Phenomenology has been the reference point that investigators have used in their efforts to understand schizophrenia. Although symptoms and signs are crucial for the diagnosis of schizophrenia, there is an ongoing debate since Kraepelin attempted to group symptoms to understand the etiology of schizophrenia. Several operational criteria have been developed to establish the diagnosis of schizophrenia, making it obvious that there are no precise symptomatological boundaries. There is little clear indication which of the systems is valid for genetic and other biological research. Despite the enormous effort to find a linkage between schizophrenia and one or more loci, the results are far from conclusive. Another approach is the search for candidate genes of which DISCI and 22q11 deletion syndrome are examples. In all studies into the genetic underpinnings of schizophrenia, however, the clinical vantage point is neglected in that a broad clinical phenotype with respect to, e.g., developmental issues, symptoms and comorbidity is narrowed down to one categorical diagnosis. This is illustrated by the lack of exclusion criteria in genetic studies and by the occurrence of schizophrenia-like psychoses in a broad array of genetic syndromes. In case of 22q11 deletion syndrome, the psy chotic symptoms emerge in the context of brain anomalies, a plethora of somatic abnormalities and specific neurocognitive deficits. Prader Willi syndrome is a hypothalamic disorder in which psychotic symptoms may occur that resemble schizophrenia. It is concluded that not only schizophrenia is a highly variable disease but that the genetic samples are even much more heterogenous.

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Keywords 4 aminobutyric acid receptor, DISC1, DISC1 protein, DiGeorge syndrome, Genetic syndrome, PWS, Prader Willi syndrome, Schizophrenia, VCFS, article, brain dysfunction, catechol methyltransferase, chromosome 15q, chromosome 22q, chromosome deletion, chromosome map, cognitive defect, comorbidity, disrupted in schizophrenia 1, gene locus, genetic analysis, genetic heterogeneity, genetic linkage, genetic predisposition, genetics, human, mitochondrial protein, nerve protein, pathogenesis, pathophysiology, phenomenology, phenotype, priority journal, psychological aspect, psychosis, review, schizophrenia, unclassified drug, uniparental disomy
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Verhoeven, W.M.A., & Tuinier, S.. (2008). Clinical perspectives on the genetics of schizophrenia: A bottom-up orientation. Neurotoxicity Research, 14(2-3), 141–150. doi:10.1007/BF03033806