m6A mRNA methylation - understanding an essential mechanism adjusting gene expression during development and differentiation

生命的信息被编码在我们染色体上的基因DNA中。染色体上的DNA是一条很长的链，由四种不同的核苷酸组成：G、a、C和t。对于大多数基因来说，这些密码通过信使RNA中间体（mRNA）转化为一条被称为蛋白质的氨基酸链，它完成了一个功能；例如，从我们吃的营养物质中产生能量的酶促反应，允许大脑神经元之间的电通信。虽然蛋白质是由长而不间断的氨基酸链组成的，但DNA中的遗传密码却不是这样。DNA中几乎所有被称为外显子的基因的编码序列都被称为内含子的非编码序列所中断。当DNA序列转录成互补的前mRNA时，需要剪接中间的内含子序列以形成成熟的mRNA。因此，pre- mrna的正确剪接是一个非常重要的过程，但是通常隐藏在非常大的内含子中的指导小外显子剪接的序列非常短，对我们人类来说是模糊的。然而，剪接体非常准确地将内含子切掉。但是剪接不仅仅是产生一个“正确”的转录本，因为外显子的内容可以通过一种称为选择性剪接的过程来改变。引人注目的是，将我们与更简单的生物体区分开来的是我们基因中可选择剪接的数量，而不是基因的数量。这些差异最显著地体现在我们最复杂的器官——大脑中表达的基因上。有趣的是，RNA序列并不仅仅由四个核苷酸组成，因为许多核苷酸可以通过添加小的化学基团来修饰。mRNA中最显著的修饰是腺苷（m6A）的甲基化，这是小鼠和植物生存所必需的。我们最近发现，缺乏m6A修饰的突变果蝇是可以存活的，尽管它们有神经和代谢缺陷。我们进一步发现，雌性的生存能力受到了损害。利用这些突变体的生存能力可能产生的敏感遗传相互作用，我们揭示了m6A在果蝇性别决定主调节因子的选择性剪接中的基本作用。Sxl还有一个额外的作用，需要调节雄性和雌性之间x染色体数量不等的基因表达；这一过程被称为剂量补偿，在缺乏m6A的女性中受到的影响最大。因此，我们现在有了理想的动物模型来解决关于这种神秘的修饰如何调节基因表达的基本问题。我们的初步数据表明，m6A修饰在早期胚胎发生中具有重要功能，此时性别命运已经确定，基因组被重新编程以用于后期发育。因此，我们将在全球范围内绘制早期胚胎发生中的m6A位点，并使用遗传敏感的Sxl范式来询问非常基本的问题，即m6A水平的动态如何用于调节细胞发育和分化过程中的基因表达。这些研究对于了解m6A修饰在基因表达调控中的重要功能以及其异常调控如何导致人类神经和代谢缺陷，或如何被利用来干扰病毒复制（如寨卡病毒）至关重要。

The information for life is encoded in the DNA of the genes harbored on our chromosomes. The DNA in a chromosome is a very long chain consisting of four different nucleotides: G, A, C and T. For most genes this code is then converted via a messenger RNA intermediate (mRNA) into a chain of amino acids called proteins, which fulfill a function; for example an enzymatic reaction to generate energy from the nutrients we eat to allow for the electrical communication among neurons in our brain. Although proteins consist of long uninterrupted chains of amino acids, this is not the case for the genetic code in the DNA. The coding sequence of almost all genes in the DNA termed exons is interrupted by non-coding sequences called introns. When the DNA sequence is transcribed into a complementary pre-mRNA the intervening intronic sequences need to be spliced out to make the mature mRNA. Thus, correct splicing of pre-mRNAs is a very important process, but the sequences directing splicing of small exons, which are generally hidden in very large introns are very short and appear fuzzy to us humans. Nevertheless the splicosome cuts introns out very accurately. But there is much more to splicing than simply producing a "correct" transcript, because the exon content can be varied by a process called alternative splicing. Strikingly, it is the amount of alternative splicing in our genes, and not the number of genes that distinguishes us from simpler organisms. These differences are most prominently manifested in the genes that are expressed in our most complex organ, the brain.Intriguingly, the sequence of RNA does not just consist of four nucleotides, because many can be modified by addition of small chemical groups. The most prominent modification in mRNA is methylation of adenosines (m6A), which is required for viability of mice and plants. We recently discovered that mutant Drosophila lacking the m6A modification are viable, although with neurological and metabolic defects. We further discovered that female viability is compromised. Using sensitive genetic interactions possible due the viability of these mutants allowed us then to reveal a fundamental role of m6A in alternative splicing of the master regulator of sex determination Sex lethal (Sxl) in the fruit fly Drosophila. Sxl has an additional role and is required to adjust gene expression of the unequal numbers of X-chromosomes between males and female; a process called dosage compensation that is most affected in m6A devoid females. Hence, we now have the ideal animal model to address the very fundamental questions about how this enigmatic modification adjusts expression of genes. Our preliminary data indicate that the m6A modification has important functions in early embryogenesis, when sexual fate is established and the genome is reprogrammed for later development. Accordingly, we will globally map m6A sites in early embryogenesis and use the genetically sensitive Sxl paradigm to ask the very fundamental question how the dynamics of m6A levels are used to adjust gene expression during development and differentiation of cells. These studies are essential to understand the vital function of the m6A modification in the regulation of gene expression and how its aberrant regulation can lead to neurological and metabolic defects in humans, or can be exploited to interfere with viral replication such as in Zika virus.