# Uncomplexed-TSC1 deploys novel mTORC1-independent pathway to exacerbate the liver glycogen storage in TSC

## TL;DR

The study reveals a new pathway in Tuberous Sclerosis Complex that causes excessive liver glycogen storage, independent of mTORC1, and suggests a potential treatment combining inhibitors.

## Contribution

Discovery of a novel mTORC1-independent pathway involving TSC1, KDM5A, METTL3, and IGF2BP2 that exacerbates glycogen storage in TSC.

## Key findings

- Excess glycogen storage occurs in TSC1 and TSC2-deficient models, with more severe effects in TSC2 defects.
- Uncomplexed-TSC1 downregulates KDM5A, increasing METTL3 expression and stabilizing GYS2 mRNA.
- Combination treatment targeting mTORC1 and METTL3 ameliorates liver lesions in TSC2 defect models.

## Abstract

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder caused by inactivating mutations in TSC1 or TSC2 gene, leading to mTORC1 hyperactivation. However, mTORC1-independent mechanisms in this disorder remain poorly understood. In the study, excess glycogen storage was found in Tsc1–/– cells, Tsc1+/– and Tsc1c.2500-2503delAACA mice, as well as in Tsc2–/– cells, Tsc2+/– and Tsc2c.1113delA mice, with more pronounced accumulation in models with TSC2 defects. Mechanistically, the deficiency of TSC1 or TSC2 gene caused redundant uncomplexed-TSC2 or TSC1 protein, respectively. Strikingly, only uncomplexed-TSC1 downregulated the histone demethylase KDM5A, which in turn increased H3K4me3 levels at the METTL3 promoter to enhance its expression. The upregulated m6A “writer” protein METTL3 cooperated with the “reader” protein IGF2BP2 to stabilize GYS2 mRNA, causing the upregulation of GYS2 resulting in the glycogen storage. Thus, our study uncovered a novel mTORC1 independent pathway (TSC1-KDM5A-METTL3-IGF2BP2-GYS2) that underlies the excess glycogen storage, and that synergy of mTORC1-dependent and independent pathways leads to the more pronounced glycogen storage with TSC2 defects compared to those with TSC1 defects, reflecting the more severer clinical phenotypes in TSC patients with TSC2 mutations. Importantly, the restoration of glycogen homeostasis and significant amelioration of liver lesion in TSC2 defect models after the combination treatment of pharmacological inhibitors targeting mTORC1 and METTL3, unveil a potential clinic intervention for TSC patients to whom mTORC1 inhibitors are less effective or even ineffective.