Genes contributing to minimal residual disease in childhood acute lymphoblastic leukemia: prognostic significance of CASP8AP2

Abstract

In childhood acute lymphoblastic leukemia (ALL), early response to treatment is a powerful prognostic indicator. To identify genes associated with this response, we analyzed gene expression of diagnostic lymphoblasts from 189 children with ALL and compared the findings with minimal residual disease (MRD) levels on days 19 and 46 of remission induction treatment. After excluding genes associated with genetic subgroups, we identified 17 genes that were significantly associated with MRD. The caspase 8-associated protein 2 (CASP8AP2) gene was studied further because of its reported role in apoptosis and glucocorticoid signaling. In a separate cohort of 99 patients not included in the comparison of gene expression profiles and MRD, low levels of CASP8AP2 expression predicted a lower event-free survival (P = .02) and a higher rate of leukemia relapse (P = .01) and were an independent predictor of outcome. High levels of CASP8AP2 expression were associated with a greater propensity of leukemic lymphoblasts to undergo apoptosis. We conclude that measurement of CASP8AP2 expression at diagnosis offers a means to identify patients whose leukemic cells are highly susceptible to chemotherapy. Therefore, this gene is a strong candidate for inclusion in gene expression arrays specifically designed for leukemia diagnosis.

Figures

Figure 1.

Expression of CASP8AP2 in leukemic lymphoblasts. (A) Expression of CASP8AP2 transcripts by RT-PCR in ALL cell lines and 10 primary ALL samples. (B) Expression of CASP8AP2 protein by flow cytometry in 2 cases of ALL. Overlay histograms indicate staining with anti-CASP8AP2 antibody and with nonreactive rabbit immunoglobulin (Control Ig) in each sample. The corresponding CASP8AP2 signal by gene array in each sample is shown. (C) Relation between CASP8AP2 transcript expression by gene array and protein expression by flow cytometry. Spearman correlation coefficient is shown.

Figure 2.

Prevalence of MRD (by level) on days 19 and 46 in 2 groups of patients with the lowest and highest expression of CASP8AP2 among 189 patients studied. Solid circles denote persistent MRD and/or hematologic relapse after day 46, while open circles denote continuous complete remission without MRD

Figure 3.

Prognostic impact of CASP8AP2 expression in a group of 99 patients enrolled in St Jude Total Therapy Study XIII. Patients were divided into 3 equal groups according to level of CASP8AP2 expression measured by gene array at diagnosis. Event-free survival and cumulative incidence of relapse are shown.

Figure 4.

Cell recovery of primary ALL cells after culture on mesenchymal cell layers according to CASP8AP2 expression. Primary ALL cells, selected from cases with known CASP8AP2 levels by GeneChip, were cultured on confluent mesenchymal cells for 7 days in serum-free medium. The number of viable ALL cells recovered at the end of the cultures was compared with the number of cells originally seeded. The dashed line indicates median cell recovery for the 24 cases. P value was calculated by Wilcoxon 2-sample test.