X-Inactivation Analysis

In females one of the two X-chromosomes becomes randomly inactivated early in embryogenesis to allow for dosage compensation of X-linked genes. In any one female somatic cell the inactive X may be either the paternal or maternal X chromosome. If the paternal X chromosome is inactivated more frequently than the maternal X chromosome or vice versa, the X inactivation pattern is skewed. If a woman carries a disease-causing mutation on just one of her X chromosomes, she normally would not show signs of the disease. However, skewed X inactivation may result in symptoms of an X-linked disease if the chromosome carrying the mutation is preferentially active. X inactivation is considered skewed if the ratio of the active to inactive X chromosome is less than 10 per cent or greater than 90 per cent.

The highly polymorphic androgen receptor (CAG)n repeat region, located on the X chromosome (Xq11-q12), is used to detect X inactivation. Methylation sensitive enzymes are used to cut the androgen receptor repeat region on the DNA strand. These enzymes digest DNA on the active X but do not cut sites on the inactive X. The quantitative PCR of androgen receptor repeats is compared with and without digestion to determine X inactivation ratio.

See related information sheet: X-Chromosome Inactivation