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. Author manuscript; available in PMC: 2009 Dec 1.
Published in final edited form as: Schizophr Res. 2008 Sep 20;106(2-3):265–267. doi: 10.1016/j.schres.2008.08.021

Sequence analysis of P21-activated kinase 3 (PAK3) in chronic schizophrenia with cognitive impairment

Eric M Morrow a,b, Anna Kane a, Donald C Goff b, Christopher A Walsh a,*
PMCID: PMC2631562  NIHMSID: NIHMS82174  PMID: 18805672

Abstract

The P21-activated kinase PAK3 is critical for cognitive development and truncating mutations cause non-syndromic mental retardation (MR). Missense mutations are also associated with psychotic disorders, most commonly with schizophrenia involving premorbid MR, namely “pfropfschizophrenie”. We set out to measure the frequency of sequence variants in PAK3 in schizophrenia without premorbid MR. We conducted complete gene reseqeuncing of all coding exons and exon-intron boundaries in patients with schizophrenia with cognitive impairment but without premorbid MR. Deleterious variants in schizophrenia alone were rare (<1/159 or 0.6%). Thereby, while PAK3 remains a strong biological candidate in psychosis, evidence from human genetics provides strongest support for a link to pfropfschizophrenie and not to schizophrenia without premorbid intellectual disability.

Keywords: Schizophrenia, Mental retardation, Pfropfschizophrenie, P21-activated kinase PAK3, cognitive development

1. Introduction

Abnormalities in GABAergic interneurons (Lewis et al., 2005) and loss of dendritic spines (Kolluri et al., 2005, Rosoklija et al., 2000) are consistent cellular abnormalities in schizophrenia. The p21 (CDKN1A)-activated kinase PAK3 is involved in both interneuron development (Cobos et al., 2007) as well as actin remodeling in dendrites (Hayashi et al., 2004, Kreis et al., 2007). In addition to being a plausible biological candidate in schizophrenia susceptibility, there are strong human genetic data that support a role for the X-linked PAK3 gene in psychotic disorders. In particular, multiple families with PAK3 mutations have members with psychosis and premorbid mild mental retardation (Gedeon et al., 2003, Peippo et al., 2007, Rejeb et al., 2008), known as “pfropfschizophrenie” (defined in (Doody et al., 1998)). Indeed, we contend that while DISC1 has received a great deal of attention based largely on the discovery of a highly penetrant translocation in a single extended pedigree (St Clair et al., 1990), highly penetrant mutations in PAK3 in psychosis have been largely overlooked. PAK3 is critical for cognitive development and truncating mutations cause moderate, non-syndromic mental retardation (MR) (Allen et al., 1998); however, missense mutations are associated with neurobehavioral disorders (and milder MR) in all pedigrees described (Donnelly et al., 1996, Gedeon et al., 2003, Peippo et al., 2007, Rejeb et al., 2008). Three of four such families have associated psychotic illness (Gedeon et al., 2003, Peippo et al., 2007, Rejeb et al., 2008). Indeed, in an extended pedigree with an A365E mutation, mental retardation was generally mild and at least two members had psychotic illness, one with chronic paranoid schizophrenia and an IQ of 80 (Gedeon et al., 2003).

In this experiment, we set out to measure the frequency of sequence variants in PAK3 in patients ascertained for schizophrenia without premorbid intellectual disability. We specifically chose a schizophrenia patient population with profound cognitive impairment with the notion that this population may be relatively enriched for PAK3 mutations.

2. Materials and methods

2.1. Patient ascertainment

Study procedures were approved by the Partners HealthCare and Massachusetts Department of Mental Health institutional review boards. After providing written informed consent, 159 outpatients with schizophrenia (43 females and 116 males) who receive treatment at an urban community mental health center in Boston were enrolled consecutively. The diagnosis of schizophrenia was confirmed by a research psychiatrist using DSM-IV criteria. Inclusion criteria included DSM-IV diagnosis of schizophrenia, age 18–70, ability to complete symptom rating scales and cognitive tests. Exclusion criteria included psychosis secondary to a substance use disorder or a general medical condition. Of enrolled subjects, 120 were Caucasian (75%), 37 were of African descent (23%), one was of East/Southeast Asian descent (1%), and one was of Latino descent (1%). All patients were medicated and stable in terms of psychotic symptoms.

2.2. Cognitive assessment

The Wechsler Adult Intelligence Scale (WAIS-III) was also administered to assess global cognitive status. Subnormal IQ was generally construed to be due to cognitive abnormalities of schizophrenia; however, psychotic symptoms and medication effects cannot be excluded as contributory factors.

2.3. Sequence analysis

All coding exons and exon-intron boundaries in PAK3 were sequenced. Each amplicon was amplified by PCR and sequenced using capillary methods. Sequence reads were made first manually and secondarily by the SNP-Detector software package.

3. Results

In female subjects, IQ ranged from 66 to 137 with six females under IQ of 70 and an overall mean female IQ of 86.3 +/− 16.7. In male subjects, IQ ranged from 59 to 130 with eight males under IQ of 70 and mean male IQ of 89.1 +/− 14.0. While no patients were known to have premorbid MR, cognitive status was significantly impaired and frequently in the mild to borderline MR range. Coding sequence across PAK3 is known to be highly invariant (http://www.ncbi.nlm.nih.gov/projects/SNP/). There are a total of 242 known SNPs across the 124561 bp genomic interval and all but 1 outside the 1635 open reading frame. One coding non-synonymous SNP (rs1805077) has been reported (but not validated), A489C, which putatively changes Arginine 31 to Serine. On re-sequencing in the schizophrenia patient sample, three total SNPs were discovered. We did not find rs1805077. Two of the SNPs had been previously discovered and were in intronic regions, namely rs16986385 and rs10521535 (Table 1). The single novel SNP discovered in the patient sample occurred in exon 12, substituting C at mRNA position 1786 for a T, and resulting in a synonymous SNP at Leucine 395. This novel SNP was found in a Caucasian female patient with normal IQ who was heterozygous for this sequence change.

Table 1.

Exon Amplicon (hg18 genomic assemby) Sequence Variation
1 chrX:110,252,938-110,253,212 No variation
2 chrX:110,271,930-110,272,130 No variation
3 chrX:110,276,355-110,276,499 No variation
4 chrX:110,277,571-110,277,824 No variation
5 chrX:110,282,244-110,282,381 rs16986385 (18 males, 10 females - heterozygous), intronic SNP, 25 bp from SA site.
6 chrX:110,292,749-110,292,980 No variation
7 chrX:110,293,396-110,293,661 No variation
8 chrX:110,302,852-110,303,015 No variation
9 chrX:110,321,961-110,322,109 No variation
10 chrX:110,322,340-110,322,552 No variation
11 chrX:110,324,135-110,324,352 No variation
12 chrX:110,325,676-110,325,875 chrX:110,325,800, C->T, Leu395->Leu395 (1 female - heterozygous), novel SNP
13 chrX:110,326,278-110,326,574 No variation
14 chrX:110,346,255-110,346,492 rs10521535 (6 males, 6 females - heterozygous), intronic SNP, 40 bp from the SD site.
15 chrX:110,350,192-110,350,381 No variation
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4. Discussion

While four mutations in PAK3 have been reported to cause MR and also to have neurobehavioral disorders in affected members of the pedigree (Donnelly et al., 1996, Gedeon et al., 2003, Peippo et al., 2007, Rejeb et al., 2008), including three with psychotic illness (Gedeon et al., 2003, Peippo et al., 2007, Rejeb et al., 2008), the frequency of potentially causative rare sequence variants in PAK3 is less than one in 159 cases of schizophrenia alone (less than 0.6%). Variants do not appear to be enriched by selecting for patients with impaired cognitive function. Kraeplin named a schizophrenia subtype in 1907 “pfropfschizophrenie” for schizophrenia engrafted on top of intellectual disability (Doody et al., 1998). Each pedigree thus far reported with PAK3 mutations has been ascertained for mild to moderate mental retardation as a primary diagnosis. The risk of psychotic illness is elevated in the presence of premorbid intellectual disability (Zammit et al., 2004) but generally quoted as less than 8% (Reid, 1980). As with mental retardation, in schizophrenia, cognitive impairment is a core feature and neurodevelopment abnormalities are prevalent. PAK3 mutations may represent a genetic susceptibility for pfropfschizophrenie and an important shared biological component between MR and schizophrenia, although our data here indicate that sequence mutations or variants are rare in schizophrenia without premorbid MR. Additional experiments are now needed to define the frequency of PAK3 sequence variants in schizophrenia with premorbid intellectual disability.

Acknowledgments

We are grateful to the patients who participated in this study. We are grateful for support from the NIMH (1K23MH080954-01 to E.M.M). E.M.M. holds a Career Award for Medical Scientists from the Burroughs Wellcome Fund and is also grateful for support from the Rappaport Research Scholarship in Neuroscience at MGH. This work was supported by a NARSAD Young Investigator award to E.M.M. who is the Sidney R. Baer Jr. Foundation Investigator. D.C.G. is supported by a mid-career development award from the NIMH (K24 MH002025-06). C.A.W. is an Investigator of the Howard Hughes Medical Institute.

Role of funding source: The funding sources 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.

Footnotes

Contributors: D.C.G. was involved in recruitment of patient subjects. E.M.M., A.K. and C.A.W. were involved in preparation of DNA, gene sequencing and data analysis. All were involved in hypothesis generation and preparation of this manuscript.

Conflicts of interest: E.M.M. has received grant funding from Pfizer and Merck. Dr. Goff has received research funding or honoraria from, or has been on an advisory board of AstraZeneca, BMS, Cephalon, Cortex, Eli Lilly, GSK, Janssen, Merck, Organon, Pfizer, Solvay and Wyeth. All other authors declare that they have no conflicts of interest.

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