Enhancer adoption caused by genomic insertion elicits interdigital Shh expression and syndactyly in mouse
Kousuke Mouri, Tomoko Sagai, Akiteru Maeno, Takanori Amano, Atsushi Toyoda, Toshihiko Shiroishi
PNAS Published online before print December 18, 2017 DOI:10.1073/pnas.1713339115
Gene expression is regulated by tissue or organ-specific enhancer(s). Acquisition of new enhancers can alter the regulation of developmental genes, and may introduce morphological novelty in evolution. Reconfiguration between a gene and pre-existing enhancers of another gene by chromosomal rearrangement, which is referred to as “enhancer adoption”, is one possible source of new enhancers.
In this study, we re-examined an old mouse mutant named Hammer toe (Hm), which arose spontaneously over a half century ago and exhibits syndactyly with interdigital webbing. We revealed that a 150-kb non-coding genomic fragment that was originally located in chromosome 14 has been inserted into a genomic region upstream to Sonic hedgehog (Shh), located in chromosome 5. ATAC-seq and subsequent reporter assays in vivo revealed that the inserted fragment contains three interdigital enhancers to induce Shh expression in the interdigital regions in Hm. This ectopic Shh upregulates Chordin (Chrd), which in turn inhibits BMP signaling, and eventually results in syndactyly and web formation. Since the donor fragment residing in chromosome 14 has enhancer activity to induce interdigital gene expression, the Hm mutation appears to be an archetypal case of enhancer adoption. Our series of genomic deletion induced by the CRISPR-Cas9 system revealed that three enhancers in 150-kb fragment induce syndactyly in a cooperative manner. Acquisition of the combination of enhancers may have important role for the enhancer adoption.
This study was carried out as a collaboration of Kousuke Mouri, Tomoko Sagai, Akiteru Maeno, Takanori Amano and Toshihiko Shiroishi of Mammalian Genetics Laboratory, and Atsushi Toyoda of Comparative Genomics Laboratory, NIG. This study was supported by JSPS KAKENHI JP15J06985, JP17K15162 and JP17K19411.
Fig. 1 (A) Hm has an insertion into the upstream region of Shh gene. Inserted fragment includes three interdigital enhancers. (B) X-ray micro-CT images of limb (P0). Shh is ectopically expressed in the interdigital region, and causes interdigital webbing in Hm.
Fig. 2 The process that the morphological alteration in Hm was caused by the enhancer adoption.
You can see the several data of micro CT-scan which is one of the methods for this research below.
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