TELOMERE END-REPLICATION PROBLEM AND CELL AGING
TELOMERE END-REPLICATION PROBLEM AND CELL AGING
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DOI：
10.1016/0022-2836(92)90096-3
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发表时间：
1992-06-20
影响因子：
5.6
通讯作者：
HARLEY, CB
中科院分区：
文献类型：
--
作者：
LEVY, MZ;ALLSOPP, RC;HARLEY, CB
由于DNA聚合酶需要一个不稳定的引物来启动单向5′-3′合成，因此每个模板链3′端的一些碱基不会被复制，除非特殊的机制绕过这个“末端复制”问题。永生的真核细胞，包括转化的人类细胞，显然使用端粒酶（一种延长端粒的酶）来克服不完全的末端复制。然而，在正常的体细胞中没有检测到端粒酶，这些细胞随着年龄的增长而失去端粒。因此，为了更好地理解不完全复制的后果，我们对分裂细胞群体的这一过程进行了建模。分析表明了四点。首先，如果由不完全复制产生的单链突出端没有降解，则平均端粒长度每代减少0.25个缺失事件。如果突出端被降解，则速率加倍。显示成纤维细胞中每代减少约50个碱基对的数据表明完全缺失事件为100至200个碱基对。第二，如果细胞在体外倍增80次后衰老，平均端粒长度减少约4000个碱基对，但每个细胞中的一个或多个端粒将丢失更多的端粒DNA。用于调节细胞生长的检查点可以在该点发出信号。第三，该模型预测的端粒长度的变化与衰老时分裂细胞的突然下降是一致的。最后，末端限制性片段长度的变化不能完全解释为不完全复制，这表明端粒或亚端粒重复序列长度的显着染色体间变异。这项分析，连同假设允许端粒酶失活的优势，表明端粒的损失可以解释人类成纤维细胞的细胞周期退出。
Since DNA polymerase requires a labile primer to initiate unidirectional 5′-3′ synthesis, some bases at the 3′ end of each template strand are not copied unless special mechanisms bypass this “end-replication” problem. Immortal eukaryotic cells, including transformed human cells, apparently use telomerase, an enzyme that elongates telomeres, to overcome incomplete end-replication. However, telomerase has not been detected in normal somatic cells, and these cells lose telomeres with age. Therefore, to better understand the consequences of incomplete replication, we modeled this process for a population of dividing cells. The analysis suggests four things. First, if single-stranded overhangs generated by incomplete replication are not degraded, then mean telomere length decreases by 0.25 of a deletion event per generation. If overhangs are degraded, the rate doubles. Data showing a decrease of about 50 base-pairs per generation in fibroblasts suggest that a full deletion event is 100 to 200 base-pairs. Second, if cells senesce after 80 doublingsin vitro, mean telomere length decreases about 4000 base-pairs, but one or more telomeres in each cell will lose significantly more telomeric DNA. A checkpoint for regulation of cell growth may be signalled at that point. Third, variation in telomere length predicted by the model is consistent with the abrupt decline in dividing cells at senescence. Finally, variation in length of terminal restriction fragments is not fully explained by incomplete replication, suggesting significant interchromosomal variation in the length of telomeric or subtelomeric repeats. This analysis, together with assumptions allowing dominance of telomerase inactivation, suggests that telomere loss could explain cell cycle exit in human fibroblasts.