细胞凋亡及周期阻滞基本信号通路

CELL DEATH AND CELL-CYCLE CHECKPOINT DURING DNA DAMAGE

细胞死亡及周期阻滞基本信号通路

有哪些因素可引起DNA损伤？DNA损伤的结局如何？

（课件）

(一)DNA损伤的原因

环境因素，化学因素，生物因素例如： UV ,离子辐射，基因毒性化学试剂引起ssDNA/dsDNA 损伤，DNA两条链交联或链内交联。正常细胞线粒体的一些代谢物（ROS）活泼氧类过多引起损伤。

(二) DNA损伤结局：

急性效应：干扰核酸代谢，触发细胞周期阻滞或死亡

长期效应：不可逆转突变导致肿瘤

细胞周期阻滞，衰老，肿瘤/癌症，有丝分裂危象

(一)DNA损伤的原因

1.DNA分子的自发性损伤

(1)DNA复制中的错误。

(2)DNA的自发性化学变化

a.碱基的异构互变性损伤

b.碱基的脱氨基作用

c.脱嘌呤与脱嘧啶

d.碱基修饰与链断裂

2.物理因素引起的DNA损伤

(1)紫外线引起的DNA损伤

(2)电离辐射引起的DNA损伤

a.碱基变化

b.脱氧核糖变化

c.DNA链断裂

d.交联

3.化学因素引起的DNA损伤

(1)烷化剂对DNA的损伤

a.碱基烷基化

b.碱基脱落

c.断链

d.交联

(2)碱基类似物、修饰剂对DNA的损伤

DNA损伤的后果

1.点突变(point mutation)指DNA上单一碱基的变异。嘌呤替代嘌呤(A与G之间的相互替代)、嘧啶替代嘧啶(C与T之间的替代)称为转换(transition)；嘌呤变嘧啶或嘧啶变嘌呤则称为颠换(transvertion)。

2.缺失(deletion)指DNA链上一个或一段核苷酸的消失。

3.插入(insertion)指一个或一段核苷酸插入到DNA链中。在为蛋白质编码的序列中如缺失及插入的核苷酸数不是3的整倍数，则发生读框移动(reading frame shift)，使其后所译读的

氨基酸序列全部混乱，称为移码突变(frame shift mutaion)。

4.倒位或转位(transposition)指DNA链重组使其中一段核苷酸链方向倒置、或从一处迁移到另一处。

5.双链断裂已如前述，对单倍体细胞一个双链断裂就是致死性事件。

（2）THE CONSEQUENCES OF DNA INJURY

The outcome of DNA damage is diverse and generally adverse（有害的）. Acute effects arise from disturbed DNA metabolism（新陈代谢）, triggering（启动，控制）cell-cycle arrest or cell death. Long term effects result from irreversible mutations（转变，突变，变异）contributing to oncogenesis（）.

●Many Lesions（损伤）Block（阻碍）Transcription（转录）— an outcome directly related to gene length. This has elicited（引出）the development of a dedicated repair system, transcription-coupled repair (TCR), which displaces or removes the stalled RNA polymerase and assures high priority repair. TRANSCRIPTIONAL STRESS, arising from persistent blockage of RNA synthesis, constitutes an efficient trigger for p53-dependent apoptosis, which may be a significant anti-cancer mechanism.

●Lesions Also Interfere With DNA Replication（复制）—Recently, a growing class of DNA polymerases（聚合酶）, numbered ζ to κ, was disc overed which seems devoted specifically to overcoming damage-induced REPLICATIONAL STRESS. These special polymerases take over temporarily from the blocked replicative DNA polymerase-δ/ε, and possibly from pol α . …But this solution generally comes at the expense of a higher error rate. In fact, this process is responsible for most of damage-induced point mutations and is thus particularly relevant for oncogenesis. Nevertheless, translesion polymerases still protect the genome.

●Double-strand DNA breaks (DSBs) induced by X-rays, chemicals or during replication of single-strand breaks (SSBs) and presumably during repair of interstrand crosslinks are particularly relevant for the recombination machinery.

Eg：Cells with specialized DNA recombination activities, such as B- and T-cells, may be very sensitive to DSBs when they are rearranging their immunoglobin or T-cell-receptor genes. This explains the frequent involvement of these genetic loci in oncogenic translocations in leukaemia(白血病) and lymphomas（淋巴瘤）and the preferential induction of these cancers by ionizing irradiation.

Eg：DSBs also pose problems during mitosis（有丝分裂）, as intact（未受损的）chromosomes are a prerequisite（先决条件）for proper chromosome segregation（分离）during cell division. Thus, these lesions（损伤）frequently induce various sorts of chromosomal aberrations（染色体病）, including aneuploidy（）, deletions（缺失）(loss of heterozygosity, LOH) and chromosomal translocations（迁移，置换）— events which are all intimately associated with carcinogenesis （癌变）.

●The cell-cycle machinery somehow senses genome injury and arrests（阻滞）at specific checkpoints in G1, S, G2 and M to allow repair of lesions before they are converted into permanent mutations. Lesion detection may occur by blocked transcription, replication or specialized sensors. When damage is too significant, a cell may opt for the ultimate mode of rescue by initiating（开始）apoptosis（凋亡）at the expense of a whole cell

什么分子可作为DNA双链断裂/损伤的标志？用什么方法测定？

（1）Senescence(衰老), can be triggered when telomeres（端粒）—the ends of linear chromosomes—cannot fulfil（执行）their normal protective functions. Here we show that senescent human fibroblasts（纤维原细胞）display molecular markers characteristic of cells bearing DNA double-strand breaks.

These markers include nuclear foci of phosphorylated histone H2AX and their co-localization with DNA repair and DNA damage checkpoint factors such as 53BP1, MDC1 and NBS1. We also show