Relationship between genes and endophenotypes

Following Gottesman and Gould’s definition Reference Gottesman and Gould(15), the reasonable criteria for a viable schizophrenia endophenotype are as follows: (a) the endophenotype is a neuropsychological or neurophysiological character associated with schizophrenia; (b) the endophenotype is heritable; (c) the endophenotype is stable and trait related: its appearance is independent of state-related fluctuations in the individuals’ conditions, although factors such as age may affect the endophenotype; (d) the endophenotype and schizophrenia show cosegregation and (e) the proband’s specific endophenotype character is found at higher rates in the proband’s relatives than in the general population.

Schizophrenia-related endophenotypes are cognitive functions such as attention, working memory, executive functions and visuospatial memory that are impaired in schizophrenic patients and in their non-schizophrenic relatives Reference Fioravanti, Carlone, Vitale, Cinti and Clare(25,Reference Sitskoorn, Aleman, Ebisch, Appels and Kahn26). Cognitive functions are associated with event-related potentials (ERPs): attention is associated with the P50 component of ERPs Reference Erwin, Turetsky, Moberg, Gur and Gur(27), which is related to sensory gating, whereas working memory and verbal fluency are associated with ERP component P300 Reference Shajahan, O’Carroll, Glabus, Ebmeier and Blackwood(28,Reference Souza, Muir and Walker29). Both P50 and P300 ERPs are altered in schizophrenic patients and in their healthy relatives Reference Patterson, Hetrick and Boutros(30,Reference Turetsky, Calkins, Light, Olincy, Radant and Swerdlow31) and can be modified by antipsychotic treatments Reference Light, Geyer, Clementz, Cadenhead and Braff(32,Reference Umbricht, Javitt and Novak33). These neuropsychological and electrophysiological endophenotypes have been related to susceptibility genes for schizophrenia.

Catechol-O-methyl transferase. Catechol-O-methyl transferase (COMT) enzyme terminates catecholamine activity in the brain by degrading these neurotransmitters. The gene encoding COMT – chromosome 22 Reference Grossman, Littrell, Weinstein, Punnett, Emanuel and Budarf(34)– has a functional polymorphism (Val108/158Met) that moderates dopamine availability in the prefrontal cortex (PFC) in an allele-dependent manner: the Met allele, which has a 3–4 times lower enzymatic activity, has been associated to higher dopamine levels in the PFC Reference Lachman, Papolos, Saito, Yu, Szumlanski and Weinshilboum(35). Several studies have investigated COMT (Val108/158Met) polymorphism as a risk factor for schizophrenia: the Val-allele, which has a higher enzymatic activity resulting in lower PFC dopamine levels, has been found to confer susceptibility to schizophrenia in Caucasian populations, while its role in other ethnic groups (e.g. Asians) is more controversial Reference Glatt, Faraone and Tsuang(36). A large number of studies have proven the association of COMT (Val108/158Met) variants with cognitive functions in schizophrenic patients and non-affected relatives (Reference Gosso, De Geus, Polderman, Boomsma, Heutink and Posthuma37–Reference Egan, Goldberg and Kolachana47). The Val-allele has also been related to P50 sensory gating Reference Lu, Martin and Edgar(48) and schizotypal personality traits Reference Ma, Sun and Yao(49,Reference Schurhoff, Szoke and Chevalier50).

Neuregulin 1. The neuregulins (NRGs) are cell-cell signalling proteins that are ligands for receptor tyrosine kinases of the ErbB family Reference Falls(51). The NRG1 proteins have been demonstrated to play important roles during the development of the nervous system Reference Britsch(52) and moderate N-methyl-D-aspartate (NMDA) receptor function Reference Bjarnadottir, Misner and Haverfield-Gross(53), and these findings support NRG1 involvement in the pathogenesis of schizophrenia Reference Corfas, Roy and Buxbaum(54). Mice with heterozygous deletion of NRG1 transmembrane domain have been characterised by behavioural phenotypes, which are related to human schizophrenia Reference O’Tuathaigh, Babovic, O’Meara, Clifford, Croke and Waddington(55,Reference Karl, Duffy, Scimone, Harvey and Schofield56). NRG1is a positional candidate gene on chromosome 8p22-p11 that several genome-wide linkage scans have identified as a susceptibility locus for schizophrenia Reference Badner and Gershon(57). NRG1has proven its association with schizophrenia in a variety of studies, although with conflicting results in Caucasian and Asian samples Reference Li, Collier and He(58). The intermediate phenotypes of the NRG1gene have been poorly investigated. Recently, Stefanis et al. reported a moderate impact of this gene on sustained attention and working memory in the general population Reference Stefanis, Trikalinos and Avramopoulos(59).

Brain-derived neurotrophic factor. Neurotrophins restore the functions of the damaged neurons and prevent apoptosis in adults, thus they are likely to be implicated in the pathophysiology of several mental disorders including schizophrenia Reference Lang, Jockers-Scherubl and Hellweg(60). Indeed, abnormal activity of the neurotrophin system has been reported in schizophrenics’ brains, in particular increased brain-derived neurotrophic factor (BDNF) levels have been found in the hippocampus and anterior cingulate cortex Reference Takahashi, Shirakawa and Toyooka(61) and decreased BDNF in the PFC Reference Weickert, Hyde, Lipska, Herman, Weinberger and Kleinman(62) of schizophrenic patients compared with healthy controls. The pathophysiological meaning of these findings is not yet fully understood. However, it has been demonstrated that BDNF can regulate the expression of Reelin Reference Ringstedt, Linnarsson and Wagner(63,Reference Alcantara, Pozas, Ibanez and Soriano64), a protein that is involved in migration and positioning of cortical and hippocampal neurons during embryonic development of the brain Reference Rice and Curran(65). Both typical and atypical antipsychotics have been shown to reduce BDNF expression in various brain regions of the rat Reference Angelucci, Mathe and Aloe(66,Reference Lipska, Khaing, Weickert and Weinberger67) and BDNF-like immunoreactivity in the serum of schizophrenics Reference Tan, Zhou, Cao, Zou and Zhang(68,Reference Grillo, Ottoni, Leke, Souza, Portela and Lara69). The gene encoding BDNF is a putative susceptibility factor for psychosis Reference Gratacos, Gonzalez, Mercader, De Cid, Urretavizcaya and Estivill(70). Two polymorphisms in the BDNFgene – Val66Met and C270T – have been associated with schizophrenia, although their effects seem to be weak Reference Zintzaras(71,Reference Kanazawa, Glatt, Kia-Keating, Yoneda and Tsuang72). The Val66Met polymorphism has also been connected with early phases of information processing in schizophrenic samples Reference Rybakowski, Borkowska and Skibinska(73) and with verbal memory in schizophrenics and healthy controls Reference Ho, Milev, O’Leary, Librant, Andreasen and Wassink(74).

Disrupted-In-Schizophrenia 1. Disrupted-In-Schizophrenia 1 (DISC1) is a gene disrupted by a balanced (1;11) (q42;q14.3) translocation that has been shown to cosegregate with major psychiatric disorders in a large Scottish family Reference Millar, Wilson-Annan and Anderson(75) DISC1 protein occurs in various subcellular compartments, which include the centrosome, microtubule fractions, postsynaptic densities, actin cytoskeletal fractions, the mitochondria and the nucleus Reference Ishizuka, Paek, Kamiya and Sawa(76). Recent studies have clarified that DISC1 is a component of a neurodevelopmentally regulated protein complex that has different functions in the developing and adult brain. In the developing brain, DISC1 has been implicated in neuronal migration Reference Brandon, Handford and Schurov(77) and in neurite outgrowth and extension Reference Kamiya, Tomoda and Chang(78). In the adult, DISC1 has been identified in multiple populations of neurons and in structures associated with synaptic function, suggesting that one of its adult functions may be synaptic plasticity Reference Duan, Chang and Ge(79). DISC1 is present in many of the brain regions known to be abnormal in schizophrenia, such as the PFC, hippocampus and thalamus Reference Roberts(80). Mutant truncated DISC1 may contribute to schizophrenia susceptibility by altering neuronal architecture and migration Reference Morris, Kandpal, Ma and Austin(81). More recently, it has been suggested that DISC1 may interact with phosphodiesterase 4B (PDE4B) and biochemical cycle of 3′,5′-cyclic adenosine monophosphate (cAMP), which appears to be implicated in learning, memory and mood. According to proposed model, DISC1 sequesters PDE4B in resting cells and releases it in an activated state in response to elevated cAMP Reference Millar, Pickard and Mackie(82). The association of the DISC1gene (chromosome 1p42) with schizophrenia, which was originally reported in a Scottish population, has been replicated in other ethnic groups Reference Roberts(80). DISC1single nucleotide polymorphisms have shown to be connected with sustained attention and working memory deficits in schizophrenic families Reference Liu, Fann and Liu(83,Reference Hennah, Tuulio-Henriksson and Paunio84).

DTNBP1. Dysbindin is an evolutionary conserved 40-kDa coiled-coil-containing protein that binds to alpha- and beta-dystrobrevin in muscle and brain. In the brain, dysbindin immunoreactivity is associated with glutamatergic mossy fibre terminals in the cerebellum and hippocampus Reference Benson, Newey, Martin-Rendon, Hawkes and Blake(85). Although most aspects of its function are still waiting to be elucidated, dysbindin seems to promote neuronal viability and protect cortical neurons against death through phosphoinositide-3 (PI3)-kinase-Akt signalling. Reference Numakawa, Yagasaki and Ishimoto(86). Significant dysbindin reductions have been found at presynaptic levels in glutamatergic afferents of the subiculum, hippocampus and dentate gyrus in the brain of schizophrenic patients Reference Talbot, Eidem and Tinsley(87). DNTBP1is the gene encoding dysbindin on chromosome 6p22.3. Sequence variations in DNTBP1determine reductions in dysbindin messenger RNA levels in the PFC of schizophrenics Reference Weickert, Straub and McClintock(88) and mediate the risk for schizophrenia by lowering dysbindin expression Reference Bray, Preece and Williams(89). A variety of linkage and association studies have supported DTNBP1 as a susceptibility gene for schizophrenia, although with different haplotypes Reference Guo, Sun, Riley, Thiselton, Kendler and Zhao(90). In samples of schizophrenics, carriers of the DNTBP1risk haplotype have been associated to early visual processing deficits and a significantly lower spatial working memory performance Reference Donohoe, Morris and De Sanctis(91,Reference Donohoe, Morris and Clarke92). In addition, genetic variation in DNTBP1has been shown to influence general cognitive ability Reference Burdick, Lencz and Funke(93).

GRM3. A large body of evidence supports the involvement of the glutamate system in schizophrenia. Glutamatergic NMDA receptor antagonist phencyclidine can cause psychotic symptoms in healthy individuals and exacerbate psychosis in schizophrenics Reference Coyle(94). Phencyclidine-evoked motor behaviours in rats are suppressed by glutamate receptor agonists Reference Clark, Johnson, Wright, Monn and Schoepp(95), which are currently studied as new agents to treat schizophrenia in phase-2 trials Reference Patil, Zhang and Martenyi(96). GRM3, a metabotropic glutamate receptor modulating synaptic glutamate, has emerged as a promising candidate gene for schizophrenia Reference Harrison, Lyon, Sartorius, Burnet and Lane(97). Variation in GRM3 mediates glutamate release in the PFC and cognitive functioning in psychotic patients Reference Egan, Straub and Goldberg(98).

CHRNA7. Sensory gating is impaired in schizophrenia most likely because of dysregulation of nicotinic cholinergic neurotransmission. Indeed, cigarette smoking and nicotine administration have been shown to reverse sensory gating deficit in individuals with schizophrenia Reference Adler, Hoffer, Griffith, Waldo and Freedman(99) and in relatives of schizophrenics Reference Adler, Hoffer, Griffith, Waldo and Freedman(99) in a transient manner. Nicotine can also reverse diminished inhibitory sensory gating in cocaine addicts Reference Adler, Olincy and Cawthra(100). One measure of sensory gating abnormalities, diminished inhibition of the P50 evoked response to repeated auditory stimuli, has been linked to the chromosome 15q14 locus of the alpha-7-nicotinic receptor gene (CHRNA7) Reference Freedman, Olincy and Ross(101). This site has shown linkage to schizophrenia across several studies. Polymorphisms in the core promoter of the gene are associated with schizophrenia and also with diminished inhibition of the P50 response Reference Houy, Raux and Thibaut(102,Reference Martin, Leonard, Hall, Tregellas, Freedman and Olincy103).

Interaction between G×E factors

In twin studies on schizophrenia, identical twins show average concordance rates of only 50%, although they share 100% of their genes Reference Tsuang, Stone and Faraone(1). This finding does not necessarily exclude a pure genetic aetiology: it is also possible that non-genetic factors consist entirely of stochastic events affecting gene expression or structure Reference McGuffin, Asherson, Owen and Farmer(104). However, there is a general agreement on the contribution of environmental factors to schizophrenia pathophysiology Reference Sullivan, Kendler and Neale(2). Identification of these factors is hampered by the lack of reliable objective measures for subjective psychological variables, thus research has focused on the few measurable environmental variables Reference McDonald and Murray(22).

Obstetric complications. It is well established that exposure to obstetric complications in the prenatal or perinatal periods increases the risk for schizophrenia with a small but significant effect Reference Boog(18). Complications of pregnancy, abnormal foetal growth and complications of delivery are more often reported in individuals who later develop schizophrenia than in non-schizophrenic controls Reference Clarke, Harley and Cannon(105). The pathogenic effect of these events generally lies in a hypoxic damage of foetal and neonatal brains (Reference Preti, Cardascia, Zen, Marchetti, Favaretto and Miotto106–Reference Byrne, Agerbo, Bennedsen, Eaton and Mortensen108). Foetal hypoxia is associated with structural brain abnormalities [reduced grey matter and increased cerebrospinal fluid (CSF)] among schizophrenic patients and their non-schizophrenic siblings but not among controls at low genetic risk Reference Cannon, VanErp and Rosso(109). This is clearly a proof that hypoxic brain damage leading to schizophrenia is mediated by genetic factors. Indeed, brain hypoxia was shown to alter the expression of 20 susceptibility genes for schizophrenia Reference Schmidt-Kastner, VanOs, Steinbusch and Schmitz(110). In particular, the most consistent finding was about the NRG1gene, which was found to be expressed at higher levels in rats exposed to brain hypoxia within the first postpartum week Reference Nadri, Belmaker and Agam(111). These models of gene-environment interaction are also confirmed by recent human studies. Thus, Nicodemus et al. reported that a few polymorphisms in the DTNBP1, AKT1, BDNF and GRM3 genes increased risk for developing schizophrenia in individuals who were exposed to obstetric complications causing foetal hypoxia Reference Nicodemus, Marenco and Batten(112).

Viral infections. Population studies indicate that exposure to influenza epidemics between the third and seventh month of gestation is associated with schizophrenia in adult life, suggesting that maternal-foetal transmission of influenza virus may be a risk factor for schizophrenia Reference Castle and Gill(113). To corroborate this hypothesis, a French study demonstrated that prenatal exposure to influenza virus occurred more frequently in schizophrenic patients than in non-schizophrenic siblings and a control sample Reference Limosin, Rouillon, Payan, Cohen and Strub(114). The pathogenic effect of influenza viral infection has been studied in rodents. Mice exposed to prenatal human influenza viral infection showed altered levels of neuronal nitric oxide synthase Reference Fatemi, Cuadra, El-Fakahany, Sidwell and Thuras(115), which has been implicated in synaptogenesis and excitotoxicity, abnormal pyramidal cell density and atrophy Reference Fatemi, Earle and Kanodia(116) and increased expression of markers of gliosis Reference Fatemi, Emamian and Sidwell(117).

Studies have suggested that also cytomegalovirus (CMV) may play an aetiological role in schizophrenia. Indeed, it has been reported that some patients experiencing initial episodes of schizophrenia had increased levels of immunoglobulin G antibodies against CMV in their sera and CSF Reference Torrey, Leweke and Schwarz(118). Treatment with antipsychotic medications may result in a decrease in CMV antibodies Reference Leweke, Gerth and Koethe(119), while treatment with antiherpes virus and anti-inflammatory medications may reduce symptoms in some individuals with schizophrenia Reference Dickerson, Boronow, Stallings, Origoni and Yolken(120). The onset of schizophrenia following prenatal exposure to viral infections is most likely mediated by genetic factors. In rats, olfactory bulb injection of a neuroadapted influenza A virus strain led to persistent changes in spatial learning and elevated transcriptional activity of the gene encoding synaptic regulatory protein RGS4 Reference Beraki, Aronsson, Karlsson, Ogren and Kristensson(121), which has been pointed to as a schizophrenia liability gene. Some polymorphisms within the chromosomal region 6p-21-p23 have been noted to confer risk for schizophrenia in conjunction with CMV exposure Reference Kim, Shirts and Dayal(122).

Substance abuse. Epidemiological data indicate that 30–60% of schizophrenics meet lifetime criteria for substance abuse or dependence (Reference Fowler, Carr, Carter and Lewin123–Reference Buhler, Hambrecht, Loffler, An DerHeiden and Hafner126). The 6-month prevalence of substance misuse in schizophrenia is 10–30% Reference Fowler, Carr, Carter and Lewin(123,Reference Buhler, Hambrecht, Loffler, An DerHeiden and Hafner126), with alcohol and cannabis having the greatest diffusion Reference Fowler, Carr, Carter and Lewin(123). In adolescents, regular use of cannabis almost doubles the risk of psychotic outcomes Reference Moore, Zammit, Lingford-Hughes, Barnes, Jones, Burke and Lewis(21). This effect is dose dependent and much stronger in individuals with predisposition for psychosis Reference Henquet, Krabbendam and Spauwen(127). Family histories of schizophrenia have been more often reported in psychotic patients who were cannabis positive on urinary screening than in controls with psychosis who screened negatively for all substances of abuse Reference McGuire, Jones, Harvey, Williams, McGuffin and Murray(128). Thus, it is necessary to postulate a gene-environment interaction to explain the onset of psychotic symptoms in response to cannabis use. Caspi et al. identified the role of the COMT (Val108/158Met) polymorphism in developing psychosis in adulthood after exposure to cannabis during adolescence: individuals who carried the Val-allele were more likely to experience psychotic symptoms or any schizophrenia-like disorder Reference Caspi, Moffitt and Cannon(129). More recently, the Val-allele has been associated not only with psychosis after exposure to delta-9-tetrahydrocannabinol (THC) but also with attention and working memory deficits Reference Henquet, Rosa and Krabbendam(130). On the contrary, Zammit et al. reported no effect of the COMT gene on association between cannabis and psychosis Reference Zammit, Spurlock and Williams(131). Another valuable candidate gene is the one encoding NRG1. NRG1-deficient knockout mice showed positive correlations with animal models of schizophrenia after administration of THC and increased c-Fos expression in the amygdala, nucleus accumbens and lateral septum but only in the subsample exposed to behavioural stimulation Reference Boucher, Arnold, Duffy, Schofield, Micheau and Karl(132). These findings might imply a complex interaction between stress, genetic factors and cannabis in substance-induced psychosis but need to be replicated in humans.

Furthermore, amphetamine-induced psychosis has been related to the dysbindin DTNBP1 gene Reference Kishimoto, Ujike and Motohashi(133).

Life stress. Exposure to stressful life events facilitates the onset of the first episode of psychosis Reference Cullberg(20) and triggers depressive exacerbation in the early course of schizophrenia Reference Ventura, Nuechterlein, Subotnik, Hardesty and Mintz(134). Schizophrenic patients are more vulnerable to the effect of daily life stress Reference Myin-Germeys, Peeters and Havermans(135). This characteristic, which is independent of cognitive deficits, may support an affective pathogenesis of schizophrenia Reference Myin-Germeys and VanOs(136).

The COMT (Val108/158Met) polymorphism has been demonstrated to modulate the impact of stress on psychotic symptoms. Stefanis et al. used a semi-experimental paradigm to evaluate the effect of stress on psychosis manifestations in a sample of young men at recruitment in the Greek army and noted that carriers of the COMT (Val108/158Met) Val-allele experienced higher levels of psychotic symptoms after stress exposure Reference Stefanis, Henquet and Avramopoulos(137). On the contrary, van Winkel et al. reported that psychosis sensitivity to stress was higher in Met/Met homozygotes Reference van Winkel, Henquet and Rosa(138).

Gene, environment and prevention

A variety of environmental factors contribute to the onset of schizophrenia. The task of primary prevention is to identify and remove those risks. The benefit of primary prevention for the population’s health may be considerable. A recent meta-analysis reported a 40% increased risk for any psychotic outcome in individuals who had ever used cannabis Reference Moore, Zammit, Lingford-Hughes, Barnes, Jones, Burke and Lewis(21): based on this finding, it has been estimated that in UK at least 800 new cases of schizophrenia yearly could be avoided if cannabis was no longer used by individuals at risk for psychosis.

Genetics may have a key role in primary prevention of schizophrenia. Accordingly, the main finding of this review, in line with more authoritative opinions of leading researchers in psychogenetics Reference Kendler and Eaves(139), is that sensitivity to environmental risk factors is under genetic control. Thus, the COMT (Val108/158Met) polymorphism has been found to modulate the risk of developing psychosis in individuals who are cannabis users Reference Caspi, Moffitt and Cannon(129) or exposed to stress conditions Reference Stefanis, Henquet and Avramopoulos(137,Reference van Winkel, Henquet and Rosa138). Various genes whose expression is regulated by hypoxia (AKT1, DTNBP1, BDNF and GRM3) have been shown to interact with obstetric complications to influence risk for schizophrenia Reference Nicodemus, Marenco and Batten(112). Although gene-environment models have surfaced to schizophrenia research, their application to prevention activity is still premature. In fact, many gene-environment associations were only reported in animal studies. Other genes were associated with environmental factors in humans, but replication studies are needed. Future research is expected to identify new models of G×E interaction. Morphometry has shown a larger proportion of myelinated fibres with atrophy of axon and swelling of periaxonal oligodendrocyte processes, a lower number of oligodendroglial satellites of pyramidal neurons and a loss of pericapillary oligodendrocytes in the PFC of schizophrenic patients compared with non-schizophrenic controls Reference Uranova, Vostrikov, Vikhreva, Zimina, Kolomeets and Orlovskaya(140). These white matter abnormalities have been associated with cognitive deficits Reference Dwork, Mancevski and Rosoklija(141). Preliminary evidence suggests that substance abuse, particularly cannabis, might interfere with the development of frontal white matter in some adolescents, thus leading to less white matter Reference Schlaepfer, Lancaster and Heidbreder(142). Therefore, it is arguable that oligodendrocyte-myelin-related genes (e.g. MAG; 2,3-cyclic nucleotide 3′-PDE; QKI and SOX10), which have been associated with schizophrenia Reference Karoutzou, Emrich and Dietrich(11), may affect risk for psychosis in substance abusers Reference Kumra(143). Communication deviance has been found to be higher in families with schizophrenic offspring (Reference Doane144–Reference Velligan, Mahurin, Eckert, Hazleton and Miller146) and has emerged as a risk factor for schizophrenia by interacting with genetic liability Reference Wahlberg, Wynne and Oja(147,Reference Wahlberg, Wynne and Hakko148). Recent studies suggest that communication deviance might be under genetic control Reference Subotnik, Goldstein, Nuechterlein, Woo and Mintz(149), although there is no information on implicated genes.

Secondary prevention involves individuals at increased risk for schizophrenia to prevent the onset of the first psychotic episode. Ultra-high-risk individuals with family histories of psychosis and a recent decline in social functioning or subthreshold psychotic manifestations Reference Phillips, Yung and McGorry(150,Reference McGorry, Yung and Phillips151) develop schizophrenia in 30–40% of cases within 1—2 years of follow-up (Reference Yung, Phillips and Yuen152–Reference Cannon, Cornblatt and McGorry154). As transition to psychosis does not occur in about two thirds of these subjects, ultra-high-risk criteria may not be specific enough to warrant secondary prevention interventions. Genetics may improve identification of at risk subjects for developing schizophrenia to an acceptable level for secondary prevention activity; this hypothesis needs to be confirmed by future research. The knowledge of gene-endophenotype interaction may increase the feasibility of prophylactic interventions. Thus, cannabis is known to alter sensory gating, thereby causing substance-induced psychosis (Reference Rentzsch, Penzhorn and Kernbichler155–Reference Patrick, Straumanis, Struve, Fitz-Gerald, Leavitt and Manno157). The COMT gene has been associated to both gating deficits in schizophrenia Reference Lu, Martin and Edgar(48) and onset of psychosis in cannabis users Reference Caspi, Moffitt and Cannon(129). Taken together, these findings would indicate that reducing sensory gating deficits among cannabis users carrying at risk variant (Val-allele) of the COMT gene, e.g. by administration of atypical antipsychotics Reference Oranje, Van Oel, Gispen-De Wied, Verbaten and Kahn(158) or nicotinic cholinergic agonists Reference Adler, Olincy and Cawthra(100), might aid in preventing psychotic complications.

Genetics and rehabilitation

In schizophrenic patients, cognitive deficits are not only related to psychopathological dimensions – significant associations have been reported between executive function deficits and negative symptoms Reference Donohoe, Corvin and Robertson(159,Reference Cameron, Oram, Geffen, Kavanagh, McGrath and Geffen160) and between working memory deficits and disorganisation Reference Cameron, Oram, Geffen, Kavanagh, McGrath and Geffen(160,Reference Daban, Amado and Bayle161)– but also affect psychosocial functioning and long-term outcome (Reference Hofer, Baumgartner and Bodner162–Reference Martinez-Aran, Penades and Vieta166). Therefore, reducing cognitive impairment is considered to be one of the main objectives in the treatment of schizophrenia Reference Green, Kern and Heaton(167).

Cognitive dysfunction can be treated by atypical antipsychotics (Reference Peuskens, Demily and Thibaut168–Reference Weiss, Bilder and Fleischhacker170). A recent meta-analysis demonstrates that atypicals produce a mild remediation of cognitive deficits in schizophrenia, each atypical having a greater effect on specific cognitive domains Reference Woodward, Purdon, Meltzer and Zald(171). Given the moderate efficacy of antipsychotics on cognitive deficits, a relatively large number of schizophrenics could have partial or no improvement in cognitive functions during antipsychotic treatment. Besides antipsychotics, cognitive deficits can also be targeted by training exercises of impaired cognitive functions Reference Green(172). These cognitive remediation interventions produce moderate improvements in cognitive performance and psychosocial outcomes, with only small effects on the symptoms of schizophrenia Reference McGurk, Twamley, Sitzer, McHugo and Mueser(173). The efficacy of cognitive remediation programmes is variable and seems to be dependent on the use of specific components of training Reference Wykes and Van DerGaag(174).

This paper summarised the contribution of genetic factors to cognitive dysfunction in schizophrenic subjects and their relatives. We argue that the same genes that have been related to cognitive deficits may influence cognitive function response to antipsychotic treatment and cognitive remediation therapy. This hypothesis is now supported by some experimental data. Indeed, in a recent study of schizophrenics treated with clozapine, cognitive improvement during antipsychotic therapy was found to be influenced by COMT (Val108/158Met) genotype Reference Woodward, Jayathilake and Meltzer(40). The COMT gene is also involved in response to cognitive remediation interventions. Fifty out-patients with chronic schizophrenia were evaluated over a 3-month follow-up on active rehabilitation treatment including cognitive remediation exercises or control treatment with standard rehabilitation alone and genotyped for COMT (Val108/158Met) variants: carriers of the Met allele showed a greater improvement in cognitive flexibility and quality of life on active treatment compared with Val/Val homozygotes on control treatment Reference Bosia, Bechi and Marino(175).

Further studies are warranted to confirm the impact of the COMT gene on cognitive response and to elucidate the role of other ‘cognitive’ genes.