Fasching Clare L, Neumann Axel A, Muntoni Alessandra, Yeager Thomas R, Reddel Roger R
Children's Medical Research Institute, Westmead, and University of Sydney, New South Wales, Sydney, Australia.
Cancer Res. 2007 Aug 1;67(15):7072-7. doi: 10.1158/0008-5472.CAN-07-1556. Epub 2007 Jul 24.
The linear chromosomes of vertebrates terminate in telomeres that consist of a tandemly repeated hexameric sequence, 5'TTAGGG3'. Telomeres form a protective loop structure (t-loop), which is thought to prevent them from being recognized as a double-strand break. Approximately 10% of human tumors prevent shortening of their telomeres by using a recombination-mediated alternative lengthening of telomeres (ALT) mechanism. ALT-positive human cells contain extrachromosomal telomere repeat (ECTR) DNA that may either be circular or linear. It has been proposed that ECTR may be generated by recombination events involving the t-loop. A proportion of the cells within ALT-positive cell populations contain promyelocytic leukemia (PML) nuclear bodies that contain telomeric DNA and telomere-binding proteins that are called ALT-associated PML bodies (APB). Although the presence of APBs is very useful for determining whether tumors and cell lines use the ALT mechanism, the function of APBs is unknown. It has previously been shown that telomeric DNA is particularly susceptible to damage by hydrogen peroxide and N-methyl-N'-nitro-N-nitrosoguanidine. We report here that these DNA-damaging agents induce both linear and circular ECTR DNA in ALT cells and increase the proportion of cells that contain APBs. We partially purified APBs and showed that the telomeric repeat DNA they contain is predominantly linear. We propose that a function of APBs is to sequester linear telomeric DNA.
脊椎动物的线性染色体末端是端粒，端粒由串联重复的六聚体序列5'TTAGGG3'组成。端粒形成一种保护性环结构（t环），人们认为这种结构可防止它们被识别为双链断裂。大约10%的人类肿瘤通过一种重组介导的端粒替代延长（ALT）机制来防止其端粒缩短。ALT阳性的人类细胞含有可能是环状或线性的染色体外端粒重复（ECTR）DNA。有人提出ECTR可能是由涉及t环的重组事件产生的。ALT阳性细胞群体中的一部分细胞含有早幼粒细胞白血病（PML）核体，其中包含端粒DNA和端粒结合蛋白，这些被称为ALT相关PML体（APB）。尽管APB的存在对于确定肿瘤和细胞系是否使用ALT机制非常有用，但其功能尚不清楚。此前已表明端粒DNA特别容易受到过氧化氢和N-甲基-N'-硝基-N-亚硝基胍的损伤。我们在此报告，这些DNA损伤剂在ALT细胞中诱导产生线性和环状ECTR DNA，并增加含有APB的细胞比例。我们对APB进行了部分纯化，并表明它们所含的端粒重复DNA主要是线性的。我们提出APB的一个功能是隔离线性端粒DNA。
