Genetics Index Glossary
Turner's Syndrome (X Monosomy, Gonadal Dysgenesis)
Turner's syndrome is a chromosomal abnormality involving absense of all or part of one X chromosome. It has been described in many species, including humans, mice and horses. Individuals with this disorder have a female phenotype, but have abnormalities in reproductive function as well as a number of other clinical manifestations of disease.
Turner's syndrome is the most common sex chromosome abnormality of human females. Its incidence in newborns is approximately 1 in 2,500. However, roughly 10% of spontaneously aborted fetuses have this disorder and the incidence has been estimated as 0.8% in zygotes, making it possibly the most common chromosomal disorder in man. It thus appears that only about 1% of human 45,X xygotes survive to term, and those individuals typically have a rather characteristic set of somatic abnormalities.
Roughly half of the females with Turner's syndrome have X chromosome monosomy (45, X). Most of the other individuals are mosaics, having varying fractions of cells with other chromosome complements (e.g. 45,X/46,XX). In roughly two-thirds of cases having a 45,X karyotype, the single X chromosome is of maternal origin.
Both the embryonic lethality and Turner's phenotype are thought to be due to haploinsufficiency - monosomy of genes common to both the X and Y chromosomes. These genes are among those on the X chromosome that escape X-inactivation and hence, would be expected to be expressed in normal females from both X chromosomes. Interestingly, at least two X-linked genes known to not undergo inactivation in man do undergo inactivation in mice - apparently, a haploid dose of these gene products is sufficient for normality in mice (see below).
Individuals with X chromosome monosomy display four cardinal features:
- Female phenotype, due to the absense of a Y chromosome.
- Short stature: Weight and height at birth are below the mean for normal infants, and a reduced growth rate is usually seen in prior to teenage years. Many individuals with Turner's syndrome are treated with recombinant human growth hormone to alleviate short stature.
- Sexual infantilism due to rudimentary ovaries: Sexual ducts and external genital structures appear like normal females, but very immature. When examined at or after birth, the gonads are long, pale streaks of tissue devoid of primordial follicles. Early in gestation the gonads appear normal and contain primordial germ cells, but after the third month, numbers become severely reduced and, in most patients oocytes are not present after birth. Fertility is rare in individuals with Turner's syndrome and usually seen in women with 45,X/46,XX mosaicism. Sex steroid replacement therapy is often applied to initiate puberty and enhance growth.
- A variety of somatic abnormalities: Frequently observed abnormalities include:
- Congenital heart disease, including cardiac valve malformations and narrowing of the aorta.
- Hypothyroidism, due to inadequate secretion of thyroid hormones. This is readily treated by replacement therapy.
- Structural malformations of the kidneys.
- Skeletal abnormalities leading to dislocations of the patella, scoliosis and chronic knee pain.
Normal intelligence is the rule for individuals with Turner's syndrome. However, a majority have some type of learning disability, often involving visual-spatial skills.
Therapy for human Turner's syndrome is directed at correcting the somatic abnormalities, augumenting the short stature and inducing secondard sexual characteristics and menses - lack of the later can be psychologically very disturbing to affected individuals. The primary treatments used are growth hormone replacement, and estrogen therapy, begun at an age of 13-14 years and administered with gaps to allow withdrawal menstruration.
Turner's syndrome is commonly diagnosed prenatally, often in women undergoing amniocentesis or chorionic villus sampling for other reasons. Advanced maternal age does not seem to be a risk factor by itself for this disorder. Ultrasonography is a valuable tool for detecting fetuses with Turner's syndrome: typical findings include a thickened neck, edema of the hands and feet and renal or left-side cardiac abnormalities. Presumptive diagnosis by ultrasound should be confirmed by chromosomal analysis.
XO mice have been reported to be fertile and have histologically normal ovaries. However, quantitative histology on such ovaries collected between 12 and 200 days of age revealed that they had approximately half the number of oocytes as their XX siblings - even at day 12 of life, the population of growing follicles was clearly retarded.
X monosomy appears to be a relatively common cytogenetic disorder in mares. Affected animals appear normal externally, but with a smaller than normal vulva. The uterus is also small, the cervix usually open and flaccid and the ovaries range in size from small to very small. Most XO mares fail to cycle, although some show sporadic or indistinct periods of estrous behavior. This abnormality should definitely be put on a list of differential diagnoses for any cases of unexplained reproductive failure in mares, particularly if associated with genital tract infantilism.References and Reviews
- Ashworth A, Rastan S, Lovell-Badge R, Kay G: X-chromosome inactivation may explain the difference in viability of XO humans and mice. Nature 351:406, 1991.
- Bruere AN, Blue MG, Jaine PM, Walker KS, etc: Preliminary observations on the occurrence of the equine XO syndrome. New Zealand Vet J 26:145-146.
- Hughes JP, Benirschke K, Kennedy PC, Smith AT: Gonadal dysgenesis in the mare. J Reprod Fertil Suppl 23:385-390, 1975.
- Saenger P: Turner's syndrome. New Eng J Med 335:1749, 1996.
- Sybert VP, McCauley E: Turner's syndrome. New Eng J Med 351:1227-1238, 2004.
- Turner HH: A syndrome of infantilism, congenital webbed neck, and cubitus valgus. Endocrinology 28:566, 1938.
Last updated on September 18, 2004
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