Telomeres

Telomeres: protecting chromosomes against genome instability : 

Telomeres: protecting chromosomes against genome instability Dr. Anand R Sullivan et al., Nat.rev. Molecular cell Biology; march 2010

Telomeres.. From Curiosity to lime light.. : 

Telomeres.. From Curiosity to lime light..

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McClintock first described the telomere in the early 1930’s- after treating maize with X-rays she found ring chromosomes, led her to this hypothesis McClintock - in 1936 described breakage fusion cycles due to telomere loss - later become a putative cause for chromosomal instability in cancer 1978 - Elizabeth Blackburn found telomeres Tetrahymena consisted of hundreds of base pairs of repetitive sequence TTGGGG 1985 - Carol Greider discovered telomerase, enzymatic activity that synthesizes telomeric repeats in Tetrahymena 1988 - Moyzis lab sequences the human telomere and determines the 6bprepeat (TTAGGG)n Looking back…..

Telomeres : 

Telomeres Ribonucleoprotein complexes at ends of chromosomes Around 9-15 Kbp size humans ; long tracts of double stranded TTAGGG repeats Leading strand G- rich; Lagging strand C- rich Actual end of telomere not blunt- G overhang Telomeric DNA associated with specialized binding proteins – Shelterin complex Forms specialized DNA secondary structures – T loop , D loop

Functions of telomeres : 

Functions of telomeres Protects the ends of chromosome Provides a means for complete replication of chromosomes and maintains chromosome length Serves as molecular clock controlling replicative capacity of human cells and entry into senescence Provides genome stability

Telomerase complex : 

Telomerase complex

Peculiarity of telomeres : 

Peculiarity of telomeres Nucleotide sequence DNA secondary structure formation Shelterin complex proteins Telomeric chromatin Interaction with DDR machinery proteins TERRAs

Shelterin complex proteins : 

Shelterin complex proteins

Structure of Telomeric chromatin : 

Structure of Telomeric chromatin

Interaction with DDR machinery : 

Interaction with DDR machinery Misconception Components of DDR machinery associated with telomeres both functional and dysfunctional MRN complex (MRE11, RAD50 and NBS1), ATM and ATR detected at telomeres ATM- TRF2 ; ATR- POT1 ; MRN- ?TRF2 Functional telomeres- efficient replication, processing of the ends, formation of a protective cap Dysfunctional telomeres- detection and processing of uncapped telomeres for repair and for mobility enhancing the efficiency of repair.

TERRAs : 

TERRAs Telomeric repeats containing RNA Earlier thought telomeres – transcriptionally inert Transcription of C­strand by RNA polymerase II produces long UUAGGG transcripts Noncoding, structural RNAs- maintain higher­order telomeric chromatin structures Display strong inverse correlation with telomerase activity

TELOMERE DYSFUNCTION : 

TELOMERE DYSFUNCTION When does it arise? Telomere shortening, telomere uncapping What does it lead to? Cell cycle arrest, Replicative senescence, cell death

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Why are telomeres important? Telomeres allow cells to distinguish natural chromosomes ends from broken DNA

Structural changes in dysfunctional telomeres : 

Structural changes in dysfunctional telomeres Result in formation of TIF (Telomere dysfunction induced loci) Accumulation of DDR machinery components Binding of Tumor suppressor p53 binding protein 1 (TP53BP1) Changes in local telomeric chromatin structure - γH2AX

Telomere shortening : 

Telomere shortening Due to DNA loss ‘End replication problem’ ‘End processing’ to generate G’ overhang Net loss 100-200 bp / round of replication Process of telomere shortening progresses till it reaches a critical size* Critically short telomeres recognized by DDR machinery Cells enter senescence / cell death

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“While 5ʹ to 3ʹ oriented growth should proceed smoothly to the end of its template, I see no simple way for 3ʹ to 5ʹ growth to reach the 3ʹ end of its template” James Watson, 1972

Replicative senescence : 

Replicative senescence Function of initial telomere length and replication associated shortening Imposes a limit to number of times a cell can divide ?????? Hayflick Limit Powerful antitumor mechanism However telomere length not the only criterion for senescence

Telomere uncapping : 

Telomere uncapping Refers to loss of the one / more shelterin complex proteins Important proteins – TRF 2 ; POT 1 Results in chromosome end deprotection Either cell death/ senescence

Telomere dysfunction and genomic instability : 

Telomere dysfunction and genomic instability

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NHEJ- Fusion break- bridge cycles

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Other possible mechanisms : Uncapped telomeres – candidates for Homologous recombination exposed G­overhang engages in rampant HR, resulting in sister telomere loss, sister chromatid exchange and gross chromosomal aberrations Overexpression of telomeric proteins – destabilizes shelterin complex Eg. TRF 1 &2 overexpression – gastric carcinoma

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Role of telomerase

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Telomere dysfunction – crucial step in carcinogenesis

Telomere dysfunction and disease : 

Telomere dysfunction and disease

Conclusions : 

Conclusions Telomeres- protective cap at end of chromosomes Telomere shortening occurs with each cycle of replication Telomeres essential to maintain genomic stability Telomere dysfunction- telomere shortening / uncapping DDR proteins play dual role