X-chromosome inactivation in healthy females: incidence of excessive lyonization with age and comparison of assays involving DNA methylation and transcript polymorphisms.

Skewed lyonization in healthy females represents the major disadvantage of X-chromosome-based clonality assays. Because most techniques are based on the difference in DNA methylation between active and inactive X-chromosomes, incomplete DNA digestion may occur, giving an unreliable clonality result. Here, we compare two different techniques carried out in healthy females belonging to three age groups and in a group of patients with essential thrombocythemia. The first technique involved the human androgen receptor gene, the second the transcript analysis of the iduronate-2-sulfatase, P55, and glucose-6-phospate dehydrogenase genes. Results between both techniques were concordant in most cases except in neonates, and the same pattern was observed in all fractions in healthy females. We conclude that: (a) clonality assays involving DNA and RNA polymorphisms are usually concordant except in neonates; (b) appropriate control tissue embryologically related to the sample must be chosen to eliminate excessive lyonization; (c) acquired skewing increases with age, whereas nonrandom lyonization is a rare phenomenon.