Endophenotypes and biological markers of schizophrenia: from biological signs of illness to novel treatment targets.

Schizophrenia is a chronic, often disabling mental illness with a lifetime prevalence of ~1% worldwide, and 2-to-3 times higher mortality rates are reported in schizophrenia patients compared to the general population. Although research has been increasingly focusing on identifying novel diagnostic and treatment resources for this illness, the diagnosis of schizophrenia is still based on clinical criteria, which are subjectively assessed and tend to vary across the course of the illness. Endophenotypes are commonly described as molecular, neuropsychological, neuro-imaging, and electrophysiological parameters that are closely associated to the genetic underpinnings of a specific disorder. Putative endophenotypes for psychiatric disorders should: 1) be associated with a specific illness in the population, 2) be heritable, 3) be present regardless of the patients clinical status, 4) co-segregate with the illness within families, and 5) be detected in non-affected family members of psychiatric patients at higher rates than in the general population. Whenever a genetic association is not present, or has not been investigated, the term biomarker is usually preferred. Endophenotypes and biomarkers are stable over time and are largely symptom independent, thus enabling an objective diagnosis of schizophrenia. Furthermore, these measures could be utilized to assess the risk of developing this disorder, and to identify novel pharmacological targets for its treatment. In this article I will present some of the most promising endophenotypes and biological markers of schizophrenia. For each of them, I will briefly describe abnormal findings in schizophrenia patients and, whenever available, in their first-degree relatives. I will then review the ability of each of these measures to identify individuals with schizophrenia (diagnostic value) and to assess the risk for schizophrenia (predictive value). Finally, I will discuss how some of these endophenotypes and biological markers could be utilized to develop novel treatment targets for schizophrenia, as well as to further the current understanding of the neurobiology of this disorder.