The Effects of the X Chromosome on Intrinsic Functional Connectivity in the Human Brain: Evidence from Turner Syndrome Patients

The Hypothesis of the Prolonged Cell Cycle in Turner Syndrome

Turner syndrome (TS) is a chromosomal disorder that is caused by a missing or structurally abnormal second sex chromosome. Subjects with TS are at an increased risk of developing intrauterine growth retardation, low birth weight, short stature, congenital heart diseases, infertility, obesity, dyslipidemia, hypertension, insulin resistance, type 2 diabetes mellitus, metabolic syndrome, and cardiovascular diseases (stroke and myocardial infarction). The underlying pathogenetic mechanism of TS is unknown. The assumption that X chromosome-linked gene haploinsufficiency is associated with the TS phenotype is questioned since such genes have not been identified. Thus, other pathogenic mechanisms have been suggested to explain this phenotype. Morphogenesis encompasses a series of events that includes cell division, the production of migratory precursors and their progeny, differentiation, programmed cell death, and integration into organs and systems. The precise control of the growth and differentiation of cells is essential for normal development. The cell cycle frequency and the number of proliferating cells are essential in cell growth. 45,X cells have a failure to proliferate at a normal rate, leading to a decreased cell number in a given tissue during organogenesis. A convergence of data indicates an association between a prolonged cell cycle and the phenotypical features in Turner syndrome. This review aims to examine old and new findings concerning the relationship between a prolonged cell cycle and TS phenotype. These studies reveal a diversity of phenotypic features in TS that could be explained by reduced cell proliferation. The implications of this hypothesis for our understanding of the TS phenotype and its pathogenesis are discussed. It is not surprising that 45,X monosomy leads to cellular growth pathway dysregulation with profound deleterious effects on both embryonic and later stages of development. The prolonged cell cycle could represent the beginning of the pathogenesis of TS, leading to a series of phenotypic consequences in embryonic/fetal, neonatal, pediatric, adolescence, and adulthood life.

Genetic conditions of short stature: A review of three classic examples

Noonan, Turner, and Prader-Willi syndromes are classical genetic disorders that are marked by short stature. Each disorder has been recognized for several decades and is backed by extensive published literature describing its features, genetic origins, and optimal treatment strategies. These disorders are accompanied by a multitude of comorbidities, including cardiovascular issues, endocrinopathies, and infertility. Diagnostic delays, syndrome-associated comorbidities, and inefficient communication among the members of a patient's health care team can affect a patient's well-being from birth through adulthood. Insufficient information is available to help patients and their multidisciplinary team of providers transition from pediatric to adult health care systems. The aim of this review is to summarize the clinical features and genetics associated with each syndrome, describe best practices for diagnosis and treatment, and emphasize the importance of multidisciplinary teams and appropriate care plans for the pediatric to adult health care transition.

Effects of hypogonadism on brain development during adolescence in girls with Turner syndrome

Gonadal steroids play an important role in brain development, particularly during puberty. Girls with Turner syndrome (TS), a genetic disorder characterized by the absence of all or part of the second X chromosome, mostly present a loss of ovarian function and estrogen deficiency, as well as neuroanatomical abnormalities. However, few studies have attempted to isolate the indirect effects of hormones from the direct genetic effects of X chromosome insufficiency. Brain structural (i.e., gray matter [GM] morphology and white matter [WM] connectivity) and functional phenotypes (i.e., resting-state functional measures) were investigated in 23 adolescent girls with TS using multimodal MRI to assess the role of hypogonadism in brain development in TS. Specifically, all girls with TS were divided into a hormonally subnormal group and an abnormal subgroup according to their serum follicle-stimulating hormone (FSH) levels, with the karyotypes approximately matched between the two groups. Statistical analyses revealed significant effects of the "group-by-age" interaction on GM volume around the left medial orbitofrontal cortex and WM diffusion parameters around the bilateral corticospinal tract, anterior thalamic radiation, left superior longitudinal fasciculus, and cingulum bundle, but no significant "group-by-age" or group differences were observed in resting-state functional measures. Based on these findings, estrogen deficiency has a nontrivial impact on the development of the brain structure during adolescence in girls with TS. Our present study provides novel insights into the mechanism by which hypogonadism influences brain development during adolescence in girls with TS, and highlights the important role of estrogen replacement therapy in treating TS.

X-Chromosome Effects on Attention Networks: Insights from Imaging Resting-State Networks in Turner Syndrome

Attention deficit hyperactivity disorder (ADHD) is strongly affected by sex, but sex chromosomes' effect on brain attention networks and cognition are difficult to examine in humans. This is due to significant etiologic heterogeneity among diagnosed individuals. In contrast, individuals with Turner syndrome (TS), who have substantially increased risk for ADHD symptoms, share a common genetic risk factor related to the absence of the X-chromosome, thus serving as a more homogeneous genetic model. Resting-state functional MRI was employed to examine differences in attention networks between girls with TS (n = 40) and age- sex- and Tanner-matched controls (n = 33). We compared groups on resting-state functional connectivity measures from data-driven independent components analysis (ICA) and hypothesis-based seed analysis. Using ICA, reduced connectivity was observed in both frontoparietal and dorsal attention networks. Similarly, using seeds in the bilateral intraparietal sulcus (IPS), reduced connectivity was observed between IPS and frontal and cerebellar regions. Finally, we observed a brain-behavior correlation between IPS-cerebellar connectivity and cognitive attention measures. These findings indicate that X-monosomy contributes affects to attention networks and cognitive dysfunction that might increase risk for ADHD. Our findings not only have clinical relevance for girls with TS, but might also serve as a biological marker in future research examining the effects of the intervention that targets attention skills.

A Synopsis of Current Practice in the Diagnosis and Management of Patients with Turner Syndrome in Turkey: A Survey of 18 Pediatric Endocrinology Centers

OBJECTIVE

A comprehensive survey was conducted to evaluate the shortcomings of clinical care in patients with Turner syndrome (TS) in Turkey.

METHODS

A structured questionnaire prepared by the Turner study group in Turkey, which covered relevant aspects of patient care in TS was sent to 44 pediatric endocrinology centers.

RESULTS

Eighteen centers (41%) responded to the questionnaire. In the majority of the centers, diagnostic genetic testing, screening for Y chromosomal material, protocols regarding the timing and posology of growth hormone (GH) and estrogen, thrombophilia screening, fertility information and screening for glucose intolerance, thyroid, and coeliac diseases in patients with TS were in line with the current consensus. Thirteen centers (72.2%) performed GH stimulation tests. Only four centers (22.2%) used oxandrolone in patients with TS with very short stature. The majority of the centers relied on bone age and breast development to assess estrogen adequacy, though together with variable combinations of oestrogen surrogates. Two centers (11.1%) reported performing serum estradiol measurements. Eight centers (44.4%) routinely conducted cardiac/thoracic aorta magnetic resonance imaging. Screening for hearing, dental and ophthalmologic problems were performed by thirteen (72.2%), six (33.3%) and ten (55.6%) centers, respectively. Psychiatric assessments were made by four centers (22.2%) at diagnosis, with only one center (5.6%) requiring annual reassessments.

CONCLUSION

Although we found some conformity between the current consensus and practice of the participating centers in Turkey regarding TS, further improvements are mandatory in the multi-disciplinary approach to address co-morbidities, which if unrecognized, may be associated with reduced quality of life and even mortality.

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.

Altered gray matter volume and functional connectivity of the motor network in young divers

BACKGROUND

Motor learning and professional sports training can induce plastic changes in brain structures that are associated with distinct training demands.

OBJECTIVE

To testify the hypothesis of that regional gray matter structures in the motor-related cortex and its functional connectivity (FC) are altered in young divers.

METHODS

We undertook T1-voxel-based morphometry (VBM) structural and resting-state functional magnetic resonance imaging in groups of diving athletes (DAs) and demographically-matched healthy controls.

RESULTS

Gray matter volume was lower in some regions in Das. By selecting the five most reduced regions, i.e. superior frontal gyrus, orbitofrontal cortex (OFC), insula, hippocampus, and cerebellum posterior lobe, as regions of interest (ROIs) for FC analysis, results showed that DAs had greater FC between the inferior temporal gyrus and superior frontal gyrus, OFC and cerebellum posterior lobe. Conversely, the divers had lesser FC between OFC and putamen, superior frontal gyrus and caudate.

CONCLUSIONS

VBM differences suggest that diving training entails more effective synaptic and/or neuronal pruning processes in motor structures. Indeed, cortical volumetric decreases in the DAs group are associated with increased FC among certain motor-related regions. We conclude that motor learning in adolescence alters brain structure in association with changes in FC between the relevant cortical and subcortical regions.