PDE4B polymorphisms and decreased PDE4B expression are associated with schizophrenia

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Abstract

Schizophrenia has a complex genetic underpinning and variations in a number of candidate genes have been identified that confer risk of developing the disorder. We report in the present studies that several single nucleotide polymorphisms (SNPs) and a two-SNP haplotype in PDE4B are associated with an increased incidence of schizophrenia in two large populations of Caucasian and African American patients. The SNPs in PDE4B associated with schizophrenia occur in intronic sequences in the vicinity of a critical splice junction that gives rise to the expression of PDE4B isoforms with distinct regulation and function. We also observed specific decreases in phosphodiesterase 4B (PDE4B) isoforms in brain tissue obtained postmortem from patients diagnosed with schizophrenia and bipolar disorder. PDE4B metabolically inactivates the second messenger cAMP to regulate intracellular signaling in neurons throughout the brain. Thus, the present observations suggest that dysregulation of intracellular signaling mediated by PDE4B is a significant factor in the cause and expression, respectively, of schizophrenia and bipolar disorder and that targeting PDE4B-regulated signaling pathways may yield new therapies to treat the totality of these disorders.

Introduction

Schizophrenia is a devastating neuropsychiatric disease affecting approximately 1% of the world's population (Mueser and McGurk, 2004). Typically, first hospitalization occurs when individuals manifest a psychotic break in young adulthood, although for many individuals there is a clear prodromal syndrome that is evident from childhood. Whereas there are a number of medications that can be used to control the positive symptoms of schizophrenia, most patients experiencing negative/cognitive symptoms do not respond to the majority of antipsychotic agents, and the overall prognosis remains poor for these individuals. Thus, there is a need for the development of new therapies to treat symptom clusters not adequately controlled by current antipsychotic medications and, ideally, to treat the underlying pathology. Towards this latter end, it has long been recognized that schizophrenia has a clear but complex genetic underpinning (McGue and Gottesman, 1991, Sullivan et al., 2003). Variations in a number of candidate genes have been identified that confer risk of developing the disorder (Harrison and Weinberger, 2005, Ross et al., 2006). These findings present a pool of potential targets for the development of new therapeutic agents.

The phosphodiesterases are a superfamily of enzymes that regulate intracellular signaling through metabolic inactivation of the ubiquitous second messengers cAMP and cGMP (Bender and Beavo, 2006). Phosphodiesterase 4B (PDE4B) is one of four genes encoding for a family of phosphodiesterases (PDE4A-D) that are major regulators of cAMP signaling throughout the mammalian organism (Conti et al., 1992, Houslay, 2001, Houslay and Adams, 2003), also known for its lack of modulation of cGMP and its sensitivity to the antidepressant rolipram (Beavo et al., 1994, Bolger, 1994, Beavo, 1995, Loughney and Ferguson, 1996). Like all of the phosphodiesterases, the organization of PDE4B is complex (Szpirer et al., 1995), comprising 17 exons spanning 580 kb located on human chromosome 1 at 1p31. Alternative splicing gives rise to five known PDE4B (PDE4B1–5) isoforms that vary in length and composition of the regulatory domain N-terminus to a common catalytic domain. Members of the PDE4 family have been shown to be differentially and widely expressed in brain tissue (Iona et al., 1998), including areas associated with reward and affect (Iona et al., 1998, Cherry and Davis, 1999). All of the PDE4B isoforms are expressed in the mammalian brain (Conti and Jin, 1999, Sette and Conti, 1996, Shepherd et al., 2003). These enzymes are particularly important for the complex signal transduction and integration in the brain (Menniti et al., 2006). Inhibitors of PDE4, including rolipram, have anti-depressant properties in humans (Houslay et al., 1998, O'Donnell and Zhang, 2004). PDE4B and PDE4D knockout mice display an anxiogenic profile and an antidepressant profile, respectively, indicating that PDE4 inhibition has profound effects on behavior (Zhang et al., 2002, Zhang et al., in press) and thus altered PDE4 expression may be involved in the pathology of psychiatric disorders.

Recently, Millar et al. (2005) described an individual with schizophrenia who carries a chromosomal translocation that disrupts the locus encoding PDE4B. The observation that disruption of PDE4B is associated with severe neuropsychiatric pathology (Millar et al., 2005) in two patients is consistent with the significant role played by PDE4B in regulating cAMP signaling and thereby neuronal function. Moreover, Millar et al (2005) demonstrated that 71 kDa isoform of disabled in schizophrenia 1 (DISC1) interacted with PDE4B, an interaction that was disrupted by elevation in levels of cAMP (Millar et al., 2005). DISC1 has been identified as a risk factor for schizophrenia and other psychiatric disorders, including bipolar disorder and depression (Hashimoto et al., 2007, Liu et al., 2006, Porteous and Millar, 2006, Qu et al., 2007). More recently, Murdoch et al. (2007) has shown that the 100 kDa full-length DISC1 isoform binding is not specific to PDE4B but common to all members of the PDE4 family (Murdoch et al., 2007). Elevation of cAMP did not affect binding of PDE4A5 and PDE4B1 isoforms to DISC1 while PDE4D3 and PDE4C2 dissociated from DISC1 (Murdoch et al., 2007). However, it remains to be determined whether aberration in the physiology of PDE4B plays a more widespread role in the etiology of neuropsychiatric disorders, including schizophrenia in particular.

We report in the present studies an association of several single nucleotide polymorphisms (SNPs) in PDE4B with an increased incidence of schizophrenia in the general population. To further examine the potential involvement of phosphodiesterase 4B (PDE4B) in schizophrenia, as well as bipolar disorder and major depression, we investigated protein expression levels in cerebellum. The cerebellum has been underemphasized as a key brain region in psychiatric illnesses, with most of the focus on areas such as the frontal cortex, amygdala, hippocampus, which are related to emotions, memory, and executive function, respectively. However, it is now becoming clear that the cerebellum plays a more important role than previously thought, particularly with regard to schizophrenia (Andreasen et al., 1996, Konarski et al., 2004, Picard et al., 2008) but also with bipolar disorder (Krüger et al., 2003, Loeber et al., 2002, Nasrallah et al., 1981) and major depression (Beauregard et al., 1998, Liotti et al., 2002, Smith et al., 2002). Recently, our laboratory observed decreased expression of PDE4B isoforms in cerebellum of subjects with autism when compared to controls (Braun et al., 2007). In the current study, we observe that the expression of specific isoforms of the PDE4B protein are reduced in different regions of brain tissue obtained postmortem from patients suffering from schizophrenia and bipolar disorder as compared to well-matched healthy individuals. These observations strongly support the further investigation of PDE4B as a target for developing novel therapies to treat schizophrenia and bipolar disorder.

Section snippets

DNA sample procurement

878 schizophrenia or schizoaffective patients were analyzed for this study, including 644 Caucasians and 234 African Americans. 604 control subjects were also evaluated, including 407 Caucasians and 197 African Americans. All subjects were from Pfizer/Pharmacia clinical trials or acquired from Precision Medicine. Ethnicity was determined by self-report. All patients with schizophrenia and schizoaffective disorder were diagnosed by clinical interview using Diagnostic Statistical Manual of Mental

Results

To evaluate whether common variation in PDE4B may be involved in the etiology of schizophrenia, we performed a case–control genetic association study in two large schizophrenia populations with ethnically matched controls. We analyzed DNA from 644 Caucasian (CA) and 234 African American (AA) subjects with schizophrenia or schizoaffective disorder, diagnosed using DSM-IV criteria, and 604 control subjects (407 Caucasians and 197 African Americans). Caucasian control subjects were screened to

Discussion

The results of the present study strongly support in two ways the hypothesis that aberration in intracellular signaling regulated by PDE4B plays a role in the etiology and/or expression of the symptoms of neuropsychiatric diseases and in particular schizophrenia and bipolar disorder. We report several SNPs within PDE4B that are associated with schizophrenia in two large populations of Caucasians and African Americans suffering from schizophrenia. Furthermore, a two-SNP haplotype comprising

Role of the funding source

Funding for this study was provided by the Stanley Medical Research Institute (SMRI), Grant #06R-1406 (SHF). SMRI had no further role in study design; in the collection, analysis, and interpretation of data; in the writing of the report; and in the decision to submit the paper for publication.

Contributors

Authors S. Hossein Fatemi, David P. King and Frank S. Menniti designed the study, wrote the protocol, analyzed the data and wrote the manuscript. Authors Teri J. Reutiman and Jessica A. Laurence performed the Western blotting experiments. Author Timothy D. Folsom assisted with literature searches and the revision of later drafts of the manuscript. Authors David P. King and Sara A. Paciga conducted the genetic analyses. Author Mario Conti provided the K118 antibody and contributed to provision

Conflict of interest

All authors declare that they have no conflicts of interest.

Acknowledgment

Grant support by the Stanley Medical Research Institute (Grant #06R-1406) to SHF is gratefully acknowledged. Postmortem brain tissue was donated by The Stanley Medical Research Institute's brain collection courtesy of Drs. Michael B. Knable, E. Fuller Torrey, Maree J. Webster, and Robert H. Yolken.

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    These individuals contributed equally to completion of this project.

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