VEGFR3 (FLT4) is crucial for embryonic lymphangiogenesis, and defects in this receptor can lead to congenital lymphedema type 1A (Milroy disease). This study analyses FLT4 gene sequence in 24 primary lymphedema patients, identifying genetic variants in five patients resembling typical Milroy disease. A novel likely pathogenic variant (c.3028A>C) was identified, and the pathogenicity of two previously described variants (c.3175G>C and c.3298T>C) was supported.

Primary lymphedema is characterized by lymphatic dysplasia in which one variant is Milroy disease. The association with congenital chylothorax is even rarer, with poor outcome. This is the first report to use peripheral lymphovenous anastomosis (LVA) and lymph node-to-vein anastomosis (LNVA) for the management of such condition. A retrospective chart review of patients with Milroy disease with complication of chylothorax between 2019 until 2023 was performed. Clinical assessment and radiologic investigations were reviewed. Six patients with a mean age of 12 ± 3.9 years and disease duration of 10.5 ± 2.8 years were reviewed. Three had International Society of Lymphology stage 3 disease, and the others had stage 2 (late) disease. All had bilateral lower extremity lymphedema and chylothorax with history of chest tube drainage. After LVA and LNVA, significant reduction in extremity volume ( P = 0.028) along with nearly complete resolution of chylothorax were noted during the long-term follow-up (32 ± 17.9 months). Milroy disease complicated with chylothorax remains challenging. This is the first report using peripheral bypass (LVA and LNVA), which resulted in improvement of both lower extremity lymphedema and chylothorax. The utility of this approach represents a promising modality in the management of this devastating condition. Therapeutic, IV.

Rare, deleterious genetic variants in FLT4 are associated with Tetralogy of Fallot (TOF), the most common cyanotic congenital heart disease. The distinct genetic variants in FLT4 are also an established cause of Milroy disease, the most prevalent form of primary hereditary lymphoedema. The phenotypic features of these two conditions are non-overlapping, implying pleiotropic cellular mechanisms during development. In this study, we show that FLT4 variants identified in patients with TOF, when expressed in primary human endothelial cells, cause aggregation of FLT4 protein in the perinuclear endoplasmic reticulum, activating proteostatic and metabolic signalling, whereas lymphoedema-associated FLT4 variants and wild-type (WT) FLT4 do not. FLT4 TOF variants display characteristic gene expression profiles in key developmental signalling pathways, revealing a role for FLT4 in cardiogenesis distinct from its role in lymphatic development. Inhibition of proteostatic signalling abrogates these effects, identifying potential avenues for therapeutic intervention. Depletion of flt4 in zebrafish caused cardiac phenotypes of reduced heart size and altered heart looping. These phenotypes were rescued with coinjection of WT human FLT4 mRNA, but incompletely or not at all by mRNA harbouring FLT4 TOF variants. Taken together, we identify a pathogenic mechanism for FLT4 variants predisposing to TOF that is distinct from the known dominant negative mechanism of Milroy-causative variants. FLT4 variants give rise to conditions of the two circulatory subdivisions of the vascular system via distinct developmental pleiotropic molecular mechanisms.

Hereditary lymphedema 1 is a rare congenital condition, characterized by the development of chronic swelling in body parts. It is highly variable in expression and age of onset with different presentations: from feet edema to hydrops fetalis. This affection is genetically heterogeneous with autosomal dominant inheritance and incomplete penetrance due to a mutation in the FLT4 gene in most cases. In our study, we report on two fetuses harboring congenital lymphedema with FLT4 variation and review the prenatal confirmed ones of the literatures. Our cases were selected within fetuses explored by exome sequencing in a diagnosis setting. Prenatal ultrasonography showed hydrops fetalis in one case and an increased nuchal translucency with hydrothorax in the other. Comparative genomic hybridization array on amniocentesis was normal in both cases. Exome sequencing identified a variation p.(Ser1275Thr) and p.(Ser1275Arg) in fetus 1 and fetus 2 in the FLT4 gene, respectively. A de novo mutation at the same codon was reported in prenatal literature suggesting possible genotype phenotype correlation. Cystic hygroma/hydrops fetalis are possible manifestations of several disorders. This study illustrates how the integration of exome sequencing in prenatal clinical practice can facilitate the diagnosis and genetic counseling of heterogeneous developmental affections.

Milroy disease, known as primary congenital lymphedema, is characterized by chronic tissue swelling due to impaired lymphatic drainage and is inherited in an autosomal dominant manner. This study reports a rare case of Milroy disease affecting siblings from unaffected parents. A one-month-old female infant presented with swelling of the bilateral calf and the dorsum of the feet which had been present since birth. Her 14-month-old brother had a similar presentation since birth with swelling of the bilateral calf and the dorsum of the feet. Milroy disease was diagnosed based on the clinical findings of bilateral lower limb swelling and confirmed by molecular genetic testing. The patient and her family, including her brother, parents, and maternal grandfather, were genetically tested, and two novel missense mutations (NM_182925.4: c.2534T>C; p.Leu845Pro, c.4006G>A; p.Glu1336Lys) were found in the Fms-related tyrosine kinase (FLT4) gene. Mutations segregated by the parents who carried each mutation in the heterozygous state were identified in the patient and her brother. The present case report in which Milroy disease developed in all offspring of parents with a normal phenotype suggests the possibility of a compound heterozygous mutation or non-penetrance during the process of inheritance of Milroy disease.