[tri-med] Re: Mosaicism

----- Original Message ----- 
From: "Vicki Christensen" <vchristensen@xxxxxxxxxx>
 >I have been hearing a lot about mosaicism lately. My question is how can 
you tell if there >is mosaicism? Luke had 20 blood cells tested, 5 of which 
were fully analyzed, 15 partially >analyzed and 6  were imaged.

That's not very many cells to check at all Vicki. Most places these days 
check 100.
The first test Alex had done was 20 cells and they missed the trisomy.
Next test 50 were checked.

And in a mosaic situation each test will show a different precentage. It all 
depends on how many are captured and seen. As far as skin tests etc, same 
thing applies. And the percentages can vary wildly from blood, to skin, to 
an organ.....it all depends on how many trisomy cells made up whatever body 
part they are checking.
Gee , hope this makes sense.

Don't know if this post from List Mum Karen will help or not but read on to 
this copy of one she did a while ago:
~~~~~~~~~~~~
There seems to be some confusion at present about the different types of
trisomy, how they occur and risk factors.

In general there is a difference in theories as to how "full" trisomy,
partial trisomy and mosaicism occur. They are not the same and the risks
are
not the same. I have pulled out most of my genetics 101, 102 posts so I am
going to attempt to pull them all together again for the new folk.

For more detailed explanations (with pictures!!) please visit both my
homepage
http://homepages.tig.com.au/~karens
and CDO's web site
http://members.aol.com/cdousa/intro.htm

First up there are essentially a number of different types of trisomy: This
refers to ALL chromosomes even though in general I am using 18 as the
example.

1. "full" trisomy (and when people simply refer to trisomy this is what
they
should mean - putting "full" in front is actually misleading)
This is where every cell in the body (as far as they can tell) has the
additional chromosome.

2. Partial Trisomy
now there are two different types of partial trisomy (though this is often
not distinguished.
This is where there is part of an extra chromosome in every cell (as far as
they can tell). Very basically a chromosome can be divided into two parts
the p (for petite or short arm) and the q arm (named because scientists are
imaginative and its the next letter in the alphabet!!)

So Trisomy 18p for example refers to there being an extra p arm and not the
q. Trisomy 18q refers to there being an extra q arm and not a p.

There is also partial Trisomy p or q. What that means is that there is part
of the p or q arm not the whole arm. This is the type of partial that
occurs
more frequently.

This sort of trisomy often has some "funny" numbers attached :-) so to
write
it "correctly it might be partial trisomy 18 (32.1, ter). meaning that it
is
only a "small" extra piece of the chromosome from the band 32.1 to the end.

3. Unbalanced translocations.
This is the other side of balanced translocations. In a balanced
translocati
on all the genetic material is there its just not in the right place. This
can happen in any format. But for example part of the 18th chromosome may
be
attached to the 16th chromosome and part of the 16th is where the 18th
should be.

Because all of the genetic information is there (and provided there are no
inversions) there shouldn't be a problem with the person. However problems
to begin when they want to have children. Because of the way the
chromosomes
come together and the egg divides it is possible for the baby to get only
part of the "balanced" translocation. So for example the baby may get the
part of the "extra" material that is on the 16th chromosome and be missing
that part of the 16th that is on the mother's 18th.

This is called an unbalanced translocation - because all of the genetic
material is not there - and in this example would be partial trisomy 18 and
monosomy 16. That is there is some extra 18th chromosome and some missing
16th chromosome. It effects the baby essentially by giving them a mix of
problems - they can have some of the problems of Trisomy 18 and some of the
problems of monosomy 16.......... How much depends exactly where the break
points occur.

While unbalanced translocations CAN mean a that one of the parents is a
"carrier" it can also happen "de nova" that is part of a chromosome just
breaks off and attaches to another chromosome during cell division.

When someone refers to a carrier they USUALLY are referring to balanced and
unbalanced translocations. Technically no other type of trisomy (other than
a Robertsonian translocation) can have a "carrier".

4. Robertsonian Translocations
These are in a totally different category to balanced and unbalanced
translocations.

A Robertsonian translocation only effects chromosomes 13, 14, 15, 21 and 22

These chromosomes have very short "p" arms. and a Robertsonian
translocation
happens when the two "q" arms join together to form one long chromosome.

A balanced translocation happens when part of one chromosome breaks off and
attaches itself to another chromosome. With a Robertsonian translocation
there is no "breakage" (hence no break points) the two complete chromosomes
attach themselves together.


5. Mosaicism
This is where there is a whole extra chromosome in some of the cells and
some of the cells have the normal compliment of chromosomes. How the child
is effected depends on how many cells have the extra chromosome AND where
those cells are (distribution). This is the very hard part of mosaicism
because NO-ONE can tell ahead of time where those cells lie.

Of course mosaicism can happen in all of the above different types of
Trisomy (except "full" trisomy) as well. So a person can be mosaic partial
Trisomy 18p. Rare but can and does happen.

6. Miscellaneous other :-)
there are lots of other problems that can effect chromosomes - some have no
bearing on Trisomy, some such as inversions, only very rarely. Most of
these
types of problems are "new" and they are only just starting to understand
them because of new technology such as FISH. If I tried to explain them
this
post would get even more complicated than it already is!!!!

I hope all of that makes sense.

Take Care and Keep Looking for Rainbows!!!
Karen, mum to Alex
~~~~~~~~~~~~~~~~~~~~~~~~~~~

Michelle mom to Alex (19, partial trisomy 14 mosaic) and Molly (15)
MichiganUSA


                  Building ___ooOOoo__ Rainbows
                       www.trisomyonline.org
                  Families Helping Families On-line

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