We have studied the role of AKT1, a highly relevant candidate gene for SCZ and BPD, in relation to quantitative cognitive and neuroanatomical phenotypes in Finnish twin pairs. We obtained evidence for association between AKT1 and four quantitative verbal memory-related functions as well as prefrontal cortical gray matter density. The association between AKT1 short-term verbal memory reached statistical significance (original P=0.0005, corrected P=0.03) even after applying conservative measures to correct for multiple testing. Importantly, the same AKT1 allele was associated with worse cognitive functioning and with decreased gray matter density in the medial, dorsolateral, and inferior pre-frontal cortex, suggesting a consistent biological effect. This study design, profiting from information content of discordant twin pairs in analysis of intermediate phenotypes, is obviously not suitable for addressing plausible association with the disease diagnosis; it rather facilitates monitoring for the impact of the disease status on the phenotypes followed. Not surprisingly, we did not see evidence of association between AKT1 alleles and psychosis or the DSM-IV based end state diagnoses of SCZ spectrum psychosis and affective psychosis (data not shown).
The strongest contribution of AKT1 to verbal memory was observed among the control pairs and unaffected co-twins of SCZ and BPD pairs, but similar trend was also seen in affected co-twins, whereas the association to gray matter density was strongest in the patients, having the largest measurable changes. There are numerous potential explanations for this, including statistical power due to differences in sample sizes and in magnitude of the measurable changes for applied methods. Furthermore, these parameters (verbal memory and gray matter density), potentially affected by AKT1, could also be independent from each other and be altered at a different and individual pace during the disease process.
We obtained support for the association by taking advantage of the twin design of the study that allowed us to compare intra-pair resemblance between co-twins relative to their AKT1 genotypes. Pairs sharing the rs1130214 genotype had a significantly lower intra-pair variation in verbal memory compared to co-twins not sharing the genotype. Interestingly, genotype-related quantitative differences can also be observed in the gray matter in the prefrontal cortex, the allele associating with lower performance in verbal memory also associating with the reduced gray matter density.
Neuropsychological functions are known to be impaired both in patients suffering from SCZ, and to a less extent, in their first degree relatives [Cannon et al., 1994
]. Some of these deficits seem to be heritable and indicate increased disease risk, whereas others probably are more directly (even diagnostically) a result of the disease itself [Tuulio-Henriksson et al., 2002
]. SCZ patients typically have a more generalized and severe impairment whereas BPD patients are found to differ from control population especially in verbal memory functions [van Gorp et al., 1999
; Seidman et al., 2002
]. Verbal memory functions are shown to have a fundamental genetic component explaining 56% of the total variation in a study of elderly twins [Swan et al., 1999
]. The performance of the healthy siblings or co-twins of BPD patients is reported to be comparable to healthy individuals or show only mild impairment [Kieseppa et al., 2005
; Antila et al., 2007
] whereas the siblings of SCZ patients perform worse in CVLT than healthy controls [Egan et al., 2001
]. Previous studies in twins discordant for SCZ [Cannon et al., 2000
], as well as heritability estimates in a family based study [Tuulio-Henriksson et al., 2002
], have indicated that verbal memory deficits are strongly influenced by the SCZ disease process. Furthermore, in line with these observations, in a recent study of five domains of cognition (processing speed, reasoning, verbal memory, working memory, and vigilance) and AKT1
conducted in patients with SCZ no evidence for association was detected [Pinheiro et al., 2007
]. The current study supports these findings. Intra-pair comparisons of verbal memory revealed a decrease in intra-pair resemblance among discordant SCZ pairs compared to controls. However, the current study does not enable us to make a distinction between influences secondary to the illness expression and treatment, nor can we differentiate these from etiologically relevant non-genetic factors underlying verbal memory.
The finding that AKT1
associates specifically with gray matter reduction in the prefrontal cortex is in line with prior work showing that gray matter density in this region correlates with the degree of genetic proximity to an individual affected with SCZ [Rapoport et al., 2005
] or BPD [Shastry, 2005
] and with evidence of a role of prefrontal cortex in verbal memory encoding and retrieval. In addition, SCZ patients are reported to have lower AKT1 protein levels and decreased phosphorylation of GSK-3β, a substrate of AKT1, in lymphocytes and prefrontal cortex [Emamian et al., 2004
]. Furthermore, several three-SNP haplotypes with a common two-SNP core in AKT1
were recently associated with SCZ and decreased AKT1 levels in lymphocyte-derived cell lines [Emamian et al., 2004
]. Several genetic studies have since then replicated the association of AKT1
in the etiology of SCZ [Ikeda et al., 2004
; Schwab et al., 2005
; Bajestan et al., 2006
; Norton et al., 2007
] but also negative reports exist [Ohtsuki et al., 2004
; Ide et al., 2006
; Liu et al., 2006
; Turunen et al., 2007
]. Our associations between verbal memory, prefrontal cortical gray matter, and AKT1
in Finns are all with the same “risk” allele A of rs1130214 which is also part of the originally reported “risk” core haplotype [Emamian et al., 2004
]. Previously the 3′ end of AKT1
specifically the SNP rs2498799/rs1130233 was associated with a factor phenotype containing measures of IQ, executive function and processing speed, and with inefficient prefrontal activation and gray matter density [Tan et al., 2008b
]. It is highly intriguing that two independent studies show evidence for a role of AKT1 in cognitive functions and brain morphology. However, we were not able to directly replicate the association with the same SNP as Tan et al. This divergence between the studies is unlikely explained by genetic proximity of the two SNPs. In our sample the LD between rs1130214, we found to be associated, and rs2498799 is low (D′ =0.157 and r2
=0.016) which is in agreement with the Caucasian haplotype structure of the International HapMap Project (http://www.hapmap.org
). In contrast the differences might rather be suggestive of multiple different causative variants in the gene affecting to a relatively broad range of nervous functions.
More direct functional data also support the role of AKT1 in complex brain functions, disturbed in psychosis. Phosphorylation levels of AKT1 are increased in hippocampus of rats, trained in a maze for several days compared to untrained controls [Mizuno et al., 2003
]. Akt1 deficient mice exhibit changes in the expression of genes related to synaptic function, neuronal development, myelination, and actin polymerization in prefrontal cortex. Furthermore, there were morphological changes in the dendritic architecture and complexity of V-pyramidal neurons [Lai et al., 2006
]. Further, alterations in blood glucose concentration have been reported to influence higher brain functions such as memory, while oscillations in glucose concentration affect the phosphorylation states of AKT1 and GSK3β in mouse brain, suggesting that this pathway might play an important role in memory deficits occurring in some patients with diabetes [Clodfelder-Miller et al., 2005
]. Similarly, insulin dependent AKT1-GSK3β signaling cascade has been found to be significantly reduced in dorsolateral prefrontal cortex of patients suffering from SCZ [Zhao et al., 2006
]. Lithium, commonly used to treat mood disorders, has been related to decreased performance in verbal memory tasks, although the effect appears to be somewhat modest and a few negative findings have also been reported [Macqueen and Young, 2003
; Pachet and Wisniewski, 2003
]. AKT1-GSK3β signaling cascade is a target of lithium [Coyle and Duman, 2003
; Beaulieu et al., 2008
] raising the possibility that the proposed lithium related defects in verbal memory are mediated through the same pathway that we now report to be genetically associated with verbal memory. In addition, dopamine, a neurotransmitter tightly linked to multiple neurological and psychiatric disorders, is suggested to exert at least some of its effect on behavior via AKT1/GSK3β pathway. Mice knocked out for gene encoding for a dopamine transporter (DAT) have reduced levels of active phosphorylated AKT1 (pAKT1) while the amount of pAKT1 increases in depletion of dopamine. Importantly, the dopamine associated hyperactivity in the DAT−/−
mice is antagonized by lithium treatment [Beaulieu et al., 2004
]. AKT1 has been implicated in a variety of functions in the central nervous system such as neuronal migration [Beffert et al., 2002
], growth factor dependent survival of neurons, as well as axon growth and branching [Markus et al., 2002
], while alterations in these functions may be the cause of changes in cognitive functions.
In conclusion, using a unique study design of same sex twin pairs, we obtained evidence that AKT1 is connected to several quantitative measures of verbal learning and memory as well as to gray matter density in the prefrontal cortex. The association was fortified by a decrease in variance in verbal memory among pairs sharing the same critical AKT1 allele. Our study further highlights the effectiveness of the use of measurable quantitative cognitive and neuroanatomical traits as endophenotypes for SCZ and BPD. Recognition of the genetic variation affecting cognitive functioning and brain structure in the population level may provide tools to identify pathways related to disease pathogenesis.