Mouse BAZ1A (ACF1) is dispensable for double-strand break repair but is essential for averting improper gene expression during spermatogenesis.
Mouse BAZ1A (ACF1) is dispensable for double-strand break repair but is essential for averting improper gene expression during spermatogenesis.
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DOI：
10.1371/journal.pgen.1003945
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发表时间：
2013-11
期刊：
影响因子：
4.5
通讯作者：
Keeney S
中科院分区：
文献类型：
--
作者：
Dowdle JA;Mehta M;Kass EM;Vuong BQ;Inagaki A;Egli D;Jasin M;Keeney S
ATP依赖性染色质重塑控制DNA进入转录、重组和其他过程。Acf 1（在哺乳动物中也称为BAZ 1A）是保守的ISWI家族染色质重塑物ACF和CHRAC的定义亚基，15年前首次从果蝇胚胎中纯化。ACF和CHRAC的生物化学特性已为人们所知，它们可以在体外和体内移动核小体以建立有序的染色质阵列。在酵母，苍蝇和培养的人类细胞中的遗传研究清楚地表明，这些复合物通过控制染色质结构参与转录抑制。在培养的转化哺乳动物细胞中的RNAi实验也暗示ACF和CHRAC在DNA损伤检查点和双链断裂修复中。然而，它们在哺乳动物体内的重要作用尚不清楚。在这里，我们表明，Baz 1a基因敲除小鼠是可行的，并能够修复发育编程的DNA双链断裂的免疫系统和生殖系，I-SceI核酸内切酶诱导的断裂在原代成纤维细胞通过同源重组，和DNA损伤的丝裂霉素C暴露在体内。然而，Baz 1a缺陷导致雄性特异性不育，这与其在雄性生殖细胞中的高表达雅阁，在那里它显示动态的、阶段特异性的染色体定位模式。不育是由精子发育中的明显缺陷引起的，最有可能是在BAZ 1A缺失的情况下精母细胞和圆形精子细胞中基因表达的大规模干扰的结果：正常的生精转录程序基本上是完整的，但超过900个其他基因被错误调节，主要反映了不适当的上调。我们提出，在精子发生过程中发生的染色质组成的大规模变化创建了一个窗口的脆弱性混杂的转录变化，与ACF和/或CHRAC染色质重塑活动的基本功能，以防止这些改变。真核生物的基因组被包装成一个周期性的核蛋白复合体，称为染色质。DNA包裹在核小体（染色质的基本重复单位）周围，使得长段DNA能够包装成紧凑的核，但也阻碍了参与基本细胞过程（如转录、复制、重组和修复）的蛋白质因子的进入。染色质重塑因子是利用ATP水解过程中释放的能量组装、重新定位、重组和分解核小体的多蛋白复合物。这些复合物破坏组蛋白-DNA接触以“重塑”染色质并允许进入基因组。或者，例如，也可以拒绝访问以抑制转录。精子发生，产生精子的发育程序，包括一个戏剧性的染色质改造和转录程序的诱导，涉及近三分之一的基因组。在这里，我们提供的证据表明，这些大规模的改变离开基因组材料容易受到虚假的转录变化，通常被抑制的ACF 1（BAZ 1A在哺乳动物），定义成员的研究ACF/CHRAC染色质重塑复合物。这些发现表明Baz 1a在男性生育中发挥了以前未意识到的作用，并可能代表男性避孕药开发的新靶点。
ATP-dependent chromatin remodelers control DNA access for transcription, recombination, and other processes. Acf1 (also known as BAZ1A in mammals) is a defining subunit of the conserved ISWI-family chromatin remodelers ACF and CHRAC, first purified over 15 years ago from Drosophila melanogaster embryos. Much is known about biochemical properties of ACF and CHRAC, which move nucleosomes in vitro and in vivo to establish ordered chromatin arrays. Genetic studies in yeast, flies and cultured human cells clearly implicate these complexes in transcriptional repression via control of chromatin structures. RNAi experiments in transformed mammalian cells in culture also implicate ACF and CHRAC in DNA damage checkpoints and double-strand break repair. However, their essential in vivo roles in mammals are unknown. Here, we show that Baz1a-knockout mice are viable and able to repair developmentally programmed DNA double-strand breaks in the immune system and germ line, I-SceI endonuclease-induced breaks in primary fibroblasts via homologous recombination, and DNA damage from mitomycin C exposure in vivo. However, Baz1a deficiency causes male-specific sterility in accord with its high expression in male germ cells, where it displays dynamic, stage-specific patterns of chromosomal localization. Sterility is caused by pronounced defects in sperm development, most likely a consequence of massively perturbed gene expression in spermatocytes and round spermatids in the absence of BAZ1A: the normal spermiogenic transcription program is largely intact but more than 900 other genes are mis-regulated, primarily reflecting inappropriate up-regulation. We propose that large-scale changes in chromatin composition that occur during spermatogenesis create a window of vulnerability to promiscuous transcription changes, with an essential function of ACF and/or CHRAC chromatin remodeling activities being to safeguard against these alterations. The eukaryotic genome is packaged into a periodic nucleoprotein complex known as chromatin. Wrapping of DNA around nucleosomes, the basic repeat unit of chromatin, enables packing of long stretches of DNA into a compact nucleus but also impedes access by protein factors involved in essential cellular processes such as transcription, replication, recombination and repair. Chromatin remodeling factors are multi-protein complexes that utilize the energy released during ATP-hydrolysis to assemble, reposition, restructure and disassemble nucleosomes. These complexes disrupt histone-DNA contacts to ‘remodel’ the chromatin and grant access to the genome. Alternatively, access can also be denied to repress transcription, for example. Spermatogenesis, the developmental program that produces sperm, comprises a dramatic chromatin makeover and the induction of a transcriptional program that engages nearly one-third of the genome. Here we provide evidence suggesting that these large-scale alterations leave the genomic material vulnerable to spurious transcriptional changes which are normally repressed by ACF1 (BAZ1A in mammals), the defining member of the well-studied ACF/CHRAC chromatin remodeling complex. These findings indicate that Baz1a plays a previously unrealized role in male fertility and may represent a novel target for male contraceptive development.
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