Schizophrenia is a severe mental disorder with a life long course in a vast majority of patients. Symptoms can be categorized in 4 dimensions: positive symptoms (including delusions, hallucinations, disorganized behavior, and disorders of thought and speech), negative symptoms (including flattening of affect, loss of motivation and drive, and social withdrawal), cognitive symptoms, and affective symptoms. The etiology of schizophrenia remains unclear however to date the evidence for a neurobiological basis is accumulating. The results of family, twin and adoption studies have shown that genetic risk of schizophrenia is increased in family members of affected individuals 1-3 . However inheritance does not follow a simple Mendelian pattern. Therefore schizophrenia is a complex genetic disease, i.e. multiple disease genes each with a small effect add up to establish overall genetic vulnerability. Molecular genetic research has yielded several chromosomal regions and candidate genes associated with increased risk of in schizophrenia. In this thesis several studies are described aimed at elucidating the genetic variation underlying schizophrenia risk.
In Chapter 1 the outline of this thesis is presented and current methods and results of molecular genetic research in schizophrenia are described. Chapter 2 reviews the present knowledge on brain function and brain structure in people with 22q11 Deletion Syndrome (22q11DS), also called velocardiofacial syndrome (VCFS). 22q11DS is caused by a microdeletion of chromosome 22q11 and is associated with cognitive, behavioral and psychiatric problems and is known to affect brain structure. People with 22q11DS have a 25 times higher chance of developing schizophrenia in comparison to the normal population. Therefore the 22q11DS has been proposed as a disease model for a genetic subtype of schizophrenia. In this chapter we discuss the available literature on neurocognitive functioning and brain anatomy in patients with 22q11DS and how this contributes to our understanding of the neurobiology of schizophrenia. Research on cognitive functioning in 22q11DS patients suggests a specific cognitive profile with impairments on arithmetical, visuo-spatial and executive tasks and relatively preserved language skills. Prominent findings of neuroimaging studies in 22q11 DS patients are: reduction of overall brain volume, midline defects, structural alterations of cerebellum and frontal lobe, white matter abnormalities and decreased gray matter volumes in parietal and temporal areas. We describe how brain abnormalities in patients with 22q11DS may contribute to the understanding of the clinical syndrome including cognitive impairments, psychotic symptoms and social and communication problems. In Chapter 3 the current research on effects of a functional polymorphism in the catechol-O-methyltransferase (COMT) gene on various neuropsychiatric disorders including schizophrenia is reviewed. Biochemical and pharmacological studies indicate that schizophrenia is associated with disturbances of dopaminergic neurotransmission. COMT is an enzyme that is critical in the metabolic degradation of dopamine, particularly in the prefrontal cortex (PFC). The COMT gene contains a functional (Val 158 Met) polymorphism resulting in decreased enzymatic activity, and thereby in increased dopamine levels in the PFC. Disturbances of dopaminergic neurotransmission, for example by variations in COMT activity, may lead to increased susceptibility for psychiatric diseases in which dopamine is assumed to play a key role. Therefore, COMT may play a role in schizophrenia susceptibility by modulating dopaminergic neurotransmission. This hypothesis is supported by data suggesting that the COMT polymorphism may influence prefrontal cognition, aggressive behavior, and the risk of developing schizophrenia in humans. The COMT gene is located on chromosome 22q11. Deletions of this region are associated with VCFS and with a 25 times higher chance of developing schizophrenia (see chapter 2). As a result COMT has been a focus of interest in schizophrenia research. The COMT Val 158 Met polymorphism has previously been associated with several neuropsychiatric (endo)phenotypes, such as cognitive performance and anxiety. In Chapter 3 the available literature is reviewed, the neurobiology of COMT and the impact of the functional polymorphism in people with or at risk for schizophrenia is discussed, and directions for future research investigating COMT as a candidate gene for schizophrenia are discussed.Chapter 4 describes the effects of the functional COMT Val 158 Met polymorphism on brain anatomy and cognition in adults with 22q11DS. The COMT Val 158 Met polymorphism was genotyped in a sample of 26 adult 22q11DS patients. We explored its effects on regional brain volumes using hand tracing approaches, on regional gray and white matter density using computerized voxel-based analyses, and measures of attention, IQ, memory, executive and visuospatial function using a comprehensive neuropsychological test battery. After corrections for multiple comparisons Val-hemizygous subjects, compared to Met-hemizygotes, had a significantly larger volume of frontal lobes. Also, Val-hemizygotes had significantly increased gray matter density in cerebellum, brain stem, and parahippocampal gyrus and decreased white matter density in cerebellum. No significant effects of COMT genotype on neurocognitive performance were found. These data suggest that effects of COMT genotype on brain anatomy in 22q11DS are not limited to frontal regions but also involve other structures previously implicated in 22q11DS. Therefore, variation in COMT activity may be implicated in brain development in 22q11DS. Chapter 5 describes the results of a study investigating the effect of the COMT Val 158 Met polymorphism on obsessive-compulsive symptoms (OCS) in young patients with schizophrenia. Severity of OCS in 77 male patients with recent-onset schizophrenia was assessed using the Yale-Brown Obsessive Compulsive Scale (Y-BOCS) and the COMT Val 158 Met polymorphism was genotyped for these patients. Y-BOCS scores were significantly higher in patients with Val/Val genotype when compared to those with Val/Met and Met/Met genotype; patients with Val/Met genotype had intermediate Y-BOCS scores. Thus, in our sample of young patients with schizophrenia the COMT high-activity Val allele is associated with more OCS.
In Chapter 6 results of a study with a combined genetics and neuroimaging approach are described. The COMT low-activity Met allele has been associated with better performance on cognitive tasks relying on the prefrontal cortex but whether COMT also affects brain structure, is still unclear. This study investigates the relationship between the COMT Val 158 Met polymorphism and brain anatomy in healthy young adults. In a cross-sectional study, s tructural MRI data and DNA for COMT genotyping were obtained from 154 healthy young adults. Statistical Parametric Mapping software (SPM2) and optimized voxel-based morphometry were used to determine total and regional gray and white matter density differences between genotype groups, and age-related gray and white matter density differences within the genotype groups. We found a significant effect of COMT genotype on age-related differences in gray and white matter density in females but not in males. In female Val carriers increased gray matter in the temporal and parietal lobe and the cerebellum and increased white matter in the frontal lobes were positively correlated with age; in female Met homozygotes decreased gray matter density in the parietal lobe and decreased white matter density in the frontal lobes, the parahippocampal gyrus and the corpus callosum were positively correlated with age. Thus the COMT Val 158 Met polymorphism may affect age-related differences in gray and white matter density in females.
In Chapter 7 genetic and imaging data are combined to explore effects of genetic variation in genes on chromosome 22q11 on gray and whiter matter density in patients with schizophrenia. Haploinsufficiency of 22q11 genes including COMT and proline dehydrogenase (PRODH) may result in structural and functional brain abnormalities and increased vulnerability to schizophrenia as observed in patients with microdeletions of 22q11. We examined association of polymorphisms in COMT and PRODH with brain anatomy in young patients with schizophrenia. We acquired structural magnetic resonance imaging data from 51 male patients with recent-onset schizophrenia and genotyped two single nucleotide polymorphisms (SNPs) in the COMT gene and three in the PRODH gene. SPM2 software and optimized voxel based morphometry were used to determine regional gray (GM) and white-matter (WM) density differences, and total GM and WM volume differences between genotype groups. Two functional SNPs in the PRODH gene were associated with bilateral frontal WM density reductions and a SNP in the P2 promoter region of COMT (rs2097603) was associated with GM increase in the right superior temporal gyrus. Furthermore, we found evidence for interaction between COMT and PRODH: in patients with a COMT Val allele (rs4680) and with one or two mutated PRODH alleles we observed increased WM density in the left inferior frontal lobe. These results suggest that genetic variation in COMT and PRODH has significant effects on brain regions known to be affected in schizophrenia.Chapter 8 investigates the effect of another candidate gene for schizophrenia: the dystrobrevin-binding protein 1 (DTNBP1) gene. There is growing evidence that DTNPB1 contributes to schizophrenia and to intelligence and cognition. In this study, we investigated association between SNPs in the DTNBP1 gene and intellectual functioning in patients with a first episode of schizophrenia or related psychotic disorder, their healthy siblings, and unrelated controls. From all subjects IQ measurements were obtained (verbal IQ [VIQ], performance IQ [PIQ], and full scale IQ [FSIQ]). Seven SNPs in the DTNBP1 gene were genotyped using single base primer extension and analyzed by matrix-assisted laser deionization mass spectrometry (MALDI-TOF). Mean VIQ, PIQ, and FSIQ scores differed significantly (p<0.001) between patients, siblings, and controls. Using a family-based and a case-control design, several single SNPs were significantly associated with IQ scores in patients, siblings, and controls. Although preliminary, our results provide evidence for association between the DTNBP1 gene and intelligence in patients with a first psychotic episode and their unaffected siblings. Thus genetic variation in the DTNBP1 gene may increase schizophrenia susceptibility by affecting intellectual functioning.
The overall aim of the studies described in this thesis was to increase our knowledge on the genetic variation underlying schizophrenia risk by examining association between genetic variation in candidate genes for schizophrenia and various (endo)phenotypes including clinical symptoms, brain structure, and brain function. We focused on known candidate genes for schizophrenia, in particular the COMT and PRODH genes because of their location on 22q11, and the 22q11DS being a disease model for a genetic subtype of schizophrenia. In addition, we investigated effects of one of the strongest candidate genes for schizophrenia, the dysbindin gene. Since the biological mechanisms underlying schizophrenia etiology are not fully known, relating genetic variables to clinical, neuropsychological, electrophysiological and neuroimaging data will increase our understanding of the neurobiology of schizophrenia. Also, research on endophenotypes may lead to a better classification of the various clinical representations of schizophrenia. In the near future, new tools and strategies including whole-genome association studies and ‘imaging genetics' may result in identification and validation of susceptibility genes for schizophrenia. The ultimate challenge will be to identify functional variants, functionally relevant pathways, and networks of genes underlying schizophrenia etiology. In the future this may allow us to better address the needs of individual patients based on their genetic background, e.g. genes known to increase schizophrenia susceptibility could be potential targets for therapeutic strategies.