肥厚型心肌病可变剪接图谱分析

目的绘制人心脏组织mRNA可变剪接图谱,探究肥厚型心肌病(HCM)中可变剪接的变化情况。方法收集HCM患者和正常人心肌组织,提取RNA并进行链特异性RNA深度测序,检测可变剪接变化,并通过功能富集分析初步了解HCM中受可变剪接调控的信号通路。实时荧光定量PCR检测可变剪接导致的不同转录本的表达变化,并在心肌肥厚动物模型中验证。结果在人类心肌组织中共发现11604个基因的94990个可变剪接事件,其中124个基因的可变剪接在HCM心肌组织中发生显著改变;GO和KEGG通路富集分析,显示可变剪接主要影响肌丝滑动、心肌收缩、肥厚型心肌病、扩张型心肌病、钙离子信号通路等功能或通路。RNA测序和定量PCR均显示虽然CAMK2δ表达总量不变,但是它的两个外显子互斥转录本,CAMK2δ3和CAMK2δ9,表达量发生了相反的改变;小鼠肥厚心肌模型也验证了这一发现。结论本研究首次绘制了人类心脏可变剪接图谱,明确HCM中多个基因发生可变剪接改变,发现即使基因表达总量没有变化,但基因不同转录本却可能通过可变剪接发生显著改变。提示可变剪接在HCM心肌肥厚中可能发挥重要作用,可变剪接图谱分析有助于更好地识别疾病相关的基因和通路。     

Sequence conservation, relative isoform frequencies, and nonsense-mediated decay in evolutionarily conserved alternative splicing

Studies of expressed sequence tag data sets have revealed large numbers of splicing variants for human genes, but it remains challenging to distinguish functionally important variants from aberrant splicing, clarify the nature of the alternative functions, and understand the signals that regulate splicing choices. To help address these issues, we have constructed and analyzed a large data set of 1,478 exon-skipping alternative splicing (AS) variants evolutionarily conserved in human and mouse. In about one-fifth of cases, one isoform appears subject to nonsense-mediated mRNA decay (NMD), supporting the idea that a major role of AS is to regulate gene expression; one-quarter of these NMD-inducing cases involve a conserved exon whose apparent sole purpose is to mediate destruction of the message when included. We explore sequence conservation likely related to splicing regulation, using in part a measure of the overall amount of conserved information in a sequence, and find that the increased conservation that has been observed within AS exons primarily affects synonymous sites, suggesting that regulatory signals significantly constrain synonymous substitution rates. We show that a lower frequency of the inclusion isoform relative to the exclusion isoform tends to be associated with weaker splice site signals, smaller exon size, and higher intronic sequence conservation, and provide evidence that all of these factors are under selection to control relative isoform frequencies. Some conserved instances of AS appear to represent aberrant splicing events that by chance have occurred in both species, and we develop a nonparametric likelihood approach to identify these.     

Smg1 haploinsufficiency predisposes to tumor formation and inflammation

SMG1 is a member of the phosphoinositide kinase-like kinase family of proteins that includes ATM, ATR, and DNA-PK, proteins with known roles in DNA damage and cellular stress responses. SMG1 has a well-characterized role in nonsense-mediated decay as well as suggested roles in the DNA damage response, resistance to oxidative stress, regulation of hypoxic responses, and apoptosis. To understand the roles of SMG1 further, we generated a Genetrap Smg1 mouse model. Smg1 homozygous KO mice were early embryonic lethal, but Smg1 heterozygous mice showed a predisposition to a range of cancers, particularly lung and hematopoietic malignancies, as well as development of chronic inflammation. These mice did not display deficiencies in known roles of SMG1, including nonsense-mediated decay. However, they showed elevated basal tissue and serum cytokine levels, indicating low-level inflammation before the development of tumors. Smg1 heterozygous mice also showed evidence of oxidative damage in tissues. These data suggest that the inflammation observed in Smg1 haploinsufficiency contributes to susceptibility to cancer and that Smg1-deficient animals represent a model of inflammation-enhanced cancer development.     

Identification of alternative splicing and negative splicing activity of a nonsegmented negative-strand RNA virus, Borna disease virus

Borna disease virus (BDV) is a nonsegmented negative-strand RNA virus that belongs to the Mononegavirales. Unlike other animal viruses of this order, BDV replicates and transcribes in the nucleus of infected cells. Previous studies have shown that BDV uses RNA splicing machinery for its mRNA expression. In the present study, we identified spliced RNAs that use an alternative 3' splice site, SA3, in BDV-infected cell lines as well as infected animal brain cells. Transient transfection analysis of cDNA clones of BDV RNA revealed that although SA3 is a favorable splice site in mammalian cells, utilization of SA3 is negatively regulated in infected cells. This negative splicing activity of the SA3 site is regulated by a putative cis-acting region, the exon splicing suppressor (ESS), within the polymerase exon of BDV. The BDV ESS contains similar motifs to other known ESSs present in viral and cellular genes. Furthermore, our results indicated that a functional polyadenylation signal just upstream of the BDV ESS is also involved in the regulation of alternative splicing of BDV. These observations represent the first documentation of complex RNA splicing in animal RNA viruses and also provide new insight into the mechanism of regulation of alternative splicing in animal viruses.     

A postnatal switch of CELF and MBNL proteins reprograms alternative splicing in the developing heart

From a large-scale screen using splicing microarrays and RT-PCR, we identified 63 alternative splicing (AS) events that are coordinated in 3 distinct temporal patterns during mouse heart development. More than half of these splicing transitions are evolutionarily conserved between mouse and chicken. Computational analysis of the introns flanking these splicing events identified enriched and conserved motifs including binding sites for CUGBP and ETR-3-like factors (CELF), muscleblind-like (MBNL) and Fox proteins. We show that CELF proteins are down-regulated >10-fold during heart development, and MBNL1 protein is concomitantly up-regulated nearly 4-fold. Using transgenic and knockout mice, we show that reproducing the embryonic expression patterns for CUGBP1 and MBNL1 in adult heart induces the embryonic splicing patterns for more than half of the developmentally regulated AS transitions. These findings indicate that CELF and MBNL proteins are determinative for a large subset of splicing transitions that occur during postnatal heart development.