Candidate gene analysis in primary lymphedema

Primary lymphedema, the accumulation of protein-rich fluid in the interstitial space, is the clinical manifestation of mutations involved in lymphatic development and function. Mutations in three genes, VEGFR3, FOXC2, and SOX18, cause primary lymphedema. However, mutations in these three genes only account for a fraction of primary lymphedema. To identify other genes mutated in primary lymphedema, we resequenced twenty-five biologically plausible candidate genes for lymphedema in a large collection of primary lymphedema families.

Candidate genes were selected on the basis of gene expression in lymphatic endothelial cells, differential antigenic expression in lymphatics, and mouse studies of lymphatic development. The gene sequence was downloaded from GenBank and sequence primers designed to amplify 1 Kb of the 5' sequence, exons and flanking intron-exon boundaries, and 500 bp of the UTR of each gene. No common causative mutations were observed among the 25 genes screened. Single mutations were observed in elastin microfibril interfacer (EMILIN1), lymphocyte cytosolic protein 2 (LCP2), fatty acid binding protein 4 (FABP4), protein tyrosine kinase SYK (SYK), neuropilin-2 (NRP2), SpSRY-box 17 (SOX17), vascular cell adhesion molecule 1 (VCAM1), ROR orphan receptor C (RORC), and vascular endothelial growth factor B (VEGFB). Among these, the mutations in EMILIN1, RORC, LCP2, SYK, and VEGFB failed to segregate with lymphedema. The mutations in FABP4 (2), NRP2, SOX17, and VACM1 are consistent with being causative mutations, but occur in families too small to convincingly confirm cosegregation of mutation and phenotype.

We excluded mutation in 21 biological candidate genes as a common cause of primary lymphedema. Mutations in FABP4, NRP2, SOX17 and VCAM1 are consistent with causality and follow up of these four genes are warranted. The evidence for FABP4 harboring lymphedema mutations is discussed.