SOX genes encode transcription factors which act as critical cell fate determinants during embryogenesis. They function as both architectural and modular transcription factors, and are related by a highly conserved HMG domain that sequence-specifically binds and bends DNA. Mutations in SOX genes can cause severe developmental defects and may be associated with human congenital diseases. Sox9 and Sox10 are two closely related SOX genes with genetic mutations causing the human developmental disorders, campomelic dysplasia (CD) and Waardenburg- Hirschsprung disease, respectively. Sox10 controls neural crest lineage specification, delineating three distinct neural crest cell types, including peripheral nervous system glial cells, Schwann cells and melanocytes. Sox9 is expressed in condensing chondrocytes and is crucial for skeletogenesis, regulating the expression of several chondrocyte-specific structural proteins. CD is characterised by severe skeletal dysmorphogenesis, neonatal death and XY sex reversal.
This thesis documents the cloning and characterisation of Sox8, demonstrating functional redundancy between SOX8 and SOX9 during sex differentiation, and adding to the global understanding of SOX biology.
Structurally very similar to Sox9 and Sox10, Sox8 is presumed to function as a modular transcription factor, containing two trans-activation domains which synergistically activate transcription in vitro. The human gene is located on chromosome 16p, approximately 1 Mb from the telomere, and is deleted in all ATR- 16 patients characterised to date. ATR-16 is an œ-thalassemia variant that also displays developmental defects including mental retardation, facial malformation, short stature and some genital abnormalities. Based on expression analysis, SOX8 is a good candidate gene for the developmental defects in ATR-16 patients.
The association of SOX genes with sex determination is well documented, with the eponymous member of the SOX gene family, Sry, the pivotal Y-linked male sex determining gene. The involvement of Sox9 in sex determination became apparent from the sex reversal phenotype of many XY CD patients. Here I report on the sexually dimorphic expression of Sox8 in Sertoli cells around the time of sex determination and throughout sexual differentiation. Like SOX9, SOX8 binds the Amh promoter and interacts with SF1 to synergistically activate Amh expression, demonstrating functional redundancy between SOXS and SOX9 during sex differentiation. However, activation of Amh expression by SOX8 is consistently lower than that achieved by SOX9. This observation was explained by the reduced DNA binding affinity and less potent trans-activation domains of SOX8, compared with SOX9.
Functional redundancy has been suggested for a number of SOX genes, and is often considered to reflect evolution in progress, with shared expression patterns and functionality a remnant of a common ancestor. Sox8 and Sox9 are closely related and are believed to have diverged from a recent common ancestor. The shared function of SOX8 and SOX9 in eutherian sex differentiation, provides a good model to study, not only SOX gene redundancy but also SOX protein function. The function of Sox8 in other species remains to be studied, specifically in species where Sox9 expression is too late to directly activate Amh expression.
Finally, not all XY CD patients undergo sex reversal. We hypothesise that SOX8 ameliorates for the loss of SOX9 in the XY patients that do not undergo sex reversal, and that the predominance of sex reversal in CD patients is due to the diminished efficiency of SOX8 to enhance Amh expression, compared with SOX9.