Impact of common KIBRA allele on human cognitive functions

Resting heart rate and risk of dementia: a Mendelian randomization study in the international genomics of Alzheimer's Project and UK Biobank

Background

Observational studies have demonstrated that a higher resting heart rate (RHR) is associated with an increased risk of dementia. However, it is not clear whether the association is causal. This study aimed to determine the causal effects of higher genetically predicted RHR on the risk of dementia.

Methods

We performed a two-sample Mendelian randomization analysis to investigate the causal effect of higher genetically predicted RHR on Alzheimer's disease (AD) using summary statistics from genome-wide association studies. The generalized summary Mendelian randomization (GSMR) analysis was used to analyze the corresponding effects of RHR on following different outcomes: 1) diagnosis of AD (International Genomics of Alzheimer's Project), 2) family history (maternal and paternal) of AD from UK Biobank, 3) combined meta-analysis including these three GWAS results. Further analyses were conducted to determine the possibility of reverse causal association by adjusting for RHR modifying medication.

Results

The results of GSMR showed no significant causal effect of higher genetically predicted RHR on the risk of AD (βGSMR = 0.12, P = 0.30). GSMR applied to the maternal family history of AD (βGSMR = -0.18, P = 0.13) and to the paternal family history of AD (βGSMR = -0.14, P = 0.39) showed the same results. Furthermore, the results were robust after adjusting for RHR modifying drugs (βGSMR = -0.03, P = 0.72).

Conclusion

Our study did not find any evidence that supports a causal effect of RHR on dementia. Previous observational associations between RHR and dementia are likely attributed to the correlation between RHR and other cardiovascular diseases.

Associations of WWC1 variants with Alzheimer's disease and vascular dementia among rural older adults in China: A population-based study

We sought to examine the associations of common WWC1 variants with Alzheimer's disease (AD) and vascular dementia (VaD) among rural-dwelling older adults in China. This population-based study used data from the baseline assessments (March -September 2018) of MIND-China. AD and VaD were diagnosed following the international criteria. Of the 5455 participants (age≥60 years, 57.27% women), 182 were diagnosed with AD and 88 with VaD. Logistic regression analysis suggested that WWC1 rs17070145 C allele (vs. T) was associated with multivariable-adjusted odds ratio of 1.23 (95% confidence interval 0.96-1.58) for AD, and that CC genotype (vs. TT) was associated with multivariable-adjusted odds ratio of 2.19(1.10-4.39) for VaD, but the association with VaD became non-significant when further adjusting for stroke history. Furthermore, exonic SNPs rs3822660 and rs3822659 were in strong linkage disequilibrium (LD) with rs17070145 (D' = 0.88). These results suggest that the strong LD between rs17070145 and 2 exonic SNPs may explain the association of WWC1 rs17070145 C allele with AD and that stroke may partly explain the association of WWC1 rs17070145 CC genotype with VaD.

Genetic Variants behind Cardiovascular Diseases and Dementia

Cardiovascular diseases (CVDs) and dementia are the leading causes of disability and mortality. Genetic connections between cardiovascular risk factors and dementia have not been elucidated. We conducted a scoping review and pathway analysis to reveal the genetic associations underlying both CVDs and dementia. In the PubMed database, literature was searched using keywords associated with diabetes mellitus, hypertension, dyslipidemia, white matter hyperintensities, cerebral microbleeds, and covert infarctions. Gene lists were extracted from these publications to identify shared genes and pathways for each group. This included high penetrance genes and single nucleotide polymorphisms (SNPs) identified through genome wide association studies. Most risk SNPs to both diabetes and dementia participate in the phospholipase C enzyme system and the downstream nositol 1,4,5-trisphosphate and diacylglycerol activities. Interestingly, AP-2 (TFAP2) transcription factor family and metabolism of vitamins and cofactors were associated with genetic variants that were shared by white matter hyperintensities and dementia, and by microbleeds and dementia. Variants shared by covert infarctions and dementia were related to VEGF ligand-receptor interactions and anti-inflammatory cytokine pathways. Our review sheds light on future investigations into the causative relationships behind CVDs and dementia, and can be a paradigm of the identification of dementia treatments.

Is the Brain an Early or Late Component of Essential Hypertension?

The brain's relationship to essential hypertension is primarily understood to be that of an end-organ, damaged late in life by stroke or dementia. Emerging evidence, however, shows that heightened blood pressure (BP) early in life and prior to traditionally defined hypertension, relates to altered brain structure, cerebrovascular function, and cognitive processing. Deficits in cognitive function, cerebral blood flow responsivity, volumes of brain areas, and white matter integrity all relate to increased but prehypertensive levels of BP. Such relationships may be observed as early as childhood. In this review, we consider the basis of these relationships by examining the emergence of putative causative factors for hypertension that would impact or involve brain function/structure, e.g., sympathetic nervous system activation and related endocrine and inflammatory activation. Currently, however, available evidence is not sufficient to fully explain the specific pattern of brain deficits related to heightened BP. Despite this uncertainty, the evidence reviewed suggests the value that early intervention may have, not only for reducing BP, but also for maintaining brain function.

Interaction of COMT and KIBRA modulates the association between hippocampal structure and episodic memory performance in healthy young adults

Genetic variations of COMT and KIBRA, which were reported to be expressed in the hippocampus, have been linked to memory function. However, their interaction on the hippocampal structure remains unknown. This study aimed to explore the interaction effects of COMT rs4680 and KIBRA rs17070145 on the hippocampal subfield volumes and test their associations with hippocampus-memory relationship in 187 healthy young adults. Two-way analysis of covariance was applied to the alterations in hippocampal subfield volumes among COMT and KIBRA genotypes. Significant interaction effects of these two genes were found in the right CA1 and CA3 subfields. Among KIBRA C-allele carriers, COMT Val/Val homozygotes showed greater volume in these regions than COMT Met-allele carriers. Furthermore, the slope of the correlation between right CA1 volume and immediate recall on the California Verbal Learning Test-II (CVLT-II) (F = 4.36, p = 0.041) as well as CVLT-II delayed recall (F = 6.44, p = 0.014) were significantly different between COMT Val/Val homozygotes and Met-allele carriers, which were positive or tend to be positive in COMT Val/Val group (CVLT immediate recall, r = 0.319, p = 0.040; CVLT delayed recall, r = 0.304, p = 0.051), but absent in COMT Met-allele carriers (CVLT immediate recall, r = -0.263, p = 0.205; CVLT delayed recall, r = -0.351, p = 0.086). These findings may provide a novel insight into the genetic effects upon the hippocampal structure and suggest that the conjoint effects of COMT and KIBRA played a modulatory role in the hippocampus-episodic memory correlation.

KIBRA T allele influences memory performance and progression of cognitive decline: a 7-year follow-up study in subjective cognitive decline and mild cognitive impairment

KIBRA is a signal transducer protein, mainly expressed in the kidney and brain. A single-nucleotide polymorphism (SNP rs17070145, T → C exchange) has been linked to different cognitive function. In 2008, we studied 70 subjects who complained of subjective cognitive decline (SCD) and found that CT/TT carriers performed worse than CC carriers on a long-term memory test. We followed up the 70 SCD subjects and also 31 subjects affected by mild cognitive impairment (MCI) for a mean follow-up time of 7 years, during which 16 SCD subjects progressed to MCI and 14 MCI subjects progressed to Alzheimer's disease (AD). Carrying the T allele was associated with MCI and with a two times-higher risk of developing MCI than CC carriers. In the SCD sample, CT/TT carriers showed a greater worsening on Rivermead Behavioral Memory Test (RBMT) compared to CC carriers. In the MCI sample, CT/TT carriers performed worse than CC carriers on RBMT. There is a lack of consensus on the effect of KIBRA gene variants on cognitive performances in episodic memory and on the risk of AD. Our results confirm a role of T allele on progression of cognitive decline.

KIBRA polymorphism modulates gray matter volume to influence cognitive ability in the elderly

Genetic variation in the kidney and brain expressed protein (KIBRA) rs17070145 gene has been linked to episodic memory and cognitive aging; yet, the neural mechanism underlying this association remains to be fully understood. Using the magnetic resonance imaging (MRI) technique, this study investigated the effect of KIBRA polymorphism on gray matter volume in 37 healthy, Chinese adults from the older population. Voxel-based morphometry (VBM) analysis revealed that KIBRA gene selectivity influences the prefrontal cortex and the parahippocampal cortex. The gray matter volume (GMV) in these structures is significantly lower in KIBRA C-allele carriers than in TT carriers. Moreover, multi-voxel pattern correlation analysis revealed that decreased prefrontal GMV could in turn affect individual cognitive function in C-allele carriers; whereas, TT individuals utilized more integrated gray matter volume in whole-brain voxels to achieve relatively better cognitive function. Overall, the findings suggest that the KIBRA rs17070145 polymorphism modulates gray matter volume, which in turn further influences cognitive function in the elderly.

KIBRA and APOE Gene Variants Affect Brain Functional Network Connectivity in Healthy Older People

Genetic factors play a critical role in the development of Alzheimer’s disease (AD). Kidney and brain expressed protein (KIBRA) and apolipoprotein E (APOE) are involved in episodic memory performance and AD. However, the interactions between KIBRA and APOE on brain functional network connectivity (FNC) remain unknown in healthy older people. Using independent component analysis, we systematically investigated additive and epistatic interactions of KIBRA rs1707045 and APOE on FNC in 170 healthy older Chinese people of Han ethnicity. We found significant additive KIBRA–APOE interactions on brain FNC in the right medial prefrontal cortex, the posterior cingulate cortex in the default-mode network, and the dorsal anterior cingulate cortex in the salience network. We also found significant epistatic KIBRA–APOE interactions on brain FNC in the left superior frontal gyrus and left angular gyrus in default-mode network. No significant KIBRA–APOE interactions were detected in other brain resting-state networks. These findings suggest that healthy older people have additive and epistatic interactions of KIBRA and APOE gene variants, which modulate brain FNC and may partly elucidate their association with episodic memory performance and AD.

The role of KIBRA in reconstructive episodic memory

BACKGROUND

In order to retrieve episodic past events, the missing information needs to be reconstructed using information stored in semantic memory. Failures in these reconstructive processes are expressed as false memories. KIBRA single nucleotide polymorphism (rs17070145) has been linked to episodic memory performance as well as an increased risk of Alzheimer's disease and post-traumatic stress disorder (PTSD).

METHODS

Here, the role of KIBRA rs17070145 polymorphism (male and female CC vs. CT/TT carriers) in reconstructive episodic memory in the Deese-Roediger-McDermott (DRM) paradigm was investigated in N = 219 healthy individuals.

RESULTS

Female participants outperformed males in the free recall condition. Furthermore, a trend towards a gender x genotype interaction was found for false recognition rates. Female CT/TT carriers exhibited a lower proportion of false recognition rates for associated critical lures as compared to male CT/TT. Additionally, an association between KIBRA rs17070145 genotype, familiarity and recollection based recognition performance was found. In trials with correct recognition of listed items CT/TT carriers showed more "remember", but fewer "know" responses as compared to CC carriers.

DISCUSSION AND CONCLUSION

Our findings suggest that the T-allele of KIBRA rs17070145 supports recollection based episodic memory retrieval and contributes to memory accuracy in a gender dependent manner. Findings are discussed in the context of the specific contribution of KIBRA related SNPs to reconstructive episodic memory and its implications for cognitive and emotional symptoms in dementia and PTSD.

KIBRA is associated with accelerated cognitive decline and hippocampal atrophy in APOE ε4-positive cognitively normal adults with high Aβ-amyloid burden

A single nucleotide polymorphism, rs17070145, in the KIdney and BRAin expressed protein (KIBRA) gene has been associated with cognition and hippocampal volume in cognitively normal (CN) individuals. However, the impact of rs17070145 on longitudinal cognitive decline and hippocampal atrophy in CN adults at greatest risk of developing Alzheimer's disease is unknown. We investigated the impact rs17070145 has on the rate of cognitive decline and hippocampal atrophy over six years in 602 CN adults, with known brain Aβ-amyloid levels and whether there is an interactive effect with APOE genotype. We reveal that whilst limited independent effects of KIBRA genotype were observed, there was an interaction with APOE in CN adults who presented with high Aβ-amyloid levels across study duration. In comparison to APOE ε4-ve individuals carrying the rs17070145-T allele, significantly faster rates of cognitive decline (global, p = 0.006; verbal episodic memory, p = 0.004), and hippocampal atrophy (p = 0.04) were observed in individuals who were APOE ε4 + ve and did not carry the rs17070145-T allele. The observation of APOE effects in only non-carriers of the rs17070145-T allele, in the presence of high Aβ-amyloid suggest that carriers of the rs17070145-T allele are conferred a level of resilience to the detrimental effects of high Aβ-amyloid and APOE ε4.

Age-Modulated Associations between KIBRA, Brain Volume, and Verbal Memory among Healthy Older Adults

The resource modulation hypothesis suggests that the influence of genes on cognitive functioning increases with age. The KIBRA single nucleotide polymorphism rs17070145, associated with episodic memory and working memory, has been suggested to follow such a pattern, but few studies have tested this assertion directly. The present study investigated the relationship between KIBRA alleles (T carriers vs. CC homozygotes), cognitive performance, and brain volumes in three groups of cognitively healthy adults-middle aged (ages 52-64, n = 38), young old (ages 65-72, n = 45), and older old (ages 73-92, n = 62)-who were carefully matched on potentially confounding variables including apolipoprotein ε4 status and hypertension. Consistent with our prediction, T carriers maintained verbal memory performance with increasing age while CC homozygotes declined. Voxel-based morphometric analysis of magnetic resonance images showed an advantage for T carriers in frontal white matter volume that increased with age. Focusing on the older old group, this advantage for T carriers was also evident in left lingual gyrus gray matter and several additional frontal white matter regions. Contrary to expectations, neither KIBRA nor the interaction between KIBRA and age predicted hippocampal volumes. None of the brain regions investigated showed a CC homozygote advantage. Taken together, these data suggest that KIBRA results in decreased verbal memory performance and lower brain volumes in CC homozygotes compared to T carriers, particularly among the oldest old, consistent with the resource modulation hypothesis.

No differences in brain microstructure between young KIBRA-C carriers and non-carriers

KIBRA rs17070145 polymorphism is associated with variations in memory function and the microstructure of related brain areas. Diffusion kurtosis imaging (DKI) as an extension of diffusion tensor imaging that can provide more information about changes in microstructure, based on the idea that water diffusion in biological tissues is heterogeneous due to structural hindrance and restriction. We used DKI to explore the relationship between KIBRA gene polymorphism and brain microstructure in young adults. We recruited 100 healthy young volunteers, including 53 TT carriers and 47 C allele carriers. No differences were detected between the TT homozygotes and C-allele carriers for any diffusion and kurtosis parameter. These results indicate KIBRA rs17070145 polymorphism likely has little or no effect on brain microstructure in young adults.

Hippo Signaling: Emerging Pathway in Stress-Related Psychiatric Disorders?

Discovery of the Hippo pathway and its core components has made a significant impact on our progress in the understanding of organ development, tissue homeostasis, and regeneration. Upon diverse extracellular and intracellular stimuli, Hippo signaling regulates stemness, cell proliferation and apoptosis by a well-conserved signaling cascade, and disruption of these systems has been implicated in cancer as well as metabolic and neurodegenerative diseases. The central role of Hippo signaling in cell biology also results in prominent links to stress-regulated pathways. Genetic variations, epigenetically provoked upregulation of Hippo pathway members and dysregulation of cellular processes implicated in learning and memory, are linked to an increased risk of stress-related psychiatric disorders (SRPDs). In this review, we summarize recent findings, supporting the role of Hippo signaling in SRPDs by canonical and non-canonical Hippo pathway interactions.

Aging and a genetic KIBRA polymorphism interactively affect feedback- and observation-based probabilistic classification learning

Probabilistic category learning involves complex interactions between the hippocampus and striatum that may depend on whether acquisition occurs via feedback or observation. Little is known about how healthy aging affects these processes. We tested whether age-related behavioral differences in probabilistic category learning from feedback or observation depend on a genetic factor known to influence individual differences in hippocampal function, the KIBRA gene (single nucleotide polymorphism rs17070145). Results showed comparable age-related performance impairments in observational as well as feedback-based learning. Moreover, genetic analyses indicated an age-related interactive effect of KIBRA on learning: among older adults, the beneficial T-allele was positively associated with learning from feedback, but negatively with learning from observation. In younger adults, no effects of KIBRA were found. Our results add behavioral genetic evidence to emerging data showing age-related differences in how neural resources relate to memory functions, namely that hippocampal and striatal contributions to probabilistic category learning may vary with age. Our findings highlight the effects genetic factors can have on differential age-related decline of different memory functions.

Age is no barrier: predictors of academic success in older learners

Although predictors of academic success have been identified in young adults, such predictors are unlikely to translate directly to an older student population, where such information is scarce. The current study aimed to examine cognitive, psychosocial, lifetime, and genetic predictors of university-level academic performance in older adults (50-79 years old). Participants were mostly female (71%) and had a greater than high school education level (M = 14.06 years, SD = 2.76), on average. Two multiple linear regression analyses were conducted. The first examined all potential predictors of grade point average (GPA) in the subset of participants who had volunteered samples for genetic analysis (N = 181). Significant predictors of GPA were then re-examined in a second multiple linear regression using the full sample (N = 329). Our data show that the cognitive domains of episodic memory and language processing, in conjunction with midlife engagement in cognitively stimulating activities, have a role in predicting academic performance as measured by GPA in the first year of study. In contrast, it was determined that age, IQ, gender, working memory, psychosocial factors, and common brain gene polymorphisms linked to brain function, plasticity and degeneration (APOE, BDNF, COMT, KIBRA, SERT) did not influence academic performance. These findings demonstrate that ageing does not impede academic achievement, and that discrete cognitive skills as well as lifetime engagement in cognitively stimulating activities can promote academic success in older adults.

Impact of Hypertension on Cognitive Function: A Scientific Statement From the American Heart Association

BACKGROUND

Age-related dementia, most commonly caused by Alzheimer disease or cerebrovascular factors (vascular dementia), is a major public health threat. Chronic arterial hypertension is a well-established risk factor for both types of dementia, but the link between hypertension and its treatment and cognition remains poorly understood. In this scientific statement, a multidisciplinary team of experts examines the impact of hypertension on cognition to assess the state of the knowledge, to identify gaps, and to provide future directions.

METHODS

Authors with relevant expertise were selected to contribute to this statement in accordance with the American Heart Association conflict-of-interest management policy. Panel members were assigned topics relevant to their areas of expertise, reviewed the literature, and summarized the available data.

RESULTS

Hypertension disrupts the structure and function of cerebral blood vessels, leads to ischemic damage of white matter regions critical for cognitive function, and may promote Alzheimer pathology. There is strong evidence of a deleterious influence of midlife hypertension on late-life cognitive function, but the cognitive impact of late-life hypertension is less clear. Observational studies demonstrated a cumulative effect of hypertension on cerebrovascular damage, but evidence from clinical trials that antihypertensive treatment improves cognition is not conclusive.

CONCLUSIONS

After carefully reviewing the literature, the group concluded that there were insufficient data to make evidence-based recommendations. However, judicious treatment of hypertension, taking into account goals of care and individual characteristics (eg, age and comorbidities), seems justified to safeguard vascular health and, as a consequence, brain health.

Impact of KIBRA Polymorphism on Memory Function and the Hippocampus in Older Adults

The single nucleotide polymorphism rs17070145 within the KIBRA gene (kidney and brain expressed protein) has been associated with variations in memory functions and related brain areas. However, previous studies yielded conflicting results, which might be due to divergent sample characteristics or task-specific effects. Therefore, we aimed to determine the impact of KIBRA genotype on learning and memory formation, and volume, microstructural integrity and functional connectivity (FC) of the hippocampus and its subfields in a well-characterized cohort of healthy older adults. One-hundred and forty subjects (72 women, age 50-80) were KIBRA genotyped and memory was tested using the Auditory Verbal Learning Task. Also, subjects underwent structural and resting-state functional magnetic resonance imaging at 3T. Subfields were delineated using automated segmentation (FreeSurfer software). Microstructural integrity was measured using mean diffusivity (MD) derived from diffusion tensor images. Seed-based analyses were used to assess FC patterns of the hippocampus. KIBRA T-allele carriers showed a trend for better memory performance, and in the hippocampus significantly higher volumes and partly lower MD, indicative for better microstructure, compared with non-T-allele carriers in the cornu ammonis (CA)2/3 and CA4/dentate gyrus subfields (all P⩽0.008, Bonferroni corrected). Also, T-allele carriers exhibited lower FC of the left hippocampus with areas outside the synchronized HC network. In sum, we could show for the first time that older T-allele carriers exhibited larger volumes and better microstructure within those hippocampus subfields that are implicated in long-term potentiation and neurogenesis, key features of memory processes. Moreover, T-allele carriers showed a more selective FC network of the hippocampus.

Serum KIBRA mRNA and Protein Expression and Cognitive Functions in Depression

BACKGROUND

Genes participating in synaptic signalling or plasticity in brain regions such as the prefrontal cortex (PFC) and the hippocampus have been implicated in cognition. Recently, a new gene (KIBRA, WWC1) has been added to this group due to its impact on memory performance. Recurrent depressive disorder (rDD) is a multifactorial disease, that one of the typical features is cognitive impairment. The main objective of this study was to perform an analysis of the KIBRA gene on both mRNA and protein levels in patients suffering from rDD and to investigate the relationship between KIBRA expression and cognitive performance.

MATERIAL/METHODS

The study comprised 236 subjects: patients with rDD (n=131) and healthy subjects (n=105, HS). Cognitive function assessment was based on: Trail Making Test, The Stroop Test, Verbal Fluency Test and Auditory Verbal Learning Test.

RESULTS

Both mRNA and protein expression levels of KIBRA gene were significantly higher in healthy subjects when compared to rDD. The presented relationship is clear even after taking age, education and sex of the examined subjects into consideration. No statistically significant relationship was found in the experiments between any of the conducted tests and KIBRA gene expression on mRNA level for both the rDD and HS groups. The presented study has limitations related to the fact that patients were being treated with antidepressant. This is relevant due to the fact that some antidepressants may affect mRNA expression. Number of patients and healthy subjects may result in the lack of statistical significance in some cases.

CONCLUSIONS

1. The results of our study show decreased expression of the KIBRA gene on both mRNA and protein levels in depression. 2. We did not find any significant relationship between KIBRA gene expression and cognitive functions in case of both the healthy subjects and the patients affected by rDD.

Investigating the influence of KIBRA and CLSTN2 genetic polymorphisms on cross-sectional and longitudinal measures of memory performance and hippocampal volume in older individuals

The variability of episodic memory decline and hippocampal atrophy observed with increasing age may partly be explained by genetic factors. KIBRA (kidney and brain expressed protein) and CLSTN2 (calsyntenin 2) are two candidate genes previously linked to episodic memory performance and volume of the hippocampus, a key memory structure. However, whether polymorphisms in these two genes also influence age-related longitudinal memory decline and hippocampal atrophy is still unknown. Using data from two independent cohorts, the Sydney Memory and Ageing Study and the Older Australian Twins Study, we investigated whether the KIBRA and CLSTN2 genetic polymorphisms (rs17070145 and rs6439886) are associated with episodic memory performance and hippocampal volume in older adults (65-90 years at baseline). We were able to examine these polymorphisms in relation to memory and hippocampal volume using cross-sectional data and, more importantly, also using longitudinal data (2 years between testing occasions). Overall we did not find support for an association of KIBRA either alone or in combination with CLSTN2 with memory performance or hippocampal volume, nor did variation in these genes influence longitudinal memory decline or hippocampal atrophy in two cohorts of older adults.

Aging-related magnification of genetic effects on cognitive and brain integrity

Heritability studies document substantial genetic influences on cognitive performance and decline in old age. Increasing evidence shows that effects of genetic variations on cognition, brain structure, and brain function become stronger as people age. Disproportionate impairments are typically observed for older individuals carrying disadvantageous genotypes of different candidate genes. These data support the resource-modulation hypothesis, which states that genetic effects are magnified in persons with constrained neural resources, such as older adults. However, given that findings are not unequivocal, we discuss the need to address several factors that may resolve inconsistencies in the extant literature (gene-gene and gene-environment interactions, study populations, gene-environment correlations, and epigenetic mechanisms).

KIBRA gene polymorphism has no association with verbal or visual episodic memory performance

Inter-individual variability in memory performance has been suggested to result, in part, from genetic differences in the coding of proteins involved in long-term potentiation (LTP). The present study examined the effect of a single-nucleotide polymorphism (SNP) in the KIBRA gene (rs17070145) on episodic memory performance, using multiple measures of verbal and visual episodic memory. A total of 256 female and 130 male healthy, older adults (mean age = 60.86 years) were recruited from the Tasmanian Healthy Brain Project (THBP), undergoing both neuropsychological and genetic testing. The current study showed no significant effect of the KIBRA polymorphism on performance on the Rey Auditory Verbal Learning Task, Logical Memory test, Paired Associates Learning test or Rey Complex Figure Task. The results suggest there is little to no functional significance of KIBRA genotype on episodic memory performance, regardless of modality.

Interactive effects of KIBRA and CLSTN2 polymorphisms on episodic memory in old-age unipolar depression

The KIBRA (rs17070145) C-allele and the CLSTN2 (rs6439886) T-allele have both been associated with poorer episodic memory performance. Given that episodic memory is affected in depression, we hypothesized that the combination of these risk alleles would be particularly detrimental to episodic memory performance in depressed persons. In the population-based SNAC-K study, 2170 participants (≥ 60 years) without dementia (DSM-IV criteria) and antidepressant pharmacotherapy were clinically examined and diagnosed following ICD-10 criteria for unipolar depression, and genotyped for KIBRA and CLSTN2. Participants were categorized according to unipolar depression status (yes, no) and genotype combinations (KIBRA: CC, any T; CLSTN2: TT, any C). Critically, a three-way interaction effect showed that the CC/TT genotype combination was associated with poorer episodic recall and recognition performance only in depressed elderly persons, with depressed CC/TT carriers consistently performing at the lowest level. This finding supports the view that effects of genetic polymorphisms on cognitive functioning may be most easily disclosed at suboptimal levels of cognitive ability, such as in old-age depression.

WWC1 genotype modulates age-related decline in episodic memory function across the adult life span

BACKGROUND

Episodic memory (EM) declines with age and the rate of decline is variable across individuals. A single nucleotide polymorphism (rs17070145) in the WWC1 gene that encodes the KIBRA protein critical for long-term potentiation and memory consolidation has previously been associated with EM performance, as well as differences in hippocampal engagement during EM tasks using functional magnetic resonance imaging (fMRI). In the current study, we explore the effect of this polymorphism on EM-related activity and cognitive performance across the adult life span using fMRI.

METHODS

Two hundred thirty-two healthy, Caucasian subjects (18-89 years) completed a battery of cognitive tests, as well as an EM task during an fMRI scan.

RESULTS

WWC1 T carriers had significantly better delayed recall performance than CC individuals (p = .006). The relationship between increasing age and recall scores (immediate and delayed) was also significantly different between WWC1 genotype groups (p = .01). In addition to the age-related decline in hippocampal formation (HF) activation (p < .05; false discovery ratesmall volume correction-HF-region of interest), we observed an age by WWC1 genotype interaction on HF activation during encoding and retrieval. The CC group showed a significant negative association between HF activity and increasing age, while no such association was observed in the T carrier group (left HF p = .04; r-z correlation difference during encoding and retrieval; right HF p = .0008; r-z correlation difference during retrieval).

CONCLUSIONS

Our results show a dynamic relationship between rs17070145 polymorphism and increasing age on neuronal activity in the hippocampal region.

KIBRA: In the brain and beyond

In mammals, the KIBRA locus has been associated with memory performance and cognition by genome-wide single nucleotide polymorphism screening. Genetic studies in Drosophila and human cells have identified KIBRA as a novel regulator of the Hippo signaling pathway, which plays a critical role in human tumorigenesis. Recent studies also indicated that KIBRA is involved in other physiological processes including cell polarity, membrane/vesicular trafficking, mitosis and cell migration. At the biochemical level, KIBRA protein is highly phosphorylated by various kinases in epithelial cells. Here, we discuss the updates concerning the function and regulation of KIBRA in the brain and beyond.

KIBRA polymorphism is associated with individual differences in hippocampal subregions: evidence from anatomical segmentation using high-resolution MRI

The KIBRA gene has been associated with episodic memory in several recent reports; carriers of the T-allele show enhanced episodic memory performance relative to noncarriers. Gene expression studies in human and rodent species show high levels of KIBRA in the hippocampus, particularly in the subfields. The goal of the present study was to determine whether the KIBRA C→T polymorphism is also associated with volume differences in the human hippocampus and whether specific subfields are differentially affected by KIBRA genotype. High-resolution magnetic resonance imaging (T2-weighted, voxel size=0.4×0.4 mm, in-plane) was used to manually segment hippocampal cornu ammonis (CA) subfields, dentate gyrus (DG), and the subiculum as well as adjacent medial temporal lobe cortices in healthy carriers and noncarriers of the KIBRA T-allele (rs17070145). Overall, we found that T-carriers had a larger hippocampal volume relative to noncarriers. The structural differences observed were specific to the CA fields and DG regions of the hippocampus, suggesting a potential neural mechanism for the effects of KIBRA on episodic memory performance reported previously.

The role of memory-related gene WWC1 (KIBRA) in lifetime posttraumatic stress disorder: evidence from two independent samples from African conflict regions

BACKGROUND

Posttraumatic stress disorder (PTSD) results from the formation of a strong memory for the sensory-perceptual and affective representations of traumatic experiences, which is detached from the corresponding autobiographical context information. Because WWC1, the gene encoding protein KIBRA, is associated with long-term memory performance, we hypothesized that common WWC1 alleles influence the risk for a lifetime diagnosis of PTSD.

METHODS

Traumatic load and diagnosis of current and lifetime PTSD were assessed in two independent African samples of survivors from conflict zones who had faced severe trauma (n = 392, Rwanda, and n = 399, Northern Uganda, respectively). Array-based single nucleotide polymorphism (SNP) genotyping was performed. The influence of WWC1 tagging SNPs and traumatic load on lifetime PTSD was estimated by means of logistic regression models with correction for multiple comparisons in the Rwandan sample. Replication analysis was performed in the independent Ugandan sample.

RESULTS

An association of two neighboring SNPs in almost complete linkage disequilibrium, rs10038727 and rs4576167, with lifetime PTSD was discovered in the Rwandan sample. Although each traumatic event added to the probability of lifetime PTSD in a dose-dependent manner in both genotype groups, carriers of the minor allele of both SNPs displayed a diminished risk (p = .007, odds ratio = .29 [95% confidence interval = .15-.54]). This effect was confirmed in the independent Ugandan sample.

CONCLUSIONS

This study reveals an association between two WWC1 SNPs and the likelihood of PTSD development, indicating that this memory-related gene might be involved in processes that occur in response to traumatic stress and influence the strengthening of fear memories.

Common exonic missense variants in the C2 domain of the human KIBRA protein modify lipid binding and cognitive performance

The human KIBRA gene has been linked to human cognition through a lead intronic single-nucleotide polymorphism (SNP; rs17070145) that is associated with episodic memory performance and the risk to develop Alzheimer's disease. However, it remains unknown how this relates to the function of the KIBRA protein. Here, we identified two common missense SNPs (rs3822660G/T [M734I], rs3822659T/G [S735A]) in exon 15 of the human KIBRA gene to affect cognitive performance, and to be in almost complete linkage disequilibrium with rs17070145. The identified SNPs encode variants of the KIBRA C2 domain with distinct Ca(2+) dependent binding preferences for monophosphorylated phosphatidylinositols likely due to differences in the dynamics and folding of the lipid-binding pocket. Our results further implicate the KIBRA protein in higher brain function and provide direction to the cellular pathways involved.

KIBRA gene variants are associated with synchronization within the default-mode and executive control networks

Genetic variation at the KIBRA rs17070145 polymorphism has been linked to episodic memory, executive function, and Alzheimer's disease (AD), which are related to the structural and functional integrity of the default-mode network (DMN) and executive control network (ECN). We hypothesize that the KIBRA polymorphism could modulate the structure and function of the DMN and ECN in healthy young subjects, which might underlie the association between this gene and cognitive function. To test our hypothesis, we analyzed the resting-state synchronization in the DMN and ECN in 288 young, healthy Chinese Han subjects. We found that carriers of the KIBRA C-allele demonstrated an increased synchronization in the posterior cingulate cortex (PCC) and medial prefrontal cortex (MPFC) of the DMN and in the right anterior insula, bilateral caudate nuclei, and bilateral dorsal anterior cingulate cortices (dACC) of the ECN compared to individuals with a TT genotype. Moreover, KIBRA C-allele carriers also showed a smaller gray matter volume (GMV) in the MPFC and bilateral dACCs than TT individuals. In contrast, there were no significant genotype differences in the synchronization of either the visual network or the sensorimotor network. These findings suggest that the polymorphism in the KIBRA gene affects GMV and the function of the DMN and ECN. This increased synchronization is likely a reflection of compensation for the regional gray matter deficits in these networks in young healthy subjects. The association between KIBRA polymorphisms and the DMN and ECN should be further explored in a healthy older population and in patients with AD.

Association of KIBRA with episodic and working memory: a meta-analysis

WWC1 was first implicated in human cognition through a genome wide association study in 2006 that reported an association of the intronic single nucleotide polymorphism (SNP) rs17070145 with episodic memory performance. WWC1 encodes the protein KIBRA, which is almost ubiquitously expressed. Together with its binding partners, KIBRA is assumed to play a role in synaptic plasticity. T-allele carriers of SNP rs17070145 have been reported to outperform individuals that are homozygous for the C-allele in episodic memory tasks. Here we report two random effects meta-analyses testing the association of rs17070145 with episodic and working memory. All currently available population-based association studies that investigated effects of rs17070145 on episodic or working memory were included in the analyses. Where performance measures for multiple domain-specific tasks were available for a given study population, averaged effect size estimates were calculated. The performed meta-analyses relied on 17 samples that were tested for episodic memory performance (N = 8,909) and 9 samples that had performed working memory tasks (N = 4,696). We report a significant association of rs17070145 with both episodic (r = 0.068, P = 0.001) and working memory (r = 0.035, P = 0.018). In summary, our findings indicate that SNP rs17070145 located within KIBRA explains 0.5% of the variance for episodic memory tasks and 0.1% of the variance for working memory tasks in samples of primarily Caucasian background.

Interaction of BDNF and COMT polymorphisms on paired-associative stimulation-induced cortical plasticity

The common single-nucleotide polymorphism (SNP) brain-derived neurotrophic factor (BDNF) valine-to-methionine substitution at codon 66 (Val66Met) has been associated with differences in memory functions and cortical plasticity following brain stimulation. Other studies could not confirm these results, though, and potential interactions of BDNF carrier status with other learning-relevant SNPs are largely unknown. The present study aimed to evaluate the effects of BDNF Val66Met genotype on paired associative stimulation (PAS)-induced motor cortex plasticity, while additionally taking catechol-O-methyltransferase (COMT) Val158Met and kidney and brain (KIBRA) rs17070145 carrier status into account. Therefore, a cohort of 2 × 16 age- and education-matched healthy young females underwent transcranial magnetic stimulation using an excitatory PAS(25) protocol to induce cortical plasticity. Cognitive performance was assessed using implicit grammar- and motor-learning tasks and a detailed neuropsychological test battery. While BDNF carrier status alone did not significantly influence PAS-induced cortical plasticity, we found a significant BDNF × COMT interaction, showing higher plasticity immediately following the PAS(25) protocol for the BDNF Val/Val vs Met genotype in COMT Met homozygotes only (ANOVA, p = 0.027). A similar advantage for this group was noted for implicit grammar learning (ANOVA, p = 0.021). Accounting for KIBRA rs17070145 did not explain significant variance. Our findings for the first time demonstrate an interaction of BDNF by COMT on human cortical plasticity. Moreover, they show that genotype-related differences in neurophysiology translate into behavioral differences. These findings might contribute to a better understanding of the mechanisms of interindividual differences in cognition.