doi: medRxiv preprint NOTE: This preprint reports new research that has not been certified by peer review and should not be used to guide clinical practice.

despite aggressive treatment. [1] Currently, maximal resection followed by radiation therapy with concurrent temozolomide (TMZ) and adjuvant TMZ treatment is the standard of care. Post-treatment surveillance involves serial MRI. A challenge faced by clinicians is the diagnosis and management of a gadolinium enhancing lesion on a follow-up MRI post-treatment. This suspicious lesion could be progressive disease (PD) or a mere post-treatment radiation effects such as pseudoprogression or radiation necrosis (RN). Pseudoprogression and RN are distinct clinical entities, which when identified and managed appropriately result in better outcomes, while PD of the tumor is often dismal. Patients with PD have a median survival of 3-6 months, [2] and there is no standard of care. Systemic options include TMZ rechallenge, lomustine, and antiangiogenic therapy such as bevacizumab, but their effectiveness is limited. Reradiation and reresection can be considered depending on the location of the tumor and the condition of the patient. [3] Conversely, antiangiogenic drugs like bevacizumab or cediranib decrease contrast enhancement by altering permeability of tumor vasculature without actual reduction in tumor burden, referred to as pseudoresponse. Distinguishing these clinical entities from PD is crucial to avoid unnecessary reoperations, premature discontinuation of adjuvant TMZ or its substitution with second line agents. MR imaging based monitoring is the current standard of care for post-surgical monitoring. Contrast enhancement on imaging is indicative of disrupted blood brain barrier (BBB), but not tumor presence. [4] Currently, MRI-based Response Assessment in Neuro-Oncology (RANO) criteria is used to monitor treatment response in GBM patients. The criteria included T1 gadolinium enhancing disease, T2/FLAIR changes, new lesions, corticosteroid use, and clinical status. [5] Adoption of RANO criteria for monitoring response is not without limitations. There is ambiguity in identifying radiation effects, enrolling patients into clinical trials and monitoring immunotherapy response. [6] Advanced imaging modalities including diffusion-tensor imaging, perfusion imaging, MR spectroscopy (MRS), and positron emission tomography (PET) imaging Liquid biopsy for the detection and monitoring of central nervous system tumors is of significant clinical interest. At initial diagnosis, the majority of patients with central nervous system tumors undergo magnetic resonance imaging (MRI), followed by invasive brain biopsy to determine the molecular diagnosis of the WHO 2016 classification paradigm. Despite the importance of MRI for long-term treatment monitoring, in the majority of patients who receive chemoradiation therapy for glioblastoma, it can be challenging to distinguish between radiation treatment effects including pseudoprogression, radiation necrosis, and recurrent/progressive disease based on imaging alone. Tissue biopsy-based monitoring is high risk and not always feasible. However, distinguishing these entities is of critical im...

Glioblastoma (GBM) is the most aggressive primary brain tumor with a median survival of 15 months despite standard care therapy consisting of maximal surgical debulking, followed by radiation therapy with concurrent and adjuvant temozolomide treatment. The natural history of GBM is characterized by inevitable recurrence with patients dying from increasingly resistant tumor regrowth after therapy. Several mechanisms including inter- and intratumoral heterogeneity, the evolution of therapy-resistant clonal subpopulations, reacquisition of stemness in glioblastoma stem cells, multiple drug efflux mechanisms, the tumor-promoting microenvironment, metabolic adaptations, and enhanced repair of drug-induced DNA damage have been implicated in therapy failure. Extracellular vesicles (EVs) have emerged as crucial mediators in the maintenance and establishment of GBM. Multiple seminal studies have uncovered the multi-dynamic role of EVs in the acquisition of drug resistance. Mechanisms include EV-mediated cargo transfer and EVs functioning as drug efflux channels and decoys for antibody-based therapies. In this review, we discuss the various mechanisms of therapy resistance in GBM, highlighting the emerging role of EV-orchestrated drug resistance. Understanding the landscape of GBM resistance is critical in devising novel therapeutic approaches to fight this deadly disease.

Repetitive elements (REs) comprise 40–60% of the mammalian genome and have been shown to epigenetically influence the expression of genes through the formation of fusion transcript (FTs). We previously showed that an intracisternal A particle forms an FT with the agouti gene in mice, causing obesity/type 2 diabetes. To determine the frequency of FTs genome-wide, we developed a TopHat-Fusion-based analytical pipeline to identify FTs with high specificity. We applied it to an RNA-seq dataset from the nucleus accumbens (NAc) of mice repeatedly exposed to cocaine. Cocaine was previously shown to increase the expression of certain REs in this brain region. Using this pipeline that can be applied to single- or paired-end reads, we identified 438 genes expressing 813 different FTs in the NAc. Although all types of studied repeats were present in FTs, simple sequence repeats were underrepresented. Most importantly, reverse-transcription and quantitative PCR validated the expression of selected FTs in an independent cohort of animals, which also revealed that some FTs are the prominent isoforms expressed in the NAc by some genes. In other RNA-seq datasets, developmental expression as well as tissue specificity of some FTs differed from their corresponding non-fusion counterparts. Finally, in silico analysis predicted changes in the structure of proteins encoded by some FTs, potentially resulting in gain or loss of function. Collectively, these results indicate the robustness of our pipeline in detecting these new isoforms of genes, which we believe provides a valuable tool to aid in better understanding the broad role of REs in mammalian cellular biology.