Telomere Shortening has been reported to be associated with aging. Telomeres consist of repetitive DNA (TTAGCC) in vertebrates at the ends of chromosomes which, together with specific binding proteins, act as protective cap structures. In cells of the germ line, telomere shortening is prevented by telomerase, a holoenzyme complex that contains a number of subunits including an RNA template molecule and a catalytic subunit (TERT). Telomers are synthesized by the enzyme telomerase which is lacking in normal somatic cells. Cell senescence resulting from shortened telomeres in somatic cells forms the basis for the telomere theory of aging.

Interestingly, while aging is associated with loss of telomerase activity and telomere shortening in most somatic cells, most cancer cells demonstrate increased telomerase actvity with varying lenghts of telomeres. Yet increasing age is the strongest risk factor for most cancers.

How Telomere shortening occurs:

It has long been recognized that complete replication of the ends of eukaryotic chormosomes requires specialized cell components. Replication of a linerar DNA strand by conventional DNA polymerase requires an RNA primer, and can proceed only 5′ to 3′. When the RNA bound at the extreme 5′ end of eukaryotic chomosomal DNA strands is removed, a gap is introduced, leading to a progressive shortening of daughter strands with each round of replication. This shortening of telomeres, the protein-DNA structures physically located on the chromosomes, is thought to account for the phenomenon of cellular senescence or aging of normal human somatic cells in vitro and in vivo.

How Telomeres are maintained:

Telomeres are not maintained via conventional replicative processes. Complete replication of the ends of linear eukaryotic chromosomes presetns special problems for conventional methods of DNA replciation because DNA polymereases cannot begin DNA synthesis de nov but require RNA primers that are later remvoed during replication. In the case of telomeres, removal of the RNA primer from the lagging-strand end would necessarily leave a 5’terminal gap, resulting in the loss of sequence from the leading strand if the daughter telomere was subsequently blunt-ended.  Specialized factors thus exist to ensure their complemete replication. The maintenance of telomeres is a function of a telomere-specific DNA polymerase known astelomerase. Telomerase is a ribonucleoprotein (RNP) that uses a protion of its RNA moiety as a template for telomeric DNA synthesis.

Structure of Telomeres:

Typically, telomers consist of arrays of simple DNA sequence repeats, ranging from about 50 copies of 5′-TTGGGG-3′ in Tetrahymena to about 1000 copies of 5′-TTAGGG-3′ in humans and other vetebrates. The sequence of telomeric repeats is specified by an RNA template (TER), which varies in lenght form about 160 nts in ciliates to about 1500 nts in vetebrates, and is an essential component of the catazlytically active form of telomerase (discussed next).

Structure of Telomerase:

Telomerase is a ribonucleoprotein complex (RNP) comprising an RNA component (TR; telomerase RNA) which serves as a template for telomeric repeat synthesis, and a catalytic protein component with RT (reverse transcriptase) activity (TERT; telomerase RT). The telomerase reverse transcriptase component, TERT, is generally thought to consist of 4 functional domains, the essential N-terminal (TEN) domain, an RNA-binding domain (TRBD), reverse transcriptase (RT), and a C-terminal extension (TEC). The catalytic protein component of human telomerase, referred to as telomerase reverse transcriptase (“hTRT”), has been cloned, and protein, cDNA and genomic sequences determined. The catalytic subunit protein of human telomerase has also been referred to as “hEST2”, “hTCS1”, “TP2”, and “hTERT”. Human TRT is of extraordinary interest and value because telomerase activity n human cells correlates with cell proliferative capacity, cell immortality, and the development of a neoplastic phenotype. The RNA component of human telomerase (hTR) has also been characterized.

Major differences in TERT genomic strcutre are apparent when comparing species (Sykorova, Biol. Cell, 2009, 101, 375-392, p. 379, last ¶). Telomerases differ in catalytic properties such as processivity and fidelity, and telomerases of some organisms function as dimers. (Sykorova, p. 376, 2nd column, lines 1-2). Alternative splicing provides a major source of pro0tein diversity within a given organism. Isoforms generated by alternative splicing may show change or loss of specific function(s) or localization of the respective product, or even a gain of a novel unexpected function (Sykorova, p. 381, 2nd column, lines 1-6).

Regulation of hTERT

hTERT appears to be regulated by different transcription factors in various cellular contexts. For example, two important c-Myc binding sites (CACGTG, referred toa s E boxes) are present in the hTERT core promoter and c-Myc is considered an important activator of telomerase. E6 human papilloma virus type 16 protein and other factors appear to be able to cooperate with c-Myc to activate transcription of hTERT.

Treatment strategies for Telomere shortening

In telomerase-negative differentiated somatic cells, cell senescence can actually be delayed and reversed by transfecting the cells with vectors encoding the human telomerase reverse transcriptase (hTERT)which is the active catyltic component of telomerase.

–Upregulation of Telmoerase Reverse Transcriptase (TERT) 

Andrews (US2005/0059622) teaches upregulating the expression of telomerase reverse transcriptase (TERT) by blocking repression of TERT transcriptation, e.g., by inhibiting binding of represssor factor to a Site C repressor binding site located in the TERT minimal promoter. Enhancing TERT expression finds uses in a number of applications such as treating disease conditions reulsting from a lack of TERT expression such as in AIDS  (body proudces a lot of CD8 cells which accelerates telomere shortening).

Esslinger (US12/991422) discloses lentivector-mediated gene transfer of human Telomerase-Reverse-Transcriptase (hTert), the catalytic protein subunit of human telomerase, into blood dervied membory B cells to immortalize or prolong the life span of the human memory B cells.