Phosphatase and tensin homolog (PTEN) is a major negative regulator of the phosphatidylinositol-3-kinase (PI3K)/Akt/mechanistic target of rapamycin (mTOR) pathway. Loss-of-function mutations in PTEN have been found in a subset of patients with macrocephaly and autism spectrum disorder (ASD). PTEN loss in neurons leads to somal hypertrophy, aberrant migration, dendritic overgrowth, increased spine density, and hyperactivity of neuronal circuits. These neuronal overgrowth phenotypes are present on Pten knock-out (KO) and reconstitution with autism-associated point mutations. The mechanism underlying dendritic overgrowth in Pten deficient neurons is unclear. In this study, we examined how Pten loss impacts microtubule (MT) dynamics in both sexes using retroviral infection and transfection strategies to manipulate PTEN expression and tag the plus-end MT binding protein, end-binding protein 3 (EB3). We found Pten KO neurons sprout more new processes over time compared with wild-type (WT) neurons. We also found an increase in MT polymerization rate in Pten KO dendritic growth cones. Reducing MT polymerization rate to the WT level was sufficient to reduce dendritic overgrowth in Pten KO neurons in vitro and in vivo. Finally, we found that rescue of dendritic overgrowth via inhibition of MT polymerization was sufficient to improve the performance of Pten KO mice in a spatial memory task. Taken together, our data suggests that one factor underlying PTEN loss dependent dendritic overgrowth is increased MT polymerization. This opens the possibility for an intersectional approach targeting MT polymerization and mTOR with low doses of inhibitors to achieve therapeutic gains with minimal side effects in pathologies associated with loss of neuronal PTEN function.

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Novel therapies are needed to for the treatment of atypical teratoid/rhabdoid tumor (ATRT), an aggressive brain tumor that predominantly affects young children and has an average 5-year survival under 50%. Claudin-6 (CLDN6) is a tight junction protein present during development and is expressed in up to 70% of ATRT specimens but not in normal tissue, making it a promising immunotherapeutic target. CLDN6-targeted chimeric antigen receptor (CAR) T cells in combination with a CAR T cell-amplifying mRNA vaccine have demonstrated antitumor activity against other CLDN6-expressing cancers in pre-clinical and a phase I adult trial (NCT04503278; Haanen J et al AACR, 2022). To assess the effectiveness of CLDN6-targeted CAR T cells against ATRT, we utilized a second-generation mRNA CAR with a 4-1BB costimulatory domain and single-chain variable fragment against CLDN6 (Reinhard et al, 2020). CLDN6 expression in patient-derived ATRT specimens was profiled by RNAseq (mean FPKM 11.4) and immunohistochemistry (positive staining in 53% of specimens). Tumor-derived cell lines were assessed for CLDN6 expression by flow cytometry. Co-culture of CLDN6-directed mRNA CAR T cells resulted in tumor-specific cytotoxicity compared to CD19-directed control CAR T cells in CLDN6-positive ATRT cell lines 7316-2187 (92% versus 15% at 10:1, p < 0.0001; 86% versus 0% at 5:1, p<0.0001) and 7316-2141 (75% versus 7% at 10:1, p<0.0001; 53% versus 0% at 5:1, p< 0.0001). Patient-derived, orthotopic xenograft ATRT models were created through intracranial injection of tumor cells into the cerebellum of NSG mice. Following engraftment with 7316-2141, repeated intratumoral administration of mRNA CLDN6 CAR T cells resulted in significant tumor regression (-1.9x108 vs. +2.4x109 p/sec/cm2/sr, p<0.001) and improved survival compared to mRNA CD19 CAR T cells (median OS not reached vs. 13 days, p=0.002). This work highlights the potential for targeting CLDN6 via CAR T cell therapy in patients with ATRT as a novel therapeutic strategy.