Analysis of clonal cell growth using X-chromosome inactivation patterns

Studies of X-chromosome inactivation (XCI) patterns are central to many aspects of our current understanding of the pathogenesis of malignancies. They are based on the fact that only one X chromosome is active in each somatic female cell. The activation of one and the inactivation of the other X chromosome occur randomly very early in embryogenesis and are stable throughout the lifetime of a cell. Consequently, every woman exhibits a chimeric population of active X chromosomes of paternal and maternal origin. A clonal expansion of one cell results in a deviation of the random activation pattern. Therefore, analysis of XCI patterns provides a means of assessing clonality without the requirement for tumor-specific genetic or cytogenetic markers. XCI analyses are potentially applicable to all women and for air types of neoplasms. In numerous studies on patients with myeloproliferative disorders and myelodysplastic syndromes, the demonstration of skewed XCI in the blood has been taken as indication for the stem cell origin in these groups of diseases. However, recent data have made it very clear that appropriate controls are essential for a meaningful interpretation of the results. Constitutionally skewed XCI patterns have been found in approximately 20% of normal women. Recently, acquired skewing and apparent clonal hemopoiesis of myeloid cells has been observed in approximately 20% of hemotologically normal erderly women. This acquired skewing increases with age and may not indicate transformation of a multipotent stem cell. Possibly, subtle allelic differences between the two X chromosomes lead to a selective advantage of one X-chromosomal population that becomes evident with age. Also stem cell depletion might be more prevalent with increasing age and could explain the higher frequency of clonal inactivation patterns in healthy elderly women. The clonal cells might precede the occurrence of mutations which then can give rise to disease phenotype. Therefore, the question has to be addressed whether XCI analysis, even with the inclusion of appropriate controls, provides a specific diagnostic method to prove monoclonal derivation of cells in myeloproliferative disorders or myelodysplastic syndromes in elderly women. To clarify the unresolved issue of the nature and implications of acquired as well as constitutional skewing, extensive investigations will be needed. Recently, it could be demonstrated that XCI patterns are genetically determined and heritable. The genetic locus has been mapped to Xq28.