Genetic Disease Flashcards
Why do mutations in NHEJ genes give rise to RS-SCID?
DNA DSBs are induced during immune system development for antibody generation
Eg. V(D)J recombination and class switch recombination
RAG1/2 creates the DSB in the VDJ gene segments
The NHEJ pathway joined the ends of the DNA, excising a piece a DNA (which is made cyclic)
Imperfect NHEJ gives more diversity for antibody variability
No NHEJ cause B and T cell death by p53 stabilisation leading to no immune system
How might you rescue someone with NHEJ-associated SCID?
Rescue the B and T cells by RAG1/2 knockout
Rescue immune system function by administering VDJ genes
What is the most common reason for tumourigenesis in NHEJ deficient patients?
Miss-repair of DSB can generate an oncogenic chromosomal translocation eg. IgH/c-Myc
Which NHEJ deficient patients are less likely to develop SCID?
XRCC4 and ligase4 patients
Possibly because in these patients NHEJ is affected but not absent- a relative amount of retained function- threshold for immune compromisation?
Is Alt-EJ enough?
Why does mutations in NHEJ lead to microcephaly?
Spontaneous DSB generated by rapid proliferation (collapsed replication forks) associated with developing organs like the brain- neural progenitor cells- cannot be fixed
This leads to apoptosis of many cells so there are fewer cells with which to build a brain with resulting in a smaller size
Genetic mutations also can slow the cell cycle of neuro-progenitor cells which reduces the pool of cells for brain development
Severity of defect correlates with the severity of reduction in head size
Where are the most apoptotic cells observed in the developing brain under TUNNEL staining?
The subventricular zone (SVZ) rather than the ventricular zone (VZ) the proliferative layer
Possibly because NHEJ is mostly needed to repair the breaks not repaired by HR- so it is needed as the cells exit S-phase and start to migrate and differentiate- therefore excess apoptosis would be triggered in the SVZ and reduce the pool of cells for development
What the common clinical features of patients with patients with HR mutations?
Immunodeficiency
Microcephaly- mostly not ATM and Mre11
Progressive cerebellar ataxia- ATM and Mre11
Describe the immunological problems experienced by some HR- deficient patients
AT and NBS- Reduced B and T cells but mostly reduction in antibody production- primarily IgG and IgA
ATM, MRN or RNF168 does not result in major problems on V(D)J recombination
AT, ATLD, NBS, NBSLD patients have concurrent spontaneous chromosomal translocations involving immune receptor genes
Why do mutations in HR lead to immunodeficiency?
B cells have to go through class switch to start expressing different immunoglobulin isotypes IgM➡️IgG or IgA
AID targets Smu and turns cytosine in to uracil
Uracil is removed from both strands forming s staggered DSB
DNA is excised- removing Smu
Sgamma is transcribed instead so IgG is mad instead of IgM
Misrepair by Alt-EJ or by NHEJ results in non-productive switched IgH loci that cannot produce IgG transcripts
Why do mutations in HR genes give rise to microcephaly?
Highly proliferative cell have spontaneous replication fork collapsed during S phase so the HR pathway is heavily relied on
What is a common clinical feature of mutations in the NHEJ pathway?
RS-SCID
Radiosensitive severe combined immune deficiency
(Also RS-CID- not ‘severe’)
Microcephaly- small head
