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The Genom of Homo sapiens.pdf

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Genetics <strong>of</strong> Schizophrenia and Bipolar Affective Disorder:Strategies to Identify Candidate GenesD.J. PORTEOUS,* K.L. EVANS,* J.K. MILLAR ,* B.S. PICKARD,* P.A. THOMSON,* R. JAMES,*S. MACGREGOR, † N.R. WRAY,* P.M. VISSCHER, † W.J. MUIR, ‡ AND D.H. BLACKWOOD ‡*Medical Genetics Section, University <strong>of</strong> Edinburgh, Western General Hospital, Edinburgh, Scotland, EH4 2XU;† Institute <strong>of</strong> Cell, Animal and Population Biology, Ashworth Laboratories, School <strong>of</strong> Biology, <strong>The</strong> University <strong>of</strong>Edinburgh, <strong>The</strong> King’s Buildings, Edinburgh, Scotland, EH9 3JT; ‡ Division <strong>of</strong> Psychiatry, University <strong>of</strong>Edinburgh, Royal Edinburgh Hospital, Edinburgh, Scotland, EH10 5HFSchizophrenia (SCZ) and bipolar affective disorder(BPAD) (formerly termed manic-depressive illness) aresevere, disabling psychiatric illnesses that feature prominentlyin the top ten causes <strong>of</strong> disability worldwide (Lopezand Murray 1998). Each will affect about 1% <strong>of</strong> the populationin their lifetime. <strong>The</strong> cost <strong>of</strong> providing treatmentfor mental illness in the UK National Health Service is estimatedat 10% <strong>of</strong> total expenditure. <strong>The</strong> total costs (medical,social, economic) are estimated at £32 billion per annumfor the population <strong>of</strong> 50 million in England (Bird1999; see also www.mentalhealth.org.uk). World HealthOrganization predictions indicate that major depressionwill be second only to heart disease in terms <strong>of</strong> disabilityadjustedlife years (DALYs) by 2020 (Lopez and Murray1998). Research into the causes <strong>of</strong> these devastating disordersand the development <strong>of</strong> improved interventions is ahigh scientific, social, individual, and public health priority.Unfortunately, these remain Cinderella disorders comparedto cancer and heart disease, both in terms <strong>of</strong> governmentaland societal recognition and national andinternational research support. Despite their high prevalenceand, indeed, decades <strong>of</strong> neuroscience research, littleis known with certainty about their cellular and molecularbases. Consequently, treatments remain largely empiricaland palliative. This apparent impasse, however, justifiesneither complacency nor despondency, because the oneconsistent, replicable finding is that family, twin, andadoption studies demonstrate a major genetic componentin both SCZ and BPAD (Merikangas and Risch 2003). Adecade ago only a handful <strong>of</strong> genes for monogenic disordershad been positionally cloned through linkage studies.Over the past five years, as the Human <strong>Genom</strong>e Projectand associated tools have developed, that number hassoared to over 1000. As the Human <strong>Genom</strong>e Projectpasses from formal completion to full development as auniversal biological tool, we can expect acceleratedprogress in tackling the much more difficult task <strong>of</strong> geneticallydissecting the common complex disorders includingmajor mental illness.EVIDENCE FOR A GENETIC COMPONENTTO SCZ AND BPAD<strong>The</strong> risk to a first-degree relative <strong>of</strong> a person affectedby SCZ or BPAD is an order <strong>of</strong> magnitude higher thanthat <strong>of</strong> the general population (from ~1% to 10–15% lifetimerisk for each disorder) (Kendler and Diehl 1993;Merikangas and Risch 2003). Further evidence for thehigh heritability <strong>of</strong> SCZ and <strong>of</strong> BPAD comes from twinand adoption studies. Concordance rates for monozygotictwins are around 50% for SCZ and 60–80% for BPAD.Importantly, these concordance rates are much higherthan for dizygotic twins (10–15%), and the biological riskis unaffected by adoption. Several chromosomal regionsthat may harbor susceptibility genes for SCZ and forBPAD have been identified by a range <strong>of</strong> genetic strategies(Owen et al. 2000; Potash and DePaulo 2000; Rileyand McGuffin 2000). <strong>The</strong>re is also growing evidence thatrelatives <strong>of</strong> sufferers are at higher risk <strong>of</strong> other psychiatricdiagnoses within the schizophrenia–affective disorderspectrum (Kendler et al. 1998; Wildenauer et al. 1999;Berrettini 2000; Valles et al. 2000). This suggests thatsome genetic risk factors may contribute to a range <strong>of</strong>psychotic symptoms that cross the traditional diagnosticboundaries <strong>of</strong> SCZ and affective disorders. In part, thismay reflect the fact that diagnosis <strong>of</strong> psychiatric illness isan imprecise science; psychiatric phenotypes are almostentirely based on pr<strong>of</strong>iles <strong>of</strong> behavioral indices and communicationpatterns. <strong>The</strong> use <strong>of</strong> standardized diagnosticcriteria (such as the Diagnostic and Statistical Manual <strong>of</strong>Mental Disorders, DSM-IV, published by the AmericanPsychiatric Association, and <strong>The</strong> ICD-10 Classification<strong>of</strong> Mental Health and Behavioural Disorders, publishedby the World Health Organization) has ensured good reproducibility<strong>of</strong> diagnoses between researchers, but thereis wide overlap <strong>of</strong> symptoms between the diagnostic categories<strong>of</strong> SCZ, BPAD, and recurrent, major (unipolar)depression. In the absence <strong>of</strong> reliable biological or geneticmarkers specific for SCZ or BPAD, the validity <strong>of</strong>existing classification remains uncertain. It would be amajor advance if genetic studies yielded molecular diagnosticmethods that could not only resolve some <strong>of</strong> thepresent uncertainties in psychiatric diagnosis, but also informtreatment choice and disease prognosis. Until suchtime, we can only speculate that variability in individualdiagnoses reflects a combination <strong>of</strong> genetic (and possiblyallelic) heterogeneity, polygenic inheritance, and variationin individual life trajectories/environmental exposures.Nevertheless, post-genome science most certainly<strong>of</strong>fers the best hope for determining the biological basisCold Spring Harbor Symposia on Quantitative Biology, Volume LXVIII. © 2003 Cold Spring Harbor Laboratory Press 0-87969-709-1/04. 383

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