Reverse genetics 2 Flashcards
RNA interference
Gene silencing process that is sequence specific and mediated by double-stranded RNA molecules (siRNAs and shRNA/microRNA).
Silencing is at the ______ level
Posttranscriptional
- mRNA degradation and translation repression
When was RNA interference initially observed?
In petunias where overexpression of a pigmentation gene resulted in its own down regulation (co-suppression of gene expression)
The molecular basis for RNA interference was discovered by…
Craig Mellow and Andrew Fire in C. elegans who determined that it was double-stranded RNA and not sense/antisense RNA that cause gene silencing
True or false: RNA interference is ubiquitous in almost all eukaryotes examined so far
True
- It is thought that this molecular mechanism evolved to control spread of viruses and transposons.
Small interfering RNAs (siRNAs)
Long double-stranded RNAs (400-700 bp) are cleaved to 21-23 bp by the Dicer Rnase III (endoribonuclease that only cuts dsRNA) to generate siRNAs.
- siRNAs are unwound and one of the strands functions as a template in RISC (RNA indicuing-silending complex) to guide cleavage of the omplementary mRNA
MicroRNA
Noncoding RNA located in intergenic regions or within genes are transcribed by RNA polymerase II
- Single-stranded RNA is cleaved in the nucleus by Drosha generating a short hairpin mRNA which is then processed by DICER and the RISC complex similar to siRNA
True or false: microRNA is not a major regulator of gene expression, and is not implicated in numerous cellular processes/diseases
False
- MicroRNA was discovered recently to be major regulators of gene expression (>50% of all genes)
- If you delete microRNA genes, leads to misregulation of genes which can lead to diseases.
- Implicated in numerous cellular processes and diseases including differentiation, proliferation, apoptosis, cancer, heart disease.
What is introduced into cells/organisms for systematic knockdown of genes
Introduction of synthetically-designed siRNA/shRNA/microRNA
- Chemically synthesize various RNA molecules for gene silencing
True or false: targeted gene knockdown in multicellular organisms can be adapted for large-scale reverse genetics
True
- complex eukaryotes
True or false: RNAi alleles are knockout/loss of function
False
- RNAi alleles are knockdowns/partial loss of function (hypomorph alleles, not null alleles)
True or false: Every gene is susceptible to RNAi, like deletion mutagens are sensitive to deletions
False
- Unlike other mutagens, not every gene is susceptible to RNAi (Resistant tissues and genes encoding proteins with long half lives)
Multiple genes with shared sequences can be knocked down.
What is an advantage or disadvantage of this?
Advantage: Uncovers redundancy
Disadvantage: off-target knockdowns make phenotypes difficult to interpret
If a gene is knocked out but has a compensatory gene (functional redundancy), will a phenotype be observed?
A phenotype will not be observed. To deal with functional redundancy, you would need to design a different RNAi for the compensatory gene.
True or false: Knockdown siNA are usually heritable
False
- Knockdown with siRNA is usually not heritable and transient unless the transgene producing the siRNA is integrated in the gene.
What does introducing siRNA at different developmental stages allow for?
Makes it easier to study maternal genes with essential zygotic functions (knockdown maternal genes and determine the function of these in development of the fetus; can’t do a deletion because that would be lethal to the fetus so can’t be studied)
How is siRNA introduced into worms?
By microinjection or feeding bacteria expressing the siRNA
Describe how bacteria expressing the siRNA is produced (5 steps)
- Amplify >19,000 fragments from genomic DNA using PCR
- Insert siRNA gene into T7/fire vector (produces dsRNA, promoter on one side makes sense strand and promoter on other sides makes antisense strand)
- Anneal sense and antisense strand the make dsRNA (anneals because they’re complementary)
- Clone vectors into E.coli
- Put bacteria on plate for worms to eat
Describe the full genome RNAi profiling of early embryogenesis in C. elegans (4)
- 20, 326 dsRNA were designed and used to target 19075 (98%) of genes
- Screen was performed to identify all genes for mitotic cell division in a metazoan
- Phenotypic analysis was performed by 40,000 differential interference contrast microscopy (movies) about 20 minutes after fertilization until the four cell stage.
- Identified 661 genes involved in early embryogenesis
Why does RNAi identify a smaller number of essential genes compared to yeast mutant deletion strain experiments?
Knockdowns result in cell viability unlike knockouts done in essential genes.
- Essential genes are still there in RNAi, just not being detected because there’s still some expression of the essential genes in knockdowns (so can’t determine which genes are essential because knockdown of some essential genes doesn’t lead to cell death)
Eliminating any proteins that compromises the function of the same protein complex should result in…
The same consequence or phenotype (guilty by association)
In the full genome RNAi profiling of early embryogenesis in C.elegans, what were 79% of all known ribosomal genes classified as? Explain why
69% of all known ribosomal genes were classified as “severe pleitotrophic defect”
- Means that phenotype is mixed (not very uniform
- Knocking down protein synthesis affects many processes
Genes with common mutant phenotypes in the RNAi profiling experiment are associated with…
Similar chemical pathways and key events of the first cell divisions
What is the goal regarding genes with common mutant phenotypes?
The goal is to further characterize these genes to see how they interact with other genes/proteins and form molecular networks governing embryogenesis.
What do terminal phenotypes tell you in gene silencing for cell division genes?
Allows you to determine where in development the gene actually functions based on the terminal phenotype
True or false: Large scale RNAi screening is practical for mammalian animal models (e.g. mouse, chimps)
False
- RNAi screening is not practical for these types of animal models
Since large scale RNAi screening is not practical for mammalian animal models, what do we have to use instead for full genome RNAi screening?
Cell lines (e.g. knocking down all the cells in the cancer cell line, or the kidney cell line, etc.)
Describe how to get temporary vs stable effect of perturbation in RNAi screening in mammalian cell lines
Temporary (transient):
Transfecting siRNA directly into target cell.
- plasmid with shRNA or shRNA-mir gene can also be transfected into target cell
Not transient:
shRNA or shRNA-mir (microRNA structure in shRNA) get cloned into plasmid, transfected into virus or host cell directly, which infects the target cell and inserts DNA into host genome from virus.
What are the two formats for high-throughput mammalian RNAi screens?
- Multi-well-plate-based RNAi
- RNAi-based cell microarrays
Describe multi-well-plate-based RNAi (5 steps)
- Library prepared of bacterial glycerol stock or siRNAs (1 specific siRNA in each well targetting a specific gene)
- Preparation of transfection-quality DNA or siRNAs
- Re-array siRNAs or plasmid shRNAs into 384 - well plates for high-throughput screening (transient) OR prepare shRNA-expressing virus and then re-array virus into 384-well plates for high throughput screening
- Transfection (or reverse transfection) of siRNAs or plasmid shRNAs into target cell lines (transient) or infect shRNA-expressing virus into target cell lines
- Assay phenotype of interest
- Now you know exactly which knockdown contributes to the phenotype (b/c this is reverse genetics)
Describe RNA-based cell microarrays
- Library of siRNAs, plasmid shRNAs, esiRNAs, and virus shRNAs mixed with printing buffer
- Library is arrayed onto glass slide with microarraying robot
- Printed microarrays of siRNA/esiRNA microarray (reverse transfection), plasmid shRNA microarray (reverse transfection) and viral shRNA microarray (reverse infection) can be stored or used directly (instead of probes, it will be siRNA)
- Microarrays can be incubated with cells and then iamged live, or fixed and stained, imaged and analysed.
On an RNAi-based cell microarray, if there is no cell growth in one region of the microarray, what does this indicate?
The siRNA/shRNA viruses in this region knockdown essential genes
What are two ways of visualizing RNAi screens?
- Reporter assays (Can see the phenotype in response to knockdowns (e.g. luciferase gene, protein modification, protein interaction, morphology, etc)
- Image-based screening
What is the issue with interpreting the results (reporter assays/image-based screening) phenotypic assays for RNAi screens in mammalian cells?
Looking at thousands of cells -> big biology
- can’t do this manually, it’s very tedious
What is needed in order to interpret RNAi screen results?
High-throughput microscopy
- Automated microscope for acquisition of thousands of cell images (so you don’t have to manually take pictures of the wells)
- Automated image analysis with software to find cells and carry out assays including cell size measurements, morphology recognition and fluorescence intensity of biomarkers
- Require specialized hardware and software for handling terabytes of data