Loss of the neural-specific BAF subunit ACTL6B relieves repression of early response genes and causes recessive autism

Synaptic activity in neurons leads to the rapid activation of genes involved in mammalian behavior. ATP-dependent chromatin remodelers such as the BAF complex contribute to these responses and are generally thought to activate transcription. However, the mechanisms keeping such “early activation” genes silent have been a mystery. In the course of investigating Mendelian recessive autism, we identified 6 families with segregating loss of function mutations in the neuronal BAF (nBAF) subunit ACTL6B (originally named BAF53b). Accordingly, ACTL6B was the most significantly mutated gene in the Simons Recessive Autism Cohort. At least 14 subunits of the nBAF complex are mutated in autism, collectively making it a major contributor to autism spectrum disorder (ASD). Patient mutations destabilized ACTL6B protein in neurons and rerouted dendrites to the wrong glomerulus in the fly olfactory system. Humans and mice lacking ACTL6B showed corpus callosum hypoplasia, indicating a conserved role for ACTL6B in facilitating neural connectivity. Actl6b knockout mice on two genetic backgrounds exhibited ASD-related behaviors, including social and memory impairments, repetitive behaviors, and hyperactivity. Surprisingly, mutation of Actl6b led to the relief of repression of early response genes including AP1 transcription factors (Fos, Fosl2, Fosb, and Junb), increased chromatin accessibility at AP1 binding sites, and transcriptional changes in late response genes associated with early response transcription factor activity. ACTL6B loss is thus an important cause of recessive ASD, with impaired neuron-specific chromatin repression indicated as a potential mechanism. Overall design: RNA- and ATAC-sequencing on primary cortical neurons from wildtype or Actl6b-/- E16.5 embryos cultured for 7 days in vitro (DIV7). Neurons were collected from n=5 wildtype and n=7 knockout biological replicates.

神经元的突触活动可快速激活参与哺乳动物行为调控的基因。依赖ATP的染色质重塑因子（如BAF复合物）参与此类细胞应答过程，且通常被认为可激活转录。然而，维持这类“早期激活”基因处于沉默状态的分子机制长期以来一直是未解之谜。在研究孟德尔隐性孤独症的过程中，我们鉴定出6个家系，其成员携带呈分离遗传模式的功能丧失突变，突变位点位于神经元特异性BAF（nBAF）亚基ACTL6B（最初命名为BAF53b）。据此，ACTL6B是西蒙隐性孤独症队列（Simons Recessive Autism Cohort）中突变显著性最高的基因。目前已有至少14种nBAF复合物亚基在孤独症患者中发生突变，该复合物因此成为孤独症谱系障碍（ASD）的重要致病贡献因素。患者来源的突变可降低神经元内ACTL6B蛋白的稳定性，并使果蝇嗅觉系统中的树突投射至错误的肾小球。缺失ACTL6B的人类和小鼠均表现出胼胝体发育不全，提示ACTL6B在促进神经连接方面具有保守性功能。在两种遗传背景下构建的Actl6b敲除小鼠均表现出孤独症谱系障碍相关行为表型，包括社交与记忆缺陷、重复刻板行为以及多动症状。令人意外的是，Actl6b突变可解除早期应答基因（包括AP1转录因子家族成员Fos、Fosl2、Fosb及Junb）的转录抑制，增加AP1结合位点处的染色质开放程度，并改变与早期应答转录因子活性相关的晚期应答基因的转录谱。因此，ACTL6B功能缺失是隐性孤独症谱系障碍的重要致病原因，神经元特异性染色质抑制功能受损可能是其潜在的分子机制。实验整体设计：从野生型或Actl6b基因敲除（Actl6b-/-）的胚胎期16.5天（E16.5）小鼠体内分离原代皮层神经元，经7天体外培养（DIV7）后进行RNA测序与ATAC测序。本实验共采集5份野生型生物学重复样本与7份敲除型生物学重复样本。