Andreas Meyer-Lindenberg’s group examine the association between a single nucleotide polymorphism (SNP), rs1344706 in gene ZNF804A, recently identified as a risk factor for schizophrenia in a genome-wide association study (GWAS) (O'Donovan et al., 2008) and functional connectivity as measured by fMRI. The attraction of this polymorphism for a study of this kind is twofold. First, statistically speaking it is the most robust SNP association with schizophrenia reported to date. Second, because a single variant shows strong evidence for association, which is not the case for other reported associations, it is possible to specify a priori for the gene in question directional hypotheses in relation to potential neurocognitive correlates. This militates against the generation of false positives through the testing of multiple SNPs and haplotypes which has rendered problematic the interpretation of at least some previous genetic imaging studies (Walters and Owen, 2007). The function...  Read more

Andreas Meyer-Lindenberg’s group examine the association between a single nucleotide polymorphism (SNP), rs1344706 in gene ZNF804A, recently identified as a risk factor for schizophrenia in a genome-wide association study (GWAS) (O'Donovan et al., 2008) and functional connectivity as measured by fMRI. The attraction of this polymorphism for a study of this kind is twofold. First, statistically speaking it is the most robust SNP association with schizophrenia reported to date. Second, because a single variant shows strong evidence for association, which is not the case for other reported associations, it is possible to specify a priori for the gene in question directional hypotheses in relation to potential neurocognitive correlates. This militates against the generation of false positives through the testing of multiple SNPs and haplotypes which has rendered problematic the interpretation of at least some previous genetic imaging studies (Walters and Owen, 2007). The function of ZNF804A is unknown but the fact that it contains a zinc finger domain suggests that it may be a transcription factor. It is hoped that the characterization of the actions of SNPs identified by GWAS will identify new pathogenic mechanisms of psychosis. One way in which this can be achieved is via approaches such as that taken in this article.

Esslinger et al. report variations in functional connectivity in 115 healthy individuals according to rs1344706 risk variant status. Given the association of ZNF804A with both schizophrenia and bipolar disorder they employed two fMRI tasks thought to be sensitive to altered function in these disorders: the N Back (2back) task is sensitive to deficits of dorsolateral prefrontal cortex (DLPFC) function in schizophrenia and an emotional face-matching task is linked with amygdala function and thought to be relevant to mood disorder. They compared the activity in these regions and functional connectivity (using time-series correlation) between the three rs1344706 genotype groups.

No differences between genotype groups were found for activation, but the authors did identify altered connectivity with the most activated DLPFC locale. Risk-allele carriers were shown to exhibit a lack of uncoupling of activity (increased functional connectivity) between the right DLPFC and left hippocampus during the 2-back task as well as decreased connectivity within right DLPFC and between right and left DLPFC. Risk variant carriers also showed wide ranging increased connectivity between right amygdala and other anatomical regions. The majority of these findings showed a risk allele dose effect.

The increased DLPFC/hippocampus functional connectivity in carriers of the risk allele is potentially the most interesting finding given that Meyer-Lindenberg’s group has previously shown that those with schizophrenia show increased functional connectivity between DLPFC and hippocampus during working memory (Meyer-Lindenberg et al., 2005). Notes of caution in this regard are that 1) the biological, anatomical or functional significance of fMRI determined functional connectivity is yet to be established and 2) other functional connectivity studies in schizophrenia have produced conflicting results Lawrie et al., 2002. Nonetheless, it is interesting that rs1344706 may affect co-ordination of activity between these two brain regions given their seeming importance in psychotic conditions. The significance of these findings to cognitive deficits and other symptom domains needs further investigation particularly as others have postulated dysconnectivity has more relevance to first rank psychotic symptoms (Stephan et al., 2009).

It is likely that genome-wide association approaches will continue to identify genes with unknown neural function and so approaches such as this are likely to be a valuable way of identifying the biological/neural pathways that involve these genes. It is also imperative that as in this study methodology is employed to allow for multiple testing and also that negative findings are reported. We would also suggest caution until these findings are replicated. As well as such approaches in humans, it is also important to investigate the effects of identified variants at other levels of analysis from gene expression to behavioural genetics work. Finally we find it reassuring that GWAS approaches seem to be successful in identifying risk variants whose functions can be investigated using methods such as that taken by Esslinger et al.

References:

O'Donovan MC, Craddock N, Norton N, et al. Identification of loci associated with schizophrenia by genome-wide association and follow-up. Nature Genetics. 2008;40(9):1053-1055. Abstract

Walters JT, Owen MJ. Endophenotypes in psychiatric genetics. Mol Psychiatry. 2007;12(10):886-890. Abstract

Meyer-Lindenberg AS, Olsen RK, Kohn PD, et al. Regionally Specific Disturbance of Dorsolateral Prefrontal-Hippocampal Functional Connectivity in Schizophrenia. Archives of General Psychiatry. 2005;62(4):379-386. Abstract

Lawrie SM, Buechel C, Whalley HC, Frith CD, Friston KJ, Johnstone EC. Reduced frontotemporal functional connectivity in schizophrenia associated with auditory hallucinations. Biological Psychiatry. 2002;51(12):1008-1011. Abstract

Stephan KE, Friston KJ, Frith CD. Dysconnection in Schizophrenia: From Abnormal Synaptic Plasticity to Failures of Self-monitoring. Schizophr Bull. 2009;35(3):509-527. Abstract

The two recent papers in Nature, from the Icelandic group (Stefansson et al., 2008), and the International Schizophrenia Consortium (2008) led by Pamela Sklar, represent a landmark in psychiatric genetics. For the first time two large studies have yielded highly significant consistent results using multiple population samples. Furthermore, they arrived at these results using quite different methods. The Icelandic group used transmission screening and focused on de novo events, using the Illumina platform in both a discovery population and a replication population. By contrast, the ISC study was a large population-based case-control study using the Affymetrix platform, which did not specifically search for de novo events.

Both identified the same two regions on chromosome 1 and chromosome 15, as well as replicating the previously well studied VCFS region on chromosome 22. Thus, we now have three copy number variants which are replicated and consistent across studies. This provides data on rare highly penetrant variants complementary to the family based study of DISC1 (  Read more

The two recent papers in Nature, from the Icelandic group (Stefansson et al., 2008), and the International Schizophrenia Consortium (2008) led by Pamela Sklar, represent a landmark in psychiatric genetics. For the first time two large studies have yielded highly significant consistent results using multiple population samples. Furthermore, they arrived at these results using quite different methods. The Icelandic group used transmission screening and focused on de novo events, using the Illumina platform in both a discovery population and a replication population. By contrast, the ISC study was a large population-based case-control study using the Affymetrix platform, which did not specifically search for de novo events.

Both identified the same two regions on chromosome 1 and chromosome 15, as well as replicating the previously well studied VCFS region on chromosome 22. Thus, we now have three copy number variants which are replicated and consistent across studies. This provides data on rare highly penetrant variants complementary to the family based study of DISC1 (Porteous et al., 2006), in which the chromosomal translocation clearly segregates with disease, but in only one family. In addition, they are in general congruent with three other studies (Walsh et al., 2008; Kirov et al., 2008; Xu et al., 2008) which also demonstrate a role for copy number variation in schizophrenia. These studies together should put to rest many of the arguments about the value of genetics in psychiatry, so that future studies can now begin from a firmer base.

However, these studies also raise at least as many questions as they answer. One is the role of copy number variation in schizophrenia in the general population. The number of cases accounted for by the deletions on chromosome 1 and 15 in the ISC and Icelandic studies is extremely small--on the order of 1% or less. The extent to which copy number variation, including very rare or even private de novo variants, will account for the genetic risk for schizophrenia in the general population is still unknown. The ISC study indicated that there is a higher overall load of copy number variations in schizophrenia, broadly consistent with Walsh et al and Xu et al but backed up by a much larger sample size, allowing the results to achieve high statistical significance. The implications of these findings are still undeveloped,

Another issue is the relationship to the phenotype of schizophrenia in the general population. Many more genotype-phenotype studies will need to be done. It will be important to determine whether there is a higher rate of mental retardation in the schizophrenia in these studies than in other populations.

Another question is the relationship between these copy number variations (and other rare events) and the more common variants accounting for smaller increases in risk, as in the recent O’Donovan et al. (2008) association study in Nature Genetics. It is far too early to know, but there may well be some combination of rare mutations plus risk alleles that account for cases in the general population. This would then be highly reminiscent of Alzheimer’s disease, Parkinson’s disease, and other diseases which have been studied for a longer period of time.

For instance, in Alzheimer’s disease there are rare mutations in APP and presenilin, as well as copy number variation in APP, with duplications causing the accelerated Alzheimer’s disease seen in Down syndrome. These appear to interact with the risk allele in APOE, and possibly other risk alleles, and are part of a pathogenic pathway (Tanzi and Bertram, 2005). Similarly in Parkinson’s disease, rare mutations in α-synuclein, LRRK2 and other genes can be causative of PD, though notably the G2019S mutation in LRRK2 has incomplete penetrance. In addition, duplications or triplications of α-synuclein can cause familial PD, and altered expression due to promoter variants may contribute to risk. By contrast, deletions in Parkin cause an early onset Parkinsonian syndrome (Hardy et al., 2006). Finally, much of PD may be due to genetic risk factors or environmental causes that have not yet been identified. Further studies will likely lead to the elucidation of pathogenic pathways. These diseases can provide a paradigm for the study of schizophrenia and other psychiatric diseases. One difference is that the copy number variations in the neurodegenerative diseases are often increases in copies (as in APP and α-synuclein), consistent with gain of function mechanisms, while the schizophrenia associations were predominantly with deletions, suggesting loss of function mechanisms. The hope is that as genes are identified, they can be linked together in pathways, leading to understanding of the neurobiology of schizophrenia (Ross et al., 2006).

The key unanswered questions, of course, are what genes or other functional domains are deleted at the chromosome 1, 15, and 22 loci, whether the deletions at these loci are sufficient in themselves to cause schizophrenia, and, if sufficient, the extent to which the deletions are penetrant. Both of the current studies identified deletions large enough to include several genes. The hope is that at least a subset of copy number variations (unlike SNP associations identified in schizophrenia to date) may be causative, making the identification of the relevant genes or other functional domains—at least in principle—more feasible.

Another tantalizing observation is that the copy number variations associated with schizophrenia were defined by flanking repeat regions. This raises the question of the extent to which undetected smaller insertions, deletions or other copy number variations related to other repetitive motifs, such as long tandem repeats, may also be associated with schizophrenia. Identification and testing of these loci may prove a fruitful approach to finding additional genetic risk factors for schizophrenia.

References:

Hardy J, Cai H, Cookson MR, Gwinn-Hardy K, Singleton A. Genetics of Parkinson's disease and parkinsonism. Ann Neurol. 2006 Oct;60(4):389-98. Abstract

Kirov G, Gumus D, Chen W, Norton N, Georgieva L, Sari M, O'Donovan MC, Erdogan F, Owen MJ, Ropers HH, Ullmann R. Comparative genome hybridization suggests a role for NRXN1 and APBA2 in schizophrenia. Hum Mol Genet . 2008 Feb 1 ; 17(3):458-65. Abstract

Porteous DJ, Thomson P, Brandon NJ, Millar JK. The genetics and biology of DISC1—an emerging role in psychosis and cognition. Biol Psychiatry. 2006 Jul 15;60(2):123-31. Abstract

Ross CA, Margolis RL, Reading SA, Pletnikov M, Coyle JT. Neurobiology of schizophrenia. Neuron. 2006 Oct 5;52(1):139-53. Abstract

Singleton A, Myers A, Hardy J. The law of mass action applied to neurodegenerative disease: a hypothesis concerning the etiology and pathogenesis of complex diseases. Hum Mol Genet. 2004 Apr 1;13 Spec No 1:R123-6. Abstract

Tanzi RE, Bertram L. Twenty years of the Alzheimer's disease amyloid hypothesis: a genetic perspective. Cell. 2005 Feb 25;120(4):545-55. Abstract

Walsh T, McClellan JM, McCarthy SE, Addington AM, Pierce SB, Cooper GM, Nord AS, Kusenda M, Malhotra D, Bhandari A, Stray SM, Rippey CF, Roccanova P, Makarov V, Lakshmi B, Findling RL, Sikich L, Stromberg T, Merriman B, Gogtay N, Butler P, Eckstrand K, Noory L, Gochman P, Long R, Chen Z, Davis S, Baker C, Eichler EE, Meltzer PS, Nelson SF, Singleton AB, Lee MK, Rapoport JL, King MC, Sebat J. Rare structural variants disrupt multiple genes in neurodevelopmental pathways in schizophrenia. Science. 2008 Apr 25;320(5875):539-43. Abstract

Xu B, Roos JL, Levy S, van Rensburg EJ, Gogos JA, Karayiorgou M. Strong association of de novo copy number mutations with sporadic schizophrenia. Nat Genet. 2008 Jul;40(7):880-5. Abstract

Several recent reports have suggested that rare CNVs may be highly penetrant genetic factors in the pathogenesis of schizophrenia, perhaps even singular etiologic events in those cases of schizophrenia who have them. This is potentially of enormous importance, as the definitive identification of such a “causative” factor may be a major step in unraveling the biologic mystery of the condition. I would stress several issues that need to be considered in putting these recent findings into a broader perspective.

It is very difficult to attribute illness to a private CNV, i.e., one found only in a single individual. This point has been potently illustrated by a study of clinically discordant MZ twins who share CNVs (Bruder et al., AJHG, 2008). Inherited CNVs, such as those that made up almost all of the CNVs described in the childhood onset cases of the study by Walsh et al. (Science, 2008), are by definition not highly penetrant (since they are inherited from unaffected parents). The finding by Xu et al. (Nat Gen, 2008) that de novo (i.e., non-inherited) CNVs are much...  Read more

Several recent reports have suggested that rare CNVs may be highly penetrant genetic factors in the pathogenesis of schizophrenia, perhaps even singular etiologic events in those cases of schizophrenia who have them. This is potentially of enormous importance, as the definitive identification of such a “causative” factor may be a major step in unraveling the biologic mystery of the condition. I would stress several issues that need to be considered in putting these recent findings into a broader perspective.

It is very difficult to attribute illness to a private CNV, i.e., one found only in a single individual. This point has been potently illustrated by a study of clinically discordant MZ twins who share CNVs (Bruder et al., AJHG, 2008). Inherited CNVs, such as those that made up almost all of the CNVs described in the childhood onset cases of the study by Walsh et al. (Science, 2008), are by definition not highly penetrant (since they are inherited from unaffected parents). The finding by Xu et al. (Nat Gen, 2008) that de novo (i.e., non-inherited) CNVs are much more likely to be associated with cases lacking a family history is provocative but difficult to interpret as no data are given about the size of the families having a family history and those not having such a history. Unless these family samples are of comparable size and obtained by a comparable ascertainment strategy, it is hard to know how conclusive the finding is. Indeed, in the study of Walsh et al., rare CNVs were just as likely to be found in patients with a positive family history. Finally, in contrast to private CNVs, recurrent (but still rare) CNVs, such as those identified on 1q and 15q in the studies of the International Schizophrenia Consortium (Nature, 2008) and Stefansson et al. (Nature, 2008), are strongly implicated as being associated with the diagnosis of schizophrenia and therefore likely involved in the causation of the illnesses in the cases having these CNVs. In all, these new CNV regions, combined with the VCFS region on 22q, suggest that approximately five to 10 patients out of 1,000 who carry the diagnosis of schizophrenia may have a well-defined genetic lesion (i.e., a substantial deletion or duplication).

The overarching question now is how relevant these findings are to the other 99 percent of individuals with this diagnosis who do not have these recurrent CNVs. Before we had the capability to perform high-density DNA hybridization and SNP array analyses, chromosomal anomalies associated with the diagnosis of schizophrenia were identified using cytogenetic techniques. Indeed, VCFS, XXX, XXY (Kleinfelter’s syndrome), and XO (Turner syndrome) have been found with similarly increased frequency in cases with this diagnosis in a number of studies. Now that we have greater resolution to identify smaller structural anomalies, the list of congenital syndromes that increase the possibility that people will manifest symptoms that earn them this diagnosis appears to be growing rapidly. Are we finding causes for the form of schizophrenia that most psychiatrists see in their offices, or are we instead carving out a new set of rare congenital syndromes that share some clinical characteristics, as syphilis was carved out from the diagnosis of schizophrenia at the turn of the twentieth century? Is schizophrenia a primary expression of these anomalies or a secondary manifestation? VCFS is associated with schizophrenia-like phenomena but even more often with mild mental retardation, autism spectrum, and other psychiatric manifestations. The same is true of the aneuploidies that increase the probability of manifesting schizophrenia symptoms. The two new papers in Nature allude to the possibility that epilepsy and intellectual limitations may also be associated with these CNVs. The diagnostic potential of any of these new findings cannot be determined until the full spectrum of their clinical manifestations is clarified.

One of the important insights that might emerge from identification of these new CNV syndromes is the identification of candidate genes that may show association with schizophrenia based on SNPs in these regions. VCFS has been an important source of promising candidate genes with broader clinical relevance (e.g., PRODH, COMT). Stefansson et al. report, however, that none of the 319 SNPs in the CNV regions showed significant association with schizophrenia in quite a large sample of individuals not having deletions in these regions. The Consortium report also presumably has the results of SNP association testing in these regions in their large sample but did not report them. It is very important to explore in greater genetic detail these regions of the genome showing association with the diagnosis of schizophrenia in samples lacking these lesions and to fully characterize the clinical picture of individuals who have them. It is hoped that insights into the pathogenesis of symptoms related to this diagnosis will emerge from these additional studies.

Anyone who has worked in a public state hospital or chronic schizophrenia care facility (where I spent over 20 years) is not surprised to find an occasional patient with a rare congenital or acquired syndrome who expresses symptoms similar to those individuals also diagnosed with schizophrenia who do not have such rare syndromes. Our diagnostic procedures are not precise, and the symptoms that earn someone this diagnosis are not specific. Schizophrenia is not something someone has; it is a diagnosis someone is given. In an earlier comment for SRF on structural variations in the genome related to autism, I suggested that, “From a genetic point of view, autism is a syndrome that can be reached from many directions, along many paths. It is not likely that autism is any more of a discrete disease entity than say, blindness or mental retardation.” These new CNV syndromes manifesting schizophrenia phenomena are probably a reminder that the same is true of what we call schizophrenia.

In this work the authors test for differences in the default mode network between healthy controls, patients with schizophrenia, and first degree relatives of the patients. They look at both the degree to which the default mode is modulated by a working memory task and also examine the strength of the functional connectivity. The controls are found to show the most default mode signal decrease during a task, with relatives and patients showing much less. The controls, relatives, and patients show increasing amounts of functional connectivity within the default mode regions. In addition, signal in some of the regions correlated with positive symptoms. The findings in the chronic patients and controls are consistent with our previous work in Garrity et al., 2007, which also showed significantly more functional connectivity in the default mode of schizophrenia patients and significant correlations in certain regions of the default mode with positive symptoms, and in both cases the regions we identified are similar to those shown in...  Read more

In this work the authors test for differences in the default mode network between healthy controls, patients with schizophrenia, and first degree relatives of the patients. They look at both the degree to which the default mode is modulated by a working memory task and also examine the strength of the functional connectivity. The controls are found to show the most default mode signal decrease during a task, with relatives and patients showing much less. The controls, relatives, and patients show increasing amounts of functional connectivity within the default mode regions. In addition, signal in some of the regions correlated with positive symptoms. The findings in the chronic patients and controls are consistent with our previous work in Garrity et al., 2007, which also showed significantly more functional connectivity in the default mode of schizophrenia patients and significant correlations in certain regions of the default mode with positive symptoms, and in both cases the regions we identified are similar to those shown in the Whitfield-Gabrieli paper. Our work in Kim et al., 2009, was a large multisite study showing significantly fewer default mode signal decreases for the auditory oddball task in chronic schizophrenia patients, again consistent with the Whitfield-Gabrieli paper, but in a different task.

The most interesting contribution of the Whitfield-Gabrieli paper is their inclusion of a first-degree relative group. They found that the first-degree relatives are “in between” the healthy controls and the chronic patients in terms of both the degree to which they modulate the default mode, as well as in their degree of functional connectivity. This has interesting implications in terms of the genetic aspects of the illness and suggests that the default mode may be a potential schizophrenia endophenotype. It will be interesting in future studies to examine both the heritability of the default mode patterns and their genetic underpinnings.

The Default Mode Network and Schizophrenia
For a long time functional imaging research has focused on brain activations. However, since 2001 it has been appreciated that there is also a network of brain regions—which includes particularly two midline regions, the medial prefrontal cortex and the posterior cingulate cortex/precuneous—which deactivates during performance of a wide range of cognitive tasks. Why some brain regions should be active at rest but deactivate when tasks have to be performed is unclear, but there is intense speculation that this network is involved in functions such as self-reflection, self-monitoring, and the maintenance of one’s sense of self.

Could the default mode network be implicated in neuropsychiatric disease states? There is evidence that this is the case in autism, and a handful of studies have been also carried out in schizophrenia. Now, Whitfield-Gabrieli and colleagues report that 13 schizophrenic patients in the early phase of illness showed a failure to deactivate the anterior medial prefrontal node of the...  Read more

The Default Mode Network and Schizophrenia
For a long time functional imaging research has focused on brain activations. However, since 2001 it has been appreciated that there is also a network of brain regions—which includes particularly two midline regions, the medial prefrontal cortex and the posterior cingulate cortex/precuneous—which deactivates during performance of a wide range of cognitive tasks. Why some brain regions should be active at rest but deactivate when tasks have to be performed is unclear, but there is intense speculation that this network is involved in functions such as self-reflection, self-monitoring, and the maintenance of one’s sense of self.

Could the default mode network be implicated in neuropsychiatric disease states? There is evidence that this is the case in autism, and a handful of studies have been also carried out in schizophrenia. Now, Whitfield-Gabrieli and colleagues report that 13 schizophrenic patients in the early phase of illness showed a failure to deactivate the anterior medial prefrontal node of the default mode network when they performed a working memory task. They also find that failure to deactivate is seen to a lesser but still significant extent in unaffected first-degree relatives of the schizophrenic patients, and that the degree of failure to deactivate is associated with both the severity of positive and negative symptoms in the patients.

Importantly, the findings of Whitfield-Gabrieli and colleagues are closely similar to those of another recent study by our group (Pomarol-Clotet et al., 2008), which found failure to deactivate in the medial prefrontal cortex node of the default mode network in 32 chronic schizophrenic patients. This is a striking convergence in the field of functional imaging studies of schizophrenia, which has previously been marked by diverse and often conflicting findings. Additionally, in both studies the magnitude of the difference between patients and controls was large and visually striking. These findings suggest that we may be dealing with an important abnormality which could be close to the disease process in schizophrenia.

If so, what does dysfunction in the default mode network mean? On the one hand, failure to deactivate part of a network whose activity normally decreases when attention has to be turned to performance of external tasks might be expected to interfere with normal cognitive operations. Consistent with this, cognitive impairment is nowadays accepted as being an important, or even a “core” feature of schizophrenia. Perhaps more importantly, could it be that default mode network dysfunction can help us understand the symptoms of schizophrenia? As Whitfield-Gabrieli and colleagues note, if the default mode network is involved in self-reflection, self-monitoring, and maintenance of one’s sense of self, then failure of deactivation might lead to an exaggerated focus on one’s own thoughts and feelings, excessive self-reference, and/or a breakdown in the boundary between the inner self and the external world. The default mode network may thus have the potential to account for two major realms of clinical abnormality in schizophrenia—its symptoms and the cognitive impairment that is frequently associated with them.

The surge in scientific interest in patterns of connectivity and activation of resting-state brain function and the default-mode network has recently extended to default-mode brain dysfunction in mental disorders (for a review, please see Broyd et al., 2008). Whitfield-Gabrieli et al. examine resting-state and (working-memory) task-related brain activity in 13 patients with early-phase schizophrenia, 13 unaffected first-degree relatives, and 13 healthy control participants. These authors report hyperconnectivity in the default-mode network in patients and relatives during rest, and note that this enhanced connectivity was correlated with psychopathology. Further, patients and relatives exhibited reduced task-related suppression (hyperactivation) of the medial prefrontal region of the default-mode network relative to the control group, even after controlling for task performance.

The findings from the Whitfield-Gabrieli paper are in accordance with those from a number of other research groups investigating possible...  Read more

The surge in scientific interest in patterns of connectivity and activation of resting-state brain function and the default-mode network has recently extended to default-mode brain dysfunction in mental disorders (for a review, please see Broyd et al., 2008). Whitfield-Gabrieli et al. examine resting-state and (working-memory) task-related brain activity in 13 patients with early-phase schizophrenia, 13 unaffected first-degree relatives, and 13 healthy control participants. These authors report hyperconnectivity in the default-mode network in patients and relatives during rest, and note that this enhanced connectivity was correlated with psychopathology. Further, patients and relatives exhibited reduced task-related suppression (hyperactivation) of the medial prefrontal region of the default-mode network relative to the control group, even after controlling for task performance.

The findings from the Whitfield-Gabrieli paper are in accordance with those from a number of other research groups investigating possible default-mode network dysfunction in schizophrenia. For example, in a similar working memory task Pomarol-Clotet and colleagues (2008) have also shown reduced task-related suppression of medial frontal nodes of the default-mode network in 32 patients with chronic schizophrenia. However, the findings are at odds with research reporting widespread reductions in functional connectivity in the resting brain of this clinical group (e.g., Bluhm et al., 2007; Liang et al., 2006). As noted by Whitfield-Gabrieli et al., increased connectivity and reduced task-related suppression of default-mode activity may redirect attentional focus from task-related events to introspective and self-referential thought processes. The reduced anti-correlation between the task-positive and default-mode network in patients further supports and helps biologically ground suggestions of the possibility of an overzealous focus on internal thought. Perhaps even more interestingly, the study by Whitfield-Gabrieli and colleagues suggests that aberrant patterns of activation and connectivity in the default-mode network, and in particular the medial frontal region of this network, may be associated with genetic risk for schizophrenia. Although there are some inconsistencies in the literature regarding the role of the default-mode network in schizophrenia, the work of Whitfield-Gabrieli and others suggests that this network may well contribute to the pathophysiology of this disorder and is relevant to contemporary models of schizophrenia. Indeed, the recent flurry in empirical research investigating the clinical relevance of this network to mental disorder has highlighted a number of possible putative mechanisms that might link the default-mode network to disorder. Firstly, effective transitioning from the resting-state to task-related activity appears to be particularly vulnerable to dysfunction in mental disorders and may be characterized by deficits in attentional control. Sonuga-Barke and Castellanos (2007) have suggested that interference arising from a reduction in the task-related deactivation of the default-mode network may underlie the disruption of attentional control. The default-mode interference hypothesis proposes that spontaneous low-frequency activity in the default-mode network, normally attenuated during goal-directed tasks, can intrude on task-specific activity and create cyclical lapses in attention resulting in increased variability and a decline in task performance (Sonuga-Barke and Castellanos, 2007). Sonuga-Barke and Castellanos (2007) suggest that the efficacious transition from rest to task and the maintenance of task-specific activity may be moderated by trait factors such as disorder. Secondly, the degree of functional connectivity in the default-mode network may highlight problems of reduced connectivity, or excess functional connectivity (e.g., schizophrenia), which suggests a zealous focus on self-referential processing and introspective thought. Thirdly, the strength of the anti-correlation between the default-mode and task-positive networks may also indicate a clinical susceptibility to introspective or extrospective orienting. Finally, future research should continue to examine the etiology of the default-mode network in schizophrenia.

References:

Bluhm, R.L., Miller, J., Lanius, R.A., Osuch, E.A., Boksman, K., Neufeld, R.W.J., Théberge, J., Schaefer, B., & Williamson, P. (2007). Spontaneous low-frequency fluctuations in the BOLD signal in schizophrenic patients: Anomalies in the default network. Schizophrenia Bulletin, 33, 1004-1012. Abstract

Broyd, S.J., Demanuele, D., Debener, S., Helps, S.K., James, C.J., & Sonuga-Barke, E.J.S. (in press). Default-mode brain dysfunction in mental disorders: a systematic review. Neurosci Biobehav Rev. 2008 Sep 9. Abstract

Liang, M., Zhou, Y., Jiang, T., Liu, Z., Tian, L., Liu, H., and Hao, Y. (2006). Widespread functional disconnectivity in schizophrenia with resting-state functional magnetic resonance imaging. NeuroReport, 17, 209-213. Abstract

Pomarol-Clotet, E., Salvador, R., Sarro, S., Gomar, J., Vila, F., Martinez, A., Guerrero, A.,Ortiz-Gil, J., Sans-Sansa, B., Capdevila, A., Cebemanos, J.M., McKenna, P.J., 2008. Failure to deactivate in the prefrontal cortex in schizophrenia: dysfunction of the default-mode network? Psychological Medicine, 38, 1185–1193. Abstract

Sonuga-Barke, E.J.S., Castellanos, F.X., 2007. Spontaneous attentional fluctuations in impaired states and pathological conditions: a neurobiological hypothesis. Neuroscience Biobehavioural Reviews, 31, 977–986. Abstract

The consistent findings on default-mode network in human brain have attracted the researcher’s attention to the task-independent activity. The component regions of the default-mode network, especially medial prefrontal cortex and posterior cingulate cortex/precuneus, are related to self-reflective activities and attention. Both of these functions are observed to be impaired in schizophrenia. And thus the default-mode network has also attracted more and more attention in the schizophrenia research community. The study of Whitfield-Gabrieli et al. shows a further step along this research streamline.

The authors found hyperactivity (reduced task suppression) and hyperconnectivity of the default network in schizophrenia, and found that hyperactivity and hyperconnectivity of the default network are associated with poor work memory performance and greater psychopathology in schizophrenia. And they found less anticorrelation between the medial prefrontal cortex and the right dorsolateral prefrontal cortex, a region showing increased task-related activity in schizophrenia,...  Read more

The consistent findings on default-mode network in human brain have attracted the researcher’s attention to the task-independent activity. The component regions of the default-mode network, especially medial prefrontal cortex and posterior cingulate cortex/precuneus, are related to self-reflective activities and attention. Both of these functions are observed to be impaired in schizophrenia. And thus the default-mode network has also attracted more and more attention in the schizophrenia research community. The study of Whitfield-Gabrieli et al. shows a further step along this research streamline.

The authors found hyperactivity (reduced task suppression) and hyperconnectivity of the default network in schizophrenia, and found that hyperactivity and hyperconnectivity of the default network are associated with poor work memory performance and greater psychopathology in schizophrenia. And they found less anticorrelation between the medial prefrontal cortex and the right dorsolateral prefrontal cortex, a region showing increased task-related activity in schizophrenia, whether during rest or task. Furthermore, the hyperactivity in medial prefrontal cortex is negatively related to the hyperconnectivity of the default network in schizophrenia.

There are two main contributions in this work. First, they found significant correlation between the abnormalities in the default mode network and impaired cognitive performance and psychopathology in schizophrenia. Thus they propose a new explanation for the impaired working memory and attention in schizophrenia, and propose a possibility that schizophrenic symptoms, such as delusions and hallucinations, may be due to the blurred boundary between internal thoughts and external perceptions. Secondly, they recruited the first-degree relatives of these patients in this study, and found that these healthy relatives showed abnormalities in the default network similar to that of patients but to a lesser extent. This is the first study investigating the default mode network of relatives of individuals with schizophrenia. This finding indicates that the dysfunction in the default mode network is associated with genetic risk for schizophrenia.

The findings in schizophrenia are consistent with our previous work (Zhou et al., 2007), in which we also found hyperconnectivity of the default mode network during rest. Considering the differences in ethnicity of participants (Chinese in our study) and methodology, the consistency in the hyperconnectivity of the default mode network in schizophrenia is exciting, which supports the possibility that abnormality in the default-mode network may be a potential imaging biomarker to assist diagnosis of schizophrenia. However, this needs to be validated in future studies with a large sample size, due to other contradictory findings, for example, the reduced resting-state functional connectivities associated with the posterior cingulate cortex in chronic, medicated schizophrenic patients (Bluhm et al., 2007). In addition, further studies should focus on default-mode function in different clinical subtypes, as schizophrenia is a complicated disorder. Finally, it should be noticed that the hyperconnectivity of the default-mode network is not exclusively contradictory with hyperconnectivity in other regions, as we previously found (Liang et al., 2006). It is possible that hyperconnectivity and hyperconnectivity coexist in the brains of individuals with schizophrenia and together lead to the complicated symptoms and cognitive deficits.

References:

Bluhm, R. L., Miller, J., Lanius, R. A., Osuch, E. A., Boksman, K., Neufeld, R. W., et al., 2007. Spontaneous low-frequency fluctuations in the BOLD signal in schizophrenic patients: anomalies in the default network. Schizophr Bull 33, 1004-1012. Abstract

Liang, M., Zhou, Y., Jiang, T., Liu, Z., Tian, L., Liu, H., et al., 2006. Widespread functional disconnectivity in schizophrenia with resting-state functional magnetic resonance imaging. Neuroreport 17, 209-213. Abstract

Whitfield-Gabrieli, S., Thermenos, H. W., Milanovic, S., Tsuang, M. T., Faraone, S. V., McCarley, R. W., et al., 2009. Hyperactivity and hyperconnectivity of the default network in schizophrenia and in first-degree relatives of persons with schizophrenia. Proc Natl Acad Sci U S A 106, 1279-1284. Abstract

Zhou, Y., Liang, M., Tian, L., Wang, K., Hao, Y., Liu, H., et al., 2007. Functional disintegration in paranoid schizophrenia using resting-state fMRI. Schizophr Res 97, 194-205. Abstract