Iron Deficiency Status in Polycythemia Vera Correlates Well with Bone Marrow Angiogenesis in Patients Treated with Phlebotomy.
Authors: Willer Andreas , Lahaye Tanja , Saussele Susanne , Hehlmann Ruediger , Hochhaus Andreas 
Published: April 2020

Abstract
Background: Polycythemia vera (PV) is a chronic myeloproliferative disorder derived from multipotent hematopoetic precursors. The pathogenesis of PV is still poorly understood; furthermore, it is not clear, why the bone marrow (BM) of patients with PV has an increased microvessel density. Aims: For these reasons, we aimed to investigate, if BM angiogenesis and pathogenesis in PV are associated, and if the increased angiogenesis can elucidate pathophysiologic mechanisms of PV. Methods: BM biopsies were taken from patients with PV before and during treatment. A total of 38 samples from 35 patients were analyzed. In three patients, we investigated biopsies in 12 months intervals. Also, eight patients with iron deficiency were studied. BM angiogenesis in thin sections was determined by immunostaining of endothelial cells (CD34, vWF) and counting the microvessels. The microvessel counts were correlated to hematologic features such as treatment modalities and iron metabolism parameters (zinc protoporphyrin (ZPP), ferritin). Iron deficiency was classified to be mild for ZPP ≤ 100 micromol/mol hem (normal value &lt; 40) and for ferritin 7 to 35 microgramm/l (normal value &gt; 35), and to be severe for ZPP &gt; 100 micromol/mol hem and for ferritin &lt; 7 microgramm/l. Results: When compared to healthy controls, the number of BM microvessels in patients with iron deficient anemia was indistinguishable from healthy individuals. As expected, the BM microvessel numbers of patients with PV at diagnosis (n= 6) or with only mild iron deficiency treated with various therapy schedules (n=20) showed no significant difference (p=n. s.). In contrast, in patients with severe iron deficiency, we detected a highly significant difference between two different treatment modalities: phlebotomy alone was associated with a normal microvessel density (n=7; mean 3.6/mm2, range 2.7 to 5.7/mm2), whereas other treatment modalites or a combination with phlebotomy were associated with a high microvessel density (n=10; mean 10.4/mm2; range 4.5 to 15.7/mm2; p=0.0004). The microvessel counts of the patients from who two sequential biopsies were investigated are in line with these results: the microvessel numbers in a patient who received an oral iron treatment increased from 9.3 to 15/mm2; the microvessels of another patient who was treated with hydroxy urea and phlebotomy showed almost unchanged microvessel counts, whereas, in a third patient, who was treated exclusively by phlebotomy, a clear decrease of BM microvessel density from 15.7 to normal value of 4.5/mm2 was observed. Conclusions: We conclude that i) iron deficiency per se does not induce increased microvessel density in the BM, ii) a severe iron deficiency induced by phlebotomy alone can reduce the BM microvessel density in patients with PV to normal. Provided that the increased BM angiogenesis is associated with pathogenesis in PV, which means that the multipotent hematologic precursors account for the increased angiogenesis, efficient iron depletion by phlebotomy represents the highly potent treatment for PV.