Wendy Shelledy

Amygdalar and hippocampal substrates of anxious temperament differ in their heritability

Anxious temperament (AT) in human and non-human primates is a trait-like phenotype evident early in life that is characterized by increased behavioural and physiological reactivity to mildly threatening stimuli. Studies in children demonstrate that AT is an important risk factor for the later development of anxiety disorders, depression and comorbid substance abuse. Despite its importance as an early predictor of psychopathology, little is known about the factors that predispose vulnerable children to develop AT and the brain systems that underlie its expression. To characterize the neural circuitry associated with AT and the extent to which the function of this circuit is heritable, we studied a large sample of rhesus monkeys phenotyped for AT. Using 238 young monkeys from a multigenerational single-family pedigree, we simultaneously assessed brain metabolic activity and AT while monkeys were exposed to the relevant ethological condition that elicits the phenotype. High-resolution 18F-labelled deoxyglucose positron-emission tomography (FDG–PET) was selected as the imaging modality because it provides semi-quantitative indices of absolute glucose metabolic rate, allows for simultaneous measurement of behaviour and brain activity, and has a time course suited for assessing temperament-associated sustained brain responses. Here we demonstrate that the central nucleus region of the amygdala and the anterior hippocampus are key components of the neural circuit predictive of AT. We also show significant heritability of the AT phenotype by using quantitative genetic analysis. Additionally, using voxelwise analyses, we reveal significant heritability of metabolic activity in AT-associated hippocampal regions. However, activity in the amygdala region predictive of AT is not significantly heritable. Furthermore, the heritabilities of the hippocampal and amygdala regions significantly differ from each other. Even though these structures are closely linked, the results suggest differential influences of genes and environment on how these brain regions mediate AT and the ongoing risk of developing anxiety and depression.

On the genetic architecture of cortical folding and brain volume in primates

Understanding the evolutionary forces that produced the human brain is a central problem in neuroscience and human biology. Comparisons across primate species show that both brain volume and gyrification (the degree of folding in the cerebral cortex) have progressively increased during primate evolution and there is a strong positive correlation between these two traits across primate species. The human brain is exceptional among primates in both total volume and gyrification, and therefore understanding the genetic mechanisms influencing variation in these traits will improve our understanding of a landmark feature of our species. Here we show that individual variation in gyrification is significantly heritable in both humans and an Old World monkey (baboons, Papio hamadryas). Furthermore, contrary to expectations based on the positive phenotypic correlation across species, the genetic correlation between cerebral volume and gyrification within both humans and baboons is estimated as negative. These results suggest that the positive relationship between cerebral volume and cortical folding across species cannot be explained by one set of selective pressures or genetic changes. Our data suggest that one set of selective pressures favored the progressive increase in brain volume documented in the primate fossil record, and that a second independent selective process, possibly related to parturition and neonatal brain size, may have favored brains with progressively greater cortical folding. Without a second separate selective pressure, natural selection favoring increased brain volume would be expected to produce less folded, more lissencephalic brains. These results provide initial evidence for the heritability of gyrification, and possibly a new perspective on the evolutionary mechanisms underlying long-term changes in the nonhuman primate and human brain.

Genetic influences on behavioral inhibition and anxiety in juvenile rhesus macaques

In humans and other animals, behavioral responses to threatening stimuli are an important component of temperament. Among children, extreme behavioral inhibition elicited by novel situations or strangers predicts the subsequent development of anxiety disorders and depression. Genetic differences among children are known to affect risk of developing behavioral inhibition and anxiety, but a more detailed understanding of genetic influences on susceptibility is needed. Nonhuman primates provide valuable models for studying the mechanisms underlying human behavior. Individual differences in threat‐induced behavioral inhibition (freezing behavior) in young rhesus monkeys are stable over time and reflect individual levels of anxiety. This study used the well‐established human intruder paradigm to elicit threat‐induced freezing behavior and other behavioral responses in 285 young pedigreed rhesus monkeys. We examined the overall influence of quantitative genetic variation and tested the specific effect of the serotonin transporter promoter repeat polymorphism. Quantitative genetic analyses indicated that the residual heritability of freezing duration (behavioral inhibition) is h2 = 0.384 (P = 0.012) and of ‘orienting to the intruder’ (vigilance) is h2 = 0.908 (P = 0.00001). Duration of locomotion and hostility and frequency of cooing were not significantly heritable. The serotonin transporter polymorphism showed no significant effect on either freezing or orienting to the intruder. Our results suggest that this species could be used for detailed studies of genetic mechanisms influencing extreme behavioral inhibition, including the identification of specific genes that are involved in predisposing individuals to such behavior.

Heritability of brain volume, surface area and shape: an MRI study in an extended pedigree of baboons.

To evaluate baboons (Papio hamadryas) as a primate model for the study of the genetic control of brain size and internal structure, we performed high resolution (<500 μm) magnetic resonance imaging on 109 pedigreed baboons. Quantitative genetic analysis of these MR images using a variance components approach indicates that native (untransformed) brain volume exhibits significant heritability among these baboons (h2 = 0.52, P = 0.0049), with age and sex also accounting for substantial variation. Using global spatial normalization, we transformed all images to a standard population‐specific reference, and recalculated the heritability of brain volume. The transformed images generated heritability estimates of h2 = 0.82 (P = 0.00022) for total brain volume, h2 = 0.86 (P = 0.0006) for cerebral volume, h2 = 0.73 (P = 0.0069) for exposed surface area of the cerebrum and h2 = 0.67 (P = 0.01) for gray matter volume. Regional differences in the genetic effects on brain structure were calculated using a voxel‐based morphometry (VBM) approach. This analysis of regional variation shows that some areas of motor cortex and the superior temporal gyrus show relatively high heritability while other regions (e.g. superior parietal cortex) exhibit lower heritability. The general pattern of regional differences is similar to that observed in previous studies of humans. The present study demonstrates that there is substantial genetic variation underlying individual variation in brain size and structure among Papio baboons, and that broad patterns of genetic influence on variation in brain structure may be similar in baboons and humans. Hum Brain Mapp, 2007.

Genetics of primary cerebral gyrification: Heritability of length, depth and area of primary sulci in an extended pedigree of Papio baboons.

Genetic control over morphological variability of primary sulci and gyri is of great interest in the evolutionary, developmental and clinical neurosciences. Primary structures emerge early in development and their morphology is thought to be related to neuronal differentiation, development of functional connections and cortical lateralization. We measured the proportional contributions of genetics and environment to regional variability, testing two theories regarding regional modulation of genetic influences by ontogenic and phenotypic factors. Our measures were surface area, and average length and depth of eleven primary cortical sulci from high-resolution MR images in 180 pedigreed baboons. Average heritability values for sulcal area, depth and length (h(2)(Area)=.38+/-.22; h(2)(Depth)=.42+/-.23; h(2)(Length)=.34+/-.22) indicated that regional cortical anatomy is under genetic control. The regional pattern of genetic contributions was complex and, contrary to previously proposed theories, did not depend upon sulcal depth, or upon the sequence in which structures appear during development. Our results imply that heritability of sulcal phenotypes may be regionally modulated by arcuate U-fiber systems. However, further research is necessary to unravel the complexity of genetic contributions to cortical morphology.

Dietary and Genetic Effects on LDL Size Measures in Baboons

Genetic and dietary effects on LDL phenotypes, including predominant LDL particle diameter, LDL size distribution, and non-HDL cholesterol and apoB concentrations, were investigated in 150 pedigreed baboons that are members of 19 sire groups. Baboons were fed a sequence of three defined diets differing in levels of fat and cholesterol. Increasing dietary fat had relatively little effect on two measures of LDL particle size. However, increasing the level of cholesterol in the diet resulted in larger increases of the predominant LDL particle diameters and in the proportion of stain on LDLs > 28 nm. As expected, apoB and non-HDL cholesterol concentrations significantly increased when levels of dietary fat and cholesterol were increased. Correlations among the LDL phenotypes suggested that several different aspects of the LDL phenotype were captured by the four LDL measures across the three diets. Genetic effects indicated by sire group membership were significant both for expression of the LDL phenotypes and for response to changes in diet.

Designing new microsatellite markers for linkage and population genetic analyses in rhesus macaques and other nonhuman primates.

Identification of polymorphic microsatellite loci in nonhuman primates is useful for various biomedical and evolutionary studies of these species. Prior methods for identifying microsatellites in nonhuman primates are inefficient. We describe a new strategy for marker development that uses the available whole genome sequence for rhesus macaques. Fifty-four novel rhesus-derived microsatellites were genotyped in large pedigrees of rhesus monkeys. Linkage analysis was used to place 51 of these loci into the existing rhesus linkage map. In addition, we find that microsatellites identified this way are polymorphic in other Old World monkeys such as baboons. This approach to marker development is more efficient than previous methods and produces polymorphisms with known locations in the rhesus genome assembly. Finally, we propose a nomenclature system that can be used for rhesus-derived microsatellites genotyped in any species or for novel loci derived from the genome sequence of any nonhuman primate.

Genetic control of apolipoprotein A-I distribution among HDL subclasses

We conducted genetic analyses to determine the components of variation for size distributions of apolipoprotein (apo) A-I among human plasma lipoproteins resolved on the basis of size. Analyses used data for 717 individuals in 26 pedigrees. Apo A-I distributions among lipoprotein size classes were measured by nondenaturing gradient gel electrophoresis (GGE) and immunoblotting procedures. Curves were fitted to apo A-I absorbance profiles to estimate fractional absorbance in each of five high-density lipoprotein (HDL) subclasses. Multivariate regression analyses revealed several covariates (sex, age, diabetes, and apo A-I concentrations) that were significantly associated with variation in one or more HDL subclasses. Female gender and elevated apo A-I concentrations were associated with increases in proportion of apo A-I in larger HDLs, while increasing age and diabetes were associated with decreases. The analyses showed significant heritabilities. h2, for each variable representing the different HDL subclasses. Both genetic and nongenetic effects on apo A-I size distributions were generally exerted across the range of lipoprotein sizes, as suggested by high genetic and environmental correlations between HDL subclass variables. Decomposition of total overall variance showed that unidentified environmental factors accounted for 48% of variation in apo A-I size distribution, while genetic factors explained about 36% and the identified covariates explained the remaining 16%. When considered separately, apo A-I concentration explained only 5% of the total variation in apo A-I size distribution, indicating that apo A-I concentration is a poor predictor of apo A-I size distribution. In summary, the data suggest that there are significant genetic and environmental effects on apo A-I size distribution in humans, and that they are general metabolic effects rather than effects on specific HDL subclasses.

Mapping of the serotonin transporter locus (SLC6A4) to rhesus chromosome 16 using genetic linkage

man primates also contains repetitive elements in the upstream promoter region (Lesch et al., 1997). Rhesus monkeys (Macaca mulatta) exhibit a common repeat unit polymorphism that is similar in structure and located close to the human repeat polymorphism, but the allelic variation among rhesus monkeys is not identical to the human polymorphism. Nevertheless, the polymorphism in rhesus also has functional effects. Trefi lov et al. (2000) found that free-ranging rhesus monkeys carrying two copies of the short allele (s) dispersed from their natal groups at an earlier age than did either the carriers of two long alleles (l) or heterozygotes. Bennett et al. (2002) showed that the rhesus polymorphism (l vs. s) alters in vitro gene expression in this species as well, and that when animals are raised in peer groups (as opposed to motherreared), the SLC6A4 polymorphism can infl uence levels of serotonin metabolites found in the cerebrospinal fl uid. Champoux et al. (2002) found that variation in the temperament of infant rhesus macaques is associated with SLC6A4 genotype. More recently, Barr et al. (2004) found an interaction between SLC6A4 genotype and early rearing experience that regulates activation of HPA axis stress response. Bethea et al. (2004) showed that infant rhesus monkeys that carry the s/s genotype exhibit increases in certain fear responses and are behaviorally inhibited when challenged with a series of tests involving stimuli that induce mild fear or anxiety. Clearly, individual variation in the SLC6A4 gene can infl uence individual variation in several aspects of neurobiology, stress reactivity and expressed behavior. The goal of the present study is to establish the chromosomal position of the SLC6A4 gene within the rhesus genome. As part of a larger program in comparative primate genomics, Rationale and signifi cance