The X-chromosome and GENDER effects IN physiology, pathophysiology, and longevity
, Head, Section on Growth and Metabolism, Section on Women’s Health Research, and Unit on Turner Syndrome
Vladimir Bakalov, MD, Staff Clinician
Clara Cheng, PhD, Biologist
Jian Zhou, MD, PhD, Staff Scientist
Eileen Lange, RN, Research Nurse
Lea Ann Matura, PhD, Postdoctoral Fellow
Alexis Sheaffer, BA, Postbaccalaureate Fellow
Nicole Wooten, BA, Postbaccalaureate Fellow
Wunan Zhou, BA, Postbaccalaureate Fellow
Our research aims to identify and define the function of X-chromosome genes involved in the differential development and function of brain, reproductive, metabolic, and immune systems in women and men. The study of girls and women with Turner syndrome (TS) provides a unique opportunity to elucidate X-chromosome gene dosage effects and to improve our understanding of this common disorder, which affects approximately 1 in 2,000 females. The research will improve our ability to care for girls and women with TS and enhance our understanding of disease processes such as women’s increased susceptibility to autoimmune disease and men’s increased risk for coronary disease.
X-chromosome, genomic imprinting, and longevity
Many of the obvious phenotypic differences between normal women and men are attributable to differential exposure to sex steroids. Since Mary Lyon advanced the elegant concept of random X-inactivation in female cells as a form of sex chromosome dosage compensation, the second X-chromosome was considered totally inert. However, it was difficult to reconcile Lyon’s view with the finding of a distinct phenotype in 45,X females with TS—evidence that the second X-chromosome is important for normal female development. We now know that epigenetic mechanisms regulate regions of X-inactivation and regions that escape both inactivation and genomic imprinting, thereby contributing to sex differences by X-chromosome gene dosage effects.
Given the traditional view that each sex had only one functional X-chromosome, it was assumed that the only genetic difference between the sexes derived from Y-chromosome, testis-determining genes. Thus, phenotypic differences between the sexes were attributed to differential exposure to gonadal secretions, e.g., estrogen in females and androgen in males. While many sex differences do clearly result from differential exposure to sex steroids, sex steroid effects do not adequately explain some important male-female distinctions, particularly women’s greater longevity. Recent studies indicate that about 20 percent of X-chromosome genes escape inactivation. Two types of X-linked genes that escape inactivation are (1) pseudoautosomal X-chromosome genes with Y-chromosome homologues that essentially behave like autosomal genes, with expression from both X-chromosomes in females and from X- and Y-chromosomes in males; and (2) genes that are unique to the X-chromosome, with no Y-allele. The expression of such genes from both X-chromosomes in females may contribute to female-specific reproductive processes such as oocyte survival and some sex-specific aspects of brain development.
Genomic imprinting may also regulate X-chromosome gene expression contributing to male-female differences and to the TS phenotype. Genomic imprinting involves the selective expression of certain genes determined by their parental origin, often associated with DNA methylation of imprinted, or silenced, alleles. Given that normal women are mosaic for maternally and paternally inherited active X-chromosomes (XM and XP), whereas men are monosomic for XM, genomic imprinting of X-linked genes causes distinct gene expression in males and females. Imprinted XM genes would still be expressed in about 50 percent of female cells but not in male cells.
Women enjoy greater longevity than men mainly because of their lower risk across all age groups for ischemic heart disease. Women’s primary advantage in this regard is their salutary “gynoid” fat distribution—adipose tissue preferentially concentrated subcutaneously in the hips and thighs. In contrast, normal men tend to concentrate fat in the abdominal area, especially intraperitoneal visceral fat, which has many adverse metabolic effects, including an atherogenic lipid profile and elevated mediators of inflammation, which are all independently associated with increased cardiovascular risk.
Parental imprinting of X-linked genes involved in regional fat distribution and lipid metabolism may have favorable effects in women. For example, an X-chromosome gene that prevents intra-abdominal fat deposition could be imprinted or silenced on XM but active from XP. Given that men receive only XM, they would be at a disadvantage compared with normal women who, because of random X inactivation, express the XP allele in about 50 percent of their cells. To test the hypothesis that X-chromosome genomic imprinting contributes to the regional fat and metabolic differences between normal women and men, we assessed these factors in groups of women monosomic for XM versus XP.
Table 5.1 X-Chromosome Parental Origin and Metabolic Profile
The XM and XP groups had similar body mass index (BMI) and total body fat, but women with XM had greater abdominal and intra-abdominal visceral fat, which was associated with higher LDL-cholesterol and triglycerides compared to those with XP. The finding of a male-type fat distribution and lipid profile in XM women supports the view that differential X-chromosome gene dosage determined by genomic imprinting contributes to the excess mortality from ischemic heart disease in 46, XMY men. Given the different parental origins of the X-chromosome in males and females, genomic imprinting is predictably associated with differential effects depending on the sex of the offspring (unlike autosomal imprinting). Thus, X-linked imprinting is not expected to regulate traits that are not sexually dimorphic, such as renal or cardiac development. We proved this prediction in a recent study showing equal prevalence of renal and cardiovascular defects in XM versus XP groups with TS. Somatic size, in contrast, is sexually dimorphic, with men typically considerably larger than women; therefore, size is related to X origin.
Bondy CA. The X-chromosome and genomic imprinting. In: Gravholt CG, Bondy CA, eds. Wellness for Girls and Women with Turner Syndrome. Elsevier Scientific Press, 2006;21-5.
Bondy CA, Wooten N, Matura LA, Zinn AR, Bakalov VB. The physical phenotype in Turner syndrome is not imprinted. Hum Genet 2007;121:469-74.
Van P, Bakalov V, Bondy CA. Monosomy for the X chromosome is associated with an atherogenic lipid profile. J Clin Endocrinol Metab 2006;91:2867-70.
Van P, Bakalov V, Zinn A, Bondy CA. Maternal X-chromosome, visceral adiposity and lipid profile. JAMA 2006;295:1373-4.
Congenital cardiovascular defects in Turner syndrome
Congenital cardiovascular disease may be the most serious medical problem in monosomy X or Turner syndrome. Pioneering the use of high-resolution magnetic resonance angiography (MRA) in this syndrome, we demonstrated cardiovascular anomalies in about half the study subjects in contrast to the previously accepted 20 to 30 percent prevalence revealed by echocardiographic studies. Whereas congenital heart defects in Turner syndrome were thought to be limited to left-sided, outflow tract defects, MRA revealed a high prevalence of major venous malformations, including partial anomalous pulmonary venous return and persistent left superior vena cava affecting over 20 percent of the study population. The most common anomaly defined in our study was a distinctive aortic deformation affecting about 50 percent of women with Turner syndrome and termed elongated transverse arch of the aorta (ETA). This newly described aortic abnormality seems to be predictive of aortic complications and hence mandates vigilant surveillance of aorta dimensions in affected patients.
Figure 5.1
Magnetic resonance angiography showing a dilated ascending aorta (solid arrow) and normal descending aorta (open arrow). The patient’s aortic arch had a distinctive squaring off and elongation of the transverse with a prominent kink at the transition to the descending aorta (arrowhead) termed ETA. Although the diameter of the ascending aorta in this woman was only 3.7 cm, which is considered at the upper limit of normal for women her age (48), it is almost three-fold larger than the descending aorta while the normal ascending/descending ratio is less than 1.5. Unfortunately, the patient experienced acute, fatal aortic dissection.
In recent years, we have encountered frequent case reports of fatal aortic dissections occurring in relatively young women with Turner syndrome. The true incidence was unclear, and it was not known whether dilatation of the ascending aorta predicts aortic dissection in Turner syndrome and, if so, what specific aortic diameters should provoke concern. In other patient groups at risk for dissection, including Marfan syndrome, dilatation of the ascending aortic is used as a predictor of an acute aortic event. Thresholds of 5.5 to 6.0 cm for the general population and 5 cm for those with Marfan syndrome guide “prophylactic” intervention to replace or stabilize the aneurysmal segment. Under this “one size fits all” prescription, however, women (who are smaller than men, for whom the guidelines were developed) have a higher likelihood of dissection/rupture and death; therefore, it has been suggested that intervention at a smaller diameter would save more female lives. Given that aortic diameter is closely correlated with somatic size, the aorta should be smaller than “normal” in very petite women with Turner syndrome because they might have significant aortic dilatation at diameters otherwise considered normal.
We recently reported the first prospective measure of the incidence of aortic dissection in Turner syndrome and proposed new guidelines to identify high-risk patients. We evaluated aortic diameters and other parameters in a large group of asymptomatic, unselected women with Turner syndrome and followed the women for an average of three years. During that period, we recorded three cases of aortic dissection among 158 patients, translating into an incidence of about 618 cases per 100,000 Turner syndrome years compared with about 6 per 100,000 non–Turner syndrome women years. The women who dissected were in their 40s and had aortic diameters ranging from 3.7 to 4.8 cm. They were under cardiologist care in their home area but, with aortic diameters of less than 5 cm, were not considered candidates for prophylactic intervention. Hence, waiting for aortic diameter to reach 5 cm is not appropriate for small patients with Turner syndrome. We investigated several methods of normalizing MRA-measured aortic diameters to individual body size and found that aortic diameter/body surface area (Aortic Size Index [ASI]) provided the highest correlation and greatest accuracy in identifying those at risk for dissection. Figure 5.1 shows the MRA study of one of the patients. In summary, 25 percent of the women with an absolute ascending aortic diameter of over 3.5 cm/m2 and 33 percent of the women with ASI over 2.5 cm/m2 experienced aortic dissection within about three years of follow-up.
Table 5.2 Monitoring for Aortic Dilatation in Turner syndrome
For screening purposes, we therefore suggest using the ASI 95th percentile of 2 cm/m2 for the ascending aorta determined by MRA, CT, or echocadiogram (Table 5.2). This method takes into account the considerable size variation of patients and identifies those 30 percent or so of women with Turner syndrome who require close monitoring. If ASI is equal to or greater than 2.5 cm/m2, the patient should be evaluated for prophylactic intervention. Clearly, further study is needed to determine if beta blocker or renin-angiotensin system blockade may prevent or retard aortic dilatation in patients with Turner syndrome and if prophylactic surgery may reduce the incidence of aortic dissection and rupture.
Bondy CA, Ceniceros I, Bakalov V, Rosing DR. Prolonged QTc and other ECG abnormalities in girls with Turner syndrome. Pediatrics 2006;118:e1220-5.
Bondy CA, Van P, Bakalov VK, Ho VB. Growth hormone treatment and aortic dimensions in Turner syndrome. J Clin Endocrinol Metab 2006;91:1785-8.
Bondy, CA, Van PL, Bakalov VK, Sachdev V, Malone CA, Ho VB, Rosing DR. Prolongation of the cardiac QTc interval in Turner syndrome. Medicine 2006;85:1-7.
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;115:732-5.
Matura LA, Ho VB, Rosing DR, Bondy CA. Aortic dilatation and dissection in Turner syndrome. Circulation 2007;116:1663-70.
Growth hormone treatment of girls with Turner syndrome
Short stature is the most common, readily recognizable clinical feature of Turner syndrome. The deficit in height is caused by haploinsufficiency of the short-stature homeobox-containing pseudoautosomal gene (SHOX). It affects virtually all individuals with Turner syndrome and results in an average adult height of about 20 cm less than predicted based on parental heights. Although girls with Turner syndrome are not usually deficient in growth hormone (GH), pharmacological GH treatment increases growth velocity and final adult stature. Thus, in 1986, the FDA approved the use of GH to enhance stature in Turner syndrome. The FDA’s approval of GH made treatment readily available to all girls diagnosed with Turner syndrome such that it has been difficult to conduct studies of GH effects in Turner syndrome with essential untreated Turner syndrome control groups. Our NIH study recruits girls and women with Turner syndrome from all over the United States and includes mostly girls who were treated with GH in private endocrinology practice settings, along with a smaller number who have never received GH owing to a very recent diagnosis or a diagnosis at an age deemed too late to start GH therapy. Thus, we have the rare and valuable opportunity to evaluate GH effects in girls with Turner syndrome compared to contemporaneous, age-matched controls.
In addition to promoting longitudinal bone growth, GH affects bone and body composition. To determine how GH treatment affects these important aspects of growth and development, we compared body composition and bone density by DXA and phalangeal cortical thickness by hand radiography in girls with Turner syndrome who had never received GH versus girls who were treated with GH for at least one year. After adjusting the sample for size and bone age, we observed no significant differences in trabecular or cortical bone density at the lumbar spine or wrist and no difference in cortical bone thickness measured at the second metacarpal. However, lean body mass percent was higher and total body fat percent lower in the GH-treated group. These effects were independent of estrogen exposure and were retained in girls who completed GH treatment a year or more before the study.
Given the high prevalence of congenital heart defects associated with GH, including problems of the aortic valve and aorta, we have been concerned about potential adverse effects of GH treatment on the cardiovascular system in Turner syndrome. Therefore, we compared aorta diameters and left ventricular (LV) dimensions and function in girls with Turner syndrome who received GH treatment versus girls who did not receive GH treatment. The two groups were similar in age and weight, but GH-treated subjects were on average 8 cm taller. The diameter of the ascending aorta increased by 7.3 percent and that of the descending aorta by 8.9 percent in the GH-treated group. However, after correcting for age, height, and weight by using multiple regressions, we noted that neither history of GH treatment nor the length of GH treatment affected aortic diameter. Likewise, cardiac dimensions corrected for age and body size did not differ significantly in the two groups, and LV function, assessed by the fractional shortening index, was similar in the two groups. Thus, it seems that GH treatment of girls with Turner syndrome does not affect cardiac or aortic dimensions beyond the proportionate increase related to larger body size, at least over intermediate-term (5 to 10 years) follow-up. More long-term follow-up is necessary to be certain that there are no residual effects on aortic and LV dimensions or function.
Ari M, Bakalov VK, Hill S, Bondy CA. The effects of growth hormone treatment on bone mineral density and body composition in girls with turner syndrome. J Clin Endocrinol Metab 2006;91:4302-5.
Bondy CA. Care of girls and women with Turner syndrome: a guideline of the Turner Syndrome Study Group. J Clin Endocrinol Metab 2007;92:10-25.
Bondy CA, Van P, Bakalov VK, Ho VB. Growth hormone treatment and aortic dimensions in Turner syndrome. J Clin Endocrinol Metab 2006;91:1785-8.
Corrigan EC, Nelson LM, Bakalov VK, Yanovski JA, Vanderhoof VH, Bondy CA. The effects of ovarian failure and X-chromosome deletion on body composition and insulin sensitivity in young women. Menopause 2006;13:911-6.
Matura LA, Sachdev V, Bakalov VB, Rosing DR, Bondy CA. Growth hormone treatment and left ventricular dimensions in Turner syndrome. J Pediatr 2007;150:587-91.
The X-chromosome, ovarian failure, and psychosocial function
Previous small surveys and anecdotal reports have indicated that many girls and women with Turner syndrome suffer from shyness and social anxiety. This “shyness trait” was traditionally attributed to stigmatization because of short stature. More recently, it has been suggested that social difficulties could have a neurobiological origin caused by genomic imprinting of X-linked genes involved in “social cognition.” We showed that women with Turner syndrome experience a rate of lifetime depression similar to that of women from gynecologic clinic samples with no “autistic spectrum” diagnoses. In open-ended interviews, girls and women identified ovarian failure, infertility, or “gender identity” as major issues related to their experience of Turner syndrome. Thus, our findings suggest that the experience of ovarian failure is a major hurdle for older girls and women with Turner syndrome while major psychological diagnoses and autistic-type disorders are uncommon.
To investigate the behavioral impact of early ovarian failure, we compared psychosocial functioning in women with Turner syndrome and women with 46,XX premature ovarian failure. We reasoned that, if the experience of premature ovarian failure per se leads to specific difficulties with social interactions in young women, then the two groups should demonstrate similar responses on tests of psychosocial function. Women with normal ovarian function served as contemporaneous controls. Our two ovarian failure groups reported identical increased shyness and social anxiety and decreased self-esteem levels compared with women with normal ovarian function. These findings suggest that the shyness and poor self-esteem seen in women with Turner syndrome reflect the experience of premature ovarian failure. The feelings of social inadequacy are associated with social isolation, difficulty in partnering, and very limited sexual functioning. Current studies target the role of infertility, childlessness, sex-steroid effects, or altered body image in the impaired social functioning in women with Turner syndrome. We plan to address the issue of how best to inform a young woman and her family about the diagnosis of premature ovarian failure so as to minimize psychological trauma. In addition, we expect to extend our studies to girls and young women who have survived cancer treatments with compromised ovarian function.
Schmidt P, Cardoso G, Haq NA, Ross JL, Rubinow DR, Bondy CA. Shyness, social anxiety, and impaired self-esteem in young women with ovarian failure. JAMA 2006;295:1374-6.
Sheaffer A, Lange E, Bondy CA. Sexual function in women with Turner syndrome. J Women’s Health, in press.
Sutton E, Young J, Bondy C, Biesecker B. Turner syndrome; four challenges across the lifespan. Amer J Med Genet 2005;139:57-66.
Sutton EJ, Young J, McInerney-Leo A, Bondy CA, Gollust SE, Biesecker BB. Truth-telling and Turner syndrome: the importance of diagnostic disclosure. J Pediatr 2006;148:102-7.
1 Mim Ari, BA, former Postbaccalaureate Fellow
2 Judith Ross, MD, former Guest Researcher, Thomas Jefferson University, PA
COLLABORATOR
Andrew Arai, MD, Laboratory of Cardiac Energetics, NHLBI, Bethesda, MD
Jeff Baron, MD, Program in Developmental Endocrinology and Genetics, NICHD, Bethesda, MD
Barbara Biesecker, MS, Social and Behavioral Research Branch, NIHGR, Bethesda, MD
Gina Cardoso, MD, Behavioral Endocrinology Branch, NIMH, Bethesda, MD
Andrew Griffith, MD, PhD, Neuro-Otology Branch, NIDCR, Bethesda, MD
Suvimol Hill, MD, Department of Radiology, NIH Clinical Center, Bethesda, MD
Vince Ho, MD, Department of Radiology, NIH Clinical Center, Bethesda, MD
James Reynolds, MD, Nuclear Medicine, NIH Clinical Center, Bethesda, MD
Douglas Rosing, MD, Cardiology Branch, NHLBI, Bethesda, MD
Vandana Sachdev, MD, Cardiology Branch, NHLBI, Bethesda, MD
Peter Schmidt, MD, Behavioral Endocrinology Branch, NIMH, Bethesda, MD
Erica Sutton, MS, Social and Behavioral Research Branch, NIHGR, Bethesda, MD
James Troendle, PhD, Biometry and Statistics Branch, NICHD, Bethesda, MD
Jack A. Yanovski, MD, PhD, Program in Developmental Endocrinology and Genetics, NICHD, Bethesda, MD
For further information, contact bondyc@mail.nih.gov.
