Turner syndrome pathophysiology: Difference between revisions
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{{Turner syndrome}} | {{Turner syndrome}} | ||
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==Overview== | |||
==Pathophysiology== | |||
Turner syndrome results from the following mechanisms. | |||
===Nondisjunction=== | |||
During [[meiosis]] in either parent, a [[nondisjunction]] event can occur that leaves the [[gamete]], either [[oocyte]] or [[spermatocyte]], with neither X nor Y chromosome. When this gamete combines with a gamete from the other parent (with a normal X chromosome), the embryo lacks the normal two chromosomes. Normally, humans have 46 chromosomes, so this leaves the embryo with 45 chromosomes and a single X chromosome, denoted <code>45,X</code> (or, sometimes <code>45,XO</code>, where the "<code>O</code>" is used as a placeholder). This is found in 50% of individuals with Turner syndrome. | |||
===Chromosomal structure=== | |||
An X chromosome can form a [[ring chromosome]] for example by losing a portion of the smaller arm, enabling the end of the long arm to wrap around. This is detrimental for the X chromosome in two ways. Either the lost portion itself makes the chromosome less functional, or it causes nondisjunction, as described above. Thus, the causes listed here are partly overlapping. | |||
When such a ring chromosome combines with another ring chromosome in fertilization, the pair is denoted as <code>46, XrXp-</code>, where <code>rXp-</code> means a ring chromosome missing the small (p) arm of the chromosome. | |||
Another variant of abnormal chromosomal structure is chromosomes with two long arms of the X chromosomes attached, and are called [[isochromosome]]s. | |||
Variants of chromosomal structure occur in 30% of individuals with Turner syndrome. | |||
===Nonfunctional Y=== | |||
Very rarely, the embryo has a normal X chromosome and a portion of the Y chromosome. In these cases, the Y chromosome does not have a functional [[SRY]] (and so develops as a female), the diagnosis is [[Swyer_syndrome|XY gonadal dysgenesis]].<ref>[http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=480000 OMIM entry for SRY]</ref> It is possible that some Turner syndrome diagnosis is due to gonadal dysgenesis, particularly when it is caused by a large deletion of the Y chromosome. | |||
===Mosaicism=== | |||
Each of the causes mentioned above can occur as a [[mosaicism]], that is, some of the cells carry the mutation and some don't. This happens if the error takes place in one cell after the very first divisions of the early embryo after [[fertilization]]. The exact mixture of the two different cell types depends on when the nondisjunction occurred. However, if the nondisjunction occurs after enough divisions, the fraction of abnormal cells is probably not large enough to show any significant effects. For instance, such a 45,X/46,XY individual will develop as a male, without Turner syndrome. Mosaicism is found in about 20% of individuals with Turner syndrome. | |||
===No single Y=== | |||
There is no equivalent syndrome which results in a Y chromosome with no X, as such a condition is fatal in utero. Because an embryo with Turner syndrome doesn't have a Y chromosome (or, doesn't have a functional SRY on the Y chromosome), it will move along the path to female development. | |||
==References== | ==References== | ||
{{Reflist|2}} | {{Reflist|2}} | ||
[[Category:Disease]] | [[Category:Disease]] | ||
[[Category:Genetic disorders]] | [[Category:Genetic disorders]] | ||
Revision as of 16:16, 20 September 2012
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
Pathophysiology
Turner syndrome results from the following mechanisms.
Nondisjunction
During meiosis in either parent, a nondisjunction event can occur that leaves the gamete, either oocyte or spermatocyte, with neither X nor Y chromosome. When this gamete combines with a gamete from the other parent (with a normal X chromosome), the embryo lacks the normal two chromosomes. Normally, humans have 46 chromosomes, so this leaves the embryo with 45 chromosomes and a single X chromosome, denoted 45,X (or, sometimes 45,XO, where the "O" is used as a placeholder). This is found in 50% of individuals with Turner syndrome.
Chromosomal structure
An X chromosome can form a ring chromosome for example by losing a portion of the smaller arm, enabling the end of the long arm to wrap around. This is detrimental for the X chromosome in two ways. Either the lost portion itself makes the chromosome less functional, or it causes nondisjunction, as described above. Thus, the causes listed here are partly overlapping.
When such a ring chromosome combines with another ring chromosome in fertilization, the pair is denoted as 46, XrXp-, where rXp- means a ring chromosome missing the small (p) arm of the chromosome.
Another variant of abnormal chromosomal structure is chromosomes with two long arms of the X chromosomes attached, and are called isochromosomes.
Variants of chromosomal structure occur in 30% of individuals with Turner syndrome.
Nonfunctional Y
Very rarely, the embryo has a normal X chromosome and a portion of the Y chromosome. In these cases, the Y chromosome does not have a functional SRY (and so develops as a female), the diagnosis is XY gonadal dysgenesis.[1] It is possible that some Turner syndrome diagnosis is due to gonadal dysgenesis, particularly when it is caused by a large deletion of the Y chromosome.
Mosaicism
Each of the causes mentioned above can occur as a mosaicism, that is, some of the cells carry the mutation and some don't. This happens if the error takes place in one cell after the very first divisions of the early embryo after fertilization. The exact mixture of the two different cell types depends on when the nondisjunction occurred. However, if the nondisjunction occurs after enough divisions, the fraction of abnormal cells is probably not large enough to show any significant effects. For instance, such a 45,X/46,XY individual will develop as a male, without Turner syndrome. Mosaicism is found in about 20% of individuals with Turner syndrome.
No single Y
There is no equivalent syndrome which results in a Y chromosome with no X, as such a condition is fatal in utero. Because an embryo with Turner syndrome doesn't have a Y chromosome (or, doesn't have a functional SRY on the Y chromosome), it will move along the path to female development.