Abstract：
Somatic mosaicism in a fraction of brain cells causes neurodevelopmental disorders, including childhood intractable epilepsy. However, the threshold for somatic mosaicism leading to brain dysfunction is unknown. In this study, we induced various mosaic burdens in focal cortical dysplasia type II (FCD II) mice, featuring mTOR somatic mosaicism and spontaneous behavioral seizures. The mosaic burdens ranged from approximately 1,000 to 40,000 neurons expressing the mTOR mutant in the somatosensory (SSC) or medial prefrontal (PFC) cortex. Surprisingly, approximately 8,000 to 9,000 neurons expressing the MTOR mutant, which are extrapolated to constitute 0.08-0.09% of total cells or roughly 0.04% of variant allele frequency (VAF) in the mouse hemicortex, were sufficient to trigger epileptic seizures. The mutational burden was correlated with seizure frequency and onset, with a higher tendency for electrographic inter-ictal spikes and beta- and gamma-frequency oscillations in FCD II mice exceeding the threshold. Moreover, mutation-negative FCD II patients in deep sequencing of their bulky brain tissues revealed somatic mosaicism of the mTOR pathway genes as low as 0.07% in resected brain tissues through ultra-deep targeted sequencing (up to 20 million reads). Thus, our study suggests that extremely low levels of somatic mosaicism can contribute to brain dysfunction.
摘要：
一小部分脑细胞中的体细胞镶嵌会导致神经发育障碍，包括儿童难治性癫痫。然而,导致脑功能障碍的躯体镶嵌性阈值尚不清楚.在这项研究中,我们在局灶性皮质发育不良II型（FCDII）小鼠中诱导了各种马赛克负担，以mTOR躯体镶嵌和自发性行为癫痫为特征。在体感（SSC）或内侧前额叶（PFC）皮层中表达mTOR突变体的马赛克负担约为1,000至40,000个神经元。令人惊讶的是,大约8,000至9,000个神经元表达MTOR突变体，外推为小鼠半皮质中总细胞的0.08-0.09％或大约0.04％的变异等位基因频率（VAF），足以引发癫痫发作。突变负荷与癫痫发作频率和发作有关，FCDII小鼠的心电图间期尖峰和β-和γ-频率振荡超过阈值的趋势更高。此外，突变阴性的FCDII患者在其庞大的脑组织的深度测序中显示，通过超深度靶向测序（高达2000万次读数），在切除的脑组织中mTOR通路基因的体细胞镶嵌性低至0.07％。因此，我们的研究表明，极低水平的躯体镶嵌可导致脑功能障碍。
