Mutations in the pre-replication complex cause Meier-Gorlin syndrome

Meier-Gorlin syndrome (ear, patella and short-stature syndrome) is an autosomal recessive primordial dwarfism syndrome characterized by absent or hypoplastic patellae and markedly small ears¹⁻³. Both pre- and post-natal growth are impaired in this disorder, and although microcephaly is often evident, intellect is usually normal in this syndrome. We report here that individuals with this disorder show marked locus heterogeneity, and we identify mutations in five separate genes: ORC1, ORC4, ORC6, CDT1 and CDC6. All of these genes encode components of the pre-replication complex, implicating defects in replication licensing as the cause of a genetic syndrome with distinct developmental abnormalities.

Mutations in ORC1, encoding the largest subunit of the origin recognition complex, cause microcephalic primordial dwarfism resembling Meier-Gorlin syndrome

Studies into disorders of extreme growth failure (for example, Seckel syndrome and Majewski osteodysplastic primordial dwarfism type II) have implicated fundamental cellular processes of DNA damage response signaling and centrosome function in the regulation of human growth. Here we report that mutations in ORC1, encoding a subunit of the origin recognition complex, cause microcephalic primordial dwarfism resembling Meier-Gorlin syndrome. We establish that these mutations disrupt known ORC1 functions including pre-replicative complex formation and origin activation. ORC1 deficiency perturbs S-phase entry and S-phase progression. Additionally, we show that Orc1 depletion in zebrafish is sufficient to markedly reduce body size during rapid embryonic growth. Our data suggest a model in which ORC1 mutations impair replication licensing, slowing cell cycle progression and consequently impeding growth during development, particularly at times of rapid proliferation. These findings establish a novel mechanism for the pathogenesis of microcephalic dwarfism and show a surprising but important developmental impact of impaired origin licensing.

APETALA2/ETHYLENE RESPONSE FACTOR and basic helix-loop-helix tobacco transcription factors cooperatively mediate jasmonate-elicited nicotine biosynthesis

Transcription factors of the plant-specific apetala2/ethylene response factor (AP2/ERF) family control plant secondary metabolism, often as part of signalling cascades induced by jasmonate (JA) or other elicitors. Here, we functionally characterized the JA-inducible tobacco (Nicotiana tabacum) AP2/ERF factor ORC1, one of the members of the NIC2-locus ERFs that control nicotine biosynthesis and a close homologue of ORCA3, a transcriptional activator of alkaloid biosynthesis in Catharanthus roseus. ORC1 positively regulated the transcription of several structural genes coding for the enzymes involved in nicotine biosynthesis. Accordingly, overexpression of ORC1 was sufficient to stimulate alkaloid biosynthesis in tobacco plants and tree tobacco (Nicotiana glauca) root cultures. In contrast to ORCA3 in C. roseus, which needs only the GCC motif in the promoters of the alkaloid synthesis genes to induce their expression, ORC1 required the presence of both GCC-motif and G-box elements in the promoters of the tobacco nicotine biosynthesis genes for maximum transactivation. Correspondingly, combined application with the JA-inducible Nicotiana basic helix-loop-helix (bHLH) factors that bind the G-box element in these promoters enhanced ORC1 action. Conversely, overaccumulation of JAZ repressor proteins that block bHLH activity reduced ORC1 functionality. Finally, the activity of both ORC1 and bHLH proteins was post-translationally upregulated by a JA-modulated phosphorylation cascade, in which a specific mitogen-activated protein kinase kinase, JA-factor stimulating MAPKK1 (JAM1), was identified. This study highlights the complexity of the molecular machinery involved in the regulation of tobacco alkaloid biosynthesis and provides mechanistic insights about its transcriptional regulators.

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.

Multi-drug-resistant Staphylococcus aureus and future chemotherapy

Staphylococcus (S.) aureus silently stays as our natural flora, and yet sometimes threatens our life as a tenacious pathogen. In addition to its ability to outwit our immune system, its multi-drug resistance phenotype makes it one of the most intractable pathogenic bacteria in the history of antibiotic chemotherapy. It conquered practically all the antibiotics that have been developed since 1940s. In 1961, the first MRSA was found among S. aureus clinical isolates. Then MRSA prevailed throughout the world as a multi-resistant hospital pathogen. In 1997, MRSA strain Mu50 with reduced susceptibility to vancomycin was isolated. Vancomycin-intermediate S. aureus (VISA), so named according to the CLSI criteria, was the product of adaptive mutation of S. aureus against vancomycin that had long been the last resort to MRSA infection. Here, we describe the genetic basis for the remarkable ability of S. aureus to acquire multi-antibiotic resistance, and propose a novel paradigm for future chemotherapy against the multi-resistant pathogens.

The bacterial chromosome segregation protein Spo0J spreads along DNA from parS nucleation sites

Regulation of chromosome inheritance is essential to ensure proper transmission of genetic information. To accomplish accurate genome segregation, cells organize their chromosomes and actively separate them prior to cytokinesis. In Bacillus subtilis the Spo0J protein is required for accurate chromosome segregation and it regulates the developmental switch from vegetative growth to sporulation. Spo0J is a DNA-binding protein that recognizes at least eight identified parS sites located near the origin of replication. As judged by fluorescence microscopy, Spo0J forms discrete foci associated with the oriC region of the chromosome throughout the cell cycle. In an attempt to determine the mechanisms utilized by Spo0J to facilitate productive chromosome segregation, we have investigated the DNA binding activity of Spo0J. In vivo we find Spo0J associates with several kilobases of DNA flanking its specific binding sites (parS) through a parS-dependent nucleation event that promotes lateral spreading of Spo0J along the chromosome. Using purified components we find that Spo0J has the ability to coat non-specific DNA substrates. These 'Spo0J domains' provide large structures near oriC that could potentially demark, organize or localize the origin region of the chromosome.

Orc1 controls centriole and centrosome copy number in human cells

Centrosomes, each containing a pair of centrioles, organize microtubules in animal cells, particularly during mitosis. DNA and centrosomes are normally duplicated once before cell division to maintain optimal genome integrity. We report a new role for the Orc1 protein, a subunit of the origin recognition complex (ORC) that is a key component of the DNA replication licensing machinery, in controlling centriole and centrosome copy number in human cells, independent of its role in DNA replication. Cyclin A promotes Orc1 localization to centrosomes where Orc1 prevents Cyclin E-dependent reduplication of both centrioles and centrosomes in a single cell division cycle. The data suggest that Orc1 is a regulator of centriole and centrosome reduplication as well as the initiation of DNA replication.

The stringent response and cell cycle arrest in Escherichia coli

The bacterial stringent response, triggered by nutritional deprivation, causes an accumulation of the signaling nucleotides pppGpp and ppGpp. We characterize the replication arrest that occurs during the stringent response in Escherichia coli. Wild type cells undergo a RelA-dependent arrest after treatment with serine hydroxamate to contain an integer number of chromosomes and a replication origin-to-terminus ratio of 1. The growth rate prior to starvation determines the number of chromosomes upon arrest. Nucleoids of these cells are decondensed; in the absence of the ability to synthesize ppGpp, nucleoids become highly condensed, similar to that seen after treatment with the translational inhibitor chloramphenicol. After induction of the stringent response, while regions corresponding to the origins of replication segregate, the termini remain colocalized in wild-type cells. In contrast, cells arrested by rifampicin and cephalexin do not show colocalized termini, suggesting that the stringent response arrests chromosome segregation at a specific point. Release from starvation causes rapid nucleoid reorganization, chromosome segregation, and resumption of replication. Arrest of replication and inhibition of colony formation by ppGpp accumulation is relieved in seqA and dam mutants, although other aspects of the stringent response appear to be intact. We propose that DNA methylation and SeqA binding to non-origin loci is necessary to enforce a full stringent arrest, affecting both initiation of replication and chromosome segregation. This is the first indication that bacterial chromosome segregation, whose mechanism is not understood, is a step that may be regulated in response to environmental conditions.

The interaction of DiaA and DnaA regulates the replication cycle in E. coli by directly promoting ATP DnaA-specific initiation complexes

Escherichia coli DiaA is a DnaA-binding protein that is required for the timely initiation of chromosomal replication during the cell cycle. In this study, we determined the crystal structure of DiaA at 1.8 A resolution. DiaA forms a homotetramer consisting of a symmetrical pair of homodimers. Mutational analysis revealed that the DnaA-binding activity and formation of homotetramers are required for the stimulation of initiation by DiaA. DiaA tetramers can bind multiple DnaA molecules simultaneously. DiaA stimulated the assembly of multiple DnaA molecules on oriC, conformational changes in ATP-DnaA-specific initiation complexes, and unwinding of oriC duplex DNA. The mutant DiaA proteins are defective in these stimulations. DiaA associated also with ADP-DnaA, and stimulated the assembly of inactive ADP-DnaA-oriC complexes. Specific residues in the putative phosphosugar-binding motif of DiaA were required for the stimulation of initiation and formation of ATP-DnaA-specific-oriC complexes. Our data indicate that DiaA regulates initiation by a novel mechanism, in which DiaA tetramers most likely bind to multiple DnaA molecules and stimulate the assembly of specific ATP-DnaA-oriC complexes. These results suggest an essential role for DiaA in the promotion of replication initiation in a cell cycle coordinated manner.

SeqA blocking of DnaA-oriC interactions ensures staged assembly of the E. coli pre-RC

DnaA occupies only the three highest-affinity binding sites in E. coli oriC throughout most of the cell cycle. Immediately prior to initiation of chromosome replication, DnaA interacts with additional recognition sites, resulting in localized DNA-strand separation. These two DnaA-oriC complexes formed during the cell cycle are functionally and temporally analogous to yeast ORC and pre-RC. After initiation, SeqA binds to hemimethylated oriC, sequestering oriC while levels of active DnaA are reduced, preventing reinitiation. In this paper, we investigate how resetting of oriC to the ORC-like complex is coordinated with SeqA-mediated sequestration. We report that oriC resets to ORC during sequestration. This was possible because SeqA blocked DnaA binding to hemimethylated oriC only at low-affinity recognition sites associated with GATC but did not interfere with occupation of higher-affinity sites. Thus, during the sequestration period, SeqA repressed pre-RC assembly while ensuring resetting of E. coli ORC.

Origin licensing requires ATP binding and hydrolysis by the MCM replicative helicase

Loading of the six related Minichromosome Maintenance (MCM) proteins as head-to-head double hexamers during DNA replication origin licensing is crucial for ensuring once-per-cell-cycle DNA replication in eukaryotic cells. Assembly of these prereplicative complexes (pre-RCs) requires the Origin Recognition Complex (ORC), Cdc6, and Cdt1. ORC, Cdc6, and MCM are members of the AAA+ family of ATPases, and pre-RC assembly requires ATP hydrolysis. Here we show that ORC and Cdc6 mutants defective in ATP hydrolysis are competent for origin licensing. However, ATP hydrolysis by Cdc6 is required to release nonproductive licensing intermediates. We show that ATP binding stabilizes the wild-type MCM hexamer. Moreover, by analyzing MCM containing mutant subunits, we show that ATP binding and hydrolysis by MCM are required for Cdt1 release and double hexamer formation. This work alters our view of how ATP is used by licensing factors to assemble pre-RCs.

Regulation of the initiation of chromosomal replication in bacteria

The initiation of chromosomal replication occurs only once during the cell cycle in both prokaryotes and eukaryotes. Initiation of chromosome replication is the first and tightly controlled step of a DNA synthesis. Bacterial chromosome replication is initiated at a single origin, oriC, by the initiator protein DnaA, which specifically interacts with 9-bp non-palindromic sequences (DnaA boxes) at oriC. In Escherichia coli, a model organism used to study the mechanism of DNA replication and its regulation, the control of initiation relies on a reduction of the availability and/or activity of the two key elements, DnaA and the oriC region. This review summarizes recent research into the regulatory mechanisms of the initiation of chromosomal replication in bacteria, with emphasis on organisms other than E. coli.

Deregulation of Cdt1 induces chromosomal damage without rereplication and leads to chromosomal instability

The activity of human Cdt1 is negatively regulated by multiple mechanisms. This suggests that Cdt1 deregulation may have a deleterious effect. Indeed, it has been suggested that overexpression of Cdt1 can induce rereplication in cancer cells and that rereplication activates Ataxia-telangiectasia-mutated (ATM) kinase and/or ATM- and Rad3-related (ATR) kinase-dependent checkpoint pathways. In this report, we highlight a new and interesting aspect of Cdt1 deregulation: data from several different systems all strongly indicate that unregulated Cdt1 overexpression at pathophysiological levels can induce chromosomal damage other than rereplication in non-transformed cells. The most important finding in these studies is that deregulated Cdt1 induces chromosomal damage and activation of the ATM-Chk2 DNA damage checkpoint pathway even in quiescent cells. These Cdt1 activities are negatively regulated by cyclin A/Cdks, probably through modification by phosphorylation. Furthermore, we found that deregulated Cdt1 induces chromosomal instability in normal human cells. Since Cdt1 is overexpressed in cancer cells, this would be a new molecular mechanism leading to carcinogenesis.

The two chromosomes of Vibrio cholerae are initiated at different time points in the cell cycle

The bacterium Vibrio cholerae, the cause of the diarrhoeal disease cholera, has its genome divided between two chromosomes, a feature uncommon for bacteria. The two chromosomes are of different sizes and different initiator molecules control their replication independently. Using novel methods for analysing flow cytometry data and marker frequency analysis, we show that the small chromosome II is replicated late in the C period of the cell cycle, where most of chromosome I has been replicated. Owing to the delay in initiation of chromosome II, the two chromosomes terminate replication at approximately the same time and the average number of replication origins per cell is higher for chromosome I than for chromosome II. Analysis of cell-cycle parameters shows that chromosome replication and segregation is exceptionally fast in V. cholerae. The divided genome and delayed replication of chromosome II may reduce the metabolic burden and complexity of chromosome replication by postponing DNA synthesis to the last part of the cell cycle and reducing the need for overlapping replication cycles during rapid proliferation.

Cell cycle dependent regulation of the origin recognition complex

The eukaryotic origin recognition complex (ORC) not only selects the sites where prereplication complexes are assembled and DNA replication begins, it is the first in a series of multiple coherent pathways that determines when prereplication complexes are assembled. Data from yeast, frogs, flies and mammals present a compelling case that one or more of the six ORC subunits undergoes cell cycle dependent modifications involving phosphorylation and ubiquitination that repress ORC activity during S, G2 and M-phases. ORC activity is not restored until mitosis is complete and a nuclear membrane is present. In yeast, frogs and mammals, the same cyclin-dependent protein kinase [Cdk1(Cdc2)] that initiates mitosis also inhibits assembly of functional ORC/chromatin sites. In yeast, ORC remains bound to chromatin throughout cell division, but in the metazoa either ORC or the Orc1 subunit appears to cycle on and off the chromatin. Thus, this "ORC cycle" is the premier step in preventing rereplication of DNA during a single cell division cycle.

Radial glial neural progenitors regulate nascent brain vascular network stabilization via inhibition of Wnt signaling

Radial glial cells, which are neural stem cells well known for their role in neurogenesis, also play an unexpected role in stabilizing nascent blood vessels in the brain.

The cerebral cortex performs complex cognitive functions at the expense of tremendous energy consumption. Blood vessels in the brain are known to form stereotypic patterns that facilitate efficient oxygen and nutrient delivery. Yet little is known about how vessel development in the brain is normally regulated. Radial glial neural progenitors are well known for their central role in orchestrating brain neurogenesis. Here we show that, in the late embryonic cortex, radial glial neural progenitors also play a key role in brain angiogenesis, by interacting with nascent blood vessels and regulating vessel stabilization via modulation of canonical Wnt signaling. We find that ablation of radial glia results in vessel regression, concomitant with ectopic activation of Wnt signaling in endothelial cells. Direct activation of Wnt signaling also results in similar vessel regression, while attenuation of Wnt signaling substantially suppresses regression. Radial glial ablation and ectopic Wnt pathway activation leads to elevated endothelial expression of matrix metalloproteinases, while inhibition of metalloproteinase activity significantly suppresses vessel regression. These results thus reveal a previously unrecognized role of radial glial progenitors in stabilizing nascent brain vascular network and provide novel insights into the molecular cascades through which target neural tissues regulate vessel stabilization and patterning during development and throughout life.

The brain is an energy-intensive organ that consumes about 10 times as much energy per unit volume as the rest of the body. It therefore requires a highly efficient vascular network for oxygen and nutrient delivery, and as a result compromises in blood vessel networks influence a wide array of brain diseases. Our current understanding is that brain-specific neural cell types are involved in shaping its vascular network, but unfortunately little is known about the cellular or molecular mechanisms involved. Using a mouse genetic model, we have found that radial glial cells, a stem cell type well known for its fundamental role in neural circuit formation, also play an unexpected role in brain vessel development. We find that radial glial cells are essential for the stabilization of newly formed blood vessels in the late embryonic brain, and do so in large part through down-regulating canonical Wnt signaling in endothelial cells (which line the interior surface of blood vessels). These findings provide new insight into how new vessels in the brain are normally stabilized and how this process may be compromised and contribute to diseases.

RNA-dependent recruitment of the origin recognition complex

The origin recognition complex (ORC) has an important function in determining the initiation sites of DNA replication. In higher eukaryotes, ORC lacks sequence-specific DNA binding, and the mechanisms of ORC recruitment and origin determination are poorly understood. ORC is recruited with high efficiency to the Epstein-Barr virus origin of plasmid replication (OriP) through a complex mechanism involving interactions with the virus-encoded EBNA1 protein. We present evidence that ORC recruitment to OriP and DNA replication function depends on RGG-like motifs, referred to as LR1 and LR2, in the EBNA1 amino-terminal domain. Moreover, we show that LR1 and LR2 recruitment of ORC is RNA dependent. HMGA1a, which can functionally substitute for LR1 and LR2 domain, can also recruit ORC in an RNA-dependent manner. EBNA1 and HMGA1a RGG motifs bound to structured G-rich RNA, as did ORC1 peptides, which interact with EBNA1. RNase A treatment of cellular chromatin released a fraction of the total ORC, suggesting that ORC association with chromatin, and possibly cellular origins, is stabilized by RNA. We propose that structural RNA molecules mediate ORC recruitment at some cellular and viral origins, similar to OriP.

The many faces of the origin recognition complex

The hetero-hexameric origin recognition complex (ORC) is well known for its separable roles in DNA replication and heterochromatin assembly. However, ORC and its individual subunits have been implicated in diverse cellular activities in both the nucleus and the cytoplasm. Some of ORC's implied functions, such as cell cycle checkpoint control and mitotic chromosome assembly, may be indirectly related to its roles in replication control and/or heterochromatin assembly. Other suggested roles in ribosomal biogenesis and in centrosome and kinetochore function are based on localization/interaction data and are as yet inconclusive. However, recent findings directly link ORC to sister chromatin cohesion, cytokinesis and neural dendritic branching.

Apoptosis in budding yeast caused by defects in initiation of DNA replication

Apoptosis in metazoans is often accompanied by the destruction of DNA replication initiation proteins, inactivation of checkpoints and activation of cyclin-dependent kinases, which are inhibited by checkpoints that directly or indirectly require initiation proteins. Here we show that, in the budding yeast Saccharomyces cerevisiae, mutations in initiation proteins that attenuate both the initiation of DNA replication and checkpoints also induce features of apoptosis similar to those observed in metazoans. The apoptosis-like phenotype of initiation mutants includes the production of reactive oxygen species (ROS) and activation of the budding-yeast metacaspase Yca1p. In contrast to a recent report that activation of Yca1p only occurs in lysed cells and does not contribute to cell death, we found that, in at least one initiation mutant, Yca1p activation occurs at an early stage of cell death (before cell lysis) and contributes to the lethal effects of the mutation harbored by this strain. Apoptosis in initiation mutants is probably caused by DNA damage associated with the combined effects of insufficient DNA replication forks to completely replicate the genome and defective checkpoints that depend on initiation proteins and/or replication forks to restrain subsequent cell-cycle events until DNA replication is complete. A similar mechanism might underlie the proapoptotic effects associated with the destruction of initiation and checkpoint proteins during apoptosis in mammals, as well as genome instability in initiation mutants of budding yeast.

Chromosomal organization and segregation in Pseudomonas aeruginosa

The study of chromosomal organization and segregation in a handful of bacteria has revealed surprising variety in the mechanisms mediating such fundamental processes. In this study, we further emphasized this diversity by revealing an original organization of the Pseudomonas aeruginosa chromosome. We analyzed the localization of 20 chromosomal markers and several components of the replication machinery in this important opportunistic γ-proteobacteria pathogen. This technique allowed us to show that the 6.3 Mb unique circular chromosome of P. aeruginosa is globally oriented from the old pole of the cell to the division plane/new pole along the oriC-dif axis. The replication machinery is positioned at mid-cell, and the chromosomal loci from oriC to dif are moved sequentially to mid-cell prior to replication. The two chromosomal copies are subsequently segregated at their final subcellular destination in the two halves of the cell. We identified two regions in which markers localize at similar positions, suggesting a bias in the distribution of chromosomal regions in the cell. The first region encompasses 1.4 Mb surrounding oriC, where loci are positioned around the 0.2/0.8 relative cell length upon segregation. The second region contains at least 800 kb surrounding dif, where loci show an extensive colocalization step following replication. We also showed that disrupting the ParABS system is very detrimental in P. aeruginosa. Possible mechanisms responsible for the coordinated chromosomal segregation process and for the presence of large distinctive regions are discussed.

The processes of chromosomal disposition, replication, and segregation in bacteria have been characterized only in a handful of species, yet there is remarkable diversity in the ways such fundamental processes are managed. In this study, we analyzed the subcellular chromosomal organization of Pseudomonas aeruginosa, an important bacterial pathogen belonging to a large bacterial group involved in plant and human diseases. Most bacterial genomes are circular molecules, and DNA replication proceeds bidirectionally from a single origin to the opposite Ter region, where the replication forks meet. Analysis by fluorescence microscopy of 20 chromosomal markers and components of the replication machinery revealed that the 6.3 Mb chromosome is globally oriented from the old pole of the cell to the division plane/new pole along the oriC-Ter axis. The replication machinery is positioned at mid-cell, and chromosomal loci from oriC to Ter are moved sequentially to mid-cell prior to replication. The two sister chromosomes are subsequently segregated at their final subcellular destination in the two halves of the cell. This study also identified two large regions in which several chromosomal loci show a biased localization pattern, suggesting that processes responsible for long-range chromosomal organization might exist in P. aeruginosa.

The BAH domain facilitates the ability of human Orc1 protein to activate replication origins in vivo

Selection of initiation sites for DNA replication in eukaryotes is determined by the interaction between the origin recognition complex (ORC) and genomic DNA. In mammalian cells, this interaction appears to be regulated by Orc1, the only ORC subunit that contains a bromo-adjacent homology (BAH) domain. Since BAH domains mediate protein-protein interactions, the human Orc1 BAH domain was mutated, and the mutant proteins expressed in human cells to determine their affects on ORC function. The BAH domain was not required for nuclear localization of Orc1, association of Orc1 with other ORC subunits, or selective degradation of Orc1 during S-phase. It did, however, facilitate reassociation of Orc1 with chromosomes during the M to G1-phase transition, and it was required for binding Orc1 to the Epstein-Barr virus oriP and stimulating oriP-dependent plasmid DNA replication. Moreover, the BAH domain affected Orc1's ability to promote binding of Orc2 to chromatin as cells exit mitosis. Thus, the BAH domain in human Orc1 facilitates its ability to activate replication origins in vivo by promoting association of ORC with chromatin.

The genetics of symptom-based phenotypes: toward a molecular classification of schizophrenia

OBJECTIVE

Genetic linkage studies in schizophrenia (SZ) have primarily focused on the phenotype of disease susceptibility. A limited number of studies, however, have reported suggestive linkage to specific SZ symptom domains including regions on chromosomes 6, 8, and 20. We examined these chromosomal regions for association to positive, negative, and disorganized symptom clusters, using a dense set of single-nucleotide polymorphisms (SNPs).

METHODS

We ascertained 178 Caucasian patients with SZ for lifetime severity of clinical symptomatology using a structured diagnostic interview. The cohort was genotyped with the Affymetrix 500K microarray, from which we selected, a priori, 4833 intragenic SNPs located within chromosomal regions previously linked to specific SZ symptom clusters. Parametric tests, corrected for multiple testing, were used to compare the effects of allelic variation within these SNPs to the lifetime severity of the specific symptom domain that had been implicated by prior linkage studies.

RESULTS

We were able to extend previous reports of linkage between chromosome 6q and both positive and disorganized symptoms. Lifetime severity of positive symptoms was significantly (P = 2.50 x 10(-5)) associated with a SNP within the origin recognition complex subunit 3-like (ORC3L) gene, a gene implicated in synaptic plasticity. Level of disorganized symptoms was significantly (P < 6.00 x 10(-5)) associated 2 SNPs within the brain-specific angiogenesis inhibitor 3 (BAI3) gene, which is highly expressed in brain during development.

CONCLUSIONS

These data point toward specific candidate genes located within previously implicated linkage peaks for clinical symptomatology. Identification of functional variants within these regions and a characterization of the effect of these risk genotypes on the treatment of specific clinical symptoms are needed.

Cdc6 ATPase activity regulates ORC x Cdc6 stability and the selection of specific DNA sequences as origins of DNA replication

DNA replication, as with all macromolecular synthesis steps, is controlled in part at the level of initiation. Although the origin recognition complex (ORC) binds to origins of DNA replication, it does not solely determine their location. To initiate DNA replication ORC requires Cdc6 to target initiation to specific DNA sequences in chromosomes and with Cdt1 loads the ring-shaped mini-chromosome maintenance (MCM) 2-7 DNA helicase component onto DNA. ORC and Cdc6 combine to form a ring-shaped complex that contains six AAA+ subunits. ORC and Cdc6 ATPase mutants are defective in MCM loading, and ORC ATPase mutants have reduced activity in ORC x Cdc6 x DNA complex formation. Here we analyzed the role of the Cdc6 ATPase on ORC x Cdc6 complex stability in the presence or absence of specific DNA sequences. Cdc6 ATPase is activated by ORC, regulates ORC x Cdc6 complex stability, and is suppressed by origin DNA. Mutations in the conserved origin A element, and to a lesser extent mutations in the B1 and B2 elements, induce Cdc6 ATPase activity and prevent stable ORC x Cdc6 formation. By analyzing ORC x Cdc6 complex stability on various DNAs, we demonstrated that specific DNA sequences control the rate of Cdc6 ATPase, which in turn controls the rate of Cdc6 dissociation from the ORC x Cdc6 x DNA complex. We propose a mechanism explaining how Cdc6 ATPase activity promotes origin DNA sequence specificity; on DNA that lacks origin activity, Cdc6 ATPase promotes dissociation of Cdc6, whereas origin DNA down-regulates Cdc6 ATPase resulting in a stable ORC x Cdc6 x DNA complex, which can then promote MCM loading. This model has relevance for origin specificity in higher eukaryotes.

DNA and origin region segregation are not affected by the transition from rod to sphere after inhibition of Escherichia coli MreB by A22

The bacterial actin homologue MreB forms a helix underneath the cytoplasmic membrane and was shown to be essential in the morphogenesis of the rod-shaped cells. Additionally, MreB was implicated to be involved in DNA segregation. However, in our hands the mreBCD deletion strain (PA340-678) grew without apparent DNA segregation defect, suggesting that the reported chromosome segregation inhibition could be caused by a temporarily effect of MreB inhibition or depletion. To assess the involvement of MreB in DNA segregation during the transition from rod to sphere, we compared the effect of A22 and the PBP2 inhibitor mecillinam on the percentage of cells with segregated nucleoids and the number of oriC foci in wild-type Escherichia coli cells. Cells became spherical in the same time window during both treatments and we could not detect any difference in the chromosome or oriC segregation between these two treatments. Additionally, flow cytometric analyses showed that A22 and mecillinam treatment gave essentially the same chromosome segregation pattern. We conclude that MreB is not directly involved in DNA segregation of E. coli.

Origins of DNA replication in eukaryotes

Errors occurring during DNA replication can result in inaccurate replication, incomplete replication, or re-replication, resulting in genome instability that can lead to diseases such as cancer or disorders such as autism. A great deal of progress has been made toward understanding the entire process of DNA replication in eukaryotes, including the mechanism of initiation and its control. This review focuses on the current understanding of how the origin recognition complex (ORC) contributes to determining the location of replication initiation in the multiple chromosomes within eukaryotic cells, as well as methods for mapping the location and temporal patterning of DNA replication. Origin specification and configuration vary substantially between eukaryotic species and in some cases co-evolved with gene-silencing mechanisms. We discuss the possibility that centromeres and origins of DNA replication were originally derived from a common element and later separated during evolution.