40 3 Site-Directed Genome Modification with Engineered Zinc Finger Proteins
DNA
Fok 1
Fok 1
PCR amplification of targeted site
Heating and reannealing of strands
Digest with surveyor nuclease
Gel electrophoresis
Control Targeted
Cut
Uncut
Cleaved
Cleaved
Mismatched duplex DNA
Mismatched duplex DNA
Figure 3.2 A nuclease assay for detecting gene targeting. ZFNs are used to create a targeted
DNA DSB. PCR is used to amplify the targeted sites and the DNA is heated and cooled to
reanneal the strands, creating mismatched heteroduplexes of DNA. The Surveyor nuclease
cleaves the heteroduplexes only at the sites of mismatched DNA, leaving the homodimers
unmodified. Gel electrophoresis can then be used to observe and quantitate the efficiency of
gene targeting using ZFNs at the genomic level.
3.4 Validating Zinc Finger Nuclease-Induced Genome
Alteration and Specificity
Methods have been developed for monitoring for endogenous gene modifica-
tion. One such assay evaluates for DNA DSB repair via using the Surveyor nucle-
ase [35]. This assay involves three steps: (i) polymerase chain reaction (PCR)
amplification of the region of interest and annealing of the strands to form
homoduplexes (no mismatches) and heteroduplexes (containing mismatches),
(ii) cleavage of the mismatched heteroduplexes by the Surveyor nuclease, and
(iii) fragment size evaluation to determine if mismatched DNA was present [61].
By only cleaving annealed complexes containing both the mutated and wild-type
DNA after amplification, the Surveyor nuclease can used to estimate the level of
mutagenesis mediated by the ZFNs (Figure 3.2).