Genetic Instability

What defense mechanisms help protect stem cells against mutation? (6)

1. Location: crypt in intestinal epithelial cells

2. Cell division rate

3. Apoptosis

4. Mucin: villi shield them from external hazards like bacterial infection, acidic pH, and mechanical stress

5. DNA strand allocation: stem cells preferentially retain older, template DNA strands during asymmetric division while passing newly synthesized, error-prone strands to daughter cells destined for differentiation

6. Mdr1: effluxing genotoxic substances before they can reach the cytoplasm or nucleus to cause DNA damage

What is the role of pol-δ? What type of mutation in pol-δ leads to cancer?

Uses 3'-5’ exonuclease activity to correct replication mistakes.

A proofreading mutation in both pol-δ alleles leads to tumor formation in transgenic mice whereas heterozygous mice are unaffected.

What two mechanisms help reduce mutation rates?

pol-δ has difficulty with repeated DNA sequences called microsatellites, leading to expansion or contractions of DNA. Caretaker mismatch repair (MMR) enzymes like MutSα and MutLα recognize mistakes in the expanded/contracted DNA and degrade it.

What does defective MMR machinery lead to?

The instability of microsatellites which can change 1000s of genomic sequences and lead to tumor formation.

Why do hereditary non-polyposis colon cancers (HNPCC) arise faster than adenomatous polyposis coli (APC)?

They do not wait for mutations to arise and have inherited mutations through defects in MMR genes

How much more damage does the inner body do to DNA compared to environmental agents? What are examples and explain them.

100,000 bases/cell/day

Water - can cause deaminations pyrimidine exchange

ROS - NADPH oxidases generate ROS as innate immunity defence and mitochrondria release them as a by-product of cellular respitation. This can lead to purine/pyrimidine exchange

How does the body protect itself against reactive oxidative species?

There are ROS detoxifiers and scavengers that can breakdown ROS

Glutathione-S-transferase (GST-π) can reduce oxidized epoxide into a reduced thiol that is more stable.

How are nucleotides of abnormal structure repaired?

Base excision repair (BER)

This can repair lesions caused by depurinations/deaminations

Repair lesions that are not significant structural distortions of the DNA

How does base excision repair work?

1. A glycolase cleaves the base

2. An endonuclease and lyase coordinate for the cleavage of the remaining sugar

3. Pol-β fills in the empty space with opposite NT

4. A ligase fills the nicks by regenerating phosphodiester bonds

What are the bodies natural defences against radiological and chemical carcinogens?

Melanin

Mucin

How does MGMT reverse the formation of DNA adducts (unrepaired dna)?

A reduction in MGMT gene expression can lead to cancer. Overexpression of MGMT reduces cancer.

How does Nucleotide excision repair (NER) work?

1. A piece of the distorted and adducted DNA strand 30 nt is excised

2. One of several enzymes like pol-δ fills the empty space using the opposing nt

3. A ligase fills the nicks

It may be coupled to transcription-coupled repair (TCR) and global genomic repair (GGR)

How do BRCA2 mutations lead to cancer?

Partial BRCA2 function loss can lead to unusual chromosomal aberrations like nonhomologous recombinations and mitotic spindle number aneuploidy and prolonged cytokines

How do BRCA1 and BRCA2 use homology-directed DNA repair?

They recruit Rad51 which helps one DNA strand invade the other during the repair process. HR occurs late in S or G2 phase when sister chromatids are available to template new DNA synthesis.