Compensatory Signaling From ROR1 and the Pre-B Cell Receptor Promote Survival of t(1;19) Acute Lymphoblastic Leukemia
Authors: Bicocca Vincent T , Chang Bill H , Muschen Markus , Druker Brian J. , Tyner Jeffrey W 
Published: October 2019

Abstract
Abstract 2466

BACKGROUND:
Aberrant tyrosine kinase activity is commonly implicated in the pathogenesis of leukemia and other cancers. Identification of these leukemogenic tyrosine kinases has proven invaluable for diagnostic and prognostic stratification of patients as well as for the development of novel strategies for therapeutic intervention. We previously demonstrated that siRNA screening of mononuclear cells from leukemia patients can determine sensitivity to individual tyrosine kinases. With the goal of uncovering novel viability-dependent tyrosine kinases in leukemia patients, we have employed an RNAi-assisted protein target identification (RAPID) assay to screen cytogenetic subtypes of acute lymphoblastic leukemia (ALL). ALL is the most common pediatric cancer, accounting for one-quarter of all childhood malignancies. Childhood ALL has a primarily B cell precursor phenotype and is characterized by chromosomal abnormalities, primarily translocations and duplications. One of the most common recurring translocations associated with pediatric ALL, t(1;19)(q23;p13.3), generates the E2A-PBX1 fusion product. Here we show unique viability-dependent expression of a receptor tyrosine kinase, ROR1, in the t(1;19) ALL background. In addition, we identify a kinase inhibitor, dasatinib, with significant activity against t(1;19) ALL cells due to its capacity to inhibit tyrosine kinases necessary for transduction of pre-B cell receptor (preBCR) signaling. Finally, we show that ROR1 and the preBCR activate mutually compensatory signaling pathways, suggesting that optimal therapeutic regimens would include agents targeting both pathways.

METHODS:
To identify targets required for viability of leukemic cells, we screened cell lines as well as primary cells from ALL patients with siRNAs and determined cell viability using an MTS assay. ROR1 expression levels were determined by RT-PCR, immunoblot analysis and flow cytometry. Kinase inhibitor screening was performed on both cells lines and primary ALL cells by treating samples with a library of small-molecule inhibitors and cell viability was assessed by MTS. Signaling pathways disrupted by inhibitor treatment or ROR1 knockdown were interrogated by phospho-protein arrays and confirmed by immunoblot analysis.

RESULTS:
The RAPID assay identified a unique sensitivity to the cell surface receptor ROR1 in a subject identified with t(1;19) pediatric ALL. Similar sensitivity was not observed in ALL patients of alternative cytogenetic subtypes. Examination of additional ALL patient samples revealed conserved overexpression of the ROR1 transcript in t(1;19)-positive specimens with absence of ROR1 expression in t(1;19)-negative samples. Cell lines and early passage xenograft cells confirmed overexpression and functional dependence of t(1;19)-positive cells on ROR1. A subsequent kinase inhibitor screen of t(1;19) ALL cell lines and patient samples revealed universal sensitivity to the FDA-approved drug dasatinib. Further examination revealed the dasatinib targets, BTK and LYN, which signal downstream of the preBCR as the viability dependent targets of dasatinib in t(1;19) ALL. Inhibition of the preBCR results in transient loss of AKT activity and, surprisingly, upregulation of ROR1. Analysis of signaling pathways after silencing of ROR1 or dasatinib treatment revealed compensatory signaling pathways emanating from ROR1 and the preBCR that both serve to regulate AKT activity. Consequently, combination of ROR1 knockdown and dasatinib treatment resulted in additive impairment of cell viability compared with either treatment alone.

CONCLUSION:
The cell surface receptor ROR1 is consistently overexpressed in t(1;19) ALL. RNAi mediated downregulation of ROR1 impairs the viability of these cells. Similarly, t(1;19) cells are sensitive to the kinase inhibitor dasatinib due to activity against the preBCR. Combined targeting of ROR1 and the preBCR with dasatinib yields an additive viability effect due to compensatory signaling pathways aimed at regulating AKT. These results demonstrate a novel mechanism of AKT regulation in t(1;19) ALL as well as a therapeutic strategy for treatment of t(1;19) ALL.

Disclosures:
Druker: MolecularMD: Equity Ownership, OHSU and Dr. Druker have a financial interest in MolecularMD. Technology used in this research has been licensed to MolecularMD. This potential conflict of interest has been reviewed and managed by the OHSU Conflict of Interest in Research Committee and t; Ariad Pharmaceuticals: Consultancy; OHSU patent #843: Mutated ABL Kinase Domains: Patents & Royalties; Bristol-Myers Squibb: OHSU has clinical trial contracts with Bristol-Myers-Squibb to pay for patient costs, nurse and data manager salaries, and institutional overhead. Dr. Druker does not derive salary, nor does his lab receive funds from these contracts.; Novartis: OHSU has clinical trial contracts with Novartis to pay for patient costs, nurse and data manager salaries, and institutional overhead. Dr. Druker does not derive salary, nor does his lab receive funds from these contracts.; Cylene: Consultancy.