Roles of SWI/SNF chromatin remodeling complex in the cellular response to DNA double strand breaks

Abstract

In response to DNA double strand breaks (DSBs) from various sources including ionizing radiation and the chemicals such as doxorubicin and etoposide, cells trigger the pathways leading to the activation of DNA repair and cell cycle checkpoints, and/or apoptosis if the cells cannot tolerate the damage. If left unrepaired, DNA DSBs can lead to chromosomal instability and cancer can arise. SWI/SNF is a chromatin remodeling complex that utilizes the energy of ATP hydrolysis to alter chromatin structure to regulate the access of proteins to their DNA targets. Increasing evidence suggests that SWI/SNF plays an important role in cancer development; for example, BRG-1, the core subunit of the complex, possesses intrinsic tumor suppressor activity. However, little is known about the mechanisms underlying the tumor suppressor function of SWI/SNF. In the present study, I investigated the role of SWI/SNF in the cellular response to DNA DSBs. First, I demonstrated that SWI/SNF plays a critical role in the resistance of cells to DNA damage-induced apopotosis. Inactivation of SWI/SNF by over-expressing dominant negative BRG-1 dramatically increased apoptosis induced by ionizing radiation, doxorubicin and etoposide. In addition, expression of BRG-1 in SW-13 cells lacking BRG-1 increased the resistance of DNA damage-induced apoptosis. The critical role of SWI/SNF in the resistance to DNA damage-induced apoptosis was also demonstrated by small interference (si) RNA approach. When BRG-1 was down-regulated by siRNA, cells became more sensitive to DNA damage. I then went on to investigate how SWI/SNF endows cells with resistance to DNA DSBs, and obtained two striking results. First, DSB induction of g-H2AX, recently shown to be critical for DSB repair, was severely compromised by inactivation or down-regulation of SWI/SNF. Second, inactivation of SWI/SNF caused a severe defect in G2/M arrest by DNA damage whereas control cells with active SWI/SNF showed a typical G2/M arrest. Taken together, these results demonstrate that the SWI/SNF chromatin remodeling complex plays a critical role in the resistance to DNA DSBs, and that this function of SWI/SNF is attributed to its ability to enhance DSB repair by both stimulating the induction of g-H2AX and activating the G2/M checkpoints. ;방사선이나, doxorubicin, etoposide 와 같은 화학 물질들에 의해 DNA의 이중 나선이 절단되면, 세포는 DNA 복구체계 활성화, 세포주기 의 정지, 세포 사멸 등의 기전을 활성화 시킴으로써, 유전자 변형이나, 암이 일어날 가능성을 최소화 시킨다.SWI/SNF Chromain Remodeling Complex 는, ATV를 가수 분해 시켜 나오는 에너지로 크로마틴(chromatin) 의 구조를 변형시킴으로써, DNA에 결합하는 단백질의 접근을 조절하며, 최근 암화(cancer development) 과정에서 중요한 역할을 하고 있을 것이라는 증거가 제시되고 있다. 그러나SWI/SNF cmplex 의 종양억제제로서의 기작은 물론, DNA 복구 체계와의 상호 기능적 관계에 대한 연구는 거의 되어 있지 않다. 본 논문에서, DNA 이중 나선 구조 절단에 의해 일어나는 여러 세포내 반응들에 있어, SWI/SNG chromatin remodeling complex 의 역할을 조사하였다. Dominant Negative 기법, FNA interference 기술 및 다양한 분자 생물학적 실험 방법을 통해 SWE/SNF chromatin remodeling complex 가 DNA 복구 체계에 결정적으로 중요하다는 것과, 세포 주기를 정지시킴으로써 손상을 입은 DNA 가 복구가 될 수 있도록 도와 주는데, 결정적인 역할을 함을 최초로, 밝혔다. 위의 결과는, 크로마틴(chromatin) 리모델링과 DNA 복구 체계를 처음으로 연관시켰다는 점에서 의의가 있으며, 나아가, 유전자 변형이나, 암의 발달이 억제되는데 있어, 크로마틴 구조의 유동성이 매우 중요함을 제시하였다.