By David Phillipi
While the technological advancements of recent decades allow us to map the human genome and look at the brain on the molecular level, the enormous amount of data that has been amassed is virtually useless for psychiatrists trying to diagnose their sick patients because the assumed biological causes of schizophrenia and manic-depressive illness have not been found. No brain abnormalities that are specific to either illness or present in all cases have been identified. Nevertheless, the experts who study and treat schizophrenia and manic-depressive illness (MDI) keep the faith (quite literally) that a breakthrough is just around the corner.
For years, genetic research has appeared to be the most promising of the recently opened avenues, but the excitement seems unwarranted by the findings. The relatively large number of chromosomal regions which may be implicated in susceptibility for bipolar disorder means that hope of finding a specific bipolar gene or even a small number of genes must be given up. Some researchers think the way to go is to narrow the search by looking for genes associated with specific aspects of the disease. Of course, this further refinement is only possible because of the huge variation in symptoms and experiences of those who fall under the MDI/bipolar umbrella, and we are once again reminded of the difficulty of defining what this illness or group of illnesses even is. Furthermore, even the distinction between schizophrenia and MDI seems to collapse in light of the genetic linkage data. In Manic-Depressive Illness: Bipolar Disorders and Recurrent Depression (2nd Edition), Drs. Frederick Goodwin and Kay Redfield Jamison write:
While the search for predisposing genes had traditionally tended to proceed under the assumption that schizophrenia and bipolar disorder are separate disease entities with different underlying etiologies, emerging findings from many fields of psychiatric research do not fit well with this model. Most notably, the pattern of findings emerging from genetic studies shows increasing evidence for an overlap in genetic susceptibility across the traditional classification categories. (49)
Genetic studies in the schizophrenia research community lead to pretty much the same hypothesis as with bipolar: genetic susceptibility is most likely polygenic, meaning dependent on the total number of certain genes which may contribute to vulnerability. It must be noted that genetic vulnerability is a condition, not a cause of schizophrenia and bipolar – something else must be acting on this vulnerability. In one way or another, this fact is usually noted in the literature that deals with genetic data, but it is often obscured by a tone of confidence which suggests the information may be more meaningful and explanatory than it truly is.
Even when a specific gene has been well studied across illnesses, its usefulness in understanding genetic susceptibility may be extremely limited. Some studies in both schizophrenia and MDI have found an increased risk of illness for those who possess the short form of the serotonin transporter promoter gene 5-HTT. The thing is, each of us has two copies of this gene, and over two-thirds of us have one long and one short form, meaning that having the normal variant of the gene is the risk factor! If most of us possess a gene which puts us at risk for an illness which only a small minority of people have, then this particular trait is obviously not much of a causal explanation.