Atypical hemolytic uremic syndrome (aHUS) is a type of thrombotic microangiopathy (TMA) characterized by acute kidney injury, thrombocytopenia, and microangiopathic hemolytic anemia. However, internationally unified diagnostic criteria for aHUS have not yet been established.

In the Kidney Disease: Improving Global Outcomes (KDIGO) classification, TMA is classified into four categories: STEC-HUS induced by Shiga toxin-producing Escherichia coli (STEC) infection, thrombotic thrombocytopenic purpura (TTP), “primary aHUS,” and “secondary aHUS.” TTP is caused by a marked deficiency in a disintegrin-like and metalloproteinase with thrombospondin type 1 motifs 13 (ADAMTS-13) activity, either because of genetic abnormalities or acquired autoantibodies. Conversely, more than 90% of HUS cases are STEC-HUS, and the remaining 10% are aHUS, which does not involve STEC infection (1,2). The term “primary aHUS” is used when an underlying abnormality of the alternative pathway of complement is strongly suspected, and other causes of “secondary aHUS” have been excluded.

Pathogenic genetic variants in a patient with aHUS include complement factor H (CFH), membrane cofactor protein (MCP; CD46), complement factor I (CFI), complement 3 (C3), complement factor B (CFB), thrombomodulin (THBD), complement factor H related protein 1 (CFHR1), complement factor H related protein 5 (CFHR5), and diacylglycerol kinase epsilon (DGKE) (3-6). Fujisawa et al. reported that, in an analysis of 118 aHUS patients in Japan, the frequencies of C3, CFH, MCP and DGKE genetic abnormalities were 32 (27%), 10 (8%), 5 (4%), and 1 (0.8%), respectively, and the frequency of anti-CFH antibodies was 20 (17%). Unidentified genetic abnormalities were reported in 36 patients (30%) (7). However, even in some patients, a complement abnormality could not be identified.

In such cases, several triggers are considered to be related to the manifestation of aHUS. The triggers include autoimmune conditions, transplants, pregnancy, infections, drugs, and metabolic conditions, which we have collectively classified as “secondary aHUS.”

We herein report a patient with an underlying genetic variant of MCP who developed aHUS triggered by the onset of acute pancreatitis.

A 49-year-old man with no medical or family history of TMA presented with pancreatic swelling and an elevated IgG4 level of 139 mg/dL. Endoscopic ultrasound-guided fine-needle aspiration (EUS-FNA) was performed for suspected autoimmune pancreatitis (AIP) on day X. The pathology results of the pancreas submitted after EUS-FNA were negative for AIP, and the pancreas was diagnosed as normal.

The laboratory data are shown in Table 1. After EUS-FNA, the patient complained of left-sided abdominal pain, and the pancreatic amylase level was found to have increased to 1,547 U/L. The patient's vital signs were as follows: body temperature, 37.5°C; and blood pressure, 136/74 mmHg. Contrast-enhanced computed tomography (CT) revealed a contrast-impaired area of the pancreatic tail and increased peripancreatic fatty tissue density (Figure A). The patient was diagnosed with acute pancreatitis after EUS-FNA. Treatment was started with fasting, analgesia with acetaminophen, treatment with nafamostat-mesylate, and large extracellular fluid replacement of 4,000 mL/day. Serum creatinine increased from 0.75 to 1.33 mg/dL, and the estimated glomerular filtration rate (eGFR) decreased from 87 to 46.5, indicating acute kidney injury. The urinary protein excretion was 6.15 g/gCr, and hematuria (3+) was observed. Blood samples showed progression of anemia from Hb 15.5 to 12.4 mg/dL, and platelets decreased from 313,000 to 5,000/μL. Schistocytes were detected at a rate of 50-70/2,000 red blood cells (RBCs), lactate dehydrogenase (LDH) levels were elevated to 2,469 U/L, and haptoglobin was 23 mg/dL.

Based on these findings, plasma exchange therapy (PE) was initiated on day X+4. Hemodialysis (HD) was initiated because of oliguria and a tendency toward weight gain. Methylprednisolone (500 mg) was administered. An analysis to evaluate the course of TMA revealed serum levels of C3, C4, and CH50 at 91, 25, and 37.8/mL, respectively. The patient had no diarrhea, and Shiga toxin levels were negative. The ADAMTS-13 activity was 44%, and ADAMTS-13 inhibitor was negative. The Coombs test results were negative. Based on these findings, we diagnosed the patient with secondary TMA secondary to pancreatitis. PE and HD were terminated on day X+7 because the TMA activity had settled and the fluid volume status improved.

The patient was found to have suffered a relapse of inflammation and showed elevated pancreatic enzymes on day X+7. CT showed pancreatic swelling and hypo-absorptive areas in the peripancreatic, gastric fold, and transverse colonic mesentery, suggesting the formation of walled-off necrosis (WON) (Figure B). The WON improved with antibiotic treatment. After confirming no recurrence of postprandial abdominal pain, the patient was discharged on day X+26.

We outsourced next-generation sequencing of complement-related genetic abnormalities to the KAZUSA DNA Research Institute. In the laboratory, CFH, CFI, MCP, C3, CFB, THBD, DGKE, and CFHR5 were analyzed using next-generation sequencing hybridization capture methods for rare single nucleotide substitutions and deletions with an allele frequency of <0.5%. In addition, complement function tests (sheep erythrocyte hemolysis test) and anti-CFH antibody tests were performed. The sheep erythrocyte hemolysis test was performed because it is useful for detecting genetic mutations in CFH and anti-CFH antibodies. The results showed the absence of anti-CFH antibodies (8). Genetic testing revealed a 493-base cytosine (C) to thymine (T) mutation in MCP (MCP P165S), a complement-related gene responsible for atypical HUS, resulting in a missense variant of proline-to-serine substitution and a heterozygous mutation in MCP. Based on the MCP findings, we considered this case to potentially be primary aHUS triggered by acute pancreatitis.

We herein report a patient with a genetic mutation in MCP, a gene associated with the complement-related pathway, who developed aHUS following pancreatitis after EUS-FNA.

In the present case, the patient had a missense variant of MCP (MCP P165S). A previous report indicated a 50% reduction in MCP expression in leukocytes in patients with the MCP P165S variant (9). Another study reported that a 50% reduction in MCP expression leads to a decrease in the binding ability of C3b to <50% compared to normal MCP expression (10). The MCP P165S variant is not registered in Clinvar but is categorized as “likely pathogenic” according to the The American College of Medical Genetics and Genomics guidelines (ACMG guidelines) (11). Esparaza-Gordillio et al. reported families with the same genetic variant as this case and found that patients with only the MCP variant did not manifest aHUS, whereas those with coexisting the CFI mutation and MCPggaac single-nucleotide variant haplotype block, another variant of MCP, did manifest aHUS (9,12,13). The clinical characteristics of the patients with the MCP P165S variant are summarized in Table 2. In the present case, only the protein-coding region exons of CFH, CFI, CD46, C3, CFB, THBD, and DGKE and their intron boundaries were searched using the next-generation sequencer, and the multiple ligation-dependent probe amplification (MLPA) method was not used to detect structural variants. The lack of a genetic analysis using MLPA is considered a limitation of this case (14).

Several triggers are thought to be involved in the manifestation of aHUS, in addition to genetic mutations, including infection, pregnancy, transplantation, metabolic diseases, vasculitis, and pancreatitis, called the “multiple hit hypothesis” (13,15-17). Among the reported triggers, cases of aHUS triggered by pancreatitis are rare, accounting for just 3% (4/110) of secondary aHUS cases (18-21). Previously reported cases of aHUS triggered by pancreatitis were not specifically investigated for complement-related gene mutations, so whether or not complement-related gene mutations other than pancreatitis have any effects on the manifestation of aHUS is unclear.

In the present case, aHUS developed in a patient with a genetic variant (MCP P165S) that originally did not meet the threshold for aHUS development due to pancreatitis after EUS-FNA. This shows that several triggers, not just genetic mutations, are involved in the development of aHUS, supporting the “multiple hit hypothesis.” Patients with MCP variants have a better prognosis and lower probability of developing end-stage kidney disease (ESKD) and death than patients with other genetic mutations, such as CFH, CFI, C3, and THBD (13,17,22).

The patient in this case responded well to plasma exchange therapy and PE. He recovered his kidney function, supporting the relatively good prognosis of patients with MCP mutations compared to other aHUS-related gene mutations. Although the efficacy of eculizumab as a treatment for aHUS has been described in recent years, eculizumab was not used in this case, as the patient had a good clinical course with plasma exchange therapy and HD (17,19,21). In addition, infection with WON after pancreatitis was suspected on day X+7; therefore, it was difficult to use eculizumab, which produces immunosuppressive effects. However, the use of eculizumab should be considered in the future if aHUS relapse is triggered by some event.

In conclusion, we encountered a case of aHUS triggered by acute pancreatitis after EUS-FNA in a patient with a heterozygous variant of MCP (MCP P165S), which is considered to not manifest aHUS. We speculate that the heterozygous variant of MCP in this case did not induce aHUS alone, instead representing a second hit by pancreatitis-induced aHUS.

The authors state that they have no Conflict of Interest (COI).