X-chromosome inactivation studies have played a pivotal role in establishing the current concepts of the pathogenesis of many hematological malignancies. These assays provide a means of assessing clonality without any requirement for tumor-specific genetic or cytogenetic markers (reviewed in ref. 26). In the female embryo each cell randomly inactivates one X-chromosome and the progeny of each cell inherits this pattern. As the inactivation is a random process, adult females are therefore mosaic with respect to this pattern of X-chromosome inactivation. Assessment of X-chromosome inactivation requires an ability to distinguish different X-linked alleles, so the various assays are based on polymorphic X-linked genes. It is also necessary to determine which X-chromosome is active. This can be achieved by monitoring expression at the RNA or protein level, or by using DNA methylation as a surrogate marker for the levels of gene activity.

The original studies of clonality in PV used a rare polymorphism of the G6PD gene that gives rise to identifiably different proteins. In two patients erythrocytes, platelets, granulocytes, and bone marrow buffy coat all showed predominant expression of one allele, whereas both alleles were expressed in skin fibroblasts.27 These data were interpreted as showing that PV results from transformation and subsequent clonal expansion of a multipotent hematopoietic stem cell. Subsequent studies using a variety of molecular methods have extended and confirmed clonality in larger numbers of PV patients.28-31 In order to distinguish a true clonal pattern in myeloid cells from extreme lyonization, the X-chromosome inactivation patterns in T cells have been concurrently examined alongside granulo-cytes or unfractionated bone marrow28,30,31 and the finding of skewed X-inactivation in granulocytes in the presence of a balanced pattern in T cells was felt to indicate a clonal disorder.

Approximately 90 percent of PV patients have a clonal pattern of X-inactivation.32 Since a clonal pattern of X-inactivation was reported to be rare in normal young females, it was hoped that X-inactivation patterns could be used as a diagnostic tool in the context of PV and the other MPDs. However, it was subsequently demonstrated that X-inactivation patterns become skewed with increasing age (acquired skewing), and 25 to 50 percent of hematologically normal women have "clonal" granulocytes and polyclonal T cells.33,34 There are several possible explanations for this observation. First, an acquired mutation could lead to clonal expansion as part of a neoplastic process. It is unlikely, however, that such a large proportion of hematolog-ically normal elderly females have a clonal hematological malignancy. Second, it has been suggested that stem cell depletion may occur with age but elderly mice have an increased number of stem cells.35 Instead current evidence favors a third explanation that entails selection for stem cells whose active X-chro-mosome is of one parental type. This concept is supported by twin studies36,37 and by the pattern of skewing observed in elderly cats.38 These data suggest the existence of one or more X-linked genes, which regulate stem cell kinetics.

Whatever the mechanism, the phenomenon of acquired skewing has important clinical and practical implications. First, the determination of clonality is clearly not a useful diagnostic tool in elderly women. Second, the data raise questions about the dogma that clonal hematopoiesis in patients with PV necessarily reflects the transformation of a multipotent stem cell.

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