Rett syndrome is a genetic neurodevelopmental disorder that almost exclusively affects females and is very rare in males.

Rett syndrome is caused by mutations in the MECP2 gene located on the X chromosome. Because males have only one X chromosome, it has long been thought that a mutation in the MECP2 gene is lethal for boys.

There are, however, rare reported cases of males with Rett syndrome. These patients have more severe symptoms that develop earlier. The first signs of the disease are seen at birth or shortly thereafter. These patients often have severe breathing problems, feeding difficulties, and seizures. They can only survive to late childhood with radical medical intervention.

There are also some other circumstances that explain the occurrence of Rett syndrome in males. They are:

Males with Klinefelter syndrome

Klinefelter syndrome is a genetic condition where males have two X chromosomes in addition to their Y chromosome. This condition affects around 1 in 500 to 1,000 males; there is a small chance that a boy with Klinefelter syndrome may have a mutation in the MECP2 gene in one of his X chromosomes and develop Rett syndrome. Because these males have a spare copy of the MECP2 gene in their second X chromosome, the clinical onset and symptoms are very similar to those seen in females. In addition to Rett syndrome symptoms, affected males also have symptoms of Klinefelter syndrome, such as underdeveloped genitalia and low production of sex hormones.

Mosaicism

Mosaicism describes the presence of two different populations of cells in the body. In some cells of males with Rett syndrome, the MECP2 gene is mutated while other cells carry a healthy copy of the gene. Every human being originates from one cell, a fertilized egg cell. This cell then divides and gives rise to the human body. Because every cell arose from the same original cell, they are genetically identical. This is true unless a genetic modification or mutation occurs during development. Because each cell gives rise to many more cells, the earlier the mutation occurs during development, the more cells will have the mutation.

In mosaic males with Rett syndrome, the MECP2 mutation occurs during embryonic development. The clinical phenotype depends on the percentage of affected cells — the more cells in which the MECP2 gene is mutated, the more severe the symptoms.

Less severe MECP2 mutations

Some MECP2 mutations do not significantly affect the function of the MECP2 protein, for which this gene encodes. Females with this mutation usually have no clinical symptoms or only very mild symptoms. In males, these mutations may cause learning difficulties or behavioral problems, but not the typical symptoms of Rett syndrome.

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