The molecular biology of thrombotic microangiopathy

Abstract

Thrombotic microangiopathy, which includes thrombotic thrombocytopenic purpura (TTP), shiga-toxin-associated hemolytic uremic syndrome (Stx-HUS) and atypical HUS, is characterized by the development of hyaline thrombi in the microvasculature resulting in thrombocytopenia, microangiopathic hemolysis, and organ dysfunction. Renal failure is a predominant complication of both Stx-HUS and atypical HUS, whereas neurological complications are more prominent in TTP. Other disorders such as lupus or bone marrow transplantations may occasionally present with features of thrombotic microangiopathy. Recent studies have found autoimmune inhibitors or genetic mutations of a von Willebrand factor (VWF) cleaving metalloprotease ADAMTS13 in patients with TTP. In approximately 30-50% of patients with atypical HUS, mutations have been detected in complement factor H, membrane cofactor protein (CD46), or factor I. All three proteins are involved in the regulation of complement activation. Additionally, autoantibodies of factor H have been described in patients without genetic mutations. These advances illustrate that dysregulation of VWF homeostasis or complement activation owing to genetic or autoimmune mechanisms may lead to the syndrome of thrombotic microangiopathy.

Figure 1

Schematic depiction of TTP due to ADAMTS13 deficiency. A. When a large VWF multimer is attached to the extracellular matrix at a site of injury, it is conformationally unfolded by high levels of wall shear stress to an elongated form, providing the substrate for platelets adhesion and hemostasis. B. In the normal circulation, VWF and platelets do not interact to form aggregates because ADAMTS13 cleaves the VWF multimer whenever one or more of its cleavage sites are exposed by shear stress. This process keeps VWF in inactive, globular forms as it become progressively smaller in size. C. In the absence of ADAMTS13, VWF multimers eventually become fully unfolded by shear stress, causing intravascular platelet thrombosis characteristic of TTP.

Figure 2

Schematic depiction of the domain structures of proteins involved in the pathogenesis of TTP and atypical HUS. A. ADAMTS13. Sig: signal peptide; PrP: propeptide; MP: metalloprotease domain; Cys: cysteine-rich region. TSR: thrombospondin type 1 repeat. The amino acid sequence of the zinc-binding motif in the MP domain and the RGDS sequence in the cysteine-rich region are shown. Brackets indicate the segments exhibiting VWF cleaving activity or binding with the IgG of TTP. Mutations of ADAMTS13 have been detected throughout the entire span of the protein. B. Factor H. The protein consists of twenty complement control protein repeats (CCPR). The three C3b binding sites and three polyanion binding domains are indicated. The first C3b binding site (gray) has cofactor activity for complement factor I. The major polyanion-binding site at CCPR20 is highlighted in black. The majority of the mutations associated with HUS cluster at the C-terminal end, particularly CCPR20, which is critical for host binding. C. Complement factor I. The single-chain precursor form is depicted. FIMAC: factor I major attack complex; Kazal_2: Kazal-type serine protease inhibitor domain; SRCR: Scavenger receptor cysteine-rich domain; LDLR-A: LDL receptor domain class A; Trypsin: trypsin type protease domain. Four mutations have been detected in the protease domain of the protein. D. MCP (membrane cofactor protein). CCPR: complement control protein repeat. STP: serine/threonine/proline-rich domain; TM: transmembranous domain; Tail: cytoplasmic anchor. The region of the protein critical for C3b binding and cofactor activity is highlighted in the bracket. HUS mutations have been detected in CCPR4.

Figure 3

A pathogenetic scheme of atypical HUS due to complement dysregulation. Mutation or autoantibodies of factor H (CFH), factor I (IF), or membrane cofactor protein (MCP, or CD46) cause defective regulation of the alternative pathway. Excessive complement activation generates cytoactive molecules such as C3a, C5a and C5b9, which injure both renal parenchymal and endothelial cells. Endothelial injury may create a prothrombotic state, leading to local accumulation of platelet-fibrin thrombi. Thrombocytopenia and hemolysis are apparent only when microvascular thrombosis is widespread. In the absence of HUS, renal parenchymal injury may continue to occur, leading to the development of renal failure. Incessant complement activation may increase the risk of pyogenic infection by depleting essential complement components.