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DNA replication regulation protein Mcm7 as a marker of proliferation in prostate cancer.

BACKGROUND: Recent studies have shown that minichromosome maintenance (MCM) proteins (Mcm2-7) may be useful proliferation markers in dysplasia and cancer in various tissues. AIMS: To investigate the use of Mcm7 as a proliferation marker in 79 lymph node negative prostate cancers and compare it with Ki-67, a commonly used cell proliferation marker. METHODS: The percentage of proliferating cells (proliferation index; PI) was calculated for basal and luminal epithelial cells in benign prostate tissue, prostatic intraepithelial neoplasia (PIN), and epithelial cells in adenocarcinoma. The PI for each biomarker was correlated with the preoperative prostate specific antigen concentration, the Gleason score, surgical resection margin status, and the AJCC pT stage for each patient. RESULTS: The mean PIs for Ki-67 and Mcm7 were: benign luminal epithelium 0.7 and 1.2 and benign basal epithelium 0.8 and 8.2; PIN non-basal epithelium 4.9 and 10.6 and PIN basal epithelium 0.7 and 3.1; adenocarcinoma 9.8 and 22.7, respectively. Mcm7 had a significantly higher mean PI (p<0.0001) than Ki-67 for all cell categories except benign luminal epithelial cells. Mcm7 was a better discriminatory marker of proliferation between benign epithelium, PIN, and invasive adenocarcinoma (p<0.0001) than Ki-67. The drop in Mcm7 mean basal cell PI from benign epithelium to PIN epithelium was significantly larger than for Ki-67 (p<0.0001). Mcm7 had a significantly higher PI than Ki-67 at each risk level. CONCLUSION: Mcm7 may be a useful proliferation marker in prostatic neoplasia and warrants further evaluation as a complementary tool in the diagnosis of PIN and prostate carcinoma.

Adenocarcinoma↗

Critical roles for non-pRb targets of human papillomavirus type 16 E7 in cervical carcinogenesis.

High-risk human papillomaviruses (HPV) encode two oncogenes, E6 and E7, expressed in nearly all cervical cancers. In vivo, HPV-16 E7 has been shown to induce multiple phenotypes in the context of transgenic mice, including cervical cancer. E7 is a multifunctional protein known best for its ability to inactivate the tumor suppressor pRb. To determine the importance of pRb inactivation by E7 in cervical cancer, we pursued studies with genetically engineered mice. E7 expression in estrogen-treated murine cervix induced dysplasia and invasive cancers as reported previously, but targeted Rb inactivation in cervical epithelium was not sufficient to induce any cervical dysplasia or neoplasia. Furthermore, E7 induced cervical cancer formation even when the E7-pRb interaction was disrupted by the use of a knock-in mouse carrying an E7-resistant mutant Rb allele. pRb inactivation was necessary but not sufficient for E7 to overcome differentiation-induced or DNA damage-induced cell cycle arrest, and expression patterns of the E2F-responsive genes Mcm7 and cyclin E indicate that other E2F regulators besides pRb are important targets of E7. Together, these data indicate that non-pRb targets of E7 play critical roles in cervical carcinogenesis.

Alleles↗

Mitogenic effects of the up-regulation of minichromosome maintenance proteins in anaplastic thyroid carcinoma.

CONTEXT: Anaplastic thyroid carcinomas (ATC) are among the most aggressive human malignancies and are characterized by high mitotic activity. Minichromosome maintenance proteins (MCM) 2-7 are required to initiate eukaryotic DNA replication, and their overexpression has been associated with dysplasia and malignancy. OBJECTIVE: In an attempt to cast light on the mechanisms governing ATC, we evaluated MCM5 and MCM7 expression in human normal, papillary (PTC), and anaplastic thyroid samples, as well as in primary culture cells and transgenic mouse models. RESULTS: MCM5 and MCM7 expression was high in 65% of ATC and negligible in normal thyroid tissue and papillary thyroid carcinomas. In ATC, high MCM5 and MCM7 expression was paralleled by high levels of MCM2 and MCM6. An analysis of human ATC primary cell cultures and of a transgenic mouse model of ATC confirmed these findings. An increased transcription rate accounted for MCM7 up-regulation, because the activity of the MCM7 promoter was more than 10-fold higher in ATC cells compared with normal thyroid cells. Adoptive overexpression of wild-type p53, but not of its inactive (R248W and R273H) mutants, strongly down-regulated transcription from the MCM7 promoter, suggesting that p53 knock-out contributes to MCM7 up-regulation in ATC. Treatment with small inhibitory duplex RNAs, which decrease MCM7 protein levels, reduced the rate of DNA synthesis in ATC cells. CONCLUSION: MCM proteins are overexpressed in ATC and sustain the high proliferative capacity of ATC cells.

Animals↗

The essential Mcm7 protein PROLIFERA is localized to the nucleus of dividing cells during the G(1) phase and is required maternally for early Arabidopsis development.

PROLIFERA (PRL) encodes a homologue of the DNA replication licensing factor Mcm7, a highly conserved protein found in all eukaryotes. Insertions in the PROLIFERA gene are lethal, resulting in decreased transmission through the female gametophyte, and homozygous embryonic lethality. We show here that PROLIFERA is specifically expressed in populations of dividing cells in sporophytic tissues of the plant body, such as the palisade layer of the leaf and founder cells of initiating flower primordia. Gene fusions with the green fluorescent protein (GFP) reveal that the PROLIFERA protein accumulates during the G(1) phase of the cell cycle, and is transiently localized to the nucleus. During mitosis, the fusion protein rapidly disappears, returning to daughter nuclei during G(1). PROLIFERA::GUS fusions are strongly expressed in the central cell nucleus of mature megagametophytes, which have a variety of arrest points reflecting a leaky lethality. Expression is also observed in the endosperm of mutant prl embryo sacs that arrest following fertilization. Crosses with wild-type pollen result in occasional embryonic lethals that also stain for GUS activity. In contrast, embryos resulting from crosses of wild-type carpels with PRL::GUS pollen do not stain and are phenotypically normal. In situ hybridization of GUS fusion RNA indicates transcription is equivalent from maternally and paternally derived alleles, so that accumulation of maternally derived gametophytic protein is likely to be responsible for the 'maternal' effect.

Arabidopsis↗

Gene expression profiling of microdissected pancreatic ductal carcinomas using high-density DNA microarrays.

Pancreatic ductal adenocarcinoma (PDAC) remains an important cause of malignancy-related death and is the eighth most common cancer with the lowest overall 5-year relative survival rate. To identify new molecular markers and candidates for new therapeutic regimens, we investigated the gene expression profile of microdissected cells from 11 normal pancreatic ducts, 14 samples of PDAC, and 4 well-characterized pancreatic cancer cell lines using the Affymetrix U133 GeneChip set. RNA was extracted from microdissected samples and cell lines, amplified, and labeled using a repetitive in vitro transcription protocol. Differentially expressed genes were identified using the significance analysis of microarrays program. We found 616 differentially expressed genes. Within these, 140 were also identified in PDAC by others, such as Galectin-1, Galectin-3, and MT-SP2. We validated the differential expression of several genes (e.g., CENPF, MCM2, MCM7, RAMP, IRAK1, and PTTG1) in PDAC by immunohistochemistry and reverse transcription polymerase chain reaction. We present a whole genome expression study of microdissected tissues from PDAC, from microdissected normal ductal pancreatic cells and pancreatic cancer cell lines using high-density microarrays. Within the panel of genes, we identified novel differentially expressed genes, which have not been associated with the pathogenesis of PDAC before.

Aged↗

The MCM complex: its role in DNA replication and implications for cancer therapy.

The MCM complex controls the once per cell cycle DNA replication in eukaryotic cells. In a process known as DNA replication licensing, it primes chromatin for DNA replication by binding origins of DNA replication during the late M to early G1 phase of the cell cycle. Activated by S phase promoting protein kinases, the origin-bound MCM complexes unwind the double stranded DNA at the origins, recruit DNA polymerases and initiate DNA synthesis. Coupled with the initiation of DNA replication in the S phase, the MCM complexes move away from replication origins as a component of the DNA replication fork, likely serving as DNA helicases. Their departure deprives replication origins the ability to re-initiate DNA replication for the reminder of the cell cycle. Because of its vital role in genome duplication in proliferating cells, deregulation of the MCM function results in chromosomal defects that may contribute to tumorigenesis. The MCM proteins are highly expressed in malignant human cancers cells and pre-cancerous cells undergoing malignant transformation. They are not expressed in differentiated somatic cells that have been withdrawn from the cell cycle. Therefore, these proteins are ideal diagnostic markers for cancer and promising targets for anti-cancer drug development. In this article, I will overview the structures and functions of the MCM complex with an effort to integrate insights from recent biochemical and structural studies. Discussions will also cover activities and structures of the complex that may be useful for the development of drug screens.

Animals↗

Dual functional regulators coordinate DNA replication and gene expression in proliferating cells.

Gene products for cell growth must meet the pace of DNA replication and vice versa during the cell division cycle, therefore coordination of DNA replication and gene expression is vital to proliferating cells. During development in multicellular organisms when rapid cell divisions must be accompanied by the expression of particular gene sets in differentiating tissues, this coordination is even more crucial. Undoubtedly, multiple strategies are used to ensure the coordination of gene expression and DNA replication. In this review, we focus on the strategy that uses dual functional factors to serve both the functions of replication initiator and transcription regulator. Classical examples are the dual functional replication initiator/transcription regulators, DnaA of E. coli and T antigen of SV40, which bind replication origins and regulate their own synthesis. Emerging examples in eukaryotes are the growth responsive transcription factor E2f, the MADS domain combinatorial transcription factor Mcm1, and a subunit of the MCM2-7 helicase, Mcm7.

Animals↗

Minichromosome maintenance proteins as biological markers of dysplasia and malignancy.

Dysplasia, an intermediate stage in the progression from normal tissue to neoplasia, is defined morphologically by a loss of normal orientation between epithelial cells, with changes in cellular and nuclear shape and size. However, little is known about the functional properties of dysplastic cells, including their replicative state, largely due to a lack of available biological markers. We have used novel antibodies against minichromosome maintenance (MCM) proteins to examine the proliferative status of a range of histological lesions and to characterize dysplastic cells in functional terms. Immunoperoxidase staining was used to localize the MCM proteins, components of the prereplicative complex that is essential for initiating eukaryotic DNA replication. These proteins are down-regulated in cells undergoing differentiation or quiescence and, thus, serve as specific markers for proliferating cells. In normal and some reactive tissues, MCM expression was present only in restricted proliferative compartments, consistent with our published findings in the uterine cervix. In dysplastic and malignant tissues, in contrast, MCM proteins were expressed in the majority of cells, extending to surface layers of dysplastic stratified epithelia. In carcinomas, the frequency of expression of MCM proteins showed an inverse correlation with the degree of tumor differentiation. Thus, we suggest that dysplastic cells may be characterized in functional terms as remaining in cell cycle, due to deregulation of normal controls over cell proliferation. Antibodies against MCM proteins have potential clinical applications, for example, in the assessment of tumor prognosis in histological sections and the identification of proliferating cells in clinical samples using biochemical or cytological assays.

Biomarkers, Tumor↗

Protein-protein interaction of FHL2, a LIM domain protein preferentially expressed in human heart, with hCDC47.

In the yeast two-hybrid library screening, the heart-specific FHL2 protein was found to interact with hCDC47. In vitro interaction study between FHL2 protein and hCDC47 was demonstrated. From the results of domain studies by the yeast two-hybrid assay, the second and third LIM domains in conjunction with the first half LIM domain of FHL2 were identified to be important in binding with hCDC47. Besides, in Northern blot hybridization of human cancer cell lines, the highest FHL2 mRNA expression was detected in colorectal adenocarcinoma SW480 and HeLa cell S3. Our results imply that FHL2 protein may associate with cancer development and may act as a molecular adapter to form a multicomplex with hCDC47 in the nucleus, thus it plays an important role in the specification or maintenance of the terminal differentiated phenotype of heart muscle cells.

Base Sequence↗

Minichromosome maintenance protein MCM7 is a direct target of the MYCN transcription factor in neuroblastoma.

The MYCN oncogene is amplified in approximately 25% of neuroblastoma tumors and is the most significant negative prognostic factor. The direct transcriptional targets of MYCN in MYCN-amplified tumors have not been defined. Microarray analysis of RNA from neuroblastoma primary cell cultures revealed 10-fold higher MCM7 expression in MYCN-amplified versus nonamplified tumors. MCM7 is an essential component of DNA replication licensing factor, a hexameric protein complex that regulates DNA synthesis during the cell cycle, preventing rereplication and ensuring maintenance of DNA euploidy. Additional experiments demonstrated markedly increased expression of MCM7 RNA and protein in MYCN-amplified neuroblastoma tumors and cell lines. Induction of MYCN in conditional cell lines results in increased expression of endogenous MCM7 mRNA and a 3-fold increase in protein levels. In addition, luciferase activity from MCM7 promoter/luciferase gene reporter constructs was significantly increased under MYCN-induced conditions. Specific electrophoretic mobility shifts of MCM7 promoter sequences are detected in extracts of MYCN-amplified cells. These findings demonstrate that in neuroblastoma, the MYCN oncogene directly activates genes required for DNA replication, and this may contribute to neoplastic transformation of these MYCN-amplified tumors.

Cell Cycle Proteins↗

[Initiation of DNA replication in higher eukaryotes].

In this review, of problems concerning initiation of DNA replication in higher eukaryotes is discussed, with special emphasis on the methods of replication origin mapping and biological tests for the activity of DNA replication origins in higher eukaryotes. Protein factors interacting with replication origins are considered in detail. The main events of replication initiation in higher eukaryotes are briefly analyzed. New data on the control of replication timing of large genomic regions are discussed.

Animals↗

Identification, characterization and chromosomal localization of the cognate human and murine DBF4 genes.

The kinase Dbf4p/Cdc7p is required for the G1/S phase transition during the cell cycle and plays a direct role in the activation of individual origins of replication in Saccharomyces cerevisiae. Here, we report the identification and characterization of mouse and human cDNAs whose products are related in sequence to Saccharomyces cerevisiae DBF4 cDNA. Both mammalian Dbf4 proteins contain a putative site for phosphorylation by CDK, PEST protease cleavage sites, nuclear localization signals and a short-looped zinc finger-like domain. Transcription of MmDBF4 is suppressed in mouse NIH3T3 fibroblasts made quiescent by serum starvation. Upon replenishment of the medium, transcript levels increase during progression through G1, peaking as cells enter S phase. MmDbf4p interacts physically with Cdc7p and Mcm2p in vivo. Using fluorescence in situ hybridization (FISH), the human DBF4 gene was localized to chromosome 7 (q21.3), whereas FISH mapped the murine counterpart to band A2 on chromosome 5. The results of chromosome mapping indicate that in both mouse and human the gene is present as a single copy. The structural conservation between Dbf4-related proteins suggests that these proteins play a key role in the regulation of DNA replication during the cell cycle in all eukaryotes.

3T3 Cells↗

Isolation and mapping of a human gene (MCM2) encoding a product homologous to yeast proteins involved in DNA replication.

From a human fetal-lung cDNA library, we isolated a gene, human MCM2, whose nucleotide sequence predicts a protein product homologous to the yeast nuclear proteins MCM2, MCM3, CDC21, and CDC46. Determination of cDNA sequences covering the entire coding region demonstrated an open reading frame of 1,629 nucleotides encoding 543 amino acids. Sequence comparison revealed 30-40% amino acid identity with these four yeast proteins, all of which are considered to play important roles in DNA replication. Analysis by Northern blotting revealed that the human MCM2 gene is expressed ubiquitously in normal tissues. Furthermore, we localized this gene to chromosomal bands 7q21.3-->q22.1 by fluorescence in situ hybridization.

Amino Acid Sequence↗

The RLF-B component of the replication licensing system is distinct from Cdc6 and functions after Cdc6 binds to chromatin.

Replication licensing factor (RLF) is an essential initiation factor that can prevent re-replication of DNA in a single cell cycle [1] [2]. It is required for the initiation of DNA replication, binds to chromatin early in the cell cycle, is removed from chromatin as DNA replicates and is unable to re-bind replicated chromatin until the following mitosis. Chromatography of RLF from Xenopus extracts has shown that it consists of two components termed RLF-B and RLF-M [3]. The RLF-M component consists of complexes of all six Xenopus minichromosome maintenance (MCM/P1) proteins (XMcm2-7), which bind to chromatin in late mitosis and are removed as replication occurs [3] [4] [5] [6] [7]. The identity of RLF-B is currently unknown. At least two factors must be present on chromatin before licensing can occur: the Xenopus origin recognition complex (XORC) [8] [9] and Xenopus Cdc6 (XCdc6) [10]. XORC saturates Xenopus sperm chromatin at approximately one copy per replication origin whereas XCdc6 binds to chromatin only if XORC is bound first [9] [10] [11]. Although XORC has been shown to be a distinct activity from RLF-B [9], the relationship between XCdc6 and RLF-B is currently unclear. Here, we show that active XCdc6 is loaded onto chromatin in extracts with defective RLF, and that both RLF-M and RLF-B are still required for the licensing of XCdc6-containing chromatin. Furthermore, RLF-B can be separated from XCdc6 by immunoprecipitation and standard chromatography. These experiments demonstrate that RLF-B is both functionally and physically distinct from XCdc6, and that XCdc6 is loaded onto chromatin before RLF-B function is executed.

Animals↗

Minichromosome maintenance protein 2 expression in normal kidney and renal cell carcinomas: relationship to tumor dormancy and potential clinical utility.

PURPOSE: A major problem in the management of patients with renal cell carcinoma is predicting tumor behavior. In the search for more accurate markers of prognosis, tumor cell proliferation has been investigated. These studies have mostly used antibodies directed against Ki-67 or proliferating cell nuclear antigen and have given conflicting results, findings that are likely because of a combination of specificity and methodological differences. Minichromosome maintenance (Mcm) proteins are a series of closely related proteins that are components of the prereplicative complex. The Mcm proteins are essential for initiating eukaryotic DNA replication and serve as useful markers of proliferating cells. EXPERIMENTAL DESIGN: The aims of this study were to determine the frequency and pattern of Mcm2 expression by immunohistochemistry in normal kidney (n = 10) and renal tumors [n = 56; clear cell n = 36; chromophil (papillary) n = 7; oncocytoma n = 5; and transitional cell carcinoma n = 8], compare its sensitivity to the established proliferation marker Ki-67, examine for differences in tumors derived from stable and labile epithelial cell populations in the kidney, and assess the relationship of Mcm2 proliferation to clinicopathological characteristics of kidney tumors. In addition, to additionally investigate the issue of tumor dormancy we wished to assess the relationship between Mcm2 labeling index (LI) and the angiogenic factors angiopoietin-1 (Ang) and Ang-2. RESULTS: In normal tissues, Mcm2 nuclear labeling was identified in both glomeruli (LI median 0.35%; range 0-1.7) and renal tubules (LI median 0.3%; range 0.1-2.9%). In tumors Mcm2 labeling was predominantly at the periphery with LIs ranging from 0.2-91.5%, which was significantly greater than Ki-67 LI (0.2-40.5%; P < 0.001). Mcm2 LI was also significantly higher in tumors derived from a labile epithelium (transitional cell carcinomas) than a stable epithelium (renal cell carcinomas; P = 0.013). A significant association was also demonstrated between Mcm2 LI and tumor grade (P = 0.0006), and angiogenic phenotype (defined by Ang expression; P = 0.03) but not with patient age (P = 0.84), patient sex (P = 0.25), tumor size (P = 0.74), or stage (P = 0.33). Furthermore, although not significant, a survival analysis demonstrated that 100% of patients with a low Mcm2 LI survived compared with 84% of those with a high Mcm2 LI over the follow-up period (up to 53.2 months; P = 0.14). CONCLUSIONS: This is the first study examining Mcm2 protein in normal and tumor kidney samples, and the first to perform histological subgroup analysis. It shows that Mcm2 is a superior marker to Ki-67 in the assessment of cell cycle entry in histological archival material and that normal kidney has a subset of cells within the glomerular and tubular compartments that are in cycle. It demonstrates that the frequency of cells in cycle in tumors formed from stable or labile epithelial populations mirrors that in the nonneoplastic epithelium. This study additionally demonstrates that the number of cells in cycle in tumors is limited by the angiogenic phenotype and supports animal models that show angiogenesis determines the likelihood of tumor dormancy. Additional study to confirm the clinical utility of Mcm2 as a prognostic marker is now indicated.

Adult↗

Phosphorylation of Mcm4 at specific sites by cyclin-dependent kinase leads to loss of Mcm4,6,7 helicase activity.

Mcm proteins that play an essential role in eukaryotic DNA replication are phosphorylated in vivo, and cyclin-dependent protein kinase is at least in part responsible for the phosphorylation of Mcm4. Our group reported that the DNA helicase activity of Mcm4,6,7 complex, which may be involved in initiation of DNA replication, is inhibited following phosphorylation by Cdk2/cyclin A in vitro. Here, we further examined the interplay between mouse Mcm4,6,7 complex and cyclin-dependent kinases and determined the sites required for the phosphorylation of Mcm4. Six Ser and Thr residues, in all, were required for the phosphorylation. Inhibition of Mcm4,6,7 helicase activity by Cdk2/cyclin A was largely relieved by introducing mutations in these residues of Mcm4. Anti-phosphothreonine antibodies raised against one of these sites reacted with Mcm4 prepared from HeLa cells at mitotic phase but did not bind to those at G(1) and G(1)/S, suggesting that this site is mainly phosphorylated in the mitotic phase. Mcm4,6,7 complex purified from HeLa cells at the mitotic phase exhibited a low level of DNA helicase activity, compared with the complexes prepared from cells at other phases. These results suggest that phosphorylation of Mcm4 at specific sites leads to loss of Mcm4,6,7 DNA helicase activity.

Amino Acid Sequence↗

The spatio-temporal organization of DNA replication sites is identical in primary, immortalized and transformed mammalian cells.

We investigated the organization of DNA replication sites in primary (young or presenescent), immortalized and transformed mammalian cells. Four different methods were used to visualize replication sites: in vivo pulse-labeling with 5-bromo-2'-deoxyuridine (BrdU), followed by either acid depurination, or incubation in nuclease cocktail to expose single-stranded BrdU-substituted DNA regions for immunolabeling; biotin-dUTP labeling of nascent DNA by run-on replication within intact nuclei and staining with fluorescent streptavidin; and, finally, immunolabeling of the replication fork proteins PCNA and RPA. All methods produced identical results, demonstrating no fundamental differences in the spatio-temporal organization of replication patterns between primary, immortal or transformed mammalian cells. In addition, we did not detect a spatial coincidence between the early firing replicons and nuclear lamin proteins, the retinoblastoma protein or the nucleolus in primary human and rodent cells. The retinoblastoma protein does not colocalize in vivo with members of the Mcm family of proteins (Mcm2, 3 and 7) at any point of the cell cycle and neither in the chromatin-bound nor in the soluble nucleoplasmic fraction. These results argue against a direct role for the retinoblastoma or nuclear lamin proteins in mammalian DNA synthesis under normal physiological conditions.

Animals↗

Biochemical activities associated with mouse Mcm2 protein.

Mcm2, a member of the Mcm2-7 protein family essential for the initiation of DNA replication, has several biochemical activities including the ability to inhibit the Mcm4,6,7 helicase. In this study, we characterized the activities associated with Mcm2 and determined the region required for them. It was found that Mcm2 deleted at an amino-terminal portion is able to bind to an Mcm4,6,7 hexameric complex and to inhibit its DNA helicase activity. The same deletion mutant of Mcm2 and the carboxyl-terminal half of Mcm2 were both able to bind to Mcm4, suggesting that the carboxyl-half of Mcm2 binds to Mcm4 to disassemble the Mcm4,6,7 hexamer. Phosphorylation of Mcm2,4,6,7 complexes with Cdc7 kinase showed that the amino-terminal region of Mcm2 is required for the phosphorylation, and it contains major Cdc7-mediated phosphorylation sites. We also found that Mcm2 itself can assemble a nucleosome-like structure in vitro in the presence of H3/H4 histones. The amino-terminal region of Mcm2 was required for the activity where a histone-binding domain is located. Finally, we identified a region required for the nuclear localization of Mcm2. The function of Mcm2 is discussed based on these biochemical characteristics.

Amino Acid Sequence↗