W14L1 Flashcards
Xist
Xist is a lncRNA that is transcribed from a region of one of the X-chromosomes
The X-chromosome that makes more Xist becomes the Xi chromosome; this chromosome is silenced
Xic (X inactivation centre) contains the Xchromosome-controllingelement (Xce)
- Xce contributes to the X chromosome that will be inactivated
- Xce is like positive feedback
Xist represents a form of imprinting in which only one chromosome or allele is expressed even though two exist
Other regions of the genome are also imprinted through paternal or maternal inheritance and lncRNAs also play a role in that type of regulation
E.g. HOTAIR
Are there factors that select which X chromosome should be inactivated?
X inactivation “skewing” means that the same X chromosome is inactivated >70% of the time (happens in 35% of women)
Extreme skewing is >90% of the time (happens in 7% of women)
WHAT CAN CAUSE THIS SKEWING?
1. different variants of the Xce gene
- variants of Xce protein that have different potentials for recruiting Xist
2. Mutations in the Xist promoter
3. Mutations in an X-linked gene that affects cell survival
Could be the underlying cause for variability in X-linked diseases. Example if that Rett syndrome (RTT), which is due to mutations in MeCP2
RTT patients shows significant variability of symptoms likely due to differences in X-inactivation
Sources of DNA damage
Normal transcription, proliferation, etc.
- Suggested that up to one million DNA lesions occur every day. Great majority are fixed
Environmental agents
- UV light
- Chemical exposure
- Chemotherapy
Internal stress
- Inflammation
- Reactive Oxygen Species (ROS)
Can lead to genomic instability, apoptosis, or senescence
Genomic instability can predispose individuals to cancer, neurological disorders and cardiovascular diseases
What are the steps required for repairing DNA?
Identify the mistake – type, extent, etc.
Remove the mistake – mismatch, unmatched base pair, non-cohesive ends
Trim back the DNA backbone
Repair the sequence
Ligate the backbone of DNA
Direct Reversal
Single step process with no excision of nucleotides
Does not break the phosphate backbone of DNA
error-free and preserves genetic information
Most energy efficient type of DNA repair
Mismatch repair
Repairs mismatched bp that may arise from mistakes in DNA replication or recombination
Failure results in microsatellite instability
DNA mismatches are recognized by MUTS and MUTL
MUTL causes an incision (nick) in the DNA
Exonuclease (EXO) removes the mismatched nucleotide and up to 1000 bps more
DNA polymerase restores the correct nucleotide and then DNA ligase joins restores the DNA backbone
Base Excision Repair
Repair of modified base pairs or nucleotides which is the most common DNA insult
Typically repairs a single nucleotide (short patch), but can replace up to 13 nucleotides (long patch)
- Removal of the damaged nucleotide by glycosylases
- Strand incision by endonuclease APE1
- Nucleotide(s) inserted by DNA polymerase
- Ligation of DNA
Nucleotide Excision Repair
Repairs DNA adducts that distort the double helix, including those induced by UV light, chemotherapy
transcription-coupled (TC)-NER or global genome (GG)-NER
four successive steps
lesion detection (different for the two processes)
local unwinding and damage verification
excision of the DNA surrounding the lesion
DNA synthesis and ligation
XP = xeroderma pigmentosum; rare genetic disorder linked to mutations in genes involved in NER
Homologous Recombination
Uses the matching chromosome as a template
DSB are recognized by a MRE11–RAD50–NBS1 (MRN) complex
MRN complex is essential for maintaining genomic integrity, cell viability, and checkpoint activation
Recruits ataxia telangiectsia mutated (ATM) protein which phosphorylates factors involved in repair (including BRCA1 and p53)
BRCA1 isrequired for the recruitment of proteins such as Rad51, BRCA2, and BARD1 to sites of DSBs
What are two possible consequences of Homologous Recombination?
- Crossing over – swapping gene regions between chromosomes
Results in non-Medelian inheritance
- Loss of heterozygosity of a specific gene
Result of HR leads to identical genomic regions – possible duplication of adverse genetic events
