Lecture 7

TELOMERES

What are they?

Why are they important?

Telomere shortening and the end-replication problem

Telomerase

Telomere hypothesis of aging

Telomeres

Ends of linear chromosomes

Centromere

TelomereTelomere

Repetitive DNA sequence(TTAGGG in vertebrates)

Specialized proteins

Form a 'capped' end structure

Telomeres 'cap' chromosome ends

TELOMERE STRUCTURE

5’ 3’

5'

3'

Telomerict loop

Telomericproteins:

TRF1TRF2TIN2RAP1

TANKS 1,2POT1

etc

NUCLEARMATRIX

Why are telomeres important?

Telomeres allow cells to distinguish chromosomesends from broken DNA

Stop cell cycle!Repair or die!! Homologous recombination

(error free, but need nearby homologue)

Non-homologous end joining(any time, but error-prone)

Why are telomeres important?Prevent chromosome fusions by NHEJ (non-homologous end joining)

NHEJ

Mitosis

FUSIONBRIDGE

BREAKAGE

Fusion-bridge-breakage cycles

Genomic instability

Cell death OR neoplastic transformation

Telomere also provide a means for "counting" cell division

Pro

lifer

ativ

e ca

paci

ty

Number of cell divisions

FiniteReplicativeLife Span"Mortal"

InfiniteReplicativeLife Span"Immortal"

How do cells "know" how many divisions they have completed??

The End Replication Problem:Telomeres shorten with each S phase

OriDNA replication is bidirectionalPolymerases move 5' to 3'Requires a labile primer

3'5'

3'5'

5'

5' 3'3' 5'

Each round of DNAreplication leaves

50-200 bp DNA unreplicatedat the 3' end

Telo

mere

Len

gth

(hum

ans)

Number of Doublings

20

10

Cellular (Replicative) Senescence

Normal Somatic Cells

(Telomerase Negative)

Telomere also provide a means for "counting" cell division: telomeres shorten with each cycle

Telomeres shorten from 10-15 kb(germ line) to 3-5 kb after 50-60 doublings

(average lengths of TRFs)

Cellular senescence is triggered whencells acquire one or a few critically short telomeres.

How do replicatively immortal cells

avoid complete loss of telomeres

(how do they solve the end-replication problem)?

TELOMERASE:Key to replicative immortality

Enzyme (reverse transcriptase) with RNA and protein components

Adds telomeric repeat DNA directly to 3' overhang (uses its own RNA as a template)

Vertebrate repeat DNA on 3' end:TTAGGG

Telomerase RNA template:AAUCCC

TELOMERASE:Key to replicative immortality

+ TELOMERASE

Overcomes telomere shortening and the end-replication problem

Expressed by germ cells, early embryonic cells

Not expressed by most somatic cells (human)

May be expressed by some stem cells, but highly controlled

Expressed by 80-90% of cancer cellsRemaining still need to overcome the end replication problem;

do so by recombinational mechanisms -- ALT (alternative lengthening of telomeres) mechanisms

Telo

mere

Len

gth

(hum

ans)

Number of Doublings

20

10

Cellular (Replicative) Senescence

Normal Somatic Cells

(Telomerase Negative)

Germ Cells (Telomerase Positive)

+ Telomerase

Telomere Length and Cell Division Potential

HOWEVER,

CELLS THAT EXPRESS TELOMERASE

STILL UNDERGO SENESCENCE

(E.G., IN RESPONSE TO DNA DAMAGE, ONCOGENES, ETC.)

Inducers of cellular senescenceCell proliferation(short telomeres)

DNA damage OncogenesStrong mitogens/

stress

Potential Cancer Causing Events

Telomerase:Biomedical uses

Expand cells for replacement therapies(burns, joint replacements, etc)

Telomerase inhibitors to selectively kill cancer cells

The telomere hypothesis of aging

Telomeres shorten with each cell division and therefore with age

TRUE

Short telomeres cause cell senescence andsenescent cells may contribute to aging

TRUE

HYPOTHESIS:Telomere shortening causes aging and

telomerase will prevent agingTRUE OR FALSE?

The telomere hypothesis of aging

Telomere length is not related to life span(mice vs human; M musculus vs M spretus)

Telomeres contribute to aging ONLY if senescent cells contribute to aging

Telomerase protects against replicativesenescence but not senescence induce by

other causes

SUMMARY

Telomeres are essential for chromosome stability

Telomere shortening occurs owing to the biochemistry ofDNA replication

Short telomeres cause replicative senescence (other senescence causes are telomere-independent)

Telomerase prevents telomere shortening andreplicative senescence

The telomere hypothesis of aging depends on the cellular senescence hypothesis of aging