Morbidity risk for psychiatric disorders in families of probands with affective disorders divided according to levels of platelet MAO activity.

In a series comprising 166 subjects with affective disorders, the lowest and highest quartiles in the male and female platelet monoamine oxidase (MAO) distribution, respectively, were included. The morbidity risk in the first-degree relatives (parents and siblings) of these low and high platelet MAO subjects was determined. First-degree relatives of low platelet MAO probands were found to have an increased morbidity risk for neurotic-reactive depressions and for alcoholism. The results seem to be in line with the biological high-risk paradigm, indicating that platelet MAO could be a biological marker for increased vulnerability. First-degree relatives of high platelet MAO probands were found to have an increased morbidity risk for bipolar affective disorders.