Overexpression of the Differentiation-Defective G-CSF Receptor Isoform IV Is Associated with a High Incidence of Relapses in Childhood Acute Myeloid Leukemia Treated with G-CSF

Introduction: In the prospective, multicenter AML-BFM 98 study for children and adolescents with acute myeloid leukemia (AML), it was randomly tested whether granulocyte colony-stimulating factor (G-CSF) can reduce infectious complications and improve outcome (161 patients with G-CSF, 156 without G-CSF). G-CSF administration did not result in less morbidity or mortality and higher event-free survival (EFS) (Lehrnbecher et al. Blood 2007). Subgroup analysis suggested an increased number of relapses in the standard risk (SR) group (i.e. AML FAB1/2 with Auer rods, FAB M4eo, AML with t(8;21) or inv16: if less than 5% of blasts on day 15) after G-CSF treatment (p=0.06). Relapses of SR patients did not correlate with initial leukocyte count, age, FAB subtype or cytogenetic aberrations.

Patients/Methods: We analyzed 50 SR patients (of n=162 SR patients in trial ALM-BFM 98) for G-CSF receptor (G-CSFR) expression. Surface expression of the G-CSFR was measured by immunophenotyping, and G-CSFR RNA isoform I and IV were analysed with real-time quantitative RT-PCR (RQ-RT-PCR). G-CSFR isoform IV overexpression was defined as expression higher than the median G-CSFR isoform IV expression in all analyzed SR patients. Minimal residual disease (MRD) was monitored by immunophenotyping and, if cytogenetic aberrations were detectable, by RQ-RT-PCR (Langebrake et al. JCO 2006; Viehmann et al. Leukemia 2003). All patients were screened for acquired mutations of the cytosolic domain of the G-CSFR by direct sequencing and subcloning.

Results: G-CSFR surface expression and MRD monitoring revealed no correlation with the risk of relapse. Notably, we found that even in complete morphologic remission after 1^st and 2^nd induction, there were still low numbers of blasts in nearly all patients. No G-CSFR mutations–as common in congenital neutropenia-were identified in this patient group.

In patients randomized to receive G-CSF after induction, the 16 patients overexpressing the differentiation-defective G-CSFR isoform IV showed an increased 5-year cumulative incidence of relapse (CI 50±13 %) as opposed to the 14 patients with low expression (CI of 14±10 %; p _{log rank}=0.04). In patients not receiving G-CSF, the level of G-CSFR isoform IV had no significant influence on CI (0±0 % in patients with high expression (n=9), and 82±12 % in patients with low expression (n=11), p=0.19.

Multivariate analyses including G-CSFR isoform IV overexpression, sex and favourable cytogenetics within the G-CSF-treated subgroup revealed prognostic relevance of G-CSFR isoform IV overexpression for EFS (p=0.031) and CI (p=0.049).

Conclusion: Our results demonstrate that children and adolescents with an AML overexpressing the differentiation-defective G-CSFR isoform IV respond to G-CSF administration after induction with significantly higher number of relapses. AML with overexpression of the differentiation-defective G-CSFR isoform IV should not be treated with G-CSF.

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