The fragile or Martin-Bell X syndrome is a genetic disorder that causes mental retardation and mild dimorphism. The cause is a mutation of the FMR1 gene on the X chromosome, affecting 1 child in 1250 and 1 child in 2500.
At the base of the disease there is a dynamic mutation that causes a deficiency of the FMRP protein. Under normal conditions, the affected DNA tract has between 6 and 50 copies of CGG triplets. In healthy carriers, copies are 50-200. In the diseased subjects, instead, this stretch of DNA expands progressively and leads to the aforementioned deficit.
In healthy subjects, the FMRP protein binds with RNA. It is found mainly in the testicles and the brain, which is why these are the organs most affected by his absence. The deficiency manifests itself gradually, in a period between the first months of life and puberty. Symptoms are varied and often not easy to identify. In some cases they are also linked to other diseases, delaying the diagnosis.
The most obvious physical symptoms are elongated face, low-planted auricles, protruding jaw. In some cases, strabismus and dental occlusions are also present. Some patients with fragile X syndrome also suffer from macrocephaly and mitral valve prolapse. In the post-pubertal phase, an increase in the volume of the testes can also occur.
Fragile X syndrome is the most common cause of inherited mental retardation. The extent of the delay, however, is variable and accompanied by other symptoms. Those suffering from Martin-Bell often show hyperactivity, attention deficit, autistic behaviors and emotional instability.
Given the wide variety of symptoms, the diagnosis can be difficult. We often get there by exclusion, or by examining similar cases in the family. In the latter case, it is also possible to resort to prenatal diagnosis.