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DARPP-32 Haplotype Affects Frontostriatal Cognition and Schizophrenia Risk

13 February 2007. The signaling molecule dopamine- and cAMP-regulated protein of 32 kilodaltons, or DARPP-32, has been the baby of Paul Greengard and colleagues at the Rockefeller University, New York, for over 2 decades; his work on this molecule led to the Nobel Prize in Physiology or Medicine in 2000. Greengard’s in vitro, animal, and postmortem brain tissue work has been pivotal in demonstrating that DARPP-32 acts as a central molecule switch integrating multiple information streams from the dopamine, glutamate, and serotonin pathways.

Now, Daniel R. Weinberger and colleagues from the NIH’s Genes, Cognition and Psychosis Program have published the first human functional data on DARPP-32, linking genetic variation at the locus to brain structure, cognitive performance, and schizophrenia. They present three lines of evidence, including cognitive testing, mRNA testing in postmortem brain tissue, and magnetic resonance imaging, to show that a common haplotype is associated with better performance on cognitive tests that call upon circuits linking frontal cortex and the striatum, perhaps as a benefit of more efficient information processing within the striatum. They also show that this haplotype is associated with schizophrenia.

DARPP-32: A Central Molecular Switch
Over the years, Greengard’s group has shown that DARPP-32 acts as a central molecular switch integrating the dopamine, glutamate, and serotonin pathways. This is accomplished through the use of different phosphorylation sites. When phosphorylated at Thr34, DARPP-32 acts as an amplifier of PKA- and PKG-mediated signaling, which leads to a potent inhibition of protein phosphatase-1 (PP-1) (Hemmings et al., 1984) PP-1 is one of the few phosphatases in the mammalian cell, and it has been implicated in the pathogenesis of neurodegenerative diseases, particularly Parkinson's disease (see Alzheimer Research Forum related news story). On the other hand, when phosphorylated at Thr75, DARPP-32 becomes an inhibitor of PKA. Thus, DARPP-32 regulates a variety of phosphorylation cascades within the cell that control many key proteins, including ion channels, neurotransmitter receptors, and transcription factors.

Greengard and colleagues have linked DARPP-32 function to schizophrenia with both animal and human studies. Following up on evidence that schizophrenia patients, as a group, display impaired sensorimotor gating (the processing and filtering of stimuli and information) they found that DARPP-32 knockout mice do not lose sensorimotor gating in response to the psychotomimetic drugs D-amphetamine, D-lysergic acid diethylamide (LSD), and phencyclidine (PCP), as wild-type mice do. This supports the idea that DARPP-32 activity mediates the actions of these agents on sensorimotor gating (see SRF related news story). In addition, Greengard’s group found that DARPP-32 protein levels were significantly reduced in the dorsolateral prefrontal cortex in postmortem brain tissue from patients with schizophrenia (Albert et al., 2002).

PPP1R1B Haplotype Influences Cognitive Performance and Schizophrenia Risk
In the current study, first author Andreas Meyer-Lindenberg and colleagues first documented genetic variability at the DARPP-32 locus (PPP1R1B). DNA from 105 white and 44 African-American patients with schizophrenia was sequenced, including all seven exons, all introns, and 2 kb upstream of the transcription start site. Within this region, the investigators found 17 single nucleotide polymorphisms (SNPs) that were present in greater than 3 percent of the white patients. None of these SNPs were coding or splice site mutations.

Next, these SNPs were genotyped in a large sample of white subjects from the Clinical Brain Disorders Branch Sibling Study (CBDB/NIMH), which is a study of neurobiological abnormalities related to genetic risk for schizophrenia (Egan et al., 2001). The sample included 257 white patients with schizophrenia, 327 siblings, 397 parents, and 243 controls. From the 17 SNPs, seven were chosen as a 7-SNP haplotype. About 75 percent of the sample carried a very frequent haplotype (CGCACTC), and 15 percent carried the haplotype GATGTCA. There were three other, rarer haplotypes present in the sample as well, at frequencies of 3.5 percent, 2.1 percent, and 1.6 percent.

Weinberger’s group then searched the CBDB/NIMH dataset for associations between PPP1R1B haplotypes and cognitive function. Cognitive function in the CBDB/NIMH sample has previously been tested using a variety of standard, validated cognitive tests. The investigators found that the most common PPP1R1B haplotype, CGCACTC, was associated with higher IQ and better scores on tests of working memory, sequencing, response alternation, and attention. In other studies, scores on these tests have been related to the function of circuits linking frontal cortex and neostriatum (caudate and putamen) via the thalamus (Pantelis et al., 1997). No association between any PPP1R1B haplotype and scores on tests of episodic memory, verbal learning, or logical memory was found. These tests are traditionally related to function of temporal-diencephalic circuitry.

Of interest to the schizophrenia community, the PPP1R1B haplotype CGCACTC was not only associated with higher scores on tests related to cortical striatal loop function, but it was also associated with schizophrenia. “Our data lead to the provocative observation that a frequent haplotype in PPP1R1B predicts increased frontostriatal interactions that appeared beneficial (as evidenced by relatively better performance on a wide range of cognitive tasks) yet contributed to risk for schizophrenia,” write Meyer-Lindenberg and colleagues. “This raises the question of whether a genetic advantage in normal subjects may translate into a disadvantage in the context of other functional impairments also associated with schizophrenia, such as abnormal function of the prefrontal cortex.”

PPP1R1B Haplotype Associated with DARPP-32 Brain Biochemistry and Structure
Next, DARPP-32 mRNA levels were tested in postmortem brain tissue from the prefrontal cortices of 16 white patients with schizophrenia and 22 controls. The data from these tests suggested that variations in levels of full-length mRNA are associated with the PPP1R1B haplotype. In particular, full-length DARPP-32 mRNA was expressed at the highest levels in subjects who were homozygous for the most common haplotype of PPP1R1B, the CGCACTC haplotype. This is the same haplotype that was associated with cognitive gains as well as schizophrenia in the earlier experiments. Expression of the full-length mRNA was lowest for carriers of at least one copy of the less frequent GATGTCA haplotype, and it was intermediate in two subjects who were heterozygous for CGCACTC in combination with a rarer haplotype. The results from this analysis were controlled for sex, smoking status, diagnostic status, postmortem interval, age, and RNA quality. As a control, the authors performed the same analysis with a well-characterized genetic variant of the catechol-O-methyltransferase (COMT) gene, and, as expected, they found no significant effect upon the expression levels of DARPP-32 mRNA.

Building their case brick by brick, Meyer-Lindenberg and colleagues next demonstrated an association between PPP1R1B haplotype and neostriatal volume using voxel-based morphometry (VBM). A cohort of 96 white, healthy volunteer subjects was selected for imaging after screening for lifetime history of psychiatric or neurological illness, psychiatric treatment, or drug or alcohol abuse. This cohort was independent from those selected for the other tests. The VBM data indicated that there was a bilateral relative decrease in neostriatal volume in subjects carrying the frequent PPP1R1B haplotype CGCACTC when compared to the volume in subjects carrying all other haplotypes.

PPP1R1B Haplotype Associated with Functional Connectivity in the Brain
Lastly, the investigators found an association between the PPP1R1B haplotype and the functional connectivity between the striatum and the prefrontal cortex using archival functional MRI (fMRI) datasets. Subjects had been given two tasks: the N-back task, a working memory task that robustly activates the dorsolateral prefrontal cortex (Callicott et al., 2000), and the emotional face-matching task using threatening visual stimuli, which engages the ventrolateral prefrontal and lateral orbitofrontal cortices (Pezawas et al., 2005). The authors noted that while neither of these tasks was specifically designed to probe striatal function, both of them show differential activation in neostriatum in the context of prefrontal cortex involvement.

Although percentage correct response and reaction time on these tasks was not correlated with PPP1R1B haplotype, the frequent haplotype was associated with other parameters. During the N-back test, carriers of the frequent CGCACTC haplotype showed significantly less reactivity in the bilateral putamen, and significantly increased functional connectivity of the bilateral striatum. During the emotional face-matching task, carriers of the frequent haplotype again showed decreased reactivity of the putamen. They also appeared to have increased functional connectivity between the left dorsolateral prefrontal cortex and striatum.

As a control for the imaging studies, the regional volume and functional activation experiments were also conducted in a sample of healthy white volunteers for which the COMT genotype was known, and no genotype associations were found for either striatal volume or activation.

“Taken together, the neuroimaging data therefore show that the frequent haplotype is associated in healthy individuals with more efficient intrastriatal processing combined with an increase of prefrontal cortical input onto a smaller striatum,” write Meyer-Lindenberg and colleagues. “We present convergent evidence in three independent datasets implicating DARPP-32 in a frontostriatal neural system for executive cognition and response selection in humans.”—Jillian Lokere.

Reference:
Meyer-Lindenberg A, Straub RE, Lipska BK, et al. Genetic evidence implicating DARPP-32 in human frontostriatal structure, function, and cognition. J Clin Invest. Published online Feb. 8, 2007. Abstract

Comments on News and Primary Papers
Comment by:  Jonathan Burns
Submitted 14 February 2007
Posted 14 February 2007

This study provides hard empirical evidence for the hypothesis that psychosis (and schizophrenia in particular) represents a costly "byproduct" of complex human (social) brain evolution. Interestingly, the activation paradigms in the fMRI study (N-back and emotional face-matching tasks) are both testing social cognition. And the demonstrated changes in frontostriatal connectivity support the hypothesis that schizophrenia is a disorder of evolved intrahemispheric circuits comprising the Social Brain in our species.

I would suggest that further candidates (conferring vulnerability to psychosis) should be sought from amongst those genes known to have played a significant role in human brain evolution.

References:

Burns J. (2007) The Descent of Madness: Evolutionary Origins of Psychosis and the Social Brain. Routledge Press: Hove, Sussex.

Burns J. The social brain hypothesis of schizophrenia. World Psychiatry. 2006 Jun;5(2):77-81. Abstract

Burns JK. Psychosis: a costly by-product of social brain evolution in Homo sapiens. Prog Neuropsychopharmacol Biol Psychiatry. 2006 Jul;30(5):797-814. Epub 2006 Mar 3. Review. Abstract

Burns JK. An evolutionary theory of schizophrenia: cortical connectivity, metarepresentation, and the social brain. Behav Brain Sci. 2004 Dec;27(6):831-55; discussion 855-85. Review. Abstract

View all comments by Jonathan BurnsComment by:  Daniel Durstewitz
Submitted 8 June 2007
Posted 8 June 2007
  I recommend the Primary Papers

The phosphoprotein DARPP-32 occupies a central position in the dopamine-regulated intracellular cascades of cortical and striatal neurons (Greengard et al., 1999). It is a point of convergence for multiple signaling pathways, is differentially affected by D1- vs. D2-class receptor activation, and mainly through inhibition of protein-phosphotase-1 mediates or contributes to a number of the dopaminergic effects on voltage- and ligand-gated ion channels. These, in turn, by regulating intracellular Ca2+ levels, themselves influence phosphorylation of DARPP-32 and thereby interact with dopamine-induced processes.

Given its central, vital role in dopamine-regulated signaling pathways, it is quite surprising that (to my knowledge) only a few studies exist on the implications of DARPP-32 variations for cognitive functions and brain activity. Therefore, this comprehensive series of studies by Meyer-Lindenberg et al. combining human genetics, structural and functional MRI, and behavioral testing represents an important milestone. Meyer-Lindenberg et al. identified different functionally relevant DARPP haplotypes, associated with differential DARPP mRNA activity in postmortem studies, and found that these were linked to significant differences on a number of cognitive tests probing “executive functions,” as well as to differences in putamen volume and activity, and structural and functional covariation between striatal and prefrontal cortical areas. Thereby, they paved the way for detailed investigations of the role of DARPP-32 in human cognition.

Since DARPP-32 is so intricately interwoven into so many intracellular and physiological feedback loops, as with dopamine itself (Durstewitz and Seamans, 2002), mechanistic accounts for the functional involvement of DARPP-32 variations in neural network dynamics may be hard to obtain. “Linear” causal thinking usually breaks down in such complex functional networks constituted of so many interacting positive and negative feedback loops on different time scales. Thus it may still be a while until we gain a deeper, biophysically based understanding of the neural processes that mediate the influence of DARPP variations on cognition, and integrative computational approaches may be required to help resolving these issues. Given the complexity of DARPP-regulated networks, I also would expect that fine-grained behavioral testing and analysis of error types of human subjects on different cognitive tasks may ultimately reveal quite subtle and differential effects of DARPP polymorphisms. Moreover, the effects on neural network dynamics may be such (e.g., changing the temporal organization of spiking patterns) that they may not always be detectable by current neuroimaging methods, meaning that while the most dramatic effects were found on activation and volume of striatum, where DARPP-32 is most abundantly expressed, a significant contribution of other brain areas in DARPP-associated cognitive differences may not be ruled out. Regardless of these difficulties in unraveling the underlying neural mechanisms, the work by Meyer-Lindenberg et al. allows us to tackle the question of how the balance in dopamine-regulated intracellular networks relates to cognition in humans, and points toward the neural structures and interactions most interesting to look at.

View all comments by Daniel Durstewitz

Comments on Related News


Related News: Biology of Reinforcement—Dopamine Linked to Three Separate Reward Paths

Comment by:  Patricia Estani
Submitted 16 November 2007
Posted 16 November 2007
  I recommend the Primary Papers

Related News: News Brief: Schizophrenia-linked AKT1 Variant Affects Brain Parameters

Comment by:  Takeo YoshikawaAkihiko Takashima
Submitted 17 June 2008
Posted 17 June 2008

Some researchers in the field of psychiatric genetics have become somewhat pessimistic about the ability to detect robust genotype-phenotype correlations using the diagnostic criteria defined by DSM-IV. If we analyze tens of thousands of samples, the ensuing results may be statistically robust, but still the effect of common variant(s) of each gene will be modest. Recently, Tan et al. (2008) reported that the AKT1 gene SNP rs1130233 and its encompassing haplotypes are significantly associated with IQ/processing speed, activities that may reflect frontal cortex function. They also showed that performance in their psychological test battery is influenced not only by AKT1 genetic variants but also the well-known COMT gene non-synonymous polymorphism (SNP rs4680, Val158Met). By undertaking fMRI analysis, they intertwined the IQ/processing speed-frontal cortex-AKT1 signal-DA system, i.e., the. integration of multidimensional disciplines. In citing references (Meyer-Lindenberg and Weinberger, 2006; Weinberger et al., 2001), they state that “there is a growing body of data showing that genes weakly associated with complex constellations of behavioral symptoms are much more strongly associated with in vivo brain measures.” Indeed, they have succeeded in explaining a possible role for AKT1 in brain execution capability, but have not provided convincing evidence for genetic associations between AKT1 and schizophrenia.

Their current results are elegantly derived from “a complex set of experiments addressing association of multiple variants in a gene with many phenotypic measures.” However, from a genetic perspective, we may still ask the following questions, irrelevant of the current study:

1. What is the genetic component (or heritability) of each psychological and imaging trait? Can variations in some of the psychological/cognitive/intellectual performances be fully captured by a single gene in an experimental set that examines, at the most, a hundred samples? We have learned the hard way from genetic association studies done in the 1990s, which examined a small number of samples, that we simply cannot trust those results. With regard to this point, the heritability calculations of so-called “endophenotypes” as reported by Greenwood et al. (2007) can give helpful information [also see Watanabe et al., 2007, supplementary Table S2]. There is the possibility that the genetic architecture of neurocognitive functions and imaging measures may not be simpler than the current disease category (entity).

2. Given the rapid advances in genotyping technology, we may be able to generate genome-wide genetic test results for every neuropsychiatric trait in the near future.

3. Because of the functional significance of AKT1 and the divergence in the signaling cascade downstream of AKT1, it would be wise to confine analysis to this gene. However, it is frustrating that we still do not know the functionally important SNP(s) of AKT1 in spite of numerous association studies.

4. Nackley et al. (2006) have convincingly demonstrated that the haplotype of the COMT gene constructed by synonymous SNPs has much more functional impact than the Val158Met polymorphism. Therefore, we would like to see the association studies examining this haplotype in future neuropsychiatric studies.

From a biochemical perspective, the following issues would be interesting and future targets for clarification:

1. The authors suggest that the coding synonymous variation of AKT1 affects protein expression, leading to the alteration of frontostriatal function and gray matter volume. The activity of AKT1 is regulated by its phosphorylation status. Therefore, readers would want to know whether the reduction of AKT1 expression levels actually affect the AKT signaling pathway. Behavioral analysis and an MRI study of Akt1 heterozygote knockout mice may provide relevant information.

2. Impairment of the AKT signal is known to result in tau hyperphosphorylation through activation of GSK3 as seen in Alzheimer disease brains. According to this idea, a reduction of AKT levels caused by SNP(s) should elicit hyperphosphorylation of tau and ultimately form neurofibrillary tangles (NFTs). In contrast, there are some reports suggesting the absence of NFTs and neuroinjury in elderly patients with schizophrenia (Arnold et al., 1998; Purohit et al., 1998). It is also reported that GSK3 is reduced in schizophrenia (Beasley et al., 2001). It would be interesting to know whether the genetic variation(s) of AKT1 that induce decreased protein expression affect tau accumulation.

3. Lithium inhibits the arrestin-Akt signal (Beaulieu et al., 2008). If so, it would be interesting to know whether lithium treatment can restore some of the effects of reduced AKT1 expression levels caused by the SNP(s) of interest.

References:

Arnold SE, Trojanowski JQ, Gur RE, Blackwell P, Han LY, Choi C. Absence of neurodegeneration and neural injury in the cerebral cortex in a sample of elderly patients with schizophrenia. Arch Gen Psychiatry 1998 55:225-232. Abstract

Beasley C, Cotter D, Khan N, Pollard C, Sheppard P, Varndell I, Lovestone S, Anderton B, Everall I. Glycogen synthase kinase-3beta immunoreactivity is reduced in the prefrontal cortex in schizophrenia. Neurosci Lett 2001 302:117-120. Abstract

Beaulieu JM, Marion S, Rodriguiz RM, Medvedev IO, Sotnikova TD, Ghisi V, Wetsel WC, Lefkowitz RJ, Gainetdinov RR, Caron MG.. A beta-arrestin 2 signaling complex mediates lithium action on behavior. Cell 2008 132:125-36. Abstract

Greenwood TA, Braff DL, Light GA, Cadenhead KS, Calkins ME, Dobie DJ, Freedman R, Green MF, Gur RE, Gur RC, Mintz J, Nuechterlein KH, Olincy A, Radant AD, Seidman LJ, Siever LJ, Silverman JM, Stone WS, Swerdlow NR, Tsuang DW, Tsuang MT, Turetsky BI, Schork NJ. Initial heritability analyses of endophenotypic measures for schizophrenia: the consortium on the genetics of schizophrenia. Arch Gen Psychiatry 2007 64:1242-1250. Abstract

Meyer-Lindenberg AS, Weinberger DR: Intermediate phenotypes and genetic mechanisms of psychiatric disorders. Nat Rev Neurosci 2006 7:818-827. Abstract

Nackley AG, Shabalina SA, Tchivileva IE, Satterfield K, Korchynskyi O, Makarov SS, Maixner W, Diatchenko L: Human catechol-O-methyltransferase haplotypes modulate protein expression by altering mRNA secondary structure. Science 2006 314:1930-1933. Abstract

Purohit DP, Perl DP, Haroutunian V, Powchik P, Davidson M, Davis KL: Alzheimer disease and related neurodegenerative diseases in elderly patients with schizophrenia: a postmortem neuropathologic study of 100 cases. Arch Gen Psychiatry 1998 55:205-211. Abstract

Tan HY, Nicodemus KK, Chen Q, Li Z, Brooke JK, Honea R, Kolachana BS, Straub RE, Meyer-Lindenberg A, Sei Y, Mattay VS, Callicott JH, Weinberger DR: Genetic variation in AKT1 is linked to dopamine-associated prefrontal cortical structure and function in humans. J Clin Invest 2008 118:2200-2208. Abstract

Watanabe A, Toyota T, Owada Y, Hayashi T, Iwayama Y, Matsumata M, Ishitsuka Y, Nakaya A, Maekawa M, Ohnishi T, Arai R, Sakurai K, Yamada K, Kondo H, Hashimoto K, Osumi N, Yoshikawa T: Fabp7 maps to a quantitative trait locus for a schizophrenia endophenotype. PLoS Biology 2007 5:e297. Abstract

Weinberger DR, Egan MF, Bertolino A, Callicott JH, Mattay VS, Lipska BK, Berman KF, Goldberg TE: Prefrontal neurons and the genetics of schizophrenia. Biol Psychiatry 2001 50:825-844. Abstract

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Related News: Special K: Primate-specific Potassium Channel Variant Implicated in Schizophrenia

Comment by:  Paul Shepard
Submitted 18 May 2009
Posted 19 May 2009
  I recommend the Primary Papers

The manuscript by Huffaker et al. extends the growing number of cardiac potassium channels that have found their way into the brain and onto the list of putative therapeutic targets for the treatment of neurological and psychiatric disease. In an extensive series of experiments, these investigators demonstrate an association between single nucleotide polymorphisms in a gene encoding an inwardly rectifying potassium channel (KCNH2), the expression of a previously unknown isoform (KCNH2-3.1), and schizophrenia. Named for the dance exhibited by ether-intoxicated fruit fly mutants in which the gene family was first identified, ether-a-go-go related gene or ERG K+ channels contribute to the repolarization of cardiac action potentials and the propensity of antipsychotic drugs to prolong the QT interval, a direct result of their ability to attenuate this current in the heart. The unique gating properties of ERG K+ channels (for review, see Shepard et al., 2007) give rise to a strong resurgent current that can profoundly alter both intermediate and slow components of neuronal signaling. Thus, ERG currents have been shown to alter spike timing (e.g., latency to first spike in a stimulus-evoked train, spike frequency adaptation) in cerebellar Purkinje (Sacco et al., 2003), medial vestibular nucleus (Pessia et al., 2008), and cultured cortical neurons, while in dopamine cells, they appear to underlie a slow afterhyperpolarization envisioned to contribute to the termination of plateau oscillations and the obligatory pause in firing after a burst of spikes (Canavier et al., 2007; Nedergaard, 2004).

Identification of a primate-specific KCNH2-3.1 isoform in hippocampus and cortex whose expression in brain alters the function of the channel begs a number of questions that will undoubtedly be the focus of subsequent research. Foremost among these is whether the therapeutic effects of antipsychotic drugs derive in some measure from their ability to block ERG channels containing the KCNH2-3.1 protein. Although the truncated KCNH2-3.1 isoform is unique to primates, phenotypic changes associated with expression of the protein result from loss of the PAS domain, a region of the protein responsible for the resurgent nature of the outward current. In addition to increasing the rate of ERG channel deactivation, expression of the truncated isoform may reduce the number of functional channels brought to the surface as suggested by the reported reduction in ERG current density in rat cortical neurons transfected with human KCNH2-3.1. The functional consequences associated with the loss of the PAS domain in individual cells can be characterized using dynamic clamp—a technique in which a computer simulation is used to introduce an artificial membrane conductance into individual neurons. However, the effects of the mutation on channel trafficking and assessment of the myriad of conductance states likely to result from heterologous expression with other ERG channel subunits will require a transgenic model, which if history serves, the Weinberger group has already begun constructing.

References:

Canavier CC, Oprisan SA, Callaway JC, Ji H, Shepard PD. Computational model predicts a role for ERG current in repolarizing plateau potentials in dopamine neurons: implications for modulation of neuronal activity. J Neurophysiol . 2007 Nov 1 ; 98(5):3006-22. Abstract

Nedergaard S. A Ca2+-independent slow afterhyperpolarization in substantia nigra compacta neurons. Neuroscience . 2004 Jan 1 ; 125(4):841-52. Abstract

Pessia M, Servettini I, Panichi R, Guasti L, Grassi S, Arcangeli A, Wanke E, Pettorossi VE. ERG voltage-gated K+ channels regulate excitability and discharge dynamics of the medial vestibular nucleus neurones. J Physiol . 2008 Oct 15 ; 586(Pt 20):4877-90. Abstract

Sacco T, Bruno A, Wanke E, Tempia F. Functional roles of an ERG current isolated in cerebellar Purkinje neurons. J Neurophysiol . 2003 Sep 1 ; 90(3):1817-28. Abstract

Shepard PD, Canavier CC, Levitan ES. Ether-a-go-go-related gene potassium channels: what's all the buzz about? Schizophr Bull . 2007 Nov 1 ; 33(6):1263-9. Abstract

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Related News: Special K: Primate-specific Potassium Channel Variant Implicated in Schizophrenia

Comment by:  Szatmar Horvath
Submitted 11 May 2009
Posted 1 June 2009
  I recommend the Primary Papers

Related News: DARPP-32 Isoform Elevated in Psychiatric Disorders

Comment by:  Jamal NasirNirmal Vadgama
Submitted 8 March 2013
Posted 8 March 2013

Kunii et al. used two different TaqMan Assays (Applied Biosystems) to compare expression levels of full-length (FL) and truncated DARPP-32 (t-DARPP-32) in various regions of the brain, and detected increased expression of t-DARPP-32 in DLPFC in both schizophrenia and bipolar disorder samples compared to controls. Overexpression of genes can cause developmental abnormalities in the brain. For example, increased LIS1 expression can lead to significant brain abnormalities in humans and mice (Bi et al., 2009).

We previously showed increased expression of DARPP-32 in human DLPFC tissue from both schizophrenia (n = 33) and bipolar disorder (n = 32) samples using the same TaqMan assay (Hs00259967_ml) as above (this detects both FL-DARPP-32 and t-DARPP-32), after excluding brain weight, age of onset, postmortem interval, time in hospital, duration of illness and antipsychotics, gender, race, smoking, alcohol, drugs, suicide status, family history, insight and psychotic features as potential confounding factors (Zhan et al., 2011). After applying Bonferroni corrections to account for multiple comparisons, our findings remained significant, and after correcting for brain pH our p-values became much more significant (p <0.001 for both schizophrenia and bipolar disorder samples vs. controls [n = 34]).

Hierarchical clustering analysis of our data revealed a distinct pattern for DARPP-32 expression in comparison to other dopamine signaling genes and dopamine receptors D1-D5, although the expression of these genes appeared to be co-regulated with the exception of dopamine receptors and D2, in particular (Zhan et al., 2011). DARPP-32 expression in relation to D2 expression is strikingly different in controls but remarkably similar in schizophrenia and bipolar samples, suggesting aberrant D2-regulated expression of DARPP-32 may be an important trigger in pathogenesis.

We found increased DARPP-32 expression in DLPFC of schizophrenia and bipolar samples by using an assay that detects both FL-DARPP-32 and t-DARPP-32. We are, therefore, unable to say whether this is attributable to increased expression of FL-DARPP-32, t-DARPP-32, or both. Kunii et al. failed to find any differences in expression levels using this assay, but since they found increased expression of t-DARPP-32, this would indicate that FL-DARPP-32 expression levels have gone down in schizophrenia and bipolar samples. However, there is inevitably considerable variability between postmortem brain samples as shown in the data presented by Kunii et al. and in other studies, so this could also account for the results. Finally, it would be useful to compare the relative expression of both isoforms of the gene in the brain during various stages of development in the same patients. This might shed useful light on their respective functions.

References:

Zhan L, Kerr JR, Lafuente M-J, Maclean A, Chibalina MV, Liu B, Burke B, Bevan S, Nasir J. (2011) Altered expression and coregulation of dopamine signalling genes in schizophrenia and bipolar disorder. Neuropathology and Applied Neurobiology 37, 206-219. Abstract

Bi W, Sapir T, Shchelochkov OA, Zhang F, Withers MA, Hunter JV, Levy T, Shinder V, Peiffer DA, Gunderson KL, Nezarati MM, Shotts VA, Amato SS, Savage SK, Harris DJ, Day-Salvatore DL, Horner M, Lu XY, Sahoo T, Yanagawa Y, Beaudet AL, Cheung SW, Martinez S, Lupski JR, Reiner O. (2009) Increased LIS1 expression affects human and mouse brain development. Nat Genet. 41:168-177. Abstract

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