Platelet monoamine oxidase values and genetic heterogeneity in schizophrenia research

Platelet monoamine oxidase (MAO) activity is significantly reduced in chronic schizophrenics with family history of schizophrenia. The degree of reduction is related to the extent of genetic load. Schizophrenics with no affected relatives do not differ from control subjects. These findings are consistent with the hypothesis of genetic heterogeneity in schizophrenia. Discrepancies among previously reported data sets can thus be explained by overrepresentation of nongenetic phenocopies with normal MAO levels. The implications for biologic and genetic research in schizophrenia are discussed.