Glioma Stem Cell Organoids: Preclinical Model of Glioblastoma Heterogeneity to Explore Resistance Mechanisms to Conventional Treatment Schedules.

Patients with glioblastoma (GM) have a median overall survival of approximately 15 months.Standard therapy for GM encompasses maximum surgical resection followed by radiation and chemotherapy using temozolomide (TMZ) (1). Regardless of initial tumor response, tumor recurrence is inevitable, after which survival drops to less than 6 months. GM tailored approaches targeting oncogenes that might drive the growth of the bulk of primary tumors, have been unsuccessful so far in clinical trials(2), creating a large unmet need warranting new approaches to overcome intrinsic and acquired resistance to current treatment schedules. The objective of this research is to establish primary patient derived organoid cultures from GM to study mechanisms that contribute to aggressive tumor growth and treatment resistance in primary and recurrent GM.

1. Inter-and intratumoral heterogeneity in glioblastoma. Tumor tailored approaches for GM are being hampered by inter-and intratumoral heterogeneity of both microenvironment and genomic alterations in GM cells. It has been shown that tumors are composed of multiple clones harboring distinct genetic alterations (3-7). The clonal evolution model posits that tumor formation is initiated in a cell of origin and is followed by the subsequent accumulation of multiple genetic and epigenetic alterations, leading to tumor cell survival and growth advantage (8). Divergent genetic alterations in early transformed cells give rise to a variety of clones under the selective pressure of the tumor microenvironment (3-7). An important microenvironmental stressor is intratumoral hypoxia, which is frequent in GM and a negative prognostic and predictive factor associated with reduced survival (9,10).

Emerging evidence implicates a subpopulation of tumor cells with characteristics of normal stem cells so-called glioma stem cells (GSC) in intrinsic and acquired treatment resistance. GSC are endowed with specific properties including high tumor initiating ability, unlimited self-renewal potential and capacity for multipotent differentiation, generating a diverse progeny(11). GSC are marked by common stem cell markers including CD133+, SOX2, Olig1, and have been shown to reside in the perivascular region as well as in hypoxic areas. GSC are expanded under hypoxia12depend on glycolysis (13,14).

Combined with their low proliferation, increased DNA repair, high anti-oxidant activity and among others, make GCS more resistant to conventional treatments (radiation and temozolomide) then non-GSC (15,16) {Jamal, 2012 #52}.

This implies that GSCs form an important driver of GM recurrence after chemoradiation. There are currently no effective treatments to eliminate glioma stem cells. Blocking hypoxia signalling in tumors (inhibiting self-renewal and survival of GSC cells)(12,17) and blocking the NOTCH stem cell pathway (rendering GSCs sensitive to radiation(18) and TMZ(19-21)), seem to promising but drugs interfering with these pathways have not passed beyond early phase clinical trials yet(22).