Hypomorphic expression ofDkk1in thedoubleridgemouse: dose dependence and compensatory interactions withLrp6

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Abstract
Doubleridge is a transgene-induced mouse mutation displaying forelimb postaxial polysyndactyly. We have cloned the doubleridge transgene insertion site and demonstrate that doubleridge acts in cis from a distance of 150 kb to reduce the expression of dickkopf 1 ( Dkk1 ), the secreted Wnt antagonist. Expression of Dkk1 from the doubleridge allele ranges from 35% of wild-type level in E7.0 head to <1% of wild type in E13.5 tail. doubleridge homozygotes and doubleridge/ null compound heterozygotes are viable. An allelic series combining the wild-type, doubleridge and null alleles of Dkk1 demonstrates the effect of varying Dkk1 concentration on development of limb, head and vertebrae. Decreasing expression of Dkk1 results in hemivertebral fusions in progressively more anterior positions, with severity increasing from tail kinks to spinal curvature. We demonstrated interaction between Dkk1 and the Wnt coreceptors Lrp5 and Lrp6 by analysis of several types of double mutants. The polydactyly of Dkk1d/d mice was corrected by reduced expression of Lrp5 or Lrp6 . The posterior digit loss and axial truncation characteristic of Lrp6 null mice was partially corrected by reduction of Dkk1 . Similarly, the anterior head truncation characteristic of Dkk1 null mice was rescued by reduction of Lrp6 . These compensatory interactions between Dkk1 and Lrp6 demonstrate the importance of correctly balancing positive and negative regulation of Wnt signaling during mammalian development.