Research Group 'Psychiatric Genetics', Head: Prof. Dr. Hans H. Stassen

Department of Psychiatry, Psychotherapy and Psychosomatics

Psychiatric Hospital, University of Zurich


MZ Twins Discordant for Schizophrenia

Within-Pair EEG Concordance

In an EEG study of 91 healthy subjects with repeated assessments, 40 pairs of healthy MZ twins, 27 pairs of MZ twins discordant for schizophrenia, and 13 pairs of MZ twins concordant for schizophrenia, we investigated (1) the trait quality of brain-wave patterns with respect to inter-individual differences, intra-individual stability over time, and within-pair MZ concordance, (2) the characteristics of brain-wave patterns that allow one to discriminate reliably between affected and unaffected individuals, and (3) the characteristics of brain-wave patterns that reflect the severity of illness. Most parameters chosen to quantify brain-wave characteristics were found to possess distinct trait-like qualities, as indicated by large inter-individual differences, great stability over time, and high within-pair concordances in healthy MZ twins.

Non-Genetic Pathologic Brain Developments

In comparison to healthy controls, MZ twins discordant and concordant for schizophrenia exhibited a much lower within-pair EEG concordance, although the majority of correlation coefficients differed significantly from zero. Accordingly, abnormalities of brain-wave patterns associated with schizophrenia and differently manifested in MZ co-twins concordant for schizophrenia seem to reflect non-genetic, idiosynchratic pathologic developments of genetically identical brains. These abnormalities allowed us to discriminate reproducibly between affected and unaffected individuals by means of a multivariate discriminant function with an overall accuracy of 80%.

Severity of Illness

The severity of illness, as derived from the brain-wave discriminant function, was closely related to the severity of illness provided by psychopathology scores and overall AXIS V social functioning. In consequence, the non-genetic, highly individual pathologic development of brain-wave patterns in schizophrenia clearly limit the usefulness of these quantities as biological markers for investigations into the genetic predisposition to this illness.


Stassen HH, Bomben G, Propping P: Genetic aspects of the EEG: an investigation into the within-pair similarity of monozygotic and dizygotic twins with a new method of analysis. Electroenceph clin Neurophysiol 1987; 66: 489-501
Stassen HH, Lykken DT, Bomben G: The within-pair similarity of twins reared apart. Eur Arch Psychiatr Neurol Sci 1988; 237: 244-252
Stassen HH, Lykken DT, Propping P, Bomben G: Genetic determination of the human EEG (survey of recent results from twins reared together and apart). Human Genetics 1988; 80: 165-176
Stassen HH, Lykken DT, Propping P: Zwillingsuntersuchungen zur Genetik des normalen Elektroenzephalogramms. In: P. Baumann (ed): Biologische Psychiatrie der Gegenwart, Wien: Springer 1993, 139-144
Kaprio J, Buchsbaum M, Gottesman II, Heath A, Körner J, Kringlen E, McGuffin P, Propping P, Rietschel M, Stassen HH: What can twin studies contribute to the understanding of adult psychopathology? In: T.J. Bouchard jr. and P. Propping: Twins as a tool for behavioral genetics. Chichester: John Wiley & Sons, Dahlem Workshop Reports, Life Sciences Research Report 1993; 53: 287-299
Stassen HH, Coppola R. Torrey EF, Gottesman II, Kuny S, Rickler KC, Hell D: EEG differences in monozygotic twins discordant and concordant for schizophrenia. Psychophysiology 1999; 36,1: 109-117
Stassen HH: EEG and evoked potentials. In: D. Cooper (ed) Nature Encyclopedia of the Human Genome. Nature Publishing Group, London 2003; 3: 266-269
Weisbrod M, Hill H, Sauer H, Niethammer R, Guggenbühl S, Stassen HH: Nongenetic pathologic developments of brain-wave patterns in monozygotic twins discordant and concordant for schizophrenia. Am J Med Genetics B 2004; 125: 1-9
Within-pair concordances of psychopathology syndrome scores in mz and dz twins where at least one co-twin suffers from a schizophrenic disorder. No more than 55% of mz and 15% of dz co-twins are concordant for schizophrenia, thus displaying highly significant deviations from the expected mz:dz ratio of 2:1 for genetically additive traits. This deviation indicates the existence of strong non-linearities. The boundary between unaffected and affected ("0.5") is somewhat arbitrary but does not change the principal finding.
Detailed analyses of our family data showed that: (1) patients with a clinical diagnosis of schizoaffective disorders have the highest genetic vulnerability; (2) the gentic vulnerability is not constant but depends on the age of onset and the severity of psychopathology scores; (3) genetic vulnerability appears to be ethnicity-independent. All this is valid on a group level but must not necessarily be true for a particular patient.
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