A Novel Heterozygous IHH c.331_333del Mutation Identified in a Fetus with Brachydactyly Type A1 Causes IHH Protein Maturation Failure in HEK293T Cells

Abstract

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.