Update on the search for DNA markers linked to manic-depressive illness in the Old Order Amish

Three Approaches to Understanding and Classifying Mental Disorder: ICD-11, DSM-5, and the National Institute of Mental Health’s Research Domain Criteria (RDoC)

The diagnosis of mental disorder initially appears relatively straightforward: Patients present with symptoms or visible signs of illness; health professionals make diagnoses based primarily on these symptoms and signs; and they prescribe medication, psychotherapy, or both, accordingly. However, despite a dramatic expansion of knowledge about mental disorders during the past half century, understanding of their components and processes remains rudimentary. We provide histories and descriptions of three systems with different purposes relevant to understanding and classifying mental disorder. Two major diagnostic manuals—the International Classification of Diseases and the Diagnostic and Statistical Manual of Mental Disorders—provide classification systems relevant to public health, clinical diagnosis, service provision, and specific research applications, the former internationally and the latter primarily for the United States. In contrast, the National Institute of Mental Health’s Research Domain Criteria provides a framework that emphasizes integration of basic behavioral and neuroscience research to deepen the understanding of mental disorder. We identify four key issues that present challenges to understanding and classifying mental disorder: etiology, including the multiple causality of mental disorder; whether the relevant phenomena are discrete categories or dimensions; thresholds, which set the boundaries between disorder and nondisorder; and comorbidity, the fact that individuals with mental illness often meet diagnostic requirements for multiple conditions. We discuss how the three systems’ approaches to these key issues correspond or diverge as a result of their different histories, purposes, and constituencies. Although the systems have varying degrees of overlap and distinguishing features, they share the goal of reducing the burden of suffering due to mental disorder.

Post-genomic era and gene discovery for psychiatric diseases: there is a new art of the trade? The example of the HUMTH01 microsatellite in the Tyrosine Hydroxylase gene

The microsatellite HUMTH01, located in the first intron of the Tyrosine Hydroxylase (TH) gene (encoding the rate-limiting enzyme in the synthesis of catecholamines), is characterized by a TCAT repeated motif and has been used in genetic studies of neuropsychiatric and other complex diseases, in which catecholaminergic neurotransmission is implicated. After reporting a positive association between HUMTH01 and bipolar disorder as well as schizophrenia, the authors established that HUMTH01 alleles display the features of regulatory elements. Thereafter, they cloned two proteins (ZNF191 and HBP1), specifically binding to HUMTH01, and demonstrated that allelic variations of HUMTH01 have a quantitative silencing effect on TH gene expression in vitro, and correlate with quantitative and qualitative changes in the binding by ZNF191. The authors aim to characterize the transduction pathway impinging on the HUMTH01 microsatellite and establish its relevance for TH gene regulation in vivo. Since the TCAT repeated sequence is widespread throughout the genome, their approach may lead to the dissection of the mechanisms underlying the quantitative expression of several genes implicated in complex genetic traits, both normal and pathological. Thus, these investigations on the possible contribution and potential role of the HUMTH01 microsatellite in neuro-pathological conditions may represent an example of the different approaches needed to validate genetic targets in the "post-genomic era."

Population isolates: their special value for locating genes for bipolar disorder

OBJECTIVES

Population isolates offer several advantages for those hoping to identify predisposition genes for bipolar disorder (BP). In this review article, the rationale for performing gene mapping studies in this type of population and the results of genetic mapping studies performed to date in population isolates are presented.

METHODS

This article begins with a brief review of the concepts involved in mapping genes for BP. The concept of populations that show some degree of historical isolation and their special utility for certain types of gene mapping is presented. Methods of statistical analysis particularly relevant for gene mapping of complex diseases like BP are presented. Finally, several BP gene studies conducted to date in several population isolates are reviewed.

RESULTS

Genetic mapping studies of BP have occurred thus far in several isolates or sub-isolates, including the Amish population, Costa Ricans, Finnish, and Canadians (in Quebec), and significant linkage scores have been identified in the latter three isolates.

CONCLUSIONS

Possible greater homogeneity and greater consistency of diagnosis are factors that have been cited in several studies of BP done in isolates to date. Another special advantage of working in certain types of population isolate is their appropriateness for using certain types of association or linkage disequilibrium-based approaches at both the genome screening and fine mapping stages. These tests include mapping by linkage disequilibrium analyses, an approach that allows mapping to occur at the population, rather than the pedigree, level.

Writing Amish culture into genes: biological reductionism in a study of manic depression

Critical realism is used to explore the problem of reductionism in a classic (the Amish Study) and widely-cited study of manic depression. Along with related ideas drawn from the works of R.C. Lewontin, Arthur Kleinman, and Byron Good, it is shown that natural and social scientists deploy atomistic and holistic reductionism; this, in turn, leads to the construction of artificially 'closed systems' through the control of variables or exogenous forces. The psychiatric genetic studies of the Amish were predicated on the assumption that Amish society is homogeneous and unchanging and, therefore, closed. We conclude by arguing that interactions between behaviors and genes, where they exist, take place only within open systems, characterized by multiple mechanisms-social and biological-that together co-determine any event. To move forward, it is argued, behavior and gene research requires recognition and resolution of the philosophical conundrums that accompany reductionism.

A systematic evaluation of linkage studies in bipolar disorder

Genetic factors have long been implicated in the aetiology of bipolar disorder (BD). During the past two decades several linkage studies have been carried out with the aim of identifying major genes. However, remarkable discrepancies in results both between and within studies have constituted a major problem. In order to elucidate some of these conflicts, we assessed the published literature on linkage studies of bipolar disorder, focusing on methodological issues. Studies published between January 1980 and December 1994 were identified by computerized literature searches and subsequent scanning of review articles, and the reference lists of the articles primarily identified. A set of defined inclusion and exclusion criteria was used to select studies for assessment. A total of 31 variables were determined, and pre-defined codes were assigned in a structured manner. More than 200 citations were reviewed, and 60 articles were included in this study. Descriptive statistical analyses of the variables, as well as associations between variables, are presented. The findings are discussed with regard to the possibility that, beyond the genetic complexity of the disorder itself, there are several other similarly complicated study design issues which should be more carefully observed. Moreover, the need for standardization of basic criteria to use and report clinical and analytical parameters employed in linkage studies is strongly suggested.

Linkage study between manic-depressive illness and chromosome 21

Chromosome 21, of interest as potentially containing a disease gene for manic-depressive illness as possible evidence for a gene pre-disposing to affective disorder, has recently been reported in a single large family as well as samples of families. The present study investigates for linkage between manic-depressive illness and markers covering the long arm of chromosome 21 in two manic-depressive families, using ten microsatellite polymorphisms as markers. No conclusive evidence for a disease gene on the long arm of chromosome 21 was found. Assuming either a dominant or recessive mode of inheritance, close linkage to the marker PFKL, which has been reported as possibly linked to affective disorder, seems unlikely in the families studied here. PFKL and more telomeric markers yielded small positive lod scores at higher recombination fractions in the largest family, and small positive lod scores at lower recombination fractions in the affected-only analyses in the smallest family.

Evaluating genetic models of cognitive evolution and behaviour

Cognitive evolution can be studied at several different levels, ranging from complex societies of interdependent persons to the DNA molecules coding for enzymes that synthesize neurotransmitter molecules. Genetic models of cognitive evolution can be fairly evaluated only if they involve one or two genetic loci, maybe three loci if a massive investment of resources is made. If a simple genetic model is seriously proposed, it ought to be tested by genetic linkage analysis so that future theorizing can be guided and constrained by facts. For more complex behavioural characteristics based on large numbers of genes and intricate interrelations with the environment, genetic analysis and genetic theories are not likely to yield conclusive results. Instead, studying individual differences in the brain and neural correlates of cognitive processes will likely provide more rapid progress toward a deeper understanding of evolution.

Complex segregation analyses of old order Amish families ascertained through bipolar I individuals

Specific genetic hypotheses about the mode of transmission of bipolar affective disorders were examined by performing complex segregation analyses of Old Order Amish families. The analyses were performed on 1) the total set of 42 families including 689 relatives, 2) a subset of 19 families consisting of those kindreds sharing common ancestors within three generations that contained 333 relatives, and 3) a subset of 23 more distantly related families with 356 relatives. When all 42 families were included in the analyses, the specific mode of transmission that could be distinguished was dependent upon the diagnostic scheme used in the analysis. An autosomal dominant mode of inheritance could be rejected when relatives with bipolar I, atypical bipolar, major depressive disorder, and hypomania were included as affected. When analyses included only the subset of families more closely related, an autosomal dominant inheritance model was found to be consistent with transmission of BP I disorder. It was not possible to distinguish between other transmission models with broader diagnostic schemes in this subset of families. Finally, results of analyses on the subset of more distantly related families suggest that there is a significant proportion of Old Order Amish families in which the genetic factors contributing to the expression of bipolar illness are either polygenic or oligogenic.

Role of genetic factors in depression based on studies of Tourette syndrome and ADHD probands and their relatives

Tourette syndrome (TS) is a common, neuropsychiatric disorder which has many similarities to attention deficit hyperactivity disorder (ADHD). TS probands have a high frequency of a variety of behavioral disorders including depression. The depression may be due to a pleiotrophic effect of the Gts genes, proband ascertainment bias, or a result of coping with the chronic tics. To distinguish between these hypotheses we examined the responses to 17 Diagnostic Interview Schedule questions to evaluate the 9 DSM-III-R criteria for major depressive episode in 1,080 adults consisting of TS and ADHD probands, their relatives and controls. Using a Bonferonni corrected p there was a significant progressive increase in 16 of 17 depressive symptoms and for a life time history of a major depressive episode in groups with increased genetic loading for Gts genes. Similar trends were seen in the small number of ADHD probands and their relatives. There was also a significant increase for these variables in non-proband TS relatives versus non-TS relatives, indicating the association of depression with Gts genes was not due to ascertainment bias or the inappropriate choice of controls. Multiple linear regression analysis indicated that obsessive-compulsive behaviors, sex, ADHD, drug abuse, and age all showed a more significant effect on depressive symptoms than the number of tics. The presence or absence of TS in the relatives had a much greater effect on risk for depression than the presence or absence of an episode of major depression in the proband. These results are consistent with the hypothesis that Gts and ADHD genes play a major role in depression.

Exclusion of close linkage of bipolar disorder to the Gs-alpha subunit gene in nine Australian pedigrees

Growing evidence suggests that guanine nucleotide binding proteins (G proteins) may be involved in both the pathogenesis and treatment of bipolar affective disorder. Both overactive G proteins and increased levels of the alpha subunit of the stimulatory form (Gs-alpha) have been demonstrated in peripheral leucocytes of manic patients while an increase of Gs-alpha subunit levels has also been found in a postmortem study of bipolar disorder. The function of Gs and Gi alpha subunits has now been shown to be affected by lithium. The present study aimed to determine whether bipolar affective disorder was linked to the Gs-alpha subunit gene which has been mapped to chromosomal region 20q13.2. Linkage analysis utilized the PCR amplification of a portion of the Gs-alpha gene that contains a dinucleotide repeat (CA repeat) polymorphism. Linkage of bipolar disorder and recurrent depression to the Gs-alpha subunit gene was tested using a series of autosomal dominant and recessive models with varying penetrance levels. Additionally, linkage was examined using a series of levels of definitions of affective illness. Close linkage to the Gs-alpha subunit gene was strongly excluded using each model and definition. Thus, our study indicates that a genetic defect in the Gs-alpha subunit gene is unlikely to be the cause of bipolar disorder.