Essential thrombocythemia in two sisters originating from different stem cell levels.

We report the rare occurrence of essential thrombocythemia (ET) in two sisters. In one patient, the clinical phenotype of the disease evolved from ET to polycythemia vera (PV) after 4 years of follow-up. Clonal hematopoiesis was established in both cases by X-chromosomal inactivation analysis using a DNA polymorphism of the phosphoglycerate-kinase (PGK) gene. Cell separation studies suggested a common ancestor for granulocytes, monocytes, and T lymphocytes in one patient; however, in her sister, monoclonality could only be demonstrated convincingly for the granulocyte fraction. Our data indicate that ET may originate from heterogenous stem cell levels.