Trisomy 21

The most frequently occurring genetic disease currently diagnosed, which causes mental handicap, is trisomy 21 (1 in 750 births), which is caused by the presence of three chromosomes 21 instead of two in the cells of the person affected.

A distinction is made between free trisomy 21 as opposed to translocation trisomy 21 (where the additional chromosome 21 is "stuck" on another chromosome), and between homogeneous trisomy 21 (present in all the cells examined) as opposed to mosaic trisomy 21 (present only in a variable proportion of the cells examined).
In the majority of cases, it is the result of a non-hereditary accident, the precise cause of which is not yet known.
In a small percentage of cases, there is a family risk factor (for example in the case of translocation of a chromosome 21 onto another chromosome).
Each trisomic 21 person is first of all itself, unique, with its own unique manner of reacting to this excess of genes.
The expression of the disease generates common signs, but with considerable variability from one person to another.
The most marked consequence is mental deficiency, which is variable, and affects the capacity for abstraction, combined with characteristic physical signs.
There may be additional congenital complications (heart or digestive malformations, ...), or occurring at a later stage in life (endocrinal, orthopaedic, visual, auditory, ...).
Most of these complications can be treated, and therefore must be screened in order to give trisomic 21 patients the best possible quality of life, since their life expectancy is tending to equal that of the general population.
Specialised medical monitoring is therefore beneficial.
In the same way, a stimulating treatment (physiotherapy, speech therapy, psychomotor therapy), enables each child to best develop its capabilities, to benefit from suitable schooling, and to acquire the greatest possible autonomy.
"Of course, they must be provided with a protected environment, a job within their abilities, and always limitless affection. But that applies to us all" (Prof. J. Lejeune).

Although medicine is able to treat the complications, and even if the various therapies are a great help, science has not yet been able to find a way to cure them. Nevertheless, significant advances in our understanding of chromosome 21 have been made over the last few years, and these give rise to reasonable hope for the improvement of their condition.

Monogenic diseases causing mental handicap

As is the case with chromosomic diseases, there exist a large number of monogenic diseases, each of which is essentially very rare.

The most frequently occurring is X-fragile syndrome, which is the most common cause of hereditary mental handicap.

The name of this disease comes from the fragility of the X chromosome observed under the microscope in certain conditions.
It is caused by a mutation of gene FMR1, located on chromosome X, and therefore affects men and women to a varying frequency and degree. The diagnosis is performed by a genetic laboratory, and must be combined with genetic counselling, which is very important for the other members of the family.

As with trisomy 21, it is now a well-known disease, and patients affected must be provided with medical monitoring and treatment (speech therapy, psychomotor therapy, ...) to enable them to best develop all their capacities. A cure has yet to be found, but the experience gained from doctors, parents and educators has made possible an improvement in the day to day treatment of these patients.

Finally, there remains still a large percentage of patients suffering from unexplained mental handicap, either isolated or associated with other symptoms, which may be sporadic or inherited. The new genetic techniques and the discovery of new genes are leading to a diagnosis for a certain number of adult patients, who are then able to benefit from a more specific treatment.