Enhancer adoption caused by genomic insertion elicits interdigital Shh expression and syndactyly in mouse

Acquisition of new cis-regulatory elements (CREs) can cause alteration of developmental gene regulation and may introduce morphological novelty in evolution. Although structural variation in the genome generated by chromosomal rearrangement is one possible source of new CREs, only a few examples are known, except for cases of retrotransposition. In this study, we show the acquisition of novel regulatory sequences as a result of large genomic insertion in the spontaneous mouse mutation Hammer toe (Hm). Hm mice exhibit syndactyly with webbing, due to suppression of interdigital cell death in limb development. We reveal that, in the Hm genome, a 150-kb noncoding DNA fragment from chromosome 14 is inserted into the region upstream of the Sonic hedgehog (Shh) promoter in chromosome 5. Phenotyping of mouse embryos with a series of CRISPR/Cas9-aided partial deletion of the 150-kb insert clearly indicated that two different regions are necessary for the syndactyly phenotype of Hm. We found that each of the two regions contains at least one enhancer for interdigital regulation. These results show that a set of enhancers brought by the large genomic insertion elicits the interdigital Shh expression and the Hm phenotype. Transcriptome analysis indicates that ectopic expression of Shh up-regulates Chordin (Chrd) that antagonizes bone morphogenetic protein signaling in the interdigital region. Indeed, Chrd-overexpressing transgenic mice recapitulated syndactyly with webbing. Thus, the Hm mutation provides an insight into enhancer acquisition as a source of creation of novel gene regulation.

Formation of normal interdigital web spaces in the hand revisited: implications for the pathogenesis of syndactyly in humans and experimental animals

The creation of the normal web spaces has been attributed to apoptosis. This paper presents evidence that lysosomal-mediated cell death and extracellular matrix degradation are important events in addition to cell death by apoptosis. The author proposes the use of the term interdigital cell death– extracellular matrix degradation instead of interdigital apoptosis. Furthermore, the concept of web creation by differential growth is introduced along with the discussion of the latest research in molecular biology and genetics on the topic.