The role of SWI/SNF chromatin remodelers in the repair of DNA double strand breaks and cancer therapy.

Switch/Sucrose non-fermenting (SWI/SNF) chromatin remodelers hydrolyze ATP to push and slide nucleosomes along the DNA thus modulating access to various genomic loci. These complexes are the most frequently mutated epigenetic regulators in human cancers. SWI/SNF complexes are well known for their function in transcription regulation, but more recent work has uncovered a role for these complexes in the repair of DNA double strand breaks (DSBs).

The SWI/SNF ATPase BRG1 stimulates DNA end resection and homologous recombination by reducing nucleosome density at DNA double strand breaks and by promoting the recruitment of the CtIP nuclease.

DNA double strand breaks (DSBs) are among the most toxic DNA lesions and can be repaired accurately through homologous recombination (HR). HR requires processing of the DNA ends by nucleases (DNA end resection) in order to generate the required single-stranded DNA (ssDNA) regions. The SWI/SNF chromatin remodelers are 10-15 subunit complexes that contain one ATPase (BRG1 or BRM).

The E2F1 transcription factor and RB tumor suppressor moonlight as DNA repair factors.

The E2F1 transcription factor and RB tumor suppressor are best known for their roles in regulating the expression of genes important for cell cycle progression but, they also have transcription-independent functions that facilitate DNA repair at sites of damage. Depending on the type of DNA damage, E2F1 can recruit either the GCN5 or p300/CBP histone acetyltransferases to deposit different histone acetylation marks in flanking chromatin. At DNA double-strand breaks, E2F1 also recruits RB and the BRG1 ATPase to remodel chromatin and promote loading of the MRE11-RAD50-NBS1 complex.

The Retinoblastoma (RB) Tumor Suppressor: Pushing Back against Genome Instability on Multiple Fronts.

The retinoblastoma (RB) tumor suppressor is known as a master regulator of the cell cycle. RB is mutated or functionally inactivated in the majority of human cancers. This transcriptional regulator exerts its function in cell cycle control through its interaction with the E2F family of transcription factors and with chromatin remodelers and modifiers that contribute to the repression of genes important for cell cycle progression.

Cockayne syndrome group B (CSB) protein: at the crossroads of transcriptional networks.

Cockayne syndrome (CS) is a rare genetic disorder characterized by a variety of growth and developmental defects, photosensitivity, cachectic dwarfism, hearing loss, skeletal abnormalities, progressive neurological degeneration, and premature aging. CS arises due to mutations in the CSA and CSB genes. Both gene products are required for the transcription-coupled (TC) branch of the nucleotide excision repair (NER) pathway, however, the severe phenotype of CS patients is hard to reconcile with a sole defect in TC-NER.

Sirt1 suppresses RNA synthesis after UV irradiation in combined xeroderma pigmentosum group D/Cockayne syndrome (XP-D/CS) cells.

Specific mutations in the XPD subunit of transcription factor IIH result in combined xeroderma pigmentosum (XP)/Cockayne syndrome (CS), a severe DNA repair disorder characterized at the cellular level by a transcriptional arrest following UV irradiation. This transcriptional arrest has always been thought to be the result of faulty transcription-coupled repair. In the present study, we showed that, following UV irradiation, XP-D/CS cells displayed a gross transcriptional dysregulation compared with "pure" XP-D cells or WT cells.

E2F1 and p53 transcription factors as accessory factors for nucleotide excision repair.

Many of the biochemical details of nucleotide excision repair (NER) have been established using purified proteins and DNA substrates. In cells however, DNA is tightly packaged around histones and other chromatin-associated proteins, which can be an obstacle to efficient repair. Several cooperating mechanisms enhance the efficiency of NER by altering chromatin structure.

CSA and CSB proteins interact with p53 and regulate its Mdm2-dependent ubiquitination.

Mutations in Cockayne syndrome (CS) A and B genes (CSA and CSB) result in a rare genetic disease that affects the development and homeostasis of a wide range of tissues and organs. We previously correlated the degenerative phenotype of patients to the enhanced apoptotic response, exhibited by CS cells, which is associated with the exceptional induction of p53 protein, upon a variety of stress stimuli. Here we showed that the elevated and persistent levels of p53 displayed by CS cells are due to the insufficient ubiquitination of the p53 protein.

NER factors are recruited to active promoters and facilitate chromatin modification for transcription in the absence of exogenous genotoxic attack.

Upon gene activation, we found that RNA polymerase II transcription machinery assembles sequentially with the nucleotide excision repair (NER) factors at the promoter. This recruitment occurs in absence of exogenous genotoxic attack, is sensitive to transcription inhibitors, and depends on the XPC protein. The presence of these repair proteins at the promoter of activated genes is necessary in order to achieve optimal DNA demethylation and histone posttranslational modifications (H3K4/H3K9 methylation, H3K9/14 acetylation) and thus efficient RNA synthesis.

Deletion of 5' sequences of the CSB gene provides insight into the pathophysiology of Cockayne syndrome.

Cockayne syndrome is an autosomal recessive neurodegenerative disorder characterized by a specific defect in the repair of UV-induced DNA lesions. Most cases of Cockayne syndrome are caused by mutations in the CSB gene but the pathophysiological mechanisms are poorly understood. We report the clinical and molecular data of two severely affected Cockayne patients with undetectable CSB protein and mRNA.

Exocyclic DNA lesions stimulate DNA cleavage mediated by human topoisomerase II alpha in vitro and in cultured cells.

DNA adducts are mutagenic and clastogenic. Because of their harmful nature, lesions are recognized by many proteins involved in DNA repair. However, mounting evidence suggests that lesions also are recognized by proteins with no obvious role in repair processes.

DNA ligation catalyzed by human topoisomerase II alpha.

The DNA ligation reaction of topoisomerase II is essential for genomic integrity. However, it has been impossible to examine many fundamental aspects of this reaction because ligation assays historically required the enzyme to cleave a DNA substrate before sealing the nucleic acid break. Recently, a cleavage-independent DNA ligation assay was developed for human topoisomerase IIalpha [Bromberg, K.