3. DNA repair

Is all DNA damage the same?

No, the way DNA can be damaged varies

How many DNA lesions do we have on average per day?

About 10^5 DNA lesions per day

Why are not all DNA repairs detected?

• Some lesions are not detected

• Some lesions are irreparable

• Sometimes repair is too late

• Some repairs are not error-free

• Repair may decline with ageing

Therefore, DNA lesions accumulate in time, causing cancer and systemic ageing

DNA repair deficiency syndromes

• DNA lesions will accumulate faster than in other people

• Causing a larger chance of cancer

Nucleotide excision repair: Global Genome Nucleotide excision repair

• This system looks for gaps in the DNA where due to lesions, base pairing has not occured and there is a hole in the DNA strand.

• When it notices this hole in the DNA strand, it recruits other proteins. On one side you get XPF and ERCC1 and on the other XPG.

• These proteins cut out the damaged strand and ligand fills the gap.

What is Chiodyoderma pigmentosia?

When you have a mutation for one of the proteins needed for Global Genome Nucleotide excision repair.

• They have a very high chance of getting skin cancer (1000x increased risk)

• Being in the sun is very risky for them and therefore they can only play out at night or with special clothing

Nucleotide excision repair: transcription-coupled nucleotide excision repair

• When DNA is being transcribed into RNA, RNA polymerase can bump into a lesion.

• This is the sensor for DNA damage.

• When this happens two other proteins are recruited: CSA and CSB to identify the problem

• Then similarly to Global genome nucleotide excision repair, proteins are recruited to repair the damage.

Cockayne syndrome and trichothiodystrophy

Caused by mutations in CSA and CSB that recognize the DNA damage and give signals to XPF, ERCC1, and XPG to repair the DNA damage.

• Cockayne syndrome: no cancer but neuro-development abnormalities, life expectancy is 12y

• Trichothiodystrophy TTD: no cancer, neuro-development, brittle hair/nails

What is the main driver of ageing?

• Genomic instability

• The majority of this has something to do with DNA damage and DNA repair

What was found in mice with Trichothiodystrophy (TTD)

• They found that the mouse aged very quickly (it got grey hairs)

• The chance of cancer however, was reduced

How is it possible that the mice with TTD had a lower chance of cancer compared to mice without TTD?

• On one side:

  • Misreplication, aberrant chromosomal segregation, causes mutations and chromosomal aberrations = more cancer

• On the other side:

  • Blocked transcription, blocked replication, the cell is not dividing

  • Therefore you get cell cycle delay or arrest or cell death.

  • Since you are not dividing, the chance of mutations becomes much smaller

  • However, less dividing = ageing

The difference between these two sides also depends on if the mutation is in a dividing or non-dividing cell.

Effect of four DNA repair pathways affected

• Transcription coupled repair (TCR) - Cockayne Syndrome

• Nucleotide excision repair (NER) - Xeroderma Pigmentosum

• Cross-link repair - Fanconi’s anemia

• Single strand annealing repair of persistent double strand breaks

Causes accelerated ageing everywher

Findings in mice with Xeroderma Pigmentosum (NER)

The genes that were expressed in mice with NER were studied and it was found that:

• Short-lived repair mutants suppress GH/IGF1 axes and prioritize resilience (e.g. anti-oxidant systems, stress resistance) above growth, likely in an attempt to extend their short lifespan

• Low IGF1 explains why these mouse mutants and corresponding human syndromes (e.g. Cockayne Syndrome children) stay tiny.

• This ‘survival’ response resembles long-lived dwarf mutants and caloric- or dietary restriction (CR/DR), which indeed delays ageing

Interventions that improve longevity of people with these mutations

Caloric- or Dietary Restriction (CR/DR):

• Reduction of dietary intake,
  without malnutrition (10-40%).

• Still the exact mechanism is unknown
  mTor, IGF1/GH, ROS, metabolism,..

• DR works in a variety of species:
  From Yeast to Monkeys
  With health benefits in human

effect of dietary restrictions on species with diseases

~200% extension of lifespan

>500% extension of neurological healthspan

Benefits of DR

• Preserves neuronal functioning

• Prevents neuronal loss: when mice were dead, the amount of neurons left were tested. Mice with DR had 50% more neurons

Does DR also protect when a single neuronal cell type is ageing faster by DNA damage?

• This is similar to what occurs during alzheimers, where the brain ages faster than the body

• It was found that in these cases a dietary restriction also helps

Does DR only work preventitively?

• We know that when applied early in life, it works

• Some of the animals died immediately when applying DR later in life.

• However, some animals that survived the adjustment period, also had a longer life.

Does DR also work in humans with progeria?

• When tested on a girl with progeria, her tremors stopped due to dietary restriction

• Additionally, she started understanding and recognizing words

• She started walking independently

Transcription stress model

• This model propposes that DNA lesions accumulate randomly and silence long genes over time. This is because long genes have a higher chance of DNA damage.

• With DR: Damage rate is low → very few genes accumulate enough lesions to be silenced at all → long genes are hit less often just by chance, but if a gene does end up silenced, it's now relatively more likely to be one of the shorter genes (since so few long-gene-silencing events happen anymore)

• This is relevant because the longer genes are for important things such as axon development, regulation of membrane potential etc. Whereas smaller genes have less relevant roles.

Relevance for cancer?

• These diseases with continual DNA damage are similar to chemotherapy

• Patients that undergo chemotherapy also age faster.

• Therefore DR can also be applied to cancer patients and result showed that this benefitted the outcome.

Are there alternatives to food restriction or fasting