Control of Human Telomere Length by TRF1 and TRF2

Telomere length in human cells is controlled by a homeostasis mechanism that involves telomerase and the negative regulator of telomere length, TRF1 (TTAGGG repeat binding factor 1). Here we report that TRF2, a TRF1-related protein previously implicated in protection of chromosome ends, is a second negative regulator of telomere length. Overexpression of TRF2 results in the progressive shortening of telomere length, similar to the phenotype observed with TRF1. However, while induction of TRF1 could be maintained over more than 300 population doublings and resulted in stable, short telomeres, the expression of exogenous TRF2 was extinguished and the telomeres eventually regained their original length. Consistent with their role in measuring telomere length, indirect immunofluorescence indicated that both TRF1 and TRF2 bind to duplex telomeric DNA in vivo and are more abundant on telomeres with long TTAGGG repeat tracts. Neither TRF1 nor TRF2 affected the expression level of telomerase. Furthermore, the presence of TRF1 or TRF2 on a short linear telomerase substrate did not inhibit the enzymatic activity of telomerase in vitro. These findings are consistent with the recently proposed t loop model of telomere length homeostasis in which telomerase-dependent telomere elongation is blocked by sequestration of the 3′ telomere terminus in TRF1- and TRF2-induced telomeric loops.

人类细胞的端粒长度受稳态机制调控，该机制涉及端粒酶与端粒长度负调控因子TRF1（TTAGGG重复结合因子1，TTAGGG Repeat Binding Factor 1）。本研究报道，与TRF1相关的蛋白TRF2——此前被证实参与染色体末端保护——是第二类端粒长度负调控因子。过表达TRF2会导致端粒长度进行性缩短，该表型与TRF1诱导的结果相似。不过，尽管TRF1的诱导表达可维持超过300次群体倍增，并形成稳定的短端粒，但外源性TRF2的表达会被沉默，且端粒最终恢复至原始长度。与二者在端粒长度测量中的功能一致，间接免疫荧光实验显示，TRF1与TRF2均可在体内结合双链端粒DNA，且在富含TTAGGG重复序列的长端粒上丰度更高。TRF1与TRF2均不影响端粒酶的表达水平。此外，在体外实验中，结合于短线性端粒酶底物的TRF1或TRF2并不会抑制端粒酶的酶活性。上述发现与近期提出的端粒长度稳态t环模型相符，该模型认为，端粒酶依赖的端粒延长会因3'端粒末端被包裹于TRF1与TRF2诱导形成的端粒环中而被阻断。