環狀RNA

環狀RNA（Circular RNA，簡稱circRNA）為生物細胞中的一類RNA，由線狀RNA5'端與3'端經共價結合（反向剪接）而形成。有些環狀RNA可編碼蛋白質，有些則為非編碼RNA，大多數環狀RNA的功能均仍未知，過去認為環狀RNA僅是RNA剪接過程中產生的副產物，在細胞中數量不多且序列保守性低，應不具重要功能，但近年許多研究已漸推翻此觀點。環狀RNA因不具5'端或3'端，不會被外切酶切割，在細胞中應較多數的線狀RNA穩定。 目前人類細胞中已有超過25,000種環狀RNA被發現，多位於細胞質中，也有些源於基因內含子的環狀RNA（環狀內含子RNA，簡稱ciRNA）會留在細胞核中調控自身基因的表現。有些環狀RNA可能可作為「miRNA海綿」（miRNA sponge）與miRNA結合，使後者無法和目標mRNA結合而阻斷RNA干擾。環狀RNA還可能與一些RNA結合蛋白（英语：RNA-binding protein）結合、由內部核糖體進入位點（IRES）啟動轉譯而編碼蛋白質、在細胞中運輸與儲存miRNA等。環狀RNA的調控異常可能與多種癌症和神經退化性疾病有關。 除真核生物經反向剪接形成的環狀RNA外，生物還有數種其他生成環狀RNA的機制，例如第二型內含子（英语：group-II intron）剪接的產物、某些藻類與古菌生成tRNA過程的中間產物等，另外D型肝炎與類病毒的基因組也是環狀RNA。 分子与细胞生物学主题 Salzman, J; Gawad, C.; Wang, P.L.; Lacayo, N; Brown, PO. Circular RNAs are the Predominant Transcript Isoform from Hundreds of Human Genes in Diverse Cell Types. PLOS ONE. 2012, 7 (2): e30733. Bibcode:2012PLoSO...730733S. PMC 3270023 . PMID 22319583. doi:10.1371/journal.pone.0030733.  New study shows circular RNA can encode for proteins. Science Daily. 2017 [3 May 2018]. （原始内容存档于2018-06-27）.  Pamudurti, Nagarjuna Reddy; Bartok, Osnat; Jens, Marvin; et al. Translation of CircRNAs. Molecular Cell. 2017, 66 (1): 9–21.e7. PMC 5387669 . PMID 28344080. doi:10.1016/j.molcel.2017.02.021.  Guo, J.U.; Agarwal, V; Guo, H; Bartel, DP. Expanded identification and characterization of mammalian circular RNAs. Genome Biology. 2014, 15 (7): 409. PMC 4165365 . PMID 25070500. doi:10.1186/s13059-014-0409-z.  Wilusz, J.E.; Sharp, PA. A Circuitous Route to Noncoding RNA (PDF). Science. 2013, 340 (6131): 440–41. Bibcode:2013Sci...340..440W. PMC 4063205 . PMID 23620042. doi:10.1126/science.1238522.  Jeck, WR; Sorrentino, JA; Wang, K; et al. Circular RNAs are abundant, conserved, and associated with ALU repeats.. RNA. 2013, 19 (2): 141–57. PMC 3543092 . PMID 23249747. doi:10.1261/rna.035667.112.  Nisar, Sabah; Bhat, Ajaz A.; Singh, Mayank; Karedath, Thasni; Rizwan, Arshi; Hashem, Sheema; Bagga, Puneet; Reddy, Ravinder; Jamal, Farrukh; Uddin, Shahab; Chand, Gyan. Insights Into the Role of CircRNAs: Biogenesis, Characterization, Functional, and Clinical Impact in Human Malignancies. Frontiers in Cell and Developmental Biology. 2021-02-05, 9. ISSN 2296-634X. PMC 7894079 . PMID 33614648. doi:10.3389/fcell.2021.617281.  Zhang, Y; Zhang, XO; Chen, T; Xiang, JF; Yin, QF; Xing, YH; Zhu, S; Yang, L; Chen, LL. Circular Intronic Long Non-coding RNAs. Molecular Cell. 2013, 51 (6): 1–15. PMID 24035497. doi:10.1016/j.molcel.2013.08.017.  Ebert, MS; Sharp, PA. MicroRNA sponges: progress and possibilities. RNA. 2010, 16 (11): 2043–50. PMC 2957044 . PMID 20855538. doi:10.1261/rna.2414110.  Hansen, T.B.; Jensen, TI; Clausen, BH; Bramsen, JB; Finsen, B; Damgaard, CK; Kjems, J. Natural RNA circles function as efficient microRNA sponges. Nature. 2013, 495 (7441): 384–88. Bibcode:2013Natur.495..384H. PMID 23446346. doi:10.1038/nature11993.  Chen, CY; Sarnow, P. Initiation of protein synthesis by the eukaryotic translational apparatus on circular RNAs. Science. 1995, 268 (5209): 415–17. PMID 7536344. doi:10.1126/science.7536344.  Hentze, MW; Preiss, T. Circular RNAs: splicing's enigma variations. The EMBO Journal. 2013, 32 (7): 923–25. PMC 3616293 . PMID 23463100. doi:10.1038/emboj.2013.53.  Burd, CE; Jeck, WR; Liu, Y; Sanoff, HK; Wang, Z; Sharpless, NE. Expression of Linear and Novel Circular Forms of an INK4/ARF-Associated Non-coding RNA Correlates with Atherosclerosis Risk. PLOS Genetics. 2010, 6 (12): e1001223. PMC 2996334 . PMID 21151960. doi:10.1371/journal.pgen.1001233.  Dube, U; Del-Aguila, JL; Li, Z; Budde, JP; Jiang, S; Hsu, S; Ibanez, L; Fernandez, MV; et al. An atlas of cortical circular RNA expression in Alzheimer disease brains demonstrates clinical and pathological associations.. Nature Neuroscience. 2019, 22 (11): 1903–1912. PMC 6858549 . PMID 31591557. doi:10.1038/s41593-019-0501-5.  Harichandran K, Shen Y, Stephenson Tsoris S, Lee SC, Casey JL. Hepatitis Delta Antigen Regulates mRNA and Antigenome RNA Levels during Hepatitis Delta Virus Replication.. J Virol. 2019, 93 (8). PMC 6450126 . PMID 30728256. doi:10.1128/JVI.01989-18.