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Question 8 of 9 Intermediate Scenario

A CRISPR knockout shows no phenotype despite strong genotyping evidence. What explanations would you test?

Short answer

Sequence-confirmed editing is not equivalent to functional knockout. Verify the actual RNA, protein and target activity before concluding that the gene is dispensable.

Why

An indel may preserve reading frame, affect only some alleles or isoforms, allow downstream initiation, or generate functional splice products. Mixed genotypes, residual protein, clonal selection and genetic compensation can also hide a phenotype. A successful editing assay establishes a DNA change, not a loss of biological function.

How to work it through

  1. Check edit allele distribution with appropriate sequencing.
  2. Test transcript isoforms, protein abundance and a functional readout.
  3. Compare independent guides and independently derived clones, plus an acute knockdown or degradation method when available.
  4. Confirm assay sensitivity using a positive phenotype control, relevant cell state and a time course.
  5. Investigate related genes or pathway adaptation only after ruling out incomplete knockout.

An example

A clone has biallelic frameshift calls yet retains a band from a downstream antibody epitope and residual enzymatic activity. That finding suggests a truncated or alternatively initiated product. It is premature to invoke compensation.

Where the answer stops holding

Essential genes can disappear during clonal selection. Polyploid cancer lines may carry additional alleles. Developmental phenotypes can be absent in the chosen model, and a lack of effect in one assay is not a universal no-function claim.

A strong answer covers

  • Sequence-confirmed editing is not equivalent to functional knockout
  • Verify the actual RNA, protein and target activity before concluding that the gene is dispensable
  • Check edit allele distribution with appropriate sequencing
  • Test transcript isoforms, protein abundance and a functional readout

Common mistakes

  • Assuming Sanger traces prove complete allele loss
  • Interpreting one clone
  • Explaining every negative result by compensation without functional evidence