4.Hallmark of Cancer - Enabling replicative immortality - Telomeres, Hayflick's limit, Telomerase

Telomeres, Function?

Protecting the end of chromosomes from degradation and recombination

= "molecular clock" for aging (counting the number of cell divisions)

Telomeres, Structure?

contain non-coding repetitive sequences, which are rich in Guanine nucleotides (GGGG)

Also a G-rich single-strand overhang of about 150-200 nucleotides

Telomeres are gene-poor areas of the genome. Containing tandem hexon nucleotide repeats

Telomere repetitive sequence in humans?

5'-TTAGGG-3', (TTAGGG repeats)

repeated multiple times. around 10-15kb (kilo bases) long

Organisation of telomeres?

in constitutive heterochromatin bound by nucleosome arrays

Octamers of histone-packaging DNA

Octamers of histone-packaging DNA, importance?

maintaining the length and structure of telomeres

Octamers of histone-packaging DNA, loss?

may lead to abnormal telomere elongation

Organisation of telomeres- g-rich overhang

folds back and forms the T-loop (mechanism for chromosome ends protection)

telomere repeat binding factors 1 and 2 (TRF1, TRF2), importance?

-DNA repair

-Maintenance of telomere length

-Telomere protection

EXCEPTION In g-rich overhang - - TRF2 protects it from DNA repair proteins

absence of telomeres?

-> The unprotected chromosome ends would fuse

-> End-to-end fusions form bicentric chromosomes

-> leading to genomic instability, aberrations and loss of cell viability

end replication problem?

During each cell division the telomeres shorten -due to incomplete replication of linear chromosomes by conventional DNA polymerases (Okazaki fragments)

=> the life span of the cell decreases, which results in cellular aging.

=> ultimately the telomeres become so short - cell is forced to retire/die

cell-cycle arrest and cellular senescence

synthesis of leading strand?

continous

synthesis of lagging strand?

discontinuous

using Okazaki fragments Require an RNA primer for each fragment

Problem with synthesis of lagging strand?

Between last Okazaki fragment and the end - there is a 3' overhang (gap)

As a result:

With every replication - progressive telomere shortening

= End-replication problem

cellular senescence

non-replicative but viable state

How is cells capability to only go through a limited number of cell divisions - mechanism of tumour suppression?

Stops cells with chromosomal instability from proliferating infinitely

Hayflick limit

limit on cell replication imposed by the shortening of telomeres with each division

Hayflick limit of humans

~50 times

replicative immortality

rare- some cells can pass crisis phase and continue proliferating

How can Telomere shortening be compensated?

telomerase

• -nucleoprotein enzyme complex

• -able to extend shortened telomeres.

• extends the life span = slows cellular aging.

Telomerase Components?

Reverse transcriptase + RNA associated molecule

Telomerase Function?

initiation factor for telomeric production. In charge of elongation of telomeric ends=adding new DNA repeats to the telomere end

Telomerase Required by?

Long-lived cells i.e. Stem cells, progenitor cells, (as well as some cancer cells)

Telomerase not expressed in?

somatic cells

Premalignant cells have.. telomerase activity?

Low / no => Progressive telomere shortening

Malignant cells have ... of telomerase?

Production / activation => Aberrant telomere elongation and replicative immortality

Ways to elongate telomeres?

reactivation of telomerae. homologous recombination

Other functions of telomerase ?

Participation in cell proliferation / Regulation of cell replication.

-Resistance to apoptosis.

-DNA repair

Germline cells and stem cells have telomerase activity because

importance of retaining all their genetic information

What is the main function of telomeres?

Protecting the end of chromosomes from degradation and recombination

Telomeres are gene-rich areas of the genome

FALSE. Gene-poor areas

Given that the telomeres are gene-poor regions, epigenetic modifications are not relevant for their maintenance

FALSE.

Are organised in constitutive heterochromatin bound by nucleosome arrays.

Loss of this modification- may lead to abnormal telomere elongation

When culturing human cells, after certain number of replications, these cells stop proliferating and become senescent (or aged)

TRUE

Which cells require the telomerase for long term survival?

Stem and progenitor cells, as well as some cancer cells

Cancer cells have an upregulated telomerase since their pre-malignant stage

False. Only during malignant stage

The telomer elongation can only be achieved through the telomerase?

FALSE. Alternative: homologous recombination

Telomerase has other functions besides elongating telomeres in stem and progenitor cells?

Yes, it is also associated with regulation of cell replication, apoptosis and DNA repair

Function of the Telomere Repeat Binding Factors?

DNA repair , maintenance of Telomere length , telomere protection

What is the name of the structure formed by the G-rich overhangs?

T-Loop

telomerase activity in healthy cells?

None -> Progressive telomere shortening

telomerase activity in premalignant cells?

Low or none -> Progressive telomere shortening

telomerase activity in malignant cells?

yes -> Aberrant telomere elongation and replicative immortality