Dying to predict outcome

Early treatment response is one of the most robust outcome predictors in childhood acute lymphoblastic leukemia (ALL). Intuitively, it is not unexpected that this factor would be an essential prognostic determinant, as it encompasses aspects of the complex interrelationship between disease, host and therapy. Early response can be measured in many ways, including morphologic reduction of blasts in the blood or bone marrow, or by more sensitive measures such as the detection of minimal residual disease (MRD) using flowcytometry or molecular techniques at time points during the initial weeks of treatment. In all cases, regardless of response measure and methodology, studies have demonstrated the favorable prognostic impact of rapid early responses to treatment. In fact, on the recently completed Pediatric Oncology Group protocol P9900, endinduction marrow MRD was the most important prognostic variable in multivariate analysis with 5-year event-free survival (EFS) rates of 88 ± 1%, 59 ± 5%, 49 ± 6%, and 30% ± 8% for those patients who were MRD negative, or MRD positive in the ranges of 0.01– 0.1%, 0.1–1%, and >1%, respectively [1]. Moreover, when used in conjunction with favorable clinical and cytogenetic features, the absence of peripheral blood MRD on day 8 combined with negative day 29 MRD, identified a subgroup of 12% of patients with a 97% ± 1% 5-year EFS [1]. Given the power of early response as a prognostic tool, numerous studies have been undertaken to dissect the factors that are essential in mediating response, with the premise that a better understanding of these factors early on in treatment could potentially provide an opportunity to modify therapy to overcome resistance. Since one of the essential ways that chemotherapeutic agents exert their effect is by inducing apoptosis, defects in apoptotic signaling have been postulated to play a key role in the acquisition of cellular drug resistance. In this issue of Pediatric Blood & Cancer, Mata et al. examined the expression profile of the apoptosis-related genes CASP3, CASP8, CASP9, FAS, and BCL2 in association with several key clinical and biological variables in children with ALL. Apoptosis is an evolutionarily conserved, active, energy dependent mechanism of cellular death and is regulated by the relative levels of both proand anti-apoptotic proteins as reviewed by Hengartner [2]. Lower expression levels of pro-apoptotic CASP3, CASP8, and FAS were associated with a poor day 7 response to treatment (white blood cell count >5,000/mm3) among 139 children ages 1–17 years with newly diagnosed ALL, treated according to the Brazilian Childhood Leukemia Treatment Group protocol. In addition, lower FAS expression was associated with lower 5-year event-free survival. It is noteworthy that while CASP3, CASP8, and FAS levels were associated with day 7 response, only FAS expression also correlated with event-free survival. The authors concluded that there is an association between lower expression of levels of pro-apoptotic genes and a poor response to therapy and poor prognosis. Somewhat counterintuitively, they found that higher expression of BCL2, an anti-apoptotic gene, was associated with a lower white count and low risk group classification; however, this finding is not inconsistent with several past studies that have failed to demonstrate an inverse correlation between BCL-2 levels at diagnosis and early response and outcome [3-5]. Despite the essential role of apoptosis in mediating response to chemotherapy, several other recent studies have also highlighted some of the challenges in defining the prognostic role of expression of individual components of the apoptotic cascade. Using gene expression microarray analyses, Flotho et al. [6] have previously shown that low expression of caspase-8 associated protein 2 (CASP8AP2), a gene that interacts with CASP8 in mediating apoptosis and which has been show to be a determinant of glucocorticoid signaling, was associated with MRD positivity on days 19 and 46 of therapy in children with ALL treated on St. Jude Total Therapy Studies. In this series, CASP8AP2 expression was lower in patients less than 1 year of age and higher in patients with hyperdiploidy, a favorable cytogenetic feature, but was not associated with other known outcome predictors. Validating these findings in an independent cohort, the authors observed that lower CASP8AP2 levels at diagnosis were a strong predictor of relapse, but were no better than age (older than 10 years) in predicting outcome. Cario et al. [7] defined gene expression profiles predictive of MRD of 10−3 or greater at week 12 of therapy, a very high risk patient subgroup with relapse-free survival rates of only 19%, and found low expression of several apoptosis-related genes, however, not members of the caspase family or FAS. More recently, Bhojwani et al. [8] identified a 24-probe set signature that was highly predictive of day 7 marrow status and a 47-probe set signature predictive of relapse among a cohort of 99 children with high risk ALL who were treated on the Children’s Oncology Group study 1961. While two of the six genes predictive of a rapid early response on day 7 were pro-apoptotic, they were distinct from the genes predictive of relapse and were not members of the caspase family or FAS. Furthermore, while the outcome