Mutations in the transcription factor gene SOX18 underlie recessive and dominant forms of hypotrichosis-lymphedema-telangiectasia

Abstract

Hereditary lymphedema is a developmental disorder characterized by chronic swelling of the extremities due to dysfunction of the lymphatic vessels. Two responsible genes have been identified: the vascular endothelial growth factor receptor 3 (VEGFR3) gene, implicated in congenital lymphedema, or Milroy disease, and the forkhead-related transcription factor gene FOXC2, causing lymphedema-distichiasis. We describe three families with an unusual association of hypotrichosis, lymphedema, and telangiectasia. Using microsatellite analysis, we first excluded both VEGFR3 and FOXC2 as causative genes; we then considered the murine ragged phenotype, caused by mutations in the Sox18 transcription factor, as a likely counterpart to the human disease, because it presents a combination of hair and cardiovascular anomalies, including symptoms of lymphatic dysfunction. Two of the families were consanguineous; in affected members of these families, we identified homozygous missense mutations in the SOX18 gene, located in 20q13. The two amino acid substitutions, W95R and A104P, affect conserved residues in the first alpha helix of the DNA-binding domain of the transcription factor. In the third family, the parents were nonconsanguineous, and both the affected child and his brother, who died in utero with hydrops fetalis, showed a heterozygous nonsense mutation that truncates the SOX18 protein in its transactivation domain; this substitution was not found in genomic DNA from either parent and hence constitutes a de novo germline mutation. Thus, we show that SOX18 mutations in humans cause both recessive and dominant hypotrichosis-lymphedema-telangiectasia, suggesting that, in addition to its established role in hair and blood vessel development, the SOX18 transcription factor plays a role in the development and/or maintenance of lymphatic vessels.

Figure  1

Physical features observed in patients. A and B, Male patient from family I at age 3.5 years. Note the sparse scalp hair and absence of eyebrows. C, Lymphedema of the legs in the female patient from family I, at age 26 years. D, Telangiectasias on the palm in the same patient (also at age 26 years). E, Male patient from family III. Note the sparse scalp hair and absence of eyebrows. F, Scrotal telangiectasias in the same patient.

Figure  2

Exclusion of VEGFR3 and FOXC2 as the gene for hypotrichosis-lymphedema-telangiectasia in family I. A, Pedigree of the family. The two affected individuals are represented by blackened symbols, and the index patient is indicated by an arrowhead. Note the consanguinity in the family. B, Microsatellite markers surrounding VEGFR3 and FOXC2. Intermarker physical distances (in kbp), marked on the right, are based on the Human Genome Project sequence draft assembly (UCSC Genome Bioinformatics), as of July 2002.

Figure  3

Families with hypotrichosis-lymphedema-telangiectasia and mutations in SOX18. A, C, and E, Electropherograms from affected members of families I, II, and III and unrelated controls, demonstrating mutations G455C, T428A, and C865A (in the protein, A104P, W95R, and C240X, respectively). WT = wild type. B, D, and F, Family pedigrees (with the individual for whom sequence is shown indicated by an arrowhead) and MnlI and DdeI restriction-fragment analysis (for families I and III). C = digested control; U = undigested PCR product; W = water control.

Figure  4

Positions of SOX18 mutations in patients with hypotrichosis-lymphedema-telangiectasia. A, Multiple alignment of protein sequences around mutated residues W95 and A104, including representative members of the different SOX protein subfamilies. * = Amino acids conserved in all 20 known SOX proteins. B, Superimposition of SOX18-mutated amino acids over the nuclear-magnetic-resonance model of the homologous HMG domain of SRY (Protein Data Bank PDB ID 1J46), with W95 shown in magenta and A104 shown in red. The structure was drawn with DINO. Positions of disease-causing amino acid substitutions in SOX9, SOX10, and SRY (obtained from Swiss-Prot [entries P48436, P56693, and Q05066, respectively]) are shown in yellow in panels A and B. C, Schematic representation of SOX18, showing positions of the three mutations. Thin boxes represent 5′ and 3′ UTRs, and thick boxes represent translated sequences. Regions corresponding to the DNA-binding domain and the transactivation domain are shown in pink and beige, respectively. Positions corresponding to murine ragged mutations (single-nucleotide deletions) are indicated by red circles.