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Simultaneous detection of WT and SNP alleles carrying a silent PAM mutation at the PSEN1 locus in a bulk-edited iPS cell population

To demonstrate the SNP-detection capabilities of the Guide-it Knockin Screening Kit, we used CRISPR/Cas editing technology to generate an iPS cell line heterozygous for a variant of the PSEN1 gene encoding an A>G substitution (M164V) associated with early-onset Alzheimer's disease. Panel A. HDR templates carrying silent PAM mutations used to perform precise editing at the PSEN1 locus. Following successful HDR, the PSEN1 locus will encode either a WT or SNP allele combined with a silent mutation in the neighboring PAM sequence. Panel B. Design of displacement and flap-probe oligos for detection of edits involving silent mutation of the PAM sequence (G>C) and introduction of the SNP (A>G). The displacement oligo probe (in purple) is designed to hybridize in a similar manner when either allele is present (HDR wt silent or HDR M164V). The two different flap-probe oligos (in orange and green) are designed to fully hybridize to either the HDR wt silent allele encoding the PAM mutation (generating a red signal) or the HDR M164V allele encoding both the PAM mutation and the SNP (generating a green signal), respectively. The fixed sequences responsible for generating the fluorescent signals are underlined for each flap-probe oligo. Assayed bases for each allele are indicated in lowercase font.

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Common outcomes when engineering SNPs

An example of a single-nucleotide edit (G>T) is shown. Panel A. Outcomes at the genomic target site. When cleavage fails to occur at the target site or is followed by accurate, nonhomologous end joining (NHEJ)-based repair, the result is the wild-type (WT) sequence. When cleavage is followed by inaccurate NHEJ-based repair, the result is an insertion or deletion (Indel) at the target site possibly causing a knockout (KO, a highly probable outcome). When cleavage is followed by accurate HDR, a SNP is introduced at the target site. Panel B. Combined allelic outcomes in diploid cells. When editing is performed in diploid cells, the outcomes for each allele can vary, generating multiple possible combinations. Cells can remain homozygous (Wild type; top), they can have one or both alleles modified via inaccurate NHEJ (Indel; middle), or they can have one or both alleles modified with the desired SNP (Successful HDR; bottom).

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SNP analysis workflow for the Guide-it Knockin Screening Kit

This example workflow demonstrates analysis of a G>A substitution, where G is the wild-type base edited to an A. After genome editing, single cells expanded to clonal cell lines can have several different genotypic outcomes at the genomic target site of interest. After PCR amplification of the target site, the PCR product is annealed simultaneously with different oligo probes: a displacement oligo (purple) in combination with either flap-probe oligo A (green; encoding the SNP allele, A) or flap-probe oligo B (orange; encoding the WT allele, G). After the annealing of the oligos to the PCR products, the Guide-it Flapase enzyme (indicated with scissors) recognizes a complete base pairing and cleaves the 5′ portion of the flap-probe oligo (shaded green or orange). The cleaved flaps are then detected by corresponding Guide-it flap detectors, which yield green or red fluorescent signals, respectively. In the example above, analysis of a clonal cell line that is homozygous WT (G/G) at the site of interest yields only a red signal, while analysis of a heterozygous clone carrying both edited and WT alleles (G/A) yields both red and green signals.

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Detection of precise editing at an endogenous locus in bulk-edited and clonal iPSC populations

Panel A. Editing outcomes following successful HDR at an anonymous locus of interest. Following successful HDR, the edited locus will encode either a SNP (in blue, lowercase) or a WT base (in purple) combined with a silent PAM mutation (in red, lowercase). Panel B. Detection of successful HDR in bulk-edited iPSCs. Displacement and flap-probe oligos were designed to detect WT silent or SNP alleles, yielding red and green fluorescent signals, respectively. In independent experiments, cells were electroporated with Cas9 protein alone (negative control), Cas9-sgRNA RNP complexes (KO), or RNP complexes combined with antisense SNP or SNP/WT silent ssODN mixtures. Synthetic oligos encoding the WT silent or SNP sequences were assayed in parallel as positive controls. For each editing scenario in which ssODNs were included in the electroporation mixture, successful HDR could be detected in the bulk population using the Guide-it Knockin Screening Kit, as indicated by the resulting fluorescent signals. Panel C. Detection of successful HDR in clonal cell lines. Clones obtained from single cells isolated by flow cytometry were screened for both edits (SNP and WT silent). While successful incorporation of either edit could be detected in separate clonal cell lines, no heterozygous clones carrying both edits were identified.

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The Guide-it Knockin Screening Kit provides a method for detecting full-length knockin insertions

After the genome editing event, bulk-edited population or clonal cell lines isolated via FACS or limiting dilution may carry wild-type, indel, or full-length insertions. After DNA extraction from the clonal cells and subsequent PCR amplification of the target site, the PCR product is annealed with two different sets of displacement and flap probes: one that hybridizes with the 5' end of the insert (Flap-probe oligo A; green), and the other with the 3' end (Flap-probe oligo B; orange). If the full-length HR event has been successful and seamless, the full hybridization of the probes at both termini will generate both green and red fluorescent signals after the cleavage of the respective flap probes by the Guide-it Flapase. Detection of only one signal (red or green) indicates an insertion truncated on either the 5' or 3' end, respectively. The lack of fluorescence is indicative of the presence of the wild-type sequence or an indel at the target site.

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Beta-tester data: successful identification of heterozygous edited clones

Genotypes determined via bioinformatic analysis of the Sanger sequencing data are indicated along the X-axis (WT, wild-type; SNP, accurate HDR; Indel, NHEJ; unknown, software could not determine). The presence of edited (SNP) and wild-type (WT) alleles in the clones as determined by the Guide-it Knockin Screening Kit are demonstrated by fluorescence signal intensities indicated in blue (green fluorescence) and purple (red fluorescence), respectively. While the results of the knockin screening were consistent with the bioinformatic analysis of the Sanger sequencing traces for a majority of clones analyzed, there were several instances where the Sanger sequencing analysis missed or called some clones incorrectly.

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632659: Guide-it Knockin Screening Kit

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Simultaneous detection of WT and SNP alleles carrying a silent PAM mutation at the PSEN1 locus in a bulk-edited iPS cell population

To demonstrate the SNP-detection capabilities of the Guide-it Knockin Screening Kit, we used CRISPR/Cas editing technology to generate an iPS cell line heterozygous for a variant of the PSEN1 gene encoding an A>G substitution (M164V) associated with early-onset Alzheimer's disease. Panel A. HDR templates carrying silent PAM mutations used to perform precise editing at the PSEN1 locus. Following successful HDR, the PSEN1 locus will encode either a WT or SNP allele combined with a silent mutation in the neighboring PAM sequence. Panel B. Design of displacement and flap-probe oligos for detection of edits involving silent mutation of the PAM sequence (G>C) and introduction of the SNP (A>G). The displacement oligo probe (in purple) is designed to hybridize in a similar manner when either allele is present (HDR wt silent or HDR M164V). The two different flap-probe oligos (in orange and green) are designed to fully hybridize to either the HDR wt silent allele encoding the PAM mutation (generating a red signal) or the HDR M164V allele encoding both the PAM mutation and the SNP (generating a green signal), respectively. The fixed sequences responsible for generating the fluorescent signals are underlined for each flap-probe oligo. Assayed bases for each allele are indicated in lowercase font.

Back

Common outcomes when engineering SNPs

An example of a single-nucleotide edit (G>T) is shown. Panel A. Outcomes at the genomic target site. When cleavage fails to occur at the target site or is followed by accurate, nonhomologous end joining (NHEJ)-based repair, the result is the wild-type (WT) sequence. When cleavage is followed by inaccurate NHEJ-based repair, the result is an insertion or deletion (Indel) at the target site possibly causing a knockout (KO, a highly probable outcome). When cleavage is followed by accurate HDR, a SNP is introduced at the target site. Panel B. Combined allelic outcomes in diploid cells. When editing is performed in diploid cells, the outcomes for each allele can vary, generating multiple possible combinations. Cells can remain homozygous (Wild type; top), they can have one or both alleles modified via inaccurate NHEJ (Indel; middle), or they can have one or both alleles modified with the desired SNP (Successful HDR; bottom).

Back

SNP analysis workflow for the Guide-it Knockin Screening Kit

This example workflow demonstrates analysis of a G>A substitution, where G is the wild-type base edited to an A. After genome editing, single cells expanded to clonal cell lines can have several different genotypic outcomes at the genomic target site of interest. After PCR amplification of the target site, the PCR product is annealed simultaneously with different oligo probes: a displacement oligo (purple) in combination with either flap-probe oligo A (green; encoding the SNP allele, A) or flap-probe oligo B (orange; encoding the WT allele, G). After the annealing of the oligos to the PCR products, the Guide-it Flapase enzyme (indicated with scissors) recognizes a complete base pairing and cleaves the 5′ portion of the flap-probe oligo (shaded green or orange). The cleaved flaps are then detected by corresponding Guide-it flap detectors, which yield green or red fluorescent signals, respectively. In the example above, analysis of a clonal cell line that is homozygous WT (G/G) at the site of interest yields only a red signal, while analysis of a heterozygous clone carrying both edited and WT alleles (G/A) yields both red and green signals.

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Detection of precise editing at an endogenous locus in bulk-edited and clonal iPSC populations

Panel A. Editing outcomes following successful HDR at an anonymous locus of interest. Following successful HDR, the edited locus will encode either a SNP (in blue, lowercase) or a WT base (in purple) combined with a silent PAM mutation (in red, lowercase). Panel B. Detection of successful HDR in bulk-edited iPSCs. Displacement and flap-probe oligos were designed to detect WT silent or SNP alleles, yielding red and green fluorescent signals, respectively. In independent experiments, cells were electroporated with Cas9 protein alone (negative control), Cas9-sgRNA RNP complexes (KO), or RNP complexes combined with antisense SNP or SNP/WT silent ssODN mixtures. Synthetic oligos encoding the WT silent or SNP sequences were assayed in parallel as positive controls. For each editing scenario in which ssODNs were included in the electroporation mixture, successful HDR could be detected in the bulk population using the Guide-it Knockin Screening Kit, as indicated by the resulting fluorescent signals. Panel C. Detection of successful HDR in clonal cell lines. Clones obtained from single cells isolated by flow cytometry were screened for both edits (SNP and WT silent). While successful incorporation of either edit could be detected in separate clonal cell lines, no heterozygous clones carrying both edits were identified.

Back

The Guide-it Knockin Screening Kit provides a method for detecting full-length knockin insertions

After the genome editing event, bulk-edited population or clonal cell lines isolated via FACS or limiting dilution may carry wild-type, indel, or full-length insertions. After DNA extraction from the clonal cells and subsequent PCR amplification of the target site, the PCR product is annealed with two different sets of displacement and flap probes: one that hybridizes with the 5' end of the insert (Flap-probe oligo A; green), and the other with the 3' end (Flap-probe oligo B; orange). If the full-length HR event has been successful and seamless, the full hybridization of the probes at both termini will generate both green and red fluorescent signals after the cleavage of the respective flap probes by the Guide-it Flapase. Detection of only one signal (red or green) indicates an insertion truncated on either the 5' or 3' end, respectively. The lack of fluorescence is indicative of the presence of the wild-type sequence or an indel at the target site.

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Beta-tester data: successful identification of heterozygous edited clones

Genotypes determined via bioinformatic analysis of the Sanger sequencing data are indicated along the X-axis (WT, wild-type; SNP, accurate HDR; Indel, NHEJ; unknown, software could not determine). The presence of edited (SNP) and wild-type (WT) alleles in the clones as determined by the Guide-it Knockin Screening Kit are demonstrated by fluorescence signal intensities indicated in blue (green fluorescence) and purple (red fluorescence), respectively. While the results of the knockin screening were consistent with the bioinformatic analysis of the Sanger sequencing traces for a majority of clones analyzed, there were several instances where the Sanger sequencing analysis missed or called some clones incorrectly.

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632660: Guide-it Knockin Screening Kit