[Autosomal recessive diseases with mental retardation]

Autosomal recessive diseases with mental retardation are disorders that affect autosomes, and their genetic expression occurs in individuals who are homozygotic for a mutation, while heterozygotic subjects are unaffected carriers. If both parents are carriers, the theoretical possibility of their children also being carriers is 50%, the risk of the children being affected by the disease is 25%, and there is a 25% chance of their being healthy. They are an important source of mental deficiencies and inborn errors of metabolism (IEM) are some of their characteristic syndromes.

The genetic disorders known as IEM can be classified according to the metabolism they affect, that is, purines, pyrimidines, amino acids, and so on. One of the lysosomal disorders is Tay-Sachs disease, which is rare among the general population but is very frequent in populations with a high rate of consanguinity, such as the Ashkenazi Jews. One of the most notable disorders affecting the metabolism of amino acids is the case of phenylketonuria due to mutations in the phenylalanine hydroxylase gene (PAH). It accounts for 0.5-1% of mental diseases and appears with a frequency rate of between 1/11,500 and 1/14,000 in newborn infants. Its early diagnosis through neonatal screening programmes makes it possible to start administering a phenylalanine-free diet and thus prevent mental retardation.

Knowledge of this kind of autosomal diseases with neurological involvement, together with their correct and early diagnosis, makes it possible to establish suitable treatment regimens in some cases and to carry out genetic counselling in all of them.