Telomeres and epigeneticsTelomeres and epigenetics

The structure of mammalian telomeresThe structure of mammalian telomeres Telomeres contain a double-stranded DNA region of TTAGGG Telomeres contain a double-stranded DNA region of TTAGGG

repeats (green arrows) repeats (green arrows) Telomeres are characterized by a 150–200-nt long single-Telomeres are characterized by a 150–200-nt long single-

stranded overhang of the G-rich strand (G-strand overhang; stranded overhang of the G-rich strand (G-strand overhang; blue arrows)blue arrows)

Telomerase recognizes the 3' OH at the end of the G-strand Telomerase recognizes the 3' OH at the end of the G-strand overhang, leading to telomere elongation. overhang, leading to telomere elongation.

Telomerase enzyme (Cc)Telomerase enzyme (Cc) Ribonuleoprotein Ribonuleoprotein

TERT, telomerase reverse TERT, telomerase reverse transcriptasetranscriptase

TERC, telomerase RNA TERC, telomerase RNA component (single RNA component (single RNA molecule provides an molecule provides an AAUCCC (in mammals) AAUCCC (in mammals) template to guide the template to guide the insertion of TTAGGGinsertion of TTAGGG

DKC1, dyskeratosis DKC1, dyskeratosis congenita 1congenita 1

Components of telomeric proteinsComponents of telomeric proteins Two main protein complexes are Two main protein complexes are

bound to telomeres, the telomere bound to telomeres, the telomere repeat binding factor 1 and 2 repeat binding factor 1 and 2 complexes, complexes, TRF1TRF1 and and TRF2TRF2

The components of the telomere The components of the telomere repeat binding factor 1 (TRF1) repeat binding factor 1 (TRF1) (Ca) and 2 (TRF2) (Cb) (Ca) and 2 (TRF2) (Cb) complexes and are shown. complexes and are shown.

human diseases in which human diseases in which expression of these components expression of these components has been shown to be altered are has been shown to be altered are indicatedindicated

A telomere in a T-loop conformationA telomere in a T-loop conformation

Strand invasion of the G-Strand invasion of the G-strand overhang is strand overhang is highlighted in redhighlighted in red

This conformation might This conformation might prevent the access of prevent the access of telomerase to the 3' OH at telomerase to the 3' OH at the chromosome end. the chromosome end.

Telomerase and telomere length in Telomerase and telomere length in ageing  ageing 

Most somatic cells show Most somatic cells show progressive telomere progressive telomere shortening owing to low or shortening owing to low or absent telomerase activity absent telomerase activity

leads to critically short leads to critically short telomeres, which triggers a telomeres, which triggers a DNA damage response that DNA damage response that results in chromosomal results in chromosomal end-to-end fusions or cell end-to-end fusions or cell arrest and apoptosis. arrest and apoptosis.

thought to contribute to the thought to contribute to the onset of degenerative onset of degenerative diseases including human diseases including human premature ageing premature ageing syndromessyndromes

Telomerase and telomere length in Telomerase and telomere length in tumourigenesis tumourigenesis

Tumour cells have shorter Tumour cells have shorter telomeres than normal cellstelomeres than normal cells

However, telomerase is However, telomerase is reactivated in more than reactivated in more than 90% of all types of human 90% of all types of human tumours tumours

therapeutic strategies aimed therapeutic strategies aimed at inhibiting telomerase will at inhibiting telomerase will preferentially kill tumour preferentially kill tumour cells and have no toxicity on cells and have no toxicity on normal cells.normal cells.

Premature ageing syndromes with short telomeres

Ataxia telangiectasia (ATM) Werner syndrome (WRN); Bloom syndrome (BLM); Dyskeratosis congenita (DKC1, Terc); aplastic anaemia (Terc, Tert); Fanconi anaemia (FANC genes); Nijmegen breakage syndrome (NBS); and ataxia telangiectasia-like disorder (MRE11).

Ataxia Telangiectasia Ataxia Telangiectasia

A-T is fatal in the second or third decade of lifeA-T is fatal in the second or third decade of life

Staggering gait, muscular unco-ordinationStaggering gait, muscular unco-ordination ImmunodeficiencyImmunodeficiency NeurodegenerationNeurodegeneration premature agingpremature aging Skin sensitivity to ionizing radiationSkin sensitivity to ionizing radiation susceptibility to certain types of cancer susceptibility to certain types of cancer

(breast cancer)(breast cancer)

Ataxia Telangiectasia Ataxia Telangiectasia Mutations in Mutations in ATMATM gene located on chromosome 11q22-q23 gene located on chromosome 11q22-q23

encodes large protein kinase involved in cell cycle checkpoint encodes large protein kinase involved in cell cycle checkpoint and genotoxic stress responsesand genotoxic stress responses

HomozygousHomozygous ATM ATM mutant alleles rare ~ 1/40,000 mutant alleles rare ~ 1/40,000

heterozygous carriers ~ 1-2% heterozygous carriers ~ 1-2%

Carriers exhibit intermediate sensitivity to radiation and Carriers exhibit intermediate sensitivity to radiation and predisposition to cancer (implications for radiotherapy)predisposition to cancer (implications for radiotherapy)

Werner syndrome (WRN short stature (common

from childhood on) wrinkled skin baldness, cataracts, muscular atrophy tendency to diabetes

mellitus

gene gene WRNWRN mapped to chromosome 8 mapped to chromosome 8 predicted helicase belonging to the RecQ familypredicted helicase belonging to the RecQ family

FANCONI/BRCA1/BRCA2/ATM/NBS1 PATHWAYFANCONI/BRCA1/BRCA2/ATM/NBS1 PATHWAY

EpigeneticsEpigenetics

epigenetic modification of epigenetic modification of DNA in mammals is DNA in mammals is methylation of cytosine methylation of cytosine at position C5 in CpG at position C5 in CpG dinucleotidesdinucleotides

Other main group is Other main group is epigenetic post-epigenetic post-translational translational modification of histonesmodification of histones

Genomic imprintingdefined as an epigenetic modification of a specific parental

chromosome in the gamete or zygote that leads to differential expression of the two alleles of a gene in the somatic cells of the offspring.

Differential expression can occur in all cells, or in specific tissues or developmental stages.

About 80 genes are known to be imprinted

Loss of imprinting (LOI)

disruption of imprinted epigenetic marks through gain or loss of DNA methylation, or simply the loss of normal allele-specific gene expression.

Epigenetics – differential Epigenetics – differential imprintingimprinting

failure to thrive during infancy, failure to thrive during infancy, hyperphagia and obesity during hyperphagia and obesity during early childhood, mental retardation, early childhood, mental retardation, and behavioural problemsand behavioural problems

molecular defect involves a molecular defect involves a paternallypaternally imprinted domain at 15q11–q13imprinted domain at 15q11–q13

Prader-Willi syndrome Angelman syndrome

abnormal gaitabnormal gait speech impairment, speech impairment, seizures, mental retardationseizures, mental retardation

inappropriate happy demeanor inappropriate happy demeanor that includes frequent laughing, that includes frequent laughing,

smiling, excitabilitysmiling, excitability defect lies within thedefect lies within the maternally maternally

imprinted domain at 15q11–q13imprinted domain at 15q11–q13

Genetic causesGenetic causes

70% have a deletion of 70% have a deletion of the PWS/AS region on the PWS/AS region on their paternal their paternal chromosome 15chromosome 15

25% have maternal 25% have maternal uniparental disomy for uniparental disomy for chromosome 15 (the chromosome 15 (the individual inherited individual inherited both chromosomes both chromosomes from the mother, and from the mother, and none from the father)none from the father)

5% have an imprinting5% have an imprintingdefectdefect

<1% have a chromosome <1% have a chromosome abnormality including abnormality including the PWS/AS regionthe PWS/AS region

Prader-Willi syndrome Angelman syndrome70% have a deletion of the PWS/AS region on their maternal chromosome 15

7% have paternal uniparental disomy  for chromosome 15 (the individual inherited both chromosomes from the father, and none from the mother)

3% have an imprinting defect

11% have a mutation in UBE3A

1% have a chromosome rearrangement

11% have a unknown genetic cause

DNA methylation and cancerDNA methylation and cancerChanges in methylation are early events in tumorigenesisChanges in methylation are early events in tumorigenesisIn tumour cells, repeat-rich heterochromatin becomes In tumour cells, repeat-rich heterochromatin becomes hypermethylatedhypermethylated and this and this

contributes to genomic instability, a hallmark of tumour cells, through contributes to genomic instability, a hallmark of tumour cells, through increased mitotic recombination events.increased mitotic recombination events.

De novo De novo hypomethylationhypomethylation of CpG islands also occurs in cancer cells, and can of CpG islands also occurs in cancer cells, and can result in the transcriptional silencing of growth-regulatory genes. result in the transcriptional silencing of growth-regulatory genes.

Region of the genome showing repeat-rich, hypermethylated pericentromeric heterochromatin and tumour suppressor gene (TSG) associated with a hypomethylated CpG island (red).

ReferencesReferences

1) Telomeres and human disease 1) Telomeres and human disease by M Blascoby M BlascoNature Reviews Genetics Aug 2005 vol 6 pp611Nature Reviews Genetics Aug 2005 vol 6 pp611

2) DNA methylation and human disease 2) DNA methylation and human disease by KD by KD RobertsonRobertson

Nature Reviews Genetics Aug 2005 vol 6 pp 597Nature Reviews Genetics Aug 2005 vol 6 pp 597