Angiogenesis in myeloproliferative disorders

The chronic myeloproliferative disorders, comprising polycythemia vera (PV), essential thrombocytosis (ET), chronic myeloid leukaemia (CML) and myelofibrosis (MF), are all characterized by pathological angiogenesis in variable degrees, e.g. increased microvascular density (MVD) and bizarre vessel architecture. The aim of this study has been to characterize the molecular background to this phenomenon by asking two questions. The first is: is there a specific marker for newly formed vessels in the bone marrow of patients with mycloproliferative disorders? The second is: what is the mechanism responsible for the abnormal vessel morphology seen in these disorders? To answer the first question, a panel of antibodies, known to identify antigens over-expressed on endothelial cells of solid tumors (Tie-2, Cox-2, endoglin, angiomodulin and glycodelin) was used to stain bone marrow biopsies from patients with PV, CML and controls. None of these markers were overexpressed in these patients, suggesting that the vessel phenotype in myeloproliferative disorders is different from that of solid tumors. Surprisingly, we found that Tie-2, Cox-2 and glycodelin were expressed by megakaryocytes in both patients and controls. Since there is an accumulation of megakaryocytes in the bone marrow in myeloproliferative disorders, the total release of angiogenic factors is probably enhanced; thus, this milieu intérieur might be of significance for the pathological angiogenesis. To reveal the mechanism for the abnormal vessel morphology, we focused on the disorder showing the most aberrant vessels, namely myelofibrosis. VEGF expression and pericyte coverage in the bone marrow were studied and we could show both an over-expression of VEGF as well as increased pericyte coverage of the aberrant vessels. The latter finding was surprising, since pericyte coverage is thought to be a sign of maturity, and decreased pericyte coverage was expected based on findings in solid tumors. To further define the molecular events orchestrating these events, we studied microvascular density and pericyte coverage in two mouse models of myelofibrosis: mice over-expressing thrombopoietin (TPO) and mice with low expression of the transcription factor GATA-1 (GATA-1low). Both TPO and GATA-1 are important factors in megakaryocyte differentiation, and both mice models show abnormal megakaryocyte maturation. We found that both these models develop morphologically aberrant vessels and increased MVD. Similar to human MF, many of the aberrant vessels were pericyte coated, suggesting that these cells also are involved in the pathological angiogenesis. By using these mice models we can demonstrate a link between deficient megakaryocyte maturation and pathological angiogenesis. It is, thus, tempting to suggest that the dysmaturation of megakaryocytes seen in human MF is responsible for the increased angiogenesis, but that remains to be proven