Attention-deficit hyperactivity disorder: a category or a continuum? Genetic analysis of a large-scale twin study

OBJECTIVE

To investigate heritability and continuum versus categorical approaches to attention-deficit hyperactivity disorder (ADHD), using a large-scale twin sample.

METHOD

A cohort of 1,938 families with twins and siblings aged 4 to 12 years, recruited from the Australian National Health and Medical Research Council Twin Registry, was assessed for ADHD using a DSM-III-R-based maternal rating scale. Probandwise concordance rates and correlations in monozygotic and dizygotic twins and siblings were calculated, and heritability was examined using the De Fries and Fulker regression technique.

RESULTS

There was a narrow (additive) heritability of 0.75 to 0.91 which was robust across familial relationships (twin, sibling, and twin-sibling) and across definitions of ADHD as part of a continuum or as a disorder with various symptom cutoffs. There was no evidence for nonadditive genetic variation or for shared family environmental effects.

CONCLUSIONS

These findings suggest that ADHD is best viewed as the extreme of a behavior that varies genetically throughout the entire population rather than as a disorder with discrete determinants. This has implications for the classification of ADHD and for the identification of genes for this behavior, as well as implications for diagnosis and treatment.

Seasonal affective disorder in Australia: is photoperiod critical?

OBJECTIVES

Seasonal affective disorder (SAD) is a variant of recurrent depression in which episodes are linked to a particular season, typically winter. SAD is understood as the extreme end of a continuum of seasonality in the general population. Photoperiod (the timing and duration of daylight) has been assumed to be aetiologically critical. The present research used a survey design to investigate the assumed centrality of photoperiod for SAD/seasonality in Australia. Two hypotheses were tested: that self-reported seasonality does not increase further from the equator and that seasonality does not stand alone from non-seasonal neurotic complaints.

METHOD

The sampling frame used was adult females on the Australian Twin Registry roll. A sample of 526 women residing across the latitudes of Australia responded to a survey based around the Seasonal Pattern Assessment Questionnaire (SPAQ). The SPAQ asks respondents to retrospectively report on season-related changes in mood and behaviour. The survey also contained three questionnaire measures of neurotic symptoms of anxiety and depression: the General Health Questionnaire (GHQ), the Community Epidemiological Survey for Depression (CES-D) and the State-Trait Anxiety Inventory-Trait (STAI-T).

RESULTS

Self-reported seasonality did not correlated with latitude (r = 0.01, NS). On the other hand, a substantial relationship was found between seasonality and each of the measures of non-seasonal complaints: GHQ (r = 0.35, p < 0.001); CES-D (r = 0.35, p < 0.001); and STAI-T (r = 0.30, p < 0.001).

CONCLUSIONS

Within the limitations of a design based on retrospective self-report, the findings of the present study suggest that the diathesis for SAD/seasonality may not be photoperiod-specific. At least in Australia, there is provisional support for the proposal that human seasonality may have a broader psychological component. The findings are discussed in terms of established research into normal mood, trait personality and non-seasonal depression.

Season of birth and schizophrenia in Tasmania

OBJECTIVE

The purpose of the present study was to determine whether there was a significant seasonal variation in the birth dates of patients with schizophrenia born in Tasmania.

METHOD

The season of birth in Tasmanian-born patients with schizophrenia (n = 1412), born between 1919-1970, was compared with general population data.

RESULTS

No significant variation in month or quarter of birth was detected for the subjects with schizophrenia. Neither was there any significant seasonal excess when data from different decades were analysed separately, nor for subsidiary analyses of gender or paranoid/non paranoid subtype.

CONCLUSION

These results are compared with those of six other published studies of Australasian-born subjects identified through registers in different States. While both positive and negative results are reported, no clear pattern emerges. The relevance of season of birth to the development of schizophrenia in Australia remains an open question.

Decline in diagnoses of schizophrenia in Tasmania during the period 1965-1990

Changes in the rates of diagnosis of schizophrenia in Tasmania, Australia during the period of 1965-1990 were examined using records from the State mental health case register. Analyses were restricted to subjects with diagnoses recorded between the ages of 20 and 29 years in order to reduce possible errors caused by age-standardized methods. There was a significant decrease in the rate of non-paranoid subtypes of schizophrenia in female subjects, accompanied by a commensurate rise in the rate of bipolar diagnoses in that group. Differential changes in male and female subjects support explanations of phenotypic shifts in presentation of psychosis as well as changes in diagnostic practice.

Characterization of the radical trapping activity of a novel series of cyclic nitrone spin traps

alpha-Phenyl-tert-butyl nitrone (PBN) is a nitrone spin trap, which has shown efficacy in animal models of oxidative stress, including stroke, aging, sepsis, and myocardial ischemia/reperfusion injury. We have prepared a series of novel cyclic variants of PBN and evaluated them for radical trapping activity in vitro. Specifically, their ability to inhibit iron-induced lipid peroxidation in liposomes was assessed, as well as superoxide anion (O2(-.)) and hydroxyl radical ((.)OH) trapping activity as determined biochemically and using electron spin resonance (ESR) spectroscopy. All cyclic nitrones tested were much more potent as inhibitors of lipid peroxidation than was PBN. The unsubstituted cyclic variant MDL 101,002 was approximately 8-fold more potent than PBN. An analysis of the analogs of MDL 101,002 revealed a direct correlation of activity with lipophilicity. However, lipophilicity does not solely account for the difference between MDL 101,002 and PBN, inasmuch as the calculated octanol/water partition coefficient for MDL 101,002 is 1.01 as compared to 1.23 for PBN. This indicated the cyclic nitrones are inherently more effective radical traps than PBN in a membrane system. The most active compound was a dichloro analog in the seven-membered ring series (MDL 104,342), which had an IC50 of 26 mum, which was 550-fold better than that of PBN. The cyclic nitrones were shown to trap (.)OH with MDL 101,002 being 20 25 times more active than PBN as assessed using 2-deoxyribose and p-nitrosodimethylaniline as substrates, respectively. Trapping of (.)OH by MDL 101,002 was also examined by using ESR spectroscopy. When Fenton's reagent was used, the (.)OH adduct of MDL 101,002 yielded a six-line spectrum with hyperfine coupling constants distinct from that of PBN. Importantly, the half-life of the adduct was nearly 5 min, while that of PBN is less than 1 min at physiologic pH. MDL 101,002 also trapped the O2(-.) radical to yield a six-line spectrum with coupling constants very distinct from that of the (.)OH adduct. In mice, the cyclic nitrones ameliorated the damaging effects of oxidative stress induced by ferrous iron injection into brain tissue. Similar protection was not afforded by the lipid peroxidation inhibitor U74006F, thus implicating radical trapping as a unique feature in the prevention of cell injury. Together, the in vivo activity, the stability of the nitroxide adducts, and the ability to distinguish between trapping of (.)OH and O2(-.) suggest the cyclic nitrones to be ideal reagents for the study of oxidative cell injury.

FRAXE and mental retardation

Mental impairment and instability of the CCG repeat at FRAXE is described in six kindreds. Cosegregation of FRAXA and FRAXE was found within one of these kindreds. Cytogenetic expression of FRAXE was shown to skip a generation when associated with a reduction in size of the CCG expansion when transmitted through a male; however, in general, transmission occurred through females and a copy number increased from one generation to the next. In these respects the behaviour of FRAXE paralleled that of FRAXA. A relationship between FRAXE and non-specific mental impairment is strongly suggested by the occurrence in these families of more mentally impaired male and female carriers, after removal of index cases, than could reasonably be expected by chance.

In vitro and in vivo activity of a novel series of radical trapping agents in model systems of CNS oxidative damage

Many laboratory and clinical studies suggest that oxygen radical formation and resultant cell damage contribute to CNS injury following stroke and neurotrauma. Accordingly, antioxidants represent a viable therapeutic approach for management of CNS oxidative damage. Recently, several investigators have reported that the spin trap PBN protects against stroked-induced damage and reduces aging-associated neurological deficits. We have prepared and tested a cyclic analog of PBN, MDL 101,002, in a number of in vitro and in vivo assays designed to assess its neuroprotective properties. MDL 101,002 was found to be an effective .OH trap, to inhibit lipid peroxidation, and to decrease infarct size in a gerbil model of stroke. These results further indicate that oxidative damage arising from stroke contributes to infarct formation, and that spin traps are effective in ameliorating ischemia and reperfusion-induced CNS injury.

Transmitting males and carrier females in fragile X--revisited

Fragile X "transmitting males" have customarily been defined as phenotypically normal hemizygotes, who show very few or no fragile sites, and who transmit the fragile X premutation to phenotypically normal daughters. However, an objective justification of this definition was lacking. The discovery of an unstable CCG repeat as the genetic basis of fragile X further emphasized the apparent distinction between the "normal transmitting males" with short repeat and expression of the FMR1 gene, and the affected males with larger repeats (delta > 0.6 kb) and a complete lack of FMR1 transcription. We have recently shown that the transition between these two groups in phenotypic expression of fragile X is gradual, mainly on account of methylation mosaicism. However, there were insufficient data on the phenotype within the short repeat (0.0 < delta < 0.6) range. In this paper we approach this problem by comparing some clinical, anthropometric, and psychometric data from a sample of normal transmitting males with those from their non-fragile X male relatives. Moreover, female carriers with short repeat are compared for the same traits with their non-fragile X female relatives. The results have shown that both males and females with a short repeat differed significantly from normal on several psychometric and physical measurements, and males only showed differences in typical facial traits. Further studies of genotype-phenotype correlations within the short repeat range, including the estimate of FMR1 gene function and a more exact estimate of repeat size, is required before genetic explanation for the clinical findings can be provided.(ABSTRACT TRUNCATED AT 250 WORDS)

Does IQ decline with age in fragile-X? A methodological critique

The recent claims for a decline in intelligence test performance in males and females with fragile-X (fra(X)) syndrome have implications both clinically and in evaluating the underlying neurological basis. This commentary identifies three key issues in evaluating evidence for a decline and in planning future, more co-ordinated efforts. These are (1) problems in combining data across different intelligence tests and/or different ages with potentially incompatible norms, task demands, and models of the structure of intelligence; (2) limitations in applying to low ability persons tests designed to discriminate best around the population average; and (3) specific cognitive deficits and behavioral problems in fra(X) individuals which may be confounded with the task demands of particular IQ tests at particular ages. While the decline in ability may be a real phenomenon rather than an artifact, recommendations are made about the psychometric requirements for a larger and more definitive collaborative study.

Twin birth is not a risk factor for seizures

There is a belief that perinatal factors are a major cause of epilepsy. We studied a community-based sample of twins, a group with a marked excess of adverse perinatal events. The observed number of non-twin siblings with seizures did not differ from that predicted by the age-specific cumulative incidence rate of seizures (4.2% at age 10 years) in the twins. The types of epilepsies in the twins were largely benign and self-limited and not those associated with brain damage. Zygosity, birth order, and birth weight did not predict affected status. Within affected sibships, the frequency of seizures in co-twins of dizygotic probands (9%) was not different from the frequency in non-twin siblings (12%) but was much less than the frequency in co-twins of monozygotic probands (38%; p < 0.001), reflecting a major genetic component to certain epilepsies. These data show that twins do not have an increased risk of seizures and strongly suggest that perinatal factors have little bearing on the etiology of the common epilepsies in the community.

Genotype-phenotype relationships in fragile X syndrome: a family study

Relationships between the measures of intellectual and physical status in the fragile X syndrome and the size of amplification of the fragile X-specific fragment, equivalent to the number of CCG repeats within the FMR1 locus, were studied by a maximum-likelihood scoring technique for analysis of pedigree data. This allows for estimation of random effects (genetic and environmental variance) concurrently with other (fixed) effects in a quantitative trait. FMR1 expression is usually shut down in males penetrant for the fragile X syndrome who have hypermethylated CCG amplifications of > or = 0.6 kb. The assumption of the step versus curvilinear function representing this relationship was tested by the likelihood-ratio criterion. The maximum-likelihood parameters were based on the most appropriate model for each measure. The results were indicative of the presence of a curvilinear relationship between the amplification size and the two intellectual scores, the Peabody Picture Vocabulary Test and Block Design Test, measuring verbal and spatial abilities, respectively. Reasons for the unexpected curvilinear regression between the amplification size and intellectual scores were explained further by methylation analysis of fragile X males with amplifications of 0.6 < delta < or = 1.2 kb who appeared to be responsible for the curvilinearity of the relationship. Four of these showed unmethylated status of the amplified bands in lymphocytes, which were presumably transcriptionally active. Removal of the aberrant individuals led to the anticipated step function between amplification and intellectual scores. For the combined anthropometric score, as well as for several single physical measures, the step function was the most appropriate model regardless of the inclusion or omission of the aberrant individuals in the pedigree sample.

Phenotypic and genetic affinities among ethnic populations in Daghestan (Caucasus, Russia): a comparison of polymorphic, physical, neurophysiological and psychological traits

Daghestan contains many distinct populations characterized by different degrees of isolation, ethnic backgrounds and ecological conditions. This report introduces a large-scale family study conducted in seven Daghestan populations as well as in Moscow, using four categories of measures: (1) single gene polymorphisms, (2) anthropometric, (3) physiological, and (4) psychological quantitative traits. Such a study permits the analysis of both phenotypic and genetic variation within and between populations. The implications of such variation are discussed, especially in the context of the measures used. These derive from Soviet conceptualizations of the nervous system and related assessment of individual differences, and vary considerably from conventional Western approaches to behaviour. From the anthropometric through the physiological to the behavioural measures there was a clear trend towards increased phenotypic variation but reduced genetic variation. Possibly because the genetic variance was less on the more complex phenotypes, measures of generalized distance indicated much greater population similarity when based on phenotypic data on the genetic component of variation. Issues that arise when using behavioural traits to study human population diversity are discussed, particularly in relation to inbreeding and the specific cultural and linguistic practices in the Daghestan region.

Between-generation differences in ascertainment and penetrance: relevance to genetic hypotheses in fragile X

Between-generation differences in ascertainment were examined in 54 extended fragile X pedigrees, where all available members were clinically, psychometrically, and cytogenetically investigated. In 24 families a diagnosis was verified by molecular characterization using the pfxa3 fragile X-specific probe. We found considerable differences between generations in relative proportions of affected fragile X subjects versus 'non-penetrant' carriers. We also found deviation in the segregation ratio in unbiased samples of relatives in pedigrees. We claim that these irregularities are influenced by different rates of ascertainment, depending on the clinical expression of the condition (penetrance) and the fertility of fragile X individuals in a pedigree, as well as by the thoroughness of clinical investigation in individual families. Penetrance and fertility were estimated in fragile X females assessed by psychometric tests, and they were compared with earlier estimates based on a subjective judgement of their intellectual status. We suggest that the standard correction for ascertainment bias, such as has been applied in segregation analysis of this condition, is not sufficient to adjust for all types of bias.

Fragile X family with unusual digital and facial abnormalities, cleft lip and palate, and epilepsy

We present a fragile X family with unusual clinical manifestations. These findings, which often occur in the X-linked FG syndrome, include minor limb anomalies, cleft lip and palate, characteristic facial appearance, gastrointestinal problems and epilepsy, and intellectual disability. In a total sample of 54 fra(X) families, the frequency of minor limb anomalies was estimated to be 32% in the affected males and 19% in the female heterozygotes. These anomalies tend to occur in several members of the same family, where some craniofacial abnormalities reported as characteristic of the FG syndrome have also been encountered. Possible mechanisms for the occurrence of these unusual manifestations in the fra(X) families are discussed.

Selective breeding for increased cholinergic function: biometrical genetic analysis of muscarinic responses

Biometric genetic analyses of behavioral and physiologic responses known to be related to muscarinic cholinergic receptors (hypothermia, hypoactivity, inhibited avoidance, and reduced responding for water) were studied in genetic crosses and backcrosses of the Flinders sensitive line (FSL) and Flinders resistant line (FRL) of rats. The FSL rats were more sensitive to the direct muscarinic agonists, arecoline and oxotremorine, and to the indirect agonist, physostigmine, than any other group. The next most sensitive group was the F1 x FSL backcross, followed by the F2, F1, F1 x FRL backcross, and the FRL, in that order. These differences between the genetic groups could be accounted for completely by either solely additive or additive plus dominance genetic factors. When dominance genetic factors contributed to the differences among groups (6 out of 15), the F1 responded like the FRL rats. The variance of the responses measured made it impossible to obtain reliable estimates of the number of genes involved in many instances; when such estimates were possible, several genes (greater than or equal to 3) appeared to be involved. We conclude that muscarinic sensitivity in rats is under genetic control, with the greatest contribution coming from additive genetic factors. Because the FSL rat appears to be a genetic animal model of depression, the finding of several genes influencing muscarinic responses may help account for the difficulties investigators have had in locating a single major gene or biological marker for human depressive disorders.

Problems in ascertainment of transmitting males in Martin-Bell syndrome

The difficulty of assigning families affected with the Martin-Bell syndrome (MBS) into the category of male transmission is emphasised and illustrated by examples of 3 MBS families. These examples demonstrate how the ability to detect transmitting males depends on the number of generations available for investigation, and also on the "spread" of clinical investigation across many branches of the family regardless of what appears to be an unremarkable family history. Some unusual properties of male transmission are shown, and the problem of selective ascertainment of the particular MBS male individuals in different generations in a set of pedigrees is discussed.

What information should the multiple birth family receive before, during and after the birth?

Advances in the management of the multiple pregnancy and delivery must be accompanied by corresponding improvements in service access outside key centres and especially in the information families receive about what may happen during or after the pregnancy. A major review of birthing services in Victoria has focussed attention on four areas where the quality of information is often inadequate. 1) Prepregnancy and the standard of counselling about the incidence of multiples as a result of fertility drugs and in vitro fertilization procedures and about problems which may accompany a multiple birth. 2) Antenatal: At what stage of the pregnancy should parents be told of the multiple pregnancy and how should monitoring of the mother and procedures such as bedrest take into account what are often conflicting demands within the family? 3) Perinatal: Families are frequently illprepared for a cesarean delivery and for the procedures for premature multiples. The problem is often compounded by separation of the mother from one or both twins. While bereavement services are improving, much still needs to be learned about handling congenital abnormalities in one or more multiples. 4) Postnatal: Irrespective of the level of prenatal advice, families greatly underestimate the workload with multiples. The resulting stress contributes to the incidence of postnatal depression, child abuse and divorce now being reported from multiple birth families. Some suggestions are made from social psychology and genetic counselling about how families can best handle risk information to achieve the goal of neither under- nor overestimating the risks at these different stages of the multiple pregnancy.

"Twins in school". An Australia-wide program

The multiple birth family is more likely to have a dispute with the education system than with any other service. So many potential areas of conflict exist over the abilities and behaviour of multiples and over such issues as separation or keeping back one twin. One reason for disputes is the lack of good data to adequately reflect the different perspectives of parents and teachers and the differing needs of families: the same solution does not apply to all. To provide the first large-scale data base and building upon an initial survey of 85% of all primary school teachers in South Australia, the LaTrobe Twin Study and AMBA worked with Education Departments to set-up in each state Education Research Teams (ERTs) of parents of multiples who were also teachers. The ERTs were crucial in three phases. 1) Developing and circulating questionnaires and publicising the nationwide survey. 784 families and 1264 teachers of their children completed these questionnaires, many reporting that simply having to address the issues raised in the questionnaire was a valuable learning experience. 2) Exploring the data base. Issues arising included the very different bases on which parents and teachers judged separation desirable, with teachers emphasising the unsubstantiated claim that separation is essential to individual development. Separation became more common over the first three years of schooling but 20-25% of twins separated one year were back together the next. 3) Running regional meetings of parents, teachers and administrators to discuss the results and to pool experiences and plan policies at the local level. A need clearly exists to improve the level of consultation between families and school personnel and to ensure the widespread availability of information which identifies key issues in making decisions for that multiple birth family.

Twin children in volunteer registries: biases in parental participation and reporting

The biases in voluntary participation by adult twins are well known but less attention has been paid to twin children where parents decide on participation and provide much of the information. Several aspects of reporting including the assessment of zygosity are compared in four large Australian data bases: 1) a nationwide compulsory (and hence representative) survey of literacy and numeracy; 2) a nationwide "Twins in School" survey of parents and teachers of twins run through Education Departments and AMBA, the parents organisation in conjunction with LaTrobe; 3) the LaTrobe Twin Study which is a longitudinal program involving frequent interactions between families and researchers, and 4) the Australian NHMRC Twin Registry which has surveyed a large sample of their families with twin children by mail. One potential bias comes when recruitment is on a continuing basis as in the LaTrobe Twin Study and the Australian Twin Registry when differences between "early" and "late" enrolling families arise. One difference between the four samples arose from parents being much more likely to contrast their twins and to report problems in one but not the other, whereas teachers' and psychologists' assessments of these same children generally reported much smaller intrapair differences. Future studies should have some common questions to provide comparative data on such biases. Key questions are proposed for this area, mainly on the perceived need for different forms of remediation, together with other recommendations about the minimal essential baseline data set for a registry.

Neuropsychological dimensions of the fragile X syndrome: support for a non-dominant hemisphere dysfunction hypothesis

This study contends that males with the fragile X syndrome feature problems in the visuospatial sphere as compared with Down syndrome males matched on vocabulary ability. Fragile X males suffer impairments of constructional functions, as demonstrated by their poor performance on block construction tests and on drawing tasks. These problems exist in association with visuoperceptive impairments, including the inability to reliably estimate angular relationships (Judgement of Line Orientation). They have shortened Digit and Corsi spans, and may feature some deficits in left hand co-ordination. The observation of a pervasive non-verbal deficit may also apply to carrier females, who despite functioning at an overall higher level, feature a similar pattern of deficits. It is possible that the deficit in non-dominant hemisphere functioning may be a pathognomonic feature of the chromosomal abnormality.

The relevance of the health belief model to Australian smokers

The Health Belief Model is one of the few models predicting health behavior which explicitly evaluates the role of cues to action from the doctor or others. Rarely have such cues to action been examined formally by the comparison of groups receiving different interventions. Initial and follow-up data covering a wide range of sociopsychological variables were gathered from typical smokers among family-practice patients participating in an Australian quit-smoking program. Patients were randomly assigned either to a control or experimental group, the latter receiving the Give-Up Smoking (GUS) kit, and quit-smoking advice from their doctor. Factor analysis of the initial data largely confirmed the clusters of the Health Belief Model. At follow-up, after the experimental and control group treatment, a totally different factor structure emerged, comprising some very specific sociopsychological variables and cues to action. Implications are discussed for the Health Belief Model relative to other health behavior models.

Clinical features and reproductive patterns in fragile X female heterozygotes

Clinical findings are presented on 113 fragile X female heterozygotes from 44 families, based on physical examination, behavioural assessment, and reproductive history. In 85% of a subsample of 92 adult females non-verbal IQ score derived from the Block Design test was 85 or less. Verbal ability deficits were much less common. Typical facial characteristics, irregular teeth, and hypermobility of finger joints occurred in approximately 40% of adult females, but facial abnormalities were less common in children. Some physical anomalies and the level of intellectual impairment were, in adult carriers, associated with the presence of fragile X sites. The commoner physical anomalies or typical facial characteristics and intellectual abilities differed significantly between the known female heterozygotes and their 40 presumed normal relatives. Frequency of miscarriages was increased in fragile X females; in spite of this, a moderate increase in the number of children has been encountered in female carriers with borderline intellectual impairment. This important problem has genetical implications and needs further investigation. The importance is emphasised of a more detailed clinical examination of the females at risk in fragile X families.

Individual variation and specific cognitive deficits in the fra(X) syndrome

Mental retardation has been associated with fra(X) but comprehensive psychological evaluation has rarely been applied to 2 major behavioral questions 1) the extent of individual variation in IQ among fra(X) males and the possibility of some fra(X) males being of normal IQ; and 2) whether there is a depression in general IQ or whether specific abilities are impaired. The problems of developing an effective battery of tests for assessing fra(X) are discussed. These questions were examined in 54 individuals, comprising fra(X) males, their obligate carrier mothers and those sisters shown to have the fra(X). Among noninstitutionalised males nonverbal IQ as measured on a Block Design test ranged from 100 to 0, and vocabulary scores while generally higher, ranged from 79-33. The males scored low on a digit span memory task, while performance on a memory of objects task was adequate. Despite lower overall scores, a similar pattern and variability emerged in institutionalised males. Daughters were extremely variable in performance and the mothers performed much better, supporting the view that women who have children are a selected subset of fra(X) syndrome individuals. The performance of one male is discussed in detail. His vocabulary and nonverbal IQ scores were normal, despite his having other specific cognitive deficits. The pattern of abilities and behavior seen in fra(X) may result in an overestimation of intelligence and underestimation of penetrance when based on clinical impressions rather than formal psychological assessment. The implications of this for molecular and for population genetic approaches to fra(X) are discussed.