Holliday junction-binding activity of human SPF45

Mechanism of Musashi2 affecting radiosensitivity of lung cancer by modulating DNA damage repair

Identifying new targets for overcoming radioresistance is crucial for improving the efficacy of lung cancer radiotherapy, given that tumor cell resistance is a leading cause of treatment failure. Recent research has spotlighted the significance of Musashi2 (MSI2) in cancer biology. In this study, we first demonstrated that MSI2 plays a key function in regulating the radiosensitivity of lung cancer. The expression of MSI2 is negatively correlated with overall survival in cancer patients, and the knockdown of MSI2 inhibits tumorigenesis and increases radiosensitivity of lung cancer cells. Cellular radiosensitivity, which is closely linked to DNA damage, is influenced by MSI2 interaction with ataxia telangiectasia mutated and Rad3-related kinase (ATR) and checkpoint kinase 1 (CHK1) post-irradiation; moreover, knockdown of MSI2 inhibits the ATR-mediated DNA damage response pathway. RNA-binding motif protein 17 (RBM17), which is implicated in DNA damage repair, exhibits increased interaction with MSI2 post-irradiation. We found that knockdown of RBM17 disrupted the interaction between MSI2 and ATR post-irradiation and increased the radiosensitivity of lung cancer cells. Furthermore, we revealed the potential mechanism of MSI2 recruitment into the nucleus with the assistance of RBM17 to activate ATR to promote radioresistance. This study provides novel insights into the potential application of MSI2 as a new target in lung cancer radiotherapy.

Alternative splicing regulation of cell-cycle genes by SPF45/SR140/CHERP complex controls cell proliferation

The regulation of pre-mRNA processing has important consequences for cell division and the control of cancer cell proliferation, but the underlying molecular mechanisms remain poorly understood. We report that three splicing factors, SPF45, SR140, and CHERP, form a tight physical and functionally coherent complex that regulates a variety of alternative splicing events, frequently by repressing short exons flanked by suboptimal 3' splice sites. These comprise alternative exons embedded in genes with important functions in cell-cycle progression, including the G2/M key regulator FOXM1 and the spindle regulator SPDL1. Knockdown of either of the three factors leads to G2/M arrest and to enhanced apoptosis in HeLa cells. Promoting the changes in FOXM1 or SPDL1 splicing induced by SPF45/SR140/CHERP knockdown partially recapitulates the effects on cell growth, arguing that the complex orchestrates a program of alternative splicing necessary for efficient cell proliferation.

Vertebrate Spt2 is a novel nucleolar histone chaperone that assists in ribosomal DNA transcription

In eukaryotes, transcription occurs in the chromatin context with the assistance of histone-binding proteins, such as chromatin/nucleosome remodeling factors and histone chaperones. However, it is unclear how each remodeling factor or histone chaperone functions in transcription. Here, we identify a novel histone-binding protein, Spt2, in higher eukaryotes. Recombinant human Spt2 binds to histones and DNA, and promotes nucleosome assembly in vitro. Spt2 accumulates in nucleoli and interacts with RNA polymerase I in chicken DT40 cells, suggesting its involvement in ribosomal RNA transcription. Consistently, Spt2-deficient chicken DT40 cells are sensitive to RNA polymerase I inhibitors and exhibit decreased transcription activity, as shown by a transcription run-on assay. Domain analyses of Spt2 revealed that the C-terminal region, containing the region homologous to yeast Spt2, is responsible for histone binding, while the central region is essential for nucleolar localization and DNA binding. Based on these results, we conclude that vertebrate Spt2 is a novel histone chaperone with a separate DNA-binding domain that facilitates ribosomal DNA transcription through chromatin remodeling during transcription.

DNA robustly stimulates FANCD2 monoubiquitylation in the complex with FANCI

FANCI and FANCD2 form a complex, and play essential roles in the repair of interstrand DNA crosslinks (ICLs) by the Fanconi anemia (FA) pathway. FANCD2 is monoubiquitylated by the FA core complex, composed of 10 FA proteins including FANCL as the catalytic E3 subunit. FANCD2 monoubiquitylation can be reconstituted with purified minimal components, such as FANCI, E1, UBE2T (E2) and FANCL (E3) in vitro; however, its efficiency is quite low as compared to the in vivo monoubiquitylation of FANCD2. In this study, we found that various forms of DNA, such as single-stranded, double-stranded and branched DNA, robustly stimulated the FANCD2 monoubiquitylation in vitro up to a level comparable to its in vivo monoubiquitylation. This stimulation of the FANCD2 monoubiquitylation strictly required FANCI, suggesting that FANCD2 monoubiquitylation may occur in the FANCI–FANCD2 complex. A FANCI mutant that was defective in DNA binding was also significantly defective in FANCD2 monoubiquitylation in vitro. In the presence of 5′ flapped DNA, a DNA substrate mimicking the arrested replication fork, about 70% of the input FANCD2 was monoubiquitylated, while less than 1% FANCD2 monoubiquitylation was observed in the absence of the DNA. Therefore, DNA may be the unidentified factor required for proper FANCD2 monoubiquitylation.

Regulation of chemoresistance via alternative messenger RNA splicing

The acquisition of resistance to chemotherapy is a significant problem in the treatment of cancer, greatly increasing patient morbidity and mortality. Tumors are often sensitive to chemotherapy upon initial treatment, but repeated treatments can select for those cells that were able to survive initial therapy and have acquired cellular mechanisms to enhance their resistance to subsequent chemotherapy treatment. Many cellular mechanisms of drug resistance have been identified, most of which result from changes in gene and protein expression. While changes at the transcriptional level have been duly noted, it is primarily the post-transcriptional processing of pre-mRNA into mature mRNA that regulates the composition of the proteome and it is the proteome that actually regulates the cell's response to chemotherapeutic insult, inducing cell survival or death. During pre-mRNA processing, intronic non-protein-coding sequences are removed and protein-coding exons are spliced to form a continuous template for protein translation. Alternative splicing involves the differential inclusion or exclusion of exonic sequences into the mature transcript, generating different mRNA templates for protein production. This regulatory mechanism enables the potential to produce many different protein isoforms from the same gene. In this review I will explain the mechanism of alternative pre-mRNA splicing and look at some specific examples of how splicing factors, splicing factor kinases and alternative splicing of specific pre-mRNAs from genes have been shown to contribute to acquisition of the drug resistant phenotype.

The splicing-factor related protein SFPQ/PSF interacts with RAD51D and is necessary for homology-directed repair and sister chromatid cohesion

DNA double-stranded breaks (DSBs) are among the most severe forms of DNA damage and responsible for chromosomal translocations that may lead to gene fusions. The RAD51 family plays an integral role in preserving genome stability by homology directed repair of DSBs. From a proteomics screen, we recently identified SFPQ/PSF as an interacting partner with the RAD51 paralogs, RAD51D, RAD51C and XRCC2. Initially discovered as a potential RNA splicing factor, SFPQ was later shown to have homologous recombination and non-homologous end joining related activities and also to bind and modulate the function of RAD51. Here, we demonstrate that SFPQ interacts directly with RAD51D and that deficiency of both proteins confers a severe loss of cell viability, indicating a synthetic lethal relationship. Surprisingly, deficiency of SFPQ alone also leads to sister chromatid cohesion defects and chromosome instability. In addition, SFPQ was demonstrated to mediate homology directed DNA repair and DNA damage response resulting from DNA crosslinking agents, alkylating agents and camptothecin. Taken together, these data indicate that SFPQ association with the RAD51 protein complex is essential for homologous recombination repair of DNA damage and maintaining genome integrity.