Kozo Tanaka

ATP-dependent chromatin remodeling is essential for replication, transcription, and DNA repair, especially DNA double-strand break (DSB) repair. However, the mechanisms underlying chromatin remodeling remain elusive. Here, we investigated the role of CHRAC17, a component of the chromatin assembly complex (CHRAC), in maintaining genome stability. In response to DSBs, the ISWI-family ATPase complex SNF2H-ACF1 is immediately recruited to DSBs through the H2B-type histone-fold protein CHRAC17, forming CHRAC, which promotes nucleosome assembly and recruits BRCA1 and RAD51 to the DSB site. CHRAC promotes later RAD51-mediated HR steps through additional nucleosome-remodeling activity, including nucleosome sliding activity. CHRAC17, also known as POLE3, is a subunit of DNA polymerase ε (Polε). Polε is recruited to DSBs via CHRAC17, facilitating RAD51-mediated homologous recombination (HR) and conferring resistance to PARP inhibitors. These findings define a mechanism of homologous recombination driven by coordinated regulation of nucleosome dynamics by CHRAC and Polε, thereby ensuring genome stability.

Target of rapamycin complex 1 (TORC1) integrates nutrient signals with cell growth. While its inactivation is known to trigger mitotic slippage via APC/C–Cdh1–dependent securin degradation and separase activation, the molecular basis of cohesion loss during nutrient stress has remained incompletely defined. In budding yeast, we show that TORC1 inactivation elicits a noncanonical, proteasome-dependent degradation of cohesin that is independent of securin and separase. Separase was itself destabilized upon TORC1 inactivation, yet Scc1 degradation persisted even in a separase-resistant mutant. Cohesin degradation proceeds when APC/C is impaired, indicating involvement of an atypical ubiquitin ligase. These results reveal a second, aberrant route to sister chromatid dissociation upon TORC1 inactivation, operating in parallel with the previously reported APC/C–Cdh1 pathway, via the unconventional degradation of mitotic key factors.

Radial glial cells (RGCs) generate cortical neurons and guide radial neuronal migration, yet how microtubule (MT) regulators coordinate progenitor maintenance, mitotic fidelity, and cortical architecture remains unclear. Here, we identify the MT depolymerase Kif2C/MCAK as an essential regulator of RGC integrity with relevance to human neurodevelopmental disorders. Kif2C is enriched in developing cortical RGCs and localizes to radial fibers, basal endfeet, and mitotic structures. Acute Kif2C depletion in embryonic mouse cortices disrupts RG fiber organization, impairs neuronal migration, reduces the RGC pool, and induces mitotic defects, chromosome segregation errors, DNA damage, and cell-cycle arrest. Kif2C -deficient cortices further exhibit focal pial basement membrane disruption and neuronal overmigration, resulting in a cobblestone-like cortical malformation. We identify two individuals with neurodevelopmental disorders carrying rare deleterious KIF2C variants and show that a patient-derived truncating variant fails to rescue Kif2C -deficient cortical phenotypes, implicating KIF2C dysfunction in human neurodevelopmental disorders. Graphical

The cGAS-STING pathway is an innate immune pathway that senses microbial and host-derived double-stranded DNA in the cytosol, and triggers type I interferon responses and proinflammatory cytokine production. Recently, activation of the cGAS-STING pathway in tumors is reported to enhance anti-tumor effects, while it is often inactivated in tumors. Therefore, to develop strategies to activate STING in tumors has become imperative in cancer therapy. Here we report that the cGAS-STING signaling is inactivated in DU145 prostate cancer cell line and find that the inactivation mechanism is distinct from the previously reported mechanisms such as DNA methylation. Treatment of DU145 cells with a lysosomal inhibitor Baf-A1 increased a STING protein level. Knockdown of VPS4, the essential factor in lysosomal microautophagy, suppresses STING degradation. Most importantly, knockdown of cGAS, the enzyme responsible for the production of the endogenous STING ligand cGAMP, fully restores a STING protein level and a response to a synthetic membrane-permeable STING agonist MSA-2. Thus, these results suggest that the constant cGAS activation causes the lysosomal degradation of STING and underlies the unresponsiveness of the STING signaling in DU145 cells. Our results may lead to new strategies to enhance STING-targeted cancer immunotherapy.

Background Polyploid giant cancer cells (PGCCs), characterized by enlarged or multiple nuclei, have long been considered non-proliferative and hallmarks of high malignancy. However, their functional contribution to tumor progression remains unclear. Methods We identified and characterized a subset of mitotically active (MA)-PGCCs in human oral squamous cell carcinoma specimens and cell lines. Mitotic activity and cell cycle was assessed using immunofluorescence, time-lapse microscopy and FUCCI. We evaluated the interactions between MA-PGCCs and cancer-associated fibroblasts (CAFs), focusing on transforming growth factor-beta (TGF-β) signaling. Chemoresistance to 5-fluorouracil (5-FU) was analyzed using cell viability assays. Results MA-PGCCs exhibited both bipolar and multipolar mitosis, generating heterogeneous progeny that contributed to genomic instability. These cells increased the number of CAFs with elevated TGF-β expression, promoting epithelial-mesenchymal transition (EMT) and enhancing resistance to 5-FU. Mechanistically, enhanced reactive oxygen species in MA-PGCCs upregulated urokinase-type plasminogen activator (uPA) and its receptor uPAR, promoting plasmin-mediated activation of TGF-β secreted from adjacent CAFs. Upregulation of TGF-β receptors in MA-PGCCs further amplified TGF-β signaling, accelerating EMT. Conclusions Our findings identify MA-PGCCs as a proliferative subpopulation that promotes EMT and chemoresistance through a TGF-β-uPA/uPAR feedback loop. Targeting this pathway may offer a novel therapeutic strategy for the treatment of aggressive tumors enriched in MA-PGCCs.

Aneuploid cells are known to increase with age. Previously, we demonstrated an increased number of aneuploid fibroblasts isolated from aged mice due to chromosomal instability (CIN), which is caused by oxidative stress. It is unclear whether this phenomenon also occurs in human cells, which are more resistant to oxidative stress than mouse cells. Here, we found that fibroblasts from aged individuals exhibited an increase in aneuploid cells. The frequency of chromosome missegregation and micronuclei increased in these cells, indicating CIN. A DNA fiber assay revealed the presence of replication stress, accompanied by an increase in 53BP1 nuclear bodies and ultrafine bridges. Increased levels of reactive oxygen species derived from mitochondria, along with reduced mitochondrial membrane potential, imply that these cells experienced oxidative stress due to mitochondrial functional decline. Antioxidant treatment reduced the frequency of chromosome missegregation and micronuclei, suggesting that oxidative stress causes CIN. Oxidative stress also causes replication stress, which precedes CIN. Spindle microtubules were stabilized in fibroblasts from aged individuals, which was alleviated by antioxidant treatment. Taken together, these findings suggest that aging-related CIN in human fibroblasts is caused by oxidative stress associated with mitochondrial dysfunction, which induces replication stress that in turn causes CIN through microtubule stabilization. Although human fibroblasts are more resistant to the ambient oxygen environment than mouse fibroblasts, our findings showed that they undergo oxidative stress that causes CIN with age in a manner similar to mouse fibroblasts, revealing a conserved phenomenon in mammalian cells.

Aneuploid cells are known to increase with age. Previously, we demonstrated that aneuploid cells increase in fibroblasts from aged mice due to chromosomal instability (CIN), which is caused by oxidative stress. It is unclear whether this phenomenon also occurs in human cells, which are more resistant to oxidative stress than mouse cells. Here, we found that fibroblasts from aged individuals exhibited an increase in aneuploid cells. The frequency of chromosome missegregation and micronuclei increased in these cells, indicating CIN. A DNA fiber assay revealed the presence of replication stress, accompanied by an increase in 53BP1 nuclear bodies and ultrafine bridges. Increased levels of reactive oxygen species derived from mitochondria, along with reduced mitochondrial membrane potential, imply that these cells experienced oxidative stress due to mitochondrial functional decline. Antioxidant treatment reduced the frequency of chromosome missegregation and micronuclei, suggesting that oxidative stress causes CIN. Oxidative stress also causes replication stress, which precedes CIN. Spindle microtubules were stabilized in fibroblasts from aged individuals, which was alleviated by antioxidant treatment. Taken together, these findings suggest that aging-related CIN in human fibroblasts is caused by oxidative stress associated with mitochondrial dysfunction, which induces replication stress that in turn causes CIN through microtubule stabilization. Although human fibroblasts are more resistant to the ambient oxygen environment than mouse fibroblasts, our findings showed that they undergo oxidative stress that causes CIN with age in a manner similar to mouse fibroblasts, revealing a conserved phenomenon in mammalian cells.

The SWI/SNF chromatin-remodeling complex that comprises multiple subunits orchestrates diverse cellular processes, including gene expression, DNA repair, and DNA replication, by sliding and releasing nucleosomes. AT-interacting domain-rich protein 1A (ARID1A) and ARID1B (ARID1A/B), a pivotal subunit, have significant relevance in cancer management because they are frequently mutated in a broad range of cancer types. To delineate the protein network involving ARID1A/B, we investigated the interactions of this with other proteins under physiological conditions. The ARID domain of ARID1A/B interacts with proteins involved in transcription and DNA/RNA metabolism. Several proteins are responsible for genome integrity maintenance, including DNA-dependent protein kinase catalytic subunit (DNA-PKcs), bound to the armadillo (ARM) domain of ARID1A/B. Introducing a knock-in mutation at the binding amino acid of DNA-PKcs in HCT116 cells reduced the autophosphorylation of DNA-PKcs and the recruitment of LIG4 in response to ionizing radiation. Our findings suggest that within the SWI/SNF complex, ARID1A couples DNA double-strand break repair processes with chromatin remodeling via the ARM domains to directly engage with DNA-PKcs to maintain genome stability.

CHAMP1 (chromosome alignment-maintaining phosphoprotein 1) plays a role in the repair of DNA double-strand breaks (DSBs) by homologous recombination (HR). The CHAMP1 gene is one of the genes mutated in individuals with intellectual disability. The majority of the mutations are premature termination codon (PTC) mutations, while missense mutations have also been reported. How these mutations affect the functions of CHAMP1 has not been clarified yet. Here we investigated the effects of the CHAMP1 mutations on HR. In Epstein-Barr virus-induced lymphoblastoid cells and fibroblasts derived from individuals with CHAMP1 PTC mutations, truncated CHAMP1 proteins of the expected sizes were detected. When DSBs were induced in fibroblasts with PTC mutations, a defect in HR was detected. U2OS cells expressing the CHAMP1 mutants did not show an HR defect in the presence of endogenous wild-type (WT) CHAMP1, whereas they were unable to restore HR activity when CHAMP1 WT was depleted, suggesting that the PTC mutations are loss-of-function mutations. On the other hand, the CHAMP1 mutants with missense mutations restored HR activity when CHAMP1 WT was depleted. In DLD-1 cells, heterozygous depletion of CHAMP1 resulted in an HR defect, indicating haploinsufficiency. These results suggest that CHAMP1 PTC mutations cause an HR defect through a haploinsufficient mechanism, while CHAMP1 missense mutations do not affect the HR function of CHAMP1.