Waarom afwijking bij trisomie
Geplaatst op 30 september 2008 - 16:18
Geplaatst op 30 september 2008 - 21:30
Which genes are involved? That's been the question researchers have asked ever since the third 21st chromosome was found. From years of research, one popular theory stated that only a small portion of the 21st chromosome actually needed to be triplicated to get the effects seen in Down syndrome; this was called the Down Syndrome Critical Region. However, this region is not one small isolated spot, but most likely several areas that are not necessarily side by side. The 21st chromosome may actually hold 200 to 250 genes (being the smallest chromosome in the body in terms of total number of genes); but it's estimated that only a small percentage of those may eventually be involved in producing the features of Down syndrome. Right now, the question of which genes do what is highly speculative. However, there are some suspects.
Genes that may have input into Down syndrome include:
* Superoxide Dismutase (SOD1)-- overexpression may cause premature aging and decreased function of the immune system; its role in Senile Dementia of the Alzheimer's type or decreased cognition is still speculative
* COL6A1 -- overexpression may be the cause of heart defects
* ETS2 -- overexpression may be the cause of skeletal abnormalities
* CAF1A -- overexpression may be detrimental to DNA synthesis
* Cystathione Beta Synthase (CBS) -- overexpression may disrupt metabolism and DNA repair
* DYRK -- overexpression may be the cause of mental retardation
* CRYA1 -- overexpression may be the cause of cataracts
* GART -- overexpression may disrupt DNA synthesis and repair
* IFNAR -- the gene for expression of Interferon, overexpression may interfere with the immune system as well as other organ systems
Other genes that are also suspects include APP, GLUR5, S100B, TAM, PFKL, and a few others. Again, it is important to note that no gene has yet been fully linked to any feature associated with Down syndrome.
One of the more notable aspects of Down syndrome is the wide variety of features and characteristics of people with trisomy 21: There is a wide range of mental retardation and developmental delay noted among children with Down syndrome. Some babies are born with heart defects and others aren't. Some children have associated illnesses such as epilepsy, hypothyroidism or celiac disease, and others don't. The first possible reason is the difference in the genes that are triplicated. As I mentioned above, genes can come in different alternate forms, called "alleles." The effect of overexpression of genes may depend on which allele is present in the person with trisomy 21. The second reason that might be involved is called "penetrance." If one allele causes a condition to be present in some people but not others, that is called "variable penetrance," and that appears to be what happens with trisomy 21: the alleles don't do the same thing to every person who has it. Both reasons may be why there is such variation in children and adults with Down syndrome.
Geplaatst op 22 oktober 2008 - 20:27
Geplaatst op 22 oktober 2008 - 21:55
--Vladimir Lenin-- (Владимир Ильич Ульянов)
Geplaatst op 22 oktober 2008 - 22:09
Een toevoeging, wat in bovenstaande niet zo goed tot uiting komt: de meeste trisomieŽn lijden tot ernstige afwijkingen - zo ernstig dat ze niet levensvatbaar zijn. Trisomie 13/18/21 zijn levensvatbaar. Ook meer kopieŽn van het X-chromosoom is levensvatbaar (XXY, XXXY), maar dat komt omdat het teveel daar wordt uitgeschakeld (dmv. X-inactivatie). Het fenomeen X-inactivatie is bedoeld om vrouwen te beschermen tegen overactiviteit van genen op het X-chromosoom (c.q. mannen te beschermen tegen een tekort).
Ook meer kopieŽn van het Y-chromosoom is levensvatbaar
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