The sexes demonstrate important differences in cognitive, immunological, and metabolic functions as well as in longevity. For example, autism is about five times more prevalent in boys than in girls, and ischemic heart disease (IHD) is about twice as common and seen at an earlier age in men than in women while autoimmune diseases are several times more common in females than in males. Our goal is to elucidate the genetic and physiological mechanisms underlying these important but poorly understood differences. Previously, it was believed that the genetic contribution to sexual dimorphism begins and ends with the action of genes specifying gonads during early embryonic development, with all phenotypic differences occurring after gonadal development attributed to sex steroids. We now realize that, owing to escape from X inactivation or to genomic imprinting, many X chromosome genes may be asymmetrically expressed in normal women and men and may thus contribute to gender differences by dosage effects. We aim to identify and define the function of X chromosome genes involved in the differential development and function of the brain and the reproductive, metabolic, and immune systems in women and men. The study of monsomy X (Turner’s syndrome) provides a unique opportunity to elucidate X chromosome gene dosage effects.
The X chromosome and longevity
Bakalov, Van, Bondy
A major reason for the greater longevity of women versus men is their relative protection from IHD across all age groups. The traditional idea that estrogen protects women against IHD has recently been discredited. To investigate possible contributions of X chromosome gene(s) to female longevity, we examined IHD risk factors in women with Turner’s syndrome (TS); these women are characterized by short stature, premature ovarian failure, cardiovascular anomalies, and premature IHD. To control for ovarian failure in TS, we compared major IHD risk factors, glucose tolerance, lipid metabolism, and blood pressure (BP) in lean, young women with TS and in age- and body composition–matched (BMI) women with 46,XX premature ovarian failure (POF). We have shown that impaired glucose tolerance (IGT) and diabetes both increase in girls and women with TS. Interestingly, a novel insulin-secretory effect, not insulin resistance, explains the glucose intolerance in the young women and girls with TS . It thus appears that the Turner “metabolic syndrome” is a distinct entity characterized by decreased insulin secretion reminiscent of mature onset diabetes of the young (MODY) syndromes. Turner “metabolic syndrome” is caused by haploinsufficiency for autosomal genes involved in pancreatic development, suggesting that haploinsufficiency for unknown X chromosome gene(s) impairs beta cell function and predisposes to diabetes mellitus in TS. We also found that LDL-cholesterol and triglycerides are all significantly higher in TS than in age- and BMI-matched women with POF. Moreover, NMR spectroscopy revealed a concentration of smaller, denser HDL and LDL lipid particles in women with TS. The data showed a distinctly atherogenic lipid profile in otherwise healthy, nonobese young women with TS.
There are at least two major mechanisms by which a second X chromosome could contribute to lipid homeostasis. First, X chromosome genes involved in lipid metabolism or clearance may escape inactivation and thus be active in two copies in 46,XX women but in only one copy in men. Alternatively, parental imprinting of X chromosome genes involved in BP control may have favorable effects in women. For example, a gene that exerts a moderating effect could be imprinted or silenced on the maternal X (Xmat) but active from the paternal X allele. Given that men receive only the Xmat, they would not experience the moderating effects on BP, whereas normal women with random X inactivation would express the Xpat allele in about 50 percent of their cells. To test this hypothesis, we compared lipid profiles in healthy young women matched for age, body mass, and the presence of ovarian failure (eliminating the sex steroid variable). The experimental variable was the X chromosome; study groups included women with TS and a paternal X (45,XP), women with TS and a maternal X (45,XM), and karyotypically normal women (46,XPXM). Figure 3.1 shows the most important results.
Our novel findings implicate parental imprinting of X chromosome gene(s) in dyslipidemia and explain the increased risk for IHD in women with TS and probably in normal XY men as compared with women. The identification of these genes clearly is of great clinical importance.
Bakalov VK, Cooley MM, Quon MJ, Luo ML, Yanovski JA, Nelson LM, Sullivan G, Bondy CA. Impaired insulin secretion in the Turner metabolic syndrome. J Clin Endocrinol Metab 2004;89:3516-3520.
Bakalov VK, Van PL, Baron J, Reynolds JC, Bondy CA. Growth hormone therapy and bone mineral density in Turner syndrome. J Clin Endocrinol Metab 2004;89:4886-4889.
Cooley M, Bakalov V, Bondy CA. Lipid profiles in women with 45,X vs 46,XX primary ovarian failure. JAMA 2003;290:2127-2128.
Ho VB, Bakalov VK, Cooley M, Van PL, Hood MN, Burklow TR, Bondy CA. Major vascular anomalies in Turner syndrome: prevalence and magnetic resonance angiographic features. Circulation 2004;110:1694-1700.
Loscalzo ML, Van PL, Ho VB, Bakalov VK, Rosing DR, Malone CA, Dietz HC, Bondy CA. Association between fetal lymphedema and congenital cardiovascular defects in Turner syndrome. Pediatrics 2005;115:732-735.
The X chromosome and gender identity
Bondy; in collaboration with Biesecker, Schmidt
Many girls and women with TS suffer from excessive shyness or social anxiety. The “shyness trait” was usually attributed to the short stature characterizing the syndrome. More recently, it has been suggested that social difficulties could be neurobiologically based, caused by reduced dosage of X-linked genes involved in autistic spectrum behaviors. In collaboration with Peter Schmidt and colleagues, we found that young women with TS do not manifest more major psychiatric diagnoses than age-matched normal women but rather experience a higher rate of life-time depression as compared with community-based samples. The depression rate was similar to that reported for women with infertility from other causes. In collaboration with Barbara Biesecker, we conducted open-ended interviews in which we asked girls and women with TS to talk about their experience with the syndrome. Responses focused most on the subjects of ovarian failure, infertility, or “gender identity” as some termed it. Thus, it seemed that the experience of ovarian failure is the major concern of older girls and women with TS. To define further the impact of early ovarian failure on psychosocial functioning in young women, we compared psychosocial symptoms in 100 women with TS and 100 women with karyotypically normal premature ovarian failure. Women with normal ovarian function served as a contemporaneous control group for responses to a specific test battery focusing on social interaction. Our two ovarian failure groups reported virtually identical scores showing greater shyness and social anxiety and lower self-esteem than women with normal ovarian function. Thus, the shyness and poor self-esteem observed in women with TS is a response to the diagnosis of premature ovarian failure, not a response to short stature and apparently not the result of neurobiological, X-linked traits. Moreover, the psychosocial impact of premature ovarian failure in young women, with or without TS, appears to be far more serious than previously recognized and deserves more attention in the medical approach to this diagnosis.
Cardoso G, Daly R, Haq NA, Hanton L, Rubinow DR, Bondy CA, Schmidt P. Current and lifetime psychiatric illness in women with Turner syndrome. Gynecol Endocrinol 2004;19:313-319.
Ross JL, Stefanatos GA, Kushner H, Bondy CA, Nelson L, Zinn A, Roeltgen D. The effect of genetic differences: intact cognitive function in adult women with premature ovarian failure versus Turner syndrome. J Clin Endocrinol Metab 2004;89:1817-1822.
Sutton E, Young J, Bondy C, Biesecker B. Truth telling and Turner syndrome: the importance of full disclosure. J Pediatr (in press)./p>
Sutton EJ, McInerney-Leo A, Bondy C, Gollust SE, King D, Biesecker B. Turner syndrome: four challenges across the lifespan. Am J Med Genet A 2005;139:57-66.
The role of IGF1 in normal brain development and aging
Cheng, Wang, Smith4, Bondy
We have shown that endogenous brain IGF1 plays an insulin-like role in promoting neuronal glucose utilization and hence growth during postnatal development. Our studies have implicated (1) IGF1-induced phosphorylation of Akt/PKB in translocation of glucose across the neuronal membrane and (2) IGF1-induced phosphorylation of GSK3b in neuronal glycogenesis, suggesting that IGF1 augments neuronal glucose uptake and storage by familiar, insulin-like pathways. We have investigated IGF1’s role in neuronal generation, survival, growth, and morphogenesis. While neuronal cell numbers are preserved throughout most brain structures in the Igf1 null brain, we observed a significant reduction in hippocampal dentate granule cell number, which is attributable to increased cell death in the Igf1 null dentate germinal zone. Neuronal numbers were preserved in the Igf1 null frontoparietal cortex, but morphometric analysis showed that, in the Igf1 null mice, pyramidal neuron soma were about 10 percent smaller while Golgi staining showed a significant reduction in pyramidal dendritic length and complexity. In addition, the density of dendritic spines, and presumably of synaptic contacts, was 16 percent lower in the Igf1 null brain. Taken together, our findings illustrate multifaceted roles for IGF1 in postnatal brain development and explain why individuals with IGF1 gene deletions demonstrate mental retardation in addition to short stature.
In the aging brain, we have shown that IGF1 normally prevents tau hyper-phosphorylation and hence accumulation of neurotoxic tangles. Igf1 is known to phosphorylate and inactivate GSK3beta, an enzyme instrumental in tau phosphorylation, suggesting that the Igf1-receptor-PI3K-Akt-GSK3beta pathway is involved in this effect as well as in Igf1’s anabolic effects on early brain development.
Bondy CA, Cheng CM. Signaling by insulin-like growth factor 1 in brain. Eur J Pharmacol 2004;490:25-31.
Cheng CM, Hicks K, Wang J, Eagles DA, Bondy CA. Caloric restriction augments brain glutamic acid decarboxylase-65 and -67 expression. J Neurosci Res 2004;77:270-276.
Cheng C, Wang J, Bondy C. Tau is hyper-phosphorylated in the IGF1 null brain. Endocrinology2005 [Epub ahead of print].
Smith A, Bourdeau I, Wang J, Bondy CA. Expression of catenin family members CTNNA1, CTNNA2, CTNNB1 and JUP in the primate prefrontal cortex and hippocampus. Brain Res Mol Brain Res 2005;135:225-231.
Wang J, Cheng CM, Zhou J, Smith A, Weickert CS, Perlman WR, Becker KG, Powell D, Bondy CA. Estradiol alters transcription factor gene expression in primate prefrontal cortex. J Neurosci Res 2004;76:306-314.
The role of endogeneous androgens in female reproduction
Zhou, Dimitrakakis, Bondy
The normal ovary produces abundant quantities of testosterone in addition to estradiol, but usual hormone “replacement” treatment (HRT) for ovarian failure consists of estrogen and progesterone for most women with a uterus or estrogen alone for smaller numbers of hysterectomized women. Given, however, that such treatment increases the risk of breast cancer in menopausal women, HRT’s usefulness is limited. We have previously shown that androgens have anti-mammogenic effects and inhibit estrogen’s mitogenic effects on the mammary epithelium. In some countries, testosterone is often prescribed for menopausal women in addition to usual HRT. The rationale for testosterone supplementation has been that estrogen treatment reduces residual ovarian androgen production in post-menopausal women and may lead to sequestration of available testosterone by increasing sex hormone–binding globulin, resulting in symptoms of asthenia and loss of libido in some women.
To investigate the role of endogenous androgen in regulating mammary epithelial proliferation, we treated normally cycling rhesus monkeys with flutamide, an androgen receptor antagonist. Mammary epithelial proliferation (MEP) increased by about 50 percent in the flutamide-treated group, indicating that androgen receptor activation normally suppresses MEP. To evaluate the efficacy of physiologic androgen supplementation in limiting estrogen replacement therapy–induced MEP, we employed an ovariectomized rhesus monkey model of menopause. MEP increased four-fold in the estradiol- and estradiol-plus-progesterone–treated groups but did not differ from vehicle-treated control in the estradiol plus testosterone group. These observations imply that endogenous androgens normally limit MEP and that androgen supplementation of estrogen therapy may reduce estrogen-induced MEP and breast cancer risk.
Arraztoa JA, Zhou J, Marcu D, Cheng C, Bonner R, Chen M, Xiang C, Brownstein M, Maisey K, Imarai M, Bondy C. Identification of genes expressed in primate primordial oocytes. Hum Reprod 2005;20:476-483.
Bondy CA, Arrazstoa JA. Insulin like growth factors and ovarian follicular growth and function. In: O’Neill K, Richards J, eds. The Physiology of Reproduction (in press).
Dimitrakakis C, Jones RA, Liu A, Bondy CA. Breast cancer incidence in menopausal women using testosterone in addition to usual hormone therapy. Menopause 2004;11:531-535.
1Santiago University, Santiago, Chile
2Athens University, Athens, Greece
3Thomas Jefferson University, Philadelphia, PA
4Alastair Smith, PhD, former Visiting Fellow
Collaborators
Jeff Baron, MD, Developmental Endocrinology Branch, NICHD, Bethesda, MD
Barbara Biesecker, MS, Medical Genetics Branch, NIHGR, Bethesda, MD
Harry Deitz, MD, The Johns Hopkins University, Baltimore, MD
Andrew Griffith, MD, PhD, Neuro-Otology Branch, NIDCD, Bethesda, MD
Suvimol Hill, MD, Department of Radiology, Warren G. Magnuson Clinical Center, NIH, Bethesda, MD
Vince Ho, MD, Department of Radiology, Warren G. Magnuson Clinical Center, NIH, Bethesda, MD
Lawrence Nelson, MD, Developmental Endocrinology Branch, NICHD, Bethesda, MD
Mike Quon, MD, PhD, Laboratory of Clinical Investigation, NCAM, Bethesda, MD
James Reynolds, MD, Nuclear Medicine, Warren G. Magnuson Clinical Center, NIH, Bethesda, MD
Douglas Rosing, MD, Cardiovascular Branch, NHLBI, Bethesda, MD
David Rubinow, MD, Behavioral Endocrinology Branch, NIMH, Bethesda, MD
Peter Schmidt, MD, Behavioral Endocrinology Branch, NIMH, Bethesda, MD
Constantine Stratakis, MD, Developmental Endocrinology Branch, NICHD, Bethesda, MD
James Troendle, PhD, Biometry and Mathematical Statistics Branch, NICHD, Bethesda, MD
Jack A. Yanovski, MD, PhD, Developmental Endocrinology Branch, NICHD, Bethesda, MD
Andrew Zinn, MD, PhD, University of Texas Southwestern Medical School, Dallas, TX
For further information, contact bondyc@mail.nih.gov.