Amygdala and hippocampal volumes in Turner syndrome: a high-resolution MRI study of X-monosomy

Insights into brain development from neurogenetic syndromes: evidence from fragile X syndrome, Williams syndrome, Turner syndrome and velocardiofacial syndrome

Over the past few decades, behavioral, neuroimaging and molecular studies of neurogenetic conditions, such as Williams, fragile X, Turner and velocardiofacial (22q11.2 deletion) syndromes, have led to important insights regarding brain development. These investigations allow researchers to examine "experiments of nature" in which the deletion or alteration of one gene or a contiguous set of genes can be linked to aberrant brain structure or function. Converging evidence across multiple imaging modalities has now begun to highlight the abnormal neural circuitry characterizing many individual neurogenetic syndromes. Furthermore, there has been renewed interest in combining analyses across neurogenetic conditions in order to search for common organizing principles in development. In this review, we highlight converging evidence across syndromes from multiple neuroimaging modalities, with a particular emphasis on functional imaging. In addition, we discuss the commonalities and differences pertaining to selective deficits in visuospatial processing that occur across four neurogenetic syndromes. We suggest avenues for future exploration, with the goal of achieving a deeper understanding of the neural abnormalities in these affected populations.

Regional sex differences in grey matter volume are associated with sex hormones in the young adult human brain

Previous studies suggest organizing effects of sex hormones on brain structure during early life and puberty, yet little is known about the adult period. The aim of the present study was to elucidate the role of 17beta-estradiol, progesterone, and testosterone on cortical sex differences in grey matter volume (GM) of the adult human brain. To assess sexual dimorphism, voxel-based morphometry (VBM) was applied on structural magnetic resonance images of 34 healthy, young adult humans (17 women, 17 men, 26.6+/-5 years) using analyses of covariance. Subsequently, circulating levels of sex hormones were associated with regional GM using linear regression analyses. After adjustment for sex and total GM, significant associations of regional GM and 17beta-estradiol were observed in the left inferior frontal gyrus (beta=0.39, p=0.02). Regional GM was inversely associated with testosterone in the left inferior frontal gyrus (beta=-0.16, p=0.04), and with progesterone in the right temporal pole (beta=-0.39, p=0.008). Our findings indicate that even in young adulthood, sex hormones exert organizing effects on regional GM. This might help to shed further light on the underlying mechanisms of both functional diversities and congruence between female and male brains.

Psychiatric phenotypes associated with neurogenetic disorders

Advances in understanding the human genome and clinical application have led to identification of genetically based disorders that have distinctive behavioral phenotypes and risk for serious psychiatric disorders. Some patients have unrecognized genetic disorders presenting as psychiatric symptoms. Practitioners must be knowledgeable about the association between symptoms and underlying genetic bases. Treatment of neurogenetic disorders includes providing information about causes and prognoses. Patients are served best if they remain long term with a multidisciplinary team of providers who recognize the realities of a lifetime course, the high risk for symptom recurrence, and the need for providing information and support to families and coordinating medical and psychiatric care.

Vulnerabilidad social en el síndrome de turner: interacción genes-ambiente

En este trabajo se exponen los fundamentos teóricos que permiten considerar al Síndrome de Turner como un “modelo genético sensible” en el cual analizar asociaciones entre caracteres psicológicos y factores genéticos y ambientales. La investigación en esta línea permitiría dilucidar mecanismos de riesgo ambiental y comprender el grado de “vulnerabilidad” que presentan las mujeres con el síndrome. El Síndrome de Turner es un trastorno genético, no heredable, que se caracteriza por la deleción total o parcial del cromosoma X en el sexo femenino. El estudio de la disfunción social que presentan las mujeres con este síndrome, es decir, de la capacidad para relacionarse con el entorno y la calidad de los vínculos tiene importantes implicaciones en la comprensión de la forma en que actúa el ambiente sobre el desarrollo psicológico, y de cómo a su vez éste se ve afectado por factores genéticos que actúan en interacción.

Moving toward an understanding of hormone replacement therapy in adolescent girls: looking through the lens of Turner syndrome

Turner syndrome (TS) is a relatively common disorder of phenotypic females caused by loss of all or part of the second sex chromosome. Most individuals with TS have short stature and gonadal dysgenesis and are at risk for many other medical and learning problems. In the 45,X ovary, germs cells multiply quite normally during fetal development, but there is accelerated atresia of oocytes in the second half of pregnancy that produces gonadal insufficiency by birth. In girls with other karyotypes, especially those mosaic for 45,X/46,XX, gonadal function may be normal or near-normal. In this chapter, management goals for gonadal insufficiency in girls with TS are presented. The effects of estrogen deficiency and its replacement on three specific problems associated with TS-short stature, cardiovascular disease, and developmental differences in brain structure and function-are explored. General guidelines for estrogen replacement therapy using transdermal estrogen, the most physiologic option, are provided and future research goals are outlined.

Sex-specific differences in expression of histone demethylases Utx and Uty in mouse brain and neurons

Although X inactivation is thought to balance gene expression between the sexes, some genes escape inactivation, potentially contributing to differences between males and females. Utx (ubiquitously transcribed tetratricopeptide repeat gene on X chromosome) is an escapee gene that encodes a demethylase specific for lysine 27 of histone H3, a mark of repressed chromatin. We found Utx to be expressed higher in females than in males in developing and adult brains and in adult liver. XX mice had a higher level of Utx than XY mice, regardless of whether they had testes or ovaries, indicating that the sexually dimorphic gene expression was a consequence of the sex chromosome complement. Females had significantly higher levels of Utx than males in most brain regions except in the amygdala. The regional expression of the Y-linked paralogue Uty (ubiquitously transcribed tetratricopeptide repeat gene on Y chromosome) was somewhat distinct from that of Utx, specifically in the paraventricular nucleus of the hypothalamus (high Uty) and the amygdala (high Utx), implying that the two paralogues may be differentially regulated. Higher expression of Utx compared with Uty was detected in P19 pluripotent embryonic carcinoma cells as well as in P19-derived neurons. This transcriptional divergence between the two paralogues was associated with high levels of histone H3 lysine 4 dimethylation at the Utx promoter and of histone H4 lysine 16 acetylation throughout the gene body, which suggests that epigenetic mechanisms control differential expression of paralogous genes.

Turner syndrome and the evolution of human sexual dimorphism

Turner syndrome is caused by loss of all or part of an X chromosome in females. A series of recent studies has characterized phenotypic differences between Turner females retaining the intact maternally inherited versus paternally inherited X chromosome, which have been interpreted as evidence for effects of X-linked imprinted genes. In this study I demonstrate that the differences between Turner females with a maternal X and a paternal X broadly parallel the differences between males and normal females for a large suite of traits, including lipid profile and visceral fat, response to growth hormone, sensorineural hearing loss, congenital heart and kidney malformations, neuroanatomy (sizes of the cerebellum, hippocampus, caudate nuclei and superior temporal gyrus), and aspects of cognition. This pattern indicates that diverse aspects of human sex differences are mediated in part by X-linked genes, via genomic imprinting of such genes, higher rates of mosaicism in Turner females with an intact X chromosome of paternal origin, karyotypic differences between Turner females with a maternal versus paternal X chromosome, or some combination of these phenomena. Determining the relative contributions of genomic imprinting, karyotype and mosaicism to variation in Turner syndrome phenotypes has important implications for both clinical treatment of individuals with this syndrome, and hypotheses for the evolution and development of human sexual dimorphism.

Wnt pathway anomalies in developing amygdalae of Turner syndrome-like mice

Certain neurobehavioral deficiencies associated with Turner Syndrome have been attributed to brain volumetric abnormalities, particularly of the amygdala. Haplo-insufficiency of a non-dosage compensated gene or genes on the X chromosome has been hypothesized to be the cause of the neuroanatomical defect. We examined gene expression levels of 6,628 genes in developing amygdalae of late-stage embryos of a mouse model for Turner Syndrome. In total, 161 genes show significant differences in expression level between TS and normal female amygdala. In silico pathway analysis of both X-linked and autosomal mis-regulated genes suggests that modulation of Wnt signaling is a critical factor in the normal growth and development of the amygdala.

Turner Syndrome

Turner syndrome is a neurogenetic disorder characterized by partial or complete monosomy-X. It is associated with certain physical and medical features, including estrogen deficiency, short stature, and increased risk for several diseases, with cardiac conditions being among the most serious. The cognitive-behavioral phenotype associated with the syndrome includes strengths in verbal domains with impairments in visuospatial, executive function, and emotion processing. Less is known regarding psychosocial and psychiatric functioning in Turner syndrome, but essential aspects of psychotherapeutic treatment plans are suggested. Future investigations should include continued genetic studies and determination of candidate genes for physical and cognitive features. Multimodal, interdisciplinary studies are essential for identifying optimal, syndrome-specific interventions for improving the lives of individuals who have Turner syndrome.

Bilateral temporal lobe epilepsy in a patient with Turner syndrome mosaicism

Turner's syndrome (TS), resulting from deletion of one X chromosome in women, is associated with cerebral development abnormalities, particularly in the temporal lobes. Symptomatic epilepsy is described only in cases with extensive malformations. Here, we report the first case of bilateral temporal epilepsy without macroscopic cerebral malformation in a woman with TS mosaicism. Bitemporal dysfunction was confirmed by the ictal and interictal EEG, PET, MR-spectroscopy and the neuropsychological examination, other causes than TS mosaicism were excluded. In rare cases, TS mosaicism may underlie non-lesional temporal lobe epilepsy, probably in relation to microanatomic structural and functional cerebral abnormalities. Further studies are needed to determine the frequency of this association.

The 39,XO mouse as a model for the neurobiology of Turner syndrome and sex-biased neuropsychiatric disorders

Turner syndrome (TS) is a developmental disorder most frequently arising from the loss of a complete X chromosome (karyotype 45,XO). The disorder is characterised by physiological abnormalities (notably short stature and ovarian dysfunction), emotional anomalies (including heightened anxiety) and by a neuropsychological profile encompassing deficits in visuospatial skills, memory, attention, social cognition and emotion recognition. Moreover, TS subjects are at significantly increased risk of developing attention deficit hyperactivity disorder (ADHD) and autism. At the neuroanatomical level, TS subjects display abnormalities across a number of brain structures, including the amygdala, hippocampus and orbitofrontal cortex. The TS phenotype arises due to reduced dosage of X-linked genes, and may also be modulated by parental origin of the single X chromosome. In this review, we discuss the utility of a mouse model of TS, the 39,XO mouse, in which the parental origin of the single X chromosome can be varied. This model provides the opportunity to investigate the effects of X-linked gene dosage/parent-of-origin effects on neurobiology in the absence of gross physiological abnormalities. Initial findings indicate that several features of the TS behavioural phenotype may be accurately recapitulated in the mouse. Furthermore, as X-linked gene dosage/imprinting can influence sex-specific neurobiology, investigations in the 39,XO mouse are also likely to offer insights into why certain neuropsychiatric disorders (including ADHD and autism) affect the sexes differently.

The development of mental state attributions in women with X-monosomy, and the role of monoamine oxidase B in the sociocognitive phenotype

We hypothesized that women with Turner syndrome (45,X) with a single X-chromosome inherited from their mother may show mentalizing deficits compared to women of normal karyotype with two X-chromosomes (46,X). Simple geometrical animation events (two triangles moving with apparent intention in relation to each other) which usually elicit mental-state descriptions in normally developing people, did not do so to the same extent in women with Turner syndrome. We then investigated the potential role in this deficit played by monoamine oxidase B enzymatic activity. MAO-B activity reflects central serotonergic activity, and by implication the functional integrity of neural circuits implicated in mentalizing. Platelet MAO-B was substantially reduced in Turner syndrome. However, contrary to prediction, in this (relatively small) sample there was no association between MAO-B enzymatic activity and mentalizing skills in participants with and without Turner syndrome.

It is not all hormones: Alternative explanations for sexual differentiation of the brain

Males and females of many species differ with regard to neurodevelopment, ongoing brain function and behavior. For many years, it was assumed that these differences primarily arose due to hormonal masculinization of the male brain (and to a lesser extent hormonal feminization of the female brain). Recent elegant experiments in model systems have revealed that, while gonadal hormones undoubtedly play an important role in sexual differentiation of the brain, they are not the only possible mechanism for this phenomenon. In the present review, we discuss the concept that genes residing upon the sex chromosomes (which are asymmetrically inherited between males and females) may influence sexually dimorphic neurobiology directly, and suggest possible mechanisms. Future work will be directed towards understanding the extent and specificity with which sex-linked genes and hormones define brain structure and function, and towards elucidating potential interactions between the two mechanisms. Ultimately, it is hoped that such studies will provide insights into why men and women are differentially vulnerable to certain mental disorders, and will enable the development of effective sex-tailored therapeutics.

Sex differences in the human olfactory system

The olfactory system (accessory) implicated in reproductive physiology and behavior in mammals is sexually dimorphic. These brain sex differences present two main characteristics: they are seen in neural circuits related to sexual behavior and sexual physiology and they take one of two opposite morphological patterns (male>female or female>male). The present work reports sex differences in the olfactory system in a large homogeneous sample of men (40) and women (51) using of voxel-based morphology. Gray matter concentration showed sexual dimorphism in several olfactory regions. Women have a higher concentration in the orbitofrontal cortex involving Brodmann's areas 10, 11 and 25 and temporomedial cortex (bilateral hippocampus and right amygdala), as well as their left basal insular cortex. In contrast, men show a higher gray matter concentration in the left entorhinal cortex (Brodmann's area 28), right ventral pallidum, dorsal left insular cortex and a region of the orbitofrontal cortex (Brodmann's area 25). This study supports the hypothesis that the mammalian olfactory system is a sexually dimorphic network and provides a theoretical framework for the morphofunctional approach to sex differences in the human brain.

Sex differences in brain expression of X- and Y-linked genes

The X chromosome plays an important role in brain development and function, as evidenced by its disproportionately high content of genes whose mutations cause mental retardation. These X-linked brain genes may play a role in sexual differentiation if they are expressed at a higher level in XX females than in XY males, due to incomplete X inactivation in females. The expression of several X escapee genes is indeed higher in brain tissues from females when compared to males. In mouse, some of the sex differences are only found in adult brains but not in other tissues. Determining the brain expression pattern of these X escapee genes is important for a better understanding of their role in the neurological phenotypes of XO Turner syndrome.

Imbalanced genomic imprinting in brain development: an evolutionary basis for the aetiology of autism

We describe a new hypothesis for the development of autism, that it is driven by imbalances in brain development involving enhanced effects of paternally expressed imprinted genes, deficits of effects from maternally expressed genes, or both. This hypothesis is supported by: (1) the strong genomic-imprinting component to the genetic and developmental mechanisms of autism, Angelman syndrome, Rett syndrome and Turner syndrome; (2) the core behavioural features of autism, such as self-focused behaviour, altered social interactions and language, and enhanced spatial and mechanistic cognition and abilities, and (3) the degree to which relevant brain functions and structures are altered in autism and related disorders. The imprinted brain theory of autism has important implications for understanding the genetic, epigenetic, neurological and cognitive bases of autism, as ultimately due to imbalances in the outcomes of intragenomic conflict between effects of maternally vs. paternally expressed genes.

Healthcare for adolescents with Turner syndrome

This review paper highlights important healthcare issues for adolescents with Turner Syndrome. Turner Syndrome potentially affects multiple organ systems including: cardiovascular, renal, endocrine, neurologic, gastrointestinal, skin, skeletal, auditory, and reproductive systems. Congenital and acquired cardiac defects remain the most significant health problem faced by women with Turner Syndrome.

Selective alterations of white matter associated with visuospatial and sensorimotor dysfunction in turner syndrome

Turner syndrome (TS) is a neurogenetic disorder characterized by impaired spatial, numerical, and motor functioning but relatively spared verbal ability. Results from previous neuroimaging studies suggest that gray matter alterations in parietal and frontal regions may contribute to atypical visuospatial and executive functioning in TS. Recent findings in TS also indicate variations in the shape of parietal gyri and white matter microstructural anomalies of the temporal lobe. Diffusion tensor imaging and structural imaging methods were used to determine whether 10 females with TS and 10 age- and gender-matched control subjects exhibited differences in fractional anisotropy, white matter density, and local brain shape. Relative to controls, females with TS had lower fractional anisotropy (FA) values in the deep white matter of the left parietal-occipital region extending anteriorly along the superior longitudinal fasciculus into the deep white matter of the frontal lobe. In addition, decreased FA values were located bilaterally in the internal capsule extending into the globus pallidus and in the right prefrontal region. Voxel-based morphometry (VBM) analysis showed corresponding white matter density differences in the internal capsules and left centrum semiovale. Tensor-based morphometry analysis indicated that the FA and VBM results were not attributable to differences in the local shape of brain structures. Compared with controls, females with TS had increases in FA values and white matter density in language-related areas of the inferior parietal and temporal lobes. These complementary analyses provide evidence for alterations in white matter pathways that subserve affected and preserved cognitive functions in TS.

X-linked genes and mental functioning

The X-chromosome has played a crucial role in the development of sexually selected characteristics for over 300 million years. During that time it has accumulated a disproportionate number of genes concerned with mental functions. Evidence is emerging, from studies of both humans and mice, for a general influence upon intelligence (as indicated by the large number of X-linked mental retardation syndromes). In addition, there is evidence for relatively specific effects of X-linked genes on social-cognition and emotional regulation. Sexually dimorphic processes could be influenced by several mechanisms. First, a small number of X-linked genes are apparently expressed differently in male and female brains in mouse models. Secondly, many human X-linked genes outside the X-Y pairing pseudoautosomal regions escape X-inactivation. Dosage differences in the expression of such genes (which might comprise at least 20% of the total) are likely to play an important role in male-female neural differentiation. To date, little is known about the process but clues can be gleaned from the study of X-monosomic females who are haploinsufficient for expression of all non-inactivated genes relative to 46,XX females. Finally, from studies of both X-monosomic humans (45,X) and mice (39,X), we are learning more about the influences of X-linked imprinted genes upon brain structure and function. Surprising specificity of effects has been described in both species, and identification of candidate genes cannot now be far off.

Neurobiology of emotion and high risk for schizophrenia: role of the amygdala and the X-chromosome

Abnormalities in emotion processing and in structure of the amygdala have consistently been documented in schizophrenia. A major question is whether amygdala abnormalities reflect a genetic vulnerability for the disease. In the present paper, we reviewed Magnetic Resonance Imaging (MRI) studies that reported amygdala measures in several high-risk populations: subjects from the general population with subclinical schizophrenia symptoms and relatives of schizophrenia patients. In addition, we reviewed the evidence regarding Klinefelter syndrome (characterised by an additional X-chromosome), which has also been related to an increased risk for schizophrenia. Overall, the evidence points to structural abnormalities of the amygdala in individuals at increased risk for schizophrenia. Although the genetic basis of amygdala deficits remains unclear, abnormalities (of genes) on the X-chromosome might play a role as suggested by the evidence from individuals with sex chromosome aneuploidies. We propose that amygdala abnormalities are an endophenotype in schizophrenia and may account for subtle emotional processing deficits that have been described in these high-risk groups.

X-linked imprinting: effects on brain and behaviour

Imprinted genes are monoallelically expressed in a parent-of-origin-dependent manner and can affect brain and behavioural phenotypes. The X chromosome is enriched for genes affecting neurodevelopment and is donated asymmetrically to male and female progeny. Hence, X-linked imprinted genes could potentially influence sexually dimorphic neurobiology. Consequently, investigations into such loci may provide new insights into the biological basis of behavioural differences between the sexes and into why men and women show different vulnerabilities to certain mental disorders. In this review, we summarise recent advances in our knowledge of X-linked imprinted genes and the brain substrates that they may act upon. In addition, we suggest strategies for identifying novel X-linked imprinted genes and their downstream effects and discuss evolutionary theories regarding the origin and maintenance of X-linked imprinting.