Strategies for integrated analysis in imaging genetics studies

Imaging Genetics (IG) integrates neuroimaging and genomic data from the same individual, deepening our knowledge of the biological mechanisms behind neurodevelopmental domains and neurological disorders. Although the literature on IG has exponentially grown over the past years, the majority of studies have mainly analyzed associations between candidate brain regions and individual genetic variants. However, this strategy is not designed to deal with the complexity of neurobiological mechanisms underlying behavioral and neurodevelopmental domains. Moreover, larger sample sizes and increased multidimensionality of this type of data represents a challenge for standardizing modeling procedures in IG research. This review provides a systematic update of the methods and strategies currently used in IG studies, and serves as an analytical framework for researchers working in this field. To complement the functionalities of the Neuroconductor framework, we also describe existing R packages that implement these methodologies. In addition, we present an overview of how these methodological approaches are applied in integrating neuroimaging and genetic data.

The 5-HTTLPR variant in the serotonin transporter gene modifies degeneration of brain regions important for emotion in behavioral variant frontotemporal dementia

The serotonin transporter length polymorphism (5-HTTLPR) short allele (5-HTTLPR-s) has been associated with differential susceptibility for anxiety and depression in multiple psychiatric disorders. 5-HTTLPR-s modifies the serotonergic systems that support emotion and behavioral regulation by reducing gene expression, which slows the reuptake of serotonin, and is associated with distinct morphological and functional effects. Serotonergic systems are also shown to be dysfunctional in behavioral variant frontotemporal dementia (bvFTD), a disease characterized by marked socioemotional dysfunction. However, studies of 5-HTTLPR-s effects in bvFTD have been inconsistent. Our objective was to investigate the patterns of gray matter volume by 5-HTTLPR-s genotype in both healthy older controls and bvFTD patients. We performed voxel-based morphometry of 179 cognitively normal older adults and 24 bvFTD cases to determine brain changes associated with dose (0/1/2) of 5-HTTLPR-s allele. 5-HTTLPR-s frequency did not differ between controls and bvFTD. We found a significant interaction effect whereby carrying more 5-HTTLPR-s alleles in bvFTD was associated with smaller volume in left inferior frontal gyrus (T = 4.86, P_FWE = 0.03) and larger volume in right temporal lobe (T = 5.01, P_FWE = 0.01). These results suggest that the 5-HTTLPR-s allele differentially influences brain morphology in bvFTD. We propose that patients with bvFTD and 5-HTTLPR-s have altered volumes in regions that support socioemotional behavior, which may be a developmental or disease-related compensation for altered serotonergic activity.

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5-HTTLPR-s correlates with greater right medial temporal lobe (R MTL) volume in FTD.

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5-HTTLPR-s correlates with lower left inferior frontal gyrus (L IFG) volume in FTD.

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R MTL and L IFG volumes are associated with neuropsychiatric symptom severity.

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5-HTTLPR-s effects on R MTL and L IFG volumes occur independently of disease severity.

Glia and immune cell signaling in bipolar disorder: insights from neuropharmacology and molecular imaging to clinical application

Bipolar disorder (BD) is a debilitating mental illness characterized by severe fluctuations in mood, sleep, energy and executive functioning. Pharmacological studies of selective serotonin reuptake inhibitors and the monoamine system have helped us to clinically understand bipolar depression. Mood stabilizers such as lithium and valproic acid, the first-line treatments for bipolar mania and depression, inhibit glycogen synthase kinase-3 beta (GSK-3β) and regulate the Wnt pathway. Recent investigations suggest that microglia, the resident immune cells of the brain, provide a physiological link between the serotonin system and the GSK-3β/Wnt pathway through neuroinflammation. We review the pharmacological, translational and brain imaging studies that support a role for microglia in regulating neurotransmitter synthesis and immune cell activation. These investigations provide a model for microglia involvement in the pathophysiology and phenotype of BD that may translate into improved therapies.

Enlarged cavum septum pellucidum as a neurodevelopmental marker in adolescent-onset opiate dependence

Objective

Adolescent-onset exposure to highly addictive substances such as opiates may induce far-reaching deleterious effects on later mental and physical health. However, little is known about the neurodevelopmental basis for adolescent-onset opiate dependence. Here we examined whether having an abnormally large cavum septum pellucidum (CSP), a putative marker of limbic structural maldevelopment, is associated with opiate dependence particularly beginning in adolescence.

Method

The overall length of the CSP and the prevalence of abnormal enlargement of the CSP were assessed and compared in 65 opiate-dependent subjects (41 adolescent-onset opiate users and 24 adult-onset opiate users) and 67 healthy subjects.

Results

Opiate-dependent subjects showed a greater prevalence of abnormal CSP enlargement relative to healthy subjects (odds ratio [OR]=3.64, p=0.034). The overall CSP length of adolescent-onset opiate-dependent subjects was greater, as compared not only with healthy subjects (F_1,104=11.03, p=0.001) but also with those who began opiate use during adulthood (F_1,61=4.43, p=0.039).

Conclusions

The current findings provide the first evidence that abnormal CSP enlargement, which reflects limbic system dysgenesis of neurodevelopmental origin, may be linked to later development of opiate dependence. In addition, a greater CSP length, which indicates more severe limbic abnormalities, appears to confer higher risk for earlier onset of opiate use.

Effects of a CACNA1C genotype on attention networks in healthy individuals

BACKGROUND

Recent genetic studies found the A allele of the variant rs1006737 in the alpha 1C subunit of the L-type voltage-gated calcium channel (CACNA1C) gene to be over-represented in patients with psychosis, including schizophrenia, bipolar disorder and major depressive disorder. In these disorders, attention deficits are among the main cognitive symptoms and have been related to altered neural activity in cerebral attention networks. The particular effect of CACNA1C on neural function, such as attention networks, remains to be elucidated.

METHOD

The current event-related functional magnetic resonance imaging (fMRI) study investigated the effect of the CACNA1C gene on brain activity in 80 subjects while performing a scanner-adapted version of the Attention Network Test (ANT). Three domains of attention were probed simultaneously: alerting, orienting and executive control of attention.

RESULTS

Risk allele carriers showed impaired performance in alerting and orienting in addition to reduced neural activity in the right inferior parietal lobule [Brodmann area (BA) 40] during orienting and in the medial frontal gyrus (BA 8) during executive control of attention. These areas belong to networks that have been related to impaired orienting and executive control mechanisms in neuropsychiatric disorders.

CONCLUSIONS

Our results suggest that CACNA1C plays a role in the development of specific attention deficits in psychiatric disorders by modulation of neural attention networks.

Neural markers of genetic vulnerability to drug addiction

This chapter will summarize genetics findings derived from various strategies and highlight important neural markers (or correlates) in some specific and extensively studied genes. Most studies highlighted here focus on alcohol and nicotine dependence (AD and ND, respectively). AD and ND are among the most prevalent addictive disorders worldwide, are among the best studied, and are also associated globally with the largest socioeconomic impact.We describe different mechanisms through which genes can have an impact on the addictive behaviors, distinguishing between the genes that inscribe the proteins affecting the metabolism of the addictive substance (e.g., ADH/ALDH for alcohol or CYP2A6 for nicotine) and genes that code for the brain transmitter systems, such as genes involved in cerebral neurotransmission thought to be involved in addiction (e.g., brain reward system, mood regulation, opioid system). Strategies include linkage analyses, association studies, whole genome association studies as well as intermediate/endophenotype studies. Moreover, some important findings derived from animal studies and from neuroimaging studies are highlighted. In conclusion, we provide the reader with an overview of most important studies related to AD and ND and give an outlook how these findings may become useful and beneficial in the future.

The effects of a DTNBP1 gene variant on attention networks: an fMRI study

BACKGROUND

Attention deficits belong to the main cognitive symptoms of schizophrenia and come along with altered neural activity in previously described cerebral networks. Given the high heritability of schizophrenia the question arises if impaired function of these networks is modulated by susceptibility genes and detectable in healthy risk allele carriers.

METHODS

The present event-related fMRI study investigated the effect of the single nucleotide polymorphism (SNP) rs1018381 of the DTNBP1 (dystrobrevin-binding protein 1) gene on brain activity in 80 subjects while performing the attention network test (ANT). In this reaction time task three domains of attention are probed simultaneously: alerting, orienting and executive control of attention.

RESULTS

Risk allele carriers showed impaired performance in the executive control condition associated with reduced neural activity in the left superior frontal gyrus [Brodmann area (BA) 9]. Risk allele carriers did not show alterations in the alerting and orienting networks.

CONCLUSIONS

BA 9 is a key region of schizophrenia pathology and belongs to a network that has been shown previously to be involved in impaired executive control mechanisms in schizophrenia. Our results identified the impact of DTNBP1 on the development of a specific attention deficit via modulation of a left prefrontal network.

Recurrent, robust and scalable patterns underlie human approach and avoidance

BACKGROUND

Approach and avoidance behavior provide a means for assessing the rewarding or aversive value of stimuli, and can be quantified by a keypress procedure whereby subjects work to increase (approach), decrease (avoid), or do nothing about time of exposure to a rewarding/aversive stimulus. To investigate whether approach/avoidance behavior might be governed by quantitative principles that meet engineering criteria for lawfulness and that encode known features of reward/aversion function, we evaluated whether keypress responses toward pictures with potential motivational value produced any regular patterns, such as a trade-off between approach and avoidance, or recurrent lawful patterns as observed with prospect theory.

METHODOLOGY/PRINCIPAL FINDINGS

Three sets of experiments employed this task with beautiful face images, a standardized set of affective photographs, and pictures of food during controlled states of hunger and satiety. An iterative modeling approach to data identified multiple law-like patterns, based on variables grounded in the individual. These patterns were consistent across stimulus types, robust to noise, describable by a simple power law, and scalable between individuals and groups. Patterns included: (i) a preference trade-off counterbalancing approach and avoidance, (ii) a value function linking preference intensity to uncertainty about preference, and (iii) a saturation function linking preference intensity to its standard deviation, thereby setting limits to both.

CONCLUSIONS/SIGNIFICANCE

These law-like patterns were compatible with critical features of prospect theory, the matching law, and alliesthesia. Furthermore, they appeared consistent with both mean-variance and expected utility approaches to the assessment of risk. Ordering of responses across categories of stimuli demonstrated three properties thought to be relevant for preference-based choice, suggesting these patterns might be grouped together as a relative preference theory. Since variables in these patterns have been associated with reward circuitry structure and function, they may provide a method for quantitative phenotyping of normative and pathological function (e.g., psychiatric illness).

New Genetic Findings in Schizophrenia: Is there Still Room for the Dopamine Hypothesis of Schizophrenia?

Schizophrenia is a highly heritable disorder, but the identification of specific genes has proven to be a difficult endeavor. Genes involved in the dopaminergic system are considered to be major candidates since the “dopamine hypothesis” of impairment in dopaminergic neurotransmission is one of the most widely accepted hypotheses of the etiology of schizophrenia. The overall findings from candidate studies do provide some support for the “dopamine hypothesis.” However, results from the first systematic genome-wide association (GWA) studies have implicated variants within ZNF804A, NRGN, TCF4, and variants in the MHC region on chromosome 6p22.1. Although these genes may not immediately impact on dopaminergic neurotransmission, it remains possible that downstream impairments in dopaminergic function are caused. Furthermore, only a very small fraction of all truly associated genetic variants have been detected and many more associated variants will be identified in the future by GWA studies and alternative approaches. The results of these studies may allow a more comprehensive re-evaluation of the dopamine hypothesis.

The impact of dystrobrevin-binding protein 1 (DTNBP1) on neural correlates of episodic memory encoding and retrieval

Episodic memory impairment is a frequently reported symptom in schizophrenia. It has been shown to be associated with reduced neural activity of the hippocampus and prefrontal cortex. Given the high heritability of schizophrenia the question arises if alterations in brain activity are modulated by susceptibility genes and might be detectable in healthy risk allele carriers. The present study investigated the effect of the single nucleotide polymorphism (SNP) rs1018381 (P1578) of the dystrobrevin-binding protein 1 (DTNBP1) on brain activity in 84 healthy subjects assessed by functional magnetic resonance imaging (fMRI) while they performed an episodic memory task comprising encoding and retrieval of faces. During encoding, the group of risk allele carriers (n = 29) showed enhanced neural activity in the left middle frontal gyrus (BA 11) and bilaterally in the cuneus (BA 17, 7) when compared with the nonrisk carrier group (n = 55). During retrieval, the risk group (compared to the non risk group) showed increased right hemispheric neural activity comprising the medial frontal gyrus (BA 9), inferior frontal gyrus (BA 9), and inferior parietal lobule (BA 40). Since there were no behavioral performance differences, increased neural activity of the risk group might be interpreted as a correlate of higher effort or differing cognitive strategies in order to compensate for a genetically determined slight cognitive deficit. Interestingly, the laterality of increased prefrontal activity is in accordance with the well known hemispheric encoding/retrieval asymmetry (HERA) model of episodic memory.

Functional brain imaging in schizophrenia: selected results and methods

Functional brain imaging studies of patients with schizophrenia may be grouped into those that assume that the signs and symptoms of schizophrenia are due to disordered circuitry within a critical brain region and studies that assume that the signs and symptoms are due to disordered connections among brain regions. Studies have investigated the disordered functional brain anatomy of both the positive and negative symptoms of schizophrenia. Studies of spontaneous hallucinations find that although hallucinations are associated with abnormal brain activity in primary and secondary sensory areas, disordered brain activation associated with hallucinations is not limited to sensory systems. Disordered activation in non-sensory regions appear to contribute to the emotional strength and valence of hallucinations, to be a factor underlying an inability to distinguish ongoing mental processing from memories, and to reflect the brain's attempt to modulate the intensity of hallucinations and resolve conflicts with other processing demands. Brain activation studies support the view that auditory/verbal hallucinations are associated with an impaired ability of internal speech plans to modulate neural activation in sensory language areas. In early studies, negative symptoms of schizophrenia were hypothesized to be associated with impaired function in frontal brain areas. In support of this hypothesis meta-analytical studies have found that resting blood flow or metabolism in frontal cortex is reduced in schizophrenia, though the magnitude of the effect is only small to moderate. Brain activation studies of working memory (WM) functioning are typically associated with large effect sizes in the frontal cortex, whereas studies of functions other than WM generally reveal smaller effects. Findings from some functional connectivity studies have supported the hypothesis that schizophrenia patients experience impaired functional connections between frontal and temporal cortex, although the nature of the disordered connectivity is complex. More recent studies have used functional brain imaging to study neural compensation in schizophrenia, to serve as endophenotypes in genetic studies and to provide biomarkers in drug development studies. These emerging trends in functional brain imaging research are likely to help stimulate the development of a general neurobiological theory of the complex symptoms of schizophrenia.