ALBERT

Beschreibung: Resistance to glucocorticoids (GCs) is a major clinical problem in the treatment of acute lymphoblastic leukemia (ALL), but the underlying mechanisms are not well understood. Although mutations in the glucocorticoid receptor (GR) gene can give rise to therapy resistance in vitro, acquired somatic mutations in the GR are rarely encountered in patients. Here we report that the protein encoded by the BTG1 gene, which is frequently deleted in (pediatric) ALL, is a key determinant of GC responsiveness. Using RNA interference, we show that loss of BTG1 expression causes GC resistance both by decimating GR expression and by controlling GR-mediated transcription. Conversely, reexpression of BTG1 restores GC sensitivity by potentiating GC-induced GR expression, a phenomenon known as GR autoinduction. In addition, the arginine methyltransferase PRMT1, a BTG1-binding partner and transcriptional coactivator, is recruited to the GR gene promoter in a BTG1-dependent manner. These results implicate the BTG1/PRMT1 complex in GR-mediated gene expression and reveal that deregulation of a nuclear receptor coactivator complex can give rise to GC resistance. Further characterization of this complex as part of the GR regulatory circuitry could offer novel opportunities for improving the efficacy of GC-based therapies in ALL and other hematologic malignancies.

Beschreibung: Abstract 272 The response to therapy as determined by minimal residual disease (MRD) is currently used for stratification in treatment protocols for pediatric acute lymphoblastic leukemia (ALL). Even though MRD classification clearly identifies patients at low or at high risk for relapse, it also results in a large intermediate group (50 to 60% of patients), which still contains approximately half of all relapse cases. To improve risk stratification, we evaluated the added value of the IKZF1 alteration status, recently identified as a prognostic factor, in precursor-B-ALL patients. In an unbiased cohort of 131 uniformly treated precursor-B-ALL patients, we determined MRD levels at 42 and 84 days after treatment initiation using RQ-PCR analysis of Ig/TCR rearrangements. Based on these levels, patients were divided into three groups: MRD-Low (MRD-L), MRD-Medium (MRD-M) and MRD-High (MRD-H). IKZF1 alterations at diagnosis were determined using multiplex ligation-dependent probe amplification and genomic sequencing. We confirmed the strong prognostic significance of MRD classification, which was independent of IKZF1 status. Importantly, in the large MRD-M group (n=81; 62% of patients) containing 46% of the relapsed patients, IKZF1 alteration status identified 8 out of 11 relapsed patients (72%). The 9 year relapse-free survival (RFS) for IKZF1 mutated patients in this MRD-M group was 27% compared to 96% for patients wild-type for IKZF1 (P

Beschreibung: Abstract 1028 Background: L-Asparaginase (ASNase) is a key component of multi-agent chemotherapy regimens used in the treatment of pediatric Acute Lymphoblastic Leukemia (ALL). This bacterially-derived enzyme lowers blood asparagine levels by catalyzing the hydrolysis of L-asparagine, leading to specific killing of leukemic blasts. However, relapses and associated therapy resistance occur in about 20% of the patients. Although the molecular mechanisms that contribute to ASNase resistance are not well understood, increased expression of asparagine synthethase (ASNS), the enzyme responsible for asparagine synthesis, has been linked to ASNase resistance both in ALL cell lines and in primary leukemias. We recently reported that micro-deletions affecting the B-cell translocation gene 1 (BTG1) occur in about 10% of pediatric pre-B ALL cases. Here we show that BTG1 loss contributes to ASNase resistance by up-regulation of ASNS. Results: Using RNA interference, we show that loss of BTG1 expression promotes cell growth and renders pre-B ALL cells completely refractory to ASNase induced cell death. Resistance to ASNase in the BTG1 knockdown cells is accompanied by increased expression of ASNS, while knockdown of this metabolic enzyme is sufficient to reverse therapy resistance, indicating that upregulation of ASNS is required for the observed resistance phenotype. We further show that BTG1 associates with and regulates the activity of the transcription factor ATF4, a key regulator of metabolic stress responses and a central component in the regulatory network that controls ASNS expression. Conclusion: Together, our experiments identify BTG1 as an important regulator of ASNS expression which acts by modulating ATF4 function. We expect that a detailed molecular understanding of how BTG1 loss contributes to ASNase resistance, will lead to the identification of pharmacological targets that can be used to improve treatment responses in therapy-resistant ALL. Disclosures: No relevant conflicts of interest to declare.