Two women with polycythemia vera and heterozygosity (GdB/GdA) at the X-chromosome-linked locus for glucose-6-phosphate dehydrogenase were studied to determine the nature of the cellular origin of their polycythemia. In contrast to unaffected tissue, such as skin fibroblasts, which consisted of both B and A types, the glucose-6-phosphate dehydrogenase of the patients' erythrocytes, granulocytes and platelets was only of Type A. These results provide direct evidence for the stem-cell nature of polycythemia vera and strongly imply a clonal origin for this disease. The fact that no descendants of the presumed normal stem cells were found in circulation suggests that bone-marrow proliferation in this disorder is influenced by local (intramarrow) regulatory factors.

Angiotensin II exerts a mitogenic effect in several in vitro models, but a direct effect on erythroid progenitors has not been documented. Angiotensin-converting enzyme inhibitors and losartan, an angiotensin II type 1 receptor (AT 1 ) antagonist, ameliorate posttransplant erythrocytosis, without altering serum erythropoietin levels. We studied erythroid differentiation and the effect of angiotensin II on proliferation of erythroid progenitors by culturing CD34 ϩ hematopoietic progenitor cells in liquid serum-free medium favoring growth of erythroid precursors. Aliquots of cells were collected every third day, and were used for RNA preparation. AT 1 mRNA was detected after 6 d. In these same samples, erythroid-specific mRNA (erythropoietin receptor) was also detected.

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Essential thrombocythemia (ET) and polycythemia vera (PV) are clonal myeloproliferative disorders that are often difficult to distinguish from other causes of elevated blood cell counts. Assays that could reliably detect clonal hematopoiesis would therefore be extremely valuable for diagnosis. We previously reported 3 X-chromosome transcription-based clonality assays (TCAs) involving the G6PD, IDS, and MPP1 genes, which together were informative in about 65% of female subjects. To increase our ability to detect clonality, we developed simple TCA for detecting the transcripts of 2 additional X-chromosome genes: Bruton tyrosine kinase (BTK) and 4-and-a-half LIM domain 1 (FHL1). The combination of TCA established the presence or absence of clonal hematopoiesis in about 90% of female subjects. We show that both genes are subject to X-chromosome inactivation and are polymorphic in all major US ethnic groups. The 5 TCAs were used to examine clonality in 46 female patients along with assays for erythropoietin-independent erythroid colonies (EECs) and granulocyte PRV-1 mRNA levels to discriminate polycythemias and thrombocytoses. Of these, all 19 patients with familial polycythemia or thrombocytosis had polyclonal hematopoiesis, whereas 22 of 26 patients with clinical evidence of myeloproliferative disorder and 1 patient with clinically obscure polycythemia were clonal. Interestingly, interferon alpha therapy in 2 patients with PV was associated with reversion of clonal to polyclonal hematopoiesis. EECs were observed in 14 of 14 patients with PV and 4 of 12 with ET, and increased granulocyte PRV-1 mRNA levels were found in 9 of 13 patients with PV and 2 of 12 with ET. Thus, these novel clonality assays are useful in the diagnosis and follow-up of polycythemic conditions and disorders with increased platelet levels.