Trisomy 20 ( Mosaic )
What is a chromosome disorder and what is its significance?
Chromosomes are the structures
in which our hereditary material (DNA) is packed.
A healthy person has 46 chromosomes (23 pairs). One chromosome of each pair (i.e.
a total of 23 chromosomes) is donated by each parent to the fetus. Twenty-two pairs
are called autosomes, and the 23rd pair consists of the sex chromosomes (X and Y).
Females have two X chromosomes and males have one X and one Y. The chromosome pairs
are arranged and numbered by size and shape, and each has different genes. Because
there are two copies of each chromosome, there is effectively a duplicate gene system
(one maternal and the other paternal), each identical in sequence to the other.
A change in the number or structure of the chromosomes in pairs 1- 22, causing an
addition or deletion of chromosomal information in these pairs compared with the
normal state, constitutes a significant medical problem. In the great majority of
cases, this change is expressed as severe structural or functional defects in the
tissues and/or mental retardation. For example, an extra number
21 chromosome, resulting in three instead of the normal two so that there are 47
chromosomes altogether, causes Down
syndrome. An extra number 18 chromosome, which is larger and therefore contains
more genes, manifests as a more severe disorder called trisomy 18. In this syndrome, there are severe defects,
and it usually ends in death soon after birth or even in the uterus. This is also
the case with a deletion of the whole or a part, however small, of chromosome 18.
Conversely, the clinical manifestation of a change in the number of sex chromosomes
(X or Y) or their structure is usually less severe. See under "chromosomes".
Trisomy 20 is an exception
Notwithstanding the above, and in contrast with the situation for other chromosomes,
the case of chromosome 20 is different.
First of all, trisomy 20, i.e. the presence of 3 number 20 chromosomes instead of
the normal 2, is only found in some of the amniotic and skin cells of the fetus
and not in all.
This situation, where there is a mixture of cells with normal chromosomes and cells
with trisomy 20, is called mosaicism.
Secondly, trisomy 20 cells never appear in the fetus's blood, so a blood test to
look for them should not be carried out.
In some cases, mosaicism of normal cells and cells with trisomy 20 may appear in
other fetal tissues, particularly in the urinary tract, and has been demonstrated
mainly in cases in which urinary tract defects have been found on ultrasound examination. In a small number
of cases defects have been found in other organs.
What is the significance for a fetus that is found to have trisomy 20?
In 10% of cases there is a risk that the fetus will have various defects. These
can be detected by ultrasound examination and are found mainly in the urinary tract,
but they have been described in other organs as well, such as the heart. Other defects
include cleft lip, defects of brain structure including small head circumference, excess cerebrospinal
fluid in the ventricles of the brain, etc.; intrauterine growth retardation,
etc. Where defects are found, they are usually caused by the presence of the trisomy
20 cells, and in most such cases the pregnancy is terminated.
What about a normal ultrasound picture without intrauterine growth retardation?
The medical literature offers relatively reassuring data about more than 125 cases
born out of 152 in which trisomy 20 was detected in the amniotic fluid cells. In
about 6.5% of such cases, the newborn was found on examination to be healthy. Some
of those who were found to be abnormal had intrauterine growth retardation, so the
problem could be anticipated. The percentage of the abnormal cells determines the
degree of severity - when there are fewer than 50% abnormal cells the risk that
the newborn will have problems is 4.5%, but in this group there are usually warning
signs such as intrauterine growth retardation. However, when there are more than
50% abnormal cells, the risk that the newborn will have problems reaches 20%.
Long-term follow-up of development has not been undertaken in a large group of patients
with this condition. However, in 18 studies that described patients who were monitored
for longer than a month, no neurological problems were observed. Among 7 patients
who were monitored for more than a year in whom the percentage of abnormal cells
was less than 50%, there were no cases of developmental delay occurring as an isolated
finding. There was, however, a single case of delayed development associated with
a defect in the structure of the brain that was detected on ultrasound examination.
Obviously with such a small number of subjects it is difficult to draw unequivocal
conclusions, but at this point in time there is no clear evidence that trisomy 20
has a detrimental effect on development when there are no accompanying defects or
intrauterine growth retardation, especially when the percentage of abnormal cells
is less than 50%. However, this must be regarded with caution, especially with respect
to the possibility of additional problems that have not been demonstrated because
of the small numbers. If the pregnancy continues, directed ultrasound examinations
in order to monitor the urinary tract, lips and brain must be performed, and growth
must also be monitored.
Fetal echocardiography should be performed.
When the percentage of abnormal cells is greater than 0%, or ultrasound examination
reveals defects or intrauterine growth retardation, there is a relatively increased
risk for problems, including developmental problems.