Key transcription factors (TFs) including SRY-related HMG box-containing protein 18 (SOX18), COUP-TFII, and prospero-related homeobox 1 (PROX1) are responsible for the molecular control of the lymphatic endothelial cell fate. Mutations or truncations in the coding sequence of these TFs can cause a variety of human diseases. In particular, mutations in the transcription factor SOX18 cause the Hypotricosis-Lymphedema-Telangiectasia syndrome (HTLS), a congenital condition characterized for presenting swelling of tissue due to malformation or malfunction of lymphatics (Lymphedema), furthermore the people who present the disease also present alopecia (Hypotricosis) and the presence of small dilated vessels near the surface of the skin (Telangiectasia). Some patients also present with renal defect and hypertension, in a new variant of the disease called HLTRS (Hypotricosis-Lymphedema-Telangiectasia renal defect syndrome). So far two types of SOX18 variants have been described to be associated with HTLS: missense mutations that lack the transactivation domain and nonsense mutations, in the DNA binding domain. We aimed at identifying how the protein-protein interactions (PPI) network is affected by these mutations and correlate interactions losses with phenotypic outcomes.
We first established a SOX18 interactome, validating interaction with 31 SOX18 potential co-factors. Then we characterized the stability of the mutated proteins by single-molecule fluorescence and measured physical interactions using different in vitro techniques. Our data show that the SOX18 mutants are thermally stable and still able to bind DNA. However they are losing a significant number of protein partners, the extent and profile of these interaction losses was found to depend on the mutations. This preliminary data set is slowly providing a molecular view of the disease and is opening the door for potential pharmaceutical treatment.