Establishment, Maintenance and Modulation of heterochromatin domains

我们基因组的DNA包含了制造细胞和制造人的指令或代码。我们细胞中的一些DNA序列（基因）编码制造蛋白质。然而，我们的大部分DNA是由不编码蛋白质的重复序列组成的。例如，人类基因组包含46%的重复序列。其他基因组包含更高比例的重复序列（如玉米的65%）。这些重复序列的功能尚不清楚。然而，可以理解的是，重复序列会给细胞带来麻烦。这是因为重复序列往往是“不稳定的”，可以与基因组中其他位置的重复序列相互作用和融合。这意味着基因组可以重新排列，导致一些序列的丢失和其他序列的复制。它还可以将两个独立的基因组片段结合在一起，从而扰乱制造适量蛋白质的指令。基因组重排是癌症和出生缺陷的标志。为了对抗DNA重复序列对基因组的潜在威胁，生物体已经开发出对抗DNA重复序列不稳定性的策略。一种策略是将重复序列包裹在保护性蛋白质中，以防止它们与其他重复序列相互作用。这种保护性蛋白质结构被称为“异染色质”。事实上，我们的DNA被包裹在一种叫做组蛋白的蛋白质中——DNA和组蛋白一起构成了“染色质”。染色质结构允许DNA被包裹起来，使其适合细胞，它还控制DNA编码的指令释放的频率和速度，以制造蛋白质。重复序列被包装在特殊的异染色质中，而异染色质更紧密地包装在组蛋白中。这意味着重复序列更难找到彼此，而可能连接基因组不同部分的蛋白质也更难找到重复序列。因此，异染色质的作用是防止基因组重排。因此，了解DNA重复序列如何被包装成异染色质是很重要的。这是我们在这项提案中要回答的问题之一。我们将研究一种简单的模式生物，单细胞酵母裂糖酵母的异染色质形成。我们之所以研究S. pombe，是因为它的异染色质与人类相似，但更简单。在S. pombe，我们最近开发了分离和建立异染色质组装过程的系统，以了解它是如何组装在一起的。使用该系统，我们的目标是鉴定DNA序列和蛋白质，允许异染色质在DNA重复序列上组装和维持。对于某些生物体，例如微生物病原体，在特定的环境条件下可以方便地重新排列它们的基因组，从而暂时消除DNA重复序列中的异染色质。其中一种微生物是真菌病原体白色念珠菌。白色念珠菌是人类最重要的真菌病原体。它通常生活在我们体内没有问题，但在某些情况下（例如免疫功能低下的患者），它可能导致危及生命的疾病。致病性白色念珠菌能有效适应不同的环境，并能获得抗真菌药物的耐药性。这是因为，与大多数生物相比，白色念珠菌可以在没有正常、正确的基因比例，甚至缺失部分染色体（一种称为基因组可塑性的现象）的情况下生存和繁衍。人们发现，DNA重复序列（称为MRS）在白色念珠菌转化为危险病原体的过程中起着重要作用。基因组重排通常发生在MRS重复序列上。我们将询问MRS重复序列是否通常通过被包裹在异染色质中而保持在“安全”状态，以及MRS的染色质类型是否改变以允许导致白色念珠菌成为病原体的基因组重排。我们将询问MRS重复序列的染色质类型是否控制白色念珠菌的致病性。

The DNA of our genome contains the instructions, or code, to make cells and to make a person. Some of the DNA sequences (genes) in our cells code for making proteins. However, much of our DNA is made up of repeat sequences that do not code for proteins. For example, the human genome contains 46% repeat sequences. Other genomes contain even higher proportions of repeats (e.g. 65% for maize). The function of these repeats is not well understood. However, it is understood that repetitive sequences spell trouble for the cell. This is because repetitive sequences tend to be 'unstable' and can interact and fuse with other repeat sequences in other places in the genome. This means the genome can become rearranged, causing loss of some sequences and duplication of others. It can also bring two separate sections of the genome together which can mess up the instructions for making the right amounts of proteins. Genome rearrangements are a hallmark of cancer and birth defects. To counteract the potential threat to the genome by DNA repeats, organisms have developed strategies to fight against the instability of DNA repeats. One strategy is to coat the repetitive sequences in protective proteins that prevent them from interacting with other repetitive sequences. This protective protein structure is called 'heterochromatin'. In fact, our DNA is packaged in proteins called histones - the DNA and histones together make 'chromatin'. The chromatin structure allows the DNA to be wrapped up so that it fits inside cells, and it also controls how often and how quickly the instructions coded by DNA are released to make proteins. Repeat sequences are packaged in specialised heterochromatin which is more tightly packaged in histones. This means that it's harder for repeats to find each other and harder for the proteins that might join up different parts of the genome to find the repeats. Therefore, heterochromatin acts to prevent genome rearrangements. For this reason it is important to know how DNA repeats are packaged into heterochromatin. This is one of the questions that we aim to answer in this proposal. We will study heterochromatin formation in a simple model organism, the unicellular yeast Schizosaccharomyces pombe. We study S. pombe because its heterochromatin is similar to what is found in humans but simpler. In S. pombe, we have recently developed systems to take apart and build up the process of heterochromatin assembly to understand how it is put together. Using this system we aim to identify the DNA sequences and the proteins that allow heterochromatin to be assembled and maintained on DNA repeats. For certain organisms, such as microbial pathogens, it could be convenient in certain environmental conditions to rearrange their genomes and therefore to temporarily erase heterochromatin from DNA repeats. One of such organism is the fungal pathogen Candida albicans. C. albicans is the most important human fungal pathogen. It normally lives inside our body without problem, but in certain situations (e.g. immunocompromised patients), it can cause life-threatening diseases. Pathogenic C. albicans adapts efficiently to different environments and it can acquire resistance to anti-fungal drugs. This is because, in contrast to most organisms, C. albicans can live and thrive without the normal, correct ratios of genes and even missing part of a chromosome (a phenomenon called genome plasticity). It was discovered that DNA repeats (called MRS) play an important role into the transformation of C. albicans into a dangerous pathogen. Rearrangements of the genome often occur at the MRS repeats. We will ask whether the MRS repeats are usually kept in a 'safe' state by being coated in heterochromatin and whether the type of chromatin at MRS changes to allow the genome rearrangements that cause C. albicans to become a pathogen. We will ask whether the type of chromatin at MRS repeats controls C. albicans pathogenicity.