Magnetization transfer ratio and volumetric analysis of the brain in macrocephalic patients with neurofibromatosis type 1

Neurobehavioral sex-related differences in Nf1+/- mice: female show a "camouflaging"-type behavior

BACKGROUND

Neurofibromatosis type 1 (NF1) is an inherited neurocutaneous disorder associated with neurodevelopmental disorders including autism spectrum disorder (ASD). This condition has been associated with an increase of gamma-aminobutyric acid (GABA) neurotransmission and, consequently, an excitation/inhibition imbalance associated with autistic-like behavior in both human and animal models. Here, we explored the influence of biological sex in the GABAergic system and behavioral alterations induced by the Nf1^+/- mutation in a murine model.

METHODS

Juvenile male and female Nf1^+/- mice and their wild-type (WT) littermates were used. Hippocampus size was assessed by conventional toluidine blue staining and structural magnetic resonance imaging (MRI). Hippocampal GABA and glutamate levels were determined by magnetic resonance spectroscopy (MRS), which was complemented by western blot for the GABA(A) receptor. Behavioral evaluation of on anxiety, memory, social communication, and repetitive behavior was performed.

RESULTS

We found that juvenile female Nf1^+/- mice exhibited increased hippocampal GABA levels. Moreover, mutant female displays a more prominent anxious-like behavior together with better memory performance and social behavior. On the other hand, juvenile Nf1^+/- male mice showed increased hippocampal volume and thickness, with a decrease in GABA(A) receptor levels. We observed that mutant males had higher tendency for repetitive behavior.

CONCLUSIONS

Our results suggested a sexually dimorphic impact of Nf1^+/- mutation in hippocampal neurochemistry, and autistic-like behaviors. For the first time, we identified a "camouflaging"-type behavior in females of an animal model of ASD, which masked their autistic traits. Accordingly, like observed in human disorder, in this animal model of ASD, females show larger anxiety levels but better executive functions and production of normative social patterns, together with an imbalance of inhibition/excitation ratio. Contrary, males have more externalizing disorders, such as hyperactivity and repetitive behaviors, with memory deficits. The ability of females to camouflage their autistic traits creates a phenotypic evaluation challenge that mimics the diagnosis difficulty observed in humans. Thus, we propose the study of the Nf1^+/- mouse model to better understand the sexual dimorphisms of ASD phenotypes and to create better diagnostic tools.

Fetal corpus callosum abnormalities: Ultrasound and magnetic resonance imaging role

The corpus callosum (CC) is the major interhemispheric commissure and its abnormalities include agenesis, hypoplasia, and hyperplasia. The CC anomalies are typically related to other central nervous system (CNS) or extra-CNS malformations. The antenatal diagnosis of complete CC agenesis is easy after mid-trimester by ultrasound (US) even in the axial plane. The non-visualization of cavum septum pellucidum and colpocephaly are critical signs in the axial view. More subtle findings (i.e., hypoplasia and partial agenesis) might also be recognized antenatally. In this review, the focus was given on the prenatal diagnosis of CC abnormalities in US and magnetic resonance imaging.

Fetal thick corpus callosum: new insights from neuroimaging and neuropathology in two cases and literature review

PURPOSE

To describe the correlation between fetal imaging (in vivo and ex vivo) and neuropathology in two fetuses at early gestational age (GA) with isolated thick corpus callosum (CC), a rare finding whose pathological significance and neuropathology data are scarce.

METHODS

Two fetuses at 21-week GA underwent fetal MRI (fMRI) for suspected callosal anomalies at ultrasound (US). After fMRI results, termination of pregnancy (TOP) was carried out and post-mortem MRI (pmMRI) was performed. Neuropathology correlation consisted in macro and microscopic evaluation with sections prepared for hematoxylin-eosin and immunohistochemistry staining.

RESULTS

Fetal imaging confirmed in both cases the presence of a shorter and thicker CC with respect to the reference standard at the same GA, without a clear distinction between its different parts. Moreover, on pmMRI, an abnormal slightly T2-weighted hyperintense layer along the superior and inferior surface of CC was noted in both cases. At histopathology, these findings corresponded to an increased amount of white matter tracts but also to an abnormal representation of embryological structures that contribute to CC development, naming induseum griseum (IG) and the glioepithelial layer (GL) of the "callosal sling." After reviewing the literature data, we confirmed the recent embryological theory regarding the CC development and provide new insights into the pathophysiology of the abnormal cases.

CONCLUSIONS

An abnormally thick CC at the early fetal period could be associated to an abnormal representation of the midline glia structures, so to result in potential disturbance of the axon guidance mechanism of callosal formation and eventually in CC dysgenesis.

PTPN11 Gain-of-Function Mutations Affect the Developing Human Brain, Memory, and Attention

The Ras-MAPK pathway has an established role in neural development and synaptic signaling. Mutations in this pathway are associated with a collection of neurodevelopmental syndromes, Rasopathies; among these, Noonan syndrome (NS) is the most common (1:2000). Prior research has focused on identifying genetic mutations and cellular mechanisms of the disorder, however, effects of NS on the human brain remain unknown. Here, imaging and cognitive data were collected from 12 children with PTPN11-related NS, ages 4.0-11.0 years (8.98 ± 2.33) and 12 age- and sex-matched typically developing controls (8.79 ± 2.17). We observe reduced gray matter volume in bilateral corpus striatum (Cohen's d = -1.0:-1.3), reduced surface area in temporal regions (d = -1.8:-2.2), increased cortical thickness in frontal regions (d = 1.2-1.3), and reduced cortical thickness in limbic regions (d = -1.6), including limbic structures integral to the circuitry of the hippocampus. Further, we find high levels of inattention, hyperactivity, and memory deficits in children with NS. Taken together, these results identify effects of NS on specific brain regions associated with ADHD and learning in children. While our research lays the groundwork for elucidating the neural and behavioral mechanisms of NS, it also adds an essential tier to understanding the Ras-MAPK pathway's role in human brain development.

Fetal midline anomalies: Diagnosis and counselling Part 1: Corpus callosum anomalies

Midline anomalies encompasses a heterogeneous group of conditions caused by an abnormal process of ventral induction after the end of primary neurulation. Advances in prenatal imaging techniques have led to an increase in the detection rate of such anomalies since the first trimester of pregnancy although a significant proportion of them remain undiagnosed until birth. Ultrasound is the primary technique in detecting such anomalies while fetal magnetic resonance imaging (MRI) is commonly performed to confirm the diagnosis and detect additional anomalies, especially those involving the cortical surface of the brain, which may potentially impact post-natal outcome. Neurodevelopmental outcome of cerebral anomalies involving the midline is directly related to the type of anomaly, cause and presence of associated anomalies. However, even in case of isolated anomalies prenatal counselling is challenging. The aim of this review is to provide an up to date on the diagnosis, counselling and management of the most common supra-tentorial anomalies involving the midline and diagnosed on prenatal ultrasound.

Quantitative microstructural cerebral changes in neurofibromatosis type 1

OBJECTIVES

To evaluate microstructural cerebral changes in children with neurofibromatosis type 1 (NF1) based on T₂ relaxation time measurements at 3Tesla.

METHODS

From our dataset of pediatric MRI examinations at 3T 19 pediatric NF1 patients (1.9-14.3 years of age, 9 girls, 10 boys) were retrospectively selected and compared with the previously published group of 44 healthy children (0-16 years of age). MRI examination included a triple echo TSE sequence as basis for T₂ maps. T₂ relaxation times were measured in 37 brain regions.

RESULTS

Compared with healthy controls, T₂ relaxation times had the tendency to be increased by 1.01% (GM) to 11.85% (dentate nucleus) for NF1 patients. Only in posterior limb of the internal capsule and parietooccipital white matter values were reduced. No differences were observed between both hemispheres. Overall, no strong evidence supporting a difference between NF1 patients with and without optic glioma or with normal and impaired neuropsychological development was observed.

CONCLUSIONS

Using T₂ relaxation times it was possible to describe measurable microstructural differences in multiple brain regions between NF1 patients and healthy children regardless of whether signal abnormalities were visible on conventional images.

Attention deficit hyperactivity disorder (ADHD) in phenotypically similar neurogenetic conditions: Turner syndrome and the RASopathies

Background

ADHD (attention deficit hyperactivity disorder) is a common neurodevelopmental disorder. There has been extensive clinical and basic research in the field of ADHD over the past 20 years, but the mechanisms underlying ADHD risk are multifactorial, complex and heterogeneous and, as yet, are poorly defined. In this review, we argue that one approach to address this challenge is to study well-defined disorders to provide insights into potential biological pathways that may be involved in idiopathic ADHD.

Main body

To address this premise, we selected two neurogenetic conditions that are associated with significantly increased ADHD risk: Turner syndrome and the RASopathies (of which Noonan syndrome and neurofibromatosis type 1 are the best-defined with regard to ADHD-related phenotypes). These syndromes were chosen for two main reasons: first, because intellectual functioning is relatively preserved, and second, because they are strikingly phenotypically similar but are etiologically distinct. We review the cognitive, behavioural, neural and cellular phenotypes associated with these conditions and examine their relevance as a model for idiopathic ADHD.

Conclusion

We conclude by discussing current and future opportunities in the clinical and basic research of these conditions, which, in turn, may shed light upon the biological pathways underlying idiopathic ADHD.

Dysfunctional mTORC1 Signaling: A Convergent Mechanism between Syndromic and Nonsyndromic Forms of Autism Spectrum Disorder?

Whereas autism spectrum disorder (ASD) exhibits striking heterogeneity in genetics and clinical presentation, dysfunction of mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway has been identified as a molecular feature common to several well-characterized syndromes with high prevalence of ASD. Additionally, recent findings have also implicated mTORC1 signaling abnormalities in a subset of nonsyndromic ASD, suggesting that defective mTORC1 pathway may be a potential converging mechanism in ASD pathology across different etiologies. However, the mechanistic evidence for a causal link between aberrant mTORC1 pathway activity and ASD neurobehavioral features varies depending on the ASD form involved. In this review, we first discuss six monogenic ASD-related syndromes, including both classical and potentially novel mTORopathies, highlighting their contribution to our understanding of the neurobiological mechanisms underlying ASD, and then we discuss existing evidence suggesting that aberrant mTORC1 signaling may also play a role in nonsyndromic ASD.

Thick Corpus Callosum in Children

BACKGROUND AND PURPOSE

A thick corpus callosum (TCC) can be associated with a very grave outcome in fetuses, but its clinical presentation in older children seems to be markedly different.

METHODS

The corpus callosum (CC) was defined as thick based on observations and impressions. We reviewed cases of children who were diagnosed as TCC based on brain magnetic resonance imaging (MRI) studies. The pertinent clinical data of these children were collected, and their CCs were measured.

RESULTS

Out of 2,552 brain MRI images, those of 37 children were initially considered as showing a TCC. Those initial imaging were reviewed by an experienced neuroradiologist, who confirmed the diagnosis in 34 children (1.3%): 13 had neurofibromatosis-1 (NF-1), 9 had epilepsy, 3 had macrocephaly capillary malformation (MCM) syndrome, 3 had autistic spectrum disorder, 1 had a Chiari-1 malformation, and 1 had increased head circumference. No specific neurologic disorder could be defined in seven children. The measured thickness of the CC in these children was comparable to those published in the literature for adults.

CONCLUSIONS

A TCC is a rare brain malformation that can be found in neuropathologies with apparently diverse pathognomonic mechanisms, such as NF-1 and MCM. It is not necessarily associated with life-threatening conditions, instead being a relatively benign finding, different in nature from that reported in fetuses.

Macrocephaly Is Not a Predictor of Optic Pathway Glioma Development or Treatment in Neurofibromatosis Type 1

Neurofibromatosis type 1 is a common neurogenetic disorder characterized by significant clinical variability. As such, numerous studies have focused on identifying clinical, radiographic, or molecular biomarkers that predict the occurrence or progression of specific clinical features in individuals with neurofibromatosis type 1. One of these clinical biomarkers, macrocephaly, has been proposed as a prognostic factor for optic pathway glioma development. In the current study, the authors demonstrate that macrocephaly is not associated with the development of these brain tumors or the need to institute treatment for clinical progression. These findings suggest that macrocephaly is not a robust biomarker of optic pathway glioma formation or progression in children with neurofibromatosis type 1.

Thick corpus callosum in the second trimester can be transient and is of uncertain significance

OBJECTIVES

Depiction of a thick corpus callosum (CC) in utero is rare, and is generally associated with severe brain anomalies. Our aim was to describe a group of fetuses diagnosed during second-trimester ultrasound examination as having an apparently isolated thick CC, which normalized subsequently in the cases followed to term.

METHODS

Among 59 fetuses referred to the Ob-Gyn Ultrasound Division of Lis Maternity Hospital with suspected callosal anomalies between January 2013 and June 2014, we identified nine cases with an apparently isolated thick CC for inclusion in this retrospective cohort study. Length and body thickness of the CC were compared with previously published nomograms. Fetuses with a suspected isolated thick CC were identified and followed until delivery or termination of pregnancy (TOP). Evaluation consisted of chromosomal analysis, at least one magnetic resonance imaging (MRI) examination and repeat ultrasound examinations. Postnatal evaluation included brain ultrasound examination, MRI when indicated and neurodevelopmental assessment through validated pediatric questionnaires.

RESULTS

The nine fetuses were diagnosed with an apparently isolated thick CC at a mean gestational age of 23 + 5 (range, 21-29) weeks. Eight exhibited a CC body thickness ≥ 2SD above the mean for gestational age and one exhibited only a thickened genu. Six also exhibited a relatively short CC. Two patients opted for TOP but declined autopsy. In five of the seven remaining fetuses, the CC thickness normalized during follow-up. In the remaining two, the increased CC thickness was a variant of the cingulate sulcus. The CC length remained ≤ 2SD in five of the six fetuses with a short CC. Fetal MRI was performed and confirmed the diagnosis in six fetuses. The karyotype was normal in all fetuses. Short-term neurodevelopmental outcome was reported as normal in all six children with complete follow-up.

CONCLUSIONS

Although the number of fetuses in our study is relatively small, it seems that an apparently isolated thick CC is not necessarily associated with poor prognosis. In such cases, a definitive diagnosis should not be reached based on a single measurement and repeat follow-up examinations during the third trimester are recommended. Copyright © 2015 ISUOG. Published by John Wiley & Sons Ltd.

Brain and behaviour phenotyping of a mouse model of neurofibromatosis type-1: an MRI/DTI study on social cognition

Neurofibromatosis type-1 (NF1) is a common neurogenetic disorder and an important cause of intellectual disability. Brain-behaviour associations can be examined in vivo using morphometric magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI) to study brain structure. Here, we studied structural and behavioural phenotypes in heterozygous Nf1 mice (Nf1(+/-) ) using T2-weighted imaging MRI and DTI, with a focus on social recognition deficits. We found that Nf1(+/-) mice have larger volumes than wild-type (WT) mice in regions of interest involved in social cognition, the prefrontal cortex (PFC) and the caudate-putamen (CPu). Higher diffusivity was found across a distributed network of cortical and subcortical brain regions, within and beyond these regions. Significant differences were observed for the social recognition test. Most importantly, significant structure-function correlations were identified concerning social recognition performance and PFC volumes in Nf1(+/-) mice. Analyses of spatial learning corroborated the previously known deficits in the mutant mice, as corroborated by platform crossings, training quadrant time and average proximity measures. Moreover, linear discriminant analysis of spatial performance identified 2 separate sub-groups in Nf1(+/-) mice. A significant correlation between quadrant time and CPu volumes was found specifically for the sub-group of Nf1(+/-) mice with lower spatial learning performance, suggesting additional evidence for reorganization of this region. We found strong evidence that social and spatial cognition deficits can be associated with PFC/CPu structural changes and reorganization in NF1.

Resting state functional MRI reveals abnormal network connectivity in neurofibromatosis 1

Neurofibromatosis type I (NF1) is a genetic disorder caused by mutations in the neurofibromin 1 gene at locus 17q11.2. Individuals with NF1 have an increased incidence of learning disabilities, attention deficits, and autism spectrum disorders. As a single-gene disorder, NF1 represents a valuable model for understanding gene-brain-behavior relationships. While mouse models have elucidated molecular and cellular mechanisms underlying learning deficits associated with this mutation, little is known about functional brain architecture in human subjects with NF1. To address this question, we used resting state functional connectivity magnetic resonance imaging (rs-fcMRI) to elucidate the intrinsic network structure of 30 NF1 participants compared with 30 healthy demographically matched controls during an eyes-open rs-fcMRI scan. Novel statistical methods were employed to quantify differences in local connectivity (edge strength) and modularity structure, in combination with traditional global graph theory applications. Our findings suggest that individuals with NF1 have reduced anterior-posterior connectivity, weaker bilateral edges, and altered modularity clustering relative to healthy controls. Further, edge strength and modular clustering indices were correlated with IQ and internalizing symptoms. These findings suggest that Ras signaling disruption may lead to abnormal functional brain connectivity; further investigation into the functional consequences of these alterations in both humans and in animal models is warranted.

Is it time to change the neurofibromatosis 1 diagnostic criteria?

Neurofibromatosis 1 is a complex inherited neurocutaneous disease that is often difficult to diagnose early because of its age-dependent presentation. The diagnosis is also extremely difficult to communicate to patients and their parents because of the disease's clinical variability, unpredictable evolution, and uncertain prognosis. Since 1988, the year of publication of the last Consensus Conference statement concerning the diagnosis of neurofibromatosis 1, our understanding of the disease has naturally increased and, in addition to the availability of increasingly precise molecular analyses, some new clinical signs have been reported such as anaemic nevi, unidentified bright objects, choroidal hamartomas, and a typical neuropsychological phenotype. We critically review the current diagnostic criteria, and suggest the addition of new signs on the basis of published findings and our own clinical experience. This proposal aims to improve diagnostic power in paediatric age, securing a better and more reliable healthcare transition toward adult age. We finally recommend a new Consensus Conference in order to revise the diagnostic criteria, possibly differentiated by age of presentation.

Characterizing the microstructural basis of "unidentified bright objects" in neurofibromatosis type 1: A combined in vivo multicomponent T2 relaxation and multi-shell diffusion MRI analysis

Introduction

The histopathological basis of “unidentified bright objects” (UBOs) (hyperintense regions seen on T2-weighted magnetic resonance (MR) brain scans in neurofibromatosis-1 (NF1)) remains unclear. New in vivo MRI-based techniques (multi-exponential T2 relaxation (MET2) and diffusion MR imaging (dMRI)) provide measures relating to microstructural change. We combined these methods and present previously unreported data on in vivo UBO microstructure in NF1.

Methods

3-Tesla dMRI data were acquired on 17 NF1 patients, covering 30 white matter UBOs. Diffusion tensor, kurtosis and neurite orientation and dispersion density imaging parameters were calculated within UBO sites and in contralateral normal appearing white matter (cNAWM). Analysis of MET2 parameters was performed on 24 UBO–cNAWM pairs.

Results

No significant alterations in the myelin water fraction and intra- and extracellular (IE) water fraction were found. Mean T2 time of IE water was significantly higher in UBOs. UBOs furthermore showed increased axial, radial and mean diffusivity, and decreased fractional anisotropy, mean kurtosis and neurite density index compared to cNAWM. Neurite orientation dispersion and isotropic fluid fraction were unaltered.

Conclusion

Our results suggest that demyelination and axonal degeneration are unlikely to be present in UBOs, which appear to be mainly caused by a shift towards a higher T2-value of the intra- and extracellular water pool. This may arise from altered microstructural compartmentalization, and an increase in ‘extracellular-like’, intracellular water, possibly due to intramyelinic edema. These findings confirm the added value of combining dMRI and MET2 to characterize the microstructural basis of T2 hyperintensities in vivo.

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We examine MRI white matter T2-weighted hyperintense lesions, “UBOs” in NF1.

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Myelin water and intra- and extracellular water fractions are unchanged in UBOs.

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Diffusivity is higher, while mean kurtosis and neurite density are lower in UBOs.

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The combined measures suggest that UBOs may arise from intramyelinic edema.

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Combining diffusion MRI and multi-exponential T2 relaxation has added value.

Quantitative multi-modal MRI of the Hippocampus and cognitive ability in community-dwelling older subjects

Hippocampal structural integrity is commonly quantified using volumetric measurements derived from brain magnetic resonance imaging (MRI). Previously reported associations with cognitive decline have not been consistent. We investigate hippocampal integrity using quantitative MRI techniques and its association with cognitive abilities in older age.

Participants from the Lothian Birth Cohort 1936 underwent brain MRI at mean age 73 years. Longitudinal relaxation time (T1), magnetization transfer ratio (MTR), fractional anisotropy (FA) and mean diffusivity (MD) were measured in the hippocampus. General factors of fluid-type intelligence (g), cognitive processing speed (speed) and memory were obtained at age 73 years, as well as childhood IQ test results at age 11 years. Amongst 565 older adults, multivariate linear regression showed that, after correcting for ICV, gender and age 11 IQ, larger left hippocampal volume was significantly associated with better memory ability (β = .11, p = .003), but not with speed or g. Using quantitative MRI and after correcting for multiple testing, higher T1 and MD were significantly associated with lower scores of g (β range = −.11 to −.14, p < .001), speed (β range = −.15 to −.20, p < .001) and memory (β range = −.10 to −.12, p < .001). Higher MTR and FA in the hippocampus were also significantly associated with higher scores of g (β range = .17 to .18, p < .0001) and speed (β range = .10 to .15, p < .0001), but not memory.

Quantitative multi-modal MRI assessments were more sensitive at detecting cognition-hippocampal integrity associations than volumetric measurements, resulting in stronger associations between MRI biomarkers and age-related cognition changes.

Bilateral posterior periventricular nodular heterotopia: a recognizable cortical malformation with a spectrum of associated brain abnormalities

BACKGROUND AND PURPOSE

Bilateral posterior PNH is a distinctive complex malformation with imaging features distinguishing it from classic bilateral PNH associated with FLNA mutations. The purpose of this study was to define the imaging features of posterior bilateral periventricular nodular heterotopia and to determine whether associated brain malformations suggest specific subcategories.

MATERIALS AND METHODS

We identified a cohort of 50 patients (31 females; mean age, 13 years) with bilateral posterior PNH and systematically reviewed and documented associated MR imaging abnormalities. Patients were negative for mutations of FLNA.

RESULTS

Nodules were often noncontiguous (n = 28) and asymmetric (n = 31). All except 1 patient showed associated developmental brain abnormalities involving a spectrum of posterior structures. A range of posterior fossa abnormalities affected the cerebellum, including cerebellar malformations and posterior fossa cysts (n = 38). Corpus callosum abnormalities (n = 40) ranged from mild dysplasia to agenesis. Posterior white matter volume was decreased (n = 22), and colpocephaly was frequent (n = 26). Most (n = 40) had associated cortical abnormalities ranging from minor to major (polymicrogyria), typically located in the cortex overlying the PNH. Abnormal Sylvian fissure morphology was common (n = 27), and hippocampal abnormalities were frequent (n = 37). Four family cases were identified-2 with concordant malformation patterns and 2 with discordant malformation patterns.

CONCLUSIONS

The associations of bilateral posterior PNH encompass a range of abnormalities involving brain structures inferior to the Sylvian fissures. We were unable to identify specific subgroups and therefore conceptualize bilateral posterior PNH as a continuum of infrasylvian malformations involving the posterior cerebral and hindbrain structures.

Alterations in white matter microstructure in neurofibromatosis-1

Neurofibromatosis (NF1) represents the most common single gene cause of learning disabilities. NF1 patients have impairments in frontal lobe based cognitive functions such as attention, working memory, and inhibition. Due to its well-characterized genetic etiology, investigations of NF1 may shed light on neural mechanisms underlying such difficulties in the general population or other patient groups. Prior neuroimaging findings indicate global brain volume increases, consistent with neural over-proliferation. However, little is known about alterations in white matter microstructure in NF1. We performed diffusion tensor imaging (DTI) analyses using tract-based spatial statistics (TBSS) in 14 young adult NF1 patients and 12 healthy controls. We also examined brain volumetric measures in the same subjects. Consistent with prior studies, we found significantly increased overall gray and white matter volume in NF1 patients. Relative to healthy controls, NF1 patients showed widespread reductions in white matter integrity across the entire brain as reflected by decreased fractional anisotropy (FA) and significantly increased absolute diffusion (ADC). When radial and axial diffusion were examined we found pronounced differences in radial diffusion in NF1 patients, indicative of either decreased myelination or increased space between axons. Secondary analyses revealed that FA and radial diffusion effects were of greatest magnitude in the frontal lobe. Such alterations of white matter tracts connecting frontal regions could contribute to the observed cognitive deficits. Furthermore, although the cellular basis of these white matter microstructural alterations remains to be determined, our findings of disproportionately increased radial diffusion against a background of increased white matter volume suggest the novel hypothesis that one potential alteration contributing to increased cortical white matter in NF1 may be looser packing of axons, with or without myelination changes. Further, this indicates that axial and radial diffusivity can uniquely contribute as markers of NF1-associated brain pathology in conjunction with the typically investigated measures.

Developmental malformation of the corpus callosum: a review of typical callosal development and examples of developmental disorders with callosal involvement

This review provides an overview of the involvement of the corpus callosum (CC) in a variety of developmental disorders that are currently defined exclusively by genetics, developmental insult, and/or behavior. I begin with a general review of CC development, connectivity, and function, followed by discussion of the research methods typically utilized to study the callosum. The bulk of the review concentrates on specific developmental disorders, beginning with agenesis of the corpus callosum (AgCC)-the only condition diagnosed exclusively by callosal anatomy. This is followed by a review of several genetic disorders that commonly result in social impairments and/or psychopathology similar to AgCC (neurofibromatosis-1, Turner syndrome, 22q11.2 deletion syndrome, Williams yndrome, and fragile X) and two forms of prenatal injury (premature birth, fetal alcohol syndrome) known to impact callosal development. Finally, I examine callosal involvement in several common developmental disorders defined exclusively by behavioral patterns (developmental language delay, dyslexia, attention-deficit hyperactive disorder, autism spectrum disorders, and Tourette syndrome).

Thick fetal corpus callosum: an ominous sign?

OBJECTIVE

Anomalies of the corpus callosum are frequently diagnosed during pregnancy, but a thick corpus callosum is a rare finding and its significance is not clear. We aimed to assess the significance of thick fetal corpus callosum by reviewing our experience of such cases.

METHODS

The records of all fetuses with anomalies of the corpus callosum referred to the prenatal diagnosis units of two university hospitals from 2000 to 2007 were reviewed. Nine fetuses with a thick corpus callosum were identified.

RESULTS

In all cases there were associated abnormalities: macrocephaly, ventriculomegaly, vermian agenesis, abnormal sulcation or encephalocele. Four pregnancies were terminated and in each of these cases the autopsy confirmed dysmorphic features and additional brain abnormalities. Five infants were delivered; two died shortly after birth, one suffers from mental retardation, one had neonatal convulsions and one is developing normally.

CONCLUSIONS

A thick fetal corpus callosum is usually associated with other brain anomalies and is part of a neurogenetic syndrome in most cases.

Neurofibromin regulates somatic growth through the hypothalamic-pituitary axis

To study the role of the neurofibromatosis-1 (NF1) gene in mammalian brain development, we recently generated mice in which Nf1 gene inactivation occurs in neuroglial progenitor cells using the brain lipid binding protein (BLBP) promoter. We found that Nf1(BLBP)CKO mice exhibit significantly reduced body weights and anterior pituitary gland sizes. We further demonstrate that the small anterior pituitary size reflects loss of neurofibromin expression in the hypothalamus, leading to reduced growth hormone releasing hormone, pituitary growth hormone (GH) and liver insulin-like growth factor-1 (IGF1) production. Since neurofibromin both negatively regulates Ras activity and positively modulates cAMP levels, we examined the signaling pathway responsible for these abnormalities. While BLBP-mediated expression of an activated Ras molecule did not recapitulate the body weight and hypothalamic/pituitary defects, treatment of Nf1(BLBP)CKO mice with rolipram to increase cAMP levels resulted in a partial restoration of the body weight phenotype. Furthermore, conditional expression of the Ras regulatory GAP domain of neurofibromin also did not rescue the body weight or Igf1 mRNA defects in Nf1(BLBP)CKO mice. Collectively, these data demonstrate a critical role for neurofibromin in hypothalamic-pituitary axis function and provide further insights into the short stature and GH deficits seen in children with NF1.

Neural phenotypes of common and rare genetic variants

Neuroimaging methods offer a powerful way to bridge the gaps between genes, neurobiology and behavior. Such investigations may be further empowered by complementary strategies involving chromosomal abnormalities associated with particular neurobehavioral phenotypes, which can help to localize causative genes and better understand the genetics of complex traits in the general population. Here we review the evidence from studies using these convergent approaches to investigate genetic influences on brain structure: (1) studies of common genetic variations associated with particular neuroanatomic phenotypes, and (2) studies of possible 'genetic subtypes' of neuropsychiatric disorders with very high penetrance, with a focus on neuroimaging studies using novel computational brain mapping algorithms. Finally, we discuss the contribution of behavioral neurogenetics research to our understanding of the genetic basis of neuropsychiatric disorders in the broader population.

Non-arteritic anterior ischaemic optic neuropathy: evaluation of the brain and optic pathway by conventional MRI and magnetisation transfer imaging

The purpose of the study was to examine the brain and the visual pathway of patients with non-arteritic anterior ischaemic optic neuropathy (NAION) by using conventional MRI (cMRI) and volumetric magnetisation transfer imaging (MTI). Thirty NAION patients, aged 67.5 +/- 8.14 years, and 28 age- and gender-matched controls were studied. MTI was used to measure the magnetisation transfer ratio (MTR) of the chiasm and for MTR histograms of the brain. The presence of areas of white matter hyperintensity (WMH) was evaluated on fluid-attenuated inversion recovery (FLAIR) images. Area of the optic nerves (ONs) and volume of the chiasm were assessed, as were coronal short-tau inversion recovery (STIR) and MTI images, respectively. More areas of WMH were observed in patients (total 419; mean 14.4; SD 19) than in controls (total 127; mean 4.7; SD 5.7), P < 0.001. Area (in square millimetres) of the affected ONs, volume(in cubic millimetres) and MTR (in percent) of the chiasm (10.7 +/- 4.6), (75.8 +/- 20.2), (56.4 +/- 6.5), respectively, were lower in patients than in controls (13.6 +/- 4.3), (158.2 +/- 75.3) (62.1 +/- 6.2), respectively, P < 0.05. Mean MTR of brain histograms was lower in patients (53.0 +/- 8.0) than in controls (58.0 +/- 5.6), P < 0.05. NAION is characterised by decreased ON and chiasmatic size. The low MTR of the chiasm and brain associated with increased areas of WMH may be suggestive of demyelination and axonal damage due to generalised cerebral vascular disease.