Primary congenital lymphedema (PCL), also called MD (OMIM: #153100), is caused by developmental lymphatic vascular anomalies, with an estimated prevalence of 1 in 160,000 individuals . We reviewed the reported phenotypes of MD (Table S2: Review of the clinical phenotypes of Milroy disease) and found that MD patients usually exhibit lymphedema at birth with swelling of the lower limbs, and most cases are bilateral . Patients often have a brawny texture and hyperkeratosis of the foot skin (Table S2: Review of the clinical phenotypes of Milroy disease). Other phenotypes associated with MD included hydrocoele in males (37%), “ski jump” toenails (14%) and bilateral pleural effusion  (Table S2: Review of the clinical phenotypes of Milroy disease). Although below-knees lymphedema is the most common phenotype of MD, edema in some patients extends to the thighs [2, 18]. Furthermore, hydrocoele is common in male patients [2, 3, 19–22].
FLT4 (NM_182925.4) gene mutations cause kinase inactivation and MD [6, 7, 22–27]. To date, all mutations have been located in two intracellular kinase domains [6, 7]. In this study, we sequenced the tyrosine kinase coding domains of the FLT4 gene in a large family with hereditary congenital lymphedema and found a missense mutation, c.T2774A, which led to a valine-to-glutamic acid substitution (Fig. 4: Sanger sequencing result of the FLT4 gene c.2774 T-A mutation p.V925E). This missense mutation caused the major phenotypes of MD in the members of our pedigree.
Currently, researchers believe that the major pathologic changes characteristic of MD are aplastic, hypoplastic and dysfunctional cutaneous lymphatic vessels [6, 12], which fail to transport fluid into the venous circulation, resulting in lymphatic fluid stasis and swelling of the extremities [28–31]. That is, lymphatic vessel malformation triggers an increase in interstitial protein-rich fluid, resulting in insufficient lymphatic drainage and transport . As a result, a large amount of protein-rich fluid accumulates in the tissue interstitial spaces and causes hyperplasia of the skin, subcutaneous tissue, and fibrous tissues, resulting in lymphatics that are more difficult to reflux to lymphatic vessels; edematous fluid and adipose tissue accumulate subcutaneously, followed by an inflammatory response that develops and forms a vicious cycle that aggravates the formation of edema [19, 33]. Moreover, the retarded lymphatic flow induces lipogenesis and fat deposition and leads to increased fibrocyte and connective tissue overgrowth [34–36]. Then, the affected skin thickens, hardens, and becomes rough and bulky, forming “elephant skin” over time.
The variability between MD families is high, and patients in one family can show heterogeneity. Two patients in our family suffered from specific clinical phenotypes compared with the other members: patient I1 complained that edema of both legs was aggravated and extended to the roots of the thighs when he had a cold and fever in childhood; patient II1 suffered from heavy edema of the lower extremities and an “elephant-like” skin hyperkeratosis of her legs (Fig. 2 − 1, 2: Bilateral lower limb lymphedema of patient II1) [4, 37, 38]. In contrast, neither the brawny texture of the skin nor the lymphedema of some patients was difficult to observe (II2, II4 and III4). The potential reasons for the heterogeneity in MD are not clear, and further research on FLT4 gene functions is needed to explore the causative factors. Further, genetic mutation might not be the only factor shaping the clinical manifestations of MD, and environmental, genetic, and epigenetic factors and their interactions should be considered.