抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Cohesin Loss Eliminates All Loop Domains

Targeted Degradation of CTCF Decouples Local Insulation of Chromosome Domains from Genomic Compartmentalization.

Multiscale 3D Genome Rewiring during Mouse Neural Development

Significance When the human genome folds up inside the cell nucleus, it is spatially partitioned into numerous loops and contact domains. How these structures form is unknown. Here, we show that data from high-resolution spatial proximity maps are consistent with a model in which a complex, including the proteins CCCTC-binding factor (CTCF) and cohesin, mediates the formation of loops by a process of extrusion. Contact domains form as a byproduct of this process. The model accurately predicts how the genome will fold, using only information about the locations at which CTCF is bound. We demonstrate the ability to reengineer loops and domains in a predictable manner by creating highly targeted mutations, some as small as a single base pair, at CTCF sites. We recently used in situ Hi-C to create kilobase-resolution 3D maps of mammalian genomes. Here, we combine these maps with new Hi-C, microscopy, and genome-editing experiments to study the physical structure of chromatin fibers, domains, and loops. We find that the observed contact domains are inconsistent with the equilibrium state for an ordinary condensed polymer. Combining Hi-C data and novel mathematical theorems, we show that contact domains are also not consistent with a fractal globule. Instead, we use physical simulations to study two models of genome folding. In one, intermonomer attraction during polymer condensation leads to formation of an anisotropic “tension globule.” In the other, CCCTC-binding factor (CTCF) and cohesin act together to extrude unknotted loops during interphase. Both models are consistent with the observed contact domains and with the observation that contact domains tend to form inside loops. However, the extrusion model explains a far wider array of observations, such as why loops tend not to overlap and why the CTCF-binding motifs at pairs of loop anchors lie in the convergent orientation. Finally, we perform 13 genome-editing experiments examining the effect of altering CTCF-binding sites on chromatin folding. The convergent rule correctly predicts the affected loops in every case. Moreover, the extrusion model accurately predicts in silico the 3D maps resulting from each experiment using only the location of CTCF-binding sites in the WT. Thus, we show that it is possible to disrupt, restore, and move loops and domains using targeted mutations as small as a single base pair.

Chromatin extrusion explains key features of loop and domain formation in wild-type and engineered genomes

Formation of Chromosomal Domains by Loop Extrusion

Summary Topologically Associating Domains (TADs) are fundamental structural and functional building blocks of human interphase chromosomes, yet mechanisms of TAD formation remain unknown. Here we propose that loop extrusion underlies TAD formation. In this process, cis-acting loop-extruding factors, likely cohesins, form progressively larger loops, but stall at TAD boundaries due to interactions with boundary proteins, including CTCF. Using polymer simulations, we show that this model can produce TADs as determined by our analyses of Hi-C data. Contrary to typical illustrations, each TAD consists of multiple dynamically formed loops, rather than a single static loop. Our model explains diverse experimental observations, including the preferential orientation of CTCF motifs, enrichments of architectural proteins at TAD boundaries, and boundary deletion experiments, and makes specific predictions for depletion of CTCF versus cohesin. The emerging picture is that TADs arise from actively forming, growing, and dissociating loops, presenting a framework for understanding interphase chromosomal organization.

Formation of Chromosomal Domains in Interphase by Loop Extrusion

Topologically Associating Domains (TADs) are fundamental structural and functional building blocks of human interphase chromosomes, yet mechanisms of TAD formation remain unknown. Here we propose that loop extrusion underlies TAD formation. In this process, cis-acting loop-extruding factors, likely cohesins, form progressively larger loops, but stall at TAD boundaries due to interactions with boundary proteins, including CTCF. Using polymer simulations, we show that this model can produce TADs as determined by our analyses of Hi-C data. Contrary to typical illustrations, each TAD consists of multiple dynamically formed loops, rather than a single static loop. Our model explains diverse experimental observations, including the preferential orientation of CTCF motifs, enrichments of architectural proteins at TAD boundaries, and boundary deletion experiments, and makes specific predictions for depletion of CTCF versus cohesin. The emerging picture is that TADs arise from actively forming, growing, and dissociating loops, presenting a framework for understanding interphase chromosomal organization.

https://www.biorxiv.org/content/biorxiv/early/2015/08/14/024620.full.pdf

Carolyn Lu

Papers

Formation of Chromosomal Domains by Loop Extrusion

Formation of Chromosomal Domains in Interphase by Loop Extrusion

Formation of Chromosomal Domains by Loop Extrusion