Aging and gene expression – Alterations of the genome due to aging

Manifestation of Novel Social Challenges of the European Unionin the Teaching Material ofMedical Biotechnology Master’s Programmesat the University of Pécs and at the University of DebrecenIdentification number: TÁMOP-4.1.2-08/1/A-2009-0011

AGING AND GENE EXPRESSION – ALTERATIONS OF THE GENOME DUE TO AGING

Krisztián KvellMolecular and Clinical Basics of Gerontology – Lecture 22

Manifestation of Novel Social Challenges of the European Unionin the Teaching Material ofMedical Biotechnology Master’s Programmesat the University of Pécs and at the University of DebrecenIdentification number: TÁMOP-4.1.2-08/1/A-2009-0011

TÁMOP-4.1.2-08/1/A-2009-0011

TT A G GT

GDNA

RNA template

Telomerase

Nucleotides

AA U C CC A

Telomere sequence andtelomerase function

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• Most favored clock, but cause or marker?

• Sequence: TTAGGG hexanucleotide > 1000x

• Polymerase leaves gap with every replication

• Oxidative stress accelerates telomere loss rate

Telomeres as biological clocks

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• Telomeres form terminal loops for stability

• Role of TRF2 in telomere stability• Issue of telomere length threshold• Issue of telomere loss rate vs. stress

rate

Factors influencing telomere loss rate

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Telomere is repetitive DNA sequence

Embyonic stem cells

Adultstem cells

Telomerelong

Telomereshort

Active telomerase

Telomerase inactive

or absent

AA T C CCTT A G GG

Changes in telomere length

Chromosome

Extending the length of a telomere

New DNAShort end of DNA

GG T T

AA U C CC A A U CRNA templateT CC C C A TAC C A A

T T A GA G G G

TelomeraseDNA polymerase

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• Counteracting (oxidative) stress conditions

• Telomerase activity increases telomere length

• ALT: alternative telomere lengthening

Slowing, reversing telomere shortening

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Telomerase reactivation

Further evolutionLoss of telomere function

Significance of telomere in cancer

Telomere lenght

Number of aberrations

Genome instability

Normal tissue Hyperplasia Carcinoma in situ

Telomerecrisis

Invasive cancer

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• Soluble factors / cell non-autonomous spreading

• Pineal clock, role of melatonin• Circadian clock mechanisms• DNA methylation, acetylation, de-

acetylation

Further clocks ticking

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• Werner-syndrome• Cockayne syndrome• Hutchinson-Guilford progeria• Xeroderma pigmentosum

Genomic instability in progeria types

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• Homozygous recessive (skin, cataract, diabetes mellitus osteoporosis)

• WRN protein (anti-recombinase, helicase, removes recombination and repair intermediates)

• Defective transcription (50%)• Relation with p53 (attenuated

apoptosis)• Increased telomere loss rate

Werner syndrome

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• Rare segmental progeria (dwarfism, photosensitivity, neurological degeneration etc.)

• Defect in transcription coupled repair (TCR)

• Defective 8-oxodG excision (50%)• Subtypes: CS-A, CS-B• Global genome repair (GGR) is

proficient

Cockayne syndrome

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• Lamin A mutation (nuclear envelope fragility)

• Primerily affects mesenchymal tissues• HGPS cells have decreased stress

resistence• Rapid progeria, premature death

Hutchinson-Guilford progeria syndrome

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DNA REPAIR

(limited synthesis:

small fragments) Cell cycle

arrest(Apoptosis) Mutations

Cancer and genetic diseases

Replication errors

X rays

UV light

Alkylating agents

Spontaneous reactions

Reactive oxygen species (ROS)

DNA damage: causes, results I

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Oxidative DNA damage• > 10,000 DNA lesions / cell / day• A variety of DNA damage types (> 50 types)• 5 distinctive groups

- Oxidized purines- Oxidized pyrimidines- Abasic sites- Single-strand breaks- Double-strand breaks

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Stochastic

Regulated

DNA damage: causes, results II

Mutations, epi-mutations

Altered regulatory circuits

DampenedGH/IGF axis

Cellular responses(apoptosis,

senescence)

Improved survival Tissue atrophy, lost regeneration

ExogenusMetabolism

DNA damage

Tissue/organ functional decline, degenerative or hyperplastic disease

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• Base excision repair (BER) is most important, subtypes: AP endonuclease or lyase repair

• Removal of oxidized purines (two types of lesions: 8-oxodG and formamido-pyrimidines)

• Removal of oxidized pyrimidines (strong block, strongly cytotoxic)

• Repair of abasic sites (most frequent) by AP endonucleases

Oxidative DNA damage repair types I

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• Repair of strand breaks (single-strand breaks occur 10x more frequently than doubles)

• Limited mitochondrial DNA repair (nuclear encoded proteins of OGG1, POLG)

• Nucleotide excision repair (NER) that is transcription-coupled repair of active genes

Oxidative DNA damage repair types II

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• Defect is lethal: APE1, FEN1, POLB, LIG1, LIG3, XRCC1

• Defect is viable: OGG1, NTHL1, MYH, ADPRT

• Severity not tested: NEIL1, 2, 3, TDG, SMUG1, APE2

Genes related to oxidative DNA damage repair

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• Elevated cancer frequencies• Werner syndrome (anti-recombinase)• Cockayne syndrome (TCR)• XPD and XPA (repair deficiency)• Base excision repair (BER) defect is

lethal• Back-up repair pathways

Oxidative DNA damage repair and aging

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• Depurination and depyrimidination• Deamination• Single-strand breaks• Spontaneous methylation• Glycation• Cross-linking

Non-oxidative DNA damage

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• Biosynthetic errors• Transcriptional errors• Translational errors• Racemization and isomerization• Deamidation (asparagine and

glutamine)• Reactive carbonyl groups (non-

oxidative)• Serine dephosphorylation

Non-oxidative protein damage