Investigating cognitive genetics and its implications for the treatment of cognitive deficit

Brain-Derived Neurotrophic Factor: A Key Molecule for Memory in the Healthy and the Pathological Brain

Brain Derived Neurotrophic Factor (BDNF) is a key molecule involved in plastic changes related to learning and memory. The expression of BDNF is highly regulated, and can lead to great variability in BDNF levels in healthy subjects. Changes in BDNF expression are associated with both normal and pathological aging and also psychiatric disease, in particular in structures important for memory processes such as the hippocampus and parahippocampal areas. Some interventions like exercise or antidepressant administration enhance the expression of BDNF in normal and pathological conditions. In this review, we will describe studies from rodents and humans to bring together research on how BDNF expression is regulated, how this expression changes in the pathological brain and also exciting work on how interventions known to enhance this neurotrophin could have clinical relevance. We propose that, although BDNF may not be a valid biomarker for neurodegenerative/neuropsychiatric diseases because of its disregulation common to many pathological conditions, it could be thought of as a marker that specifically relates to the occurrence and/or progression of the mnemonic symptoms that are common to many pathological conditions.

DJ1 expression downregulates in neuroblastoma cells (SK-N-MC) chronically exposed to HIV-1 and cocaine

Background: HIV-associated neurological disorder (HAND) has long been recognized as a consequence of human immunodeficiency virus (HIV) infection in the brain. The pathology of HAND gets more complicated with the recreational drug use such as cocaine. Recent studies have suggested multiple genetic influences involved in the pathology of addiction and HAND but only a fraction of the entire genetic risk has been investigated so far. In this regard, role of DJ1 protein (a gene linked to autosomal recessive early-onset Parkinson’s disease) in regulating dopamine (DA) transmission and reactive oxygen species (ROS) production in neuronal cells will be worth investigating in HIV-1 and cocaine exposed microenvironment. Being a very abundant protein in the brain, DJ1 could serve as a potential marker for early detection of HIV-1 and/or cocaine related neurological disorder.

Methods:In vitro analysis was done to observe the effect of HIV-1 and/or cocaine on DJ1 protein expression in neuroblastoma cells (SK-N-MC). Gene and protein expression analysis of DJ1 was done on the HIV infected and/or cocaine treated SK-N-MC and compared to untreated cells using real time PCR, Western Blot and flow cytometry. Effect of DJ1 dysregulation on oxidative stress was analyzed by measuring ROS production in these cells.

Results: Gene expression and protein analysis indicated that there was a significant decrease in DJ1 expression in SK-N-MC chronically exposed to HIV-1 and/or cocaine which is inversely proportional to ROS production.

Conclusion: This is the first study to establish that DJ1 expression level in the neuronal cells significantly decreased in presence of HIV-1 and/or cocaine indicating oxidative stress level of DA neurons.

Results of a "GWAS plus:" general cognitive ability is substantially heritable and massively polygenic

We carried out a genome-wide association study (GWAS) for general cognitive ability (GCA) plus three other analyses of GWAS data that aggregate the effects of multiple single-nucleotide polymorphisms (SNPs) in various ways. Our multigenerational sample comprised 7,100 Caucasian participants, drawn from two longitudinal family studies, who had been assessed with an age-appropriate IQ test and had provided DNA samples passing quality screens. We conducted the GWAS across ∼2.5 million SNPs (both typed and imputed), using a generalized least-squares method appropriate for the different family structures present in our sample, and subsequently conducted gene-based association tests. We also conducted polygenic prediction analyses under five-fold cross-validation, using two different schemes of weighting SNPs. Using parametric bootstrapping, we assessed the performance of this prediction procedure under the null. Finally, we estimated the proportion of variance attributable to all genotyped SNPs as random effects with software GCTA. The study is limited chiefly by its power to detect realistic single-SNP or single-gene effects, none of which reached genome-wide significance, though some genomic inflation was evident from the GWAS. Unit SNP weights performed about as well as least-squares regression weights under cross-validation, but the performance of both increased as more SNPs were included in calculating the polygenic score. Estimates from GCTA were 35% of phenotypic variance at the recommended biological-relatedness ceiling. Taken together, our results concur with other recent studies: they support a substantial heritability of GCA, arising from a very large number of causal SNPs, each of very small effect. We place our study in the context of the literature–both contemporary and historical–and provide accessible explication of our statistical methods.

BDNF val66met polymorphism affects aging of multiple types of memory

The BDNF val66met polymorphism (rs6265) influences activity-dependent secretion of brain-derived neurotrophic factor in the synapse, which is crucial for learning and memory. Individuals homozygous or heterozygous for the met allele have lower BDNF secretion than val homozygotes and may be at risk for reduced declarative memory performance, but it remains unclear which types of declarative memory may be affected and how aging of memory across the lifespan is impacted by the BDNF val66met polymorphism. This cross-sectional study investigated the effects of BDNF polymorphism on multiple indices of memory (item, associative, prospective, subjective complaints) in a lifespan sample of 116 healthy adults aged 20-93 years. Advancing age showed a negative effect on item, associative and prospective memory, but not on subjective memory complaints. For item and prospective memory, there were significant age×BDNF group interactions, indicating the adverse effect of age on memory performance across the lifespan was much stronger in the BDNF met carriers than for the val homozygotes. BDNF met carriers also endorsed significantly greater subjective memory complaints, regardless of age, and showed a trend (p<.07) toward poorer associative memory performance compared to val homozygotes. These results suggest that genetic predisposition to the availability of brain-derived neurotrophic factor, by way of the BDNF val66met polymorphism, exerts an influence on multiple indices of episodic memory - in some cases in all individuals regardless of age (subjective memory and perhaps associative memory), in others as an exacerbation of age-related differences in memory across the lifespan (item and prospective memory). This article is part of a Special Issue entitled Memory & Aging.

APOE and COMT polymorphisms are complementary biomarkers of status, stability, and transitions in normal aging and early mild cognitive impairment

Objective: Research has reported associations among selected genetic susceptibility biomarkers and risk of (a) normal cognitive aging decrements, (b) established mild cognitive impairment (MCI), and (c) sporadic Alzheimer's disease (AD). In focusing on the transitional normal-to-early MCI phase, we examine associations among three theoretically relevant polymorphisms (APOE [rs429358, rs7412], BDNF [rs6265], COMT [rs4680]) and both baseline cognitive status (MCI vs. normal aging) and two-wave (four-year) longitudinal stability or change profiles. The latter included three profiles: (a) stable as normal aging, (b) stable or chronic impairment (MCI-to-MCI), and (c) emergence of impairment (normal-to-MCI).

Method: Genotyped older adults (n = 237 at baseline; age range = 64–91; 62% women) from the Victoria Longitudinal Study were examined for (a) independent and interactive associations of three genetic polymorphisms with (b) two objectively classified cognitive status groups (not-impaired controls (NIC) and MCI) at (c) both baseline and across a two-wave (four-year) longitudinal interval.

Results: First, logistic regression revealed that the presence of at least one APOE ε4 allele (the risk factor for AD) was linked to greater baseline risk of objective MCI. Second, multinomial logistic regression revealed that (a) the presence of an APOE ε4 allele was associated with an increased risk of 4-year MCI status stability (chronicity), and (b) the COMT homozygous risk genotype (G/G or Val/Val) was associated with an increased risk of both MCI-to-MCI stability (chronicity) and emerging NIC-to-MCI conversion.

Discussion: Both chronicity and emergence of objectively classified early cognitive impairment may be genetically heterogeneous phenomena, with influences from a panel of both normal cognitive aging (COMT) and AD-related (APOE) polymorphisms.

The hunt for gene effects pertinent to behavioral traits and psychiatric disorders: from mouse to human

The field of behavioral genetics was reviewed in the classic 1960 text by Fuller and Thompson. Since then, there has been remarkable progress in the genetic analysis of animal behavior. Many molecular genetic methods in common use today were not even anticipated in 1960. Animal models for many human psychiatric disorders have been discovered or created. In human behavior genetics, however, powerful new methods have failed to reveal even one bona fide, replicable gene effect pertinent to the normal range of variation in intelligence and personality. There is no explanatory or predictive value in that genetic information. For several psychiatric disorders, including autism and schizophrenia, many large genetic effects arise from de novo mutations. Genetically, the disorders are heterogeneous; different cases with the same diagnosis have different causes. The promises of the molecular genetic revolution have not been fulfilled in behavioral domains of most interest to human psychology.

KIBRA: A New Gateway to Learning and Memory?

The genetic locus encoding KIBRA, a member of the WWC family of proteins, has recently been shown to be associated with human memory performance through genome-wide single nucleotide polymorphism screening. Gene expression analysis and a variety of functional studies have further indicated that such a role is biologically plausible for KIBRA. Here, we review the existing literature, illustrate connections between the different lines of evidence, and derive models based on KIBRA's function(s) in the brain that can be further tested experimentally.

A bivariate twin study of regional brain volumes and verbal and nonverbal intellectual skills during childhood and adolescence

Twin studies indicate that both intelligence and brain structure are moderately to highly heritable. Recent bivariate studies of adult twins also suggest that intelligence and brain morphometry are influenced by shared genetic factors. The current study examines shared genetic and environmental factors between brain morphometry and intelligence in a sample of children and adolescents (twins, twin siblings, and singletons; n = 649, ages 4-19). To extend previous studies, brain morphometric data were parsed into subregions (lobar gray/white matter volumes, caudate nucleus, lateral ventricles) and intelligence into verbal and nonverbal skills (Wechsler Vocabulary and Block Design subtests). Phenotypic relationships between brain volumes and intelligence were small. Verbal skills shared unique environmental effects with gray matter volumes while nonverbal skills shared genetic effects with both global and regional gray and white matter. These results suggest that distinct mechanisms contribute to the small phenotypic relationships between brain volumes and verbal versus nonverbal intelligence.

No association between Cholinergic Muscarinic Receptor 2 (CHRM2) genetic variation and cognitive abilities in three independent samples

Cognitive ability has a substantial genetic component and more than 15 candidate genes have been identified over the past 8 years. One gene that has been associated with general cognitive ability is the cholinergic muscarinic 2 receptor (CHRM2). In an attempt to replicate this finding we typed marker rs8191992 (the originally reported CHRM2 SNP) in two population based cohorts-one Scottish aged over 50 years (N = 2,091) and the other English comprising non-demented elderly participants (N = 758)-and a family-based Australian adolescent sample (N = 1,537). An additional 29 SNPs in CHRM2 were typed in the Australian sample and a further seven in the English cohort. No significant association was found between CHRM2 and diverse measures of cognitive ability in any of the samples. In conclusion, this study does not support a role for CHRM2 in cognitive ability.

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.

Refining psychiatric genetics: from 'mouse psychiatry' to understanding complex human disorders

Investigating the pathogenesis of psychiatric disorders is a complicated and rigorous task for psychiatric geneticists, as the disorders often involve combinations of genetic, behavioral, personality, and environmental factors. To nurture further progress in this field, a new set of conceptual tools is needed in addition to the currently accepted approaches. Concepts that consider cross-species trait genetics and the interplay between the domains of disorders, as well as the full spectrum of potential symptoms and their place along the pathogenetic continuum, are particularly important to address these needs. Here, we outline recent concepts and approaches that can help refine the field and enable more precise dissection of the genetic mechanisms contributing to psychiatric disorders.

The importance of cognitive phenotypes in experimental modeling of animal anxiety and depression

Cognitive dysfunctions are commonly seen in many stress-related disorders, including anxiety and depression—the world's most common neuropsychiatric illnesses. Various genetic, pharmacological, and behavioral animal models have long been used to establish animal anxiety-like and depression-like phenotypes, as well as to assess their memory, learning, and other cognitive functions. Mounting clinical and animal evidences strongly supports the notion that disturbed cognitions represent an important pathogenetic factor in anxiety and depression, and may also play a role in integrating the two disorders within a common stress-precipitated developmental pathway. This paper evaluates why and how the assessment of cognitive and emotional domains may improve our understanding of animal behaviors via different high-throughput tests and enable a better translation of animal phenotypes into human brain disorders.

Genome-wide quantitative trait locus association scan of general cognitive ability using pooled DNA and 500K single nucleotide polymorphism microarrays

General cognitive ability (g), which refers to what cognitive abilities have in common, is an important target for molecular genetic research because multivariate quantitative genetic analyses have shown that the same set of genes affects diverse cognitive abilities as well as learning disabilities. In this first autosomal genome-wide association scan of g, we used a two-stage quantitative trait locus (QTL) design with pooled DNA to screen more than 500 000 single nucleotide polymorphisms (SNPs) on microarrays, selecting from a sample of 7000 7-year-old children. In stage 1, we screened for allele frequency differences between groups pooled for low and high g. In stage 2, 47 SNPs nominated in stage 1 were tested by individually genotyping an independent sample of 3195 individuals, representative of the entire distribution of g scores in the full 7000 7-year-old children. Six SNPs yielded significant associations across the normal distribution of g, although only one SNP remained significant after a false discovery rate of 0.05 was imposed. However, none of these SNPs accounted for more than 0.4% of the variance of g, despite 95% power to detect associations of that size. It is likely that QTL effect sizes, even for highly heritable traits such as cognitive abilities and disabilities, are much smaller than previously assumed. Nonetheless, an aggregated ‘SNP set’ of the six SNPs correlated 0.11 (P < 0.00000003) with g. This shows that future SNP sets that will incorporate many more SNPs could be useful for predicting genetic risk and for investigating functional systems of effects from genes to brain to behavior.

The Lothian Birth Cohort 1936: a study to examine influences on cognitive ageing from age 11 to age 70 and beyond

BACKGROUND

Cognitive ageing is a major burden for society and a major influence in lowering people's independence and quality of life. It is the most feared aspect of ageing. There are large individual differences in age-related cognitive changes. Seeking the determinants of cognitive ageing is a research priority. A limitation of many studies is the lack of a sufficiently long period between cognitive assessments to examine determinants. Here, the aim is to examine influences on cognitive ageing between childhood and old age.

METHODS/DESIGN

The study is designed as a follow-up cohort study. The participants comprise surviving members of the Scottish Mental Survey of 1947 (SMS1947; N = 70,805) who reside in the Edinburgh area (Lothian) of Scotland. The SMS1947 applied a valid test of general intelligence to all children born in 1936 and attending Scottish schools in June 1947. A total of 1091 participants make up the Lothian Birth Cohort 1936. They undertook: a medical interview and examination; physical fitness testing; extensive cognitive testing (reasoning, memory, speed of information processing, and executive function); personality, quality of life and other psycho-social questionnaires; and a food frequency questionnaire. They have taken the same mental ability test (the Moray House Test No. 12) at age 11 and age 70. They provided blood samples for DNA extraction and testing and other biomarker analyses. Here we describe the background and aims of the study, the recruitment procedures and details of numbers tested, and the details of all examinations.

DISCUSSION

The principal strength of this cohort is the rarely captured phenotype of lifetime cognitive change. There is additional rich information to examine the determinants of individual differences in this lifetime cognitive change. This protocol report is important in alerting other researchers to the data available in the cohort.

Intelligence and the developing human brain

Determining the brain properties that make people 'brainier' has moved well beyond early demonstrations that increasing intelligence correlates with increasing grey and white matter volumes. Both structural and functional in vivo neuroimaging techniques delineate a distributed network of brain regions, perhaps with a focus in the lateral prefrontal cortex, which varies in extent and connectivity with individual differences in intelligence. Longitudinal studies further show that the neuroanatomic correlates of intelligence are dynamic, changing most rapidly in early childhood. Several promising candidate genes affecting neuronal development and neurotransmission have been proposed that might begin to explain the marked genetic overlap between cortical morphology and intelligence. A major future challenge is to determine the cellular events that underpin the neuroanatomic differences correlated with intelligence.

Cognitive performance in elderly women: significance of the 19bp insertion/deletion polymorphism in the 5' flank of the dopamine beta-hydroxylase gene, educational level, body fat measures, serum triglyceride, alcohol consumption and age

BACKGROUND

Genetic and environmental factors influence cognitive aging. The gene encoding dopamine beta-hydroxylase (DBH) could be one such factor since this hydroxylase converts dopamine to norepinephrine both of which are involved in cognition regulation.

OBJECTIVE

To assess the effect of the 19bp insertion/deletion polymorphism in the 5' flank of the DBH gene on cognitive performance in elderly women relative to other factors of cognitive aging.

METHODS

We examined a cross-sectional sample of 1371 postmenopausal women. Cognitive abilities were assessed by the 6-item orientation-memory-concentration test. The 19bp insertion/deletion polymorphism of the DBH gene was genotyped and apolipoprotein E (APOE) epsilon4 allele status was determined. In addition blood pressure, body fat mass and blood lipids were measured. Information was also obtained by personal interviews. Data were analyzed by regression analysis.

RESULTS

Cognition was univariately associated with DBH genotype (p = 0.04). A univariate association of borderline significance was observed for APOE epsilon4 allele status (p = 0.07). Exclusion of women with severe cognition impairment did not alter the strength of the association with the DBH gene polymorphism markedly (p = 0.06) but obliterated the weak association between APOE epsilon4 allele status and cognition. The association of the DBH gene polymorphism with cognition persisted after adjustment for other variables (p = 0.03).

CONCLUSIONS

The 19bp insertion/deletion polymorphism of the DBH gene influences cognition in elderly women and might have a stronger effect than APOE epsilon4 allele status on mild cognitive impairment. Both genetic polymorphisms had a significantly smaller impact on cognition than age, education, alcohol consumption and body fat measures.

Impaired cognition and attention in adults: pharmacological management strategies

Cognitive psychology has provided clinicians with specific tools for analyzing the processes of cognition (memory, language) and executive functions (attention-concentration, abstract reasoning, planning). Neuropsychology, coupled with the neurosciences (including neuroimaging techniques), has authenticated the existence of early disorders affecting the "superior or intellectual" functions of the human brain. The prevalence of cognitive and attention disorders is high in adults because all the diseases implicating the central nervous system are associated with cognitive correlates of variable intensity depending on the disease process and the age of the patient. In some pathologies, cognitive impairment can be a leading symptom such as in schizophrenia, posttraumatic stress disorder or an emblematic stigmata as in dementia including Alzheimer's disease. Paradoxically, public health authorities have only recognized as medications for improving cognitive symptoms those with proven efficacy in the symptomatic treatment of patients with Alzheimer's disease; the other cognitive impairments are relegated to the orphanage of syndromes and symptoms dispossessed of medication. The purpose of this review is to promote a true "pharmacology of cognition" based on the recent knowledge in neurosciences. Data from adult human beings, mainly concerning memory, language, and attention processes, will be reported. "Drug therapeutic strategies" for improving cognition (except for memory function) are currently rather scarce, but promising perspectives for a new neurobiological approach to cognitive pharmacology will be highlighted.

Neurobiology of memory and anxiety: from genes to behavior

Interaction of anxiety and memory represents an essential feature of CNS functioning. This paper reviews experimental data coming from neurogenetics, neurochemistry, and behavioral pharmacology (as well as parallel clinical findings) reflecting different mechanisms of memory-anxiety interplay, including brain neurochemistry, circuitry, pharmacology, neuroplasticity, genes, and gene-environment interactions. It emphasizes the complexity and nonlinearity of such interplay, illustrated by a survey of anxiety and learning/memory phenotypes in various genetically modified mouse models that exhibit either synergistic or reciprocal effects of the mutation on anxiety levels and memory performance. The paper also assesses the putative role of different neurotransmitter systems and neuropeptides in the regulation of memory processes and anxiety, and discusses the role of neural plasticity in these mechanisms.

Generalist genes and cognitive neuroscience

Multivariate genetic research suggests that a single set of genes affects most cognitive abilities and disabilities. This finding already has far-reaching implications for cognitive neuroscience, and will become even more revealing when this - presumably large - set of generalist genes is identified. Similar to other complex disorders and dimensions, molecular genetic research on cognitive abilities and disabilities is adopting genome-wide association strategies. These strategies involve very large samples to detect DNA associations of small effect size using microarrays that simultaneously assess hundreds of thousands of DNA markers. When this set of generalist genes is identified, it can be used to provide solid footholds in the climb towards a systems-level understanding of how genetically driven brain processes work together to affect diverse cognitive abilities and disabilities.