Campomelic dysplasia (CD, OMIM: #114290) is a rare autosomal dominant skeletal dysplasia syndrome caused by rearrangement of 17 chromosome and mutations in SOX9 gene. The diagnosis is usually made after 20 weeks of pregnancy by ultrasonography characterized by malformation of skeletal including bowing of the legs, especially the tibias, small scapular bones, and cleft palate (1). Otherwise, a small chest, eleven pairs of ribs, micrognathia, flat face, and hypertelorism are also featured. CD is a lethal syndrome due to respiratory distress related to small chest and tracheobronchial hypoplasia (2). In two-thirds of reported 46, XY karyotype affected individuals existed male to female sex reversal or had ambiguous genitalia because of the essential role of SOX9 in sex determination (3, 4).

SOX9 (SRY-related HMG-box gene 9, OMIM: 608160), located in chromosome 17q24.3, is an essential transcription factor for both sex and skeletal development. Anomalies of SOX9 are the main causes of CD, and the rearrangement involving the long arm of chromosome 17 will interrupt the upstream of SOX9 (5, 6). Most reported cases of CD are caused by intragenic heterozygous mutations in SOX9 gene, and the deletions of SOX9 represent strong evidence for the dosage-dependent action of SOX9 protein in normal chondrogenesis (7, 8).

Heterozygous de novo mutations are the main cause of CD and sex reversal. The detected shortness and bowing of tubular lower bones by ultrasonography could lead to the suspect of CD. Here we reported on novel heterozygous terminated mutation in SXO9 gene (c.1249C > T; p.Q417*) in a CD-affected individual in a Chinese family, and we present new evidence for the pathogenesis of SOX9 in CD.

In this study, a novel mutation (c.1249C > T; p.Q417*) was identified in the affected fetus of a Chinese family by WES and Sanger sequencing. Our results were consistent with previous studies in humans and animals that heterozygous mutations in SOX9 may lead to CD (10, 11). This mutation occurred within the C-terminus of SOX9 and was presumably thought to led to a truncated SOX9 protein and affected the transactivation of SOX9. Our study further supports the recommendations released by the International Society for Prenatal Diagnosis, which highlight the application of WES when a fetal anomaly was observed (12).

CD is a lethal, autosomal dominant skeletal dysplasia usually caused by a heterozygous mutation of SOX9 and often accompanied by sex reversal (4). The SOX9 protein mainly consists of several domains, including a dimerization domain (DIM), a DNA-binding domain (high-mobility group, HMG), two transactivation domains, and a proline/glutamine/alanine (PQA)-rich domain (13). The missense, nonsense, frameshift, consensus splice site, and loss of function mutations located on one of the three exons of SOX9 are the main causes of CD (14). Otherwise, the reciprocal translocation involving SOX9 or its regulatory region also led to CD (15). The reported cases of CD that SOX9 being completely deletion over the past few decades were thought to represent evidence for the dosage-dependent action of SOX9 protein in normal chondrogenesis (8, 16). However, Csukasi et al. (14) revealed dominant-negative mutations of SOX9 in CD-affected individuals, and the reported deletion of SOX9 also overlapped the upstream enhancer region (7, 17, 18). Thus, it is difficult to confirm the haploinsufficiency of SOX9 in CD.

The correlation between the type and position of mutations of SOX9 with resulting phenotype is also lacking. Kwok et al. (19) reported two patients with the same frameshift mutation, but one presented with male phenotype and the other displayed a female phenotype with XY sex reversal. The reported Y440X mutation that presumably resulted in a truncated protein but retains some transactivation function has been described as a less lethal form of CD. The patient carrying the nonsense mutation Q117X has been reported performing well over a 10-year period. However, other reported patients with missense mutations such as Q375X and E400X died in the neonatal period (20).

The CD-causing mutations are usually located in the HGM domain and transactivation domain. SOX9 gene has two transactivation domains located in the middle (TAM) and the C-terminus (TAC), interacting with transcriptional co-activators or basal transcriptional machinery components. SOX9 transactivation domains work synergistically or independently of each other to activate chondrocyte-specific genes (13). A recent study revealed an evolutionarily conserved EΦ[D/E]QYΦ motif in TAM, playing a critical role in the transactivation function of SOX9 protein (21). The missense mutation in EΦ[D/E]QYΦ motif of SOX18 showed impaired transactivation (22). In addition, the interaction of SOX9 with β-catenin and TRAP230 required TAC, and the reported mutation of TAC (R394G and R437C) which retained normal transactivation of Col2a1 caused testicular dysgenesis without CD and sex reversal, a less lethal result (23, 24). However, the terminated mutation (Q412X) causes a severe phenotype and has a dramatically reduced effect on the SOX9 activation of Col2a1 (14). In our case, we reported a novel terminated mutation (p.Q417*) of SOX9, causing severe and lethal clinical phenotypes, including the leading causes of fetal mortality such as small thoracic, micrognathia, and cleft palate. This mutation is located in the TAC transactivation domain and the terminated mutation p.Q417* presumably resulted in a truncated SOX9 protein and impaired transactivation. The phenotype caused by mutation p.Q417X located in TAC of SOX9 was consistent with CD, providing increasing evidence for the correlation between the clinical and radiographic phenotype and the extent to SOX9 mutations affect transactivation.

In conclusion, there is growing evidence to support the role of SOX9 in the pathogenesis of CD, but the link between mutations and clinical phenotypes has not been established. Recent studies have shown that the pathogenicity of SOX9 is related to the activity of transactivation of mutant SOX9 protein. Severe clinical phenotype is closely related to the reduced activation of Col2a1. Mutations in the two transactivation domains of SOX9 have received additional attention, and this study provides a truncated mutation located in the TAC domain of SOX9, which may affect the transactivation activity of SOX9. Our study confirmed the clinical phenotype of this variant. Future research on SOX9 mutations should pay more attention to the correlation between SOX9 mutation and transactivation activity.

The results of genetic and ultrasound reports supported the diagnosis of CD, and the couple decided to terminate the pregnancy. The parents of the fetus provided written informed consent to participate in this study. Written informed consent was obtained from the couple for the publication of potentially identifiable images or data included in this article.