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Robertsonian Translocation ( union of two chromosomes having only one arm forming
one common chromosome having two arms; involving chromosomes 13, 14, 15, 21 and
22 ).
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What is a chromosome disorder and what is its significance?
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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".
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What is a translocation?
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This is the exchange of chromosomal material between two chromosomes of different
pairs.
A translocation is said to be balanced when a change occurs in the location of certain
genes, but there is no gain or loss of genetic material.
If a person carries a balanced translocation, with no change in the amount of genetic
material and no damage to the genes has occurred, he will be healthy.
A parent carrying a balanced translocation can have healthy children without a translocation,
or healthy children carrying a balanced translocation like that of the parent.
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In these cases, where the chromosome arrangement is identical to that of the affected
parent, there are no clinical manifestations. The probability that a parent who
carries a balanced translocation will have healthy children is approximately 85%
per pregnancy. However, such a parent does have an increased risk (approximately
15%) of having a child with an unbalanced chromosome arrangement. In such a child,
the chromosome arrangement will include gains or losses of genetic material that
result from the parental translocation. Some fetuses that have an unbalanced chromosome
arrangement abort spontaneously at the beginning of pregnancy, so that some of the
couples where one partner carries a balanced translocation have recurrent miscarriages.
This can be diagnosed during pregnancy by amniocentesis or chorionic villus sampling.
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There are two main types of translocations:
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- Robertsonian translocation- described below.
- Reciprocal
translocation - see corresponding information sheet.
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Robertsonian translocation
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This is a condition in which there is almost complete fusion between two chromosomes.
This type of translocation is only possible with chromosomes 13, 14, 15, 21 and
22 because of their shape- they have only long arms and no short arms. The union
usually occurs in DNA sequences that are not genes, meaning that no active gene
is damaged, and there are usually no clinical consequences. It is rare for a translocation
of this type to damage a vital gene. However, if this does occur, there may be a
mutation with a functional deficit.
What is the significance of this translocation in the fetus?
The significance depends firstly on whether or not one of the parents carries the
same translocation (it is unlikely that both have it). If one of the parents does
carry the translocation, it may also be present in other family members. In most
cases, one of the parents carries a similar translocation.
If one of the parents carries such a translocation
If the parent carrying the translocation is completely healthy, it can be expected
that the fetus will also be healthy, without genetic problems arising from the translocation.
It is important to look for a specific condition known as Uni-Parental Disomy (UPD).
Here the fetus receives two chromosomes of the same pair from a single parent, instead
of the normal situation in which it receives one of each pair from each parent.
This condition is frequent in Robertsonian translocations. It can be detected by
performing molecular genetic tests involving testing at the DNA rather than the
chromosomal level. If UPD is found, the couple should be referred for genetic counseling in order to evaluate the risks
for various problems resulting from the UPD based on the chromosome involved and
other DNA tests.
If neither parent is a carrier of such a translocation
The chance of finding such a translocation in the fetus on performing amniocentesis
in pregnancies of couples where neither is a carrier of this translocation is one
in 9,000. If the translocation is not present in either parent, this is described
as a de novo translocation in the fetus, and in this situation the risk for defects
or developmental problems and mental retardation in the baby is approximately 3.7%.
Some of these defects can be detected by testing for UPD (see above), and some by
ultrasound. The defects detected by ultrasound are diverse. If defects or growth
retardation are found on
ultrasound examination, the risk that the child will be mentally retarded
is significantly increased. In genetic counseling, the risk can be assessed based
on the ultrasound findings and the UDP site.
Are cases of de novo Robertsonian translocation associated with an increased risk
for mental retardation even if there is no UPD and the ultrasound scan is normal?
Based on a number of reports in which a total of approximately 50 cases were studied,
two cases with defects that went undetected in ultrasound scans performed in pregnancy
were found. One was a baby with hypospadias
where UPD had not been looked for, and the other a baby with mild developmental
delay and convulsions.
In this case, however, the mother also had convulsions. It thus appears that the
absence of defects demonstrated by ultrasound and the absence of UPD are associated
with a greater chance that the fetus will be healthy.
It must be remembered that to date not enough studies have been carried out that
have monitored enough children with de novo Robertsonian translocations detected
in pregnancy for long enough to draw conclusions on their development.
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