DNA repair and colorectal cancer

sources of DNA damage

internal: ROS, enzymatic damage e.g. DNA polymerase - error prone

external: UV light, radiation, smoking

types of DNA damage

single strand break

abasic site

double strand break

inter/intrastrand crosslink

mispairing

bulky adducts

consequences of DNA damage

cell cycle arrest

DNA repair

programmed/non-programmed cell death

types of DNA repair

base excision - oxidative

mismatch repair - nucleotide mismatch

nucleotide excision repair - bulky adducts

double strand break repair - HR and NHEJ

base excision repair

deamination converts a cytosine base into a uracil → uracil is detected and removed, leaving a base-less nucleotide → leaving a small hole in the DNA backbone → correct nucleotide is filled with DNA polymerase and sealed by ligase

mismatch repair

mismatch detected in newly synthesised strand → small section of DNA including mismatch is removed by exonuclease → DNA replaces with new correct nucleotides → DNA ligase seals the backbone

nucleotide excision repair

UV radiation produces a thymine dimer → dimer detected, helicase opens the surrounding DNA to form a bubble → dual excision to remove the damages strand → DNA polymerase replace nucleotides, ligase seals the backbone

double strand break repair

homologous recombination: S/G2 phase, uses sister chromatid as a template strand

• broken ends are processed to produce single strands

• cross over structure is formed with DNA strand from sister chromatid

non-homologous end joining: G0/G1 phase, directly joins broken DNA ends making it faster but more error-prone

• Ku70/80 binds DNA ends and recruits DNA-PKcs

• DNA ends are aligned and processed if necessary using Artemis

• XRCC4/ligase-IV complex seals the break

DNA damage repair in colorectal cancer

base excision - inherited polyposis disorders

mismatch repair - lynch syndrome

nucleotide excision repair - oxaliplatin response

polyposis disorders

genetic predispositions which leads to the development of polyps in the GI tract at a much earlier age than the general public → increased risk of colorectal cancer and much earlier on

types:

• MUTYH associated polyposis (MAP)

• NTHL1 associated polyposis (NAP)

• polymerase proofreading associated polyposis (PPAP)

• MBD4 associated neoplastic syndrome (MANS)

MUTYH associated polyposis (MAP)

autosomal recessive

10-100 colorectal polyps

28 fold increased cancer risk - duodenal, urinary, ovarian cancer

biallelic missense variants in MUTYH - mutations in Y179C and G396D → encodes for DNA glycosylases involved in base excision repair which normally corrects oxidative DNA damage

NTHL1 associated polyposis (NAP)

autosomal recessive

10’s colorectal polyps - increased risk of bladder cancer and basal cell carcinoma

mutations in the NTHL1 gene - codes for DNA glycosylases involved in base excision repair which normally corrects oxidative DNA damage

MBD4 associated neoplastic syndrome (MANS)

autosomal recessive

10-100’s colorectal polyps - increased risk of AML, uveal melanomas and ovarian tumours

biallelic mutations in MBD4 gene - encoding DNA glycosylase involved in base excision repair which normally repairs spontaneous deamination at CpG nucleotides → accumulation of C→T transitions

proofreading polymerase associated polyposis (PAPP)

autosomal dominant

10’s colorectal polyps - increased risk of pancreas, ovarian, duodenum, endometrial cancer

mutations in POLE and POLD1 genes - normally encodes for DNA polymerases for ensuring accurate proofreading and removing of mis paired bases from the newly synthesised strand - high amount of base substitution mutations

Lynch syndrome

autosomal dominant

hereditary non-polyposis colorectal cancer - bowel cancer predisposition

mutations in MLH1, MSH2, MSH6, PMS2 (mismatch repair genes) - normally detect base-base mismatches and small insertion/deletion loops

second somatic hit inactivates the remaining allele → loss of mismatch repair, accumulation of mutations (microsatellite instability)

oxaliplatin

platinum-based chemotherapy drug for colorectal cancer

MOA:

• DNA crosslinking - forms covalent adducts with DNA, creating intra-strand crosslinks between adjacent guanine bases

• DNA damage - crosslinking distorts the double helix → interfering with replication and transcription

• cell cycle arrest and apoptosis

oxaliplatin PK

absorption: hydrolysed to displace the oxalate → reactive intermediates

distribution: highly lipophilic and readily distributes from plasma throughout the body

metabolism: oxalate is detoxified and metabolised by enzymes

excretion: renal

oxaliplatin side effects

peripheral neuropathy:

• paraesthesia

• dysesthesia

• allodynia

acute form - reversible (disruption of sodium channels)

chronic form - after several forms of chemo (direct toxicity to nerve cells via accumulation of platinum adducts in the dorsal root ganglia - enhances apoptosis)