Length-dependent gene misregulation in Rett syndrome (MeCP2)

Disruption of the MECP2 gene leads to Rett syndrome (RTT), a severe neurological disorder with features of autism. MECP2 encodes a methyl-DNA-binding protein that is proposed to function as a transcriptional repressor, but, despite numerous studies examining neuronal gene expression in MeCP2 mutants, no coherent model has emerged for how MeCP2 regulates transcription. Here we identify a genome-wide length-dependent increase in the expression of long genes in neurons lacking MeCP2. This gene misregulation occurs in human RTT brains and correlates with onset and severity of phenotypes in Mecp2 mutant mice, suggesting that the disruption of long gene expression contributes to RTT pathology. We present evidence that MeCP2 represses long genes by binding to brain-enriched, methylated CA dinucleotides within genes and show that loss of methylated CA in the brain recapitulates gene expression defects observed in MeCP2 mutants. We find that long genes encode proteins with neuronal functions, and overlap substantially with genes that have been implicated in autism and Fragile X syndrome. Reversing the overexpression of long genes in neurons lacking MeCP2 can improve some RTT-associated cellular deficits. These findings suggest that a function of MeCP2 in the mammalian brain is to temper the expression of genes in a length-dependent manner, and that mutations in MeCP2 and possibly other autism genes may cause neurological dysfunction by disrupting the expression of long genes in the brain. Total RNA was extracted from the cerebellum of 8-10 week old MeCP2 R306C mutant mice and their wild-type controls. Gene expression was analyzed using the Affymetrix Mo_Exon ST 1.0 microarray platform.MECP2基因的功能缺失会引发雷特综合征（Rett syndrome, RTT），这是一种兼具自闭症特征的重症神经系统疾病。MECP2编码一种甲基化DNA结合蛋白，被推测可作为转录抑制因子；尽管已有大量研究针对MeCP2突变体的神经元基因表达展开分析，但学界始终未能就MeCP2调控转录的机制形成统一连贯的模型。本研究在全基因组范围内发现，缺失MeCP2的神经元中，长链基因的表达呈现长度依赖性上调。这种基因表达紊乱同样存在于人类雷特综合征患者的脑组织中，且与Mecp2突变小鼠的表型发作时机及严重程度相关，提示长链基因表达异常可能参与了雷特综合征的病理发生。本研究证实，MeCP2可通过结合基因内部富集于脑组织的甲基化CA二核苷酸来抑制长链基因的表达；同时证明，脑组织中甲基化CA位点的缺失会重现MeCP2突变体中观察到的基因表达缺陷。研究还发现，长链基因所编码的蛋白多参与神经元功能活动，且与自闭症及脆性X综合征（Fragile X syndrome）的相关基因存在大量重叠。在缺失MeCP2的神经元中逆转长链基因的过度表达，可改善部分雷特综合征相关的细胞功能缺陷。上述研究结果表明，MeCP2在哺乳动物脑组织中的一项重要功能是以长度依赖的方式调控基因表达水平；MeCP2乃至其他自闭症相关基因的突变，可能通过破坏脑内长链基因的正常表达，进而引发神经系统功能障碍。本研究从8~10周龄的MeCP2 R306C突变小鼠及其野生型对照的小脑组织中提取总RNA，采用Affymetrix Mo_Exon ST 1.0微阵列平台开展基因表达分析。