What is the X syndrome
Fragile X Syndrome
Synonyms: fragile X chromosome syndrome, marker X syndrome, Martin-Bell syndrome, fra (X) syndrome
English: fragile X syndrome
The Fragile X Syndrome is a hereditary disease that occurs mainly - but not exclusively - in men and among other things leads to intellectual disabilities in those affected.
The cause of the disease is a mutation on the X chromosome: the FMR1 gene on the Xq27.3 gene locus is mutated.
Genetic anticipation is possible in Fragile X syndrome. This means that the disease occurs earlier and more severe in the course of the following generations.
Under certain conditions, a break point on the X chromosome (fragile area) can be detected, which is what gave the syndrome its name.
Furthermore, the premutation (55-200 repeats) leads to an increased risk of primary ovarian insufficiency and, in men, to the so-called fragile X tremor ataxia syndrome (FXTAS).
The literature gives very different information on the frequency of the syndrome. The information on the incidence fluctuates between 1: 1,200 to 1: 4,000 in men and 1: 2,500 to 1: 6,000 in women. This makes it the second most common form of genetic intellectual disability after trisomy 21.
Men who have a fragile X chromosome but are healthy transfer the gene to their offspring: sons do not bear any risk of disease, daughters, however, carry the premutation, and in some cases the full mutation. Carriers of the premutation are symptom-free; If there is a full mutation, there may be different degrees of intellectual disability.
The main symptom of Fragile X syndrome is a reduced intelligence, which can be differently pronounced in each affected person. In addition, there are often other abnormalities in behavior and brain function, some of which are mentioned in the following list:
In addition, there are physical abnormalities in Fragile X syndrome:
- eye-catching face
- long, oval face shape
- large and protruding ears
- Progeny: protrusion of the chin
- protruding forehead
- Testicular enlargement
- occasionally scoliosis and foot deformities
All of these symptoms can, but do not have to, occur in the context of Fragile X syndrome.
The condition can occur in both men and women. However, since one of the two X chromosomes in the body's cells is inactivated in women, the symptoms in affected women are often less severe than in men.
To diagnose fragile X syndrome, the fragile point on the X chromosome (fra (X) (q)) can be detected in the karyogram, but this is a relatively unsafe method.
The detection of a triplet expansion is often used for diagnostics: this tests the number of repetitions of a cytosine-guanine-guanine base triplet in the fraX gene sequence. The triplet repeats themselves are in the intron. Healthy people have about 50-200 repetitions of this base triplet, sick people have over 200 repetitions.
The main tests used today are PCR (polymerase chain reaction) and immunohistochemical methods for the direct determination of the protein concentration of the FMR gene in question (marking with monoclonal antibodies).
The prenatal diagnostic methods available today are chorionic villus sampling (10th-12th week of pregnancy) and amniocentesis (16th-18th week of pregnancy).
Fragile X syndrome is a congenital disease that cannot be cured in and of itself. Therapy is therefore limited to the symptoms.
It is important that various medical disciplines work together (e.g. psychiatry, pediatrics, neurology, etc.) in order to enable optimal treatment of the patient. Components of the therapy are, for example, speech therapy and behavior therapy.
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