Mutant Hoxd13 induces extra digits in a mouse model of synpolydactyly directly and by decreasing retinoic acid synthesis
Pia Kuss, … , Jochen Hecht, Stefan Mundlos
Pia Kuss, … , Jochen Hecht, Stefan Mundlos
Published December 15, 2008
Citation Information: J Clin Invest. 2009;119(1):146-156. https://doi.org/10.1172/JCI36851.
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Mutant Hoxd13 induces extra digits in a mouse model of synpolydactyly directly and by decreasing retinoic acid synthesis

Abstract

Individuals with the birth defect synpolydactyly (SPD) have 1 or more digit duplicated and 2 or more digits fused together. One form of SPD is caused by polyalanine expansions in homeobox d13 (Hoxd13). Here we have used the naturally occurring mouse mutant that has the same mutation, the SPD homolog (Spdh) allele, and a similar phenotype, to investigate the molecular pathogenesis of SPD. A transgenic approach and crossing experiments showed that the Spdh allele is a combination of loss and gain of function. Here we identify retinaldehyde dehydrogenase 2 (Raldh2), the rate-limiting enzyme for retinoic acid (RA) synthesis in the limb, as a direct Hoxd13 target and show decreased RA production in limbs from Spdh/Spdh mice. Intrauterine treatment with RA restored pentadactyly in Spdh/Spdh mice. We further show that RA and WT Hoxd13 suppress chondrogenesis in mesenchymal progenitor cells, whereas Hoxd13 encoded by Spdh promotes cartilage formation in primary cells isolated from Spdh/Spdh limbs, and that this was associated with increased expression of Sox6/9. Increased Sox9 expression and ectopic cartilage formation in the interdigital mesenchyme of limbs from Spdh/Spdh mice suggest uncontrolled differentiation of these cells into the chondrocytic lineage. Thus, we propose that mutated Hoxd13 causes polydactyly in SPD by inducing extraneous interdigital chondrogenesis, both directly and indirectly, via a reduction in RA levels.

Authors

Pia Kuss, Pablo Villavicencio-Lorini, Florian Witte, Joachim Klose, Andrea N. Albrecht, Petra Seemann, Jochen Hecht, Stefan Mundlos

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Figure 1

The Spdh allele acts as a loss and gain of function.

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The Spdh allele acts as a loss and gain of function. (A) Skeletal pr...
(A) Skeletal preparations of WT and Spdh/Spdh mice at birth (P0) and 1 week of age (P7). Spdh/Spdh mice exhibited multiple additional incompletely formed digits, fused joints, and a severe delay in ossification. Cartilage was stained with Alcian blue, bone with Alizarin red. (B) Skeletal preparations of transgenic PrxHoxd13+21Ala mice at P0 showed a severe malformation of the radius and bending of the ulna. No major changes were seen in the digits. Introducing 1 Spdh allele (Hoxd13+21Ala; Spdh/WT) did not change the phenotype. Mice expressing Hoxd13+21Ala on a homozygous Spdh background (Hoxd13+21Ala; Spdh/Spdh) showed a severe phenotype with excessive fusions and polydactyly. Mice with 1 Spdh allele and 1 inactivated Hoxd13 allele (Hoxd13st/Spdh) showed a delay in bone formation, fusion of joints, occasional postaxial polydactyly, and a short digit 2. In addition, ectopic cartilage formation between the digits, and an uneven surface of the digit cartilage (magnification of interdigital space shown on right), was present. Original magnification, ×3.2 (left), ×4.0 (right). (C) Shh expression in E10.5 WT and Spdh embryos.