Constitutive activation of mTORC1 signaling induced by biallelic loss-of-function mutations in SZT2 underlies a discernible neurodevelopmental disease

Abstract

There have been increasing number of reports of SZT2 -related neurological diseases, the main symptoms of which are epilepsy, developmental delay, macrocephaly and a dysmorphic corpus callosum. SZT2 functions as a regulator of mechanistic target of rapamycin complex 1 (mTORC1) signaling in cultured human cell lines and mouse tissues. However, it remains to be determined whether mutations in SZT2 in human patients alter mTORC1 signaling. In this study, we aimed to investigate the functional consequence of biallelic SZT2 variants in Epstein-Barr virus-induced lymphoblastoid cell lines (LCLs) established from two patients with a typical SZT2 -related neurodevelopmental disease. Increased phosphorylation of S6 kinase and S6 was identified in patient-derived cell lines under amino acid-starved condition, suggestive of constitutive hyperactivation of mTORC1 signaling. This result was validated by constitutive lysosomal localization of mTOR in patients’ LCLs. Furthermore, patients’ LCLs display an excessive response to slight amino acid stimulation. Our data suggest the loss-of-function nature of SZT2 mutations in the patients, and consequent hyperactivation of mTORC1 signaling in response to both amino acid starvation and stimulation in their LCLs. By these functional analyses, the pathogenicity of newly identified SZT2 variants can be determined, allowing for more detailed characterization of genotype-phenotype correlations.

A novel possible familial cause of epilepsy of infancy with migrating focal seizures related to SZT2 gene variant

“…SZT2 loss‐of‐function variants alter the mechanistic target of rapamycin (mTOR) signalling 7 . This may explain the role of its variants in epileptogenesis 1 .…”

“…SZT2 loss-of-function variants alter the mechanistic target of rapamycin (mTOR) signalling. 7 This may explain the role of its variants in epileptogenesis. 1 It was recently shown that SZT2 dysfunction leads to a hyperactivation of the mTORC1 signaling pathway resulting in increased cell proliferation, disturbed connectivity of the brain and epileptogenesis.…”

Determining the pathogenicity of variants of uncertain significance and identification of a founder variant in the epilepsy-associated gene,SZT2

“…Nakamura Y et al recently examined mTORC1 activity in lymphoblastoid cell lines (LCLs) from affected individuals 24 . They report elevated mTORC1 activity in cell lines derived from individuals with biallelic SZT2 variants relative to cell lines generated from healthy individuals.…”

Mouse and cellular models of KPTN-related disorder implicate mTOR signalling in cognitive and progressive overgrowth phenotypes

“…KPTN functions in a protein complex (KICSTOR) negatively regulating mTOR activity in response to nutrient availability 19,33 . Given the known contribution of mTOR pathway members in numerous brain overgrowth disorders in humans 9,[63][64][65] , this provides a plausible mechanism for KRD overgrowth.…”

“…Aberrant mTOR signalling is associated with numerous diseases such as cancer, diabetes, and neurological disorders including epilepsy, autism, developmental delay and macrocephaly [10][11][12][13] . De novo and inherited variants have been identified in numerous mTOR pathway genes, leading to the term "mTORopathies" to describe the involvement of the mTORC1 signalling pathway in these disorders [15][16][17][18][19][20] . A growing body of literature is uncovering the intricate relationship between the mTOR pathway, neural stem cell proliferation, and neuronal differentiation [21][22][23][24] , providing insights into the mechanisms of malformations of cortical development such as tuberous sclerosis complex, focal cortical dysplasia, hemimegalencephaly, and megalencephaly [25][26][27] .…”