Clonality studies in acute myeloid leukemia.

The study of X-chromosome inactivation patterns (XCIPs) to determine tumor clonality was established by Fialkow using G6PD protein isoenzymes but was limited by the low frequency of heterozygotes. Analysis was extended to most females with the demonstration of differential DNA methylation patterns on active and inactive X-chromosome alleles and uses Southern blotting or PCR of either restriction enzyme polymorphisms, eg PGK, HPRT, or variable number tandem repeat sequences, eg M27beta, HUMARA. More recently RNA polymorphisms have been reported enabling direct analysis of expressed transcripts from the two alleles. Interpretation of clonality requires knowledge of an individual's constitutive XCIP as skewing (>75% expression of one allele) occurs in a significant proportion of hematologically normal females, probably due to the stem cell pool size at the time of Lyonization. Furthermore, acquired skewing occurs with increasing age. For myeloid disorders in the young, T lymphocytes serve as a suitable control XCIP, but interpretation of imbalanced XCIPs in the elderly can be difficult. In AML, XCIPs at presentation are consistent with a clonal disorder whereas in remission comparison with normal controls suggests that true clonal remission is infrequent. Sequential analysis of samples may be helpful in some patients in order to determine evolving clonal populations.