Chromosomal deletions can be used in mutagenesis strategies that screen for
recessive phenotypes. Several mouse chromosomal deletions are available which
remove the Pax6 gene and result in the small-eye phenotype. Deletion of the
syntenic region in humans results in the phenotypically variable WAGR syndrome.
The Pax6ˢᵉʸ⁻¹ᴴ deletion spans 3 Mb and includes 15 genes. This deletion has been
used in an ENU mutagenesis screen for phenotypes revealed when hemizygous with
the deletion. One such mutation, which is lethal against the Pax6ˢᵉʸ⁻¹ᴴ deletion, has
been found from 233 pedigrees. Mice heterozygous for the mutation are
phenotypically normal. When homozygous the mutation is generally lethal by El4,
and embryos have retarded growth from as early as E9. However, some homozygous
mice are born which are small and have extra digits on one or both hind limbs.
Sequencing of the genes in the deletion interval identified a splice site mutation in
Dnajc20 a member of the J-domain family of molecular chaperones. The mutation
causes the in frame skipping of exon 4, leading to an mRNA that encodes a protein
with an internal 23 amino acid deletion.
The yeast orthologue of Dnajc20 has recently been identified as one of five
genes required for the biosynthesis of diphthamide, a post-translational modification
that is unique to translation elongation factor 2 (eEF2), the target of diphtheria toxin
and Pseudomonas exotoxin A. Here it is shown that the mutation in the mouse
Dnajc20 protein results in the abolition of diphthamide on eEF2 in these animals.
Additionally mice mutated for Dnajc20 show remarkable similarity to the phenotype
of Ovcal mutant mice. Ovcal is a tumour suppressor whose yeast orthologue was
also identified as being required for diphthamide biosynthesis. The phenotypes of
these two mutations suggest that the diphthamide biosynthesis pathway, which is
conserved from archaebacteria to eukaryotes but absent in eubacteria, is essential for
normal mammalian development and survival. Furthermore the human orthologue of
Dnajc20 is suggested as the gene responsible for the manifestation of Polydactyly in
some WAGR patients.