Raghavan Sathees C, Swanson Patrick C, Ma Yunmei, Lieber Michael R
Norris Comprehensive Cancer Center, Rm. 5428, University of Southern California Keck School of Medicine, 1441 Eastlake Ave., MC9176, Los Angeles, California 90033, USA.
Mol Cell Biol. 2005 Jul;25(14):5904-19. doi: 10.1128/MCB.25.14.5904-5919.2005.
The most common chromosomal translocation in cancer, t(14;18) at the 150-bp bcl-2 major breakpoint region (Mbr), occurs in follicular lymphomas. The bcl-2 Mbr assumes a non-B DNA conformation, thus explaining its distinctive fragility. This non-B DNA structure is a target of the RAG complex in vivo, but not because of its primary sequence. Here we report that the RAG complex generates at least two independent nicks that lead to double-strand breaks in vitro, and this requires the non-B DNA structure at the bcl-2 Mbr. A 3-bp mutation is capable of abolishing the non-B structure formation and the double-strand breaks. The observations on the bcl-2 Mbr reflect more general properties of the RAG complex, which can bind and nick at duplex-single-strand transitions of other non-B DNA structures, resulting in double-strand breaks in vitro. Hence, the present study reveals novel insight into a third mechanism of action of RAGs on DNA, besides the standard heptamer/nonamer-mediated cleavage in V(D)J recombination and the in vitro transposase activity.
癌症中最常见的染色体易位，即位于150碱基对的bcl-2主要断裂点区域（Mbr）的t(14;18)，发生在滤泡性淋巴瘤中。bcl-2 Mbr呈现非B型DNA构象，从而解释了其独特的易断裂性。这种非B型DNA结构在体内是RAG复合体的作用靶点，但并非因其一级序列。在此我们报告，RAG复合体在体外可产生至少两个独立的切口，进而导致双链断裂，而这需要bcl-2 Mbr处的非B型DNA结构。一个3碱基对的突变能够消除非B型结构的形成以及双链断裂。对bcl-2 Mbr的观察结果反映了RAG复合体更普遍的特性，即它能够在其他非B型DNA结构的双链-单链转换处结合并产生切口，从而在体外导致双链断裂。因此，本研究揭示了RAGs作用于DNA的第三种作用机制的新见解，这一机制有别于V(D)J重组中标准的七聚体/九聚体介导的切割以及体外转座酶活性。
