Atypical haemolytic uraemic syndrome: a case of rare genetic mutation

Discussion

Thrombotic microangiopathy (TMA) is a group of disorders characterised by a triad of non-immune haemolytic anaemia, thrombocytopenia and organ injury.2 aHUS is a rare variant of TMA, which particularly affects the kidneys but has potential to cause multiorgan dysfunction, as in the presenting child. Though aHUS is responsible for only 10% of cases in children, progression to end-stage renal disease (ESRD) is observed in more than 50% of affected children.3 4

The complement system is an integral part of innate immunity, responsible for fighting infections and handling damaged cells and debris. The alternative complement pathway is one of the three complement pathways that constitutively functions as an arm of our innate immune system. Activation of the alternative pathway results in the generation of C3 convertase and consequent formation of C5 convertase, which is responsible for enzymatic cleavage of C5, generating C5a (anaphylatoxin) and C5b. The sequential assembly of C5b and C6–C9 causes formation of the lytic membrane attack complex (MAC).5

To prevent unrestrained activation and consequent endothelial injury, the complement system is rigorously controlled by surface regulators. Patients with aHUS often have mutations in the key complement genes or autoantibodies against these surface regulators. As a result, they are incapable of efficiently protecting the endothelium from complement activation. Uncontrolled activation and amplification of the alternative complement pathway leads to excessive formation of the MAC, which damages the vascular endothelial cell surface.6

The presenting child had digenic mutations in CFI and C3. CFI and C3 are critical regulators of the complement system. The loss-of-function mutation in CFI leads to uncontrolled activation and subsequent consumptive depletion of C3. On the other hand, the gain-of-function mutation in C3 causes increased formation and stabilisation of the C3 degradation product, which leads to increased C3 deposition onto the endothelial cells. The net effect of these mutations is hypocomplementaemia (figure 2).7 The frequency of CFI and C3 mutations in patients with aHUS is approximately 6%, according to global aHUS registry.8

In silico predictive analysis can be performed to determine the effect of amino acid substitutions on the structure or function of a protein without performing functional studies. In addition, the potential pathogenicity of these causative mutations can be predicted by various methods.9 In our patient, the in silico predictions of the CFI variant were found to be pathogenic by Polymorphism Phenotyping V.2 (PolyPhen-2), Sorting Intolerant From Tolerant (SIFT) and Likelihood Ratio Test (LRT). Similarly, in silico predictions of the C3 variant were also found to be pathogenic by PolyPhen-2, SIFT, LRT and MutationTaster2.

These pathogenic mutations cause dysfunction in the complement cascade, which leads to uninhibited complement deposition on the endothelial cells. Subsequently, proteins and cellular infiltrates accumulate on the cell surface creating a prothrombotic state. This leads to formation of microvascular thrombi, causing shearing of red blood cells as they flow through turbulent areas with endothelial injury, creating schistocytes. The affected microvasculature ultimately get partially occluded, causing ischaemic end organ damage.10 Eventually, the triad of non-immune haemolytic anaemia, thrombocytopenia and AKI is produced.

aHUS can manifest at any age and onset is generally sudden. Among children, the most common trigger would be any type of infection. Although diarrheal episode commonly precedes STEC HUS, it may rarely be a precipitating factor for aHUS like in our patient.3 Clinical presentation of aHUS is acute pallor, oedema, oligoanuria and hypertension. aHUS may present as mild or severe multisystemic disease. Renal impairment is frequent; most common symptoms are haematuria, proteinuria, hypertension and azotaemia.11 Extrarenal manifestations may be seen in 20% of patients and a catastrophic progressive multiorgan dysfunction is seen in about 5% of patients. These include cardiac, neurological, respiratory, liver, peripheral vascular and gastrointestinal involvement.10 This child had AKI, elevated liver enzymes and neurological involvement in the form of seizures.

A similar case reported a 10-year-old girl with history of fever, abdominal pain, vomiting and bloody diarrhoea for 3 days. She had features of HUS with delayed renal and haematological recovery despite plasma therapy. Later, she was detected with mutation in CFI. She was initiated on eculizumab 600 mg/week, following which she recovered completely.12

According to the Indian Society of Paediatric Nephrology, the diagnostic criteria for aHUS is microangiopathic haemolytic anaemia (haemoglobin <100 g/L, haematocrit <30% and schistocytes ≥2% in the peripheral blood smear, with either elevated LDH >450 IU/L or undetectable haptoglobin), thrombocytopenia (platelet count <150 000/μL) and AKI (increase in serum creatinine by 50% over baseline level).13

In most of the aHUS cases, C4 levels are normal. Reduced C3 levels are frequently seen in patients with mutations in complement regulatory factors. At the same time, the demonstration of normal C3 and C4 concentrations does not exclude aHUS because not all patients with aHUS present with hypocomplementaemia.14 Genetic testing is needed to confirm the underlying mutation in the complement regulatory proteins. Renal biopsy will not be required to establish the diagnosis, when the patient presents with typical triad of aHUS.15

Conventional therapy of aHUS is supportive, with attention towards management of AKI and systemic complications. Plasmapheresis has been effective in improving haematological parameters in 78% of children with aHUS. Despite improvement in the haematological parameters, around 48% died or developed ESRD at 3-year follow-up. Patients with aHUS developing renal failure may require haemodialysis and renal transplantation.11 16 The overall rate of post-transplant recurrence in aHUS is approximately 60%. The risk tends to be highest (nearly 80%) in children with C3 mutation and nearly 50% in children with CFI mutation. Patients with ESRD and C3 or CFI mutation should be considered for hepatorenal transplantation because of the high risk of graft failure due to disease recurrence associated with renal transplantation.16

Recently, eculizumab has been identified as curative treatment. Eculizumab is an anti-C5 monoclonal antibody that inhibits MAC formation and C5b generation. Eculizumab has led to favourable outcome in the disease course. With the aim of reducing dose frequency, a longer-acting congener, ravulizumab, has also been approved for treatment of aHUS (figure 2).11 17 Currently, these drugs are unavailable in India. However, eculizumab can be procured in India through pharmaceutical companies with necessary documents.

An open-labelled non-randomised multicentre clinical trial was conducted to determine the safety and efficacy of eculizumab among children, with a sample population of 22 patients aged between 5 months and 17 years. Overall, eculizumab was found to be well tolerated with no deaths or meningococcal infections reported. By the 26th week, 14 patients recovered completely; 18 achieved haematological normalisation; and 16 had more than 25% reduction in serum creatinine.18 Another multicentre trial was conducted recently to evaluate the safety and efficacy of ravulizumab with 18 children (0–18 years) enrolled. By the end of 26th week, 14 children achieved complete remission; 16 had haematological normalisation; and 15 had more than 25% reduction in serum creatinine, with no deaths, unexpected adverse events or meningococcal infections.19

aHUS is a multifactorial disorder driven by genetic, environmental and immunological determinants. It is of utmost importance to differentiate aHUS from other TMAs and initiate adequate treatment to improve the prognosis. Though anticomplement reagents have potential to modify the natural course of this disorder, the lack of availability and high cost of currently available anticomplement therapy pose a great challenge in treating this condition. Awareness of this disorder’s broad spectrum of clinical presentations, along with development of precise diagnostic studies, will help enhance our diagnostic efficiency, thereby allowing us to optimally use the emerging array of complement-based therapies.