GATA-1 Mutation R216Q: Increased Bone Marrow Angiogenesis and Reticulin Fibrosis in X-Linked Thrombocytopenia with Thalassemia Trait (XLTT).

Abstract 5125

We describe features of a previously unpublished 4-generation family with sex-linked thrombocytopenia and large platelets, first investigated in 1993 and followed since then. Whereas there is thrombocytopenia only in 2 males (patient I and III, patelet counts 43 and 29 ×109/l, respectively) a thalassemia trait with elevated HbA2 and HbF is also found in one female. Moderate hitherto non-progressive splenomegaly is present in III. DNA sequencing of exon 4 of the GATA-1 gene was performed in the affected males, and revealed a G to A mutation corresponding to the amino acid change R216Q. This mutation has earlier been shown in 4 other kindred as the cause of XLTT, characterized by thrombocytopenia, splenomegaly and imbalanced globin chain synthesis (Balduini et al Thromb Haemost 2004;91:129-40). Bone marrow findings in our affected males include increased megakaryocytes and morphological abnormalities in megakaryocytes and erythroblasts, confirming the role of GATA-1 for their maturation. A slight reticulin fibrosis (grade 1) is also noted, not hitherto described in other families with XLTT. In an attempt to further characterize this fibrosis and underlying mechanisms we performed CD34 staining which revealed a high microvessel density (MVD) and thus increased angiogenesis. We studied the pericyte coverage of vessels by performing double staining for CD34 and SMA-a, and expression of platelet derived growth factor receptor b (PDGFR-b). Comparisons were made to results in myelofibrosis patients and controls.

Mice carrying the hypomorphic GATA-1low mutation have decreased expression of GATA-1 in megakaryoctes, defective megakaryocyte maturation and develop with age the typical traits of the human myeloproliferative disorder myelofibrosis, including reticulin fibrosis and increased angiogenesis (Vannucchi et al Semin Oncol 2005;32:365-72). In these mice as well as in the human disease myelofibrosis, immature megakaryocytes are retained in the bone marrow and pro-angiogenic and fibroblast stimulating factors are released because of pathologic emperipolesis. We show here that patients with the GATA-1 R216Q mutation also develop reticulin fibrosis and increased angiogenesis. It hasWe have previously been shown that the percentage of pericyte covered vessles is lower in the bone marrow of GATA-1low mice than in control mice, while on the contrary, patients with myelofibrosis have increased pericyte coverage as compared to controls (Zetterberg et al Haematologica 2007;92(5):597-604). We show above thatAs shown above, identical to mice with the GATA-1low mutation, patients with the GATA-1 R216Q mutation have lower pericyte coverage in their bone marrows than controls. Pericytes are cells of mesenchymal origin, important for vessel maturation and recruited to vessels by PDGF. Mice lacking either PDGF or its receptor have no pericyte coverage and die in utero because of hemorrhage from defective vessels. In contrast to malignant primary myelofibrosis, the reticulin fibrosis in our GATA-1 mutation R216Q patients appears non-progressive. We show here that the bone marrow stromal reactions are similar in patients with myelofibrosis and in patients with the GATA-1 R216Q mutation, with exception of pericyte coverage of vessels. Thus, GATA-1 dependent factors regulating pericyte recruitment might also be important for fibrosis progression in myelofibrosis and identifying these factors could forego therapeutic advances.