Non-myeloablative conditioning is sufficient to achieve complete donor myeloid chimerism following matched sibling donor bone marrow transplant for myeloproliferative leukemia virus oncogene (MPL) mutation-driven congenital amegakaryocytic thrombocytopenia: Case report

Abstract

Background: Congenital amegakaryocytic thrombocytopenia (CAMT) is a rare platelet production disorder caused mainly by loss of function biallelic mutations in myeloproliferative leukemia virus oncogene (MPL), the gene encoding the thrombopoietin receptor (TPOR). Patients with MPL-mutant CAMT are not only at risk for life-threatening bleeding events, but many affected individuals will also ultimately develop bone marrow aplasia owing to the absence of thrombopoietin/TPOR signaling required for maintenance of hematopoietic stem cells. Curative allogeneic stem cell transplant for patients with CAMT has historically used myeloablative conditioning; however, given the inherent stem cell defect in MPL-mutant CAMT, a less intensive regimen may prove equally effective with reduced morbidity, particularly in patients with evolving aplasia.

Methods: We report the case of a 2-year-old boy with MPL-mutant CAMT and bone marrow hypocellularity who underwent matched sibling donor bone marrow transplant (MSD-BMT) using a non-myeloablative regimen consisting of fludarabine, cyclophosphamide, and antithymocyte globulin (ATG).

Results: The patient achieved rapid trilinear engraftment and resolution of thrombocytopenia. While initial myeloid donor chimerism was mixed (88% donor), due to the competitive advantage of donor hematopoietic cells, myeloid chimerism increased to 100% by 4 months post-transplant. Donor chimerism and blood counts remained stable through 1-year post-transplant.

Conclusion: This experience suggests that non-myeloablative conditioning is a suitable approach for patients with MPL-mutant CAMT undergoing MSD-BMT and is associated with reduced risks of conditioning-related toxicity compared to traditional myeloablative regimens.

Keywords: MPL; aplastic anemia; bone marrow transplant; case report; congenital amegakaryocytic thrombocytopenia.

FIGURE 1

(A) Low power view (Top) of hypocellular bone marrow biopsy (∼30% cellular) taken at 26 months of age (4x, H, and E stain) in the patient with MPL-mutant CAMT. Higher power view (Middle) of hypocellular marrow with myeloid and erythroid hematopoiesis and absence of megakaryocytes (10x, H, and E stain). Immunohistochemical stain for CD42b (platelet receptor for von Willebrand factor) is negative (Bottom), confirming absent megakaryocytes (10x). (B) Total, T cell, and myeloid donor chimerism through 14 months post-bone marrow transplant showing evolution to complete myeloid donor chimerism and rising donor T cell chimerism over time.