The Fragile-X syndrome (or Martin-Bell syndrome) is the most frequent cause of mental retardation after the Down syndrome. It affects about a male on 5000 individuals and a female on 8000 individuals with a variable mental retardation level, soft or deep, soft behavior disorder (mood instability) or severe (Autism), cranio-facial dysmorphism (facial asymmetry, forehead,ears and chin prominence). In females the cognitive disorders are softer and consist in emotional and learning obstacles.
| Fig.1 – ideogramma del cromosoma X (A), cromosoma X normale (B),
cromosoma con sito fragile (C)
The fragile-X syndrome is a monogenic recessive sex-linked disease and it is caused by the loss of function of the FMR1 gene (Fragile X Mental Retardation1) located on a terminal region of the X chromosome (Xq 27.3), as a result of a mutation: expansion of CGG triplets number (succession of cytosine -guanine bases) exceeding to 200 (complete mutation) and further gene inactivation cause of the methylation. The expansion causes a bottleneck, visible throughout a microscope, in a X chromosome structure, so the name X-fragile (fig. 1).
The FMR1 gene mutation causes the non-production of a protein important for neuronal development. The FMR1 normal gene has a number of CGG triplets less to 45; it should be noted that approximately 20-30% of female individuals have a number of CGG triplets in a normal range, identical for two alleles (homozygous individuals). If the triplets number is between 45 and 54 the gene is normal but it may present instability (gray zone), that is an upward of triplets number trend in later generations.
A number of CGG triplets between 55 and 200 is called "premutation" and the gene is
unstable because it can increase the CGG triplets number in later generation until it reaches the full mutation. The risk entity of expansion of permutation to mutation is generally proportional to premutation size. About 20% of carrier women with premutation have premature ovarian failure (POF). When the gene has a number over 200 triplets and it's methylated it comes to full
mutation. The Fragile-X syndrome trasmission is defined as X-linked, that is linked to
chromosome X; premutation carrier mothers have a 50% of chance to transmit the mutated or premutated gene to children. The father premutated gene is stable and is transmitted only to daughters.
In a prenatal period, the test is performed on peripheral blood of the pregnant woman to exclude the presence of alleles with "premutation" or "mutation" that may have been transmitted to the fetus; in case the analysis highlights the state of permutation carrier, there is an increased risk , so it is appropriate to perform the test also on the fetus, analyzing the DNA extracted from chorionic villus or amniocytes.
Furthermore, if the test, performed on chorialis villus picked up before of 12 echographic week, could underline a triplets number expansion near to limit between permutation and mutation (200-230 CGG triplets), the exam isn’t usefull because the methylation process is complete after the 12 week.
Tecnobios Eurogenlab Prenatal gives a genetic counseling service to inform patients and parents, in a clear terms about the disease, the risk of disease transmission within the family, and the possibilities of diagnosis and treatments.
• DNA analysis A profile to underline a CGG triplets number in a normal range and small premutations: 20 days from sampling.
• DNA analysis B profile to underline large premutations: 20 days from sampling.
DNA is extracted from peripheral blood lymphocytes (in tubes with EDTA), from chorialis villus, amniotic fluid.