total Atr protein levels due to missplicing into the neo cassette. Under conditions of APH-induced replication stress, primary fibroblasts from homozygous mice displayed an increase in overall chromosome damage and an increase in gaps and breaks at specific common fragile sites. In addition, mutant cells display a significant delay in checkpoint induction and an increase in DNA damage as assayed by Chk1 phosphorylation and gamma-H2ax levels, respectively. These mice provide a novel model system for studies of Atr deficiency and replication stress.