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。染色体中的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，这种代码然后通过信使RNA中间体（mRNA）转化为称为蛋白质的氨基酸链，从而实现某种功能;例如，通过酶促反应从我们吃的营养物质中产生能量，以实现电通信我们大脑中的神经元。虽然蛋白质是由不间断的氨基酸长链组成的，但DNA中的遗传密码却不是这样。DNA中几乎所有基因的编码序列称为外显子，被称为内含子的非编码序列中断。当DNA序列被转录成互补的前mRNA时，插入的内含子序列需要被剪接掉以产生成熟的mRNA。因此，前mRNA的正确剪接是一个非常重要的过程，但是指导小外显子剪接的序列（通常隐藏在非常大的内含子中）非常短，对我们人类来说似乎很模糊。然而，剪接体非常精确地切割内含子。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个叫做选择性剪接的过程来改变。引人注目的是，我们的基因中选择性剪接的数量，而不是基因的数量，将我们与更简单的生物区分开来。这些差异在我们最复杂的器官--大脑中表达的基因中表现得最为突出。有趣的是，RNA的序列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进行修饰。mRNA中最突出的修饰是腺苷（m6 A）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m6 A修饰的突变果蝇是可行的，尽管有神经和代谢缺陷。我们进一步发现雌性的生存能力受到了损害。使用敏感的遗传相互作用可能是由于这些突变体的生存能力，使我们能够揭示一个基本的作用，m6 A在选择性剪接的主调节器的性别决定性致死（SXL）的果蝇。Sxl具有额外的作用，并且需要调节男性和女性之间不相等数量的X染色体的基因表达;这一过程称为剂量补偿，在m6 A缺乏的女性中受影响最大。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，m6 A修饰在早期胚胎发生中具有重要的功能，当性命运建立时，基因组被重新编程用于以后的发育。因此，我们将在早期胚胎发生中全局映射m6 A位点，并使用遗传敏感的Sxl范例来询问非常基本的问题，即m6 A水平的动态如何用于调节细胞发育和分化期间的基因表达。这些研究对于了解m6 A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.