Genetic epidemiology of major depression: review and meta-analysis

OBJECTIVE

The authors conducted a meta-analysis of relevant data from primary studies of the genetic epidemiology of major depression.

METHOD

The authors searched MEDLINE and the reference lists of previous review articles to identify relevant primary studies. On the basis of a review of family, adoption, and twin studies that met specific inclusion criteria, the authors derived quantitative summary statistics.

RESULTS

Five family studies met the inclusion criteria. The odds ratios for proband (subjects with major depression or comparison subjects) versus first-degree relative status (affected or unaffected with major depression) were homogeneous across the five studies (Mantel-Haenszel odds ratio=2.84, 95% CI=2.31-3.49). No adoption study met the inclusion criteria, but the results of two of the three reports were consistent with genetic influences on liability to major depression. Five twin studies met the inclusion criteria, and their statistical summation suggested that familial aggregation was due to additive genetic effects (point estimate of heritability of liability=37%, 95% CI=31%-42%), with a minimal contribution of environmental effects common to siblings (point estimate=0%, 95% CI=0%-5%), and substantial individual-specific environmental effects/measurement error (point estimate=63%, 95% CI=58%-67%). The literature suggests that recurrence best predicts the familial aggregation of major depression.

CONCLUSIONS

Major depression is a familial disorder, and its familiality mostly or entirely results from genetic influences. Environmental influences specific to an individual are also etiologically significant. Major depression is a complex disorder that does not result from either genetic or environmental influences alone but rather from both. These findings are notably consistent across samples and methods and are likely to be generally applicable.

Schizophrenia as a complex trait: evidence from a meta-analysis of twin studies

CONTEXT

There are many published twin studies of schizophrenia. Although these studies have been reviewed previously, to our knowledge, no review has provided quantitative summary estimates of the impact of genes and environment on liability to schizophrenia that also accounted for the different ascertainment strategies used.

OBJECTIVE

To calculate meta-analytic estimates of heritability in liability and shared and individual-specific environmental effects from the pooled twin data.

DATA SOURCES

We used a structured literature search to identify all published twin studies of schizophrenia, including MEDLINE, dissertation, and books-in-print searches.

STUDY SELECTION

Of the 14 identified studies, 12 met the minimal inclusion criteria of systematic ascertainment.

DATA SYNTHESIS

By using a multigroup twin model, we found evidence for substantial additive genetic effects-the point estimate of heritability in liability to schizophrenia was 81% (95% confidence interval, 73%-90%). Notably, there was consistent evidence across these studies for common or shared environmental influences on liability to schizophrenia-joint estimate, 11% (95% confidence interval, 3%-19%).

CONCLUSIONS

Despite evidence of heterogeneity across studies, these meta-analytic results from 12 published twin studies of schizophrenia are consistent with a view of schizophrenia as a complex trait that results from genetic and environmental etiological influences. These results are broadly informative in that they provide no information about the specific identity of these etiological influences, but they do provide a component of a unifying empirical basis supporting the rationality of searches for underlying genetic and common environmental etiological factors.

Molecular genetics of attention-deficit/hyperactivity disorder

Results of behavioral genetic and molecular genetic studies have converged to suggest that both genetic and nongenetic factors contribute to the development of attention-deficit/hyperactivity disorder (ADHD). We review this literature, with a particular emphasis on molecular genetic studies. Family, twin, and adoption studies provide compelling evidence that genes play a strong role in mediating susceptibility to ADHD. This fact is most clearly seen in the 20 extant twin studies, which estimate the heritability of ADHD to be .76. Molecular genetic studies suggest that the genetic architecture of ADHD is complex. The few genome-wide scans conducted thus far are not conclusive. In contrast, the many candidate gene studies of ADHD have produced substantial evidence implicating several genes in the etiology of the disorder. For the eight genes for which the same variant has been studied in three or more case-control or family-based studies, seven show statistically significant evidence of association with ADHD on the basis of the pooled odds ratio across studies: DRD4, DRD5, DAT, DBH, 5-HTT, HTR1B, and SNAP-25.

Genetic and environmental factors in relative body weight and human adiposity

We review the literature on the familial resemblance of body mass index (BMI) and other adiposity measures and find strikingly convergent results for a variety of relationships. Results from twin studies suggest that genetic factors explain 50 to 90% of the variance in BMI. Family studies generally report estimates of parent-offspring and sibling correlations in agreement with heritabilities of 20 to 80%. Data from adoption studies are consistent with genetic factors accounting for 20 to 60% of the variation in BMI. Based on data from more than 25,000 twin pairs and 50,000 biological and adoptive family members, the weighted mean correlations are .74 for MZ twins, .32 for DZ twins, .25 for siblings, .19 for parent-offspring pairs, .06 for adoptive relatives, and .12 for spouses. Advantages and disadvantages of twin, family, and adoption studies are reviewed. Data from the Virginia 30,000, including twins and their parents, siblings, spouses, and children, were analyzed using a structural equation model (Stealth) which estimates additive and dominance genetic variance, cultural transmission, assortative mating, nonparental shared environment, and special twin and MZ twin environmental variance. Genetic factors explained 67% of the variance in males and females, of which half is due to dominance. A small proportion of the genetic variance was attributed to the consequences of assortative mating. The remainder of the variance is accounted for by unique environmental factors, of which 7% is correlated across twins. No evidence was found for a special MZ twin environment, thereby supporting the equal environment assumption. These results are consistent with other studies in suggesting that genetic factors play a significant role in the causes of individual differences in relative body weight and human adiposity.

A review and meta-analysis of the genetic epidemiology of anxiety disorders

OBJECTIVE

The authors conducted meta-analyses of data from family and twin studies of panic disorder, generalized anxiety disorder, phobias, and obsessive-compulsive disorder (OCD) to explore the roles of genetic and environmental factors in their etiology.

METHOD

MEDLINE searches were performed to identify potential primary studies of these disorders. Data from studies that met inclusion criteria were incorporated into meta-analyses that estimated summary statistics of aggregate familial risk and heritability for each disorder.

RESULTS

For family studies, odds ratios predicting association of illness in first-degree relatives with affection status of the proband (disorder present or absent) were homogeneous across studies for all disorders. The calculated summary odds ratios ranged from 4 to 6, depending on the disorder. Only for panic disorder and generalized anxiety disorder could the authors identify more than one large-scale twin study for meta-analysis. These yielded heritabilities of 0.43 for panic disorder and 0.32 for generalized anxiety disorder. For panic disorder, the remaining variance in liability could be attributed primarily to nonshared environment. For generalized anxiety disorder, this was true for men, but for women, a potentially significant role for common familial environment was also seen.

CONCLUSIONS

Panic disorder, generalized anxiety disorder, phobias, and OCD all have significant familial aggregation. For panic disorder, generalized anxiety disorder, and probably phobias, genes largely explain this familial aggregation; the role of family environment in generalized anxiety disorder is uncertain. The role of nonshared environmental experience is significant, underscoring the importance of identifying putative environmental risk factors that predispose individuals to anxiety.

Genetic influences on brain structure

Here we report on detailed three-dimensional maps revealing how brain structure is influenced by individual genetic differences. A genetic continuum was detected in which brain structure was increasingly similar in subjects with increasing genetic affinity. Genetic factors significantly influenced cortical structure in Broca's and Wernicke's language areas, as well as frontal brain regions (r2(MZ) > 0.8, p < 0.05). Preliminary correlations were performed suggesting that frontal gray matter differences may be linked to Spearman's g, which measures successful test performance across multiple cognitive domains (p < 0.05). These genetic brain maps reveal how genes determine individual differences, and may shed light on the heritability of cognitive and linguistic skills, as well as genetic liability for diseases that affect the human cortex.

Neuroanatomical hypothesis of panic disorder, revised

OBJECTIVE

In a 1989 article, the authors provided a hypothesis for the neuroanatomical basis of panic disorder that attempted to explain why both medication and cognitive behavioral psychotherapy are effective treatments. Here they revise that hypothesis to consider developments in the preclinical understanding of the neurobiology of fear and avoidance.

METHOD

The authors review recent literature on the phenomenology, neurobiology, and treatment of panic disorder and impressive developments in documenting the neuroanatomy of conditioned fear in animals.

RESULTS

There appears to be a remarkable similarity between the physiological and behavioral consequences of response to a conditioned fear stimulus and a panic attack. In animals, these responses are mediated by a "fear network" in the brain that is centered in the amygdala and involves its interaction with the hippocampus and medial prefrontal cortex. Projections from the amygdala to hypothalamic and brainstem sites explain many of the observed signs of conditioned fear responses. It is speculated that a similar network is involved in panic disorder. A convergence of evidence suggests that both heritable factors and stressful life events, particularly in early childhood, are responsible for the onset of panic disorder.

CONCLUSIONS

Medications, particularly those that influence the serotonin system, are hypothesized to desensitize the fear network from the level of the amygdala through its projects to the hypothalamus and the brainstem. Effective psychosocial treatments may also reduce contextual fear and cognitive misattributions at the level of the prefrontal cortex and hippocampus. Neuroimaging studies should help clarify whether these hypotheses are correct.

Mapping brain maturation

Human brain maturation is a complex, lifelong process that can now be examined in detail using neuroimaging techniques. Ongoing projects scan subjects longitudinally with structural magnetic resonance imaging (MRI), enabling the time-course and anatomical sequence of development to be reconstructed. Here, we review recent progress on imaging studies of development. We focus on cortical and subcortical changes observed in healthy children, and contrast them with abnormal developmental changes in early-onset schizophrenia, fetal alcohol syndrome, attention-deficit-hyperactivity disorder (ADHD) and Williams syndrome. We relate these structural changes to the cellular processes that underlie them, and to cognitive and behavioral changes occurring throughout childhood and adolescence.

Genes, evolution, and personality

There is abundant evidence, some of it reviewed in this paper, that personality traits are substantially influenced by the genes. Much remains to be understood about how and why this is the case. We argue that placing the behavior genetics of personality in the context of epidemiology, evolutionary psychology, and neighboring psychological domains such as interests and attitudes should help lead to new insights. We suggest that important methodological advances, such as measuring traits from multiple viewpoints, using large samples, and analyzing data by modern multivariate techniques, have already led to major changes in our view of such perennial puzzles as the role of "unshared environment" in personality. In the long run, but not yet, approaches via molecular genetics and brain physiology may also make decisive contributions to understanding the heritability of personality traits. We conclude that the behavior genetics of personality is alive and flourishing but that there remains ample scope for new growth and that much social science research is seriously compromised if it does not incorporate genetic variation in its explanatory models.

Balancing life-style and genomics research for disease prevention

Genetic and environmental factors, including diet and life-style, both contribute to cardiovascular disease, cancers, and other major causes of mortality, but various lines of evidence indicate that environmental factors are most important. Overly enthusiastic expectations regarding the benefits of genetic research for disease prevention have the potential to distort research priorities and spending for health. However, integration of new genetic information into epidemiologic studies can help clarify causal relations between both life-style and genetic factors and risks of disease. Thus, a balanced approach should provide the best data to make informed choices about the most effective means to prevent disease.

Family, twin, and adoption studies of bipolar disorder

Family, twin, and adoption studies have been essential in defining the genetic epidemiology of bipolar disorder over the past several decades. Family studies have documented that first-degree relatives of affected individuals have an excess risk of the disorder, while twin studies (and to a lesser extent, adoption studies) suggest that genes are largely responsible for this familial aggregation. We review these studies, including the magnitude of familial risk and heritability estimates, efforts to identify familial subtypes of bipolar disorder, and the implications of family/genetic data for validating nosologic boundaries. Taken together, these studies indicate that bipolar disorder is phenotypically and genetically complex.

Functional anatomical abnormalities in limbic and prefrontal cortical structures in major depression

Neuroimaging studies of major depression have identified neurophysiological abnormalities in multiple areas of the prefrontal cortex (PFC), the amygdala, and related parts of the striatum and thalamus. Some of these abnormalities are mood state-dependent, and appear in regions where cerebral blood flow (CBF) increases during other normal and pathological emotional states. These neurophysiological differences between depressives and non-depressed controls may thus locate areas where physiological activity changes to mediate or respond to the emotional, behavioral and cognitive manifestations of major depressive episodes (MDE). Other abnormalities persist following symptom remission, and are found in orbital and medial PFC areas where post mortem studies demonstrate reductions in cortex volume and/or histopathological changes in primary mood disorders. These orbital and medial PFC areas have been shown by other types of evidence to modulate emotional behavior and stress responses, suggesting that dysfunction involving these regions may be involved in the pathogenesis of depressive symptoms. Finally, physiological activity is decreased during MDE in dorsal PFC areas implicated in language, selective attention, visuospatial or mnemonic processing, but these abnormalities reverse with symptom remission. These areas of 'deactivation' during the depressed state may reflect neurophysiological interactions between cognitive and emotional processing, and may relate to the subtle cognitive impairments associated with MDE.

Genetic influences on measures of the environment: a systematic review

BACKGROUND

Traditional models of psychiatric epidemiology often assume that the relationship between individuals and their environment is unidirectional, from environment to person. Accumulating evidence from developmental and genetic studies has made this perspective increasingly untenable.

METHOD

Literature search using Medline, PsycINFO, article references and contact with experts to identify all papers examining the heritability of measures of environments of relevance to psychiatry/psychology.

RESULTS

We identified 55 independent studies organized into seven categories: general and specific stressful life events (SLEs), parenting as reported by child, parenting reported by parent, family environment, social support, peer interactions, and marital quality. Thirty-five environmental measures in these categories were examined by at least two studies and produced weighted heritability estimates ranging from 7% to 39%, with most falling between 15% and 35%. The weighted heritability for all environmental measures in all studies was 27%. The weighted heritability for environmental measures by rating method was: self-report 29%, informant report 26%, and direct rater or videotape observation (typically examining 10 min of behavior) 14%.

CONCLUSION

Genetic influences on measures of the environment are pervasive in extent and modest to moderate in impact. These findings largely reflect 'actual behavior' rather than 'only perceptions'. Etiologic models for psychiatric illness need to account for the non-trivial influences of genetic factors on environmental experiences.

Neurophysiological endophenotypes of schizophrenia: the viability of selected candidate measures

In an effort to reveal susceptibility genes, schizophrenia research has turned to the endophenotype strategy. Endophenotypes are characteristics that reflect the actions of genes predisposing an individual to a disorder, even in the absence of diagnosable pathology. Individual endophenotypes are presumably determined by fewer genes than the more complex phenotype of schizophrenia and would, therefore, reduce the complexity of genetic analyses. Unfortunately, despite there being rational criteria to define a viable endophenotype, the term is sometimes applied indiscriminately to characteristics that are deviant in affected individuals. Schizophrenia patients exhibit deficits in several neurophysiological measures of information processing that have been proposed as candidate endophenotypes. Successful processing of sensory inputs requires the ability to inhibit intrinsic responses to redundant stimuli and, reciprocally, to facilitate responses to less frequent salient stimuli. There is evidence to suggest that both these processes are "impaired" in schizophrenia. Measures of inhibitory failure include prepulse inhibition of the startle reflex, P50 auditory evoked potential suppression, and antisaccade eye movements. Measures of impaired deviance detection include mismatch negativity and the P300 event-related potential. The purpose of this review is to systematically evaluate the endophenotype candidacy of these key neurophysiological abilities. For each candidate, we describe typical experimental procedures, the current understanding of the underlying neurobiology, the nature of the abnormality in schizophrenia, the reliability, stability and heritability of the measure, and any reported gene associations. We conclude with a discussion of the few studies thus far that have employed a multivariate approach with these candidates.

A meta-analysis of estimated genetic and environmental effects on smoking behavior in male and female adult twins

BACKGROUND

Numerous twin studies on smoking behavior have shown that genetic and environmental factors play significant and approximately equal roles in the determination of smoking initiation (SI) and smoking persistence (SP). However, estimates of heritability (h2), shared (c2) and unique environmental effects (e2) from the literature display considerable variability for SI and SP, due most probably to differences in statistical analysis models, age, gender, sample size, origin of cohorts and measurement of smoking behavior.

METHODS

A systematic literature search identified six studies for SI and 10 studies for SP. Data from these studies were obtained and re-analysed by meta-analytical techniques.

RESULTS

For SI, our results indicate that the parameters h2, c2 and e2 are (mean +/- SEM): 0.37 +/- 0.04, 0.49 +/- 0.04 and 0.17 +/- 0.03 in male adults, and 0.55 +/- 0.04, 0.24 +/- 0.06 and 0.16 +/- 0.01 in female adults, respectively. These values were weighted by a combination of original estimates of variance from studies reporting variances plus estimated variances from studies where variances were not reported (called the combined variance method). Using the same approach for SP, we found that the parameters h2, c2 and e2 weighted by the combined variance method for the phenotype are (mean +/- SEM): 0.59 +/- 0.02, 0.08 +/- 0.04 and 0.37 +/- 0.03 in male adults, and 0.46 +/- 0.12, 0.28 +/- 0.08 and 0.24 +/- 0.07 in female adults, respectively.

CONCLUSIONS

Our results indicate that genetic factors play a more significant role for SI but a less significant role for SP in female adults compared to male adults. Significant gender difference was also detected in shared environmental factors for SI and SP. However, no significant gender difference was detected for e2 for either phenotype. These findings suggest that genetic and environmental factors may contribute differently to the determination of smoking initiation and persistence in male and female smokers.

Heritability of polycystic ovary syndrome in a Dutch twin-family study

BACKGROUND

Polycystic ovary syndrome (PCOS) is one of the most common endocrine disorders among women of reproductive age. There is evidence for a genetic component in PCOS based on familial clustering of cases.

OBJECTIVE

In the present study, the heritability of PCOS was estimated.

DESIGN/PARTICIPANTS

Data from 1332 monozygotic twins (genetically identical) and 1873 dizygotic twins/singleton sisters of twins (who share on average 50% of their segregating genes) registered with The Netherlands Twin Register were used. PCOS was defined as less than nine menstrual cycles and acne or hirsutism in agreement with the 2003 Rotterdam consensus.

RESULTS

Results point to a strong contribution of familial factors to PCOS. The resemblance in monozygotic twin sisters (tetrachoric correlation 0.71) for PCOS was about twice as large as in dizygotic twin and other sisters (tetrachoric correlation 0.38). Univariate analyses point to strong contributions of genetic factors to the variance in PCOS. Next, a trivariate genetic analysis of oligomenorrhea, acne, and hirsutism was carried out. This analysis confirmed that the familial component in PCOS is due to genetic factors.

CONCLUSIONS

This study demonstrated a large influence of genetic factors to the pathogenesis of PCOS, justifying the search for susceptibility genes.

Family genetic studies, suicide, and suicidal behavior

The extant adoption, twin, and family studies of suicide and suicidal behavior are reviewed. Suicidal behavior is highly familial, and on the basis of twin and adoption studies, heritable as well. Both completed and attempted suicide form part of the clinical phenotype that is familially transmitted, as rates of suicide attempt are elevated in the family members of suicide completers, and completion rates are elevated in the family members of attempters. A family history of suicidal behavior is associated with suicidal behavior in the proband, even after adjusting for presence of psychiatric disorders in the proband and family, indicating transmission of attempt that is distinct from family transmission of psychiatric disorder. Impulsive aggression in probands and family members is associated with family loading for suicidal behavior, and may contribute to familial transmission of suicidal behavior. Shared environment effects such as abuse, imitation, or transmission of psychopathology are other possible explanations.

Intergenerational transmission of subthreshold autistic traits in the general population

BACKGROUND

Autistic disorder (AD) is a disabling oligogenic condition characterized by severe social impairment. Subthreshold autistic social impairments are known to aggregate in the family members of autistic probands; therefore, we conducted this study to examine the intergenerational transmission of such traits in the general population.

METHODS

The Social Responsiveness Scale (SRS), a quantitative measure of autistic traits, was completed on 285 pairs of twins (by maternal report) and on their parents (by spouse report).

RESULTS

Correlation for social impairment or competence between parents and their children and between spouses was on the order of .4. In families in which both parents scored in the upper quartile for social impairment on the SRS, mean SRS score of offspring was significantly elevated (effect size 1.5). Estimated assortative mating explained approximately 30% of the variation in parent SRS scores.

CONCLUSIONS

Children from families in which both parents manifest subthreshold autistic traits exhibit a substantial shift in the distribution of their scores for impairment in reciprocal social behavior, toward the pathological end. As has been previously demonstrated in children, heritable subthreshold autistic impairments are measurable in adults and appear continuously distributed in the general population.

Testing hypotheses on specific environmental causal effects on behavior

There have been strong critiques of the notion that environmental influences can have an important effect on psychological functioning. The substance of these criticisms is considered in order to infer the methodological challenges that have to be met. Concepts of cause and of the testing of causal effects are discussed with a particular focus on the need to consider sample selection and the value (and limitations) of longitudinal data. The designs that may be used to test hypotheses on specific environmental risk mechanisms for psychopathology are discussed in relation to a range of adoption strategies, twin designs, various types of "natural experiments," migration designs, the study of secular change, and intervention designs. In each case, consideration is given to the need for samples that "pull-apart" variables that ordinarily go together, specific hypotheses on possible causal processes, and the specification and testing of key assumptions. It is concluded that environmental risk hypotheses can be (and have been) put to the test but that it is usually necessary to use a combination of research strategies.

Visual hallucinations in Lewy body disease relate to Lewy bodies in the temporal lobe

Consensus opinion characterizes dementia with Lewy bodies (DLB) as a progressive dementing illness, with significant fluctuations in cognition, visual hallucinations and/or parkinsonism. When parkinsonism is an early dominant feature, consensus opinion recommends that dementia within the first year is necessary for a diagnosis of DLB. If dementia occurs later, a diagnosis of Parkinson's disease with dementia (PDD) is recommended. While many previous studies have correlated the neuropathology in DLB with dementia and parkinsonism, few have analysed the relationship between fluctuating cognition and/or well-formed visual hallucinations and the underlying neuropathology in DLB and PDD. The aim of the present study was to determine any relationship between these less-studied core clinical features of DLB, and the distribution and density of cortical Lewy bodies (LB). The brains of 63 cases with LB were obtained over 6 years following population-based studies of dementia and parkinsonian syndromes. Annual, internationally standardized, clinical assessment batteries were reviewed to determine the presence and onset of the core clinical features of DLB. The maximal density of LB, plaques and tangles in the amygdala, parahippocampal, anterior cingulate, superior frontal, inferior temporal, inferior parietal and visual cortices were determined. Current clinicopathological diagnostic criteria were used to classify cases into DLB (n = 29), PDD (n = 18) or parkinsonism without dementia (n = 16) groups. Predictive statistics were used to ascertain whether fluctuating cognition or visual hallucinations predicted the clinicopathological group. Analysis of variance and regressions were used to identify any significant relationship(s) between the presence and severity of neuropathological and clinical features. Cognitive fluctuations and/or visual hallucinations were not good predictors of DLB in pathologically proven patients, although the absence of these features early in the disease course was highly predictive of PDD. Cases with DLB had higher LB densities in the inferior temporal cortex than cases with PDD. There was no association across groups between any neuropathological variable and the presence or absence of fluctuating cognition. However, there was a striking association between the distribution of temporal lobe LB and well-formed visual hallucinations. Cases with well-formed visual hallucinations had high densities of LB in the amygdala and parahippocampus, with early hallucinations relating to higher densities in parahippocampal and inferior temporal cortices. These temporal regions have previously been associated with visual hallucinations in other disorders. Thus, our results suggest that the distribution of temporal lobe LB is more related to the presence and duration of visual hallucinations in cases with LB than to the presence, severity or duration of dementia.

Factor Structure of the Psychopathic Personality Inventory: Validity and Implications for Clinical Assessment

Psychopathy is a personality disorder characterized by impulsive antisocial deviance in the context of emotional and interpersonal detachment. A factor analysis of the subscales of the Psychopathic Personality Inventory (PPI) yielded evidence for 2 factors. One factor showed relations with external criteria mirroring those of the emotional-interpersonal facet of psychopathy, including high dominance, low anxiety, and venturesomeness. The other factor showed relations paralleling those of the social deviance facet of psychopathy, including positive correlations with antisocial behavior and substance abuse, negative correlations with socioeconomic status and verbal ability, and personality characteristics including high negative emotionally and low behavioral constraint. Findings support using the PPI to assess these facets of psychopathy in community samples and to explore their behavioral correlates and genetic-neurobiological underpinnings.

The immunogenetics of human infectious diseases

Twin and adoptee studies have indicated that host genetic factors are major determinants of susceptibility to infectious disease in humans. Twin studies have also found high heritabilities for many humoral and cellular immune responses to pathogen antigens, with most of the genetic component mapping outside of the major histocompatibility complex. Candidate gene studies have implicated several immunogenetic polymorphisms in human infectious diseases. HLA variation has been associated with susceptibility or resistance to malaria, tuberculosis, leprosy, AIDS, and hepatitis virus persistence. Variation in the tumor necrosis factor gene promoter has also been associated with several infectious diseases. Chemokine receptor polymorphism affects both susceptibility ot HIV-1 infection and the rate of progression to AIDS. Inactivating mutations of the gamma-interferon receptor lead to increased susceptibility to typical mycobacteria and disseminated BCG infection in homozygous children. The active form of vitamin D has immunomodulatory effects, and allelic variants of the vitamin D receptor appear to be associated with differential susceptibility to several infectious diseases. NRAMP1, a macrophage gene identified by positional cloning of its murine homologue, has been implicated in susceptibility to tuberculosis in Africans. Whole genome linkage analysis of multi-case families is now being used to map and identify new loci affecting susceptibility to infectious diseases. It is likely that susceptibility to most microorganisms is determined by a large number of polymorphic genes, and identification of these should provide insights into protective and pathogenic mechanisms in infectious diseases.

Regression models for twin studies: a critical review

Twin studies have long been recognized for their value in learning about the aetiology of disease and specifically for their potential for separating genetic effects from environmental effects. The recent upsurge of interest in life-course epidemiology and the study of developmental influences on later health has provided a new impetus to study twins as a source of unique insights. Twins are of special interest because they provide naturally matched pairs where the confounding effects of a large number of potentially causal factors (such as maternal nutrition or gestation length) may be removed by comparisons between twins who share them. The traditional tool of epidemiological 'risk factor analysis' is the regression model, but it is not straightforward to transfer standard regression methods to twin data, because the analysis needs to reflect the paired structure of the data, which induces correlation between twins. This paper reviews the use of more specialized regression methods for twin data, based on generalized least squares or linear mixed models, and explains the relationship between these methods and the commonly used approach of analysing within-twin-pair difference values. Methods and issues of interpretation are illustrated using an example from a recent study of the association between birth weight and cord blood erythropoietin. We focus on the analysis of continuous outcome measures but review additional complexities that arise with binary outcomes. We recommend the use of a general model that includes separate regression coefficients for within-twin-pair and between-pair effects, and provide guidelines for the interpretation of estimates obtained under this model.