Embryo lethal phenotype

If a molecule targets two different isoforms of a protein, either by design or by the inability to obtain selectivity among closely related isoforms, the most relevant KOs for safety evaluation should be dual KOs of the targeted isoforms. An example that illustrates the complexities of drug target KOs and that incorporates embryo lethal phenotypes, tissue-specific KO, and dual isoform KOs is provided by MEK KOs. 

Type III coronins are different from other coronins in that they consist of two coronins fused in tandem but lacking coiled-coiled domains. In humans, they are represented by coronin 7. Type III coronins from Caenorhabditis (POD-1) sxiA Drosophila (Dpodl) have been shown to be involved with actin but seem participate in different processes. The embryonic-lethal phenotype of POD-1 mutants suggests that it is required for polarized membrane trafficking necessary for the establishment of anterior-posterior polarity in the embryo. Consistent with this phenotype, coronin 7 is associated with the Golgi apparatus and has been impheated in vesicle trafficking ... 

Similar to SIP receptor mutants, individual loss of either SPHKl or SPHK2 does not produce an abnormal phenotype. However, SPHK 1 and 2 double mutant embryos lose detectable SPHK activity. As a consequence, the double mutants are lethal prior to El3.5 with severe vascular and neural tube defects (Allende et al., 2004b Mizugishi et al., 2005). 

In summary, gene knockout mouse technology has allowed us to determine the function of cell surface carbohydrates in mouse embryos in recent years. Some glycosyltransferases-deficient mutant mouse showed severe phenotypes, including embryonic lethality. However, we have not yet identified the mechanisms underlying these phenotypes. Identification of critical carbohydrate structure (s), specific proteins carrying specific carbohydrate structure(s), and the counter-receptors for those carbohydrates should be identified in future studies. 

Figure 11. Two PARP-1 dosage-linked phenotypes in a PARP-2 null background (b-c) early embryonic lethality at E 8-8.5 of double mutant (PARP-1 -/- PARP-2-/-) embryos (d) female lethality in PARP-1 PARP-2 embryos at E 12.5. (Taken from Mdnissier-de Murcia et al, with permission.)...

For assessment of dominant lethals, it is essential to avoid noise as much as possible. The largest fraction of the experimental noise comes from embryos that die from recessive lethals accumulated in the genome of the strains used. Since many recessive lethal mutations occur spontaneously and are accumulated in the genome, these cannot be distinguished phenotypically from dominant lethals. Hence, the use of inbred or narrowly maintained strains should be avoided. Instead, the test of dominant lethals using a hybrid will prove to be much more useful. 

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