Tetrasomy 9p FTNW - Unique - The Rare Chromosome Disorder ...

The karyotype
Your genetic specialist can tell you more about how much material there is in the extra
chromosome. You will almost certainly be given a karyotype, a shorthand code that
usually tells you the band where chromosome 9 has broken. The chromosome usually
breaks in the short arm close to the centromere (the point where the short arm meets
the long arm). Sometimes it breaks in the long arm, usually close to the centromere.
47,XY,+i(9p) This shows that there are 47 chromosomes, it’s a boy (XY), and the
extra (+) chromosome is an isochromosome (i) made up of material from the short (p)
arm of chromosome 9.
47,XX,iso(9)(q12)de novo This shows that it’s a girl (XX) and that the extra
chromosome is made up of the short arm and material as far as band q12 in the long
arm. De novo means that the parents’ chromosomes have been examined and
are normal. Iso was used by laboratories in the past but today would be (i) or, strictly
speaking in this case, idic (see below).
47,XY,+idic(9)(q13) This shows that the extra chromosome is made up of material
from the entire short arm and the long arm as far as q13, including the centromeres
where the short arms meet the long arms. An extra isochromosome that includes the
centromeres is called an isodicentric chromosome, shortened to idic (eye-dick).
47,XX,+mar.ish i(9)(p10)(wcp9+) dn This means that it’s a girl (XX) with a small
extra chromosome that couldn’t at first be identified. A small unidentified chromosome
like this is called a marker (mar). Using a technique called FISH (.ish) that allows
chromosomes to be examined in greater detail, the marker was found to consist of
material from chromosome 9. The chromosome has broken off at band p10, another
way of describing the centromere. The specific technique used was whole chromosome
painting (wcp), in which the test ‘recognises’ different parts of the extra chromosome
as coming from chromosome 9. You can write de novo as dn, meaning that the parents’
chromosomes have been examined and are normal.
How is tetrasomy 9p diagnosed?
Tetrasomy 9p is usually found in highest concentrations in blood, so diagnosis in a baby,
child or adult means taking and analysing a blood sample. Confirmation and an
estimation of the ratio of normal to tetrasomy 9p cells can use a sample of skin or a
scraping from inside the cheek (buccal smear) (Lloveras 2004).
Diagnosis during pregnancy is more difficult as tetrasomy 9p cells may not be found in
fetal cells in amniotic fluid. It is therefore quite possible for a pregnancy to be affected
by tetrasomy 9p but for the amniotic fluid or chorionic villus sample to show only cells
with normal chromosomes (Chen 2007; Eggermann 1998; Grass 1993).
One possible solution in this situation is to take a sample of blood from the umbilical
cord, as this blood is part of the fetal circulation (Papenhausen 1990). This procedure
can be risky, however, and one couple at high risk of having a baby with tetrasomy 9p
who lost a pregnancy after fetal blood sampling chose instead to have future
pregnancies intensively monitored by amniocentesis and ultrasound with the
sonographer looking for findings such as an enlarged head, growth delay and palate, lip
and kidney anomalies (Henriques-Coelho 2005; McAuliffe 2005; Lloveras 2004; Dhanda
2002; Xu 1998).
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