Heterogeneous Expression Patterns of the Minichromosome Maintenance Complex Members in Retinoblastoma Unveil Its Clinical Significance

Purpose

To explore the expression patterns and clinical significance of minichromosome maintenance (MCM) complex members in retinoblastoma (RB).

Methods

Single-cell RNA sequencing datasets from five normal retina, six intraocular, and five extraocular RB samples were integrated to characterize the expression patterns of MCM complex members at the single-cell level. Western blot and quantitative PCR were used to detect the expression of MCM complex members in RB cell lines. Immunohistochemistry was conducted to validate the expression of MCM complex members in RB patient samples and a RB mouse model.

Results

The expression of MCM2-7 is increased in RB tissue, with MCM2/3/7 showing particularly higher levels in extraocular RB. MCM3/7 are abundantly detected in cell types associated with oncogenesis. Both mRNA and protein levels of MCM3/4/6/7 are increased in RB cell lines. Immunohistochemistry further confirmed the elevated expression of MCM3 in extraocular RB, with MCM6 being the most abundantly expressed MCM in RB.

Conclusions

The distinct MCM expression patterns across various RB cell types suggest diverse functional roles, offering valuable insights for targeted therapeutic strategies. The upregulation of MCM3, MCM4, MCM6, and MCM7 in RB, with a specific emphasis on MCM6 as a notable marker, highlights their potential significance.

Minichromosome Maintenance Complex is Required for Checkpoint Kinase 2 Chromatin Loading and its Phosphorylation to DNA Damage Response in SCC-4 Cells

Checkpoint kinase 2 (Chk2) is a significant mediator of diverse responses to DNA damage. The present study was aimed to identify possible interactive proteins of Chk2 and try to clarify the underlying mechanism regarding Chk2 chromatin loading and its phosphorylation to DNA damage response in oral squamous cell carcinoma (OSCC). Differently tagged Chk2 and minichromosome maintenance (MCM) complex (MCM2, MCM3, MCM5, and MCM6) were overexpressed into SCC-4 cells. After 48 h of transfection cell fractionation was performed to localize proteins. In addition, immunoreactive species were detected by immunoprecipitation (IP) and immunoblot (IB) analysis, and protein-protein interaction between Chk2 and MCM complex was ensured by glutathione S-transferase (GST) pull-down assay. Expression of MCM2 and MCM6 was downregulated by small interfering RNA (siRNA), and the chromatin and non-chromatin fraction were analyzed. The expression of Chk2 phosphorylation (pT68-Chk2) was measured after administration of different dosages of siMCM2 (0.5 μg, 1 μg, and 2.5 μg) and camptothecin (CPT). Our results showed that Chk2 directly interacts with MCM2, MCM3, MCM5, and MCM6 in SCC-4 cells. Downregulation of MCM2 and MCM6 markedly reduced Chk2 chromatin fraction, and downregulation of MCM2 decreased the expression of pT68-Chk2 to DNA damage response in a dose manner. Our results suggest that the interaction between Chk2 and MCM complex is required for Chk2 chromatin loading and its phosphorylation to DNA damage response in SCC-4 cells.

Global Proteome Changes in Liver Tissue 6 Weeks after FOLFOX Treatment of Colorectal Cancer Liver Metastases

(1) Oxaliplatin-based chemotherapy for colorectal cancer liver metastasis is associated with sinusoidal injury of liver parenchyma. The effects of oxaliplatin-induced liver injury on the protein level remain unknown. (2) Protein expression in liver tissue was analyzed—from eight patients treated with FOLFOX (combination of fluorouracil, leucovorin, and oxaliplatin) and seven controls—by label-free liquid chromatography mass spectrometry. Recursive feature elimination–support vector machine and Welch t-test were used to identify classifying and relevantly changed proteins, respectively. Resulting proteins were analyzed for associations with gene ontology categories and pathways. (3) A total of 5891 proteins were detected. A set of 184 (3.1%) proteins classified the groups with a 20% error rate, but relevant change was observed only in 55 (0.9%) proteins. The classifying proteins were associated with changes in DNA replication (p < 0.05) through upregulation of the minichromosome maintenance complex and with the innate immune response (p < 0.05). The importance of DNA replication changes was supported by the results of Welch t-test (p < 0.05). (4) Six weeks after FOLFOX treatment, less than 1% of identified proteins showed changes in expression associated with DNA replication, cell cycle entry, and innate immune response. We hypothesize that the changes remain after recovery from FOLFOX treatment injury.

Transient association of MCM complex proteins with the nuclear matrix during initiation of mammalian DNA replication

The minichromosome maintenance complex (MCM2-7) is the putative DNA helicase in eukaryotes, and essential for DNA replication. By applying serial extractions to mammalian cells synchronized by release from quiescence, we reveal dynamic changes to the sub-nuclear compartmentalization of MCM2 as cells pass through late G1 and early S phase, identifying a brief window when MCM2 becomes transiently attached to the nuclear-matrix. The data distinguish 3 states that correspond to loose association with chromatin prior to DNA replication, transient highly stable binding to the nuclear-matrix coincident with initiation, and a post-initiation phase when MCM2 remains tightly associated with chromatin but not the nuclear-matrix. The data suggests that functional MCM complex loading takes place at the nuclear-matrix.