KPTN Loss and Megalencephaly: mTOR Activation as Therapeutic Target

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Mutations in select mTOR pathway genes (MPG) are associated with megalencephaly (ME) and/or hemimegalencephaly (HME) including MTOR (Smith-Kingsmore syndrome, ME), STRADA (Pretzel syndrome, ME), PI3KCA (ME/HME), AKT3 (HME), PTEN (ME), DEPDC5 (HME) and RHEB (HME). Autosomal recessive germline variants in KPTN (kaptin; 19q13.32), part of the mTOR regulatory KICSTOR complex, have been recently identified in a clinical syndrome characterized by ME, intellectual disability, autism, and severe epilepsy (Baple et al., 2014) among the Old Order Amish communities in Ohio and Pennsylvania and more recently outside of the Plain Communities. There are 8 established pathogenic KPTN variants (loss of function) all linked to the clinical syndrome Megalencephaly, Autism, Seizures Developmental Delay (KPTN/MASD). The neurological phenotype includes hypotonia in infancy, autism spectrum disorder, intellectual disability (mild to severe), and seizures (onset between 3 months-27 years). Seizures are typically refractory to standard medical treatments and not amenable to resective surgery). ME is a universal feature of KPTN/MASD by adulthood (orbitofrontal head circumference [OFC] >2 standard deviations for age, with OFC measurements up to 5.4 SDS). Strikingly, serial OFC measurements available from 31 children, revealed that OFC is typically within normal limits at birth, with a rapid increase in OFC within the first 2 years of life with ~75% of patients with OFC >2 SDS by that age. Neuroimaging in a limited number of cases revealed a globally enlarged but structurally normal brain. A Kptn -/- mouse strain was recently developed and now is under investigation in our lab. Amazingly, Kptn-/- mice, but not Kptn +/- mice, also exhibit a ME phenotype with post-natal brain overgrowth, thus mirroring the clinical KPTN/MASD phenotype. This application proposes 3 Aims to investigate the mouse and human KPTN/MASD phenotypes. In Aim 1, we will define the effects of Kptn CRISPR/Cas9 KO in vitro on mTOR pathway activation, cell morphology, and cell motility and effects of mTOR pathway inhibitors (mTORi) in murine neural progenitor cells and neurons. In Aim 2, we will analyze the Kptn -/- brain including histopathology, mTOR signaling activation, and hyperexcitability (EEG), and response to mTORi. In Aim 3 we will define the neurological phenotype of KPTN/MASD in an existing and prospectively collected cohort of individuals within the Plain Community and in the general population. Because of the close phenotypic overlap between the Kptn -/- mouse model and human KPTN/MASD, these studies provide a unique opportunity to set the stage for an interventional clinical trial in KPTN/MASD to prevent brain overgrowth.