Phenotyping analysis of p53 knockout mice produced by gene editing and comparison with conventional p53 knockout mice

Mis-splicing Drives Loss of Function of p53E224D Point Mutation

Background

Tp53 is the most commonly mutated gene in cancer. Canonical Tp53 DNA damage response pathways are well characterized and classically thought to underlie the tumor suppressive effect of Tp53. Challenging this dogma, mouse models have revealed that p53 driven apoptosis and cell cycle arrest are dispensable for tumor suppression. Here, we investigated the inverse context of a p53 mutation predicted to drive expression of canonical targets, but is detected in human cancer.

Methods

We established a novel mouse model with a single base pair mutation (GAG>GAC, p53E221D) in the DNA-Binding domain that has wild-type function in screening assays, but is paradoxically found in human cancer in Li-Fraumeni syndrome. Using mouse p53E221D and the analogous human p53E224D mutant, we evaluated expression, transcriptional activation, and tumor suppression in vitro and in vivo.

Results

Expression of human p53E224D from cDNA translated to a fully functional p53 protein. However, p53E221D/E221D RNA transcribed from the endogenous locus is mis-spliced resulting in nonsense mediated decay. Moreover, fibroblasts derived from p53E221D/E221D mice do not express a detectable protein product. Mice homozygous for p53E221D exhibited increased tumor penetrance and decreased life expectancy compared to p53 WT animals.

Conclusions

Mouse p53E221D and human p53E224D mutations lead to splice variation and a biologically relevant p53 loss of function in vitro and in vivo.

The early-life stress induced by oxytocin inhibition in p53 knockout mouse dams increases adulthood tumorigenesis in first and second generations

BACKGROUND

Early-life stress due to poor parental care has been suggested to increase cancer risk, though, so far, no experimental evidence established a link between defective parental behavior and spontaneous tumorigenesis in progeny. Essential maternal behavior is regulated, in particular, by the oxytocin (OT) hormonal circuit, which in turn responds to stimuli from the offspring and impinges on the central nervous systems.

METHODS

By providing L-368,899 OT receptor (OTR) inhibitor to lactating mothers, we set up a model of defective maternal care in p53 knockout mice.

RESULTS

The progeny of these dams showed, later in life, higher cortisol levels, shortened life span and increased tumorigenic potential of bone marrow cells (BMC). Notably, these phenotypes were transmitted to the following generation.

CONCLUSIONS

Therefore, the inhibition of OT function in mothers is a novel paradigm of early-life stress that is inherited across generations and increases cancer risk in tumor-prone mice.

The position of the target site for engineered nucleases improves the aberrant mRNA clearance in in vivo genome editing

Engineered nucleases are widely used for creating frameshift or nonsense mutations in the target genes to eliminate gene functions. The resulting mRNAs carrying premature termination codons can be eliminated by nonsense-mediated mRNA decay. However, it is unclear how effective this process would be in vivo. Here, we found that the nonsense-mediated decay was unable to remove the mutant mRNAs in twelve out of sixteen homozygous mutant mice with frameshift mutations generated using engineered nucleases, which is far beyond what we expected. The frameshift mutant proteins translated by a single nucleotide deletion within the coding region were also detected in the p53 mutant mice. Furthermore, we showed that targeting the exons present downstream of the exons with a start codon or distant from ATG is relatively effective for eliminating mutant mRNAs in vivo, whereas the exons with a start codon are targeted to express the mutant mRNAs. Of the sixteen mutant mice generated, only four mutant mice targeting the downstream exons exhibited over 80% clearance of mutant mRNAs. Since the abnormal products, either mutant RNAs or mutant proteins, expressed by the target alleles might obscure the outcome of genome editing, these findings will provide insights in the improved performance of engineered nucleases when they are applied in vivo.