Lymphatics maintain fluid homeostasis, immune surveillance, and tissue integrity. Here, we identified the E26 transformation-specific transcription factors Erg and Fli1 as essential cooperative regulators of lymphatic integrity and function. Using inducible lymphatic endothelial cell-specific deletion in mice, we demonstrated that combined loss of Erg and Fli1 in adults results in fatal lymphatic failure, including chylothorax, chylous ascites, and impaired lymphatic drainage. Single-cell transcriptomic analysis revealed that loss of Erg and Fli1 causes disrupted lymphatic heterogeneity and dysregulation of key lymphatic genes, including valve-specific gene profiles. Erg and Fli1 coordinated lymphatic-immune crosstalk by transcriptionally regulating C-C motif chemokine ligand 21, which mediates DC trafficking. Erg or Fli1 loss also induced proinflammatory and prothrombotic gene expression, further contributing to lymphatic dysfunction. During embryonic development, the codeletion led to lymphatic mispatterning and loss of valve-initiating lymphatic endothelial cell clusters. The impact of loss of Erg and Fli1 function on lymphatic development in mice is consistent with FOXC2 mutations in lymphedema-distichiasis syndrome or ERG gene variants underlying primary lymphedema in humans. Moreover, Erg and Fli1 were required for regenerative lymphangiogenesis and lymphatic repair following injury in adults. Our findings establish Erg and Fli1 as core transcriptional regulators of lymphatic identity, integrity, and function.

Lymphedema-distichiasis syndrome (LDS) is an autosomal dominant genetic disorder associated with mutations in forkhead box C2 (FOXC2) gene, critical for lymphatic endothelial cell (LEC) differentiation. LDS patients suffer from swelling of limbs (lymphedema) due to excessive lymph accumulation and are characterized by the presence of additional row of eyelashes (distichiasis). Here, we generated human induced pluripotent stem cells (hiPSCs) from LDS patient-derived peripheral blood mononuclear cells (PBMCs). LDS hiPSC line allows in vitro modeling and investigation of the molecular mechanisms of LDS upon differentiation towards LEC.

Efficient lymph flow is ensured by lymphatic valves (LVs). The mechanisms that regulate LV development are incompletely understood. Here, we show that the deletion of the GPCR sphingosine 1-phosphate receptor-1 (S1PR1) from lymphatic endothelial cells (LECs) results in fewer LVs. Interestingly, LVs that remained in the terminal ileum-draining lymphatic vessels were specifically dysfunctional. Furthermore, tertiary lymphoid organs (TLOs) formed in the terminal ileum of the mutant mice. TLOs in this location are associated with ileitis in humans and mice. However, mice lacking S1PR1 did not develop obvious characteristics of ileitis. Mechanistically, S1PR1 regulates shear stress signaling and the expression of the valve-regulatory molecules FOXC2 and connexin-37. Importantly, Foxc2+/- mice, a model for lymphedema-distichiasis syndrome, also develop TLOs in the terminal ileum. Thus, we have discovered S1PR1 as a previously unknown regulator of LV and TLO development. We also suggest that TLOs are a sign of subclinical inflammation that can form due to lymphatic disorders in the absence of ileitis.

To explore the prenatal and postnatal features and genetic characteristics of patients with Lymphedema-Distichiasis syndrome (LDS) due to variants of FOXC2 gene. A retrospective analysis was carried out on the phenotypic information, fetal ultrasound image, and genetic testing of two Chinese pedigrees diagnosed at the Third Affiliated Hospital of Zhengzhou University. A literature review was also carried out by searching the China National Knowledge Infrastructure (CNKI), Wanfang Database, and PubMed databases dated from January 2010 to June 2024 using keywords "Lymphedema-Distichiasis syndrome " and "FOXC2 ". This study has been approved by the Medical Ethics Committee of the Third Affiliated Hospital of Zhengzhou University (Ethic No. 2021-046-01). Neither family was found to harbor chromosomal aneuploidy or pathogenic CNVs larger than 100 kb. The fetuses from pedigree 1 and pedigree 2 were respectively found to be heterozygous for a c.361C>T (p.R121C) variant and a c.168C>A (p.Y56*) variant of the FOXC2 gene. Both variants were paternally derived. Based on the guidelines from the American College of Medical Genetics and Genomics (ACMG), the variants were classified as pathogenic and likely pathogenic, respectively. Literature search has identified 20 articles, and combined with our cases, a total of 117 patients were identified. Among them, 13 had shown prenatal phenotypes, primarily with increased nuchal translucency (NT) (12/13), urinary abnormalities (5/12), and fetal edema (4/13). Postnatal phenotypes were observed in 110 cases, mainly as distichiasis (87/110) and lymphedema (73/110). Only 6 cases had both prenatal and postnatal phenotypes. A total of 32 genetic variants were identified. The primary prenatal manifestations of LDS include increased NT, fetal edema, pleural and abdominal effusion, and separation of renal collecting system. Postnatal phenotypes are primarily characterized by lymphedema, distichiasis, and spinal extradural arachnoid cysts. Discovery of the c.168C>A variant has expanded the spectrum of FOXC2 gene mutations in China.

Lymphedema distichiasis syndrome is one of the most frequent phenotypes of primary lymphedema, even so, its prevalence is still low. This syndrome courses with the appearance of abnormal eyelashes and distichiasis during childhood or puberty. This can cause a notable discomfort on our patients, especially at such an early age. The clinic evaluation of this signs must make us have in mind this group of syndromes, because in the case of lymphedema distichiasis syndrome, we can certainly diagnose it with the genetic analysis of the FOXC2 gen on patient's serum. With this we could prevent, diagnose and treat the ophthalmologic syndrome alongside the rest of systemic symptoms of this syndrome in a more effective way, giving our patients a higher quality of life.