Nicotine and familial vulnerability to schizophrenia: a discordant twin study

Associations between genetic liabilities to smoking behavior and schizophrenia symptoms in patients with a psychotic disorder, their siblings and healthy controls

It is unknown how smoking behavior polygenic scores (PRS) relate to psychosis and psychotic symptoms. To elucidate this, genotype and phenotype data were collected from patients with schizophrenia, their unaffected siblings, and healthy controls in a six-year follow-up prospective cohort study. Associations between smoking behaviors, PRS and schizophrenia symptoms were explored using linear mixed-effect models. The mean number of cigarettes smoked per day were 18 for patients, 13 for siblings and 12 for controls. In the overall sample, PRSs-smoking initiation (i.e., ever smoking as a binary phenotype, PRS-SI) were positively associated with positive symptoms, negative symptoms, and depressive symptoms, whereas PRSs-AI (age at regular smoking initiation) were negatively associated with all symptom dimensions, with similar effect sizes. When considering groups separately, PRS were only associated with psychotic symptoms in siblings and controls. In conclusion, unaffected siblings show smoking behaviors at an intermediate level between patients and healthy controls. Additionally, PRS-SI and PRS-AI are associated with all symptom dimensions only in unaffected siblings and healthy controls, possibly owing to the dominant role of other (genetic) risk factors in patients. Future studies may examine mechanisms via which genetic risk for smoking affects mental health symptoms.

Impact of Nicotine on Cognition in Patients With Schizophrenia: A Narrative Review

Nicotine is the psychoactive component given tobacco has several main components and acts as an agonist for nicotinic acetylcholine receptors (nAChRs) in the nervous system. Although the ligand-gated cation channels known as nAChRs are found throughout the nervous system and body, this review focuses on neuronal nAChRs. Individuals with psychiatric diseases such as schizophrenia, comorbid substance use disorders, attention-deficit hyperactivity disorder, major depression, and bipolar disorder have increased rates of smoking. These psychiatric disorders are associated with various cognitive deficits, including working memory, deficits in attention, and response inhibition functions. The cognitive-enhancing effects of nicotine may be particularly relevant predictors of smoking initiation and continuation in this comorbid population. Individuals with schizophrenia make up a significant proportion of smokers. Literature suggests that patients smoke to alleviate cognitive deficiencies due to the stimulating effects of nicotine. This narrative review examines the role of nicotine on cognition in schizophrenia.

The relationship between nicotine and psychosis

Cigarette smoking is strongly associated with psychotic disorders such as schizophrenia. For several decades it was assumed that the relationship could be explained by reverse causation; that smoking was secondary to the illness itself, either through self-medication or a process of institutionalization, or was entirely explained by confounding by cannabis use or social factors. However, studies have exposed that such hypotheses cannot fully explain the association, and more recently a bidirectional relationship has been proposed wherein cigarette smoking may be causally related to risk of psychosis, possibly via a shared genetic liability to smoking and psychosis. We review the evidence for these candidate explanations, using findings from the latest epidemiological, neuroimaging, genetic and preclinical work.

Neural Responses to Smoking Cues in Schizophrenia

The high prevalence of nicotine dependence contributes to excess mortality in schizophrenia. Cue reactivity, or the encounter of drug-related cues or contexts, triggers craving, drug-seeking, and relapse. Prior functional magnetic resonance imaging (fMRI) research indicates that individuals with schizophrenia have blunted neural responses to rewarding stimuli in association with more severe negative symptoms. The objectives of this study are to determine if smokers with schizophrenia have altered neural reactivity to smoking cues compared with non-psychiatrically ill smokers and to evaluate the influence of negative symptoms on cue reactivity. Twenty smokers with schizophrenia and 19 control smokers underwent fMRI while viewing smoking-related and neutral cues. The primary analysis was group comparison of Smoking-Neutral contrast using whole-brain analysis (Pcorrected < .05). Smokers with schizophrenia had significantly greater baseline carbon monoxide levels and longer duration of smoking, suggesting more nicotine use. While both groups had greater brain reactivity to smoking vs neutral cues, smokers with schizophrenia had significantly decreased cue reactivity (Smoking-Neutral) compared to controls in bilateral frontal midline regions. There were significant negative correlations between negative symptoms and frontal midline reactivity. Despite greater nicotine use, smokers with schizophrenia exhibited decreased smoking cue-induced neural reactivity in frontal midline regions, suggesting that increased smoking and low cessation rates in schizophrenia are not primarily driven by responses to smoking-related cues. The finding of negative correlations between cue reactivity and negative symptoms is consistent with previous research demonstrating decreased neural responses to rewarding cues, particularly in patients with negative symptoms.

Nicotine-induced activation of caudate and anterior cingulate cortex in response to errors in schizophrenia

BACKGROUND

Nicotine improves attention and processing speed in individuals with schizophrenia. Few studies have investigated the effects of nicotine on cognitive control. Prior functional magnetic resonance imaging (fMRI) research demonstrates blunted activation of dorsal anterior cingulate cortex (dACC) and rostral anterior cingulate cortex (rACC) in response to error and decreased post-error slowing in schizophrenia.

METHODS

Participants with schizophrenia (n = 13) and healthy controls (n = 12) participated in a randomized, placebo-controlled, crossover study of the effects of transdermal nicotine on cognitive control. For each drug condition, participants underwent fMRI while performing the stop signal task where participants attempt to inhibit prepotent responses to "go (motor activation)" signals when an occasional "stop (motor inhibition)" signal appears. Error processing was evaluated by comparing "stop error" trials (failed response inhibition) to "go" trials. Resting-state fMRI data were collected prior to the task.

RESULTS

Participants with schizophrenia had increased nicotine-induced activation of right caudate in response to errors compared to controls (DRUG × GROUP effect: p _corrected < 0.05). Both groups had significant nicotine-induced activation of dACC and rACC in response to errors. Using right caudate activation to errors as a seed for resting-state functional connectivity analysis, relative to controls, participants with schizophrenia had significantly decreased connectivity between the right caudate and dACC/bilateral dorsolateral prefrontal cortices.

CONCLUSIONS

In sum, we replicated prior findings of decreased post-error slowing in schizophrenia and found that nicotine was associated with more adaptive (i.e., increased) post-error reaction time (RT). This proof-of-concept pilot study suggests a role for nicotinic agents in targeting cognitive control deficits in schizophrenia.

Tobacco smoking is associated with psychotic experiences in the general population of South London

BACKGROUND

The association between cigarette smoking and psychosis remains unexplained, but could relate to causal effects in both directions, confounding by socioeconomic factors, such as ethnicity, or use of other substances, including cannabis. Few studies have evaluated the association between cigarettes and psychotic experiences (PEs) in diverse, inner-city populations, or relationships with number of cigarettes consumed.

METHODS

We assessed associations and dose-response relationships between cigarette smoking and PEs in a cross-sectional survey of household residents (n = 1680) in South East London, using logistic regression to adjust for cannabis use, other illicit substances, and socioeconomic factors, including ethnicity.

RESULTS

We found association between any PEs and daily cigarette smoking, which remained following adjustment for age, gender, ethnicity, cannabis and use of illicit stimulant drugs (fully adjusted odds ratio 1.47, 95% confidence interval 1.01-2.15). Fully adjusted estimates for the association, and with number of PEs, increased with number of cigarettes smoked daily, implying a dose-response effect (p = 0.001 and <0.001, respectively). Odds of reporting any PEs in ex-smokers were similar to never-smokers.

CONCLUSIONS

In this diverse epidemiological sample, association between smoking and PEs was not explained by confounders such as cannabis or illicit drugs. Daily cigarette consumption showed a dose-response relationship with the odds of reporting PEs, and of reporting a greater number of PEs. There was no difference in odds of reporting PEs between ex-smokers and never-smokers, raising the possibility that the increase in PEs associated with smoking may be reversible.

Chronic prostatitis and comorbid non-urological overlapping pain conditions: A co-twin control study

OBJECTIVES

Chronic prostatitis/chronic pelvic pain syndrome (CP/CPPS) is characterized by pain and voiding symptoms in the absence of an obvious infection or other cause. CP/CPPS frequently occurs with non-urological chronic overlapping pain conditions (COPCs) of unknown etiology. We conducted a co-twin control study in men discordant for chronic prostatitis (CP), an overarching diagnosis of which approximately 90% is CP/CPPS. The primary aim was to investigate the contribution of familial factors, including shared genetic and common environmental factors, to the comorbidity of CP and COPCs.

METHODS

Data from 6824 male twins in the Vietnam Era Twin Registry were examined to evaluate the association between self-reported lifetime physician diagnosis of CP with COPCs including fibromyalgia, chronic fatigue syndrome, irritable bowel syndrome, temporomandibular disorder, tension headaches, and migraine headaches. Random effects logistic regression models were used and within-pair analyses evaluated confounding effects of familial factors on the associations.

RESULTS

There were significant associations between CP and all 6 examined COPCs. After adjusting for shared familial influences in within twin pair analyses, the associations for all COPCs diminished but remained significant. Familial confounding was strongest for the association of CP with fibromyalgia and temporomandibular disorder and smallest for irritable bowel syndrome.

CONCLUSIONS

CP and COPCs are highly comorbid. These associations can be partially explained by familial factors. The mechanisms underlying these relationships are likely diverse and multifactorial. Future longitudinal research can help to further elucidate specific genetic and environmental mechanisms and determine potentially causal relationships between CP and its comorbidities.

Neuropsychiatric Features in Primary Mitochondrial Disease

Mitochondrial diseases are a clinically heterogeneous group of disorders that ultimately result from dysfunction of the mitochondrial respiratory chain. There is some evidence to suggest that mitochondrial dysfunction plays a role in neuropsychiatric illness; however, the data are inconclusive. This article summarizes the available literature published in the area of neuropsychiatric manifestations in both children and adults with primary mitochondrial disease, with a focus on autism spectrum disorder in children and mood disorders and schizophrenia in adults.

Blood Levels of Monoamine Precursors and Smoking in Patients with Schizophrenia

Smoking is highly prevalent in patients with schizophrenia and exerts a negative impact on cardiovascular mortality in these patients. Smoking has complex interactions with monoamine metabolism through the ability of cigarette smoke to suppress Type 1 T helper cell (Th1) type immunity, the immunophenotype that is implicated in phenylalanine hydroxylase (PAH) dysfunction and tryptophan (Trp) breakdown to kynurenine (Kyn) via indoleamine 2,3-dioxygenase. Nicotine also induces tyrosine hydroxylase (TH) gene expression, leading to increased synthesis of catecholamines. Furthermore, there is evidence for PAH dysfunction in schizophrenia. This study aimed to compare the plasma levels of selected monoamine precursors and their metabolites in smokers vs. non-smokers in a large sample of patients with schizophrenia. We measured plasma phenylalanine (Phe), tyrosine (Tyr), Trp, and Kyn levels using high-performance liquid chromatography and calculated Phe:Tyr and Kyn:Trp ratios in 920 patients with schizophrenia. Analysis of variance and linear regression analyses were used to compare these endpoints between three groups of patients with schizophrenia: (1) current smokers, (2) past smokers, and (3) non-smokers. There were significant differences among the three groups with regards to Tyr levels [F_(2,789) = 3.77, p = 0.02], with current smokers having lower Tyr levels when compared with non-smokers (p = 0.02). Kyn levels and Kyn:Trp ratio were different among the three groups [F_(2,738) = 3.17, p = 0.04, F_(2,738) = 3.61, p = 0.03] with current smokers having lower Kyn levels (p = 0.04) and higher Kyn:Trp ratio (p = 0.02) when compared with past smokers. These findings need to be replicated with protocols that include healthy controls to further elucidate the neurobiological underpinnings of altered Tyr and Kyn levels in smokers. Results do suggest potential molecular links between schizophrenia and smoking that may represent biomarkers and treatment targets for reducing an important modifiable cause of general morbidity and mortality in patients with schizophrenia.

Smoking and schizophrenia in population cohorts of Swedish women and men: a prospective co-relative control study

OBJECTIVE

The purpose of this study was to clarify the causes of the smoking-schizophrenia association.

METHOD

Using Cox proportional hazard and co-relative control models, the authors predicted future risk for a diagnosis of schizophrenia or nonaffective psychosis from the smoking status of 1,413,849 women and 233,879 men from, respectively, the Swedish birth and conscript registries.

RESULTS

Smoking was assessed in women at a mean age of 27 and in men at a mean age of 18. The mean age at end of follow-up was 46 for women and 26 for men. Hazard ratios for first-onset schizophrenia were elevated both for light smoking (2.21 [95% CI=1.90-2.56] for women and 2.15 [95% CI=1.25-3.44] for men) and heavy smoking (3.45 [95% CI=2.95-4.03] for women and 3.80 [95% CI=1.19-6.60] for men). These associations did not decline when schizophrenia onsets 3-5 years after smoking assessment were censored. When age, socioeconomic status, and drug abuse were controlled for, hazard ratios declined only modestly in both samples. Women who smoked into late pregnancy had a much higher risk for schizophrenia than those who quit early. Hazard ratios predicting nonaffective psychosis in the general population, in cousins, in half siblings, and in full siblings discordant for heavy smoking were, respectively, 2.67, 2.71, 2.54, and 2.18. A model utilizing all relative pairs predicted a hazard ratio of 1.69 (95% CI=1.17-2.44) for nonaffective psychosis in the heavy-smoking member of discordant monozygotic twin pairs.

CONCLUSIONS

Smoking prospectively predicts risk for schizophrenia. This association does not arise from smoking onset during a schizophrenic prodrome and demonstrates a clear dose-response relationship. While little of this association is explained by epidemiological confounders, a portion arises from common familial/genetic risk factors. However, in full siblings and especially monozygotic twins discordant for smoking, risk for nonaffective psychosis is appreciably higher in the smoking member. These results can help in evaluating the plausibility of various etiological hypotheses for the smoking-schizophrenia association.

Resting-state functional connectivity and nicotine addiction: prospects for biomarker development

Given conceptual frameworks of addiction as a disease of intercommunicating brain networks, examinations of network interactions may provide a holistic characterization of addiction-related dysfunction. One such methodological approach is the examination of resting-state functional connectivity, which quantifies correlations in low-frequency fluctuations of the blood oxygen level-dependent magnetic resonance imaging signal between disparate brain regions in the absence of task performance. Here, evidence of differentiated effects of chronic nicotine exposure, which reduces the efficiency of network communication across the brain, and acute nicotine exposure, which increases connectivity within specific limbic circuits, is discussed. Several large-scale resting networks, including the salience, default, and executive control networks, have also been implicated in nicotine addiction. The dynamics of connectivity changes among and between these large-scale networks during nicotine withdrawal and satiety provide a heuristic framework with which to characterize the neurobiological mechanism of addiction. The ability to simultaneously quantify effects of both chronic (trait) and acute (state) nicotine exposure provides a platform to develop a neuroimaging-based addiction biomarker. While such development remains in its early stages, evidence of coherent modulations in resting-state functional connectivity at various stages of nicotine addiction suggests potential network interactions on which to focus future addiction biomarker development.

The Role of Nicotine in Schizophrenia

Schizophrenia is associated with by severe disruptions in thought, cognition, emotion, and behavior. Patients show a marked increase in rates of smoking and nicotine dependence relative to nonaffected individuals, a finding commonly ascribed to the potential ameliorative effects of nicotine on symptom severity and cognitive impairment. Indeed, many studies have demonstrated improvement in patients following the administration of nicotine. Such findings have led to an increased emphasis on the development of therapeutic agents to target the nicotinic system as well as increasing the impetus to understand the genetic basis for nicotinic dysfunction in schizophrenia. The goal of this review article is to provide a critical summary of evidence for the role of the nicotinic system in schizophrenia. The first part will review the role of nicotine in normalization of primary dysfunctions and endophenotypical changes found in schizophrenia. The second part will provide a summary of genetic evidence linking polymorphisms in nicotinic receptor genes to smoking and schizophrenia. The third part will summarize attempts to treat schizophrenia using agents specifically targeting nicotinic and nicotinic receptor subtypes. Although currently available antipsychotic treatments are generally able to manage some aspects of schizophrenia (e.g., positive symptoms) they fail to address several other critically effected aspects of the disease. As such, the search for novel mechanisms to treat this disease is necessary.

Retinal microvessels reflect familial vulnerability to psychotic symptoms: A comparison of twins discordant for psychotic symptoms and controls

Mounting evidence suggests that individuals with schizophrenia have an underlying vulnerability to cardiovascular disease, and a recent study suggested that this vulnerability might be reflected in the retinal microvasculature. The purpose of this study was to test the hypothesis that the retinal microvessels reflect familial vulnerability to psychotic symptoms. Participants were 531 adolescent and young adult twins who took part in the Brisbane Longitudinal Twin Study and the Twins Eye Study in Tasmania. The twins had photographs taken of their retina when they were adolescents or young adults (M age=20.6 years), and retinal vessel diameter was assessed using computer software. The twins completed an assessment of psychosis symptoms approximately six years later. We compared retinal venular diameters of individuals with one or more symptoms of psychosis (n=45), their unaffected co-twins (n=24), and controls (n=462). Individuals with one or more symptoms of psychosis had wider venules (standardized mean=0.29) than controls (standardized mean=-0.04; p=.03), and unaffected co-twins had venular diameters that were intermediate (standardized mean=0.13) between the two groups, suggesting that wide venules may represent a proxy marker of familial vulnerability to psychosis symptoms. Consistent with previous work, there were no differences in arteriolar diameter between individuals with and without symptoms of psychosis. Findings suggest that wide retinal venules may serve as a proxy marker of familial liability to psychosis symptoms. The pathophysiological mechanisms linking psychosis and cardiovascular disease may be operative from early in life, possibly at the level of the microvasculature.

Brain circuits that link schizophrenia to high risk of cigarette smoking

Schizophrenia is associated with a high prevalence of smoking. Functional connectivity between the dorsal anterior cingulate (dACC) and limbic regions including the ventral striatum, extended amygdala and parahippocampal areas has been previously implicated in the genetics and clinical severity of smoking. In this study, we test the hypothesis that dACC functional circuits are key paths for the high risk of smoking comorbidity in schizophrenia. Resting state functional magnetic resonance imaging (fMRI) was performed using the dACC as a seed region in smoking and nonsmoking patients with schizophrenia (n = 54), matched controls (n = 65), and nonpsychotic first-degree relatives (n = 24). Multiple regions had decreased connectivity with the dACC in schizophrenia patients when compared with matched controls (n = 65). Several of these functional circuits were also associated with nicotine addiction severity; the largest cluster included limbic areas such as the parahippocampal, extended amygdala, ventral striatal, and posterior insula regions, indicating an overlap of schizophrenia and nicotine addiction on to this circuit. These same functional connectivity-defined circuits were also significantly impaired in schizophrenia nonsmokers compared with control nonsmokers and in nonpsychotic first-degree relatives. Functional connectivity between the dACC and limbic regions is inherently abnormal in schizophrenia, related to its genetic liability regardless of smoking, and overlaps with a nicotine addiction-related circuit. Our findings establish a biologically defined brain circuit mechanism that contributes to the high prevalence of smoking.

Sex differences in the genetic risk for schizophrenia: history of the evidence for sex-specific and sex-dependent effects

Although there is a long history to examinations of sex differences in the familial (and specifically, genetic) transmission of schizophrenia, there have been few investigators who have systematically and rigorously studied this issue. This is true even in light of population and clinical studies identifying significant sex differences in incidence, expression, neuroanatomic and functional brain abnormalities, and course of schizophrenia. This review highlights the history of work in this arena from studies of family transmission patterns, linkage and twin studies to the current molecular genetic strategies of large genome-wide association studies. Taken as a whole, the evidence supports the presence of genetic risks of which some are sex-specific (i.e., presence in one sex and not the other) or sex-dependent (i.e., quantitative differences in risk between the sexes). Thus, a concerted effort to systematically investigate these questions is warranted and, as we argue here, necessary in order to fully understand the etiology of schizophrenia.

Variants in the 15q25 gene cluster are associated with risk for schizophrenia and bipolar disorder

BACKGROUND

Rates of tobacco smoking are significantly higher in patients with schizophrenia compared with the general population. The underlying mechanism for this comorbidity is unclear. One hypothesis is that there are common genetic factors that predispose to both nicotine dependence (ND) and schizophrenia. To investigate this hypothesis, we examined the association of the 15q25 gene cluster, the most significant candidate region to date implicated in ND and smoking behavior, with schizophrenia and bipolar disorder.

METHODS

Five variants in the 15q25 gene cluster (rs951266, rs16969968, rs1051730, rs8040868, and rs17477223) were selected to test for association with schizophrenia diagnosis, bipolar disorder diagnosis, and the presence of negative symptoms of schizophrenia. Effects of the variants on 15q25 gene expression were analyzed using publically available postmortem brain expression data.

RESULTS

A meta-analysis revealed four markers associated with risk for schizophrenia and bipolar disorder (rs951266, rs16969968, rs8040868, and rs17477223), and with the presence of negative symptoms of schizophrenia (rs951266, rs1051730, rs8040868, and rs17477223). The associations were in the same direction as that found for ND. Gene expression analysis indicated an association between genotypes of the rs1051730 variant and CHRNA5 expression in brain and peripheral blood mononuclear cells, and with the rs16969968 and rs17477223 variants in brain.

CONCLUSION

Variants in the 15q25 gene cluster are associated with risk for schizophrenia/bipolar illness, negative symptoms of schizophrenia, and influence CHRNA5 expression in the brain and peripheral blood mononuclear cells. These results are consistent with the notion that there are genetic mechanisms common to schizophrenia, ND, and bipolar disorder.

Relationship between tobacco and cannabis use status in outpatients with schizophrenia

BACKGROUND AND OBJECTIVE

While high prevalence of tobacco and cannabis use are well established in schizophrenia, reports on their co-morbid use is limited. We explored the links between tobacco and cannabis use in an outpatient population meeting DSM-IV criteria for schizophrenia.

METHODS

Cigarette smoking behaviors were assessed in an outpaitent population with schizophrenia (N=54) with current (n=18), former (n=24), and no lifetime cannabis dependence (n=12).

RESULTS

We found significant differences in cigarettes per day (CPD) across groups: current dependent patients smoked less CPD than patients with former dependence and those with no history of dependence; former dependent patients smoked significantly less than patients with no history of cannabis dependence.

CONCLUSIONS AND SCIENTIFIC SIGNIFICANCE

Preliminary results support an effect of cannabis use status on tobacco consumption. In the absence of cannabis, patients may increase cigarette smoking, suggesting state-dependent effects of cannabis on tobacco. Prospective designs should further examine this relationship between cannabis and tobacco in schizophrenia versus non-psychiatric controls.

Cigarette smoking, psychopathology and cognitive function in first-episode drug-naive patients with schizophrenia: a case-control study

BACKGROUND

Although patients with chronic schizophrenia have substantially higher smoking rates than either the general population or patients with other mental illnesses, drug-naive patients with a first episode of schizophrenia have received little systemic study. This study examined smoking rates, the association between smoking and symptom severity and cognitive function in Chinese first-episode schizophrenia (FES) patients using cross-sectional and case-control designs.

METHOD

Two hundred and forty-four drug-naive FES patients and 256 healthy controls matched for gender, age and education completed the Fagerström Test for Nicotine Dependence (FTND) and the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS). Patients were also rated on the Positive and Negative Symptom Scale (PANSS).

RESULTS

The rate and quantity of smoking were not significantly higher among FES patients compared to the general population. Among patients, smokers scored higher than non-smokers on the total PANSS and the positive symptom subscale scores. There were no significant associations between cognitive function and smoking in either FES patients or healthy controls.

CONCLUSIONS

In contrast to studies in patients with chronic schizophrenia, drug-naive FES patients did not smoke more frequently than the general population. Furthermore, patients with psychotic disorders who smoked did not exhibit significant cognitive differences compared with those who did not smoke. However, smoking may have other detrimental effects on physical and mental health, for example on positive symptoms.

Exploring the relationships between tobacco smoking and schizophrenia in first-degree relatives

Up to 90% of individuals with schizophrenia suffer from nicotine dependence. Both schizophrenia and nicotine consumption have strong genetic components, which may overlap. The relationship between schizophrenia and nicotine dependence remains unclear, due in part to confounding factors. Studies of the relationship between nicotine consumption and milder schizophrenia-related phenotypes, such as schizotypy, in first-degree relatives of individuals with schizophrenia could help to better understand the relationship between smoking and schizophrenia while avoiding such confounders. We assessed the proportion of smokers, their level of nicotine dependence and their level of schizotypy in a sample of 98 first-degree relatives of schizophrenic subjects and 110 healthy controls. Partial correlation analysis was used to assess the relationship between schizotypal dimensions and smoking dependence. The prevalence of smoking and nicotine dependence levels were higher in the relatives than in the healthy control group. We found no relationship between nicotine dependence and the magnitude of schizotypal features in either group. Our results support the hypothesis that the relationship between schizophrenia and smoking is largely mediated by common familial factors, which may be genetic.

Insular and anterior cingulate circuits in smokers with schizophrenia

Schizophrenia (SZ) is associated with high rates of smoking. We previously found that resting state functional connectivity (rsFC) between the dorsal anterior cingulate (dACC) and striatum is independently associated with nicotine addiction and psychiatric illness. Since the insula is implicated in nicotine dependence, we hypothesized that SZ smokers will have greater dysfunction in smoking-related insular and dACC circuits than normal control smokers (NC) independent of smoking severity, consistent with an inherent disease-related weakening of smoking-related circuits. Nicotine challenge was used to demonstrate that decreased rsFC in identified circuits reflects addiction trait and is not affected by pharmacological state. Twenty-four NC smokers and 20 smokers with SZ matched on nicotine addiction severity participated in a resting state fMRI study and were scanned during two separate sessions while receiving a placebo or nicotine patch, in a randomized, cross-over design. Using individualized, anatomically defined anterior and posterior insula and dACC as regions of interest (ROI), whole brain rsFC was performed using each ROI as a seed. Significant negative correlations between smoking severity and rsFC between insula, dACC and striatum were found for both groups. Furthermore, smokers with SZ demonstrated additive reductions in circuit strength between the dACC and insula compared to NC smokers independent of smoking severity. Nicotine challenge did not significantly alter rsFC in insula-dACC-striatal circuits. Reduced rsFC strength between the insula, dACC and striatum is associated with nicotine addiction severity in both non-psychiatrically ill and in SZ smokers. Decreased insula-dACC rsFC may index overlapping circuitry associated with smoking and SZ.

Genetics of smoking and depression

Smoking and depression are significant public health problems with multiple etiological dimensions and outcomes. Although each condition is important by itself, they are important because they often potentiate each other. Consequently, it is also essential to understand the nature their relationship. This representative review focuses on the genetic etiology of the relationship in the context of reviewing first the epidemiology of depression and smoking, and then by exploring behavioral and molecular genetic studies, and other psychiatric and medical comorbidities. At this point, epidemiological evidence for a relationship between depression and smoking/nicotine dependence is compelling. Although behavioral genetic results differ somewhat by gender and in accordance with specific definitions of depression and smoking variables, recent studies show converging evidence for common genetic factors underlying the relationship, often in addition to non-shared environmental factors. The search for underlying genes and genetic mechanisms is at an early stage, but shows promising candidate genes and genetic approaches for future studies.

Relationship between smoking and family history of smoking in schizophrenia patients

BACKGROUND

The rate of smoking is known to be high in patients with schizophrenia. The purpose of this study was to quantitatively examine the relationship between smoking and family history of smoking in patients with schizophrenia.

METHODS

The sample consisted of 172 patients (mean age: 54 years, 55% male) hospitalized in a general psychiatric hospital. They were asked by a physician whether they smoked. Family histories of smoking and schizophrenia were assessed among first- and third-degree relatives.

RESULTS

Ninety-eight patients (57%) were identified as having smoked at some point. Multiple logistic regression analysis showed that smoking was significantly associated with family histories of smoking among both first- and third-degree relatives, controlling for the effects of age, sex, and family history of schizophrenia. The smoking rates were significantly higher in those with a family history of smoking, but not a family history of schizophrenia, than among those without a family history of either smoking or schizophrenia. However, this difference did not reach statistical significance among those with a family history of schizophrenia.

CONCLUSION

Smoking is greatly influenced by family history of smoking in patients with schizophrenia, especially among those without a family history of schizophrenia. The association between smoking and family history of smoking was inconclusive among those patients with schizophrenia who also had a family history of schizophrenia.

Imaging genetic liability to schizophrenia: systematic review of FMRI studies of patients' nonpsychotic relatives

There is a growing literature on brain activity in the nonpsychotic first-degree relatives of patients with schizophrenia as measured using functional imaging. This systematic review examined 20 studies in 4 domains of cognition, including cognitive control (7 samples), working memory (5 samples), long-term memory (4 samples), and language (4 samples). While the literature was widely divergent, these studies did consistently find activation differences between patients' relatives and controls. The most consistent increases in activation within hemisphere were found in right ventral prefrontal cortex (PFC) and right parietal cortex. Abnormal activity, defined as significant increases or decreases in activation relative to controls irrespective of hemisphere, was found in about two-thirds of contrasts in the cerebellum, dorsal prefrontal, lateral temporal, and parietal cortices, and thalamus, with basal ganglia and ventral PFC showing abnormalities in approximately half of those contrasts. Anterior cingulate was generally spared in patients' relatives. The diversity of findings in studies of patients' relatives may derive from differences between the cognitive demands across studies. We identify avenues for building a more accurate and cumulative literature, including symmetrical inclusion criteria for relatives and controls, recording in-scanner responses, using both a priori and whole-brain tests, explicitly reporting threshold values, reporting main effects of task, reporting effect sizes, and quantifying the risk of false negatives. While functional imaging in the relatives of schizophrenia patients remains a promising methodology for understanding the impact of the unexpressed genetic liability to schizophrenia, no single region or mechanism of abnormalities has yet emerged.

Smoking, nicotine and neuropsychiatric disorders

Tobacco smoking is an extremely addictive and harmful form of nicotine (NIC) consumption, but unfortunately also the most prevalent. Although disproportionately high frequencies of smoking and its health consequences among psychiatric patients are widely known, the neurobiological background of this epidemiological association is still obscure. The diverse neuroactive effects of NIC and some other major tobacco smoke constituents in the central nervous system may underlie this association. This present paper summarizes the pharmacology of NIC and its receptors (nAChR) based on a systematic review of the literature. The role of the brain's reward system(s) in NIC addiction and the results of functional and structural neuroimaging studies on smoking-related states and behaviors (i.e. dependence, craving, withdrawal) are also discussed. In addition, the epidemiological, neurobiological, and genetic aspects of smoking in several specific neuropsychiatric disorders are reviewed and the clinical relevance of smoking in these disease states addressed.

CHRFAM7A copy number and 2-bp deletion polymorphisms and antisaccade performance

Chromosome 15q13-q14 harbours the gene for the alpha7 nicotinic acetylcholine receptor subunit (CHRNA7) and a related gene (CHRFAM7A) which arises from a partly duplicated portion of CHRNA7. Recent evidence suggests that CHRFAM7A is a locus with a possible role in schizophrenia and cognitive functioning. We studied an antisaccade task as a fronto-parietal measure of executive function that reflects risk for schizophrenia. Association of CHRFAM7A genotype with antisaccade performance was assessed in 103 healthy Caucasian individuals. No significant associations of 2-bp deletion or CHRFAM7A copy number with antisaccade performance parameters were observed. The failure to observe an association between antisaccade performance and polymorphisms in CHRFAM7A gene is consistent with specificity of the gene effects on hippocampal and memory functions as previously demonstrated.

Predictors of starting to smoke cigarettes in patients with first episode psychosis

INTRODUCTION

Cigarette smoking is common in psychotic disorders and may be initiated in an attempt to control features of illness. However, genetic, obstetric and early life conditions are risks for starting to smoke in the general population but their role in psychotic patients is unclear.

METHOD

Smoking history and the putative predictors of starting to smoke were assessed in a community-wide sample of 115 first episode psychosis patients. The proportion that initiated smoking was compared with that from population surveys and the impact of risk factors was assessed within the patient sample.

RESULTS

Most patients began smoking before illness onset and the proportion who initiated smoking was significantly high by the onset of a functional decline. Gestational tobacco exposure was a risk for smoking and was also associated with low birthweight, poor academic achievement, and obesity. Low familial socioeconomic position but not familial psychiatric problems also predicted smoking initiation.

DISCUSSION

In most cases, smoking preceded illness onset and was not a response to early features of illness. General population predictors of starting to smoke were also associated with smoking initiation in psychotic patients. Of these risks, exposure to tobacco during gestation is noteworthy in that it affects brain development and is associated with cognitive, behavioral, psychiatric and general health problems. In addition, nicotine interacts with other substances of abuse. The initiation of smoking before illness onset and the association with developmental problems raises the question of whether cigarette smoking influences some aspects of illness in patients with psychosis.

Elevated rates of substance use disorders in non-psychotic siblings of individuals with schizophrenia

BACKGROUND

Individuals with schizophrenia use psychoactive substances more frequently than the general population. The genetic vulnerability to develop schizophrenia may also increase risk for the development of substance use disorders. We examine this hypothesis by assessing the rates of substance use disorders and nicotine use in non-psychotic siblings of individuals with schizophrenia.

METHODS

Participants included 59 individuals with DSM-IV schizophrenia, 53 of their siblings, 80 community controls, and 75 of their siblings. Statistical regression was used to assess the rates of substance use disorders and nicotine use in study participants while controlling for age, gender, lifetime diagnosis of a mood or anxiety disorder, and a family history of substance use disorder.

RESULTS

Individuals with schizophrenia and their non-psychotic siblings reported higher rates of alcohol and cannabis use disorders and nicotine use when compared to siblings of comparison subjects.

CONCLUSIONS

The vulnerability to develop schizophrenia may also extend to substance use disorders. Future research is needed to investigate the neurobiological basis of increased substance use in non-psychotic siblings and the psychosocial mechanisms that may contribute to increased substance use in non-psychotic siblings.

Renner T. Genetische Befunde zu Schizophrenie

Zusammenfassung: Schizophrene Erkrankungen zeichnen sich durch eine sehr heterogene Symptomatik mit übergreifenden Funktionsstörungen verschiedenster kognitiver Bereiche aus. Die vielfältigen Phänotypen werden durch das Zusammenwirken von genetischer Prädisposition und Umwelteinflüssen erklärt. Pathophysiologische Modelle beinhalten die Dopamin-Überschuss- sowie Glutamat-Mangel-Hypothese, die Radikal-Hypothese und die Hypothese entwicklungsbedingter versus degenerativer Genese. Neben den neurobiologischen Erklärungsansätzen geben Kopplungsstudien mit nachfolgenden Feinkartierungen Hinweise auf potentiell an der Pathophysiologie beteiligte Gene. Den wichtigsten Kandidatengenen, wie Dysbindin (DTNBP1), Neuregulin1 (NRG1) oder DISC-1 (disrupted-in schizophrenia-1), werden Einfluss auf die Signalübertragung sowie der Ausbildung und dem Erhalt der Struktur von neuronalen Netzwerken zugeschrieben. Zu ihnen werden zahlreiche weitere Gene gezählt. Insgesamt ist bei der Pathogenese der Schizophrenie von einer multifaktoriellen Entstehung mit der Interaktion von verschiedenen genetischen und neurobiologischen sowie exogenen Komponenten auszugehen.

[Smoking and schizophrenia: epidemiological and clinical features]

FREQUENCY: The prevalence of cigarette smoking is significantly higher among patients with schizophrenia (60-90%) than in the general population (23-30%). While tobacco smoking decreases in the general population (from 45% in the 1960's to 23-30% in the 2000's), smoking in patients with schizophrenia remains high. Patients with schizophrenia smoke more cigarettes than control subjects. Patients smoke more deeply, thereby increasing their exposure to the harmful elements in tobacco smoke. IMPACT OF SMOKING IN SCHIZOPHRENIC PATIENTS: As in the general population, smoking contributes to the reduced life expectancy in patients with schizophrenia. Patients with schizophrenia are at increased risk for cardiovascular disease due to high rates of cigarette smoking. In the Department of Mental Health of the commonwealth of Massachusetts, cardiovascular disease was the factor the most strongly associated with excess mortality. Cardiac deaths were elevated more than six-fold. Weight gain, insulin resistance, metabolic syndrome and diabetes mellitus are frequent in patients with schizophrenia, and may worsen the risk of cardiovascular diseases. It has been reported that the risk for lung cancer in patients with schizophrenia is lower than that of the general population, despite increased smoking. However, in a study conducted in Finland, a slightly increased cancer risk was found in patients with schizophrenia. Half of the excess cases were attributable to lung cancer. IMPROVEMENT OF COGNITIVE DEFICITS: Patients with schizophrenia may use nicotine to reduce cognitive deficits and negative symptoms or neuroleptic side effects. Smoking may transiently alleviate negative symptoms in schizophrenic patients by increasing dopaminergic and glutamatergic neurotransmission in the prefrontal cortex. In patients with schizophrenia, nicotine improves some cognitive deficits: (1) sensory gating deficits and abnormalities in smooth pursuit eye movements associated with schizophrenia are transiently normalized with the administration of nicotine ; (2) high-dose nicotine transiently normalizes the abnormality in P50 inhibition in patients with schizophrenia and in their relatives; (3) in tasks that tax working memory and selective attention, nicotine may improve performance in schizophrenia patients by enhancing activation of and functional connectivity between brain regions that mediate task performance (Jacobsen et al. 2004; Paktar et al.2002); (4) cigarette smoking may selectively enhance visuospatial working memory and attentional deficits in smokers with schizophrenia. However, Harris et al., found that nicotine affects only the attention without effects of nicotine on learning, memory or visuospatial/constructional abilities. In addition, smoking could facilitate disinhibition in schizophrenic patients.

Nicotine consumption and schizotypy in first-degree relatives of individuals with schizophrenia and non-psychiatric controls

Individuals with schizophrenia have very high rates of cigarette smoking, and much has been discovered about the influence of nicotine on brain functioning in schizophrenia. However, less is understood about the relationship between nicotine consumption and milder phenotypes related to schizophrenia, specifically schizotypy. This study examined the relationship between nicotine consumption and schizotypy in two unmedicated samples that included first-degree relatives and non-psychiatric controls. Forty-two first-degree relatives and 50 control participants were administered a self-report questionnaire on schizotypal features as well as a self-report questionnaire on smoking behavior. A positive relationship was found between smoking status and level of schizotypy, and higher levels of schizotypy significantly predicted the odds of being a smoker after controlling for gender and group status. Interestingly, group status was a significant moderator in the relationship between level of schizotypy and smoking status, such that the relationship between these two variables was only significant in the first-degree relatives. This is the first study to investigate the relationship between these variables in a sample of first-degree relatives of individuals with schizophrenia. Those individuals with more schizotypal features are presumably at greater risk for schizophrenia-spectrum disorders and thus may be more likely to smoke cigarettes given the known biochemical effects of nicotine on overt positive and negative symptoms of schizophrenia. Although relatives did not differ from controls in their level of self-reported schizotypy, the significant relationship between smoking status and schizotypy in the former group is likely explained by their genetic vulnerability to schizophrenia-spectrum disorders.

Genetics of nicotine dependence and pharmacotherapy

Nicotine dependence is substantially heritable. Several regions across the genome have been implicated in containing genes that confer liability to nicotine dependence and variation in individual genes has been associated with nicotine dependence. Smoking cessation measures are also heritable, and measured genetic variation is associated with nicotine dependence treatment efficacy. Despite significant strides in the understanding of the relative contribution of genetic and environmental factors to nicotine dependence and treatment, emergent challenges necessitate interdisciplinary coordinated effort for effective problem solving. These challenges include refinement of the nicotine dependence phenotype, better understanding of the dynamic interplay between genes and environment in nicotine dependence etiology, application and development of molecular and statistical methodology that can adequately address vast amounts of data, and continuous translational cross-talk.

The vulnerability to alcohol and substance abuse in individuals diagnosed with schizophrenia

Individuals with schizophrenia are at increased risk for developing substance abuse disorders. Here, we consider factors that might elevate their risk for substance abuse. The tendency among schizophrenic individuals to overvalue drug-like rewards and to devalue the potential negative consequences of substance abuse may be a contributing factor to their substance abuse risk. This bias, which may partly reflect the convergence of glutamatergic and dopaminergic input to the limbic striatum, also may contribute to disadvantageous decision-making and other impulsive behavior. This propensity to seek drug-like rewards is augmented by alterations in nicotinic cholinergic, GABAergic, glutamatergic, and cannabinnoid receptor function associated with schizophrenia that increase the abuse liability of low doses of nicotine, ethanol, and perhaps cannabis, and augment the dysphoric effects of higher doses of ethanol and cannabis. The distortions in reward processing and altered response to substances of abuse also increase the likelihood that individuals with schizophrenia will self-medicate their subjective distress with abused substances. The focus on distinctions between motivation and reward with respect to substance abuse risk by schizophrenic patients suggests a need for a reconsideration of the construct of "negative symptoms" for this dually-diagnosed patient group.

Genetics of chromosome 15q13-q14 in schizophrenia

Positive genetic linkage to the 15q13-q14 region has been found in 11 studies, and several association reports support this locus as a candidate region for schizophrenia. The locus is unusual in that it was first linked to an endophenotype found in schizophrenia, the P50 deficit, and subsequently to schizophrenia. There is also biological data showing that a candidate gene in the region, the alpha7 nicotinic receptor CHRNA7, plays a seminal role in the linked endophenotype, and is decreased in expression in the patient population. The 15q13-q14 region is complicated by a partial duplication of the CHRNA7 gene that includes exons 5-10 and considerable sequence downstream. Evidence from multiple studies supports a broad region of genetic linkage around the marker D15S1360.

Nicotine use in schizophrenia: the self medication hypotheses

The behavioural and cognitive effects of nicotine in schizophrenia have received much interest in recent years. The rate of smoking in patients with schizophrenia is estimated to be two- to four-fold the rate seen in the general population. Furthermore such patients favour stronger cigarettes and may also extract more nicotine from their cigarettes than other smokers. The question has been raised whether the widespread smoking behaviour seen in this patient group is in fact a manifestation of a common underlying physiology, and that these patients smoke in an attempt to self-medicate. We present an overview of the explanations for elevated rates of smoking in schizophrenia, with particular emphasis on the theories relating this behaviour to sensory gating and cognitive deficits in this disorder that have been viewed as major support for the self-medication hypotheses.

A meta-analysis of worldwide studies demonstrates an association between schizophrenia and tobacco smoking behaviors

A meta-analysis of worldwide studies, found by a 10-year literature follow-up and/or by searching PubMed, was performed. Forty-two studies across 20 nations consistently demonstrated an association between schizophrenia and current smoking (weighted average odds ratio, OR=5.9; 95% confidence interval, CI 4.9--5.7). In 32 male studies across 18 nations, the weighted average OR was 7.2 (CI, 6.1--8.3). In 25 female studies across 15 nations, the weighted average OR was 3.3 (CI, 3.0--3.6). The association between schizophrenia and current smoking remained after using severe mentally ill controls (18 studies across 9 countries, weighted average OR was 1.9, CI 1.7--2.1) and controlling for other variables (3 studies, adjusted ORs ranged 2-3). Heavy smoking (6 studies across 4 countries, ORs ranged 1.9--6.4) and high nicotine dependence were more frequent in smokers with schizophrenia versus the general population. There was no consistent evidence that heavy smoking or high nicotine dependence was more frequent in smokers with schizophrenia versus severe mentally ill controls. Cessation rates were lower in schizophrenia smokers versus the general population. Schizophrenia patients had a higher prevalence of ever smoking than the general population (9 studies across 6 countries, weighted average OR=3.1, CI 2.4--3.8) and than severe mentally ill patients (5 studies across 5 countries, OR=2.0, CI 1.6--2.4). Moreover, in two studies adjusting for confounders schizophrenia patients had an increased risk of starting daily smoking than controls. Thus, people who are going to develop schizophrenia have risk factors that make them more vulnerable to start smoking.

Subjective and physiological responses to smoking cues in smokers with schizophrenia

The prevalence of smoking is high among people with schizophrenia. Although several research groups are developing smoking treatments for these smokers, abstinence rates to date have been modest. Methodological tools such as cue exposure are useful in clinical research with smokers in general, but the value of these paradigms with smokers with schizophrenia has yet to be established. The aim of the present study was to determine the subjective and physiological effects of exposure to in vivo smoking cues in smokers with schizophrenia. A total of 25 heavy smokers with schizophrenia or schizoaffective disorder were assessed while nonabstinent and after 2-hr smoking abstinence. Urge to smoke, mood, nicotine withdrawal symptoms, heart rate, and blood pressure were measured during a precue relaxation period, after exposure to neutral cues, and after exposure to smoking cues. Results indicate that both exposure to smoking cues and brief abstinence increased urge levels, nicotine withdrawal symptom levels, and negative affect. Abstinence did not amplify the effects of cues on urges or other cue reactivity measures. These results indicate that smoking cue reactivity laboratory models may be useful for investigating potential smoking treatments for, or neurobiological contributions to, smoking behavior in smokers with schizophrenia.

Metabolism and biochemical effects of nicotine for primary care providers

Nicotine is a colorless and volatile liquid alkaloid naturally occurring in the leaves and stems of Nicotiana tabacum and Nicotiana rustica. Nicotine, the primary component of tobacco, is responsible for both tobacco product addiction (with chronic exposure) and the odor associated with tobacco. In addition to cigarettes, nicotine is found in chewing gum, transdermal patches, nasal spray, and sublingual tablets. Following its inhalation and absorption, nicotine and its metabolic products exert diverse physiologic and pharmacologic effects. This article covers the absorption and metabolism of nicotine, nicotine toxicity, pharmacologic effects of nicotine, nicotine-drug interactions, and the use of nicotine for the treatment of disease.

Alpha-7 nicotinic receptor agonists: potential new candidates for the treatment of schizophrenia

RATIONALE AND OBJECTIVE

Auditory sensory gating, a biological measurement of the ability to suppress the evoked response to the second of two auditory stimuli, is diminished in people with schizophrenia. Deficits in sensory gating are associated with attentional impairment, and may contribute to cognitive symptoms and perceptual disturbances. This inhibitory process, which involves the alpha(7) nicotinic receptor mediated release of gamma-aminobutyric acid (GABA) by hippocampal interneurons, represents a potential new target for therapeutic intervention in schizophrenia.

METHOD

This paper will review several lines of evidence implicating the nicotinic-cholinergic, and specifically, the alpha(7) nicotinic receptor system in the pathology of schizophrenia and the evidence that alpha(7) nicotinic receptor agonists may ameliorate some of these deficits.

RESULTS

Impaired auditory sensory gating has been linked to the alpha(7) nicotinic receptor gene on the chromosome 15q14 locus. Single nucleotide polymorphisms of the promoter region of this gene are more frequent in people with schizophrenia. Although nicotine can acutely reverse diminished auditory sensory gating in people with schizophrenia, this effect is lost on a chronic basis due to receptor desensitization. Clozapine is able to reverse auditory sensory gating impairment, probably through an alpha(7) nicotinic receptor mechanism, in both humans and animal models with repeated dosing. The alpha(7) nicotinic agonist 3-2,4 dimethoxybenzylidene anabaseine (DMXBA) can also enhance auditory sensory gating in animal models. DMXBA is well tolerated in humans and improves several cognitive measures.

CONCLUSION

Alpha-7 nicotinic receptor agonists appear to be reasonable candidates for the treatment of cognitive and perceptual disturbances in schizophrenia.

Environmental factors and membrane polyunsaturated fatty acids in schizophrenia

There is accumulating evidence of reductions in red blood cell membrane essential fatty acids in patients with schizophrenia. The mechanisms that may underlie these reductions have yet to be determined. It is possible that the observed membrane fatty acid deficits are associated with the development of schizophrenia. Alternatively, the membrane fatty acid deficits may be due to environmental factors, such as smoking and variations in diet, which may not be associated specifically with the pathophysiology of schizophrenia. Patients with schizophrenia smoke cigarettes at very high rates. Cigarette smoke contains many pro-oxidants that contribute directly to oxidative stress. Polyunsaturated fatty acids (PUFAs) are very susceptible to oxidative effects of free radicals. Thus, smoke-induced oxidative stress could plausibly account for reductions in membrane fatty acid in schizophrenia. Recent studies provide conflicting evidence for smoking effects on membrane fatty acid deficits. Likewise, the effects of diet on membrane PUFAs in schizophrenia are not entirely clear. Essential PUFAs need to be consumed in diet. Thus, differences in membrane PUFAs observed between patients and control subjects may be due to dietary variation. Few studies that have examined dietary effects differ in their interpretation of the effects of diet on membrane PUFAs. Thus, the jury is still out whether smoking or dietary effects are the primary causes of membrane PUFA deficits in patients with schizophrenia. Future studies will need to systematically examine the potential effects of smoking and diet, as well as other environmental factors such exercise, to definitively establish whether or not PUFA abnormalities are inherent to schizophrenia.

Adolescent twins discordant for major depressive disorder: shared familial liability to externalizing and other internalizing disorders

BACKGROUND

In adolescents, as in adults, there is often comorbidity between major depressive disorder (MDD) and many other disorders. In this study, the discordant twin method was used to investigate whether this comorbidity in adolescents may be due to a shared familial liability between MDD and other internalizing disorders, and between MDD and externalizing disorders.

METHODS

We examined prevalence rates of anxiety disorders, substance use disorders, and childhood externalizing disorders in 624 seventeen-year-old same-sex monozygotic and dizygotic twin pairs. Overall prevalence of MDD in this sample was 12.9%. Twenty-seven twin pairs were concordant for MDD, 107 were discordant for MDD, and 490 were concordant for no MDD.

RESULTS

Prevalence rates for disorders other than MDD were elevated in the depressed twins relative to the control twins from pairs concordant for no MDD. Prevalence rates were also elevated in the nondepressed co-twins of depressed twins. Twin concordance for disorders other than MDD was higher in twin pairs where at least one twin was depressed than in pairs where neither twin was depressed.

CONCLUSIONS

These results support a shared familial liability between MDD and other internalizing disorders and between MDD and externalizing disorders in adolescents.