A mutation in Ihh that causes digit abnormalities alters its signalling capacity and range

Abstract

Brachydactyly type A1 (BDA1) was the first recorded disorder of the autosomal dominant Mendelian trait in humans, characterized by shortened or absent middle phalanges in digits. It is associated with heterozygous missense mutations in indian hedgehog (IHH). Hedgehog proteins are important morphogens for a wide range of developmental processes. The capacity and range of signalling is thought to be regulated by its interaction with the receptor PTCH1 and antagonist HIP1. Here we show that a BDA1 mutation (E95K) in Ihh impairs the interaction of IHH with PTCH1 and HIP1. This is consistent with a recent paper showing that BDA1 mutations cluster in a calcium-binding site essential for the interaction with its receptor and cell-surface partners. Furthermore, we show that in a mouse model that recapitulates the E95K mutation, there is a change in the potency and range of signalling. The mice have digit abnormalities consistent with the human disorder.

Mechanisms of digit formation: Human malformation syndromes tell the story

“…This phenotype, presence of a terminal phalanx but absence of more proximal structures is seen in human brachydactylies type A1, A2 and in some cases of BDB1. However, analyses of mouse models for BDA1 and BDB1 (Raz et al, 2008;Gao et al, 2009) have found no indication of AER/Fgf involvement in the pathogenesis of these syndromes. It thus appeared likely that another mechanism must be involved in the elongation of the digits, namely by driving chondrogenesis in a distally orientated manner.…”

“…In studying a mouse model for BDA1 (see Fig. 3), caused in humans by mutations in Indian hedgehog (IHH; Gao et al, 2001), Gao et al (2009) showed that this condition was caused by an impaired distal outgrowth of the digit condensation due to reduced commitment of distal progenitor cells. Ihh is produced by cells within the cartilage condensation and then diffuses across the growth plate, thereby forming a gradient .…”

“…In the growth plate, Ihh exhibits short-range signaling to the proliferative chondrocytes expressing high levels of Ptc1, and long-range signaling to the periarticular perichondrium of the joints, where it induces parathyroid hormonelike hormone (Pthlh). Gao et al (2009) showed that a BDA1 mutation in Ihh (Ihh p.E95K) led to a reduced binding affinity of Ihh to both Ptc1 and Hip1, thereby increasing the Ihh signaling range. This resulted in reduced shortrange Ihh signaling in the growth plate chondrocytes but conversely caused elevated long-range Ihh signaling in the periarticular perichondrium, leading to increased expression of Pthlh.…”

Indian hedgehog mutations causing brachydactyly type A1 impair Hedgehog signal transduction at multiple levels

“…In a mouse model for BDA1, with a human-equivalent p.E95K mutation, signaling potency was reduced, and the range of signaling was enhanced. This mutation affected chondrocyte differentiation in the growth plate, delaying endochondral bone formation, and impaired recruitment of mesenchymal cells into the distal cartilage of the developing digit, causing the brachydactyly phenotype [27]. However, detailed biochemical characterization of the three key mutations that line the surface of this calciumbinding interaction groove has not been performed, and whether these mutations lead to a similar physical and biochemical outcome is not clear.…”

“…We have previously shown that the E95K mutation affected the affinity of this binding [27]. Thus, we performed the same cell-based competition assay for the D100E and E131K mutants, and compared the results with those obtained from the WT and E95K IHH proteins.…”