NOTE The report of the Committee without its annexes appears as Official Records of the General Assembly, Sixty-third Session, Supplement No. 46. The designations employed and the presentation of material in this publication do not imply the expression of any opinion whatsoever on the part of the Secretariat of the United Nations concerning the legal status of any country, territory, city or area, or of its authorities, or concerning the delimitation of its frontiers or boundaries. The country names used in this document are, in most cases, those that were in use at the time the data were collected or the text prepared. In other cases, however, the names have been updated, where this was possible and appropriate, to reflect political changes. Scientific Annexes Annex A. Medical radiation exposures Annex B. Exposures of the public and workers from various sources of radiation INTROdUCTION 1. In the course of the research and development for and the application of atomic energy and nuclear technologies, a number of radiation accidents have occurred. Some of these accidents have resulted in significant health effects and occasionally in fatal outcomes. The application of technologies that make use of radiation is increasingly widespread around the world. Millions of people have occupations related to the use of radiation, and hundreds of millions of individuals benefit from these uses. Facilities using intense radiation sources for energy production and for purposes such as radiotherapy, sterilization of products, preservation of foodstuffs and gamma radiography require special care in the design and operation of equipment to avoid radiation injury to workers or to the public. Experience has shown that such technology is generally used safely, but on occasion controls have been circumvented and serious radiation accidents have ensued. 2. Reviews of radiation exposures from accidents have been presented in previous UNSCEAR reports. The last report containing an exclusive chapter on exposures from accidents was the UNSCEAR 1993 Report [U6]. 3. This annex is aimed at providing a sound basis for conclusions regarding the number of significant radiation accidents that have occurred, the corresponding levels of radiation exposures and numbers of deaths and injuries, and the general trends for various practices. Its conclusions are to be seen in the context of the Committee's overall evaluations of the levels and effects of exposure to ionizing radiation. 4. The Committee's evaluations of public, occupational and medical diagnostic exposures are mostly concerned with chronic exposures of …

sources and effects of ionizing radiation

DNA Methylation and Its Basic Function

Prader-Willi syndrome

The discovery that expansion of unstable repeats can cause a variety of neurological disorders has changed the landscape of disease-oriented research for several forms of mental retardation, Huntington disease, inherited ataxias, and muscular dystrophy. The dynamic nature of these mutations provided an explanation for the variable phenotype expressivity within a family. Beyond diagnosis and genetic counseling, the benefits from studying these disorders have been noted in both neurobiology and cell biology. Examples include insight about the role of translational control in synaptic plasticity, the role of RNA processing in the integrity of muscle and neuronal function, the importance of Fe-S-containing enzymes for cellular energy, and the dramatic effects of altering protein conformations on neuronal function and survival. It is exciting that within a span of 15 years, pathogenesis studies of this class of disorders are beginning to reveal pathways that are potential therapeutic targets.

Trinucleotide Repeat Disorders

Angelman syndrome (AS), characterized by mental retardation, seizures, frequent smiling and laughter, and abnormal gait, is one of the best examples of human disease in which genetic imprinting plays a role. In about 70% of cases, AS is caused by de novo maternal deletions at 15q11-q13 (ref. 2). Approximately 2% of AS cases are caused by paternal uniparental disomy (UPD) of chromosome 15 (ref. 3) and 2-3% are caused by "imprinting mutations'. In the remaining 25% of AS cases, no deletion, uniparental disomy (UPD), or methylation abnormality is detectable, and these cases, unlike deletions or UPD, can be familial. These cases are likely to result from mutations in a gene that is expressed either exclusively or preferentially from the maternal chromosome 15. We have found that a 15q inversion inherited by an AS child from her normal mother disrupts the 5' end of the UBE3A (E6-AP) gene, the product of which functions in protein ubiquitination. We have looked for novel UBE3A mutations in nondeletion/non-UPD/non-imprinting mutation (NDUI) AS patients and have found one patient who is heterozygous for a 5-bp de novo tandem duplication. We have also found in two brothers a heterozygous mutation, an A to G transition that creates a new 3' splice junction 7 bp upstream from the normal splice junction. Both mutations are predicted to cause a frameshift and premature termination of translation. Our results demonstrate that UBE3A mutations are one cause of AS and indicate a possible abnormality in ubiquitin-mediated protein degradation during brain development in this disease.

UBE3A/E6-AP mutations cause Angelman syndrome

The much-quoted prevalence figure of 1:1,000 males for fragile X syndrome is an overestimate in a mixed ethnic population. A reexamination of the individuals from whom those data were derived using molecular diagnostic techniques demonstrates a more realistic figure of 1:4,000 males.

Prevalence of fragile X syndrome.

Editor—Prader-Willi syndrome (PWS) is a genetically determined disorder in which the absence of expression of one or more maternally imprinted gene(s) in the chromosomal region 15q11-13 results in a characteristic facial appearance, learning disabilities (mental retardation), and severe overeating behaviour owing to an abnormal satiety response to food intake, together with a range of other behaviours. Initially, as reported by Prader et al ,1 PWS was conceived as a syndrome of obesity, short growth, cryptorchidism, and mental retardation following hypotonia in the neonatal period. As more and more people with PWS were reported and research into the syndrome began, behavioural characteristics and other clinical features were added, culminating in the consensus diagnostic criteria.2Concurrently, the genetics of the disorder were receiving attention. First was the discovery that for many there was a visible chromosomal deletion in the proximal part of the long arm of chromosome 15 (15q11-13). Reports of an apparently similar deletion being associated with a phenotypically very different syndrome (Angelman syndrome, AS),3 and the observation that PWS was the result of a deletion on the chromosome 15 of paternal origin, and AS the chromosome 15 of maternal origin, led to the recognition that gender specific imprinting of genes at that locus accounted for two diverse syndromes being associated with apparently similar chromosomal deletions.4 Maternal chromosome 15 disomies, mutations of an imprinting centre, and chromosomal translocations accounted for non-deletion cases of PWS.5 

In published reports on Prader-Willi syndrome (PWS), prevalence has been variously quoted as “about 1 in 25 000 live births”,6 “between one in 25 000 and one in 10 000 live born children”,7 “[estimates] vary 6-fold from 1 in 5000 to 10 000; 1 in 10 000; 1 in 15 000; 1 in 25 000; to 1 in 10 000 to 30 000”.8 Only two estimates …

/pdf/population-prevalence-and-estimated-birth-incidence-and-3nplb54490.pdf

Population prevalence and estimated birth incidence and mortality rate for people with Prader-Willi syndrome in one UK Health Region

Angelman's syndrome and Prader-Willi syndrome are both causes of mental retardation with recognisable, but quite different, clinical phenotypes. Both are associated with deletions of chromosome 15q11-13, of maternal origin in Angelman's and paternal in Prader-Willi. Prader-Willi can arise by inheritance of two chromosomes 15 from the mother and none from the father (uniparental maternal disomy). In 2 patients with Angelman's syndrome we found evidence of uniparental paternal disomy. The phenotypic effects of maternal and paternal disomy of chromosome 15 are very different and inheritance of two normal 15s from one parent does not lead to normal development—strong evidence in man for genomic imprinting, in which the same gene has different effects dependent upon its parental origin.

Uniparental paternal disomy in Angelman's syndrome

Psychotic illness in people with Prader Willi syndrome due to chromosome 15 maternal uniparental disomy

Background Prader–Willi syndrome (PWS) is characterized by extreme floppiness at birth, impaired sexual development, short stature, severe over-eating, characteristic physical features and learning disabilities (LD). Impaired social cognition, literal mindedness and cognitive inflexibility are also present. The syndrome has two main genetic subtypes that both result in the failure of expression of maternally imprinted genes on chromosome 15 at the locus q11-13.



Methods  Through multiple sources, we attempted to identify all people with PWS living in one health region in the UK. Additional people with PWS identified in other regions were also recruited to augment the study sample. A comparison group of people with LD as a result of aetiologies other than PWS was also identified. All people from these three groups, over age three, who gave their consent, were assessed using tests of ability and attainment. In addition, their main carers were interviewed using a semistructured interview. Blood samples for genetic diagnosis were obtained from all consenting participants.



Findings  The IQ distribution of the population sample was approximately normal with a mean IQ 40 points below that of the general population. There were systematic differences between the two main genetic subtypes. Those with disomies differed in cognitive profiles from both those with deletions and the comparison LD group (the latter two groups were very similar) in terms of better verbal abilities and impaired coding ability. Some people with PWS deletions had strong visuospatial skills.



Interpretation  We propose that the normal distribution of IQ, shifted downwards relative to that of the general population, is the result of a global effect on IQ of the PWS gene(s), and that the different cognitive profile seen in those with chromosome 15 maternal disomies is a specific effect of a gene, or genes, on chromosome 15 which is differentially either expressed or not expressed in those with disomies relative to those with deletions. One hypothesis is that these subtle cognitive differences are a manifestation of the genetic influences of gender-specific imprinted genes on cerebral lateralization. This requires further investigation.

Tessa Webb

Papers

Prevalence of fragile X syndrome.

Population prevalence and estimated birth incidence and mortality rate for people with Prader-Willi syndrome in one UK Health Region

Uniparental paternal disomy in Angelman's syndrome

Psychotic illness in people with Prader Willi syndrome due to chromosome 15 maternal uniparental disomy

Cognitive abilities and genotype in a population-based sample of people with Prader-Willi syndrome.