Interactive effects of APOE and CHRNA4 on attention and white matter volume in healthy middle-aged and older adults

Increased sensitivity to effects of normal aging and Alzheimer's disease on cortical thickness by adjustment for local variability in gray/white contrast: a multi-sample MRI study

MRI-based estimates of cerebral morphometric properties, e.g. cortical thickness, are pivotal to studies of normal and pathological brain changes. These measures are based on automated or manual segmentation procedures, which utilize the tissue contrast between gray and white matter on T(1)-weighted MR images. Tissue contrast is unlikely to remain a constant property across groups of different age and health. An important question is therefore how the sensitivity of cortical thickness estimates is influenced by variability in WM/GM contrast. The effect of adjusting for variability in WM/GM contrast on age sensitivity of cortical thickness was tested in 1189 healthy subjects from six different samples, enabling evaluation of consistency of effects within and between sites and scanners. Further, the influence of Alzheimer's disease (AD) diagnosis on cortical thickness with and without correction for contrast was tested in an additional sample of 96 patients. In healthy controls, regional increases in the sensitivity of the cortical thickness measure to age were found after correcting for contrast. Across samples, the strongest effects were observed in frontal, lateral temporal and parietal areas. Controlling for contrast variability also increased the cortical thickness estimates' sensitivity to AD, thus replicating the finding in an independent clinical sample. The results showed increased sensitivity of cortical estimates to AD in areas earlier reported to be compromised in AD, including medial temporal, inferior and superior parietal regions. In sum, the findings indicate that adjusting for contrast can increase the sensitivity of MR morphometry to variables of interest.

Genetic and vascular modifiers of age-sensitive cognitive skills: effects of COMT, BDNF, ApoE, and hypertension

Several single nucleotide polymorphisms have been linked to neural and cognitive variation in healthy adults. We examined contribution of three polymorphisms frequently associated with individual differences in cognition (Catechol-O-Methyl-Transferase Val158Met, Brain-Derived-Neurotrophic-Factor Val66Met, and Apolipoprotein E epsilon4) and a vascular risk factor (hypertension) in a sample of 189 volunteers (age 18-82). Genotypes were determined from buccal culture samples, and cognitive performance was assessed in 4 age-sensitive domains?fluid intelligence, executive function (inhibition), associative memory, and processing speed. We found that younger age and COMT Met/Met genotype, associated with low COMT activity and higher prefrontal dopamine content, were independently linked to better performance in most of the tested domains. Homozygotes for Val allele of BDNF polymorphism exhibited better associative memory and faster speed of processing than the Met allele carriers, with greater effect for women and persons with hypertension. Carriers of ApoE epsilon4 allele evidenced steeper age-related increase in costs of Stroop color interference, but showed no negative effects on memory. The findings indicate that age-related cognitive performance is differentially affected by distinct genetic factors and their interactions with vascular health status.

The influence of apolipoprotein E genotype on visuospatial attention dissipates after age 80

Although it is established that apolipoprotein E (APOE) e4 allele increases the risk of Alzheimer's disease (AD), epidemiological studies indicate that genetic risk decreases late in life. This raises the question of whether the effects of APOE on cognition that are seen in midlife arise from a cognitive phenotype of APOE or from the presence of early AD in some APOE-e4 carriers. The authors addressed this question by comparing the cognitive consequences of variation in the APOE gene between individuals over the age of 80 (old-old) and middle-aged and young-old individuals. A spatially cued discrimination paradigm--previously shown to be sensitive to AD and to APOE genotype--required a speeded categorization of a target letter following cues that were valid, invalid, or neutral in predicting target location. Results revealed greater costs of invalid cues in the APOE-e4 carriers of middle-aged and young-old, but not old-old, groups. The dissipation of the APOE effect in old-old individuals at lower risk of AD suggests that visuospatial attention impairments seen as early as midlife in APOE-e4 carriers may be a preclinical marker of AD.

Using genetic data in cognitive neuroscience: from growing pains to genuine insights

Research that combines genetic and cognitive neuroscience data aims to elucidate the mechanisms that underlie human behaviour and experience by way of 'intermediate phenotypes': variations in brain function. Using neuroimaging and other methods, this approach is poised to make the transition from health-focused investigations to inquiries into cognitive, affective and social functions, including ones that do not readily lend themselves to animal models. The growing pains of this emerging field are evident, yet there are also reasons for a measured optimism.

Synergistic effects of genetic variation in nicotinic and muscarinic receptors on visual attention but not working memory

It is widely appreciated that neurotransmission systems interact in their effects on human cognition, but those interactions have been little studied. We used genetics to investigate pharmacological evidence of synergisms in nicotinic/muscarinic interactions on cognition. We hypothesized that joint influences of nicotinic and muscarinic systems would be reflected in cognitive effects of normal variation in known SNPs in nicotinic (CHRNA4 rs1044396) and muscarinic (CHRM2 rs8191992) receptor genes. Exp. 1 used a task of cued visual search. The slope of the cue size/reaction time function showed a trend level effect of the muscarinic CHRM2 SNP, no effect of the nicotinic CHRNA4 SNP, but a significant interaction between the 2 SNPs. Slopes were steepest in individuals who were both CHRNA4 C/C and CHRM2 T/T homozygotes. To determine the specificity of this synergism, Exp. 2 assessed working memory for 1-3 locations over 3 s and found no significant effects on either SNP. Interpreting these results in light of Sarter's [Briand LA, et al. (2007) Modulators in concert for cognition: Modulator interactions in the prefrontal cortex. Prog Neurobiol 83:69-91] claims of tonic and phasic modes of cholinergic activity, we argue that reorienting attention to the target after invalid cues requires a phasic response, dependent on the nicotinic system, whereas orienting attention to valid cues requires a tonic response, dependent on the muscarinic system. Consistent with that, shifting and scaling after valid cues (tonic) were strongest in CHRNA4 C/C homozygotes who were also CHRM2 T/T homozygotes. This shows synergistic effects within the human cholinergic system.

Auditory hallucinations in schizophrenia: the role of cognitive, brain structural and genetic disturbances in the left temporal lobe

In this article we review research in our laboratory on auditory hallucinations using behavioral and MRI measure. The review consists of both previously published and new data that for the first time is presented together in a cohesive way. Auditory hallucinations are among the most common symptoms in schizophrenia, affecting more than 70% of the patients. We here advance the hypothesis that auditory hallucinations are internally generated speech perceptions that are lateralized to the left temporal lobe, in the peri-Sylvian region. From this we predict that hallucinating patients should have problems identifying a simultaneously presented external speech sound, as measured through performance on the dichotic listening (DL) paradigm with consonant-vowel syllables, since this technique lateralizes the stimulus input. Across a series of behavioral experiments, we have shown that patients with schizophrenia who experience frequent auditory hallucinations fail to demonstrate an expected right ear advantage on the dichotic listening test. Absence of a right ear advantage is indicative of a functional deficit in the left peri-Sylvian region. The results also revealed that patients with ongoing auditory hallucinations were more impaired than patients with previous hallucinations, and that a higher score on the hallucination item in a standard symptom rating scale (BPRS) correlated negatively with number of correct reports for the right ear stimulus. Moreover, we have found that schizophrenia patients fail to shift attention to the left ear stimulus, when explicitly instructed to focus on the right or left ear stimulus only, thus showing a deficit in inhibition of attention and response-inhibition. The behavioral DL data are substantiated in two MR morphometry studies that revealed significant reductions in grey matter density in the left peri-Sylvian region in hallucinating patients, and patients with reduced left temporal lobe grey matter density. Hallucinating patients also failed to show a right ear advantage in the dichotic listening test. Ongoing fMRI studies are focused on the underlying synaptic and molecular mechanisms by investigating the effects of the glutamate antagonist drug memantine on auditory perception and speech lateralization, and examination of temporal cortex-specific gene expression in the left peri-Sylvian region.

Neuroimaging and APOE genotype: a systematic qualitative review

Apolipoprotein E (APOE) is the major genetic risk factor for late-onset Alzheimer's disease (AD) and has also been implicated in cardiovascular disease, cognitive decline and cognitive changes in healthy ageing. The aim of this paper is to systematically review and critically assess the association between the APOE genotype and structural/functional cerebral changes as evidenced by brain imaging studies. A second aim is to determine whether these observed associations between APOE and the brain reflect changes which are consistent with the progression of AD neurodegenerative changes described in Braak stages. A search of Pubmed, Psycinfo, and Web of Science databases identified 64 articles available for qualitative review. The review found that presence of the APOE epsilon4 allele is associated with (1) hippocampal, amygdalar and entorhinal cortex atrophy, (2) increased brain atrophy, (3) increased white matter hyperintensity volumes and (4) altered cerebral blood flow and glucose metabolism patterns. It is possible that there are critical age ranges when these effects are evident and that the APOE epsilon2 genotype might present a risk. We conclude that structural brain change is associated with the APOE genotype and that it is more salient in younger ageing individuals.

APOE status and its association to learning and memory performance in middle aged and older Norwegians seeking assessment for memory deficits

BACKGROUND

We examined the hypothesis that deficits in learning, memory, and other cognitive functions are associated with the epsilon4 allele of the Apolipoprotein E (APOE) gene in a non-demented sample with memory complaints recruited from a population with a high prevalence of this allele.

METHODS

The study group comprised 70 consecutively referred patients aged 50-75 seeking assessment due to memory complaints. They were screened for dementia, for neurological and psychiatric disease, and for cerebral infarction using Magnet Resonance Imaging (MRI). Participants were classified as non-demented based on clinical evaluation and results on cognitive tests.

RESULTS

APOE epsilon4 carriers (56% of the sample) showed poorer performance than non-carriers on the Mini Mental State Examination, a number of measures of verbal memory function from the California Verbal Learning Test, and visual recall. In 46% of the participants, psychometric criteria for amnestic Mild Cognitive Impairment (aMCI) were satisfied.

CONCLUSION

Findings may be partly explained by a significant number of participants being in a preclinical phase of Alzheimer's disease. The observed deficits in learning performance and the lack of significant age modulation of the genetic association suggest a more general genetic effect. The findings are consistent with known neurobiological function of APOE epsilon4, including both increased risk of neurodegenerative disease and reduced synaptic integrity in older age.

Nicotine receptor gene CHRNA4 modulates early event-related potentials in auditory and visual oddball target detection tasks

The present study seeks to identify effects of a common genetic polymorphism in the human nicotinic alpha4beta2 receptor on components of the cognitive event-related potentials in auditory and visual modalities. The same sense thymine-to-cytosine polymorphism (c.1629T-C; Ser543Ser) was shown to preferentially modulate early components in both modalities. Specifically, the auditory N1 component amplitude was higher for T allele homozygotes than for C allele carriers. The visual P1 component revealed the same pattern of significant polymorphic modulation, but the later N1 amplitude differences were only marginally significant. There was no reliable indication of interactions between genotype and task factors. Parallel modulation of early latency modality-specific event-related potential (ERP) components in vision and audition may indicate that the CHRNA4 polymorphism affects factors that are common to top-down modulation of sensory processing across modalities.

Effects of APOE and CHRNA4 genotypes on retinal nerve fibre layer thickness at the optic disc and on risk for developing exfoliation syndrome

PURPOSE

To investigate whether CHRNA4 and APOE genotypes influence retinal nerve fibre layer (RNFL) thickness at the optic disc, intraocular pressure (IOP) and the development of exfoliation syndrome (XFS).

METHODS

A sample of 88 healthy adults (aged 50-75 years) genotyped for polymorphisms of APOE and CHRNA4 underwent an eye examination including slit-lamp examination and fundus photography, as well as measurements of visual acuity, refraction, IOP and RNFL thickness at the optic disc by optical coherence tomography. The Fisher-Boschloo unconditional full multinomial test and two sample t-tests were used in the statistical analyses.

RESULTS

There was no correlation between CHRNA4 and APOE genotypes and average RNFL thickness at the optic disc in the two eyes. Mean IOP in the eyes of APOE2 carriers was 13.18 mmHg, whereas that in the eyes of non-APOE2 carriers, at 14.82 mmHg, was significantly higher (p = 0.014). Exfoliation syndrome was found in one or both eyes in 15 persons. The presence of XFS was less likely in CC carriers of the CHRNA4 gene than in TT and TC carriers (p = 0.049).

CONCLUSIONS

We found no significant difference in RNFL thickness at the optic disc in the different genotype carriers of the APOE and CHRNA4 genes, and thereby no evidence for increased loss of ganglion cells in the retina as an effect of these genes. APOE2 carriers had significantly lower IOP than non-APOE2 carriers. However, both values are within the normal range and RNFL thickness measurements at the optic disc showed no difference between these two groups. In our study population, CC carriers of the CHRNA4 gene were less likely to develop XFS.

Microbial genome dynamics in CNS pathogenesis

The balancing act between microbes and their host in commensal and disease states needs to be deciphered in order to fully treat and combat infectious diseases. The elucidation of microbial genome dynamics in each instance is therefore required. In this context, the major bacterial meningitis pathogens are Neisseria meningitidis, Haemophilus influenzae and Streptococcus pneumoniae. In prokaryotic CNS pathogenesis both the intact organism as well as its released components can elicit disease, often resulting in neurological sequelae, neurodegeneration or fatal outcome. The study of microbial virulence in CNS disease is expected to generate findings that yield new information on the general mechanisms of brain edema and excitatory neuronal disturbances due to meningitis, with significant potential for discoveries that can directly influence and inspire new strategies for prevention and treatment of this serious disease.

Accelerated age-related cortical thinning in healthy carriers of apolipoprotein E epsilon 4

Effects of APOE genotype on age-related slopes of cortical thinning was estimated by measuring the thickness of the cerebral cortex on a point-by-point basis across the cortical mantle in 96 healthy non-demented volunteers aged 48-75 years. Fifty nine were APOE epsilon 4- (no epsilon 4 allele) and 37 were epsilon 4+ (1 or 2 epsilon 4 alleles). The genotype groups had similar age, sex and IQ. Two T(1)-weighted MP-RAGE sequences were averaged for each participant to yield images with high signal-to-noise ratio, and quantified using semi-automated analysis tools. epsilon 4 carriers had thicker cortex than non-carriers in several frontal and temporal areas in both hemispheres, but showed a steeper age-related decline in adjacent areas. Upon comparison of the epsilon 4-specific age-related thinning with previously published patterns of thinning in normal aging and Alzheimer's disease (AD), we conclude that APOE epsilon 4 may function to accelerate thinning in areas found to decline in aging (medial prefrontal and pericentral cortex), but also to initiate thinning in areas associated with AD and amyloid-beta aggregation (occipitotemporal and basal temporal cortex).

Genes, brain, and behavior: bridging disciplines

With excitement surrounding the publication of the human genome, scientists have set out to uncover the functions of specific genes. This special issue on Genes, Brain, and Behavior attempts to present research strategies that connect major avenues of genetic research across disciplines. For example, anatomical information provided by brain imaging can serve as a convenient link between anatomical abnormalities seen in knockout/transgenic mouse models and abnormal patterns of brain activity seen in certain patient populations. Identifying genetic risk factors for disorders with carefully designed cognitive assays is another strategy that has gained increasing attention. These approaches are being combined with behavioral studies of mouse models of gene function. Alone, each of these approaches provides limited information on gene function in complex human behavior, but together, they are forming bridges between animal models and human psychiatric disorders.