Turner syndrome-Missing X Chromosone

Turner syndrome - Wikipedia, the free encyclopedia:

Turner syndrome or Ullrich–Turner syndrome (also known as "Gonadal dysgenesis"^[1]:550), 45,X, encompasses several conditions in humanfemales, of which monosomy X (absence of an entire sex chromosome, the Barr body) is most common. It is a chromosomal abnormality in which all or part of one of the sex chromosomes is absent (unaffected humans have 46 chromosomes, of which two are sex chromosomes). Normal females have two X chromosomes, but in Turner syndrome, one of those sex chromosomes is missing or has other abnormalities. In some cases, the chromosome is missing in some cells but not others, a condition referred to as mosaicism^[2] or "Turner mosaicism".

Occurring in 1 in 2000^[3] – 1 in 5000 phenotypic females,^[4] the syndrome manifests itself in a number of ways. There are characteristic physical abnormalities, such as short stature, swelling, broad chest, low hairline, low-set ears, and webbed necks.^[5] Girls with Turner syndrome typically experience gonadal dysfunction (non-working ovaries), which results in amenorrhea (absence of menstrual cycle) and sterility. Concurrent health concerns are also frequently present, including congenital heart disease, hypothyroidism (reduced hormone secretion by the thyroid), diabetes, visionproblems, hearing concerns, and many autoimmune diseases.^[6] Finally, a specific pattern of cognitive deficits is often observed, with particular difficulties in visuospatial, mathematical, and memory areas.^[7]

Turner syndrome is named after Henry H. Turner.

Signs and symptoms

Lymphedema, puffy legs of a newborn with Turner syndrome

Common symptoms of Turner syndrome include:

• Short stature
• Lymphedema (swelling) of the hands and feet
• Broad chest (shield chest) and widely spaced nipples
• Low hairline
• Low-set ears
• Reproductive sterility
• Rudimentary ovaries gonadal streak (underdeveloped gonadal structures that later become fibrosed)
• Amenorrhoea, or the absence of a menstrual period
• Increased weight, obesity
• Shield shaped thorax of heart
• Shortened metacarpal IV
• Small fingernails
• Characteristic facial features
• Webbed neck from cystic hygroma in infancy
• Coarctation of the aorta
• Bicuspid aortic valve
• Poor breast development
• Horseshoe kidney
• Visual impairments sclera, cornea, glaucoma, etc.
• Ear infections and hearing loss
• High waist-to-hip ratio (the hips are not much bigger than the waist)
• Attention Deficit/Hyperactivity Disorder or ADHD (problems with concentration, memory, attention with hyperactivity seen mostly in childhood and adolescence)
• Nonverbal Learning Disability (problems with math, social skills and spatial relations)

Other features may include a small lower jaw (micrognathia), cubitus valgus,^[8] soft upturned nails, palmar crease, and drooping eyelids. Less common are pigmented moles, hearing loss, and a high-arch palate (narrow maxilla). Turner syndrome manifests itself differently in each female affected by the condition, therefore, no two individuals will share the same features.

[edit]Cause

Turner's syndrome is caused by the absence of two complete copies of the X chromosome in some, or all the cells. The abnormal cells may have only one X (monosomy) (45,X) or they may be affected by one of several types of partial monosomy like a deletion of the short p arm of one X chromosome (46,XdelXp) or the presence of an isochromosome with two q arms (46XiXq)^[9] In mosaic individuals, cells with X monosomy (45,X) may occur along with cells that are normal (46,XX), cells that have partial monosomies, or cells that have a Y chromosome (46,XY).^[9] The presence of mosaicism is estimated to be relatively common in affected individuals (67-90%).^[9]

In the majority of cases where monosomy occurs, the X chromosome comes from the mother.^[10] This may be due to a nondisjunction in the father. Meiotic errors that lead to the production of X with p arm deletions or abnormal Y chromosomes are also mostly found in the father.^[11] Isochromosome X or ring chromosome X on the other hand are formed equally often by both parents.^[11]Overall, the functional X chromosome mostly comes from the mother.

[edit]Diagnosis

[edit]Prenatal

45,X karyotype, showing an unpaired X at the lower right

Turner syndrome may be diagnosed by amniocentesis or chorionic villus sampling during pregnancy.

Often, fetuses with Turner syndrome can be identified by abnormal ultrasound findings (i.e., heart defect, kidney abnormality, cystic hygroma, ascites). In a study of 19 European registries, 67.2% of prenatally diagnosed cases of Turner Syndrome were detected by abnormalities on ultrasound. 69.1% of cases had one anomaly present, and 30.9% had two or more anomalies.^[12]

An increased risk of Turner Syndrome may also be indicated by abnormal triple or quadruple maternal serum screen. The fetuses diagnosed through positive maternal serum screening are more often found to have a mosaic karyotype than those diagnosed based on ultrasonographic abnormalities, and conversely those with mosaic karyotypes are less likely to have associated ultrasound abnormalities.^[12]

Although the recurrence risk is not increased, genetic counseling is often recommended for families who have had a pregnancy or child with Turner syndrome.

[edit]Postnatal

Turner Syndrome can be diagnosed postnatally at any age. Often, it is diagnosed at birth because of heart problems, an unusually wide neck or swelling of the hands and feet. However, it is also common for it to go undiagnosed for several years, typically until the girl reaches the age of puberty/adolescence and she fails to develop properly (the changes associated with puberty do not occur). In childhood, a short stature can be indicative of Turner syndrome.^[13]

A test, called a karyotype or a chromosome analysis, analyzes the chromosomal composition of the individual. This is the test of choice to diagnose Turner syndrome.

[edit]Prognosis

While most of the physical findings are harmless, there can be significant medical problems associated with the syndrome.

[edit]Prenatal

Despite the excellent postnatal prognosis, 99% of Turner-syndrome conceptions are thought to end in spontaneous abortion or stillbirth,^[14] and as many as 15% of all spontaneous abortions have the XO karyotype.^[15] Among cases that are detected by routine amniocentesis or chorionic villus sampling, one study found that the prevalence of Turner syndrome among tested pregnancies was 5.58 and 13.3 times higher respectively than among live neonates in a similar population.^[16]

[edit]Cardiovascular

Price et al. (1986 study of 156 female patients with Turner syndrome) showed a significantly greater number of deaths from diseases of the circulatory system than expected, half of them due tocongenital heart defects — mostly coarctation of the aorta. When patients with congenital heart disease were omitted from the sample of the study, the mortality from circulatory disorders was not significantly increased.^[17]

Cardiovascular malformations are a serious concern as it is the most common cause of death in adults with Turner syndrome. They play an important part in the 3-fold increase in overall mortality and the reduced life expectancy (up to 13 years) associated with Turner syndrome.

[edit]Cause

According to Sybert, 1998 the data are inadequate to allow conclusions about phenotype-karyotype correlations in regard to cardiovascular malformations in Turner syndrome because the number of individuals studied within the less common karyotype groups is too small. Other studies also suggest the presence of hidden mosaicisms that are not diagnosed on usual karyotypic analyses in some patients with 45,X karyotype.

In conclusion, the associations between karyotype and phenotypic characteristics, including cardiovascular malformations, remain questionable.

[edit]Prevalence of cardiovascular malformations

The prevalence of cardiovascular malformations among patients with Turner syndrome ranges from 17% (Landin-Wilhelmsen et al., 2001) to 45% (Dawson-Falk et al., 1992).

The variations found in the different studies are mainly attributable to variations in non-invasive methods used for screening and the types of lesions that they can characterize (Ho et al., 2004). However Sybert, 1998 suggests that it could be simply attributable to the small number of subjects in most studies.

Different karyotypes may have differing prevalence of cardiovascular malformations. Two studies found a prevalence of cardiovascular malformations of 30%^[18] and 38%^[19] in a group of pure 45,X monosomy. But considering other karyotype groups, they reported a prevalence of 24.3%^[18] and 11%^[19] in patients with mosaic X monosomy, and a prevalence of 11% in patients with X chromosomal structural abnormalities.^[18]

The higher prevalence in the group of pure 45,X monosomy is primarily due to a significant difference in the prevalence of aortic valve abnormalities and coarctation of the aorta, the two most common cardiovascular malformations.

[edit]Congenital heart disease

The most commonly observed are congenital obstructive lesions of the left side of the heart, leading to reduced flow on this side of the heart. This includes bicuspid aortic valve and coarctation(narrowing) of the aorta. Sybert, 1998 found that more than 50% of the cardiovascular malformations observed in her study of individuals with Turner syndrome were bicuspid aortic valves or coarctation of the aorta, alone or in combination.

Other congenital cardiovascular malformations, such as partial anomalous venous drainage and aortic valve stenosis or aortic regurgitation, are also more common in Turner syndrome than in the general population. Hypoplastic left heart syndrome represents the most severe reduction in left-sided structures

[edit]Bicuspid aortic valve

Up to 15% of adults with Turner syndrome have bicuspid aortic valves, meaning that there are only two, instead of three, parts to the valves in the main blood vessel leading from the heart. Since bicuspid valves are capable of regulating blood flow properly, this condition may go undetected without regular screening. However, bicuspid valves are more likely to deteriorate and later fail.Calcification also occurs in the valves,^[20] which may lead to a progressive valvular dysfunction as evidenced by aortic stenosis or regurgitation.^[21]

With a prevalence from 12.5%^[18] to 17.5% (Dawson-Falk et al., 1992), bicuspid aortic valve is the most common congenital malformation affecting the heart in this syndrome. It is usually isolated but it may be seen in combination with other anomalies, particularly coarctation of the aorta.

[edit]Coarctation of the aorta

Between 5% and 10% of those born with Turner syndrome have coarctation of the aorta, a congenital narrowing of the descending aorta, usually just distal to the origin of the left subclavian artery(the artery that branches off the arch of the aorta to the left arm) and opposite to the duct (and so termed "juxtaductal"). Estimates of the prevalence of this malformation in patients with Turner syndrome ranges from 6.9%^[18] to 12.5% (Dawson-Falk et al., 1992). A coarctation of the aorta in a female is suggestive of Turner syndrome, and suggests the need for further tests, such as a karyotype.

[edit]Partial anomalous venous drainage

This abnormality is a relatively rare congenital heart disease in the general population. The prevalence of this abnormality also is low (around 2.9%) in Turner syndrome. However, its relative risk is 320 in comparison with the general population. Strangely, Turner syndrome seems to be associated with unusual forms of partial anomalous venous drainage.^[18][22]

In the management of a patient with Turner syndrome it is essential to keep in mind that these left-sided cardiovascular malformations in Turner syndrome result in an increased susceptibility to bacterial endocarditis. Therefore prophylactic antibiotics should be considered when procedures with high risk endocarditis are performed, such as dental cleaning.^[21]

Turner syndrome is often associated with persistent hypertension, sometimes in childhood. In the majority of Turner syndrome patients with hypertension, there is no specific cause. In the remainder, it is usually associated with cardiovascular or kidney abnormalities, including coarctation of the aorta.

[edit]Aortic dilation, dissection, and rupture

Two studies have suggested aortic dilatation in Turner syndrome, typically involving the root of the ascending aorta and occasionally extending through the aortic arch to the descending aorta, or at the site of previous coarctation of the aorta repair.^[23]

• Allen et al., 1986 who evaluated 28 girls with Turner syndrome, found a significantly greater mean aortic root diameter in patients with Turner syndrome than in the control group (matched for body surface area). Nonetheless, the aortic root diameter found in Turner syndrome patients were still well within the limits.^[24]
• This has been confirmed by the study of Dawson-Falk et al., 1992 who evaluated 40 patients with Turner syndrome.^[25] They presented basically the same findings: a greater mean aortic root diameter, which nevertheless remains within the normal range for body surface area.

Sybert, 1998 points out that it remains unproven that aortic root diameters that are relatively large for body surface area but still well within normal limits imply a risk for progressive dilatation.^[26]

[edit]Prevalence of aortic abnormalities

The prevalence of aortic root dilatation ranges from 8.8%^[23] to 42%^[21] in patients with Turner syndrome. Even if not every aortic root dilatation necessarily goes on to an aortic dissection(circumferential or transverse tear of the intima), complications such as dissection, aortic rupture resulting in death may occur. The natural history of aortic root dilatation is still unknown, but it is a fact that it is linked to aortic dissection and rupture, which has a high mortality rate.^[27]

Aortic dissection affects 1% to 2% of patients with Turner syndrome. As a result any aortic root dilatation should be seriously taken into account as it could become a fatal aortic dissection. Routine surveillance is highly recommended.^[21]

[edit]Risk factors for aortic rupture

It is well established that cardiovascular malformations (typically bicuspid aortic valve, coarctation of the aorta and some other left-sided cardiac malformations) and hypertension predispose to aortic dilatation and dissection in the general population. At the same time it has been shown that these risk factors are common in Turner syndrome. Indeed these same risk factors are found in more than 90% of patients with Turner syndrome who develop aortic dilatation. Only a small number of patients (around 10%) have no apparent predisposing risk factors. It is important to note that the risk of hypertension is increased 3-fold in patients with Turner syndrome. Because of its relation to aortic dissection blood pressure needs to be regularly monitored and hypertension should be treated aggressively with an aim to keep blood pressure below 140/80 mmHg. It has to be noted that as with the other cardiovascular malformations, complications of aortic dilatation is commonly associated with 45,X karyotype.^[21]

[edit]Pathogenesis of aortic dissection and rupture

The exact role that all these risk factors play in the process leading to such fatal complications is still quite unclear. Aortic root dilatation is thought to be due to a mesenchymal defect as pathological evidence of cystic medial necrosis has been found by several studies. The association between a similar defect and aortic dilatation is well established in such conditions such asMarfan syndrome. Also, abnormalities in other mesenchymal tissues (bone matrix and lymphatic vessels) suggests a similar primary mesenchymal defect in patients with Turner syndrome.^[23]However there is no evidence to suggest that patients with Turner syndrome have a significantly higher risk of aortic dilatation and dissection in absence of predisposing factors. So the risk of aortic dissection in Turner syndrome appears to be a consequence of structural cardiovascular malformations and hemodynamic risk factors rather than a reflection of an inherent abnormality in connective tissue (Sybert, 1998). The natural history of aortic root dilatation is unknown, but because of its lethal potential, this aortic abnormality needs to be carefully followed.

[edit]Pregnancy

Turner syndrome is characterized by primary amenorrhea, premature ovarian failure, streak gonads and infertility. However, technology (especially oocyte donation) provides the opportunity of pregnancy in these patients.

As more women with Turner syndrome complete pregnancy thanks to modern techniques to treat infertility, it has to be noted that pregnancy may be a risk of cardiovascular complications for the mother. Indeed several studies had suggested an increased risk for aortic dissection in pregnancy.^[23] Three deaths have even been reported. The influence of estrogen has been examined but remains unclear. It seems that the high risk of aortic dissection during pregnancy in women with Turner syndrome may be due to the increased hemodynamic load rather than the high estrogen rate.^[21] Of course these findings are important and need to be remembered while following a pregnant patient with Turner syndrome.

Cardiovascular malformations in Turner syndrome are also very serious, not only because of their high prevalence in that particular population but mainly because of their high lethal potential and their great implication in the increased mortality found in patients with Turner syndrome. Congenital heart disease needs to be explored in every female newly diagnosed with Turner syndrome. As adults are concerned close surveillance of blood pressure is needed to avoid a high risk of fatal complications due to aortic dissection and rupture.

[edit]Skeletal

Normal skeletal development is inhibited due to a large variety of factors, mostly hormonal. The average height of a woman with Turner syndrome, in the absence of growth hormone treatment, is4 ft 7 in (140 cm). Patients with Turner's mosaicism can reach normal average height.

The fourth metacarpal bone (fourth toe and ring finger) may be unusually short, as may the fifth.

Due to inadequate production of estrogen, many of those with Turner syndrome develop osteoporosis. This can decrease height further, as well as exacerbate the curvature of the spine, possibly leading to scoliosis. It is also associated with an increased risk of bone fractures.

[edit]Kidney

Approximately one-third of all women with Turner syndrome have one of three kidney abnormalities:

1. A single, horseshoe-shaped kidney on one side of the body.
2. An abnormal urine-collecting system.
3. Poor blood flow to the kidneys.

Some of these conditions can be corrected surgically. Even with these abnormalities, the kidneys of most women with Turner syndrome function normally. However, as noted above, kidney problems may be associated with hypertension.

[edit]Thyroid

Approximately one-third of all women with Turner syndrome have a thyroid disorder.^{[citation needed]} Usually it is hypothyroidism, specifically Hashimoto's thyroiditis. If detected, it can be easily treated with thyroid hormone supplements.

[edit]Diabetes

Women with Turner syndrome are at a moderately increased risk of developing type 1 diabetes in childhood and a substantially increased risk of developing type 2 diabetes by adult years. The risk of developing type 2 diabetes can be substantially reduced by maintaining a healthy weight.

[edit]Cognitive

Turner syndrome does not typically cause mental retardation or impair cognition. However, learning difficulties are common among women with Turner syndrome, particularly a specific difficulty in perceiving spatial relationships, such as nonverbal learning disorder. This may also manifest itself as a difficulty with motor control or with mathematics. While it is non-correctable, in most cases it does not cause difficulty in daily living. Most Turner Syndrome patients are employed as adults and lead productive lives.

There is also a rare variety of Turner Syndrome, known as "Ring-X Turner Syndrome", which has an approximate 60 percent association with mental retardation. This variety accounts for approximately 2–4% of all Turner Syndrome cases.^[28]

[edit]Reproductive

Women with Turner syndrome are almost universally infertile. While some women with Turner syndrome have successfully become pregnant and carried their pregnancies to term, this is very rare and is generally limited to those women whose karyotypes are not 45,XO.^[29][30] Even when such pregnancies do occur, there is a higher than average risk of miscarriage or birth defects, including Turner Syndrome or Down Syndrome.^[31] Some women with Turner syndrome who are unable to conceive without medical intervention may be able to use IVF or other fertility treatments.^[32]

Usually estrogen replacement therapy is used to spur growth of secondary sexual characteristics at the time when puberty should onset. While very few women with Turner Syndrome menstruate spontaneously, estrogen therapy requires a regular shedding of the uterine lining ("withdrawal bleeding") to prevent its overgrowth. Withdrawal bleeding can be induced monthly, like menstruation, or less often, usually every three months, if the patient desires. Estrogen therapy does not make a woman with nonfunctional ovaries fertile, but it plays an important role in assisted reproduction; the health of the uterus must be maintained with estrogen if an eligible woman with Turner Syndrome wishes to use IVF (using donated oocytes).

Turner syndrome is a cause of primary amenorrhea, premature ovarian failure (hypergonadotropic hypogonadism), streak gonads and infertility. Failure to develop secondary sex characteristics (sexual infantilism) is typical.

Especially in mosaic cases of Turner syndrome that contains Y-chromosome (e.g. 45,X0/46,XY) due to the risk of development of ovarian malignancy (most common is gonadoblastoma) gonadectomy is recommended.^{[citation needed]}

[edit]Treatment

As a chromosomal condition, there is no cure for Turner syndrome. However, much can be done to minimize the symptoms. For example:^[33]

• Growth hormone, either alone or with a low dose of androgen, will increase growth and probably final adult height. Growth hormone is approved by the U.S. Food and Drug Administration for treatment of Turner syndrome and is covered by many insurance plans.^[33][34] There is evidence that this is effective, even in toddlers.^[35]

• Estrogen replacement therapy such as the birth control pill, has been used since the condition was described in 1938 to promote development of secondary sexual characteristics. Estrogens are crucial for maintaining good bone integrity, cardiovascular health and tissue health.^[33] Women with Turner Syndrome who do not have spontaneous puberty and who are not treated with estrogen are at high risk for osteoporosis and heart conditions.

• Modern reproductive technologies have also been used to help women with Turner syndrome become pregnant if they desire. For example, a donor egg can be used to create an embryo, which is carried by the Turner syndrome woman.^[33]

• Uterine maturity is positively associated with years of estrogen use, history of spontaneous menarche, and negatively associated with the lack of current hormone replacement therapy.^[36]

[edit]Epidemiology

This section requires expansion. (July 2012)

Approximately 99 percent of all fetuses with Turner syndrome result in spontaneous termination during the first trimester.^[37] Turner syndrome accounts for about 10 percent of the total number of spontaneous abortions in the United States.^{[citation needed]} The incidence of Turner syndrome in live female births is believed to be around 1 in 2000.^[3]

[edit]History

This section requires expansion. (July 2012)

The syndrome is named after Henry Turner, an endocrinologist from Illinois, who described it in 1938.^[38] In Europe, it is often called Ullrich–Turner syndrome or even Bonnevie–Ullrich–Turner syndrome to acknowledge that earlier cases had also been described by European doctors.

The first published report of a female with a 45,X karyotype was in 1959 by Dr. Charles Ford and colleagues in Harwell, Oxfordshire and Guy's Hospital in London.^[39] It was found in a 14-year-old girl with signs of Turner syndrome.