A comparison of the incidence of undiagnosed congenital heart disease in hospital born and home born children.

OBJECTIVE: To evaluate the incidence of otherwise undiagnosed congenital heart disease (CHD) in a population of children born in a hospital with routine pulse oximetry (RPO) screening compared to children born at home. METHODS: We reviewed 15 years of births at 2 hospitals for incidence of undiagnosed CHD with RPO. The Health Department reviewed the same data for out of hospital births. RESULTS: A total of 50,545 hospital births were screened and 1,274 children were born outside the hospital. There were 28 hospital-born babies diagnosed with cyanotic CHD prior to nursery discharge. Only one of these babies would not have been diagnosed without RPO. Three children were missed and there were 3 false positives. Sensitivity and positive predictive value of RPO was 25%, specificity and negative predictive value of RPO exceed 99%. The incidence of CHD requiring RPO diagnosis was roughly one birth per 50,000. Two children born at home with undiagnosed CHD were missed. One of these children presented with neonatal demise. CONCLUSION: RPO screening is still valuable in diagnosing CHD only diagnosable with RPO. However, the incidence of CHD requiring RPO to diagnose is similar to other congenital diseases which are not mandated national screening tests. In our limited experience a patient is roughly 25 times more likely to have undiagnosed CHD if they are born outside of a hospital.

Analysis of genetic control elements in eukaryotes: transcriptional activity or nuclear hitchhiking?

A common way to analyse basal and stimulated activity of eukaryotic genetic control elements, such as promoters and enhancers, is to introduce them into cells via DNA vectors containing an easily assayable reporter gene. Activity is then studied by measurement of transiently produced mRNA or reporter protein. In such assays, it is assumed that the variable measured is proportional to the transcriptional activity of the control element under investigation. Here we question the validity of this generally accepted assumption. Specifically, recent observations indicate that control elements, in addition to modulating transgene transcription, can facilitate the nuclear uptake of their carrier plasmids. This process is mediated by transcription factors or other nuclear proteins harbouring nuclear localisation signals, which bind to the control elements in the cytoplasm and transport the DNA into the nucleus through the protein nuclear import machinery. As the number of mRNA transcripts produced for an epi-chromosomally expressed transgene is directly related to its copy number inside the nucleus, such transport activity may lead to substantial overestimation of the transcriptional potency of the control element(s) studied.

A regulated, NFkappaB-assisted import of plasmid DNA into mammalian cell nuclei.

The success of synthetic DNA delivery systems in human gene therapy will be enhanced by increasing transfection efficiencies and by providing tighter control over targeting of the DNA into the nucleus. Here, we used DNA vectors that contain repetitive binding sites for the inducible transcription factor NFkappaB, which is transported into the nucleus by the nuclear import machinery. Nuclear entry of the modified vectors was augmented 12-fold and was associated with corresponding increase in gene expression. Depending on their position, the binding sites could also function as transcriptional enhancers, increasing gene expression levels up to an additional 19-fold. Notably, nuclear targeting of the DNA and transgene transcription could both be regulated by exogenous stimulators which modulate the intracellular distribution of NFkappaB. The approach provides a framework for the controlled targeting of constitutive or transcriptionally regulated synthetic vectors into mammalian cell nuclei.

Scanning force microscopy in the applied biological sciences.

Fifteen years after its invention, the scanning force microscope (SFM) is rooted deep in the biological sciences. Here we discuss the use of SFM in biotechnology and biomedical research. The spectrum of applications reviewed includes imaging, force spectroscopy and mapping, as well as sensor applications. It is our hope that this review will be useful for researchers considering the use of SFM in their studies but are uncertain about its scope of capabilities. For the benefit of readers unfamiliar with SFM technology, the fundamentals of SFM imaging and force measurement are also briefly introduced.

Thermodynamics of T cell receptor binding to peptide-MHC: evidence for a general mechanism of molecular scanning.

Antigen-dependent activation of T lymphocytes requires T cell receptor (TCR)-mediated recognition of specific peptides, together with the MHC molecules to which they are bound. To achieve this recognition in a reasonable time frame, the TCR must scan and discriminate rapidly between thousands of MHC molecules differing from each other only in their bound peptides. Kinetic analysis of the interaction between a TCR and its cognate peptide-MHC complex indicates that both association and dissociation depend heavily on the temperature, indicating the presence of large energy barriers in both phases. Thermodynamic analysis reveals changes in heat capacity and entropy that are characteristic of protein-ligand associations in which local folding is coupled to binding. Such an "induced-fit" mechanism is characteristic of sequence-specific DNA-binding proteins that must also recognize specific ligands in the presence of a high background of competing elements. Here, we propose that induced fit may endow the TCR with its requisite discriminatory capacity and suggest a model whereby the loosely structured antigen-binding loops of the TCR rapidly explore peptide-MHC complexes on the cell surface until some critical structural complementarity is achieved through localized folding transitions. We further suggest that conformational changes, implicit in this model, may also propagate beyond the TCR antigen-binding site and directly affect self-association of ligated TCRs or TCR-CD3 interactions required for signaling.

Initiation of signal transduction through the T cell receptor requires the multivalent engagement of peptide/MHC ligands [corrected].

While much is known about intracellular signaling events in T cells when T cell receptors (TCRs) are engaged, the mechanism by which signaling is initiated is unclear. We have constructed defined oligomers of soluble antigen-major histocompatibility complex (MHC) molecules, the natural ligands for the TCR. Using these to stimulate specific T cells in vitro, we find that agonist peptide/MHC ligands are nonstimulatory as monomers and minimally stimulatory as dimers. Similarly, a partial-agonist ligand is very weakly active as a tetramer. In contrast, trimeric or tetrameric agonist ligands that engage multiple TCRs for a sustained duration are potent stimuli. Ligand-driven formation of TCR clusters seems required for effective activation and helps to explain the specificity and sensitivity of T cells.

Secondary structure composition and pH-dependent conformational changes of soluble recombinant HLA-DM.

HLA-DM catalyzes the release of invariant chain fragments from newly synthesized major histocompatibility complex (MHC) class II molecules, stabilizes empty class II molecules, and edits class II-associated peptides by preferentially releasing those that are loosely bound. The ability of HLA-DM to carry out these functions in vitro is pH dependent, with an optimum at pH 4.5-5.5 and poor activity at pH 7. The structural basis for these properties of HLA-DM is unknown. Sequence homology suggests that HLA-DM resembles classical, peptide-binding MHC class II molecules. In this study, we examined whether HLA-DM has a secondary structure composition consistent with an MHC fold and whether HLA-DM changes conformation between pH 5 and pH 7. Far-UV circular dichroism (CD) spectra of recombinant soluble HLA-DM (sDM) indicate that HLA-DM belongs to the alpha/beta class of proteins and structurally resembles both MHC class I and class II molecules. The CD peak around 198 nm increases upon going from neutral to endosomal pH and drops sharply upon denaturation below pH 3.5, distinguishing at least three states of sDM: the denatured state and two highly similar folded states. Fluorescence emission spectra show a slight blue-shift and a approximately 20% drop in intensity at pH 5 compared with pH 7. Unfolding experiments using guanidinium chloride show that the stability of sDM is somewhat reduced but not lost at pH 5. These results indicate that sDM undergoes a pH-dependent conformational change between neutral and endosomal pH. The change seems to involve both hydrogen bonding patterns and the hydrophobic core of sDM and may contribute to the pH dependence of DM activity.

Ligand recognition by alpha beta T cell receptors.

While still incomplete, the first data concerning the biochemistry of T cell receptor-ligand interactions in cell-free systems seem to have considerable predictive value regarding whether a T cell response is strong or weak or suppressive. This data will help considerably in elucidating the mechanisms behind T cell responsiveness. Also of great interest are the first structures of T cell receptor molecules and, particularly, TCR-ligand complexes. These appear to confirm earlier suggestions of a fixed orientation for TCR engagement with peptide/MHC and should form the basis for understanding higher oligomers, evidence for which has also just emerged. We conclude with an analysis of the highly diverse CDR3 loops found in all antigen receptor molecules and suggest that such regions form the core of both TCR and antibody specificity.

Nucleosomes: a solution to a crowded intracellular environment?

The emergence of eukaryotes was accompanied by two major events that concern their genome and are of crucial significance when considered in terms of macromolecular crowding: (i) a substantial increase in the amount of DNA, and (ii) its confinement within a defined space. The resulting highly crowded environment would have strongly promoted DNA self-assembly processes, leading to extremely condensed and thermodynamically stable DNA aggregates. Such structural transitions have indeed been observed in vitro, as well as in virtually all cellular systems in which a nucleosomal assembly is absent. In this appear we raise the hypothesis that upon transition from prokaryotic systems to eukaryotes, the nucleosomes were rendered essential in order to negate extensive DNA condensation processes that would have resulted from excluded volume effects. By suppressing such processes, the nucleosomes act to maintain and regulate the conformational space of the DNA, thus enabling conformational flexibility and reversible structural modulations.

Ligand-specific oligomerization of T-cell receptor molecules.

T cells initiate many immune responses through the interaction of their T-cell antigen receptors (TCR) with antigenic peptides bound to major histocompatibility complex (MHC) molecules. This interaction sends a biochemical signal into the T cell by a mechanism that is not clearly understood. We have used quasielastic light scattering (QELS) to show that, in the presence of MHC molecules bound to a full agonist peptide, TCR/peptide-MHC complexes oligomerize in solution to form supramolecular structures at concentrations near the dissociation constant of the binding reaction. The size of the oligomers is concentration dependent and is calculated to contain two to six ternary complexes for the concentrations tested here. This effect is specific as neither molecule forms oligomers by itself, nor were oligomers observed unless the correct peptide was bound to the MHC. These results provide direct evidence for models of T-cell signalling based on the specific assembly of multiple TCR/peptide-MHC complexes in which the degree of assembly determines the extent and qualitative nature of the transduced signal. They may also explain how T cells maintain sensitivity to antigens present in only low abundance on the antigen-presenting cell.

The recognition of the nonclassical major histocompatibility complex (MHC) class I molecule, T10, by the gammadelta T cell, G8.

Recent studies have shown that many nonclassical major histocompatibility complex (MHC) (class 1b) molecules have distinct antigen-binding capabilities, including the binding of nonpeptide moieties and the binding of peptides that are different from those bound to classical MHC molecules. Here, we show that one of the H-2T region-encoded molecules, T10, when produced in Escherichia coli, can be folded in vitro with beta2-microglobulin (beta2m) to form a stable heterodimer in the absence of peptide or nonpeptide moieties. This heterodimer can be recognized by specific antibodies and is stimulatory to the gammadelta T cell clone, G8. Circular dichroism analysis indicates that T10/beta2m has structural features distinct from those of classical MHC class I molecules. These results suggest a new way for MHC-like molecules to adopt a peptide-free structure and to function in the immune system.

Stability of empty and peptide-loaded class II major histocompatibility complex molecules at neutral and endosomal pH: comparison to class I proteins.

The structure and thermal stability of empty and peptide-filled forms of the murine class II major histocompatibility complex (MHC) molecule I-E(k) were studied at neutral and mildly acidic pH. The two forms have distinct circular dichroic spectra, suggesting that a conformational change may accompany peptide binding. Thermal stability profiles indicate that binding of peptide significantly increases the thermal stability of the empty heterodimers at both neutral and mildly acidic pH. Free energies calculated from these data provide a direct measure of this stabilization and show that the empty form of I-E(k) is significantly more stable than that of class I MHC proteins. Furthermore, for the two MHC class II proteins that were analyzed (I-E(k) and I-A(d)), thermal stability was not significantly altered by acidification. In contrast, of four class I MHC molecules studied, three have shown a significant loss in complex stability at low pH. The marked stability exhibited by their empty form, as well as their resistance to low pH, as observed in this study, correlate well with the ability of class II MHC molecules to traverse and bind peptides in acidic endosomal vesicles.

Ala244Val is a common, probably ancient mutation causing factor VII deficiency in Moroccan and Iranian Jews.

We investigated the molecular basis for factor VII (FVII) deficiency in Israel and found that 13 patients were homozygous and 10 heterozygous for a C to T substitution at nucleotide 10648 of the FVII gene. This predicted an Ala244Val change and was associated with decreased FVII activity and antigen level. Of the 36 Ala244Val positive alleles, 20 were observed in patients of Moroccan origin, 10 in Iranian-Jewish patients and 6 in patients of other origins. A computer model of the serine protease domain of FVII suggested that the Ala244Val substitution may cause distortion of the entire protein structure. Intragenic polymorphic sites analyses disclosed a founder effect for the Moroccan and Iranian-Jewish patients. A survey of the Ala244Val mutation revealed an allele frequency of 1:42.5 in Moroccan Jews and 1:40 in Iranian Jews. As Moroccan Jews have been separated from Iranian Jews for more than two millennia, the data suggest that the Ala244Val mutation occurred in ancient times.

Flow of structural information between four DNA conformational levels.

Closed-circular supercoiled DNA molecules have been shown to form a cholesteric assembly within bacteria as well as in vitro under physiological DNA and salt concentrations. Circular dichroism and X-ray scattering studies indicate that the macroscopic structural properties of the chiral mesophase are directly and uniquely dictated by the supercoiling parameters of the constituent molecules. Specifically, we find that the pitch of the DNA cholesteric phase derived from supercoiled DNA is determined by the superhelical density, which, in turn, is modulated by secondary conformational changes. A direct interrelationship among four DNA structural levels, namely, DNA sequence, secondary structural transitions, the tertiary superhelical conformation, and the quaternary, supramolecular organization is accordingly pointed out. Since secondary conformational changes are both sequence and environment dependent, alterations of cellular conditions may effectively modulate the properties of the packed DNA organization, through their effects on secondary structural transitions and hence on the superhelical parameters. On the basis of these results we suggest that liquid crystallinity represents an effectively regulated packaging mode of plectonemic, nucleosome-free DNA molecules in living systems.

In vivo quantitative characterization of intermolecular interactions.

Extensive effort has been directed toward a quantitative evaluation of forces which operate between biomacromolecules since the characterization of such forces is essential to a thorough understanding of fundamental biological processes. However, all studies hitherto reported were conducted in vitro, using isolated species. Here we report the first quantitative characterization of forces operating between DNA molecules within living bacteria. Evaluation of x-ray scattering studies conducted on intact bacteria indicates that, at DNA-DNA surface separations characteristic of DNA assemblies, interactions are dominated by repulsive hydration forces which originate from the structuring of water molecules. The results support the notion that the mechanisms by means of which macromolecules function, fold, and interact with each other crucially depend upon their hydration properties.

Supercoiling-regulated liquid-crystalline packaging of topologically-constrained, nucleosome-free DNA molecules.

Electron microscopy and circular dichroism studies of cholesteric aggregates derived from topologically-constrained DNA molecules indicate that the overall morphology and structural properties of these aggregates are fundamentally different from those characterizing condensed structures of nonconstrained DNA species. Specifically, the cholesteric pitch and twist of all hitherto characterized lyotropic mesophases of biopolymers--including those obtained from linear DNA--depend predominantly upon environmental parameters such as the dielectric constant of the solvent. In contrast, the properties of aggregates derived from closed circular supercoiled DNA are found to be solely and directly dictated by the superhelical density and handedness. On the basis of these results, as well as on the demonstrated ubiquity of liquid-crystalline DNA organizations in vivo, we suggest that supercoiling-regulated liquid crystallinity represents an effective packaging mode of nucleosome-free, topologically-constrained DNA molecules in living systems.

Liquid-crystalline mesophases of plasmid DNA in bacteria.

Bacterial plasmids may often reach a copy number larger than 1000 per cell, corresponding to a total amount of DNA that may exceed the amount of DNA within the bacterial chromosome. This observation highlights the problem of cellular accommodation of large amounts of closed-circular nucleic acids, whose interwound conformation offers negligible DNA compaction. As determined by x-ray scattering experiments conducted on intact bacteria, supercoiled plasmids segregate within the cells into dense clusters characterized by a long-range order. In vitro studies performed at physiological DNA concentrations indicated that interwound DNA spontaneously forms liquid crystalline phases whose macroscopic structural properties are determined by the features of the molecular supercoiling. Because these features respond to cellular factors, DNA supercoiling may provide a sensitive regulatory link between cellular parameters and the packaging modes of interwound DNA in vivo.

Effects of adenine tracts on the B-Z transition. Fine tuning of DNA conformational transition processes.

The effects exerted by short runs of adenines (A-tracts), alternating (AT)n segments, and single-stranded DNA upon the right- to left-handed DNA transition, as well as upon the energetic and structural parameters of the B/Z junctions, were investigated by using synthetic segments in which these motifs are coupled to a potentially Z-forming core. UV, CD, and 31P NMR studies of the salt-induced B to Z transition occurring in the various segments indicate that the transition is composed of two phases: a slow rate-determining induction of an initial structural deformation followed by a cooperative propagation of this "nucleus" in the form of a left-handed Z-DNA. The first phase is found to be crucially affected by the nature of the sequences coupled to the potentially Z-forming core. Thus, a higher rigidity of the flanking segments, such as that characterizing adenine tracts, is associated with higher energy values required for the induction of the initial conformational deformation, as well as with more defined structural parameters of the ultimate B/Z junctions. The second phase is affected mainly by the composition and sequence of the Z-forming segment. The observations that DNA conformational changes can be finely tuned and modulated by parameters pertaining to both the segment which undergoes the transition and the flanking sequences support the notion that DNA secondary motifs, such as the Z form and A-tracts, might be involved in the regulation of cellular processes.

On the metastability of left-handed DNA motifs.

Alternating purine-pyrimidine DNA sequences such as poly [d(C-G)] or poly[d(m5C-G)] undergo a cooperative, salt-induced structural transition from a right-handed B conformation, which prevails at relatively low ionic strength, into a left-handed Z form, generally believed to be stabilized by high salt concentrations. We report here that upon a monotonous increase of the ionic strength, the well-established B to Z transition is followed by a second, hitherto unobserved conformational change leading from Z-DNA back into a right-handed B-like form. This observation indicates that, in contrast with the current convention, the Z motif represents an unstable configuration relative to the B form at both low and high salt concentrations and that the occurrence of a left-handed DNA structure, presently depicted as a step function of the ionic strength, should rather be treated in terms of a pulse. The reported transition underscores the inherent metastability of the Z configuration, and indicates, consequently, that this motif is ideally suited to act as a structural regulatory element, as such an element should be endowed with a large susceptibility to cellular parameters.

Prediction of protein folding pathways.

Recent 1H nuclear magnetic resonance (n.m.r.) hydrogen exchange experiments on five different proteins have delineated the secondary structures formed in trapped, partially folded intermediates. The early forming structural elements are identifiable through a technique described in this work to predict folding pathways. The method assumes that the sequential selection of structural fragments such as alpha-helices and beta-strands involved in the folding process is founded upon the maximal burial of solvent accessible surface from both the formation of internal structure and substructure association. The substructural elements were defined objectively by major changes in main-chain direction. The predicted folding pathways are in complete correspondence with the n.m.r. results in that the formed structural fragments found in the folding intermediates are those predicted earliest in the pathways. The technique was also applied to proteins of known tertiary structure and with fold similar to one of the five proteins examined by 1H n.m.r. The pathways for these structures also showed general consistency with the n.m.r. observations, suggesting conservation of a secondary structural framework or molten globule about which folding nucleates and proceeds.