In a 2013 Diffusion article,1 I highlighted the report by Dr. Mathieu Lemaire and colleagues on recessive mutations in DGKE, the gene for diacylglycerol kinase-ε, as a cause of atypical hemolytic uremic syndrome (aHUS).2 Recall, aHUS is a form of thrombotic microangiopathy (TMA). TMA is a histopathologic term for a distinctive lesion of small vessels characterized by swelling and detachment of the endothelium, accumulation of protein and cellular debris in the subendothelium, and obstruction of vessels by fibrin and platelet-rich thrombi. TMA also refers to the conditions themselves that produce the histopathologic TMA lesion (the “thrombotic microangiopathies”). The phenotypic spectrum of TMA includes microangiopathic hemolytic anemia, thrombocytopenia, and ischemia and injury of organs (especially the kidney, in the case of aHUS). Practically, aHUS refers to a form of TMA that is not thrombotic thrombocytopenic purpura (TTP; which can be differentiated from HUS in most cases by a severe reduction in ADAMTS13 activity in TTP but not HUS), not HUS caused by Shiga toxin (verocytotoxin) -producing Escherichia coli (STEC-HUS), and not a secondary form of aHUS, like S. pneumoniae–related HUS. The 2013 report by Dr. Lemaire and colleagues was noteworthy because abnormal activation of the alternative pathway of complement was then a feature of all known specific forms of primary aHUS. Indeed, some have used the term aHUS to be synonymous with complement-mediated HUS (complement-HUS). In contrast, the patients with DGKE-related aHUS had no evidence of complement dysregulation. This distinction is important therapeutically, because complement-directed therapy with eculizumab is an effective life- and kidney-sparing therapy for many patients with aHUS, but it does not appear to benefit most patients with DGKE-related aHUS.
In a new report in Blood, Dr. Sarah Bruneau and colleagues (some of whom contributed to the report by Dr. Lemaire and colleagues) describe mechanisms by which inactivating mutations of DGKE could cause aHUS without dysregulation of the complement system.3 DGKε is an intracellular lipid kinase, present in glomerular endothelial cells, podocytes, and platelets, that preferentially phosphorylates arachidonic acid-containing diacylglycerol (DAG) and terminates DAG-mediated signaling. Although the exact regulatory roles of DGKε are not known, arachidonic acid-containing DAG is known to activate protein kinase C (PKC), so loss of DGKε in endothelial cells could increase the activation of PKC and signaling through downstream networks, such as MAP kinase–mediated pathways.
Using cultured endothelial cells, Dr. Bruneau and colleagues showed that siRNA knockdown of DGKE induces ICAM-1, E-selectin, and tissue factor expression through the upregulation of p38-MAPK–mediated signals, findings consistent with the activated and prothrombotic endothelial phenotype of aHUS. DGKE knockdown also induced endothelial cell apoptosis and impaired endothelial cell migration and angiogenic responses, supporting the hypothesis that inactivating DGKE mutations could promote the development and maintenance of vascular endothelial injury. In contrast, endothelial injury is caused by abnormal activation and deposition of complement in most other forms of aHUS. The investigators then showed that DGKε knockdown modulated the expression of some endothelial cell membrane complement-regulatory proteins, markedly decreasing MCP (CD46) and slightly increasing DAF (CD55) expression. Notably, it did so without increasing C3b deposition on those cells.
In summary, the experiments of Dr. Bruneau and colleagues bolster the argument that DGKE deficiency, caused by inactivating, recessive DGKE mutations, results in activation and damage of endothelial cells and causes aHUS independently of complement activation. Of note, consumption of serum C3 has been reported in some patients with DGKE-associated aHUS. The authors speculate that apoptotic DGKε-deficient endothelial cells could release microparticles that promote cleavage of circulating C3, and that this secondary activation of complement could amplify the endothelial damage that results from inactivation of DGKE. Some patients with DGKE-associated aHUS have also been reported to harbor additional mutations in genes of the complement system, which could modify disease severity and possibly explain the low serum C3 levels of some patients. Complement-directed therapy with eculizumab may not be effective for patients with isolated DGKE mutations. However, this possibility should not prevent or delay empiric therapy with eculizumab when a clinical diagnosis of aHUS is first made, because genetic diagnosis takes time, and some patients with DGKE-associated aHUS may have secondary complement dysregulation. Empiric eculizumab therapy can be stopped at a later time if it proves ineffective, and if a non–complement-mediated form of aHUS is eventually established. Finally, the reports of aHUS associated with mutations in THBD (thrombomodulin),5 PLG (plasminogen),6 and DGKE indicate that both the complement and coagulation systems can play a role in the development of aHUS.
Dr. Quinn indicated no relevant conflicts of interest.