Genetics of platelet MAO activity in discordant schizophrenic and normal twins

Platelet monoamine oxidase (MAO) activity was compared in four age and sex-matched groups: monozygotic (MZ) twins discordant for schizophrenia, normal MZ twins, normal dizygotic (DZ) twins and unrelated individuals. Among the twin groups, schizophrenic and normal there was a remarkably consistent degree of genetic control amounting to 70-80 per cent of the variation in activity. The mean platelet MAO activity of the schizophrenic twins was significantly lower than that of controls, but not than that of their psychiatrically well, neuroleptic-free cotwins; indeed the correlation for the MZ twins discordant for schizophrenia was almost exactly the same as that for the normal MZs. Thus, lower platelet MAO activity in schizophrenia, where it is found, is genetically modulated and not the result of the illness or its treatment.