出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/01/29 18:21:10」(JST)
| Triple X syndrome | |
|---|---|
| Classification and external resources | |
| ICD-10 | Q97.0 |
| DiseasesDB | 13386 |
Triple X syndrome (also known as triplo-X, trisomy X, XXX syndrome, 47,XXX aneuploidy) is a form of chromosomal variation characterized by the presence of an extra X chromosome in each cell of a human female. Females with triple X syndrome have three X chromosomes instead of two. The karyotype reads 47,XXX because the affected individual has 47 chromosomes, as opposed to the usual 46. A mosaic form also occurs where only a percentage of the body cells contain XXX while the remainder carry XX. The extent to which an individual is affected by the condition will depend upon the proportion of XXX to XX throughout.[1] Triple X results during division of a parent's reproductive cells and occurs about once in every 1,000 female births. Unlike most other chromosomal conditions (such as Down syndrome), there is usually no distinguishable difference to the naked eye between those with triple X and the rest of the female population.
Triple X syndrome is not inherited, but usually occurs as an event during the formation of reproductive cells (ovum and sperm). An error in cell division called nondisjunction can result in reproductive cells with additional chromosomes. For example, an oocyte or sperm cell may gain an extra copy of the X chromosome as a result of the non-disjunction. If one of these cells contributes to the genetic makeup of a child, the child will have an extra X chromosome in each of her cells. In some cases, trisomy X occurs during cell division in early embryonic development.
Some females with triple X syndrome have an extra X chromosome in only some of their cells. These cases are called 46,XX/47,XXX mosaics.
Because the vast majority of Triple X females are never diagnosed, it may be very difficult to make generalizations about the effects of this syndrome. The samples that were studied were small and may be nonrepresentative or biased.
Because of the lyonization, inactivation, and formation of Barr bodies in all female cells, only one X chromosome is active at any time. Thus, Triple X syndrome most often has only mild effects, or has no unusual effects at all. Symptoms may include tall stature; small head (microcephaly); vertical skinfolds that may cover the inner corners of the eyes (epicanthal folds); speech and language learning disabilities, such as dyslexia; or weak muscle tone.[2] The symptoms vary from person to person, with some women being more affected than others. There are seldom any observable physical anomalies in Triple X females, other than being taller than average.
Females with Triple X syndrome are at increased risk of delayed language development, EEG abnormalities, motor coordination problems and auditory processing disorders, and scoliosis. They tend to show accelerated growth until puberty. Premature ovarian failure seems to be more prevalent in these women, but most Triple X females seem to have normal fertility. They are more likely to struggle with personality and psychological problems, and low self-esteem, but these respond well to treatment. Triple X females are at increased risk of poor academic results at school, and some may need special education. Sometimes, they may suffer from anxiety and be very shy, and this may affect their relations with school peers. They seem to feel much better after leaving school. They benefit very much from a stable home environment.[3]
Triple X syndrome occurs in around 1 in 1,000 girls. On average, five to ten girls with triple X syndrome are born in the United States each day.[4]
The first published report of a woman with a 47,XXX karyotype was by Patricia A. Jacobs, et al. at Western General Hospital in Edinburgh, Scotland, in 1959. It was found in a 35‑year-old, 5 ft. 9 in. (176 cm) tall, 128 lb. (58.2 kg) woman who had premature ovarian failure at age 19; her mother was age 41 and her father was 40 at the time of her conception.[5] Jacobs, et al. called the 47,XXX woman a "superfemale", a term which was immediately criticized, did not gain acceptance, and was based on the incorrect assumption that the sex-determination system in mammals was the same as in the fruit fly Drosophila.[6] British pathologist and geneticist Bernard Lennox, the principal consultant on medical terms for the Oxford English Dictionary, suggested the term "XXX syndrome".[7]
The vast majority of Triple X women are never diagnosed, unless they undergo tests for other medical reasons later in life. Triple X can be diagnosed by a blood test which is able to look at a person’s chromosomes (karyotype).
Triple X syndrome can be diagnosed prenatally through amniocentesis or chorionic villus sampling. In Denmark, between 1970 and 1984, 76% of the prenatally diagnosed fetuses with triple-X were aborted. Between 1985-1987, this figure dropped to 56%. With improved information, the number of abortions diminished. In the Netherlands, between 1991 and 2000, 33% (18/54) of the couples that were confronted with a prenatal diagnosis of 47,XXX elected to abort. If balanced information is provided to prospective parents, pre-natally, the incidence of voluntary termination (abortion) is reduced.[8]
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| リンク元 | 「XXX症候群」 |
| 関連記事 | 「X」「syndrome」 |