Pathophysiology and Molecular Research in Lymphedema

Introduction

Lymphedema can develop as a result of developmental or genetic abnormalities intrinsic to the lymphatic system (primary lymphedema) or secondary to extrinsic factors (infections, trauma, malignancy, obesity, etc.) that impair lymphatic function. Although the common denominator in both lymphedema subtypes is inadequate lymphatic function, the pathophysiology of the disease is complex, and the phenotype that is displayed clinically is highly variable.

Recent research has shed light on some of the molecular mechanisms that regulate disease progression and translate lymphatic abnormalities or injury to fibroadipose deposition. While this research is exciting and has provided some hope for therapies that may be effective in treating lymphedema, there is much that remains unknown. It is not clear, for example, why some patients have a mild, smoldering form of lymphedema while others develop a rapidly progressive, fulminant disease course. We do not understand why some areas of the limb are more prone to fibroadipose deposition and disease progression than others are. Similarly, it is unclear why a subset of patients develop collateral lymphatic vessels that bypass the zone of lymphatic injury, thus avoiding lymphedema development. Cellular and molecular studies with clinical biopsy specimens and preclinical models of lymphedema are therefore a critical and significant biomedical need and can help elucidate the mechanisms that regulate these pathologic changes. This understanding will improve not only our ability to prevent lymphedema development but also our ability to treat this disease medically and surgically. This chapter will serve as a review of the recent cellular and molecular studies that have shed light into the pathophysiology of lymphedema.

Pathophysiology of Primary Lymphedema

DNA sequencing studies have identified genetic abnormalities in patients with primary lymphedema. These genetic defects can be transmitted in families as a result of germline mutations or can develop from spontaneous somatic mutations. Genetic defects in patients with primary lymphedema have a broad range of effects: abnormal expression or function of growth factors that regulate lymphatic growth and repair, lymphatic valve abnormalities, leaky lymphatics, and changes in lymphatic smooth muscle cells. It is important to note, however, that the genetic defects that cause the development of lymphedema in most patients with primary lymphedema remain unknown. Nevertheless, the most common cause of primary lymphedema is deficits in the number and function of collecting lymphatics. These defects likely are present at birth and worsen over time, eventually resulting in overt lymphedema development.

The timing and severity of primary lymphedema are highly variable and may be modulated by the penetrance of genetic abnormalities, environmental factors, or hormonal fluctuations. For example, patients with subtle genetic lymphatic abnormalities may be asymptomatic due to the excess capacity of the lymphatic system to transport macromolecules. However, additional insults to the lymphatic system—for example, surgical injury or obesity—may lead to further declines in lymphatic function and overt development of lymphedema. In other patients with germline mutations of the lymphatic system, genetic abnormalities may have limited penetrance and involve only a subset of lymphatic channels. These patients may develop lymphedema later in life, as the pathophysiology of the disease advances to a point where the capacity of the lymphatic system to transport macromolecules is exceeded. Thus, the timing of lymphedema development in primary lymphedema is variable, with some patients exhibiting the disease shortly after birth— congenital lymphedema —or, more commonly, disease development later in life, termed lymphedema praecox or lymphedema tarda .

Congenital lymphedema, most commonly manifesting as lower extremity swelling in female patients, accounts for approximately 10–25% of all cases of primary lymphedema. These patients have variable severity of the disease, with some exhibiting mild swelling, while others display severe pathology. Further, primary lymphedema may present unilaterally or bilaterally, with differential severity between limbs. Milroy disease is a familial, sex-linked disease and is the classic form of congenital lymphedema, accounting for 2–3% of all patients with congenital primary lymphedema. Patients with Milroy disease have heterozygous inactivating mutations of vascular endothelial growth factor receptor 3 (VEGFR3) and usually present with leg swelling and chylothorax shortly after birth. VEGFR3 activation by its ligands—vascular endothelial growth factor C (VEGF-C) and VEGF-D—is a critical mechanism regulating lymphatic endothelial cell differentiation, proliferation, migration, and function. Homozygous mutations of VEGFR3 are embryologically lethal in mice; in contrast, heterozygous inactivating mutations allow for some lymphatic development. As a result, patients with Milroy disease have hypoplastic lymphatic vessels of the extremities, usually affecting the lower extremities more severely.

Lymphedema development before the age of 35 is referred to as lymphedema praecox . Patients with this form of lymphedema are most commonly female (ratio of females to males is 4:1) and usually develop unilateral lower extremity lymphedema around the time of puberty. The timing of disease development and its overlap with female hormone fluctuations have led some investigators to hypothesize that sex hormones may play a role in disease development/progression. However, the cellular mechanisms that regulate this response remain unknown, and only a few studies have investigated the role of female sex hormones in regulating lymphatic function. Pathologically, most patients with lymphedema praecox have fewer initial lymphatics and hypoplastic collecting vessels—although the severity of the disease is highly variable.

Patients who develop primary lymphedema after the age of 35 have lymphedema tarda . This presentation of primary lymphedema is infrequent and is a diagnosis of exclusion. Lymphedema tarda most commonly involves the lower extremity of women, and the genetic causes of the disease remain largely unknown. However, recent genetic studies have shown that some patients with lymphedema tarda have mutations in FOXC2 , a gene that regulates lymphatic valve development.

Most researchers concede that an arbitrary classification of primary lymphedema based on the age at which symptoms present is not particularly helpful when evaluating individual patients. This approach does not, for example, delineate the primary pathophysiology of the disease and is therefore not useful for developing targeted interventions. Instead, most patients are treated with palliative measures such as compression and physiotherapy, aiming to prevent disease progression. More recent studies have developed classification schemes that combine the clinical presentation of the disease with genetic sequencing. This approach categorizes congenital lymphedemas into five main groups: syndromic, systemic or visceral, disturbed growth, congenital onset, and late onset. This classification is more precise and provides more pathological information that may aid diagnostic or therapeutic interventions.