Is X-linked infantile spasm syndrome inherited ?
X-linked infantile spasm syndrome can have different inheritance patterns depending on the genetic cause.  When caused by mutations in the ARX gene, this condition is inherited in an X-linked recessive pattern. The ARX gene is located on the X chromosome, which is one of the two sex chromosomes. In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. Usually in females (who have two X chromosomes), a mutation would have to occur in both copies of the gene to cause the disorder. However, in some instances, one altered copy of the ARX gene is sufficient because the X chromosome with the normal copy of the ARX gene is turned off through a process called X-inactivation. Early in embryonic development in females, one of the two X chromosomes is permanently inactivated in somatic cells (cells other than egg and sperm cells). X-inactivation ensures that females, like males, have only one active copy of the X chromosome in each body cell. Usually X-inactivation occurs randomly, such that each X chromosome is active in about half of the body cells. Sometimes X-inactivation is not random, and one X chromosome is active in more than half of cells. When X-inactivation does not occur randomly, it is called skewed X-inactivation. Some ARX gene mutations may be associated with skewed X-inactivation, which results in the inactivation of the X chromosome with the normal copy of the ARX gene in most cells of the body. This skewed X-inactivation causes the chromosome with the mutated ARX gene to be expressed in more than half of cells, causing X-linked infantile spasm syndrome.  When caused by mutations in the CDKL5 gene, this condition is thought to have an X-linked dominant inheritance pattern. The CDKL5 gene is also located on the X chromosome, making this condition X-linked. The inheritance is dominant because one copy of the altered gene in each cell is sufficient to cause the condition in both males and females.  X-linked infantile spasm syndrome caused by CDKL5 gene mutations usually occurs in individuals with no history of the disorder in their family. These mutations likely occur in early embryonic development (called de novo mutations). Because males have only one X chromosome, X-linked dominant disorders are often more severe in males than in females. Male fetuses with CDKL5-related X-linked infantile spasm syndrome may not survive to birth, so more females are diagnosed with the condition. In females, the distribution of active and inactive X chromosomes due to X-inactivation may affect whether a woman develops the condition or the severity of the signs and symptoms. Generally, the larger the proportion of active X chromosomes that contain the mutated CDKL5 gene, the more severe the signs and symptoms of the condition are.  A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.