User:Marsden.da/Shelterin

The shelterin complex is a group of 6 protein subunits that interact with each other and with other cellular components to protect the telomere, the end region of a chromosome, from DNA repair mechanisms, to bring the enzyme telomerase to the telomere, and to protect telomere structural integrity. The subunits have been identified as TRF1, TRF2, POT1, TIN2, TPP1, and Rap1. The functions of the shelterin complex prevent senescence, cellular aging or the inability to divide, and apoptosis, the death of dysfunctional cells. TRF1, TRF2, and POT1 execute these primary functions of the shelterin complex whereas TIN2, TPP1, and Rap1 are important in bringing the previous three to the telomere. The absence of even a single subunit can have catastrophic effects on the phenotype of a cell and on that of an organism, and several human tumors are characterized by the absence of shelterin subunit TRF2.

TRF1
TRF1 is a double-stranded DNA binding protein that regulates telomere length and prevents telomeric fusion. Low levels of TRF1 in mouse embryonic fibroblasts results in telomere elongation while abnormally high levels of TRF1 causes telomere shortening. Also, reduced TRF1 expression in replicating cells results in an increase of chromosomes with fragile telomeres, thereby increasing telomere fusion. Thus, the TRF1 subunit is responsible for halting DNA repair mechanisms for creating a complementary DNA strand for the single-stranded telomeric overhang, the end portion of the telomere that is essential for the binding of telomerase.

The TRF1 DNA strand is activated by Gcn5, a transcription coregulator, a protein that interacts with a DNA strand to activate or repress transcription. Knockdown, or reduced expression, of Gcn5 is associated with an increase in telomere fusion due to its inability to activate TRF1. Gcn5 exerts its effects through the tumor suppressor gene, p53. When there are insufficient quantities of Gcn5, p53 is over-expressed, which promotes senescence and apoptosis.

TRF2
TRF2 prevents DNA repair mechanisms from identifying telomeres as broken DNA. It will bind with any region rich in the nucleotide sequence TTAGGG, the sequence found as tandem repeats in human telomeres, and it has been found to bring Rap1 to binding sites outside the telomere as a result. TRF2 mRNA is modified by the protein Siah 1 after it has been transcribed. This gene is regulated by p53 too, as upregulation of p53 results in upregulation of Siah 1. This increase in Siah 1, a common occurrence in patients suffering from Li-Fraumeni syndrome, leads to a decrease in TRF2. This decrease causes the increased levels of tumor formation, tumorigenesis, seen in Li-Fraumeni patients.

When TRF2 is unable to bind to the telomere a positive feedback loop occurs in which the ensuing p53 upregulation causes lower levels of TRF2. This inhibition of TRF2 prevents Rap1 from reaching its destinations on the telomere and away from it as p53 levels continue to rise. This persists until there is enough p53 in the cell to cause apoptosis.

POT1
POT1 is a complex of POT1a and POT1b that mediates repression of the ATR kinase, an enzyme that responds to DNA damage and inhibits extensive telomere elongation. It binds to the single stranded overhang region of the telomere. Low levels of POT1a in mouse cells causes an increase in DNA damage repair proteins being brought to the telomere because the overhang is perceived as a damaged strand. Much like TRF1, POT1a depletion results in telomere fusion and, ultimately, apoptosis.

POT1b is involved in stopping telomere elongation once the telomere has reached its proper length. When chromosomes are replicated, the ends are shortened, so the enzyme telomerase elongates the telomere to its pre-replicated state. Low levels of POT1b causes extensive telomere elongation, thus POT1b prevents telomerase from becoming too active and adding too much to the telomere.

TPP1 and TIN2
TPP1 binds to the POT1 complex to increase its affinity for the telomere and to TIN2 to bring telomerase to its proper site at the telomere. Low levels of TPP1 has been shown not to cause a significant decrease in the amount or stability of POT1 protein complexes in the cell, but it does result in diminished levels at their proper telomeric sites. Much like with POT1, reduction in levels of TPP1 does not cause any significant decrease in the levels of telomerase synthesized by the cell, but it does result in diminished levels at the telomere. Reduction in levels of TIN2 is associated with similar cellular effects, indicating that TIN2 and TPP1 form a complex that increases their affinity for telomerase and brings the enzyme to the telomere.

Rap1
Rap1 is more important for its effects on regions outside the telomere than for its functions at the telomere and it is brought to the telomere by TRF2. While Rap1 reduction in mouse embryos causes severe telomere fragility and increased telomere fusion, when Rap1 expression is decreased in mature mouse skin cells the only effect is mild darkening of the skin. Also, the other 5 shelterin subunits were found in their proper levels and locations in Rap1 knockout cells. Thus, in mature cells Rap1 is not critical to the major functions of the shelterin complex as a whole.

Rap1 has been found to contain several binding sites outside the telomere, however, and may play a role in the expression of genes important in metabolism, cell adhesion, and cancer. In particular, it has been shown that Rap1 controls expression of metabolic enzymes, as Rap1-deficient adult female mice are consistently 10% heavier than age-controlled normal mice. Lastly, Rap1 has been associated with introns, the non-coding regions of DNA, found near the telomere. Its relevance at the introns is not known, but because of its role at the telomere it is possible that Rap1 prevents intronic fragility and recombination.

Shelterin Dysfunction and Cancer
Researchers suggest that shelterin dysfunction could play a major role in tumor formation. However, the only evidence so far of tumorigenesis is through TRF2 overexpression. Because subunit TRF2 deactivates the tumor-suppressor gene p53, overexpression of TRF2 is a common factor in many human tumors, most notably liver hepatocarcinomas, breast carcinomas, and lung carcinomas. Furthermore, mice with upregulated TRF2 expression in their skin cells spontaneously develop tumors while mice with downregulated TRF2 or normal amounts of mutated TRF2 expression display the opposite. More research must be done to explore the roles of the other shelterin subunits in the development of human cancers.