Overview

Replicative cell senescence is a property of cells that allows them to divide a finite number of times throughout the organism's lifespan while preventing excessive proliferation. Replicative senescence is associated with the gradual loss of the telomere — short, repetitive DNA sequences found at the end of the chromosomes. Telomeres are bound by a group of proteins to form a protective cap on the ends of chromosomes. Embryonic stem cells express telomerase — an enzyme that adds the telomeric repeat sequence and enables repetitive cell division; however, in adults, telomerase is active only in the cells that need to divide regularly.

Because telomerase is inactive in most human somatic cells, the length of the telomere decreases with every cell division. After a critical length, telomere shortening leads to permanent cell cycle arrest. This mechanism is assumed to protect against cancer development by limiting the abnormal proliferation of tumors; however, a rare mutation can activate telomerase, which reconstructs the telomere region, allowing the cells to proliferate. Thus, telomerase is a perfect target for specific anticancer therapy, as most cancer cells express telomerase while normal cells do not.

The relation between telomere length and tumor formation has been experimentally verified using oncogenic mice. Oncogenic mice are mouse models that have cancer-causing genes. When such oncogenic mice are crossed with telomerase-deficient mice that lack telomerase activity, the resultant progeny mice express shorter telomeres than the oncogenic parent. These progeny mice, when interbred, generate successive generations that have progressively shorter telomeres. The frequency of tumor formation is studied by treating the progeny mice with carcinogens at every generation. As per the results, the late generation mice with shorter telomeres exhibit a reduced frequency of tumor formation compared to the early generation mice that maintain longer telomeres. This proves that limiting the replicative capacity of cells suppresses tumor formation.

Procedure

Most animal cells divide a finite number of times before they stop and undergo permanent cell cycle arrest.

In a mitogenic medium, for example, human fibroblast cells divide about 25-50 times. As a cell approaches this finite number of divisions, the rate of cell division slows down and finally halts with cells entering a permanent non-dividing state. This phenomenon is called replicative cell senescence.

Replicative cell senescence is a result of changes in the structure of telomeres. Telomeres are located at the ends of the chromosomes and consist of long repetitive DNA sequences and protein complexes.

In the absence of telomeres, chromosome ends could be recognized as double-strand breaks. These ends could fuse to one another forming abnormal structures like a ring chromosome. The telomeres act as caps, protecting the ends of the chromosomes from degradation by nucleases and preventing the aberrant fusion of chromosome ends to one another.

Shelterin is a telomere-associated protein complex that protects the chromosome ends.

Shelterin helps DNA ends form a lariat-like structure called a telomerase-loop, or T-loop. This T-loop masks the DNA ends, preventing degradation.

During cell division, telomeres are shortened by 25-200 bases due to the inability of the polymerase to completely replicate DNA ends. As the length of telomeres becomes shorter, the shelterin components are displaced from the telomere region. Shrinking of the telomere eventually destabilizes the t-loop conformation.

The change in the T-loop structure leaves the chromosome ends exposed, which are sensed as DNA damage by the DNA damage response pathway.

The persistent DNA damage response that ensues induces replicative cell senescence which helps limit genomic instability and malignant transformation.