Aplastic anemia (AA) is bone marrow failure characterized by a hypocellular marrow and peripheral pancytopenia. Immunosuppressive therapy (IST) results in hematologic recovery in the majority, decreased short term cytopenia related complications, and increased survival. However, clonal evolution to a secondary myeloid malignancy is a major complication in long-term survivors. Clonal evolution occurs in 10-15% of severe aplastic anemia (SAA) patients after IST. Most high risk clonal evolution is associated with complete or partial loss of chromosome 7, which is also frequent in other marrow failure syndromes. Previous studies have found older age, multiple rounds of IST, and severity of cytopenia as risk factors for clonal evolution. Shorter telomere length at diagnosis of AA and accelerated telomere attrition preceding the malignant transformation are associated with chromosome 7 clonal evolution, suggesting genomic instability as a possible mechanism. Frank myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML) without chromosome 7 involvement is much less common. Mutations in myeloid neoplasia related genes are present in 20-30% of patients with SAA. Excluding PIGA mutations, the most frequently affected genes are epigenetic regulators: ASXL1, DNMT3A and BCOR. The presence of unfavorable somatic mutations as a group is associated with a higher risk of clonal evolution and poorer survival. Surprisingly, somatic mutations were not frequently detected in clonal evolution to monosomy including in patients treated with eltrombopag (EPAG). We retrospectively assessed all subjects with SAA treated at the NIH Clinical Center on Hematology Branch protocols, with either horse- anti thymocyte globulin (ATG), rabbit-ATG or alemtuzumab based regimens for treatment-naïve or relapsed/refractory disease from 1989 to 2019. A total of 666 subjects were identified, of whom 96 had clonal evolution. The definition of clonal evolution was consistent across protocols. Clonal evolution was defined as an acquisition of new cytogenetic abnormalities with or without morphologic evidence of a myeloid malignancy. It was considered a high-risk clonal evolution when there was a diagnosis of a myeloid malignancy, an isolated acquisition of chromosome 7 abnormality or complex karyotype. High-risk evolution was noted in 58 of 96 subjects. Fifteen of these 58 subjects had evolved at 5 years or later after initial IST treatment. Among these 15 patients, eight subjects evolved with cytogenetic abnormalities and seven had MDS/AML with normal karyotype. We further investigated whether the late evolution to MDS/AML in the absence of cytogenetic changes was associated with somatic mutations in myeloid genes. Three subjects evolved at outside institutions and were excluded from the analysis due to insufficient data. One subject with loss of Y, a common age-related cytogenetic abnormality, was included.
