Splicing together sister chromatids

Mosaic variegated aneuploidy in development, ageing and cancer

Mosaic variegated aneuploidy (MVA) is a rare condition in which abnormal chromosome counts (that is, aneuploidies), affecting different chromosomes in each cell (making it variegated) are found only in a certain number of cells (making it mosaic). MVA is characterized by various developmental defects and, despite its rarity, presents a unique clinical scenario to understand the consequences of chromosomal instability and copy number variation in humans. Research from patients with MVA, genetically engineered mouse models and functional cellular studies have found the genetic causes to be mutations in components of the spindle-assembly checkpoint as well as in related proteins involved in centrosome dynamics during mitosis. MVA is accompanied by tumour susceptibility (depending on the genetic basis) as well as cellular and systemic stress, including chronic immune response and the associated clinical implications.

Isolation of mutant alleles of the U6 snRNA m 6 A methyltransferase Mtl16 and characterization of their genetic interactions with splicing mutants in Schizosaccharomyces pombe

Schizosaccharomyces pombe Dim1 is a conserved essential component of the U4/U6.U5 tri-snRNP complex essential for pre-mRNA splicing. In a synthetic lethal screen with the temperature-sensitive dim1-35 mutant, we isolated multiple alleles of non-essential mtl16 that encodes the U6 snRNA m 6 A methyltransferase. Further genetic analysis revealed strong and specific negative genetic interactions between mtl16 and a mutation in the Dim1 binding partner, Prp31, and between dim1-35 and a mutation in the Prp31 binding partner, Prp6. Our work provides additional tools to study pre-mRNA splicing in S. pombe and biological confirmation of the importance of the Prp6-Prp31-Dim1-U6 snRNA interactions for pre-mRNA splicing.

CDCA4 interacts with IGF2BP1 to regulate lung adenocarcinoma proliferation via the PI3K/AKT pathway

BACKGROUND

Lung adenocarcinomas (LUAD) remain the leading cause of death in many countries. In this study, we investigated the role of division cycle-associated 4 (CDCA4) in the carcinogenesis of LUADs.

METHODS

Real-time fluorescent quantitative polymerase chain reaction and western blot were performed to detect the messenger RNA and protein levels of CDCA4 in cells. Cell counting kit 8, real-time cell analysis, clone formation, EdU assays, and cell-cycle assays were used to preliminarily investigate the proliferation and cell-cycle-related functions of CDCA4 in lung adenocarcinoma. Immunoprecipitation assays were used to identify possible targets of CDCA4. A xenograft model was used to examine how CDCA4 knockdown affects LUAD cells growth in vivo.

RESULTS

We found that the expression of CDCA4 was upregulated in LUAD cell lines. When CDCA4 was knocked out, the ability of LUAD cells to proliferate was dramatically reduced, and the cell cycle was stalled in the S phase. Meanwhile, boosting the CDCA4 expression had the opposite effect. The critical protein levels of phosphatidylinositol 3 kinase (PI3K)/protein kinase B (AKT) signaling pathway were subsequently examined. The findings demonstrated that elevated CDCA4 lowered the phosphate and tensin homolog expression and increased the p-PI3K and p-AKT levels. Moreover, we demonstrated that CDCA4 favorably regulated IGF2BP1, a downstream target. The downregulation of the IGF2BP1 expression could reverse the proliferation promotion effect induced by the CDCA4 overexpression.

CONCLUSIONS

CDCA4 can operate as an oncogenic factor to control the growth of lung adenocarcinoma via the PI3K/AKT pathway.

DHX15-independent roles for TFIP11 in U6 snRNA modification, U4/U6.U5 tri-snRNP assembly and pre-mRNA splicing fidelity

The U6 snRNA, the core catalytic component of the spliceosome, is extensively modified post-transcriptionally, with 2'-O-methylation being most common. However, how U6 2'-O-methylation is regulated remains largely unknown. Here we report that TFIP11, the human homolog of the yeast spliceosome disassembly factor Ntr1, localizes to nucleoli and Cajal Bodies and is essential for the 2'-O-methylation of U6. Mechanistically, we demonstrate that TFIP11 knockdown reduces the association of U6 snRNA with fibrillarin and associated snoRNAs, therefore altering U6 2'-O-methylation. We show U6 snRNA hypomethylation is associated with changes in assembly of the U4/U6.U5 tri-snRNP leading to defects in spliceosome assembly and alterations in splicing fidelity. Strikingly, this function of TFIP11 is independent of the RNA helicase DHX15, its known partner in yeast. In sum, our study demonstrates an unrecognized function for TFIP11 in U6 snRNP modification and U4/U6.U5 tri-snRNP assembly, identifying TFIP11 as a critical spliceosome assembly regulator.

Identification of small molecules that mitigate vincristine-induced neurotoxicity while sensitizing leukemia cells to vincristine

Vincristine (VCR) is one of the most widely prescribed medications for treating solid tumors and acute lymphoblastic leukemia (ALL) in children and adults. However, its major dose-limiting toxicity is peripheral neuropathy that can disrupt curative therapy. Peripheral neuropathy can also persist into adulthood, compromising quality of life of childhood cancer survivors. Reducing VCR-induced neurotoxicity without compromising its anticancer effects would be ideal. Here, we show that low expression of NHP2L1 is associated with increased sensitivity of primary leukemia cells to VCR, and that concomitant administration of VCR with inhibitors of NHP2L1 increases VCR cytotoxicity in leukemia cells, prolongs survival of ALL xenograft mice, but decreases VCR effects on human-induced pluripotent stem cell-derived neurons and mitigates neurotoxicity in mice. These findings offer a strategy for increasing VCR's antileukemic effects while reducing peripheral neuropathy in patients treated with this widely prescribed medication.

Splicing events in the control of genome integrity: role of SLU7 and truncated SRSF3 proteins

Genome instability is related to disease development and carcinogenesis. DNA lesions are caused by genotoxic compounds but also by the dysregulation of fundamental processes like transcription, DNA replication and mitosis. Recent evidence indicates that impaired expression of RNA-binding proteins results in mitotic aberrations and the formation of transcription-associated RNA-DNA hybrids (R-loops), events strongly associated with DNA injury. We identify the splicing regulator SLU7 as a key mediator of genome stability. SLU7 knockdown results in R-loops formation, DNA damage, cell-cycle arrest and severe mitotic derangements with loss of sister chromatid cohesion (SCC). We define a molecular pathway through which SLU7 keeps in check the generation of truncated forms of the splicing factor SRSF3 (SRp20) (SRSF3-TR). Behaving as dominant negative, or by gain-of-function, SRSF3-TR impair the correct splicing and expression of the splicing regulator SRSF1 (ASF/SF2) and the crucial SCC protein sororin. This unique function of SLU7 was found in cancer cells of different tissue origin and also in the normal mouse liver, demonstrating a conserved and fundamental role of SLU7 in the preservation of genome integrity. Therefore, the dowregulation of SLU7 and the alterations of this pathway that we observe in the cirrhotic liver could be involved in the process of hepatocarcinogenesis.

USP15 regulates dynamic protein-protein interactions of the spliceosome through deubiquitination of PRP31

Post-translational modifications contribute to the spliceosome dynamics by facilitating the physical rearrangements of the spliceosome. Here, we report USP15, a deubiquitinating enzyme, as a regulator of protein-protein interactions for the spliceosome dynamics. We show that PRP31, a component of U4 snRNP, is modified with K63-linked ubiquitin chains by the PRP19 complex and deubiquitinated by USP15 and its substrate targeting factor SART3. USP15SART3 makes a complex with USP4 and this ternary complex serves as a platform to deubiquitinate PRP31 and PRP3. The ubiquitination and deubiquitination status of PRP31 regulates its interaction with the U5 snRNP component PRP8, which is required for the efficient splicing of chromosome segregation related genes, probably by stabilizing the U4/U6.U5 tri-snRNP complex. Collectively, our data suggest that USP15 plays a key role in the regulation of dynamic protein-protein interactions of the spliceosome.

The cohesion stabilizer sororin favors DNA repair and chromosome segregation during mouse oocyte meiosis

Maintenance and timely termination of cohesion on chromosomes ensures accurate chromosome segregation to guard against aneuploidy in mammalian oocytes and subsequent chromosomally abnormal pregnancies. Sororin, a cohesion stabilizer whose relevance in antagonizing the anti-cohesive property of Wings-apart like protein (Wapl), has been characterized in mitosis; however, the role of Sororin remains unclear during mammalian oocyte meiosis. Here, we show that Sororin is required for DNA damage repair and cohesion maintenance on chromosomes, and consequently, for mouse oocyte meiotic program. Sororin is constantly expressed throughout meiosis and accumulates on chromatins at germinal vesicle (GV) stage/G2 phase. It localizes onto centromeres from germinal vesicle breakdown (GVBD) to metaphase II stage. Inactivation of Sororin compromises the GVBD and first polar body extrusion (PBE). Furthermore, Sororin inactivation induces DNA damage indicated by positive γH2AX foci in GV oocytes and precocious chromatin segregation in MII oocytes. Finally, our data indicate that PlK1 and MPF dissociate Sororin from chromosome arms without affecting its centromeric localization. Our results define Sororin as a determinant during mouse oocyte meiotic maturation by favoring DNA damage repair and chromosome separation, and thereby, maintaining the genome stability and generating haploid gametes.

SORORIN and PLK1 as potential therapeutic targets in malignant pleural mesothelioma

Malignant pleural mesothelioma (MPM) is an aggressive type of cancer of the thoracic cavity commonly associated with asbestos exposure and a high mortality rate. There is a need for new molecular targets for the development of more effective therapies for MPM. Using quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR) and an RNA interference-based screening, we examined the SORORIN gene as potential therapeutic targets for MPM in addition to the PLK1 gene, which is known for kinase of SORORIN. Following in vitro investigation of the effects of target silencing on MPM cells, cell cycle analyses were performed. SORORIN expression was analyzed immunohistochemically using a total of 53 MPM samples on tissue microarray. SORORIN was found to be overexpressed in the majority of clinical MPM samples and human MPM cell lines as determined by qRT-PCR. Gene suppression of each SORORIN and PLK1 led to growth inhibition in MPM cell lines. Knockdown of SORORIN showed an increased number of G2M-phase population and a larger nuclear size, suggesting mitotic arrest. High expression of SORORIN (SORORIN-H) was found in 50.9% of all the MPM cases, and there is a tendency towards poorer prognosis for the SORORIN-H group but the difference is not significant. Suppression of SORORIN with PLK1 inhibitor BI 6727 showed a combinational growth suppressive effect on MPM cell growth. Given high-dose PLK1 inhibitor induced drug-related adverse effects in several clinical trials, our results suggest inhibition SORORIN-PLK1 axis may hold promise for the treatment of MPMs.