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Author Topic: Mosaicism & ring chromosome 22  (Read 5117 times)

Offline DoctorBeaver

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Mosaicism & ring chromosome 22
« on: 06/06/2005 13:33:53 »
A friends of mine has an autistic 10yo daughter with Mosaic Chromosome Disorder and has asked if I know, or can find out, anything about that particular chromosome. My ignorance on such a subject is total but I couldn't think of anywhere more likely to have people with knowldege of it than here. So... any offers?


 

Offline DoctorBeaver

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Re: Mosaicism & ring chromosome 22
« Reply #1 on: 07/06/2005 20:00:03 »
I've managed to find some info on a few sites but I'm as confused as ever. Is chromosome ring 22 a form of mosaicism or is it in addition to? I've also found references to monosomy, trisomy & tetrasomy. I've managed to sort out what they mean & what the symptoms of monosomy can be, but trisomy & tetrasomy are proving somewhat more elusive.
 

Offline rosy

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Re: Mosaicism & ring chromosome 22
« Reply #2 on: 07/06/2005 20:39:04 »
DrBeaver, I don't know if you found this site when you were searching, but it seems to have a lot of stuff about chromosome 22 disorders including the ring disorder.
http://www.nt.net/~a815/mosaict22.htm

Here is some first year undergrad cell biology... no idea if it'll tell anyone anything they don't know already, and not all of it is strictly relevant, but some of it's fairly interesting (I think).
OK, trisomy is simply where you have three copies of a chromosome where you should have two. This means that as chromosomes are what the cell copies to make protein in the cell you get too much of some proteins (the ones encoded by that chromosome) relative to what you've got of others. As cell function is based on having the right amount of all the proteins relative to each other having three copies made instead of two can seriously mess things up.

Mosaicism is where, rather than triploidy occuring throughout the cells of the body due to an error the gametes (as in Downs' syndrome) only some of the cells are triploid, due to a cell division error or similar in the embryo and that error being copied every time the faulty cell divides (so the earlier the error the more cells will be affected).


Basically you're supposed to have two of each chromosome (or in the case of the sex chromosomes an X and a Y) to give you the right amount of RNA to give you the right amount of protein synthesis in the cells. Too much (as caused by having three copies of a chromosome) or too little (caused by having only one) tends to mess up the balance and normally doesn't result in a viable foetus.
Chromosome 21 is very small and doesn't code for many important proteins so babies with three copies can come to term and are born with Downs' Syndrome (they're "not right" but they can survive).

The sex chromosomes are a bit more complicated. You only need one functioning chromosome to be fine (otherwise men wouldn't do too well) but women have two. The reason it is possible to have one *or* two and do OK is that, in women, one of the X-chromosomes is folded up out of the way and almost entirely unused.*
Some girls are born with more than two X chromosomes (Kleinfelter syndrome), or boys with more than one (I forget the name). Although they, like the Downs' children, function OK they too have characteristic features such as short stature, a particular facial type, and below aveage IQ.


* An interesting (I think) thing about this is that in the very early embryo they *aren't* folded up and they pack themselves away when the embryo has a few hundred cells. After this, at cell division the same chromosome in each daughter cell is folded up as in the dividing cell (so it is perpetuated as the copying error is in mosaicism), which means that, for example, mothers of colour-blind sons (therefore carriers of colour blindness) are colour blind on some parts of the retina, because patches of the cells have the defective gene active and so don't make the functionning protein.
« Last Edit: 07/06/2005 20:56:00 by rosy »
 

Offline DoctorBeaver

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Re: Mosaicism & ring chromosome 22
« Reply #3 on: 07/06/2005 21:30:34 »
Rosy - thanks for your comments. You've clarified a few points for me.
I've emailed your reply to my friend. I hope you don't mind.
I still can't quite figure out where mosaicism comes into it, though. Is chromosome 22 ring a form of mosaicism? None of the sites I've visited make that clear.
I also haven't found anywhere that compares autism with chromosomic disorders. Is it a form of autism or just that the symptoms are the same? My friend's daughter displays classic signs of autism - obsession with routine, lack of verbal communication, tantrums etc: but she also has a slightly small head and her 2nd & 3rd toes are webbed which are both typical of chromosome 22 disorder.
quote:
As cell function is based on having the right amount of all the proteins relative to each other having three copies made instead of two can seriously mess things up

That's somewhat of an understatement!
I knew Down's Syndrome was chromosome-based but wasn't sure exactly in what way.
 

Offline rosy

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Re: Mosaicism & ring chromosome 22
« Reply #4 on: 07/06/2005 22:06:24 »
I found this reference:
Ring Chromosome 22 and Autism: Report and Review. Am J Med Genet 2000 Feb 28;90(5):382-5. MacLean JE et al.
On this page:
http://www.nt.net/~a815/ring22refs.htm
It's available as a PDF here:
http://www3.interscience.wiley.com/cgi-bin/fulltext/71001932/PDFSTART

No idea if it's got anything useful in it, but it looks like someone's reporting something...


I'd guess the ring/mosaicism link is that the ring chromosome occurs as a mosaic, as in the ring formed in one cell tissue formed by division of that cell have the ring chromosome, but that's just me applying logic to the name...
 

Offline DoctorBeaver

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Re: Mosaicism & ring chromosome 22
« Reply #5 on: 07/06/2005 22:45:42 »
I'm gradually accruing info but so much of it is in technical language that I'm spending half my time finding out what the words mean. Then by the time I've looked up the 2nd word I've forgotten what the 1st 1 meant! grrrrrrr. Why can't someone write an authoritative text in idiot language for people like me who want to know but don't have a PhD in genetics?
 

Offline lawrenceleichtman

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Re: Mosaicism & ring chromosome 22
« Reply #6 on: 04/01/2006 20:46:03 »
quote:
Originally posted by DoctorBeaver

I've managed to find some info on a few sites but I'm as confused as ever. Is chromosome ring 22 a form of mosaicism or is it in addition to? I've also found references to monosomy, trisomy & tetrasomy. I've managed to sort out what they mean & what the symptoms of monosomy can be, but trisomy & tetrasomy are proving somewhat more elusive.



Rings are formed when both ends of the chromosome have been lost making them sticky thus forming a ring and causing deletions of genetic material from each end of the chromosome. 22q deletions are known common causes of developmental and behavioral problems. Whether or not they are mosaic depends on whether there is more than one cell line in the body. This can happen at each of the first 8-10 divisions post fertilization. A new clonal line can form that being the normal or abnormal complement. If it is the first division it is generally a 50/50 mosaicism and down the line from there.

Lawrence G. Leichtman, MD, FAAP, FACMG
PO Box 4548
Virginia Beach, VA 23454-0548
 

Offline DoctorBeaver

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Re: Mosaicism & ring chromosome 22
« Reply #7 on: 04/01/2006 23:27:29 »
Lawrence - thank you very much for that. I'm 6 months further down the line so I've learned a lot more since I started this thread; but that is the 1st definitive explanation of ring disorder that I've seen.
I decided to do that "Idiot's guide" myself in the end - although I haven't actually called it that. The difficulty I'm having is the same I always had when writing essays as an undergrad - knowing where to stop! I'm forever coming across little snippets that I think should be included, but when I look into them a bit further it turns into another major task. For instance, someone recently was asking me about Prada-Willi Syndrome. When I looked into it I found there were similarities with some forms of ASD and, indeed, there seems to have been quite some debate about whether it should be classified as an ASD.
The line is stubbornly refusing to be drawn! [xx(]
 

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Re: Mosaicism & ring chromosome 22
« Reply #7 on: 04/01/2006 23:27:29 »

 

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