Mechanism of aberrant brain development induced by exposure to the reverse transcriptase inhibitor during fetal stages

<p>Autism spectrum disorder (ASD) is a developmental disorder characterized by impaired social communication, repetitive behaviors, and obsessiveness. Its prevalence has been increasing year by year and is currently very high: about 1 in 50 people. Although several causative genes for ASD have been identified, in most cases the causative gene has not even been identified, and effective treatment strategies have yet to be established.</p><p>ASD is known to be caused by abnormalities in brain development beginning in the fetal period. Previous studies have reported abnormal expression of the retrotransposon LINE1 (L1) in the postmortem brains of ASD patients, but the impact of L1 expression on normal brain development is unknown. It has been reported that L1 expression is basically maintained at a low level, but exceptionally, its expression is upregulated in neural progenitor cells in the brain and germ cells. Recent studies have also revealed that L1-cDNA, which is reverse-transcribed from L1-mRNA, regulates intracellular signaling by acting as a functional molecule. Therefore, we analyzed the role of L1 in normal brain development focusing on L1-cDNA.</p><p>First, we examined L1 expression during brain development and found that L1 expression increased as brain development progressed. Next, the reverse transcription of L1 mRNA to cDNA was inhibited by administering a reverse transcriptase inhibitor (3TC) to the mother mice from day 9 of gestation, and the proliferative ability of neural stem cells in the fetal brain at day 15 of gestation was evaluated. The results showed that treatment with 3TC increased the number of Sox2-positive (stem cell marker) and Ki67-positive (proliferation marker) neural stem cells. To elucidate the mechanism by which 3TC treatment increases neural stem cells, we administered 3TC to mice expressing EGFP downstream of the Sox2 promoter, collected EGFP-positive neural stem cells, and performed RNA-sequencing. GO analysis of the genes upregulated by 3TC treatment revealed that the expression of genes related to cell proliferation downstream of Yap signaling was increased. Furthermore, since the cGAS-STING pathway is known to be a major pathway for recognizing intracellular DNA, and activation of this pathway has been reported to inhibit Yap activation, we investigated whether the phenotype observed by 3TC treatment is also observed in STING-Knockout (KO) mice. We found that the number of Sox2-positive and Ki67-positive neural stem cells was increased in STING-KO mice as well as in 3TC-treated mice. Furthermore, when we waited until the embryonic 3TC-exposed mice reached adulthood to analyze their behavior, we observed abnormalities in their social behavior. The above results suggest that the presence of a certain amount of endogenous DNA is essential for normal brain development and that when this endogenous DNA is reduced by drugs or environmental changes, the activity of the cGAS-STING pathway is suppressed and YAP is activated, causing abnormal brain development and resulting in the observation of abnormal social behavior. </p>