表观遗传学

亨廷顿氏舞蹈病是一种常染色体显性遗传的神经退行性疾病，其主要病理特征是大脑纹状体神经元的渐进性死亡。亨廷顿基因突变导致纹状体神经元选择性死亡的机制还不清楚，目前也没有任何治疗手段。DNA胞嘧啶的甲基化修饰是一重要的表观修饰方式，在染色体结构重塑、基因沉默、发育与分化等过程中扮演着关键角色。最近有研究结果证实，胞嘧啶的甲基化可被进一步氧化为羟甲基化修饰，有证据显示胞嘧啶羟甲基化修饰（5hmC）在胚胎发育和胚胎干细胞的增殖分化、神经细胞的分化成熟中有重要作用。关于DNA的5hmC修饰与神经退行性疾病的关系还未见报道。

动物研究所唐铁山研究员领导的分子神经生物学研究组以酵母人工染色体转基因舞蹈病（YAC128-HD）小鼠为研究模型，系统分析了不同月龄的疾病模型鼠脑组织和脑组织不同区域的5hmC修饰水平。研究发现小鼠出生后随月龄增长，神经细胞的5hmC水平快速上升，在三个月龄达到最高值；随后随月龄增加5hmC水平逐渐降低。与正常小鼠相比，疾病小鼠的纹状体、皮层、海马区域的胞嘧啶羟甲基化水平显著降低，而小脑和周边组织的羟甲基化水平未见改变。这些结果提示舞蹈病脑组织DNA在5hmC构建过程中存在缺陷。全基因组范围的5hmC分析鉴定了一系列舞蹈病相关的差异5hmC富集区域和差异表达的基因。进一步通路分析发现神经干细胞增殖及发育分化相关通路（如Wnt/b-catenin/Sox pathway, axonal guidance signaling pathway）和神经元功能以及存活相关通路（glutamate receptor/calcium/CREB, GABA receptor signaling, dopamine-DARPP32 feedback pathway etc）与疾病的发生密切相关。这些结果首次揭示了5hmC丢失可能是舞蹈病新的表观特征标记，同时也为该疾病的治疗开辟了一个新的方向。

该成果在线发表于 Human Molecular Genetics杂志(First published online: May 12, 2013, doi: 10.1093/hmg/ddt214)。研究组博士研究生王凤丽等同学为该论文共同第一作者；动物所唐铁山研究员、韩春生研究员和北京基因组研究所郭彩霞研究员为共同通讯作者，合作者有北京基因组研究所杨运桂研究员、动物所孙钦秒研究员和昆明植物所罗怀容研究员。该研究得到了科技部、国家自然科学基金委和中国科学院的资助。

原文摘要：

Genome-wide Loss of 5-hmC is a Novel Epigenetic Feature of Huntington's Disease
5-hydroxymethylcytosine (5-hmC) may represent a new epigenetic modification of cytosine. While the dynamics of 5-hmC during neurodevelopment have recently been reported, little is known about its genomic distribution and function(s) in neurodegenerative diseases such as Huntington's disease (HD). We here observed a marked reduction of 5-hmC signal in YAC128 (yeast artificial chromosome transgene with 128 CAG repeats) HD mouse brain tissues when compared with age-matched wild type (WT) mice, suggesting a deficiency of 5-hmC reconstruction in HD brains during postnatal development. Genome-wide distribution analysis of 5-hmC further confirmed the diminishment of 5-hmC signal in striatum and cortex in YAC128 HD mice. General genomic features of 5-hmC are highly conserved, not being affected by either disease or brain regions. Intriguingly, we have identified disease-specific (YAC128 versus WT) differentially hydroxymethylated regions (DhMRs), and found that acquisition of DhmRs in gene body is a positive epigenetic regulator for gene expression. Ingenuity pathway analysis of genotype-specific DhMR-annotated genes revealed that alternation of a number of canonical pathways involving neuronal development/differentiation (Wnt/β-catenin/Sox pathway, axonal guidance signaling pathway) and neuronal function/survival (glutamate receptor/calcium/CREB, GABA receptor signaling, dopamine-DARPP32 feedback pathway etc) could be important for the onset of HD. Our results indicate that loss of the 5-hmC marker is a novel epigenetic feature in Huntington's disease, and that this aberrant epigenetic regulation may impair the neurogenesis, neuronal function and survival in HD brain. Our study also opens a new avenue for HD treatment; re-establishing the native 5-hmC landscape may have the potential to slow/halt the progression of HD.