Brachydactyly A1 (BDA1) is a rare disorder characterized by the disproportionate shortening of fingers and/or toes with or without symphalangism. Mutations in Indian hedgehog signaling molecule (IHH), which impair the effect of functional IHH protein derived from its precursor IHH, are commonly identified in patients with BDA1 or acrocapitofemoral dysplasia (ACFD). The ultrasound phenotype of fetuses with IHH mutations has rarely been described. To better understand the consequences of IHH mutation, we analyzed the characteristics of a Chinese fetus with BDA1 caused by a novel heterozygous IHH mutation. Clinical data and genomic DNA were collected from the proband and family members. Whole-exome sequencing (WES) was performed to identify potential causative mutations. Sequence analysis was performed to investigate the conservation of the affected leucine residue in IHH. Protein 3D modeling was performed to predict the effects of the mutation on protein structure. In vitro overexpression transfection experiments in human embryonic kidney 293T (HEK293T) cell lines were performed to evaluate the pathogenicity of the identified mutation. The fetal proband carried a novel heterozygous mutation in IHH (NM_002181.4: c.331_333delCTG, NP_002172.2: p.Leu111del) inherited from the father; this mutation manifested as shortening of the limbs, with more severe shortening observed in the proximal extremities than in the distal extremities, as evidenced by ultrasound. The Leu111 residue is highly conserved among vertebrates, and deletion of this residue destabilizes the protein structure. Western blotting analysis of HEK293T cells in overexpression transfection experiments revealed that the Leu111del mutation led to an increase in the level of the IHH precursor and a reduction in the level of functional IHH protein compared with those in HEK293T cells expressing wild-type IHH, indicating that this mutation might cause IHH protein dysmaturity. The novel heterozygous mutation c.331_333delCTG (p.Leu111del) in the IHH gene is the likely cause of BDA1 in this Chinese fetus. This mutation causes IHH protein maturation failure. These findings contribute to our understanding of the molecular pathogenesis of BDA1 and the clinical identification of fetal BDA1.

The etiology of short stature is heterogeneous. The disturbance of endochondral ossification and cartilage matrix synthesis caused by genetic mutations often causes short height combined with skeletal deformities in children. Some patients with minor skeletal abnormalities, such as short fingers and mild limb shortening, may be overlooked by clinicians and misdiagnosed as idiopathic short stature (ISS) or growth hormone deficiency (GHD). We conducted a detailed investigation of laboratory and imaging examinations on a family with short stature and non-classical brachydactyly type A1 (BDA1) and summarized the clinical features. They received whole exome sequencing (WES) to reveal the possible genetic variation. A heterozygous mutation in the Indian hedgehog gene (IHH) (c.387_388insC, p.Thr130Hisfs*18) was found in the two siblings and their mother. The siblings both started recombinant human growth hormone (rhGH) therapy (rhGH: 33 µg/kg/day) and followed up for 4 years. After treatment, the siblings' height improved significantly, and they acquired a significant increase in the height standard deviation score (SDS) (the boy: +2.54, the girl: +1.86) during the 4-year therapy. No noticeable adverse effect was observed during rhGH treatment. We found a novel heterozygous pathogenic mutation in the IHH gene in a family and detailed the phenotype with short stature and non-classical BDA1. The therapy of rhGH showed promising effects. To avoid misdiagnosis, clinicians should not overlook minor skeletal anomalies in patients with short stature, especially those with a family history.

Apoptosis occurs during development when a separation of tissues is needed. Synovial joint formation is initiated at the presumptive site (interzone) within a cartilage anlagen, with changes in cellular differentiation leading to cavitation and tissue separation. Apoptosis has been detected in phalangeal joints during development, but its role and regulation have not been defined. Here, we use a mouse model of brachydactyly type A1 (BDA1) with an IhhE95K mutation, to show that a missing middle phalangeal bone is due to the failure of the developing joint to cavitate, associated with reduced apoptosis, and a joint is not formed. We showed an intricate relationship between IHH and interacting partners, CDON and GAS1, in the interzone that regulates apoptosis. We propose a model in which CDON/GAS1 may act as dependence receptors in this context. Normally, the IHH level is low at the center of the interzone, enabling the "ligand-free" CDON/GAS1 to activate cell death for cavitation. In BDA1, a high concentration of IHH suppresses apoptosis. Our findings provided new insights into the role of IHH and CDON in joint formation, with relevance to hedgehog signaling in developmental biology and diseases.

Heterozygous Indian Hedgehog gene (IHH) variants are associated with brachydactyly type A1 (BDA1). However, in recent years, numerous variants have been identified in patients with short stature and more variable forms of brachydactyly. Many are located in the C-terminal domain of IHH (IHH-C), which lacks signaling activity but is critical for auto-cleavage and activation of the N-terminal (IHH-N) peptide. The absence of functional studies of IHH variants, particularly for those located in IHH-C, has led to these variants being classified as variants of uncertain significance (VUS). To establish a simple functional assay to determine the pathogenicity of IHH VUS and confirm that variants in the C-terminal domain affect protein function. In vitro studies were performed for 9 IHH heterozygous variants, to test their effect on secretion and IHH intracellular processing by western blot of cells expressing each variant. IHH secretion was significantly reduced in all mutants, regardless of the location. Similarly, intracellular levels of N-terminal and C-terminal IHH peptides were severely reduced in comparison with the control. Two variants present at a relatively high frequency in the general population also reduced secretion but to a lesser degree in the heterozygous state. These studies provide the first evidence that variants in the C-terminal domain affect the secretion capacity of IHH and thus, reduce availability of IHH ligand, resulting in short stature and mild skeletal defects. The secretion assay permits a relatively easy test to determine the pathogenicity of IHH variants. All studied variants affected secretion and interestingly, more frequent population variants appear to have a deleterious effect and thus contribute to height variation.

This study aimed to identify the molecular defects and clinical manifestations in a Chinese family with brachydactyly (BD) type A1 (BDA1) and multiple-synostoses syndrome 2 (SYNS2). A Chinese family with BDA1 and SYNS2 was enrolled in this study. Whole-exome sequencing was used to analyze the gene variants in the proband. The sequences of the candidate pathogenic variant in GDF5 was validated via Sanger sequencing. I-TASSER and PyMOL were used to analyze the functional domains of the corresponding mutant proteins. The family was found to have an autosomal-dominantly inherited combination of BDA1 and SYNS2 caused by the S475N variant in the GDF5 gene. The variant was located within the functional region, and the mutated residue was found to be highly conserved among species. Via bioinformatic analyses, we predicted this variant to be deleterious, which perturb the protein function. The substitution of the negatively charged amino acid S475 with the neutral N475 was predicted to disrupt the formation of salt bridges with Y487 and impair the structure, stability, and function of the protein, consequently, the abnormalities in cartilage and bone development ensue. A single genetic variant (S475N) which disrupt the formation of salt bridges with Y487, in the interface of the antagonist- and receptor-binding sites of GDF5 concurrently causes two pathological mechanisms. This is the first report of this variant, identified in a Chinese family with BDA1 and SYNS2.