DNA Mutation and Repair Overview Flashcards
(29 cards)
How did we start the lecture
There is a constant battle going on with DNA repair mechanisms and the natural environment where the DNA gets damaged due to ultraviolet rays and other factors. Some DNA damage is required for evolution but that is counter to the goals of a physician which is to maintain a healthy population
How is damage to the DNA caused
Damage is routine. It is caused by physical, chemical factors as well as the innate errors made by the DNA replication machinery.
Define mutation
Mutation is defined as the unrepaired damage to the DNA that becomes part of the genome.
Effect of mutation on somatic cells and germ line cells
It is important here to differentiate between somatic cells and germline cells. Mutations in somatic cells only affects those cells individually whereas mutation in germline cells affect every offspring that arises directly or indirectly from that specific germline cell.
Comments on DNA repair
DNA repair mechanism are required by all species. Some mutation can be repaired by different routes, by multiple repair mechanisms whereas other mutation can be repaired by only 1 specific mechanism
Important questions discussed in this lecture
What damages the DNA, what types of damages occur, how are different types of damaged repaired, how failure in the repair mechanism can contribute to a disease and how can repair mechanisms be exploited to treat a disease (of course).
Define DNA adduct
A DNA adduct is the part of DNA that is bound to a cancerous agent or to a mutagen. This can initiate carcinogenesis. Many DNA adducts are reactive species that can either break the double stranded DNA at that location or modify the DNA that can cause errors in base pair readings.
How does ROS affect the DNA
ROS that are naturally present in a cell can also cause damage to the DNA and can result in various forms of damages to the cell. They can break the strands or lead to changes in bases
Methyl sulfonate
Adds an alkyl group to the CG pair which leads to a conversion such that now this part of the DNA is read as an AT pair.
The side effect of this cancer drug is that it can lead to secondary cancers especially in pediatric patients
Methyl SulfonaTe makes it an AT pair. Rememba dat!
Cisplatin as an anti cancer drug
Cisplatin is another common cancer drug that has platinum as the active agent. Pt binds with the base pairs either on the same strand or across the strand. DNA replication machinery cannot bypass this bond so the cell is unable to divide
Physical agents that damage the DNA
Physical agents can cause significant DNA damage such as UA light. These are called UVA and UVB which are in the range of absorbance for DNA, leading to the production of ROS. UVB is more dangerous than UVA since UVB does not only make ROS but also directly damages the DNA by making covalent bonds.
Further details about UVB
Chromophores in skin (melanin) are the first line in defense.
Remember UVB, B is for bad.
Remember that UV light leads to several DNA mutation. We only talk about thymidine dimers that can be formed (note that a structure of the dimer looks like inside the DNA strand as it is 4 carbons, 2 carbons on each thymidine making a box). When encountered in DNA replication, this can be repaired. Sometimes the DNA pol can jump over it without repairing this site which can lead to point mutation in the daughter strand.
Most common type of DNA damage is from UV light.
Effects of ionizing radiation on DNA
Ionizing radiation can lead to single stranded and double stranded breaks. This leads to most damaging form of breaks in the DNA as there is great difficulty for the cell to repair it.
Unique aspect about DNA pol
DNA pol exhibits 3’ to 5’ exonuclease ability where it can reverse and correct these mistakes. But still some mistakes happen.
Summary of DNA damage
Summary of DNA damage is that chemicals such as adducts, cross linkers (such as ones used in cancer drugs) and ROS can lead to DNA damage as well as physical agents such as UV light and ionizing radiation. Lastly DNA pol can make mistakes and incorporate the wrong base pairs
DNA Lesion Repair
There’s only 3 things going on, recognize the lesion, remove the lesion and then replace it with the correct base.
Types of DNA Lesion Repair
There are 2 types of simple DNA lesions repair:
- Mismatch repair: mismatch of a single nucleotide, it can be due to mismatch or deletion. This triggers the following events: there is insertion in the single strand, nuclease cuts the DNA, DNA pol fills in with the correct nucleotide and then DNA ligase connect the correct nucleotide with the rest of the DNA backbone.
- Base Excision repair: In here we simply remove the base. An enzyme called DNA glycosylase cuts out the base from the nucleotide and then we have the same events take place, nuclease, DNA pol and then DNA ligase.
Details on Mismatch Repair
He then showed us a basic schematic diagram of mismatch repair. It involved recognition, removing the lesion, that part of the DNA and then DNA pol theta coming in to join the correct nucelotides.
However, it is important to know if there are mutations in the part of the proteins that are associated with DNA repair mechanisms, then that patient is said to have lynch syndrome or hereditary nonpolyposis colorectal cancer. This is autosomal dominant and the individuals have about 60 to 80% higher risk of colon cancer. This disease is associated with congenital absence of any one of the 4 proteins that are associated with mismatch repair.
You track these patients very closely and they need a much higher levels of screening.
DNA glycosylase removes the base (it also recognized the mismatch), then AP endonuclease breaks the phosphodiester bond of this base and then DNA polymerase and DNA ligase come in to correctly fill in the space.
Congenital defects in BER
Congenital defects in Base Excision Repair are rare, there is only 1 documented case so far.
Complex lesion types and how are they repaired
Complex lesions consists of pyrimidine dimers (NER), Intrastrand cross links (NER) and Interstrand cross links (ICL repair, associated with Fanconi’s Anemia Proteins).
Explain NER
NER involves recognizing distorting lesions in the helix, removing about 20 to 30 nts, and then pol and ligase come in to add in the correct nts. Loss of pathway results in a variety of syndromes.
NER disease
Here we have to know about 1 disease that is called Xeroderma Pigmentosum. In XP disease the NER mechanism does not function properly due to the absence or defective in one of the involved proteins.
FA
There are 13 of these proteins, if any of these proteins are lost, then the individual gets Fanconi’s Anemia
FA is extremely rare, prevalence is 5000 worldwide.
Bone marorw failure is the biggest challenge here. They often end up with various types of leukemia. They have a high sensitivity to cross-linking chemotherapeutic agents.
SSB repair
We will leave SSB at that since it is a simpler process and it is not involved with a lot of pathology and diseases
