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Fragile X syndrome
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Types
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Fragile X syndrome causes mental retardation, and is the second most common cause
of hereditary mental retardation after
Down syndrome.
The disease has this name because a break occurs in the lower part of the X chromosome in
cytogenetic testing (a chromosome test carried out on blood cells),
when the blood cells are grown on a special medium that lacks folic acid. In the
past, this test was the only way to diagnose these children. However, it is technically
difficult and is unreliable, especially in women and in fetal testing, so it is
not routinely used nowadays. Today, diagnosis is made by molecular testing as described
below.
2 different types are distinguished:
In the great majority of cases there is a mutation in the gene called FMR1, or FRAX-A,
- this is the gene on the X chromosome that is routinely tested.
Rarely there are families with fragile X syndrome that is due to a defect in other
genes on the X chromosome known as Frax-E, Frax-F, etc. These are genes that are
not tested for routinely and that require a specific investigation that is not included
in the regular fragile X test. Tests for these must be specifically requested if
there is a suspicion that there are mutations in these genes.
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Clinical signs
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The syndrome is characterized by varying degrees of mental retardation, learning
disorders and some autistic traits.
It is very difficult to differentiate between a child with this syndrome and one
with another syndrome of nonspecific mental retardation or from another child with
autism/PDD.
The facial appearance is not different from the normal apart from mild signs such
as a slightly prominent jaw and somewhat large ears.
Sometimes the testes are slightly larger than normal in postadolescent males.
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Special properties of the fragile X gene
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This gene is located on the X chromosome, of which women have two and men only one,
and it produces a protein known as fraxin. The gene, like all genes, is composed
of a sequence of building blocks called bases. It contains a region that is composed
of many copies (repeats) of the base sequence CGG. Usually there are no more than
54 repeats. Males tend to be more mentally retarded than females because females
have a second, normal X chromosome that partially protects them from the influence
of the abnormal gene on the first X chromosome.
The mutation in this gene is the number of CGG repeats. When the number of repeats
exceeds 54, this is abnormal, although there is no effect on mental functioning
until the number of repeats exceeds 200. The degree of functional damage increases
the greater the number of repeats, and it is significant when there are hundreds.
When the number of repeats exceeds 54, the region is unstable, and during the copying
of genetic material from a mother with more than 54 repeats to a fetus, the number
of repeats can multiply (expand) several times. This phenomenon is known as increased
anticipation. For some reason, this occurs only if the mother has the mutation -
if the father has the mutation and he transmits the affected allele to the fetus,
the number of repeats does not expand and the fetus will have the same number as
the father. The greater the number of repeats, the more severe is the damage to
the gene and the protein. In severe cases the number of CGG repeats can reach several
thousand - the damage (degree of retardation) here is very severe. A smaller number
of repeats manifests as a milder disease. Therefore the same family can have members
affected to differing degrees of severity - each with an abnormal number of repeats
that is different from that of relatives. Members of later generations are usually
more affected due to the increase in the number of repeats from generation to generation,
transmitted by mothers to their offspring. This is therefore a mutation that worsens
from generation to generation (dynamic mutation).
The number of repeats can be counted in the laboratory, and women undergoing the
test are divided into three categories based on the number.
Women with 54 or fewer CGG repeats in both their FMR1 genes. This is normal.
Women with between 55 and 200 CGG repeats in one of their FMR1 genes. These women
are not mentally retarded, but they are carriers and can transmit the abnormal gene
to their offspring. A CGG repeat number of between 55 and 200 is called a premutation,
and women in this category are called premutation carriers.
The transmission by a premutation carrier mother of the X chromosome containing
the abnormal FMR1 gene will only cause mental retardation in the offspring if the
number of the CGG repeats expands - that is, increases - to more than 200 during
transmission to the fetus (see number 3 below). A man with between 55 and 200 CGG
repeats in the FMR1 gene on his X chromosome is healthy, but because he transmits
his X chromosome to all of his daughters, they will all be carriers. However, all
of his sons will be healthy because they receive his Y chromosome and not his X
chromosome.
Women with more than 200 CGG repeats in one of their FMR1 genes. This is called
a full mutation and women in this category are called full mutation carriers. All
the sons of full mutation carrier mothers who receive her affected X chromosome
will be mentally retarded, since they only have one X chromosome, which is the one
that they received from her. However, only about 50% of the daughters of full mutation
carrier mothers who receive the affected X chromosome will be mentally retarded,
since their second X chromosome, which they have received from their father, "protects"
them from the full effect of the abnormal X chromosome that they have received from
their mother. The degree of retardation is directly dependent (both in males and
females) on the severity of the mutation, i.e. on the number of CGG repeats in the
FMR1 gene.
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Inheritance pattern
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Fragile X syndrome is transmitted by
X-linked inheritance with dynamic mutation.
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Penetrance
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Depends on the severity of the mutation, i.e. the number of CGG repeats.
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Associated features that can be demonstrated in imaging tests performed during pregnancy
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There are no diagnostic signs on
ultrasound examination or in biochemical tests apart from the molecular
genetic test that determines the number of CGG repeats in the gene - see below.
In all cases, genetic counseling
is important before embarking on a pregnancy. Preliminary tests must be performed
on family members, and the risk of recurrence determined based on the number of
patients and their relationship to the affected individual. If molecular testing
of the couple is required, it must be taken into account that even though this is
relatively not a long process, it should nevertheless be commenced before pregnancy.
Only after completing the molecular analysis before pregnancy can prenatal diagnoses
be undertaken in subsequent pregnancies.
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What is the risk of recurrence in a subsequent pregnancy?
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A carrier mother who has one affected and one normal gene has a 50% chance of transmitting
the normal gene, in which case the fetus will be healthy. During the transmission
of a gene with an abnormal number of repeats from a carrier mother to a child of
either sex, there may be a worsening of the severity of the disease due to expansion
of the number of repeats. The risk for expansion depends on the number of repeats
in the mother's affected gene. For example, if the mother has 54 - 60 repeats, a
fetus receiving this gene may have a minimal increase in the number of repeats in
families without cases of mental retardation due to fragile X syndrome. Conversely,
if a woman who has more than 70 repeats transmits this gene to her fetus, there
is a high risk that the number of repeats will expand significantly, even to over
200. When a woman with a number of repeats in excess of 80 transmits the affected
gene to her fetus, there is a significant risk that the number of repeats will expand
to over 200, and in these cases there is a distinct risk that the child will be
mentally retarded.
This expansion does not occur when the affected parent is the father. Therefore
if the father is not mentally retarded, all his offspring will be healthy, but all
his daughters will be fragile X carriers.
For more distant relatives, the risk is determined within genetic counseling, based
on the clinical picture, the pedigree, the results of tests including DNA testing,
etc.
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Molecular genetic information
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The gene for the disease
FMR1, which produces a protein called fraxin.
Location
Chromosome Xq28.
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Genetic testing
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Diagnostic testing
A direct test can be performed to identify the mutation - this is usually an increase
in the number of CGG repeats in the FMR1 gene. This is determined by means of a
direct test and is demonstrable in patients using a relatively simple method. See:
">Testing the disease-causing gene for mutations that are common in
a specific ethnic group - X-linked diseases.
Carrier testing
The same as diagnostic testing.
For the reasons mentioned above, it is recommended that only women undergo the test
for carrier status of fragile X syndrome. If a woman is found to have 55 or more
CGG repeats in one of her FMR1 genes, she is invited to come for genetic counseling,
and is advised to undergo prenatal diagnosis in every pregnancy because of the risk
of expansion of the number of CGG repeats to over 200, which causes mental retardation,
if the fetus receives her abnormal X chromosome. The number of CGG repeats can be
counted in the fetus by means of
amniocentesis or
chorionic villus sampling. The laboratory test of the mother's blood
takes about 5 weeks, testing of the chorionic villi about 3 - 4 weeks, and amniocentesis
7 - 8 weeks. In special cases, the time can be reduced to 7 - 10 days.
Several studies in the medical literature have reported that one in every 250 women
in the population is a carrier (either premutation or full mutation) of this condition.
However, in widespread surveys conducted in Israel, double the frequency has been
found. It is therefore advisable for every woman to be tested for carrier status,
regardless of ethnic origin. Like other carrier tests, it only needs to be performed
once, and not in every pregnancy.
In families with a history of mental retardation, the test is especially recommended.
In such families, if the mother is found to have more than 51 CGG repeats it is
advisable to test the fetus for fragile X syndrome.
Fetal testing
As for diagnostic testing.
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