Hypoxia Induces Tumor-Derived Exosome SNHG16 to Mediate Nasopharyngeal Carcinoma Progression through the miR-23b-5p/MCM6 Pathway

This study aims to investigate the mechanism of tumor-derived exosomal (EVs) SNHG16 in promoting the progression of nasopharyngeal carcinoma (NPC). QRT-PCR was used to detect the expression of SNHG16, miR-23b-5p and MCM6 in NPC. MTT, flow cytometry and transwell were used to detect the effects of them on the proliferation, cycle, apoptosis and invasion ability of NPC. Transmission electron microscopy, Western blotting and BCA were used to verify the regulation of exosome secretion under different oxygen environments. Our results showed that hypoxia induces tumor-derived exosome SNHG16 to mediate NPC progression through the miR-23b-5p/MCM6 pathway.

Identification of the role of MCM6 in bladder cancer prognosis, immunotherapy response, and in vitro experimental investigation using multi-omics analysis

BACKGROUND

The tumor-promoting effects of MCM6 in numerous tumors have been widely revealed, yet its specific role in bladder cancer (BLCA) is still elusive. The objective of this research was to explore the underlying impact of MCM6 on BLCA.

METHODS

Integrating transcriptomic and proteomic data, MCM6 was identified to be strongly correlated with BLCA through weighted gene co-expression network analysis(WGCNA) and venn analyses. Then, the clinical value of MCM6 was validated with public database data. The different molecular/immune characteristics and the benefit of immunotherapy were also found in MCM6-defined subgroups. Additionally, single-cell RNA sequencing (scRNA-seq) data was choose for quantify MCM6 expression in the distinct BLCA cell types. The biological role of MCM6 were evaluated via in vitro functional experiments.

RESULTS

It was testified that the MCM6 could distinguish patients outcome in TCGA and GEO cohorts. Moreover, compared with the MCM6 low-expression group, the MCM6 high-expression group was related to more tumor-promoting related pathways, aggressive phenotypes, and benefit from immunotherapy. Analysis of scRNA-seq data resulted in MCM6 was mainly expressed in BLCA epithelial cells and the proportion of MCM6-expressing tumor epithelial cells is higher than the normal epithelial cells. Moreover, vitro experiments demonstrated that MCM6 knockdown repressed proliferation, cell cycle, migration, and invasion of BLCA cells.

CONCLUSION

This research indicated MCM6 is a promising marker for both prognosis and immunotherapy benefit and could promote the cells proliferation, invasion and migration in BLCA.

MCM6 is a Poor Prognostic Biomarker and Promotes Progression in Breast Cancer

BACKGROUND

Breast cancer is the commonest global malignancy and the primary cause of carcinoma death. MCM6 is vital to carcinogenesis, but the pathogenesis of MCM6 remains unclear.

METHODS

MCM6 expression in patients with breast cancer was examined through The Cancer Genome Atlas (TCGA) database, immunohistochemistry, Quantitative Real-Time PCR (qRT‒PCR) and Western blotting. The prognostic factors were assessed by the Kaplan‒Meier method and Cox regression. On the basis of the key factors selected by multivariable Cox regression analysis, a nomogram risk prediction model was adopted for clinical risk assessment. The TCGA database was utilized to determine how MCM6 is correlated with chemotherapy sensitivity, immune checkpoint-related genes (ICGs), tumor-infiltrating immune cells, along with tumor mutation burden (TMB) and methylation. The impact of MCM6 on carcinoma cells was investigated in terms of proliferation, cell cycle as well as migrating and invasive behavior through CCK assays, flow cytometry, wound healing assays, Transwell assays and xenotransplantation experiments.

RESULTS

MCM6 expression was upregulated, which is closely associated with the size of the tumor (p = 0.001) and lymph node metastasis (p = 0.012) in patients with breast cancer. Multivariate analysis revealed MCM6 to be an independent risk factor for prognosis in patients with breast carcinoma. The nomograph prediction model included MCM6, age, ER, M and N stage, which displayed good discrimination with a C index of 0.817 and good calibration. Overexpression of MCM6 correlated with chemotherapy sensitivity, immune checkpoint-related genes (ICGs), tumor-infiltrating immune cells, tumor mutation burden (TMB), and methylation. Silencing MCM6 significantly inhibited proliferation, prolonged the G1 phase of the cell cycle, and restrained the proliferation, migration and invasive behavior of cancerous cells and inhibited tumor growth in vivo.

CONCLUSIONS

Our research shows that MCM6 is highly expressed in breast cancer and can be used as an independent prognostic factor, which is expected to become a new target for the treatment of breast cancer in the future.

An in vitro method for inducing titan cells reveals novel features of yeast-to-titan switching in the human fungal pathogen Cryptococcus gattii

Cryptococcosis is a potentially lethal fungal infection of humans caused by organisms within the Cryptococcus neoformans/gattii species complex. Whilst C. neoformans is a relatively common pathogen of immunocompromised individuals, C. gattii is capable of acting as a primary pathogen of immunocompetent individuals. Within the host, both species undergo morphogenesis to form titan cells: exceptionally large cells that are critical for disease establishment. To date, the induction, defining attributes, and underlying mechanism of titanisation have been mainly characterized in C. neoformans. Here, we report the serendipitous discovery of a simple and robust protocol for in vitro induction of titan cells in C. gattii. Using this in vitro approach, we reveal a remarkably high capacity for titanisation within C. gattii, especially in strains associated with the Pacific Northwest Outbreak, and characterise strain-specific differences within the clade. In particular, this approach demonstrates for the first time that cell size changes, DNA amplification, and budding are not always synchronous during titanisation. Interestingly, however, exhibition of these cell cycle phenotypes was correlated with genes associated with cell cycle progression including CDC11, CLN1, BUB2, and MCM6. Finally, our findings reveal exogenous p-Aminobenzoic acid to be a key inducer of titanisation in this organism. Consequently, this approach offers significant opportunities for future exploration of the underlying mechanism of titanisation in this genus.

Diversity of lactase persistence alleles in Ethiopia: signature of a soft selective sweep

The persistent expression of lactase into adulthood in humans is a recent genetic adaptation that allows the consumption of milk from other mammals after weaning. In Europe, a single allele (-13910(∗)T, rs4988235) in an upstream region that acts as an enhancer to the expression of the lactase gene LCT is responsible for lactase persistence and appears to have been under strong directional selection in the last 5,000 years, evidenced by the widespread occurrence of this allele on an extended haplotype. In Africa and the Middle East, the situation is more complicated and at least three other alleles (-13907(∗)G, rs41525747; -13915(∗)G, rs41380347; -14010(∗)C, rs145946881) in the same LCT enhancer region can cause continued lactase expression. Here we examine the LCT enhancer sequence in a large lactose-tolerance-tested Ethiopian cohort of more than 350 individuals. We show that a further SNP, -14009T>G (ss 820486563), is significantly associated with lactose-digester status, and in vitro functional tests confirm that the -14009(∗)G allele also increases expression of an LCT promoter construct. The derived alleles in the LCT enhancer region are spread through several ethnic groups, and we report a greater genetic diversity in lactose digesters than in nondigesters. By examining flanking markers to control for the effects of mutation and demography, we further describe, from empirical evidence, the signature of a soft selective sweep.

Adult-type hypolactasia and calcium intake in polycystic ovary syndrome

OBJECTIVE

Adult-type hypolactasia (ATH) is related to lower calcium and milk intake, which might be associated with obesity and metabolic disturbances. Women with polycystic ovary syndrome (PCOS) frequently suffer from metabolic disturbances including central obesity. We aimed to examine the association of ATH and calcium intake with anthropometric, metabolic and endocrine parameters in a cohort of PCOS and control women.

DESIGN

Metabolic, endocrine and anthropometric measurements and oral glucose tolerance tests were performed in 504 PCOS and 366 control women. Genotyping of ATH, defined by the -13910 variant of the MCM6 gene, was performed. Calcium intake was assessed by questionnaires.

RESULTS

Adult-type hypolactasia was more prevalent in PCOS women (29·8%) than in controls (23·5%) (P = 0·040). PCOS women with ATH had higher waist-to-hip ratio (WHR) (0·80 [0·75-0·88] vs 0·78 [0·73-0·85], P = 0·046), glucose 2 h (5·28 [4·57-6·33] mmol/l vs 5·67 [4·68-6·78] mmol/l, P = 0·037), HbA1c (5·2 [5·0-5·4]%vs 5·1 [5·0-5·3]%, P = 0·009), parathyroid hormone (3·72(2·91-4·86] pmol/l vs 3·61 [2·94-4·63] pmol/l, P = 0·030) and Ferriman-Gallwey-Scores (FG Scores) (7 [3-12] vs 4 [1-9], P = 0·002) and lower 25(OH)D levels (54·4 [35·2-80·6] nmol/l vs 68·4 [49·7-89·4] nmol/l, P < 0·001) than PCOS women without ATH. The association of 25(OH)D and FG-Scores with ATH remained significant in age-, BMI- and WHR-adjusted analyses. PCOS women within the highest quartile of calcium intake had significantly lower testosterone (P = 0·023) and androstenedione (P = 0·032) and significantly higher high-density lipoprotein (HDL) levels (P = 0·035) than PCOS women with lower calcium intake.

CONCLUSION

Our results indicate an association of ATH with PCOS susceptibility. Moreover, ATH might influence WHR, HbA1c and FG-Scores as well as 25(OH)D levels. Higher calcium intake was associated with lower androgens and higher HDL levels.

Lactase persistence and lipid pathway selection in the Maasai

The Maasai are a pastoral people in Kenya and Tanzania, whose traditional diet of milk, blood and meat is rich in lactose, fat and cholesterol. In spite of this, they have low levels of blood cholesterol, and seldom suffer from gallstones or cardiac diseases. Field studies in the 1970s suggested that the Maasai have a genetic adaptation for cholesterol homeostasis. Analysis of HapMap 3 data using Fixation Index (Fst) and two metrics of haplotype diversity: the integrated Haplotype Score (iHS) and the Cross Population Extended Haplotype Homozygosity (XP-EHH), identified genomic regions and single nucleotide polymorphisms (SNPs) as strong candidates for recent selection for lactase persistence and cholesterol regulation in 143-156 founder individuals from the Maasai population in Kinyawa, Kenya (MKK). The non-synonmous SNP with the highest genome-wide Fst was the TC polymorphism at rs2241883 in Fatty Acid Binding Protein 1(FABP1), known to reduce low density lipoprotein and tri-glyceride levels in Europeans. The strongest signal identified by all three metrics was a 1.7 Mb region on Chr2q21. This region contains the genes LCT (Lactase) and MCM6 (Minichromosome Maintenance Complex Component) involved in lactase persistence, and the gene Rab3GAP1 (Rab3 GTPase-activating Protein Catalytic Subunit), which contains polymorphisms associated with total cholesterol levels in a genome-wide association study of >100,000 individuals of European ancestry. Sanger sequencing of DNA from six MKK samples showed that the GC-14010 polymorphism in the MCM6 gene, known to be associated with lactase persistence in Africans, is segregating in MKK at high frequency (∼58%). The Cytochrome P450 Family 3 Subfamily A (CYP3A) cluster of genes, involved in cholesterol metabolism, was identified by Fst and iHS as candidate loci under selection. Overall, our study identified several specific genomic regions under selection in the Maasai which contain polymorphisms in genes associated with lactase persistence and cholesterol regulation.

Structural insights into the Cdt1-mediated MCM2-7 chromatin loading

Initiation of DNA replication in eukaryotes is exquisitely regulated to ensure that DNA replication occurs exactly once in each cell division. A conserved and essential step for the initiation of eukaryotic DNA replication is the loading of the mini-chromosome maintenance 2-7 (MCM2-7) helicase onto chromatin at replication origins by Cdt1. To elucidate the molecular mechanism of this event, we determined the structure of the human Cdt1-Mcm6 binding domains, the Cdt1(410-440)/MCM6(708-821) complex by NMR. Our structural and site-directed mutagenesis studies showed that charge complementarity is a key determinant for the specific interaction between Cdt1 and Mcm2-7. When this interaction was interrupted by alanine substitutions of the conserved interacting residues, the corresponding yeast Cdt1 and Mcm6 mutants were defective in DNA replication and the chromatin loading of Mcm2, resulting in cell death. Having shown that Cdt1 and Mcm6 interact through their C-termini, and knowing that Cdt1 is tethered to Orc6 during the loading of MCM2-7, our results suggest that the MCM2-7 hexamer is loaded with its C terminal end facing the ORC complex. These results provide a structural basis for the Cdt1-mediated MCM2-7 chromatin loading.

Decreased MCM2-6 in Drosophila S2 cells does not generate significant DNA damage or cause a marked increase in sensitivity to replication interference

A reduction in the level of some MCM proteins in human cancer cells (MCM5 in U20S cells or MCM3 in Hela cells) causes a rapid increase in the level of DNA damage under normal conditions of cell proliferation and a loss of viability when the cells are subjected to replication interference. Here we show that Drosophila S2 cells do not appear to show the same degree of sensitivity to MCM2-6 reduction. Under normal cell growth conditions a reduction of >95% in the levels of MCM3, 5, and 6 causes no significant short term alteration in the parameters of DNA replication or increase in DNA damage. MCM depleted cells challenged with HU do show a decrease in the density of replication forks compared to cells with normal levels of MCM proteins, but this produces no consistent change in the levels of DNA damage observed. In contrast a comparable reduction of MCM7 levels has marked effects on viability, replication parameters and DNA damage in the absence of HU treatment.

1H, 15N and 13C chemical shift assignments of the Cdt1 binding domain of human Mcm6

The eukaryotic minichromsome maintenance (Mcm) proteins (Mcm2-7) are evolutionally conserved from yeast to human. These proteins are essential for DNA replication and Mcm6 is one subunit of Mcm2-7 complex that serves as the replicative helicase in DNA replication. Cdt1 is a critical member of pre-replicative complex (pre-RC), which directs the chromatin loading of Mcm2-7 complex. The Cdt1 binding domain (CBD) of human Mcm6 was found to directly interact with Cdt1 and this interaction may mediate the chromatin loading of Mcm2-7 complex. The structure of CBD exhibits a typical "winged-helix" fold which is generally involved in protein-nucleic acid interaction. Here we report the (1)H, (15)N and (13)C chemical shift assignments of human Mcm6 CBD determined by triple resonance experiments. The resonance assignments obtained in this work were required for the structure-function studies of CBD by NMR spectroscopy (BMRB deposits with accession number 16396).

Characterization and structure determination of the Cdt1 binding domain of human minichromosome maintenance (Mcm) 6

The minichromosome maintenance (Mcm) 2-7 complex is the replicative helicase in eukaryotic species, and it plays essential roles in the initiation and elongation phases of DNA replication. During late M and early G(1), the Mcm2-7 complex is loaded onto chromatin to form prereplicative complex in a Cdt1-dependent manner. However, the detailed molecular mechanism of this loading process is still elusive. In this study, we demonstrate that the previously uncharacterized C-terminal domain of human Mcm6 is the Cdt1 binding domain (CBD) and present its high resolution NMR structure. The structure of CBD exhibits a typical "winged helix" fold that is generally involved in protein-nucleic acid interaction. Nevertheless, the CBD failed to interact with DNA in our studies, indicating that it is specific for protein-protein interaction. The CBD-Cdt1 interaction involves the helix-turn-helix motif of CBD. The results reported here provide insight into the molecular mechanism of Mcm2-7 chromatin loading and prereplicative complex assembly.

Milk, rather than other foods, is associated with vertebral bone mass and circulating IGF-1 in female adolescents

SUMMARY

Low calcium intake hampers bone mineral acquisition in adolescent girls. This study explores dietary calcium sources and nutrients possibly associated with vertebral mass. Milk intake is not influenced by genetic variants of the lactase gene and is positively associated with serum IGF-1 and with lumbar vertebrae mineral content and density.

INTRODUCTION

Low calcium intake hampers bone mineral acquisition during adolescence. We identified calcium sources and nutrients possibly associated with lumbar bone mineralization and calcium metabolism in adolescent girls and evaluated the possible influence of a genetic polymorphic trait associated with adult-type hypolactasia.

METHODS

Lumbar bone mineral content (BMC), bone mineral density (BMD), and area, circulating IGF-1, markers of bone metabolism, and -13910 LCT (lactase gene) polymorphism; and intakes of milk, dairy products, calcium, phosphorus, magnesium, proteins, and energy were evaluated in 192 healthy adolescent girls.

RESULTS

After menarche, BMC, BMD, serum IGF-1, and serum PTH were tightly associated with milk consumption, but not with other calcium sources. All four parameters were also associated with phosphorus, magnesium, protein, and energy from milk, but not from other sources. Girls with milk intakes below 55 mL/day have significantly lower BMD, BMC, and IGF-1 and higher PTH compared to girls consuming over 260 mL/day. Neither BMC, BMD, calcium intakes, nor milk consumption were associated with -13910 LCT polymorphism.

CONCLUSIONS

Milk consumption, preferably to other calcium sources, is associated with lumbar BMC and BMD in postmenarcheal girls. Aside from being a major source of calcium, milk provides phosphates, magnesium, proteins, and as yet unidentified nutrients likely to favor bone health.

High expression of minichromosome maintenance protein 6 in classic Hodgkin's lymphoma points to a cell cycle arrest in G1 phase

BACKGROUND

Minichromosome maintenance protein 6 (MCM6) is one of the six proteins of minichromosome maintenance family that are involved in the initiation of DNA replication and thus represent a marker for proliferating cells. The aim of this study was to determine the proliferation characteristics of neoplastic cells in patients with classic Hodgkin's lymphoma.

METHODS

Paraffin-embedded blocks of lymph node, mediastinal, subcutaneous chest wall, and lung mass biopsies of 55 patients with classic Hodgkin's lymphoma were immunostained by the proliferation-associated monoclonal antibodies; Ki-S5 (Ki-67 antigen) and Ki-MCM6 (MCM6 antigen).

RESULTS

High MCM6 antigen expression was a striking feature of Hodgkin's and Reed-Sternberg cells (median: 85%, range: 35 - 99%) in comparison with lower Ki-67 expression (median: 63.5%, range: 1 - 98%, P<0.001). This indicates that MCM6 is already expressed in the early G1 phase, a cell cycle fraction that is not covered by antibodies specific to the Ki-67 antigen. The proliferation rates were determined by two markers, independent of histologic subtype, stage, presence of B symptoms, and size.

CONCLUSION

These data show that a subset of Reed-Sternberg and Hodgkin's cells is arrested in the early G1 phase and the MCM6-positive cells do not necessarily represent the real proliferating compartment of Hodgkin's lymphoma. Clinical relevance of this marker in patients with Hodgkin's lymphoma should be investigated.

Accumulation and dynamics of proteins of the MCM family during mouse oogenesis and the first embryonic cell cycle

We describe the localization of three proteins of the minichromosome maintenance (MCM) family, Mcm2, -6 and -7 in mouse ovarian oocytes. We showed that Mcm proteins are stored in two forms: soluble and insoluble. Soluble Mcm2, -6 and -7 were uniformly distributed in the nuclei of ovarian oocytes. Insoluble Mcm2 and Mcm7 (but not Mcm6) were detected in the nuclei of resting, growing and fully-grown transcribing oocytes. In transcriptionally inactive fully-grown oocytes, Mcm2 underwent redistribution and Mcm7 disappeared. A similar effect was observed when transcription in growing oocytes was inhibited with alpha-amanitin. We postulate that in mouse oogenesis, the insoluble Mcm proteins are engaged in processes related to regulation of transcription and/or chromatin organization. In oocytes preparing for meiotic maturation, aggregates of the insoluble form of Mcm2 fragmented, dispersed and ultimately disappeared from the nuclei. Numerous Mcm2-positive deposits were observed in the cytoplasm of maturing oocytes. In the one-cell embryo, insoluble Mcm2 appeared in the G1 nucleus, persisted in the S phase and was undetectable in the G2 nucleus. Such behavior of Mcm2 supports its involvement in chromatin licensing in the first embryonic cell cycle.

Differences in the single-stranded DNA binding activities of MCM2-7 and MCM467: MCM2 and MCM5 define a slow ATP-dependent step

The MCM2-7 complex, a hexamer containing six distinct and essential subunits, is postulated to be the eukaryotic replicative DNA helicase. Although all six subunits function at the replication fork, only a specific subcomplex consisting of the MCM4, 6, and 7 subunits (MCM467) and not the MCM2-7 complex exhibits DNA helicase activity in vitro. To understand why MCM2-7 lacks helicase activity and to address the possible function of the MCM2, 3, and 5 subunits, we have compared the biochemical properties of the Saccharomyces cerevisiae MCM2-7 and MCM467 complexes. We demonstrate that both complexes are toroidal and possess a similar ATP-dependent single-stranded DNA (ssDNA) binding activity, indicating that the lack of helicase activity by MCM2-7 is not due to ineffective ssDNA binding. We identify two important differences between them. MCM467 binds dsDNA better than MCM2-7. In addition, we find that the rate of MCM2-7/ssDNA association is slow compared with MCM467; the association rate can be dramatically increased either by preincubation with ATP or by inclusion of mutations that ablate the MCM2/5 active site. We propose that the DNA binding differences between MCM2-7 and MCM467 correspond to a conformational change at the MCM2/5 active site with putative regulatory significance.

A novel checkpoint mechanism regulating the G1/S transition

Ultraviolet irradiation of fission yeast cells in G1 phase induced a delay in chromatin binding of replication initiation factors and, consistently, a transient delay in S-phase entry. The cell cycle delay was totally dependent on the Gcn2 kinase, a sensor of the nutritional status, and was accompanied by phosphorylation of the translation initiation factor eIF2alpha and by a general depression of translation. However, the G1-specific synthesis of factors required for DNA replication was not reduced by ultraviolet radiation. The cell cycle delay represents a novel checkpoint with a novel mechanism of action that is not activated by ionizing radiation.

Expression of human MCM6 and DNA Topo II alpha in craniopharyngiomas and its correlation with recurrence of the tumor

Craniopharyngioma often recurs after resection resulting in poor outcome for the affected patients. The reliable criteria for predicting the tumor behavior are still lacking. It has been suggested that proliferative potential of the tumor cells is necessary for recurrence. Present study evaluated the cell multiplication activity, which is possibly related to relapse in 32 patients with adamantine epithelioma (AE) and 31 patients with squamous papillary tumor (SP). For this tissue specimens from their primary and recurring tumors were collected and immunohistochemical analysis of the expression of minichromosome maintenance protein 6 (MCM6) and DNA topoisomerase II alphalpha (DNA Topo II alpha) in the tumor sections was performed by quantitative microscopy. It was found that the MCM6 label index (LI) was significantly higher in AE than that of SP, and in the primary tumors of both subtypes the LIs of tumors with recurrence were higher than those without recurrence. There was a strong linear positive correlation between MCM6 LI and DNA Topo II alpha LI (r = 0.713; p = 0.000) in the craniopharyngiomas studied. The median MCM6 LI of the total 20 recurrent craniopharyngiomas (31.49%) was not significantly different from that of their primary tumors (29.65%). The long term risk of tumor recurrence is higher in AE than SP and it is associated with MCM6 and DNA Topo II alpha expression.

Angiogenesis and cell proliferation in human craniopharyngioma xenografts in nude mice

OBJECT

Craniopharyngioma is one of the most common congenital tumors of the sellar and suprasellar regions and accounts for between 4 and 6% of all intracranial tumors. Its oncogenesis and biological behavior have not been well studied, and neither a cell line nor an animal model have been established. To better understand the tumor and improve its clinical management, the authors investigated the angiogenesis and cellular proliferation in subcutaneous craniopharyngioma xenografts obtained by implanting human tumor cells into athymic nude mice.

METHODS

Human craniopharyngioma cells obtained from surgical specimens were subcutaneously implanted into BALB/c-nu/nu nude mice to establish a preliminary animal model of a transplanted tumor. Immunohistochemical staining with streptavidin-peroxidase complex was used to identify the cell phenotype and to evaluate the angiogenesis and proliferation in the xenografts. Expression of cytokeratin, minichromosome maintenance deficient 6 (MCM6) protein, and endothelial cell marker CD34 on the xenograft sections were assayed quantitatively by computer-assisted microscopy. Twenty-seven surviving subcutaneous xenografts were obtained in 15 nude mice. The total implantation success rate was 28.12% (adamantine epithelioma [AE], 37.50%; squamous papillary tumor [SPT], 18.75%). Formation of capillaries and cell proliferation were observed in all of these xenografts. Microvessel density and degree of MCM6 immunostaining were positively correlated in the surviving grafts (r = 0.410, p < 0.05), but there was no significant difference in these variables between the AE and SPT groups (p > 0.05).

CONCLUSIONS

A preliminary animal model of human craniopharyngioma was established in the nude mouse by heterotopic implantation. Surviving xenografts maintained their vascularization and proliferation activities until harvesting at 12 weeks.

Nascent transcription of MCM2-7 is important for nuclear localization of the minichromosome maintenance complex in G1

The minichromosome maintenance genes (MCM2-7) are transcribed at M/G1 just as the Mcm complex is imported into the nucleus to be assembled into prereplication complexes, during a period of low cyclin-dependent kinase (CDK) activity. The CDKs trigger DNA replication and prevent rereplication in part by exporting Mcm2-7 from the nucleus during S phase. We have found that repression of MCM2-7 transcription in a single cell cycle interferes with the nuclear import of Mcms in the subsequent M/G1 phase. This suggests that nascent Mcm proteins are preferentially imported into the nucleus. Consistent with this, we find that loss of CDK activity in G2/M is not sufficient for nuclear import, there is also a requirement for new protein synthesis. This requirement is not met by constitutive production of Cdc6 and does not involve synthesis of new transport machinery. The Mcm proteins generated in the previous cell cycle, which are unable to reaccumulate in the nucleus, are predominantly turned over by ubiquitin-mediated proteolysis in late mitosis/early G1. Therefore, the nuclear localization of Mcm2-7 is dependent on nascent transcription and translation of Mcm2-7 and the elimination of CDK activity which occurs simultaneously as cells enter G1.

Genome-wide localization of pre-RC sites and identification of replication origins in fission yeast

DNA replication of eukaryotic chromosomes initiates at a number of discrete loci, called replication origins. Distribution and regulation of origins are important for complete duplication of the genome. Here, we determined locations of Orc1 and Mcm6, components of pre-replicative complex (pre-RC), on the whole genome of Schizosaccharomyces pombe using a high-resolution tiling array. Pre-RC sites were identified in 460 intergenic regions, where Orc1 and Mcm6 colocalized. By mapping of 5-bromo-2'-deoxyuridine (BrdU)-incorporated DNA in the presence of hydroxyurea (HU), 307 pre-RC sites were identified as early-firing origins. In contrast, 153 pre-RC sites without BrdU incorporation were considered to be late and/or inefficient origins. Inactivation of replication checkpoint by Cds1 deletion resulted in BrdU incorporation with HU specifically at the late origins. Early and late origins tend to distribute separately in large chromosome regions. Interestingly, pericentromeric heterochromatin and the silent mating-type locus replicated in the presence of HU, whereas the inner centromere or subtelomeric heterochromatin did not. Notably, MCM did not bind to inner centromeres where origin recognition complex was located. Thus, replication is differentially regulated in chromosome domains.

Identification and characterization of a novel component of the human minichromosome maintenance complex

Minichromosome maintenance (MCM) complex replicative helicase complexes play essential roles in DNA replication in all eukaryotes. Using a tandem affinity purification-tagging approach in human cells, we discovered a form of the MCM complex that contains a previously unstudied protein, MCM binding protein (MCM-BP). MCM-BP is conserved in multicellular eukaryotes and shares limited homology with MCM proteins. MCM-BP formed a complex with MCM3 to MCM7, which excluded MCM2; and, conversely, hexameric complexes of MCM2 to MCM7 lacked MCM-BP, indicating that MCM-BP can replace MCM2 in the MCM complex. MCM-BP-containing complexes exhibited increased stability under experimental conditions relative to those containing MCM2. MCM-BP also formed a complex with the MCM4/6/7 core helicase in vitro, but, unlike MCM2, did not inhibit this helicase activity. A proportion of MCM-BP bound to cellular chromatin in a cell cycle-dependent manner typical of MCM proteins, and, like other MCM subunits, preferentially associated with a cellular origin in G(1) but not in S phase. In addition, down-regulation of MCM-BP decreased the association of MCM4 with chromatin, and the chromatin association of MCM-BP was at least partially dependent on MCM4 and cdc6. The results indicate that multicellular eukaryotes contain two types of hexameric MCM complexes with unique properties and functions.

[Expression of minichromosome maintenance protein 6 in craniopharyngioma and its correlation with prognosis]

OBJECTIVE

To investigate the expression of minichromosome maintenance protein 6 (MCM6) in tissue sections of craniopharyngioma and observe its relation with the outcome of patients with craniopharyngiomas.

METHODS

Prospective cohorts were composed of 32 adamantine epithelioma (AE) patients and 31 squamous papillary tumor (SP) patients. The average of follow-up phase was 84. 26 months, of 60 patients with craniopharyngioma, 20 suffered from recurrence and underwent operation again for removal of tumor, and the specimens of the tumors patients were collected. MCM6 as proliferative marker expression in the specimen sections was measured by immunohistochemical method (avidin-biotin-peroxidase); quantitatively, scoring for MCM6 protein variation was performed by TE2000-U inverted biological microscope and Image-Pro Plus professional image analysis software. Oncocyte proliferation potential was evaluated for inter-group comparison in three pair of groups, including AE/SP, recurrence/recurrence-free, and primary/relapse groups.

RESULTS

14 of 32 AE patients and 6 of 31 SP patients had recurrence during follow-up. MCM6 protein expression showed significant difference between AE/SP groups and between recurrence/recurrence-free groups (P < 0.05, two-tailed), but there was no statistically significant difference between primary and recurrent craniopharyngiomas.

CONCLUSION

The subtype and MCM6 protein expression in craniopharyngiomas are related to the prognosis of tumor and thus may be useful in predicting the risk of tumor relapse.

Analysis of Mcm2–7 chromatin binding during anaphase and in the transition to quiescence in fission yeast

Mcm2-7 proteins are generally considered to function as a heterohexameric complex, providing helicase activity for the elongation step of DNA replication. These proteins are loaded onto replication origins in M-G1 phase in a process termed licensing or pre-replicative complex formation. It is likely that Mcm2-7 proteins are loaded onto chromatin simultaneously as a pre-formed hexamer although some studies suggest that subcomplexes are recruited sequentially. To analyze this process in fission yeast, we have compared the levels and chromatin binding of Mcm2-7 proteins during the fission yeast cell cycle. Mcm subunits are present at approximately 1 x 10(4) molecules/cell and are bound with approximately equal stoichiometry on chromatin in G1/S phase cells. Using a single cell assay, we have correlated the timing of chromatin association of individual Mcm subunits with progression through mitosis. This showed that Mcm2, 4 and 7 associate with chromatin at about the same stage of anaphase, suggesting that licensing involves the simultaneous binding of these subunits. We also examined Mcm2-7 chromatin association when cells enter a G0-like quiescent state. Chromatin binding is lost in this transition in a process that does not require DNA replication or the selective degradation of specific subunits.

The replicative helicases of bacteria, archaea, and eukarya can unwind RNA-DNA hybrid substrates

Replicative helicases are hexameric enzymes that unwind DNA during chromosomal replication. They use energy from nucleoside triphosphate hydrolysis to translocate along one strand of the duplex DNA and displace the complementary strand. Here, the ability of a replicative helicase from each of the three domains, bacteria, archaea, and eukarya, to unwind RNA-containing substrate was determined. It is shown that all three helicases can unwind DNA-RNA hybrids while translocating along the single-stranded DNA. No unwinding could be observed when the helicases were provided with a single-stranded RNA overhang. Using DNA, RNA, and DNA-RNA chimeric oligonucleotides it was found that whereas the enzymes can bind both DNA and RNA, they could translocate only along DNA and only DNA stimulates the ATPase activity of the enzymes. Recent observations suggest that helicases may interact with enzymes participating in RNA metabolism and that RNA-DNA hybrids may be present on the chromosomes. Thus, the results presented here may suggest a new role for the replicative helicases during chromosomal replication or in other cellular processes.

Site-specific phosphorylation of MCM4 during the cell cycle in mammalian cells

MCM4, a subunit of a putative replicative helicase, is phosphorylated during the cell cycle, at least in part by cyclin-dependent kinases (CDK), which play a central role in the regulation of DNA replication. However, detailed characterization of the phosphorylation of MCM4 remains to be performed. We examined the phosphorylation of human MCM4 at Ser3, Thr7, Thr19, Ser32, Ser54, Ser88 and Thr110 using anti-phosphoMCM4 sera. Western blot analysis of HeLa cells indicated that phosphorylation of MCM4 at these seven sites can be classified into two groups: (a) phosphorylation that is greatly enhanced in the G2 and M phases (Thr7, Thr19, Ser32, Ser54, Ser88 and Thr110), and (b) phosphorylation that is firmly detected during interphase (Ser3). We present data indicating that phosphorylation at Thr7, Thr19, Ser32, Ser88 and Thr110 in the M phase requires CDK1, using a temperature-sensitive mutant of mouse CDK1, and phosphorylation at sites 3 and 32 during interphase requires CDK2, using a dominant-negative mutant of human CDK2. Based on these results and those from in vitro phosphorylation of MCM4 with CDK2/cyclin A, we discuss the kinases responsible for MCM4 phosphorylation. Phosphorylated MCM4 detected using anti-phospho sera exhibited different affinities for chromatin. Studies on the nuclear localization of chromatin-bound MCM4 phosphorylated at sites 3 and 32 suggested that they are not generally colocalized with replicating DNA. Unexpectedly, MCM4 phosphorylated at site 32 was enriched in the nucleolus through the cell cycle. These results suggest that phosphorylation of MCM4 has several distinct and site-specific roles in the function of MCM during the mammalian cell cycle.

Gene expression profiling of primary cutaneous melanoma and clinical outcome

BACKGROUND

Gene expression profiling data for human primary cutaneous melanomas are scarce because of the lack of retrospective collections of frozen tumors. To identify differentially expressed genes that may be involved in melanoma progression and prognosis, we investigated the relationship between gene expression profiles and clinical outcome in a cohort of patients with primary melanoma.

METHODS

Labeled complementary RNA (cRNA) from each tissue sample was hybridized to a pangenomic 44K 60-mer oligonucleotide microarray. Class comparison and class prediction analyses were performed to identify genes whose expression in primary melanomas was associated with 4-year distant metastasis-free survival among 58 patients with at least 4 years of follow-up, distant metastasis, or death. Results were validated immunohistochemically at the protein level in 176 independent primary melanomas from patients with a median clinical follow-up of 8.5 years. Survival was analyzed with a Cox multivariable model and stratified log-rank test. All statistical tests were two-sided.

RESULTS

We identified 254 genes that were associated with distant metastasis-free survival of patients with primary melanoma. These 254 genes include genes involved in activating DNA replication origins, such as minichromosome maintenance genes and geminin. Twenty-three of these genes were studied at the protein level; expression of five (MCM4, P = .002; MCM3, P = .030; MCM6, P = .004; KPNA2, P = .021; and geminin, P = .004) was statistically significantly associated with overall survival in the validation set. In a multivariable Cox model adjusted for tumor thickness, ulceration, age, and sex, expression of MCM4 (hazard ratio [HR] of death = 4.04, 95% confidence interval [CI] = 1.39 to 11.76; P = .010) and MCM6 (HR of death = 7.42, 95% CI = 1.99 to 27.64; P = .003) proteins was still statistically significantly associated with overall survival.

CONCLUSION

We identified 254 genes whose expression was associated with metastatic dissemination of cutaneous melanomas. These genes may shed light on the molecular mechanisms underlying poor prognosis in melanoma patients.

Direct interaction between cohesin complex and DNA replication machinery

Structural maintenance of chromosome 1 (Smc1) is a multifunctional protein, which has been implicated in sister chromatid cohesion, DNA recombination and repair, and the activation of cell cycle checkpoints by ionizing radiation, ultraviolet light, and other genotoxic agents. In order to identify the proteins that interact with Smc1, we conducted the Tandem affinity purification (TAP) technique and analyzed the Smc1-interacting proteins via MALDI-TOF mass spectrometry. We identified minichromosome maintenance 7 (Mcm7), an essential component of the pre-replication complex, as a novel Smc1-interacting protein. Co-immunoprecipitation revealed an interaction occurring between Smc1 and Mcm7, both in vitro and in vivo. Using a GST pull-down assay, we determined that Smc1 interacts physically with Mcm7 via its N-terminal and hinge regions, and Mcm7 interacts with Smc1 via its middle region. Interestingly, we also discovered that Smc1 interacts with other DNA replication proteins, including Mcm6, RFC1, and DNA polymerase alpha. These results suggest that a functional link exists between the cohesin complex and DNA replication proteins.

Pre-replication complex organization in the atypical DNA replication cycle of Plasmodium falciparum: Characterization of the mini-chromosome maintenance (MCM) complex formation

The overall organization of cell division in Plasmodium is unique compared to that observed in model organisms because DNA replicates more than once per cell cycle at several points of its life cycle. The sequencing of the Plasmodium genome has also revealed the apparent absence of many key components (e.g. Cdt1, DDK and Cdc45) of the eukaryotic cell cycle machinery that are responsible for the formation of the pre-replication complex (pre-RC). We have characterized the Plasmodium falciparum minichromosome maintenance complex (MCM) that plays a key role in the transition of pre-RC to the RC. Similar to other eukaryotes, the Plasmodium genome encodes six MCM subunits. Here, we show that expression levels of at least three of the PfMCM subunits, the homologues of MCM2, MCM6 and MCM7, change during the intraerythrocytic development cycle, peaking in schizont and decreasing in the ring and trophozoite stages. PfMCM2, 6 and 7 subunits interact with each other to form a developmentally regulated complex: these interactions are detectable in rings and schizonts, but not in trophozoites. PfMCM2, 6 and 7 subunits are localized in both cytosolic and nucleosolic fractions during all intraerythrocytic stages of P. falciparum development, with increased nuclear localization in schizonts. Only PfMCM6 is associated with the chromatin fraction at all stages of growth. No phosphorylation of PfMCM2, 6 and 7 was detected, but two as yet unidentified threonine-phosphosphorylated proteins were present in the complex, whose pattern of phosphorylation varied during parasite development.

Minichromosome maintenance protein 6, a proliferation marker superior to Ki-67 and independent predictor of survival in patients with mantle cell lymphoma

Minichromosome maintenance protein 6 (MCM6) is one of six proteins of the MCM family which are involved in the initiation of DNA replication and thus represent a marker of proliferating cells. Since the level of cell proliferation is the most valuable predictor of survival in mantle cell lymphoma (MCL), we investigated lymph node biopsy specimens from 70 patients immunohistochemically with a monoclonal antibody against MCM6. The percentage of MCM6 expressing lymphoma cells ranged from 12.0 to 95.6%, with a mean of 61.0%, and was significantly higher than the percentage of Ki-67-positive cells (P<0.0001). Surprisingly, the ratio of MCM6-positive cells to Ki-67-positive cells was higher than in normal stimulated peripheral blood mononuclear cells, indicating a cell early G1-phase arrest in MCL. A high MCM6 expression level of more than 75% positive cells was associated with a significantly shorter overall survival time (16 months) compared to MCL with a low MCM6 expression level of less than 25% (no median reached, P<0.0001). Multivariate analysis revealed MCM6 to be an independent predictor of survival that is superior to the international prognostic factor and the Ki-67 index. Therefore, aside from gene expression profiling, immunohistochemical detection of MCM6 seems to be the most promising marker for predicting the outcome in MCL.

CDK phosphorylation of a novel NLS-NES module distributed between two subunits of the Mcm2-7 complex prevents chromosomal rereplication

Cyclin-dependent kinases (CDKs) use multiple mechanisms to block reassembly of prereplicative complexes (pre-RCs) at replication origins to prevent inappropriate rereplication. In Saccharomyces cerevisiae, one of these mechanisms promotes the net nuclear export of a pre-RC component, the Mcm2-7 complex, during S, G2, and M phases. Here we identify two partial nuclear localization signals (NLSs) on Mcm2 and Mcm3 that are each necessary, but not sufficient, for nuclear localization of the Mcm2-7 complex. When brought together in cis, however, the two partial signals constitute a potent NLS, sufficient for robust nuclear localization when fused to an otherwise cytoplasmic protein. We also identify a Crm1-dependent nuclear export signal (NES) adjacent to the Mcm3 NLS. Remarkably, the Mcm2-Mcm3 NLS and the Mcm3 NES are sufficient to form a transport module that recapitulates the cell cycle-regulated localization of the entire Mcm2-7 complex. Moreover, we show that CDK regulation promotes net export by phosphorylation of the Mcm3 portion of this module and that nuclear export of the Mcm2-7 complex is sufficient to disrupt replication initiation. We speculate that the distribution of partial transport signals among distinct subunits of a complex may enhance the specificity of protein localization and raises the possibility that previously undetected distributed transport signals are used by other multiprotein complexes.

Levels of the origin-binding protein Double parked and its inhibitor Geminin increase in response to replication stress

The regulation of a pre-replicative complex (pre-RC) at origins ensures that the genome is replicated only once per cell cycle. Cdt1 is an essential component of the pre-RC that is rapidly degraded at G1-S and also inhibited by Geminin (Gem) protein to prevent re-replication. We have previously shown that destruction of the Drosophila homolog of Cdt1, Double-parked (Dup), at G1-S is dependent upon cyclin-E/CDK2 and important to prevent re-replication and cell death. Dup is phosphorylated by cyclin-E/Cdk2, but this direct phosphorylation was not sufficient to explain the rapid destruction of Dup at G1-S. Here, we present evidence that it is DNA replication itself that triggers rapid Dup destruction. We find that a range of defects in DNA replication stabilize Dup protein and that this stabilization is not dependent on ATM/ATR checkpoint kinases. This response to replication stress was cell-type specific, with neuroblast stem cells of the larval brain having the largest increase in Dup protein. Defects at different steps in replication also increased Dup protein during an S-phase-like amplification cell cycle in the ovary, suggesting that Dup stabilization is sensitive to DNA replication and not an indirect consequence of a cell-cycle arrest. Finally, we find that cells with high levels of Dup also have elevated levels of Gem protein. We propose that, in cycling cells, Dup destruction is coupled to DNA replication and that increased levels of Gem balance elevated Dup levels to prevent pre-RC reformation when Dup degradation fails.

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.

Notch-Dependent Cell Cycle Arrest Is Associated With Downregulation of Minichromosome Maintenance Proteins

Perturbation of the Notch signaling pathway has been implicated in the pathogenesis of human cardiovascular diseases, and animal models have confirmed the requirement of Notch during cardiovascular development. We recently demonstrated that Notch activation delays S-phase entry and contributes to endothelial contact inhibition. Minichromosome maintenance (MCM) proteins, components of the prereplicative complex (pre-RC), are essential for DNA replication. Here, we report that Notch-mediated cell cycle arrest is associated with downregulation of MCM2 and MCM6 in endothelial cells and human fibroblasts. Downregulation of MCM proteins is also observed on activation of C promoter binding factor (CBF1) and is mediated by inhibition of Rb phosphorylation, as demonstrated using a constitutively active Rb mutant. Although the effects of the Notch pathway are cell-type specific and context-dependent, in cell types where Notch has an antiproliferative effect, downregulation of MCM proteins may be a common mechanism to inhibit DNA replication.

DNA binding and helicase actions of mouse MCM4/6/7 helicase

Helicases play central roles in initiation and elongation of DNA replication. We previously reported that helicase and ATPase activities of the mammalian Mcm4/6/7 complex are activated specifically by thymine-rich single-stranded DNA. Here, we examined its substrate preference and helicase actions using various synthetic DNAs. On a bubble substrate, Mcm4/6/7 makes symmetric dual contacts with the 5′-proximal 25 nt single-stranded segments adjacent to the branch points, presumably generating double hexamers. Loss of thymine residues from one single-strand results in significant decrease of unwinding efficacy, suggesting that concurrent bidirectional unwinding by a single double hexameric Mcm4/6/7 may play a role in efficient unwinding of the bubble. Mcm4/6/7 binds and unwinds various fork and extension structures carrying a single-stranded 3′-tail DNA. The extent of helicase activation depends on the sequence context of the 3′-tail, and the maximum level is achieved by DNA with 50% or more thymine content. Strand displacement by Mcm4/6/7 is inhibited, as the GC content of the duplex region increases. Replacement of cytosine–guanine pairs with cytosine–inosine pairs in the duplex restored unwinding, suggesting that mammalian Mcm4/6/7 helicase has difficulties in unwinding stably base-paired duplex. Taken together, these findings reveal important features on activation and substrate preference of the eukaryotic replicative helicase.

Caenorhabditis elegans Geminin Homologue Participates in Cell Cycle Regulation and Germ Line Development

Cdt1 is an essential component for the assembly of a pre-replicative complex. Cdt1 activity is inhibited by geminin, which also participates in neural development and embryonic differentiation in many eukaryotes. Although Cdt1 homologues have been identified in organisms ranging from yeast to human, geminin homologues had not been described for Caenorhabditis elegans and fungi. Here, we identify the C. elegans geminin, GMN-1. Biochemical analysis reveals that GMN-1 associates with C. elegans CDT-1, the Hox protein NOB-1, and the Six protein CEH-32. GMN-1 inhibits not only the interaction between mouse Cdt1 and Mcm6 but also licensing activity in Xenopus egg extracts. RNA interference-mediated reduction of GMN-1 is associated with enlarged germ nuclei with aberrant nucleolar morphology, severely impaired gametogenesis, and chromosome bridging in intestinal cells. We conclude that the Cdt1-geminin system is conserved throughout metazoans and that geminin has evolved in these taxa to regulate proliferation and differentiation by directly interacting with Cdt1 and homeobox proteins.

The Mcm467 complex of Saccharomyces cerevisiae is preferentially activated by autonomously replicating DNA sequences

We have analyzed the role of single-stranded DNA (ssDNA) in the modulation of the ATPase activity of Mcm467 helicase of the yeast Saccharomyces cerevisiae. The ATPase activity of the Mcm467 complex is modulated in a sequence-specific manner and that the ssDNA sequences derived from the origin of DNA replication of S. cerevisiae autonomously replicating sequence 1 (ARS1) are the most effective stimulators. Synthetic oligonucleotides, such as oligo(dA) and oligo(dT), also stimulated the ATPase activity of the Mcm467 complex, where oligo(dT) was more effective than oligo(dA). However, the preference of a thymidine stretch appeared unimportant, because with yeast ARS1 derived sequences, the A-rich strand was as effective in stimulating the ATPase activity, as was the T-rich strand. Both of these strands were more effective stimulators than either oligo(dA)( )()or oligo(dT). The DNA helicase activity of Mcm467 complex is also significantly stimulated by the ARS1-derived sequences. These results indicate that the ssDNA sequences containing A and B1 motifs of ARS1, activate the Mcm467 complex and stimulate its ATPase and DNA helicase activities. Our results also indicate that the yeast replication protein A stimulated the ATPase activity of the Mcm467 complex.

Molecular genetics of adult-type hypolactasia

Adult-type hypolactasia (lactase non-persistence; primary lactose malabsorption) is characterized by the down-regulation of the lactase enzyme activity in the intestinal wall after weaning. The down-regulation is genetically determined and a mutation has occurred that has made part of mankind tolerate milk (lactase persistence). A DNA-variant, single nucleotide polymorphism C/T-13910 located 13 910 base pairs (bp) upstream of the lactase gene (LCT) at chromosome 2q21-22 has been shown to associate with the lactase persistence/non-persistence trait both in family and case-control studies. The C/T-13910 variant is located in a non-coding region in the genome in intron 13 of the minichromosome maintenance type 6 gene (MCM6). Significant correlation between the C/T-13910-variant and lactase activity in the intestinal biopsy specimens has been demonstrated. Molecular epidemiological studies on the prevalence of the C/C-13910 genotype associated with low lactase activity are in agreement with the prevalence figures for adult type hypolactasia in>70 diverse ethnic groups studied. Recent functional studies have suggested that this variant has an enhancer effect over the lactase gene. Based on the biochemical, functional, genetic and molecular epidemiological studies of the C/T-13910 variant, genetic testing for adult type hypolactasia has been introduced into clinical practice in Finland. Identification of the genetic change has highlighted the role of non-coding variants in the regulation of common genes and created new tools to study the mechanism of lactase enzyme activation.

Liver X receptor agonists suppress vascular smooth muscle cell proliferation and inhibit neointima formation in balloon-injured rat carotid arteries

The liver X receptors alpha and beta (LXRalpha and LXRbeta) are important regulators of cholesterol homeostasis in liver and macrophages. Synthetic LXR ligands prevent the development of atherosclerosis in murine models; however, the potential functional relevance of LXRs in vascular smooth muscle cells (VSMCs) has not been investigated. In the present study, we demonstrate that LXRs are expressed and functional in primary human coronary artery VSMCs (CASMCs). LXR ligands inhibited mitogen-induced VSMC proliferation and G1-->S phase progression of the cell cycle. Inhibition of G1 exit by LXR ligands was accompanied by a dose-dependent inhibition of retinoblastoma protein (Rb) phosphorylation, which functions as the key switch for G1-->S cell cycle progression. LXR ligands suppressed mitogen-induced degradation of the cyclin-dependent kinase inhibitor p27Kip1, attenuated cyclin D1 and cyclin A expression, and inhibited the expression of S phase-regulatory minichromosome maintenance protein 6. Stabilization of p27kip1 by LXR ligands was mediated by supressing the transcriptional activation of the S phase kinase-associated protein 2 (Skp2), an F-box protein that targets p27Kip1 for degradation. Inhibition of Rb phosphorylation and G1-->S cell cycle progression by LXR ligands was reversed in VSMCs overexpressing Skp2, indicating that Skp2 as an upstream regulator of p27Kip1 degradation plays a central role in LXR ligand-mediated inhibition of VSMC proliferation. Furthermore, adenovirus-mediated overexpression of the S phase transcription factor E2F, which is released after Rb phosphorylation, reversed the inhibitory effect of LXR ligands on VSMC proliferation and S phase gene expression, suggesting that the primary mechanisms by which LXR ligands inhibit VSMC proliferation occur upstream of Rb phosphorylation. Finally, neointima formation in a model of rat carotid artery balloon injury was significantly attenuated after treatment with the LXR ligand T1317 compared with vehicle-treated animals. These data demonstrate that LXR ligands inhibit VSMC proliferation and neointima formation after balloon injury and suggest that LXR ligands may constitute a novel therapy for proliferative vascular diseases. The full text of this article is available online at http://circres.ahajournals.org.

Site-specific Loading of an MCM Protein Complex in a DNA Replication Initiation Zone Upstream of the c-MYC Gene in the HeLa Cell Cycle

The MCM proteins participate in an orderly association, beginning with the origin recognition complex, that culminates in the initiation of chromosomal DNA replication. Among these, MCM proteins 4, 6, and 7 constitute a subcomplex that reportedly possesses DNA helicase activity. Little is known about DNA sequences initially bound by these MCM proteins or about their cell cycle distribution in the chromatin. We have determined the locations of certain MCM and associated proteins by chromatin immunoprecipitation (ChIP) in a zone of initiation of DNA replication upstream of the c-MYC gene in the HeLa cell cycle. MCM7 and its clamp-loading partner Cdc6 are highly specifically colocalized by ChIP and re-ChIP in G(1) and early S on a 198-bp segment located near the center of the initiation zone. ChIP and Re-ChIP colocalizes MCM7 and ORC1 to the same segment specifically in late G(1). MCM proteins 6 and 7 can be coimmunoprecipitated throughout the cell cycle, whereas MCM4 is reduced in the complex in late S and G(2), reappearing upon mitosis. MCM7 is not visualized by immunohistochemistry on metaphase chromosomes. MCM7 is recruited to multiple sites in chromatin in S and G(2), at which time it is not detected with ORC1. The rate of dissemination is surprisingly slow and is unlikely to be simply attributed to progression with replication forks. Results indicate sequence-specific loading of MCM proteins onto DNA in late G(1) followed by a recruitment to multiple sites in chromatin subsequent to replication.

Self-assembling protein microarrays

Protein microarrays provide a powerful tool for the study of protein function. However, they are not widely used, in part because of the challenges in producing proteins to spot on the arrays. We generated protein microarrays by printing complementary DNAs onto glass slides and then translating target proteins with mammalian reticulocyte lysate. Epitope tags fused to the proteins allowed them to be immobilized in situ. This obviated the need to purify proteins, avoided protein stability problems during storage, and captured sufficient protein for functional studies. We used the technology to map pairwise interactions among 29 human DNA replication initiation proteins, recapitulate the regulation of Cdt1 binding to select replication proteins, and map its geminin-binding domain.

A novel intermediate in initiation complex assembly for fission yeast DNA replication

Assembly of initiation factors on individual replication origins at onset of S phase is crucial for regulation of replication timing and repression of initiation by S-phase checkpoint control. We dissected the process of preinitiation complex formation using a point mutation in fission yeast nda4-108/mcm5 that shows tight genetic interactions with sna41(+)/cdc45(+). The mutation does not affect loading of MCM complex onto origins, but impairs Cdc45-loading, presumably because of a defect in interaction of MCM with Cdc45. In the mcm5 mutant, however, Sld3, which is required for Cdc45-loading, proficiently associates with origins. Origin-association of Sld3 without Cdc45 is also observed in the sna41/cdc45 mutant. These results suggest that Sld3-loading is independent of Cdc45-loading, which is different from those observed in budding yeast. Interestingly, returning the arrested mcm5 cells to the permissive temperature results in immediate loading of Cdc45 to the origin and resumption of DNA replication. These results suggest that the complex containing MCM and Sld3 is an intermediate for initiation of DNA replication in fission yeast.

Polyomavirus tumorantigens have a profound effect on gene expression in mouse fibroblasts

Polyomavirus (Py) large and small tumorantigens together are competent to induce S phase in growth-arrested mouse fibroblasts. The capacity of the large tumorantigen to bind the pocket proteins, pRB, p130 and p107, is important for the transactivation of DNA synthesis enzymes and the cyclins E and A, while the interference of small tumorantigen with protein phosphatase PP2A causes a destabilization of the cdk2 inhibitor p27, and thus leads to strong cyclin E- and cyclin A-dependent cdk2 activity. Py small tumorantigen, in addition, is able to transactivate cyclin A. Hence, this protein might have a much wider effect on gene expression in arrested mouse fibroblasts than hitherto suspected. This may have a profound part in the known capacity of Py to form tumors in mice. Therefore, it was interesting to gain an insight into the spectrum of transcriptional deregulation by Py tumorantigens. Accordingly, we performed microarray analysis of quiescent mouse fibroblasts in the absence and presence of small or large tumorantigen. We found that the viral proteins can induce or repress a great variety of genes beyond those involved in the S phase induction and DNA synthesis. The results of the microarray analysis were confirmed for selected genes by several methods, including real-time PCR. Interestingly, a mutation of the binding site for pocket proteins in case of LT and for PP2A in case of ST has a variable effect on the deregulation of genes by the viral proteins depending on the gene in question. In fact, some genes are transactivated by LT as well as ST completely independent of an interaction with their major cellular targets, pocket proteins and PP2A, respectively.

A novel method for diagnosis of adult hypolactasia by genotyping of the -13910 C/T polymorphism with Pyrosequencing technology

The lactose-loading test and other functional tests that have been the most widely used clinically in the diagnosis of adult hypolactasia up to now are labour intensive and costly, and suffer from low sensitivity as well as low specificity. In addition, lactose-loading tests may be painful to the patient. Here, a new genotyping method for the diagnosis of adult hypolactasia is described. The method utilizes Pyrosequencing technology, which gives the DNA sequence around the recently identified C/T polymorphic site in the MCM6 gene. Among the advantages compared to the other genotyping methods published are less staff hands-on time than for example RFLP analyses, and the avoidance of radioactivity, as in the originally described isotope-minisequencing. Most importantly, Pyrosequencing, which is a direct DNA sequencing technique, gives unambiguous genotyping results as well as some redundant sequence information beyond the SNP position, which serves as a valuable internal control, obtained for each sample.

Thymine-rich single-stranded DNA activates Mcm4/6/7 helicase on Y-fork and bubble-like substrates

The presence of multiple clusters of runs of asymmetric adenine or thymine is a feature commonly found in eukaryotic replication origins. Here we report that the helicase and ATPase activities of the mammalian Mcm4/6/7 complex are activated specifically by thymine stretches. The Mcm helicase is specifically activated by a synthetic bubble structure which mimics an activated replication origin, as well as by a Y-fork structure, provided that a single-stranded DNA region of sufficient length is present in the unwound segment or 3' tail, respectively, and that it carries clusters of thymines. Sequences derived from the human lamin B2 origin can serve as a potent activator for the Mcm helicase, and substitution of its thymine clusters with guanine leads to loss of this activation. At the fork, Mcm displays marked processivity, expected for a replicative helicase. These findings lead us to propose that selective activation by stretches of thymine sequences of a fraction of Mcm helicases loaded onto chromatin may be the determinant for selection of initiation sites on mammalian genomes.

Expression of minichromosome maintenance proteins in vascular smooth muscle cells is ERK/MAPK dependent

Proliferation of vascular smooth muscle cells (VSMC) represents a key event for the pathogenesis of postangioplasty restenosis. Minichromosome maintenance proteins (MCM) form essential components of the prereplicative complex at DNA replication origins and are regulated by E2F. The present studies were designed to investigate the signal transduction pathways controlling the expression of MCM6 and MCM7 in VSMC in response to mitogenic stimuli. MCM6 and MCM7 expression was substantially increased after stimulation with platelet-derived growth factor-BB and insulin. Pretreatment with PD98059, a specific inhibitor of the extracellular signal-regulated kinases (ERK)-mitogen-activated protein kinase (MAPK), competely inhibited the mitogen-induced MCM6 and MCM7 mRNA and protein expression, demonstrating a critical role for this pathway in transmitting transmembrane signals required for the initiation of DNA replication. The p38MAPK inhibitor SB203580, the phosphatidylinositol 3 kinase (PI3-kinase) pathway inhibitor wortmannin, and the protein kinase C pathway (PKC) inhibitor Gö 6976 did not significantly affect mitogen-induced MCM6 and MCM7 expression. Transient transfection experiments revealed that PD98059 inhibited mitogen-induced MCM6 and MCM7 transcriptional activation. In addition, blockade of ERK/MAPK signaling with PD98059 strongly inhibited phosphorylation of the retinoblastoma protein (Rb) and activity of a luciferase reporter plasmid driven by multiple E2F elements. Inhibition of mitogen-induced MCM6 and MCM7 expression by PD98059 was reversed by ectopic overexpression of E2F, indicating that ERK/MAPK signaling is required for events that occur upstream of E2F release from phosphorylated Rb. In combination, these data demonstrate that the ERK/MAPK signal transduction pathway plays a central role in regulating E2F-dependent MCM expression and DNA replication in VSMC.

Identification of MCM4 as a target of the DNA replication block checkpoint system

Inhibition of the progression of DNA replication prevents further initiation of DNA replication and allows cells to maintain arrested replication forks, but the proteins that are targets of the replication checkpoint system remain to be identified. We report here that human MCM4, a subunit of the putative DNA replicative helicase, is extensively phosphorylated in HeLa cells when they are incubated in the presence of inhibitors of DNA synthesis or are exposed to UV irradiation. The data presented here indicate that the consecutive actions of ATR-CHK1 and CDK2 kinases are involved in this phosphorylation in the presence of hydroxyurea. The phosphorylation sites in MCM4 were identified using specific anti-phosphoantibodies. Based on results that showed that the DNA helicase activity of the MCM4-6-7 complex is negatively regulated by CDK2 phosphorylation, we suggest that the phosphorylation of MCM4 in the checkpoint control inhibits DNA replication, which includes blockage of DNA fork progression, through inactivation of the MCM complex.

Mammalian Mcm2/4/6/7 complex forms a toroidal structure

BACKGROUND

The Mcm proteins are a family of six homologous proteins (Mcm2-7) that play an important role in DNA replication. They form Mcm4/6/7 and Mcm2/4/6/7 complexes, but their structures are not known.

RESULTS

We found that the human Mcm2/4/6/7 tetramer forms a toroidal structure, with a central cavity about 3-4 nm in diameter. Observations were made using electron microscopy, employing the image analysis of single particles. The most predominant averaged image displayed a toroid harbouring four bulges forming corners, one of which was larger than the others. This structure was very similar to the mouse Mcm2/4/6/7 tetramer that was independently prepared and analysed by electron microscopy. These toroidal structures are distinct from that of the Mcm4/6/7 hexamer, which was also examined by electron microscopy. GST(glutathione S-transferase)-pull down and two hybrid experiments suggest that a putative Mcm6-Mcm6 hinge contributes to the formation of the Mcm7/4/6/6/4/7 heterohexamer.

CONCLUSIONS

The Mcm2/4/6/7 tetramer forms a toroidal structure that is distinct from that of the Mcm4/6/7 hexamer in size and shape.

Reconstitution of the Mcm2-7p heterohexamer, subunit arrangement, and ATP site architecture

The Mcm2-7p heterohexamer is the presumed replicative helicase in eukaryotic cells. Each of the six subunits is required for replication. We have purified the six Saccharomyces cerevisiae MCM proteins as recombinant proteins in Escherichia coli and have reconstituted the Mcm2-7p complex from individual subunits. Study of MCM ATPase activity demonstrates that no MCM protein hydrolyzes ATP efficiently. ATP hydrolysis requires a combination of two MCM proteins. The fifteen possible pairwise mixtures of MCM proteins yield only three pairs of MCM proteins that produce ATPase activity. Study of the Mcm3/7p ATPase shows that an essential arginine in Mcm3p is required for hydrolysis of the ATP bound to Mcm7p. Study of the pairwise interactions between MCM proteins connects the remaining MCM proteins to the Mcm3/7p pair. The data predict which subunits in the ATPase pairs bind the ATP that is hydrolyzed and indicate the arrangement of subunits in the Mcm2-7p heterohexamer.