Sudden unexpected death and covert homicide in infancy.

It is impossible to be certain, but it is estimated that each year in England and Wales there may be about 30-40 infant deaths from covert homicide, which represents about 10% of the current annual total of sudden unexpected deaths in infancy. This paper reviews the features that have been suggested as possible indicators of covert homicide, describes the difficulties in its identification and the need for better evidence, and emphasises the importance of thorough medical investigation of all sudden infant deaths.

Changes in psychological distress after cancer genetic counselling: a comparison of affected and unaffected women.

This study sought to examine changes in psychological distress following cancer genetic counselling. Women attending a family cancer clinic completed questionnaires before their appointment and at 2 weeks, 6 months and 12 months after their appointment. Twenty-six women were at low risk of developing breast or ovarian cancer, 76 were at moderate risk, 46 were at high risk and 46 women had previously had breast or ovarian cancer. All groups were compared with regard to measures of anxiety, depression, general psychological distress, worry about developing breast and ovarian cancer, and perceived risk of developing breast/ovarian cancer and perceived likelihood of carrying a genetic mutation. General psychological distress did not change over the course of the study and the groups did not differ on these measures. Worry about developing breast cancer and perceptions of the likelihood of carrying a genetic mutation significantly reduced following genetic counselling. On the whole women who had already had breast/ovarian cancer showed more concerns about ovarian cancer and raised perceptions of risk in comparison with the other groups, indicating the need for sensitive counselling of such women.

Telomere shortening is proportional to the size of the G-rich telomeric 3'-overhang.

Most normal diploid human cells do not express telomerase activity and are unable to maintain telomere length with ongoing cell divisions. We show that the length of the single-stranded G-rich telomeric 3'-overhang is proportional to the rate of shortening in four human cell types that exhibit different rates of telomere shortening in culture. These results provide direct evidence that the size of the G-rich overhang is not fixed but subject to regulation. The potential ability to manipulate this rate has profound implications both for slowing the rate of replicative aging in normal cells and for accelerating the rate of telomere loss in cancer cells in combination with anti-telomerase therapies.

Children are not goldfish--mark/recapture techniques and their application to injury data.

OBJECTIVES: Mark/recapture (or capture-recapture) is a simple technique commonly applied to estimate the hypothetical total (including undercount) in a register composed of cases from two or more independent and separately incomplete case lists. This paper seeks to illustrate serious drawbacks in the use of the mark/recapture technique when applied to injuries. SETTING AND SUBJECTS: Northumbrian children under 15 years of age who were seriously injured in motor vehicle accidents (MVAs) over a five year period ascertained from two data sources: police reports and hospital inpatient records. METHODS: Individuals (n) appearing in both police (S) and hospital (H) case lists are identified using various matching criteria. The separate and combined influence of age, sex, and casualty class (cyclist, passengers, pedestrians) on the probability of such matching is estimated using multivariate techniques. The hypothetical total incidence of child MVA victims (N) is calculated from N = (S x H)/n. MAIN OUTCOMES: Estimates of the incidences of "serious" injuries in MVAs under various conditions of stratification and matching. The overall procedure is tested for conformity with accepted criteria for valid use of mark/recapture. RESULTS: About one third of the 1009 police and 836 hospital records could be exactly matched. There were significant variations in matching proportions by class of accident (pedestrian v passenger v cyclist). This selective recapture or "heterogeneity" was not affected by sex, but was independently influenced by the age of the child. Further uncertainty was introduced when matching criteria were slightly relaxed. Estimates of the total population of children with serious injuries vary accordingly from 1729 to 2743. A number of plausible reasons why these two data sources might not be unbiased or mutually independent samples of the total target population are proposed as explanations for this heterogeneity. CONCLUSION: This typical example of two sample mark/recapture estimation in an epidemiological setting can be shown to violate virtually all the requirements for valid use of the technique. Very little can be deduced accurately about the scale or characteristics of an unobserved group by the use of mark/recapture applied to two overlapping health event registers.

DNA-sequence asymmetry directs the alignment of recombination sites in the FLP synaptic complex.

The FLP recombinase promotes site-specific recombination in the 2 micrometer circle of Saccharomyces cerevisiae. FLP recognizes a 48 bp target site (FLP recombination target, or FRT) consisting of three 13 bp protein binding sites, or symmetry elements, flanking an 8 bp spacer region. Efficient recombination also occurs with DNA substrates that have minimal FRT sites, consisting only of the spacer and two surrounding 13 bp symmetry elements arranged in inverse orientation; thus, the wild-type spacer sequence is the main asymmetric feature of the minimal recombination site. FLP carries out recombination with many minimal target sites bearing symmetric or asymmetric mutant spacer sequences; however, the overall directionality of recombination defined in terms of inversion or excision of a DNA domain is determined by spacer-sequence asymmetry. In order to evaluate the potential influence of spacer-sequence asymmetry on structures formed during early steps in recombination, we used electron microscopy to investigate the structure of the FLP synaptic complex, which is the intermediate protein-DNA complex involved in site pairing and strand exchange. Using linear substrate DNAs that have minimal FRTs with wild-type spacer sequences, we find that 85 to 90% of the FLP synaptic complexes examined contain the two FRTs aligned in parallel. This strong preference for parallel site alignment stands in contrast with prevailing models for lambda integrase-class recombination systems, which postulate antiparallel site alignment, and results from biophysical studies on synthetic, immobile four-way DNA junctions. Our results show that the strong preference for parallel alignment can be attributed to conformational preferences of Holliday junctions present in the synaptosome.

Normal human chromosomes have long G-rich telomeric overhangs at one end.

Telomeres protect the ends of linear chromosomes from degradation and abnormal recombination events, and in vertebrates may be important in cellular senescence and cancer. However, very little is known about the structure of human telomeres. In this report we purify telomeres and analyze their termini. We show that following replication the daughter telomeres have different terminal overhangs in normal diploid telomerase-negative human fibroblasts. Electron microscopy of those telomeres that have long overhangs yields 200 +/- 75 nucleotides of single-stranded DNA. This overhang is four times greater than the amount of telomere shortening per division found in these cells. These results are consistent with models of telomere replication in which leading-strand synthesis generates a blunt end while lagging-strand synthesis produces a long G-rich 3' overhang, and suggest that variations in lagging-strand synthesis may regulate the rate of telomere shortening in normal diploid human cells. Our results do not exclude the possibility that nuclease processing events following leading strand synthesis result in short overhangs on one end.

Supercoiling-dependent flexibility of adenosine-tract-containing DNA detected by a topological method.

Intrinsically bent DNA sequences have been implicated in the activation of transcription by promoting juxtaposition of DNA sequences near the terminal loop of a superhelical domain. We have developed a novel topological assay for DNA looping based on lambda integrative recombination to study the effects of intrinsically bent DNA sequences on the tertiary structure of negatively supercoiled DNA. Remarkably, the localization of adenosine-tract (A-tract) sequences in the terminal loop of a supercoiled plasmid is independent of the extent of intrinsic bending. The results suggest that A-tract-containing sequences have other properties that organize the structure of superhelical domains apart from intrinsic bending and may explain the lack of conservation in the degree of A-tract-dependent bending among DNA sequences located upstream of bacterial promoters.

Anomalous rapid electrophoretic mobility of DNA containing triplet repeats associated with human disease genes.

Eight human genetic diseases have been associated with the expansion of CTG or CGG triplet repeats. The molecular etiology behind expansion is unknown but may involve participation of an unusual DNA structure in replication, repair, or recombination. We show that DNA fragments containing CTG triplet repeats derived from the human myotonic dystrophy gene migrate up to 20% faster than expected in nondenaturing polyacrylamide gels, suggesting the presence of an unusual DNA helix structure within the CTG triplet repeats. The anomalous migration is dependent upon the number of triplet repeats, the length of the flanking DNA, and the percentage and temperature of the polyacrylamide. The effect could be reduced by the addition of actinomycin D. Applying a reptation model for electrophoresis, the results are consistent with a 20% increase in persistence length of the DNA. PCR products containing CTG or CGG repeats from the spinocerebellar ataxia type I gene (SCA1) or the fragile X FMR1 gene, respectively, also showed higher electrophoretic mobility. These are the first sequences of defined length for which a dramatic increase in mobility can be attributed to sequence-dependent structural elements in DNA.

Analysis of the structure of dimeric DNA catenanes by electron microscopy.

We analyzed the structure of open-circular and supercoiled dimeric DNA catenanes generated by site-specific recombination in vitro. Electron microscopy of open-circular catenanes shows that the number of duplex crossings in a plane is a linear function of the number of catenane interlinks (Ca/2), and that the length of the catenane axis is constant, independent of Ca. These relationships are similar to those observed with supercoiled DNA. Statistical analyses reveal, however, that the conformations of the individual rings of the catenanes are similar to those of unlinked circles. The distribution of distances between randomly chosen points on separate rings depends strongly on Ca and is consistent with a sharp decrease in the center-of-mass separation between rings with increasing Ca. Singly linked supercoiled catenanes are seen by microscopy to be linked predominantly through terminal loops in the respective superhelices. The observations suggest that chain entropy is a major factor determining the conformation of DNA catenanes.

Suffocation, choking, and strangulation in childhood in England and Wales: epidemiology and prevention.

The causes, classification, and prevention of mechanical asphyxial death in children were examined. The Office of Population Censuses and Surveys (OPCS) identified children, under 15 years of age, who had died as a result of choking, suffocation, or strangulation in England and Wales during the years 1990 and 1991. Cases in the International Classification of Diseases (ICD) codes of E911-3, E953, E963, and E983 were selected and case details from HM coroners' records and the death certificates were extracted. The OPCS identified 136 children (99 boys, 37 girls) in the two year period, 65% were under 3 years of age. The children were classified as dying from choking (21 cases), aspirating gastric contents (39 cases), suffocation (29 cases), strangulation (11 cases), and hanging (36 cases). The strangulation cases could be further subdivided into a group of 12 younger children who were suspended from ligatures around the home and a group of 21 boys (8-14 years) who died of self initiated hanging. Overall, 11 children were deliberately killed and 31 children died in beds or cots. Children whose deaths are classified as being due to aspiration of vomit appear to be cases of the sudden infant death syndrome or background medical conditions. This study suggests the need for advice on maintaining a safe sleeping environment. Only one child choked on a toy and European Standards for Toy Safety appear to have been successful. The prevention of hanging in the group of older boys needs further exploration.

Accident prevention: the health visitor's role.

The health of the nation white paper sets targets in five key areas for reductions in both mortality and morbidity: coronary heart disease and stroke, cancers, mental illness, HIV/Aids and sexual health and accidents. In a series of articles in Health visitor, experts will be considering the opportunities the white paper offers for community nurses in each of the key areas. Here Dr Sara Levene, medical consultant to the Child Accident Prevention Trust, considers accidents, a major problem which health visitors can do much to control. She reviews how accidents are presented in the white paper, what health visitors can do and what resources are available to help them. She offers particular advice on special accident prevention initiatives and discusses some of the opportunities created by the white paper.

Conformational and thermodynamic properties of supercoiled DNA.

We used Monte Carlo simulations to investigate the conformational and thermodynamic properties of DNA molecules with physiological levels of supercoiling. Three parameters determine the properties of DNA in this model: Kuhn statistical length, torsional rigidity and effective double-helix diameter. The chains in the simulation resemble strongly those observed by electron microscopy and have the conformation of an interwound superhelix whose axis is often branched. We compared the geometry of simulated chains with that determined experimentally by electron microscopy and by topological methods. We found a very close agreement between the Monte Carlo and experimental values for writhe, superhelix axis length and the number of superhelical turns. The computed number of superhelix branches was found to be dependent on superhelix density, DNA chain length and double-helix diameter. We investigated the thermodynamics of supercoiling and found that at low superhelix density the entropic contribution to superhelix free energy is negligible, whereas at high superhelix density, the entropic and enthalpic contributions are nearly equal. We calculated the effect of supercoiling on the spatial distribution of DNA segments. The probability that a pair of DNA sites separated along the chain contour by at least 50 nm are juxtaposed is about two orders of magnitude greater in supercoiled DNA than in relaxed DNA. This increase in the effective local concentration of DNA is not strongly dependent on the contour separation between the sites. We discuss the implications of this enhancement of site juxtaposition by supercoiling in the context of protein-DNA interactions involving multiple DNA-binding sites.