INTRODUCTION

Campomelic dysplasia (CD; OMIM #114290), a rare genetic disorder character- ized by skeletal abnormalities and male to female sex reversal, was first described by Maroteaux et al. in 1971 [1]. Due to the skeletal anomalies in the airway and thoracic cage, patients with CD often present with life-threatening respiratory failure, particularly during neonatal periods [2]. The incidence rate of CD has been reported at approximately 0.05-0.09 per 10,000 live births [3], and no reliable data exists on the prevalence which is currently estimated as 1:40,000 to 1:80,000 [2]. Data have been reported across case reports and small series, and three Korean cases have been published [4-6]. The SOX9 (17q 24.3-q25.1) gene is the only known causal gene for CD [7-9]. To date, approximately 90 causative mutations have been identified in SOX9, which are listed on the Human Gene Mutation Database (HGMD; http://www.hgmd.org). Most mutations are sporadic and familial CD is rarely found, although CD is inherited in an autosomal dominant manner.

The term “campomelic” comes from the Greek for “bent limb”, but this characteristic is neither specific to CD nor a mandatory finding for diagnosis. Bowing of the long bone is present in many other skeletal dysplasias (e.g., osteoporosis imperfections), and is absent in about 10% of cases of CD, a condition called acampomelic CD, which shares the same OMIM number as CD [2].

CLINICAL CHARACTERISTICS

A wide spectrum of pathologic clinical features involving multiple organs is observed in CD, not only skeletal anomalies but also dysmorphic facial features and male sex reversal (Table 1). Skeletal dysplasia, which is the most representative characteristic of CD, includes bowing and shortening of long bones, hip dislocation, clubfoot, and pretibial skin dimples due to bowing of the lower legs [2]. Skeletal radiographs of patients with CD typically show spine anomalies, scoliosis, hypoplastic scapulae, bell-shaped thorax with 11 pairs of ribs, and vertically oriented narrow iliac wings (Fig. 1).

Fig. 1.

Radiographs of a 7-month-old girl with campomelic dysplasia showing (A) a bell-shaped thoracic cage, 11 pairs of ribs, scapular hypoplasia, (B) vertical ischia, bowing of the right proximal femur, and (C) thoracic kyphosis.

Patients with CD also exhibit dysmorphic facial features known as the Pierre-Robin sequence (PRS) with cleft palate, including high forehead, midface hypoplasia, hypertelorism, long philtrum, micrognathia, and low-set ears. Another specific characteristic of CD is male to female sex reversal that results from the abnormal development of male reproductive organs. Approximately 75% of 46,XY patients have ambiguous or complete female genitals [2]. Sensorineural and conductive hearing impairment and laryngotracheomalacia are common, and ventriculomegaly, which is usually of the communicating type, is also found in patients with CD. Some patients have reported minor congenital heart defects, such as an atrioseptal defect [4,5].

DIAGNOSIS

There is no consensus on the diagnostic criteria for CD, but clinical diagnosis can be established based on clinical features and radiographic interpretation [2]. In particular, radiographic findings, including short bent limbs, bell-shaped thoracic cages with 11 pairs of ribs, or hypoplastic scapulars, are consistent and reliable diagnostic cues for CD.

Similar clinical manifestations between CD and some diseases should be considered during differential diagnosis. For example, patients with spondyloepiphyseal dysplasia congenita, one of the type 2 collagen disorders caused by COL2A1 mutations, present a dysmorphic faces with cleft palate and short limbs [10]. Also, patients with Stickler syndrome, a mild form of type 2 collagen disorder, have similar facial features to CD. Bowing long bones suggest a rare but fatal skeletal disorders not only in CD but also in other conditions. In a study by Tonni et al. [11] molecular and histopathological investigations revealed that four fetuses presented bowing limbs on prenatal ultrasound and were diagnosed with CD with a de novo mutation in SOX9, osteogenesis imperfecta type II, Cumming syndrome, and femoral-facial syndrome.

SOX9 is the only known causative gene of CD, and the presence of a heterozygous pathogenic variant in SOX9 can confirm the diagnosis of CD [2]. Sequence analysis of SOX9 allows the detection of the majority of pathogenic variants. Using chromosomal microarray analysis, large deletions and duplications that cannot be detected by sequence analysis can be identified. Rarely, a reciprocal translocation that involves the SOX9 locus but does not result in SOX9 copy number changes can be found through a karyotype [12].

GENES AND MOLECULAR PATHOGENICITY

The SOX9 gene codes for a high mobility group (HMG) DNA-binding domain-containing transcription factor that plays an important role in multiple organ development, especially in chondrogenesis and sex determination. In 1994, Foster et al. established a high resolution map across a 20 Mb region of chromosome 17q24.1-q25.1 that was previously revealed to contain the locus that is responsible for CD [7-9].

SOX9 is a master regulator of chondrogenesis [13]. SOX9 is expressed in prechondrocytes and chondrocytes during embryogenic development, and is an essential transcription factor that controls chondrocyte differentiation and cartilage formation. SOX9 is co-expressed with COL2A1, which encodes type II collagen, the major cartilage matrix protein, and SOX9 protein binds to sequences in COL2A1 and activates chondrocyte-specific enhancers in nonchondrocytic cells [14,15]. As COL2A1 expression is directly regulated by the SOX9 protein, mutations in SOX9 lead to decreased COL2A1 expression, resulting in skeletal dysplasia.

Moreover, SOX9 expression is also detected in non-chondrogenic tissues, such as the neural crest, gonad, otic vesicle, lung, notochord, neural tube, pancreas, and cardiac cushions during embryonic development in vertebrates [14,16,17]. These results suggest that SOX9 mutations are related to deformities that include multiple organs not only skeletal systems in CD. Embryonically, the expression of SOX9 is closely correlated with the formation of neural crest cells, a group of cells unique to vertebrates that contribute to the development of the craniofacial complex [18]. SOX9 acts as a craniofacial regulatory element and is associated with large deletions or translocations found in non-syndromic PRS [19]. This can be the basis for explaining the formation of cleft palate and the typical facial features of CD.

In mammalian male development, the SRY (sex-determining region on the Y chromosome) gene is fundamental and initiates testis differentiation of the undetermined male gonads [20]. SOX9 (SRY-related HMG-box gene 9) is another key gene involved in a cascade for testicular formation. Shortly after SRY activation, SOX9 is activated and involved in the male sex development process at various stages, including glycogenesis in pre-Sertoli cells, coelomic epithelium proliferation, mesonephric migration, vasculogenesis, and testicular cord formation [21]. However, SOX9 gene mutations do not cause sex reversal in all male patients. Approximately 25% of male patients with CD do not exhibit sex reversal [2], as seen in the patient with a normal male phenotype of the two 46,XY karyotypes of Korean patients (Table 2). The cooperative dimerization of SOX9 with other genes is essential for chondrogenesis but not for gonadal development [22]. In sex development, SOX9 proteins act as monomers and bind sex-determining genes, such as SF1, which might explain why CD is not always accompanied by sex reversal.

GENOTYPE-PHENOTYPE CORRELATIONS

To date, 140 SOX9 gene mutations have been reported on the HGMD, among which only 87 have been identified as CD-causing mutations. Of these 87 mutations, almost all of them were detected using a sequence analysis of SOX9: 39 missense/nonsense mutations, 16 small insertions, nine small deletions, three splicing mutations, one small indel mutation, and one regulatory abnormality. Ten large deletion mutations and eight complex rearrangements have been also reported. In three Korean patients with CD, three different novel heterozygous mutations in the SOX9 gene have been reported; two were small deletions [4,6] and one was a nonsense mutation [5].

In CD, there is no clear genotypic phenotype correlation regarding mutation type or location and clinical severity [23]. However, some degree of correlation is observed in some cases. The farther the translocation breakpoint is from SOX9, the milder the phenotype, including its impact on male genitalia and skeletal anomalies [24]. This property was observed in a family with acampomelic CD which was transmitted through several generations [25]. Moreover, several cases with isolated disorders of sex development were identified with duplications or deletions involving a region located approximately 600 kb upstream of SOX9 [26]. Notably, among the 53 mutations not related to CD out of 140 SOX9 mutations, more than half have been classified as isolated disorders of sex development (n= 29), and other phenotypes of PRS with or without minor skeletal anomalies (n= 17). Most of these mutations are associated with large deletions, large insertions/duplications, and chromosomal rearrangements. These types of SOX9 mutations appear to be associated with mild phenotypes that do not involve multiple organs.

MANAGEMENT OF MANIFESTATIONS

During the neonatal period, CD is a life-threatening condition primarily because it causes respiratory failure. Fatal respiratory failure is associated with tracheobronchomalacia or cervical instability, rather than with a hypoplastic thoracic cage [2]. Many of the surviving patients require a tracheostomy to relieve laryngotracheomalacia, and home mechanical ventilator support is needed because of the restrictive lung condition caused by hypoplastic thoracic cage, scoliosis, and tracheobron-chomalacia. All three Korean patients with CD needed tracheostomy for laryngotracheomalacia. One patient died of respiratory failure at 4 months of age [4]. Of the two surviving patients, one was able to discontinue mechanical ventilation after a closure operation of the atrioseptal defect [5], while the other patient required home ventilator support [6].

Although fatal complications decrease after infancy, mental retardation, global developmental delay, feeding problems, scoliosis, and respiratory complications will continue to need medical intervention. For patients with medical conditions such as cleft palate, cervical instability, hip subluxation, and clubfoot, proper management is required. Hearing aids are needed in patients with hearing impairments. Regular radiographic examinations of the spinal curvature are necessary to determine the progression of scoliosis. For cervical instability and progressive kyphoscoliosis that compromises lung function, surgical treatment is required, whereas bracing is usually not helpful [27]. Patients that are 46,XY phenotypic females mostly have internal male genital organs. Gonadectomy is recommended because of the increased risk of malignant changes, as reported in a phenotypic female who developed gonadoblastoma at age 3 [28].

FAMILY COUNSELING

Most reported SOX9 mutations are de novo mutations found in sporadic cases. However, due to the autosomal dominant inheritance manner of CD, a genetic approach to the proband’s parents and siblings is also required. As some families with mosaicism have been reported [29], prenatal evaluation and diagnosis are recommended for future pregnancies as there is a risk of recurrence in these families. For fetuses at risk of CD, fetal ultrasound allows the detection of clues, such as increased nuchal transparency, micrognathia, short bowed limbs, and hypoplastic scapulae [30]. Molecular genetic testing through DNA analysis or chromosomal analysis from amniocentesis is confirmative.

CONCLUSION

CD is a very rare genetic disorder, but it is lethal in affected patients, especially during the neonatal period. This review focused on the clinical and genetic characteristics of patients with CD. A better understanding of patients with CD will allow clnicians to provide more appropriate care to patients. Early diagnosis and management of the associated complications in affected patients can prevent deterioration of their clinical conditions and improve their quality of life.