Results of transplanting bone marrow from genetically identical twins into patients with aplastic anemia

Background: Aplastic anemia is caused by several diverse factors, including a lack of or defective hematopoietic stem cells, immune abnormalities, and disorders of the bone marrow microenvironment. The outcome of transplanting bone marrow from genetically identical twins into patients with aplastic anemia may help define how frequently these factors play a role in this condition. Objective: To determine the outcome of transplanting bone marrow from genetically identical twins into patients with aplastic anemia. Design: Observational study. Setting: 31 centers participating in the International Bone Marrow Transplant Registry. Patients: 40 patients with aplastic anemia who received bone marrow transplants from their genetically identical twins between 1964 and 1992. Intervention: 23 patients received their first bone marrow transplant without pretransplantation conditioning; 17 received it after pretransplantation conditioning with cyclophosphamide atone or combined with other drugs or radiation. Six patients received post- transplantation immunosuppressive therapy with methotrexate, cyclosporine, and corticosteroids, alone or in combination. Measurements: Outcomes of transplantation, including hematologic recovery and survival. Results: Seven of 23 patients who received their first transplant without receiving conditioning had sustained complete hematologic recovery. One of 16 patients who did not have complete recovery after the first transplantation recovered after a second transplantation, which was not preceded by conditioning. The other 15 patients had two to five transplantations that were preceded by conditioning; in 13 patients, sustained bone marrow function was recovered. Twelve of 17 patients whose first transplantation was preceded by conditioning had sustained complete hematologic recovery. The likelihood of hematologic recovery was greater in patients who had conditioning before the first transplantation (P = 0.033). The actuarial 10-year survival rate for the 40 patients was 78% (95% CI, 59% to 92%). The survival rate was higher in patients who did not have conditioning before the first transplantation (patients without conditioning, 87% [range, 65% to 99%]; patients with conditioning, 70% [range, 47% to 89%]; P = 0.037). Conclusions: Most patients with aplastic anemia recover bone marrow function after receiving a transplant from a genetically identical twin. Pretransplantation conditioning may increase the chance of bone marrow recovery but does not seem to improve survival.