Blinding the CYCLOPS - Neuroblastoma vulnerabilities unveiled by 1p loss
AbstractNeuroblastoma is the most common solid tumor in infants arising during embryonal development or early post-natal life. A frequently recurrent event is the deletion of chromosome arm 1p, which accounts for ~35% of all high stage cases. In the past years research focused on identification of potential 1p tumor suppressor genes but therapeutic targeting of these was shown to be difficult. With tumor suppressor gene deletion also a wide range of passenger genes get lost. As some of these are cell essential, hemizygous loss and associated reduced expression renders cells vulnerable to further impairment. Genes fulfilling these requirements are referred to as CYCLOPS (copy number alterations yielding cancer liabilities owing to partial loss) genes and may open a new therapeutic window.
In this study we aimed at identifying CYCLOPS genes on chromosome arm 1p in neuroblastoma. After detailed characterization of the 1p status in 35 neuroblastoma cell lines, we selected five cell lines with and five without 1p-deletion. For candidate gene identification, an siRNA screen for 184 druggable genes mapping to the distal end of 1p was done. Six candidates which showed high dependency in 1p-deleted but not in 1p non-deleted cells and differentially expression (1pdel < 1pnorm) were selected for further validation. In the end one gene met our requirements, EPHB2. This gene is especially important for embryonic cells and the developing nervous system. In 1p-deleted cell lines, EphB2 knock-down induced cell cycle arrest in G1/G0 and impaired cell survival. A small proportion of cells remained alive after activating HGF-induced c-MET signaling and MAPK/Akt pathway-mediated survival mechanisms. Induced EphB2 overexpression rescued the cells from cell death upon knock-down, supporting that EPHB2 expression is at a minimum level for survival in 1p-deleted cell lines.
In the control group of 1p non-deleted cell lines the impact on viability and gene expression after EphB2 knock-down was minimal. We observed also G1/G0 arrest but viability was not impaired.
Taking together, this study revealed EPHB2 as a promising 1p CYCLOPS candidate in neuroblastoma. As 1p is deleted in ~35% of all high-risk cases, a much wider range of patients may benefit from therapy approaches compared to strategies targeting other neuroblastoma-specific aberrations. Side effects of such approaches may be reduced as 1p non-deleted cells are not affected negatively. In general, this is a proof-of-principle for new drug target identification and is expandable to all cancers carrying frequent 1p-deletions