Relevance of Abnormal KCNN1 Expression and Osmotic Hypersensitivity in Ewing Sarcoma

SIMPLE SUMMARY: The main oncogene in Ewing sarcoma directly drives a high expression of a previously unknown variant KCNN1 (encoding the K(Ca)2.1 channel) that we also verified in samples from >200 patients. Yet, we found that the channel is not functional and does not modulate Ewing sarcoma cell behavior. We could explain this lack of functional impact by the surprising absence of any K(Ca)2.1-carried K(+) current in Ewing sarcoma cells. However, we show in a proof-of-principle study that the essential lack of a K(+) conductance can be exploited by applying hypoosmotic stress and effectively and selectively killing the Ewing sarcoma cells. ABSTRACT: Ewing sarcoma (EwS) is a rare and highly malignant bone tumor occurring mainly in childhood and adolescence. Physiologically, the bone is a central hub for Ca(2+) homeostasis, which is severely disturbed by osteolytic processes in EwS. Therefore, we aimed to investigate how ion transport proteins involved in Ca(2+) homeostasis affect EwS pathophysiology. We characterized the expression of 22 candidate genes of Ca(2+)-permeable or Ca(2+)-regulated ion channels in three EwS cell lines and found the Ca(2+)-activated K(+) channel K(Ca)2.1 (KCNN1) to be exceptionally highly expressed. We revealed that KCNN1 expression is directly regulated by the disease-driving oncoprotein EWSR1-FL1. Due to its consistent overexpression in EwS, KCNN1 mRNA could be a prognostic marker in EwS. In a large cohort of EwS patients, however, KCNN1 mRNA quantity does not correlate with clinical parameters. Several functional studies including patch clamp electrophysiology revealed no evidence for K(Ca)2.1 function in EwS cells. Thus, elevated KCNN1 expression is not translated to K(Ca)2.1 channel activity in EwS cells. However, we found that the low K(+) conductance of EwS cells renders them susceptible to hypoosmotic solutions. The absence of a relevant K(+) conductance in EwS thereby provides an opportunity for hypoosmotic therapy that can be exploited during tumor surgery.