7 - Telomeres and Telomerase Flashcards
Senescence
No proliferation but also no dying
Immortalisation
- Hallmark of cancer, endless replication capacity
- Characterized by having short telomeres (ends of
chromosomes). - After an immortalizing event, that results in increased telomere length, the cells will again start dividing.
- Cancerous cells divide continuously in culture and do not enter a senescence phase as they have high telomerase activity.
Normal cells that need to replicate many times
- Stem cells need to be able to replicate throughout your life, but at a very low rate, to replace specific cell types.
- Germ cells need to be able to replicate over generations so we `can reproduce, but at a low rate
Mititic phase
Can visibly see the cell and chromosomes dividing
G0
- Stage of the cell cycle when cells are not dividing
- Can be in senescence or entering terminal differentiation
- Mitogens stimulate cells in G0 to enter cell cycle
- The lack of appropriate mitogens/nutrients or specific anti-growth signals will result in a cell exiting the cell cycle
G1 phase
Gap phase between end of cytokinesis and beginning of DNA synthesis.
G2 phase
Gap phase between end of DNA synthesis and beginning of mitosis.
How long is human genome
3 x 10^9 bp long
Direction of DNA synthesis
5’ to 3’ direction
Telomeres
- Region at end of chromosome
- Gets progressively shorter with each round of cell division
- The reduction in telomere length is associated with senescence.
- An enzyme called telomerase (hTERT) attempts to maintain telomere length.
- Cancer cells have high telomerase activity enabling cells to keep dividing
Four functions of telomeres
- Protect the chromosome ends from degradation and fusion.
- Mask chromosome ends from DNA damage response mechanisms that might trigger apoptosis.
- Help position the chromosomes in the nucleus.
- Provide a means of maintaining chromosome length through many generations of replication.
Telomerase
- A ribonucleoprotein with reverse transcriptase activity that catalyzes the extension or lengthening of telomeres
- Carries its own RNA template
Unequal Telomeric SCE (Sister Chromatid Exchange)
Homologous recombination dependent telomere maintenance, but one daughter cell suffers even shorter telomeres
Strand invasion
- Similar to the D-loop structure at the ends of telomeres
- Can lead to initiation of DNA synthesis
Shelterin complex
- Protein complex that binds to the telomere and control telomere elongation as well as protect/prevent unwanted responses from ssDNA.
- Required to recruit hTERT complex to telomere
Proteins that make up shelterin complex
- TRF1
- TRF2
- RAP1
- TIN2
- POT1
- TPP1
TRF1 complex
- Interacts with TPP1 (via TRF1/POT1) to recruit hTERT to the telomere and regulate telomere extension
- When tankyrase is active it polyA-ribosylates TRF1 to release it from telomeres and allows more hTERT in to elongate the telomeres.
TRF2 complex
- Prevents NHEJ and homologous recombination from the ssDNA at the telomere
- Interacts with TRF1 complexes to prevent DNA damage checkpoint activation
Pathology of defective telomerase function
- Dyskeratosis congenita (DKC)
- Werner syndrome (WS)
- Bloom syndrome (BS)
Dyskeratosis congenita (DKC)
- Produces short telomere
- Rare progressive congenital disorder with a highly variable phenotype, sometimes resembling premature aging, initially affecting skin, progressing to bone marrow failure and early mortality.
Werner syndrome (WS)
- Rare, autosomal recessive syndrome, characterized by premature aging.
- The WRN gene produces a helicase (unwinds DNA) primarily during repair of double strand breaks and during stalled replication.
Bloom syndrome
- Rare autosomal recessive disorder characterized by short stature, genomic instability, and development cancer early in life.
- The BS gene (called BLM) is a DNA helicase.
- BS patients exhibit a striking genomic instability including excessive homologous recombination and sister chromatid exchanges (SCE).
Telomerase knockout mice
5th - 6th generation has decreased wound-healing capacity, premature ageing, telomeres very short (even at birth), and have an increased risk of developing cancer.
Telomerase inhibitors
- Anti-hTERT cancer Vaccines
- Competitive RNA binding antisense oligonucleotides
- BIBR1532 inhibits RNA template translocation
- Nucleoside analogues
- G-quadruplex-stabilizing ligands
