Brachydactyly A-1 mutations restricted to the central region of the N-terminal active fragment of Indian Hedgehog

Abstract

Mutations in the gene Indian Hedgehog (IHH) that cause Brachydactyly A-1 (BDA1) have been restricted to a specific region of the N-terminal active fragment of Indian Hedgehog involving codons 95, 100, 131, and 154. We describe two novel mutations in codons 128 and 130, not previously implicated in BDA1. Furthermore, we identified an independent mutation at codon 131 and we also describe a New Zealand family, which carries the 'Farabee' founder mutation and haplotype. All of the BDA1 mutations occur in a restricted area of the N-terminal active fragment of the IHH and are in contrast to those mutations causing an autosomal recessive acrocapitofemoral dysplasia, whose mutations are located at the distal N- and C-terminal regions of IHH-N and are physically separated from the BDA1-causing mutations. The identification of multiple independent mutations in codons 95, 100, and now in 131, implicate a discrete function for this region of the protein. Finally, we present a clinical review of all reported and confirmed cases of BDA1, highlighting features of the disorder, which add to the spectrum of the IHH mutations.

Figure 1

Alignment of the amino acid sequence of IHH with those of other species, as well as with human SHH and DHH. The amino acid sequence of the human IHH was aligned with those of mouse Ihh, chicken Ihh, African clawed frog (banded hedgehog), and zebrafish (echidna hedgehog). The alignment has been anchored to the Homo sapien IHH amino acid sequence, and the amino acid numbers are listed above. These were further aligned with the amino acid sequences of human SHH and DHH. Residues associated with disease are indicated by an arrow and by the wild-type amino acids in red. Heterozygous mutations D100N, R128Q, T130N, and E131K are associated with BDA1 in this study (denoted by an asterisk^*). Other BDA1-causing heterozygous mutations E95K, D100E, and E131K were described by Gao et al; D100N was described by McCready et al, Giordano et al, and McCready et al; E95G was described by Kirkpatrick et al; T154I was described by Liu et al; and delE95 was described by Lodder et al. Homozygous mutations, P46L and V190A, were described by Hellemans et al and are associated with ACFD.

Figure 2

Pedigrees of four families with BDA1. Brachydactyly type A1 is transmitted as an autosomal dominant trait in all families. Probands are denoted by arrows. Numbers represent the sample number assigned to the DNA of individuals who participated in this study. Those members of family 4 above the dashed line represent members who were described earlier by Nissen.

Figure 3

Hand radiographs of affected members of families 1, 3, and 4.

Figure 4

Mutations in IHH. (a) Family 1, c.383G>A; (b) Family 2, c.389C>A; (c) Family 3, c.391G>A; (d) Family 4, c.298G>A.

Figure 5

Three-dimensional reconstruction of the N-terminal active fragment of Indian Hedgehog defining the positions of the amino acids, whose mutation have been implicated in Brachydactyly type A1 or Acrocapitofemoral dysplasia pathogenesis. To compare the locations of the Indian Hedgehog mutations, the crystal structure of the amino-terminal domain of mouse Shh (1VHH.pdb) was used because of its high similarity to IHH. The equivalent positions were utilized. Numbers indicate the amino acid positions. ACFD mutations (blue); BDA1 mutations at codon 100 (red), codons 95, 131, and 154 (green), and codons 128, 130 (orange) are shown. N and C termini are indicated. The ribbons represent α-helices. The three representations are shown from a perspective, in which the N and C termini are closest to the viewer; at the top of the molecule; and furthest away from the viewer, projecting into the page. All the BDA1 mutations appear to cluster in the central portion of IHH-N.