Risk Factors For Clonal Evolution Of Acquired Bone Marrow Failure After Immunosuppressive Therapy In Children

Long-term survivors of acquired aplastic anemia (AA) have an increased risk of developing clonal evolution after immunosuppressive therapy (IST). We previously reported that the duration of granulocyte colony-stimulating factor (G-CSF) treatment and non-response to IST at 6 months were risk factors for clonal evolution. In 2008, the revised World Health Organization (WHO) classification proposed a new entity, “refractory cytopenia of childhood” (RCC), which is often difficult to differentiate from aplastic anemia (AA). The spectrum of patients with RCC is wide, ranging from patients with severe hypocellular bone marrow (BM) with mild dysplasia to those with normocellular BM with distinct dysplasia meeting the criteria for refractory cytopenia with multilineage dysplasia (RCMD) in adults. Few studies have investigated the correlation between morphological classifications of bone marrow failure (BMF) and clonal evolution. Before introducing the criteria of RCC, we conducted a prospective study with antithymocyte globulin (ATG) and cyclosporine for patients with acquired BMF (AA 97 study), which provided a unique opportunity both to compare the long-term outcome of patients with RCC and AA and to analyze risk factors for clonal evolution.

We retrospectively reviewed BM morphology in 186 children (median age, 8 years;range, 1–16 years) who were enrolled in the AA 97 study between July 1999 and November 2008. The median follow-up period was 87 months (range, 1–146 months). Fifty patients had non-severe, 54 had severe, and 82 had very severe disease. RCC criteria were defined as persistent cytopenia with <2% blasts in the peripheral blood (PB) and <5% blasts in the BM. BM aspirate smears showed dysplastic changes in >2 cell lineages or >10% within 1 cell lineage. RCMD criteria were defined as persistent cytopenia with <1% blasts in the PB and <5% blasts in the BM. BM smears showed >10% dysplastic changes in >2 cell lineages.

Morphologically, 62 patients (33%) were classified as AA, 94 (49%) as RCC, and 34 (18%) as RCMD. Disease severity differed among the three groups as follows: 76% of the AA patients had very severe disease, while 41% of the RCMD patients had non-severe disease (p<0.001). AA patients received G-CSF more frequently and for a longer duration than other patients (p=0.002). After 6 months, the response rates to IST were not significantly different among the three groups (AA, 52%; RCC, 59%; RCMD, 56%). Predictors of IST response were investigated by multivariate analysis. Morphological classification was not associated with IST response (p=0.519). Acquisition of clonal chromosomal abnormalities was observed in five patients in the AA group (monosomy 7, n=4; monosomy X, n=1), four patients in the RCC group (monosomy 7, n=1; trisomy 8, n=1; other, n=2), and three patients in the RCMD group (trisomy 8, n=3). Although the cumulative incidence (CI) of total clonal evolution at 10 years was not significantly different among the three groups, the CI of monosomy 7 was significantly higher in the AA group than in the other groups (p=0.02). Multivariate analysis revealed that morphology was not related to clonal evolution (p=0.23), and that only duration of G-CSF administration for >40 days was a significant risk factor for the development of monosomy 7 (p=0.016). Death, relapse, development of myelodysplastic syndrome or acute myeloid leukemia (AML), or disease progression requiring clinical intervention was considered to represent treatment failure. A second therapeutic intervention was required in some children. A second IST was therefore performed in 20 children, including five with AA, 11 with RCC, and four with RCMD. Hematopoietic stem cell transplantation was performed in 64 children, including 25 with AA, 29 with RCC, and 10 with RCMD. The rate of failure-free survival at 10 years was not significantly different among the three groups. On the other hand, the rate of overall survival at 10 years was significantly lower in the AA group (85±5.1%) than in the RCC (97±1.9%) and RCMD (100%) groups (p=0.01). Two patients died due to AML, and five patients died due to transplantation-related complications in the AA group.

To confirm these results, we are currently conducting a prospective study involving IST in patients with acquired BMF classified according to WHO classification before treatment.