1 Transfection agents: CaPO4 (co-precipitates with DNA) Electroporation (naked DNA, high voltage pulsetransient holes) Lipofection (multilamellar liposomes) Polybrene (detergent) Ballistic (DNA-coated gold particles) DEAE-dextran (toxic, OK for transient) Poly-ethylenimine (PEI, cheap) Effectene (non-liposomal lipid) Must traverse cytoplasm. Much engulfed in lysosomes. Inhibition of lysosomal function often helps (chloroquin). Co-integration of high MW DNA . Can reach 2000 KB. Separate plasmids transfected together same site (co-integration). Separate transfections separate locations Random or semi‑random (many) integration sites (unless targeted) Low but real homologous recombination rate. DNA transfection DEAE= diethyl-amino-ethyl (positively charged) DNA DNA polybrene Linear PEI Last updated: Nov. 21, 2011 1:10 AM
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
1
Transfection agents:
CaPO4 (co-precipitates with DNA)Electroporation (naked DNA, high voltage pulse transient holes)Lipofection (multilamellar liposomes)Polybrene (detergent)Ballistic (DNA-coated gold particles)DEAE-dextran (toxic, OK for transient)Poly-ethylenimine (PEI, cheap)Effectene (non-liposomal lipid)
Must traverse cytoplasm. Much engulfed in lysosomes. Inhibition of lysosomal function often helps (chloroquin).
Co-integration of high MW DNA . Can reach 2000 KB. Separate plasmids transfected together same site (co-integration). Separate transfections separate locationsRandom or semi‑random (many) integration sites (unless targeted)Low but real homologous recombination rate.
DNA transfection
DEAE= diethyl-amino-ethyl (positively charged)
DNA
DNA
polybrene
Linear PEI
Last updated: Nov. 21, 2011 1:10 AM
2
Transient transfection vs. Permanent transfection: cloned genes chromosomally integratedunintegrated DNA. position effects ?unnatural ? (so analyze a pool of many tosuper-physiological expression average)levels (per transfected cell) ?
Permanents more like 0.001 transfectants per μg DNA per cell (~high). i.e., 106 treated cells -> 1000 colonies; could be much less for certain types of cells
3
One the most dramatic first applications of gene transfection from total DNA:
Transfer of the growth‑transformed phenotype: ability to grow in multilayers or in suspension in soft agar: (Weinberg; Wigler)
DNA from tumor transfected into growth-controlled mouse 3T3 cells. Look for foci (one = focus).Make a library from growth‑transformed transfectant.Screen for human Alu repeat. Verify that cloned DNA yields a high frequency of focus‑forming transfectants.Isolate cDNA by hybridization to the cloned genomic DNA.Sequence. Identify gene/protein: = a dominant oncogene.
Ras, a signaling protein in a transducing pathway for sensing growth factors
Mouse 3T3 cells Transformed Mouse 3T3 cells transfected with an EGFreceptor gene
ES cells and transgenic mice. Selection for homologous recombinants via the loss of HSV TK genes (Capecchi): ----------– hsvtk – homol. region – drugR – homol. region – hsvtk –----------Non-homologous recombination favors ends: tk is inserted, conferring sensitivity to the drug gancyclovir (HSVtk specific, not a substrate for human tk)
7
Most K.O. work in ES cells mice homozygosis via F1 breedingLittle work in cultured lines:
Gene amplification for high level production in CHO dhfr- cells.
DHFR system (dihydrofolate reductase): Selection for resistance to marginal levels of methotrexate
Folate DHFR
“FH2” “FH4”dihydrofolate tetrahydrofolate
DHFRGlycine
Purine nucleotides(AMP and GMP)
Thymidylic acid(TMP)
Methotrexate (MTX, amethopterin)
Resistance to MTX can occur via 3 different mechanaisms:1) Methotrexate permeation mutants (incl. MDR, increased efflux))2) Altered DHFR with lower MTX binding affinity3) Increased levels of P-glycoprotein efflux pump (MDR)4) Overproduction of DHFR protein
MDR
MDR = multiple drug resistance
11Gene amplification: dhfr
Historically: • Methotrexate resistance• MTX inhibits dihydrofolate reductase (DHFR)• MTX-resistant cells have (in order of discovery):• High DHFR enzyme activity• High DHFR protein• High protein synthetic rate• High in vitro translatable mRNA• High mRNA level (by hybridization) • High DNA level.
Homogeneously staining, expanded chromosomal regions (HSRs)
HSRs are the location of the high number of dhfr genes.Double minute chromosomes are an occasional alternative form.
Amplicons (distance between repeated genes) are large (300 KB).(dhfr gene = ~ 25 kb)
Arnaud Coquelle, Lorène Rozier, Bernard Dutrillaux and Michelle Debatisse ONCOGENE, 31 October 2002, Volume 21, Number 50, Pages 7671-7679Induction of multiple double-strand breaks within an hsr by meganuclease I-SceI expression or fragile site activation leads to formation of double minutes and other chromosomal rearrangements
+FISH
Original locus?
Restriction-type enzyme with a very long recognition sequence ( ~20 bp)
17
Ampification models: over-replication, unequal sister chromatid exchange, breakage and fusion (Tanaka et al. Mol. Cell. Biol. 2007 27:1993-2002 .).
Map dhfr amplicons: ~ 300 kb , but wide range (Nunberg et al., Proc Natl Acad Sci U S A. 1978 Nov;75(11):5553-6; Looney et al., Mol Cell Biol. 1988. 8:5268-79)
Gene amplification is rare in normal cells p53- mutation allows it. (Livingstone et al., Cell. 1992.70:923-35; Yin et al., Cell. 1992. 70:937-48).
In nature:
rDNA in oocytes; Drosophila chorion genes.
In medicine:
chemotherapy resistance (MDR, P-glycoprotein, efflux pump) cancer (myc, ras)
In biotechnology:
high level recombinant protein production in mammalian cells
Gene amplification for high level recombinant protein production in mammalian cells.
Principal system = dhfr- CHO cells
Facilitated by the availability of DHFR-deficient mutant CHO cells
CHO dhfr- cells + vector with dhfr minigene + YFG
-GHT mediumMost cells die.Transfectants live.
+ gradually increasing concentrations of MTX
Cells with gradually amplified dhfr transgenes survive.YFG is co-amplified along with the dhfr minigene.
-GHT = without glycine, hypoxanthine (a purine source) and thymdine
19DHFR- cells require G,H,T
DHFR- cells selected by their resistance to radioactive 3H-deoxyuridine:3H-dU 3H-dUMP 3H-TMP 3H-DNA death from radioactive decay.DHFR- cells require glycine, hypoxanthine and thymidine (GHT). In GHT-free medium CHO dhfr- cells die, but transfectants that have received a dhfr minigene, +/- YFG, survive.
21A different major system for high level Mab production
NS0 cells:
Mouse myeloma cells, high IgG producers IgG- variants = NS0 No endogenous IgG, but cell is a natural IgG secretor.
Lack glutamine synthetase (GS): glutamate + NH3 + ATP glutamine + ADP + Pi
Vector = MAb genes driven by strong promoters (H-chain, L-chain)+ GS cDNA gene (Bebbington)
Select on glutamine-free medium
Inhibit GS with methionine sulfoximine (gln analog)
Select for GS overproducers --->--> (gene amplification does not seem to be operating in this system of the GS cDNA gene and linked Mab genes)
Proprietary (Lonza Biologics)
22
Transfection strategies
1. YFG (Your Favorite Gene) linked to a dhfr minigene on a single plasmidA. ~Insures co-integrationB. ~Insures co-amplification
2. YFG and dhfr on separate plasmidsA. Allows a high ratio of YFG to dhfr to startB. Co-amplification not assured but commonly occurs.
23
Linked amp
dhfrYFG
Select in purine-free
m edium
DH FR+
D H FR-
DH FR+++++YFG +++++
Increas ing [M TX]
CHO cells
24Co-amp1
DHFR
DHFR YFG
YFG
Co-amplification of genes on unlinked plasmids
Transfection Co-integration, usually
Step-wiseLow MTX --> High MTXCo-am plification
W igler et al., PN AS, 1980 . 77: 3567-70 (Principle of co-amplification).
25
DHFR YFG
YFG YFG
YFG
YFG
YFG
YFGYFG
YFG
YFG
YFG
YFG
YFG
YFG
YFG
YFG
Use a high ratio (e.g., 1000X) of YFG plasm id DNA to dhfr plasm id DNA
Transfect with 10 of and 10 of ug YFG ng dhfrCo-integration
(W igler et al, Cell, 1979: X174 DNA)
Multiple copies of from the startYFG
Step-w iseLow MTX --> High MTXCo-am plification
Co-amp3
(with or without pre-ligation)
76(11): 5684–5688
76: 5684–5688
26kaufman
Y.F.G.
Y.F.G.
DHFR
DHFRDicistronic mRNA
Transfection with both genes in one vector and even in one transcription unit
Poor translation initiationLow DHFRSupersensitive to MTXSelect initially in low MTXMore room for amplification
Kaufman, RJ, et al, NAR, 1993
(ribosome read-through)
Y.F.G. DHFRAlso, later, better dhfr translationusing an IRES,Internal ribosome initiation site, used mostly in viral but also in some cellular genes.In theory, not an advantage.
Y.F.G. DHFRDicistronic mRNA
IRES
27
Co-amp2
DHFR
DHFR YFG
YFG
Use a weak dhfr promoter to confer supersensitivity to MTX
Transfection, Co-integration, usually
Stepwise low MTX --> high MTX
Co-amplificationVery
W igler et al., PN AS, 1980
More room for am plification
28Co-amp4
DHFR YFG
YFG YFG
YFG
YFG
YFG
YFGYFG
YFG
YFG
YFG
YFG
YFG
YFG
YFG
YFG
Use a high ratio (e.g., 1000X) of YFG plasm id DNA to dhfr plasmid DNAand a poorly expressed dhfr minigene
Transfect with 10 of and 10 of ug YFG ng dhfr
StepwiseVery low MTX --> high MTXCo-amplification
Co-integration
(W igler et al, Cell, 1979: X174 DNA)
Multiple copies of from the startYFG
29
Pool of transfectants selected for growth in purine-free medium