Abstract
Objective
The delivery of biological markers for schizophrenia would greatly assist preventative strategies by identifying at-risk individuals who could then be monitored and treated in a manner with a view to minimising subsequent morbidity. This paper aims to present a selection of biological measures that may indicate risk of schizophrenia.
Method
A selective and brief review is provided of intensively studied putative markers, including enlarged cerebral ventricles, dopamine D2 receptor density, amphetamine-stimulated central nervous system dopamine release, plasma homovanillic acid and smooth pursuit eye tracking dysfunction.
Results
A number of biological measures have been reported to be correlated with schizophrenia.
Conclusions
Presently, none of these measures has satisfactory performance characteristics in terms of predictive validity, noninvasiveness, ease of testing and low cost that would enable their widespread use. However, a few have potential for further investigation and development.
Keywords
The search for the use of neurobiological markers is, in part, motivated by the need to better identify individuals at risk of schizophrenia. For example, in relation to patients seeking advice about their use of cannabis [1], it would be very helpful to have a simple test that could predict whether they were at heightened risk of developing psychosis and should therefore be discouraged, even more actively than the rest of the community, from using the drug.
The search for valid markers in schizophrenia is limited by the lack of a set of diagnostic criteria that are both widely agreed to and are based on discernible pathophysiologies. In the absence of such criteria, one approach is to use several competing diagnostic systems and a multitude of different potential biological markers with the aim of obtaining the best fit between a particular diagnostic system and a particular biological marker [2].
Biological markers can be defined as biochemical, physiological or anatomical traits that are specific to particular conditions. Examples of potential biological markers for schizophrenia include: enlarged lateral ventricular volumes, indices of disturbed dopamine neurochemistry and abnormal smooth pursuit eye movements.
Ventricular Enlargement
Lateral ventricular volume enlargement is the most consistently replicated neuroanatomical finding in schizophrenia. Lawrie and Abukmeil summarised 40 magnetic resonance imaging (MRI) studies involving more than 1300 patients with schizophrenia and examined several brain regions, whole brain volume, and grey and white matter differentiation [3]. Of the seven studies that examined ventricular volume changes, people with schizophrenia had a volume approximately 20% larger than controls. The main changes were seen in the body of the ventricles (approximately 50% increase) and the occipital horn of the ventricles (approximately 30% increase), even though it is commonly thought that the enlargement primarily occurs in the temporal horns (approximately 15% increase).
It is important to note that the effect size of this change is not very big. For example, an analysis by Raz and Raz of more than 93 computed tomography (CT) studies suggested that the effect size is of the order of 0.7 [4]. This is a modest effect size, which indicates that there is a substantial overlap (approximately 60%) between the schizophrenia population and the control population. Andreasen and colleagues undertook a CT study of 108 patients and reported that only 29% of people with schizophrenia had ventricular volumes that were larger than one standard deviation above the mean of controls, while only 6% had volumes that were greater than two standard deviations [5]. It is also important to note that ventricular enlargement in psychiatric disorders is not specific to schizophrenia; larger than normal ventricular size has also been reported in several other conditions, especially affective disorders, including bipolar [6], and schizoaffective disorders [7]. Furthermore, there is no evidence of a bimodal distribution of ventricular sizes in schizophrenia [8,9], meaning that it is not possible to specify a subgroup of individuals with a ‘ventricular enlargement’ endophenotype, which is a defining phenotypic trait that might be coupled to other specific biological and prognostic markers.
An alternative approach to using ventricular enlargement as a potential biological marker in the general population is to focus on its use in families at risk. The potential value of such a strategy follows from earlier research, which indicated that there are differences in ventricular volume between identical twins discordant for schizophrenia, with the affected twins having greater (approximately 15%) ventricular volumes [10]. This points to the possibility that presently unidentified environmental factors result in increased ventricular volume, and that such increases are related to the schizophrenia phenotype and may be valuable as diagnostic or predictive markers. However, the prospects of success for such a strategy are rather limited because it has been shown that presumed obligate carriers for schizophrenia (i.e. those who appear to be transmitting the disorder but are not affected themselves) have enlarged lateral ventricles that are similar in volume to subjects with schizophrenia and larger than those in nonobligate relatives and normal controls [11]. This suggests that ventricular enlargement is unlikely to be a good predictor for the development of schizophrenia in multiply affected families because it is insufficient to result in schizophrenia unless it is associated with other factors.
Another possible use of ventricular volume data relates to the prospect of using changing volumes over time as predictors of course and outcome. This follows from studies which indicate that although the majority of subjects with schizophrenia failed to demonstrate progressive increases in ventricular volume [12], a subset of subjects demonstrate such increases on follow up. For example, Knoll and coworkers examined four studies that reported changes in ventricle to brain ratios (VBR) using baseline and follow-up MRI or CT scans in 44 subjects [13]. While the majority of the subjects (32) exhibited no change or only a small decrease in VBR over two years, 12 of the 44 patients showed an increase in VBR over time. It is interesting to note that there was a tendency for the larger increases to occur in younger subjects. This supports the hypothesis that early in the disorder there may be a significant plasticity of brain structure.
An important goal arising from these data is to determine whether subjects showing progressive ventricular enlargement during the course of their disorder, manifest different outcomes or response to treatment. Lieberman and coworkers examined baseline and 18 monthly follow-up MRI scans in 62 first-episode patients [14]. The worst clinical outcomes were observed in subjects who had large ventricles initially with no subsequent change, and an intermediate outcome was seen in subjects who had small ventricles initially with subsequent ventricular enlargement. The subjects with small ventricles and no subsequent enlargement of their ventricles had a relatively robust response to treatment. It is important to note that, in the study by Lieberman and coworkers, the size of the ventricles was assessed using qualitative analyses. Although there is a need to replicate this work using quantitative measures, this appears to be a promising research strategy to pursue.
Dopaminergic Neurotransmission
Another set of potential biological markers relates to measures of increased dopaminergic neurotransmission. Although the dopaminergic hypothesis of schizophrenia retains considerable theoretical strength, few studies have provided convincing evidence of altered dopaminergic activity, especially that reflected by changes in dopamine receptors. Of the 10 positron emission tomography (PET) studies and three single photon emission computerised tomography (SPECT) studies which measured dopamine D2 receptors [15], only two studies, that of the PET study by Wong et al. [16] and the SPECT study by Crawley et al. [17], demonstrated statistically significant dopamine D2 receptor density increase. Laurelle noted, however, that the majority of the other studies showed a trend in the direction of increased D2 receptor density, with an average increase of approximately 12% and an effect size of 0.54 [15]. He estimated that if this effect was ‘real’, to be statistically significant with 80% power, 64 subjects with schizophrenia and 64 controls would be needed per study. In contrast, the maximum number of patients with schizophrenia in any of the studies was 20, an average of 13. Because of the modest effect and the expense of PET and SPECT scans, it is unlikely that the in vivo quantification of D2 receptor density in the baseline state is likely to prove useful as a diagnostic marker in schizophrenia.
A more promising use of D2 receptor numbers and occupancy relates to the use of PET analysis of D2 dopamine receptors following the administration of antipsychotic drugs. Studies by Farde et al. [18], Nordstrom et al. [19], Nyberg et al. [20] and Kapur et al. [21] indicate that an occupancy threshold of 60–70% is required to induce antipsychotic effect using typical antipsychotic drugs, although clozapine is associated with a substantially lower D2 receptor occupancy [21]. In theory, the estimation of D2 receptor occupancy might be useful in the management of patients who are not responding to standard doses of antipsychotic drugs, although logistical and cost factors limit the utility of this strategy.
Another component of dopaminergic neurotransmission – that of dopamine release – has been studied by Laruelle and colleagues [22]. Dopamine release was induced using amphetamine and measured by estimating the binding of the dopamine antagonist radio-tracer IBZM to D2 receptors before and after the administration of amphetamine. Patients with schizophrenia showed a greater displacement of IBZM after amphetamine than comparison subjects, which suggests that schizophrenia is associated with a greater dopamine release from presynaptic nerve terminals following stimulation of that release. These findings have now been replicated [23] but now they are of more theoretical than practical significance.
An alternative and much less costly approach to research for markers of dopamine dysfunction in schizophrenia relates to the investigation of plasma or cerebrospinal fluid metabolites of dopamine [24] or neuroendocrine markers of hypothalamic pituitary dopaminergic function [25,26]. Although the studies in these areas have reported differences between subjects with schizophrenia and controls, none of the approaches has resulted in findings that are strong and consistent enough to warrant their application as likely biological markers. While four studies report lower homovanillic acid (HVA) levels in drug-free patients with schizophrenia compared to controls [27–30], many other studies do not support this conclusion [31–36]. Furthermore, although some studies indicate that there are differences between plasma HVA in schizophrenia and controls (e.g. lower levels in schizophrenia that are thought to be reflective of ‘hypofrontality’ [37]) or that there is a relationship between a reduction in plasma HVA and treatment responsiveness [38], plasma HVA is a poor indicator of central nervous system dopaminergic function. The sympathetic nervous system makes a significant contribution to peripheral HVA levels, and only approximately 25% of plasma HVA is of central nervous system origin [39].
Smooth Pursuit Eye Movements
Another potential biological marker is that of abnormal smooth pursuit eye movements. A significant body of literature indicates an overrepresentation of eye tracking dysfunction in schizophrenia. The nature of this dysfunction appears to be an abnormality of smooth pursuit (in contrast to saccadic eye movement) and appears to involve reduced gain. The specificity of this finding for schizophrenia remains less clear in that smooth pursuit abnormalities have also been shown to occur in depressed, bipolar and obsessive compulsive disorders [40]. Although there is reasonable evidence that antipsychotic drugs do not cause the smooth pursuit abnormalities in patients (as suggested by a lack of correlation between neuroleptic drug dose and smooth pursuit abnormalities) [41], such abnormalities have been shown to occur in normal subjects following the administration of typical antipsychotic drugs [42].
Family studies have demonstrated that smooth pursuit abnormalities are common in relatives of people with schizophrenia [43,44], suggesting that eye tracking dysfunction may be a phenotypic marker for a genetic liability to schizophrenia and maybe useful in genetic linkage studies. Arolt and colleagues provided an example of such a strategy by demonstrating linkage between chromosomal markers on the short arm of chromosome 6 (6p21–23) and eye tracking dysfunction [45]. In contrast, there was no linkage between schizophrenia and markers on this chromosomal region. These findings are reminiscent of recent data from Freedman's group, which demonstrates linkage between a failure to inhibit the p50 auditory evoked response to repeated stimuli and the locus of α7 nicotinic acetylcholine receptor on chromosome 15 (15q14) in families affected by schizophrenia, but not between the locus and schizophrenia itself [46].
At the present time, it is difficult to interpret the linkages between candidate genes and neurophysiological abnormalities in families where schizophrenia is manifest, when such linkage is not apparent to the disorder itself. For progress to be made in this area, it will be necessary to demonstrate that the neurophysiological abnormalities have validity either as predictors of the onset, course or treatment responsiveness of schizophrenia.
Epilepsy Research: The Importance of Phenotype Elucidation
It is hoped that potential biological markers may be able to lead to progress in schizophrenia in the same way that such markers have led to major advances in epilepsy. For example, Steinlein et al. was able to identify the role that the nicotinic acetylcholine receptor α4 subunit plays in autosomal dominant nocturnal frontal lobe epilepsy as a result of characterising the epilepsy based on age of onset of the disorder, the clinical symptoms (epilepsy occurring at night), and the interictal electroencephalogram (EEG) pattern [47]. Characterising this subtype of the disorder (in five families) has enabled the linkage studies to be conducted in a ‘homogenous’ group of families. Importantly, other epilepsy researchers have also adopted this strategy. As a result, there has now been a number of genetic abnormalities determined in rare types of epilepsy, such as benign familial neonatal convulsions and generalised epilepsy with febrile seizures plus (FS+), which involve sodium and potassium channels, as well as cholinergic receptors. The characterisation of at least some forms of epilepsy as ‘channelopothies’ is a demonstration of how clinical and endophenotype characterisation followed by genetic linkage investigations can lead to major advances in the understanding of specific neurological disorders. It is hoped that similar progress can be made in schizophrenia.