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[1]。有些環狀RNA可編碼蛋白質[2][3],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[4],但近年許多研究已漸推翻此觀點[1][5][6]。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[6]

circRNA由線狀RNA經反向剪接而生成

目前人類細胞中已有超過25,000種環狀RNA被發現[7],多位於細胞質[6],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[8]。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[9][10]。環狀RNA還可能與一些RNA結合蛋白結合[5]、由內部核糖體進入位點(IRES)啟動轉譯而編碼蛋白質[11]、在細胞中運輸與儲存miRNA等[12]。環狀RNA的調控異常可能與多種癌症神經退化性疾病有關[13][14]

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,例如第二型內含子剪接的產物、某些藻類與古菌生成tRNA過程的中間產物等[7],另外D型肝炎類病毒的基因組也是環狀RNA[15]

參考文獻

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