What gene change causes Aicardi syndrome?
The gene responsible for causing Aicardi syndrome is currently unknown. However, because the condition primarily occurs in females, it is believed to be caused by a mutation of a gene on the X chromosome. More specficially, the short (p) arm of the X chromosome between bands 22.2 and 22.3 is being studied.
Humans have 23 pairs, or 46 chromosomes, in each cell. The last pair of chromosomes are called the sex chromosomes because they help to determine whether an individual will have male or female characteristics. Female individuals generally have two X chromosomes (46,XX) and males generally have one X and one Y chromosome (46,XY).
In order to equalize the amount of X chromosomes that have influence on the body, early in embryonic development, one copy of the X chromosome in cells other than egg cells will be permanently inactivated in female individuals. This process occurs randomly, meaning that the X chromosome that is inactivated in one cell might not be the same X chromosome that is inactivated in another cell. Each copy of the X chromosome in females is inactivated equally, or about 50% of the time. However, sometimes X-inactivation is not random, and one copy of the X chromosome is active in more than half of cells. When X-inactivation does not occur randomly, it is called skewed X-inactivation.
When there is a severe gene mutation in one of the X chromosomes, skewed X-inactivation can occur. The cells in which the chromosome that has the mutation are active will not be able to survive as well making it appear as though X-inactivation is skewed. Skewed X-inactivation has been identified in girls with Aicardi syndrome, further supporting the idea that the disorder is caused by a mutation in a gene on the X chromosome.