A case of campomelic dysplasia in whom a new mutation was found in the SOX9 gene

Diverse Regulation but Conserved Function: SOX9 in Vertebrate Sex Determination

Sex determination occurs early during embryogenesis among vertebrates. It involves the differentiation of the bipotential gonad to ovaries or testes by a fascinating diversity of molecular switches. In most mammals, the switch is SRY (sex determining region Y); in other vertebrates it could be one of a variety of genes including Dmrt1 or dmy. Downstream of the switch gene, SOX9 upregulation is a central event in testes development, controlled by gonad-specific enhancers across the 2 Mb SOX9 locus. SOX9 is a ‘hub’ gene of gonadal development, regulated positively in males and negatively in females. Despite this diversity, SOX9 protein sequence and function among vertebrates remains highly conserved. This article explores the cellular, morphological, and genetic mechanisms initiated by SOX9 for male gonad differentiation.

De Novo SOX4 Variants Cause a Neurodevelopmental Disease Associated with Mild Dysmorphism

SOX4, together with SOX11 and SOX12, forms group C of SRY-related (SOX) transcription factors. They play key roles, often in redundancy, in multiple developmental pathways, including neurogenesis and skeletogenesis. De novo SOX11 heterozygous mutations have been shown to cause intellectual disability, growth deficiency, and dysmorphic features compatible with mild Coffin-Siris syndrome. Using trio-based exome sequencing, we here identify de novo SOX4 heterozygous missense variants in four children who share developmental delay, intellectual disability, and mild facial and digital morphological abnormalities. SOX4 is highly expressed in areas of active neurogenesis in human fetuses, and sox4 knockdown in Xenopus embryos diminishes brain and whole-body size. The SOX4 variants cluster in the highly conserved, SOX family-specific HMG domain, but each alters a different residue. In silico tools predict that each variant affects a distinct structural feature of this DNA-binding domain, and functional assays demonstrate that these SOX4 proteins carrying these variants are unable to bind DNA in vitro and transactivate SOX reporter genes in cultured cells. These variants are not found in the gnomAD database of individuals with presumably normal development, but 12 other SOX4 HMG-domain missense variants are recorded and all demonstrate partial to full activity in the reporter assay. Taken together, these findings point to specific SOX4 HMG-domain missense variants as the cause of a characteristic human neurodevelopmental disorder associated with mild facial and digital dysmorphism.

The presence of diminished white matter and corpus callosal thinning in a case with a SOX9 mutation

SOX9 is responsible for campomelic dysplasia (CMPD). Symptoms of CMPD include recurrent apnea, upper respiratory infection, facial features, and shortening of the lower extremities. The variant acampomelic CMPD (ACMPD) lacks long bone curvature. A patient showed macrocephaly (+3.9 standard deviations [SD]) and minor anomalies, such as hypertelorism, palpebronasal fold, small mandible, and a cleft of soft palate without long bone curvature. From three months of age, he required tracheal intubation and artificial respiration under sedation because of tracheomalacia. Cranial magnetic resonance imaging was normal at one month of age but showed ventriculomegaly, hydrocephaly, and the corpus callosum thinning at two years of age. Exome sequencing revealed a de novo novel mutation, c. 236A>C, p (Q79P), in SOX9. Sox9 is thought to be crucial in neural stem cell development in the central and peripheral nervous system along with Sox8 and Sox10 in mice. In humans, neuronal abnormalities have been reported in cases of CMPD and ACMPD, including relative macrocephaly in 11 out of 22 and mild lateral ventriculomegaly in 2 out of 22 patients. We encountered a two-year old boy with ACMPD presenting with tracheomalacia and macrocephaly with a SOX9 mutation. We described for the first time an ACMPD patient with acquired diminished white matter and corpus callosal thinning, indicating the failure of oligodendrocyte/astrocyte development postnatally. This phenotype suggests that SOX9 plays a crucial role in human central nervous system development. Further cases are needed to clarify the relationship between human neural development and SOX9 mutations.