Excess tyrosine hydroxylase restriction fragment length polymorphism homozygosity in unipolar but not bipolar patients: a preliminary report

ADHD co-morbidities: A review of implication of gene × environment effects with dopamine-related genes

ADHD is a major burden in adulthood, where co-morbid conditions such as depression, substance use disorder and obesity often dominate the clinical picture. ADHD has substantial shared heritability with other mental disorders, contributing to comorbidity. However, environmental risk factors exist but their interaction with genetic makeup, especially in relation to comorbid disorders, remains elusive. This review for the first time summarizes present knowledge on gene x environment (GxE) interactions regarding the dopamine system. Hitherto, mainly candidate (GxE) studies were performed, focusing on the genes DRD4, DAT1 and MAOA. Some evidence suggest that the variable number tandem repeats in DRD4 and MAOA may mediate GxE interactions in ADHD generally, and comorbid conditions specifically. Nevertheless, even for these genes, common variants are bound to suggest risk only in the context of gender and specific environments. For other polymorphisms, evidence is contradictory and less convincing. Particularly lacking are longitudinal studies testing the interaction of well-defined environmental with polygenic risk scores reflecting the dopamine system in its entirety.

Genetic biomarkers of depression

Depression is a term that has been used to describe a variety of ailments, ranging from minor to incapacitating. Clinically significant depression, termed as major depression, is a serious condition characterized not only by depressed mood but also by a cluster of somatic, cognitive, and motivational symptoms. Significant research efforts are aimed to understand the neurobiological as well as psychiatric disorders, and the evaluation of treatment of these disorders is still based solely on the assessment of symptoms. In order to identify the biological markers for depression, we have focused on gathering information on different factors responsible for depression including stress, genetic variations, neurotransmitters, and cytokines and chemokines previously suggested to be involved in the pathophysiology of depression. The present review illustrates the potential of biomarker profiling for psychiatric disorders, when conducted in large collections. The review highlighted the biomarker signatures for depression, warranting further investigation.

Molecular epidemiology of major depressive disorder

Major depressive disorder causes significant morbidity, affecting people's ability to work, function in relationships, and engage in social activities. Moreover, major depressive disorder increases the risk of suicidal ideation, attempted suicide and death by completed suicide. There is evidence that chronic stress can cause major depressive disorder. As for genetic factors, only minor susceptibility genes have been reliably identified. The serotonin system provides a logical source of susceptibility genes for depression, because this system is the target of selective serotonin reuptake-inhibitor drugs that are effective in treating depression. The 5-hydroxytryptamine (serotonin) transporter (5-HTT) has received particular attention because it is involved in the reuptake of serotonin at brain synapses. One common polymorphic variant of the 5-HTT-linked polymorphic region (5-HTTLPR), which affects the promoter of the 5-HTT gene, causes reduced uptake of the neurotransmitter serotonin into the presynaptic cells in the brain. The authors discussed the relationship between genetic polymorphisms and major depressive disorder, with special emphasis on the 5-HTTTLPR polymorphism. As the 5-HTTLPR polymorphism was significantly associated with an increased risk of major depressive disorder, the 5-HTT gene may be a candidate for a major depressive disorder susceptibility gene. As major depressive disorder is a multifactorial disease, an improved understanding of the interplay of environmental and genetic polymorphisms at multiple loci may help identify individuals who are at increased risk for major depressive disorder. Hopefully, in the future we will be able to screen for major depressive disorder susceptibility by using specific biomarkers.

Will genetic polymorphism of tetranucleotide sequences help in the diagnostics of major psychiatric disorders?

Expecting a significant breakthrough in the diagnosis of complex disorders of neuropsychiatric background, intensive efforts are taking place to establish genetic markers correlated with these disorders. During the last decade, this research was focused on code regions connected with neurotransmission and metabolism of catecholamines. Nowadays big diagnostic expectations are associated with sequences of STR type, which are widespread throughout the genome. These microsatellite sequences do not code proteins, but may have function of regulatory elements in the process of gene transcription and expression. One of these is polymorphic TH01 locus with TCAT tetranucleotide repetitive motive. It is located in chromosomal position 11p15 in the first intron of the tyrosine hydroxylase gene (TH). We examined the existence of the association between polymorphism of TH01 marker and schizophrenia. The results of statistical comparative analysis between neuropsychiatric patients from Poland and their regionally matched healthy subjects were presented.

Nicotine dependence in a prospective population-based study of adolescents

Dopamine is a key neurotransmitter of the mesolimbic reward pathway in the human brain, and tyrosine hydroxylase (TH) is the rate-limiting enzyme in dopamine biosynthesis. Consequently, the gene encoding TH is a strong candidate for involvement in the genetic component of addiction. The importance of this gene in nicotine dependence is supported by many studies showing a link between nicotine administration and TH expression. A functional tetranucleotide repeat polymorphism within intron 1 of the TH gene (HUMTH01-VNTR) has been shown to modify tobacco use in two independent Caucasian samples from the USA and Australia. Using information drawn from an eight-wave Australian population-based longitudinal study of adolescent health, we tested the effect of the HUMTH01-VNTR on nicotine dependence. Comparisons were made between dependent smokers and non-dependent smokers. These data provide further support for a protective association between the K4 allele and dependent smoking (odds ratio 0.54, 95% confidence interval 0.28-1.0). No associations were observed at any of three other common TH polymorphisms (rs6356, rs6357 and HUMTH01-PstI). Including these data, three independent studies, two of which use identical phenotypes, have now identified a protective relationship between the K4 allele of the functional HUMTH01-VNTR polymorphism and high-level smoking.

Search for the influence of the tyrosine hydroxylase (TCAT)(n) repeat polymorphism on personality traits

A putatively functional tetranucleotide repeat polymorphism in the tyrosine hydroxylase gene (TH) has been investigated with regard to different aspects of psychopathology. We investigated whether reported associations of this TH polymorphism may reflect associations with common personality traits. Personality was assessed by the NEO Personality Inventory-Revised version (NEO PI-R), in 205 healthy Caucasian volunteers. Tendencies for higher scores in the neuroticism (N) facets, Angry hostility (P=0.008) and Vulnerability (P=0.021), were observed among carriers of one of the alleles (T8). Healthy women with the T6/T10 genotype had significantly higher scores (P=0.001) in the Deliberation and Dutifulness facets (P=0.031) (the Conscientiousness dimension, C) and lower scores (P=0.031) in the Feelings facet (the Openness dimension, O). We concluded that: (1) higher mean scores in the Neuroticism facets among T8 allele carriers are consistent with previous data and warrants further research; (2) the T6/T10 genotype may influence personality among women; (3) these data should be cautiously interpreted in the absence of corroborating data.

Tyrosine hydroxylase polymorphism and phenotypic heterogeneity in bipolar affective disorder: a multicenter association study

Tyrosine hydroxylase (TH), the rate-limiting enzyme in the metabolism of catecholamines, is considered a candidate gene in bipolar affective disorder (BPAD) and has been the subject of numerous linkage and association studies. Taken together, most results do not support a major gene effect for the TH gene in BPAD. Genetic and phenotypic heterogeneity may partially explain the difficulty of confirming the exact role of this gene using both association and linkage methods. Four hundred one BPAD patients and 401 unrelated matched controls were recruited within a European collaborative project (BIOMED1 project in the area of brain research, European Community grant number CT 92-1217, project leader: J. Mendlewicz) involving 14 centers for a case-control association study with a tetranucleotide polymorphism in the TH gene. Patients and controls were carefully matched for geographical origin. Phenotypic heterogeneity was considered and subgroup analyses were performed with relevant variables: age at onset, family history, and diagnostic stability. No association was observed in the total sample or for subgroups according to age at onset (n = 172), family history alone (n = 159), or high degree of diagnostic stability and a positive family history (n = 131). The results of this association study do not confirm the possible implication of TH polymorphism in the susceptibility to BPAD.

Analysis and metaanalysis of two polymorphisms within the tyrosine hydroxylase gene in bipolar and unipolar affective disorders

Tyrosine hydroxylase (TH) is the rate-limiting enzyme in the synthesis of dopamine and noradrenaline. While positive associations between TH and bipolar affective disorder have been found in several studies, many studies have failed to reproduce these results. In order to clarify this situation, association studies of bipolar and unipolar affective disorder groups and metaanalyses of published data on the TH tetranucleotide repeat polymorphism were done. The association studies used the TH tetranucleotide repeat polymorphism in intron 1 and a PstI polymorphism at the 3' end of the gene. The study comprised 124 unrelated bipolar patients, 126 unipolar patients, and 242 controls. There was no significant association of either bipolar or unipolar affective disorder with the TH tetranucleotide repeat polymorphism. However, a weak association (chi2 = 3.946, 1 df, P = 0.047; odds ratio, allele 2 vs. allele 1 = 0.71 (95% CI, 0.51-0.996)) was observed in the unipolar sample with the TH-PstI polymorphism. Three metaanalyses of published data on the TH tetranucleotide repeat polymorphism in major affective disorder were performed: bipolar I + II vs. control using 583 cases and 745 controls; unipolar vs. control using 204 cases and 359 controls; and bipolar + unipolar vs. control using 846 cases and 823 controls. In each analysis there was no association of the TH tetranucleotide repeat polymorphism and affective disorder. These results do not support the tyrosine hydroxylase gene having a major role in the etiology of bipolar affective disorder. However, our data suggest that this locus should be examined in larger samples of unipolar affective disorder.

Mode of inheritance in mood disorder families according to fluvoxamine response

Mood disorders are known to cluster within families, but the mode of transmission remains largely unknown. The purpose of our analysis was to determine whether selection of a sample that was homogeneous in its response to an antidepressant provided stronger evidence for a single major locus. Complex segregation analysis was applied to a sample of 171 Italian families of bipolar and unipolar probands that were responsive to the antidepressant fluvoxamine. We used regressive logistic analyses to determine the best fit from among environmental, arbitrary Mendelian, dominant, recessive and additive models. For the 171 affective families with probands that were responsive to the antidepressant fluvoxamine, a Mendelian model of inheritance was rejected. When considering 68 families of bipolar probands, the best fit was obtained for a Mendelian dominant model of transmission. The identification of a Mendelian mode of transmission in bipolar subjects who were selected according to their response to fluvoxamine supports the use of a pharmacological criterion as a tool for identifying true genetic disorders.