Identification of a new CD46 gene mutation site in a family with atypical hemolytic uremic syndrome

Abstract

Background

Atypical hemolytic uremic syndrome (aHUS) is a thrombotic microangiopathy resulting from the dysregulation of the alternative complement pathway. Pathogenic variants in complement regulators (e.g., CFH, CFI, CD46, THBD), effectors (C3, CFB), CFHR genes, and non-complement genes (e.g., DGKE, INF2), as well as anti-factor H autoantibodies, play significant roles in disease pathogenesis. Furthermore, numerous CFH-CFHR hybrid genes are increasingly recognized as significant contributors to aHUS pathogenesis. Among these, epidemiological data on CD46-associated aHUS remain limited. Here, we present a case of aHUS associated with a rare novel homozygous mutation in the CD46 gene (c.1127 + 2T > A).

Case presentation

We present a case of a 27-year-old Chinese male diagnosed with atypical Hemolytic Uremic Syndrome (aHUS) at the age of 8, who has experienced seven relapses over a span of 19 years. Whole-exome sequencing (WES) revealed a novel homozygous mutation in the CD46 gene (c.1127 + 2T > A; intron 12 splice site), which is classified as pathogenic according to ACMG guidelines and has not been previously reported. Sanger sequencing confirmed the presence of this variant. Further analyses demonstrated significantly reduced CD46 mRNA and protein expression in the patient’s peripheral blood compared to healthy controls and his mother, as assessed by qPCR and ELISA.

Conclusion

In our study, a novel mutation in the CD46 gene (c.1127 + 2T > A) was identified via WES and confirmed to affect the transcription and translation of CD46, thereby contributing to the pathogenesis of aHUS. This finding broadens the spectrum of CD46 gene variants associated with aHUS, providing a critical basis for clinical diagnosis, genetic counseling, and treatment.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12882-025-04458-9.

Background

aHUS is a thrombotic microangiopathy (TMA) caused by complement dysregulation and is characterized by the triad of thrombocytopenia, microangiopathic hemolytic anemia, and end-organ (primarily renal) damage [1, 2]. aHUS has an annual incidence of 0.23 to 0.42 per million [3] and is commonly observed in children and adolescents, often with familial or recurrent cases. Triggering events, such as infections or pregnancies in susceptible individuals, lead to uncontrollable and persistent activation of the alternative complement pathway and the formation of membrane attack complexes, causing disease [2]. The recurrence rates of atypical aHUS vary significantly among different complement gene mutations. Specifically, mutations in CFI, THBD, CFH or CFH-CFHR hybrid, C3, and CD46 exhibit recurrence rates of 10%-30%, 30%, 50%, 50%, and 70%-90%, respectively [4]. The condition has a poor prognosis, with acute-phase mortality of approximately 25% and progression to end-stage renal disease (ESRD) in approximately 50% of cases [3]. The main causes of aHUS are congenital or acquired abnormalities in the complement alternative pathway. Approximately 30–60% of patients with aHUS exhibit mutations in genes that encode complement regulatory proteins, while anti-factor H autoantibodies are identified in about 10% of cases [4, 5]. Additionally, mutations in the DGKE (diacylglycerol kinase epsilon) gene, which encodes a cytoplasmic signaling protein, and the INF2 (inverted formin 2) gene, which encodes an intracellular actin-binding protein, are also associated with aHUS [6, 7]. Furthermore, numerous CFH-CFHR hybrid genes have been reported in the literature and are increasingly recognised as contributing factors to aHUS pathogenesis [8]. Mutations in the CD46 gene alone are the second most common genetic abnormality in aHUS, accounting for 10–15% of cases [1]. The CD46 gene is located on the long arm of chromosome 1 and is composed of 14 exons spanning over 43 kb. The CD46 protein encoded by this gene is a transmembrane protein consisting of four N-terminal short consensus repeat (SCR) sequences, a serine/threonine (ST)-rich domain, a transmembrane (TM) domain, and a cytoplasmic tail (CYT). The CD46 protein acts as a cofactor for serine protease factor I, cleaving and inactivating C3b and C4b deposited on host cells. CD46 is widely expressed in all nucleated cells and is particularly highly expressed in the kidneys [9]. The majority of CD46 mutations lead to a quantitative protein defect [10–13], and these mutations are associated with various glomerular diseases, especially aHUS, pregnancy-related diseases, and lupus nephritis [14].

In this case report, we presented a patient with aHUS who harbored a rare novel homozygous mutation in the CD46 gene (c.1127 + 2T > A), a pathogenic variant not previously documented in the literature. This discovery enriches the genetic database of aHUS and further suggests that genetic testing is the most effective method.

Case presentation

The patient is a 27-year-old male who was admitted to our hospital on March 24, 2022, owing to “diagnosed aHUS for 16 years, abdominal pain for 2 days, and brown urine for 2 hours”. A timeline of the patient’s medical history is shown in Fig. 1.

Fig. 1.

Patient history timeline. In the diagram, the blue thick arrows represent the treatment timeline, and the grey arrows represent the key time points. The upper part of the arrows indicates recurrence triggers: clinical events that precipitated aHUS episodes, while the lower part indicates treatment measures

The patient first presented to Shanghai Children’s Hospital affiliated with Fudan University on March 6, 2006, due to fever accompanied by cough, sputum production, jaundice, and gross hematuria. Laboratory tests indicated renal function abnormalities, anemia, and thrombocytopenia. He was diagnosed with “aHUS and bronchopneumonia”. Treatment included antibiotics such as cefotaxime and penicillin, plasma transfusions and urine alkalinization. The patient’s renal function, hemoglobin levels, and platelet counts returned to normal, and he was discharged. The patient returned to Fudan University Children’s Hospital on four occasions (October 28, 2006; February 10, 2012; June 24, 2015; and June 6, 2018) due to fever, cough, sputum production, and gross hematuria, with all cases considered aHUS triggered by acute upper respiratory infections. For the episodes in February 2012 and June 2015, the patient underwent plasmapheresis, along with anti-infective and symptomatic supportive therapy. For the other two episodes, the patient received plasma transfusions, anti-infective treatment, and supportive therapy. All four episodes ultimately fully resolved, and renal function and complete blood count tests remained normal at follow-up visits between episodes. Two days prior to this latest admission, the patient experienced upper abdominal pain without any obvious trigger, accompanied by nausea and dry heaving, with no diarrhea or melena. The pain gradually worsened, and two hours before admission, the patient developed tea-colored urine along with lumbar pain, leading to a visit to our hospital’s emergency department. The examination results were as follows: urinalysis (2022-03-24, Emergency): urine protein 2+, urine occult blood 3+; blood amylase, 866 U/L; urine amylase, 1831 U/L; blood creatinine, 281.4 µmol/L; lactate dehydrogenase, 2454.4 U/L; schistocytes, 10%; and complete blood count: white blood cell count, 11.79 × 10⁹/L; neutrophil percentage, 80.8%; hemoglobin, 142 g/L; and platelet count, 78 × 10⁹/L. On the basis of these findings, aHUS was suspected, and the patient was admitted to the nephrology ward. The patient’s medical history included hypertension for 3 years and a history of plasma and red blood cell transfusions. The patient’s family history was as follows: his parents were affected by blood (cousins), and the patient’s grandmother, mother, and cousin had a history of anemia with specific causes unknown. No other obvious familial or hereditary diseases were noted. The physical examination results were as follows: temperature, 36.5 °C; pulse, 74 beats/min; respiration, 19 breaths/min; blood pressure, 124/80 mmHg; and weight, 90 kg. The patient was alert and in good spirits. Heart, lung, and abdominal examinations were normal, and there was no swelling in the lower limbs. Pathological signs were negative. Key laboratory abnormalities upon admission are summarized in Table 1. Additional investigations, including antinuclear antibody (ANA), antineutrophil cytoplasmic antibodies (ANCA), anti-glomerular basement membrane antibody (anti-GBM), serum glucose, electrolytes (sodium, potassium, chloride, calcium), coagulation profile, and serological markers for hepatitis and HIV, as well as an electrocardiogram and chest computed tomography, yielded unremarkable results.

Table 1.

Laboratory data at admission

Examination item	Test value	Reference value
Complete blood count
White blood cell (×109/L)	5.57	3.50–9.50
Neutrophil (%)	82	40–75%
Hemoglobin (g/L)	53	130–175
Platelet (×109/L)	16	125–350
Hemolysis indices
Lactate dehydrogenase (U/L)	2454.4	120–250
Schistocytes (%)	10	< 1%
Coombs test	Negative	Negative
Liver Function
Total bilirubin (µmol/L)	117.7	0–26
Direct bilirubin (µmol/L)	18.7	0–8
Indirect bilirubin (µmol/L)	99	0–17
ALT (U/L)	38	9–50
AST (U/L)	122	15–40
Albumin (g/L)	39.3	40–55
Renal function
Serum creatinine (µmol/L)	462.9	57–97
eGFR (mL/min/1.73 m2)	14.17	90–120
Blood urea nitrogen (mmol/L)	34.6	3.1-8
Complement system
C3 (g/L)	0.99	0.7–1.4
C4 (g/L)	0.23	0.1–0.4
CFI (ng/mL)	76.50	12.26-333.02
Anti-CFH antibodies (ng/mL)	429.13	474.38-1346.75
sC5b-9 (ng/mL)	267	75–219
ADAMTS13 (%)	153.76	42.16-126.37%
Inflammation
C-reactive protein(mg/L)	122.2	0–6
Procalcitonin(ng/mL)	2.5	0-0.1
Urinalysis
Urine Occult Blood	3+	Negative
Urine red blood cells	20/µl	0–17
Urine protein	2+	Negative
Stool Analysis
Salmonella/Shigella	Negative	Negative

Abbreviations: eGFR: estimated glomerular filtration rate; CFI: complement factor I; Anti-CFH antibodies: anti-complement factor H antibodies; sC5b-9: Soluble C5b-9 complex; ADAMTS13: a disintegrin-like and metalloproteinase with thrombospondin type 1 motifs 13

Diagnosis and treatment process The patient was admitted with upper abdominal pain and markedly elevated amylase levels (serum: 866 U/L; urine: 1831 U/L). Initial laboratory results revealed a hemoglobin level of 160 g/L, a platelet count of 242 × 10⁹/L, and a creatinine level of 187 µmol/L. On the 3rd day, a follow-up test revealed that the hemoglobin level decreased to 53 g/L, the platelet count decreased to 16 × 10⁹/L, and the creatinine level increased to 462.9 µmol/L. The patient experienced progressively worsening hemolytic anemia and deteriorating kidney function. Considering the patient’s medical history and the results of relevant tests conducted after admission, a diagnosis of aHUS and acute pancreatitis was established. During hospitalization, the patient underwent 6 plasma exchanges and 2 continuous renal replacement therapy (CRRT) sessions. Supportive care included antibiotic treatment, red blood cell transfusions, and symptomatic management. On April 9, 2022, follow-up tests revealed that total bilirubin was 10.5 µmol/L, direct bilirubin was 3.7 µmol/L, creatinine decreased to 128.0 µmol/L, hemoglobin improved to 83 g/L, and the platelet count returned to 266 × 10⁹/L. The patient’s condition improved significantly, and he was discharged. After discharge, the patient was regularly followed up in our department every month for liver and kidney function tests, complete blood count, all of which were normal (Fig. 2A). On December 25, 2022, the patient visited Eastern Theater General Hospital again due to fever, cough with sputum, and gross hematuria. A routine blood test revealed a white blood cell count of 11.49 × 10⁹/L, a neutrophil percentage of 82%, a hemoglobin level of 60 g/L, a platelet count of 18 × 10⁹/L, and a serum creatinine level of 231 µmol/L. Concurrently, the patient tested positive for SARS-CoV-2 via nucleic acid amplification. Based on the clinical presentation and laboratory findings, a diagnosis of aHUS secondary to COVID-19 infection was suspected. Owing to insufficient blood supply, plasma exchange was not performed. During hospitalization, symptomatic treatments, including infection control and plasma transfusion, were administered, and the patient’s condition completely improved before discharge. However, on April 22, 2025, the patient was readmitted due to a relapse of aHUS triggered by acute gastroenteritis. Admission laboratories revealed hemoglobin 102 g/L, platelets 12 × 10⁹/L, and serum creatinine 183 µmol/L. Notably, eculizumab had been included in China’s National Reimbursement Drug List (NRDL) for aHUS on January 1, 2024, substantially reducing financial barriers to treatment. Given this improved accessibility, eculizumab was recommended as first-line therapy. Due to critical disease progression requiring immediate intervention, timely meningococcal vaccination could not be administered. After obtaining written informed consent from the patient and his parents, eculizumab therapy was initiated within 24 h of relapse onset, with concurrent antibiotic prophylaxis using piperacillin-tazobactam. Meningococcal vaccination was subsequently completed within two weeks of initial eculizumab administration. Treatment outcomes demonstrated a rapid increase in platelet counts within 24 h of initiation, achieving normalization within one week. Hemoglobin levels and renal function also showed improvement beginning in the first week, reaching complete normalization within two weeks. As of the last follow-up in July 2025, hepatic and renal function along with complete blood count parameters remained within normal ranges, while the patient continued on biweekly eculizumab maintenance therapy (Fig. 2B).

Fig. 2.

Treatment and follow-up process. (A) Clinical course during the seventh episode (March 2022). “1d” indicates initiation of plasma exchange (PE; dark blue rhombuses). The patient received 6 PE sessions and 2 continuous renal replacement therapy (CRRT; dark blue squares) sessions. (B) Clinical course during the eighth episode (April 2025). “1d” indicates initiation of eculizumab therapy (dark blue triangles) with the following regimen: 900 mg weekly on Days 1, 8, 15, and 22, 1200 mg on Day 36, then 1200 mg every 14 days thereafter

Line indicators: Blue, serum creatinine (Scr); purple, hemoglobin (HB); orange, platelet count (PLT). Intervention markers: Dark blue rhombuses, PE; Dark blue squares, CRRT; Dark blue triangles, eculizumab therapy.

We performed extensive diagnostic testing to identify the cause of our patient’s HUS. The study was performed in accordance with the Declaration of Helsinki and was approved by the ethics committee of the Fuyang People’s Hospital Affiliated with Anhui Medical University (Number: 2025-19), and all participants signed written informed consent forms.

Genetic investigation method

Whole-exome sequencing (WES)

Relevant data from the patient, including medical history and family history, physical examination results upon admission, and auxiliary examination results, were collected. A GenCap^® Whole Exome Gene Capture Probe V6.0 (Beijing Maikeno Gene Technology Co., Ltd.) was used to perform hybrid capture of the patient’s whole exome. The target area included 23,000 genes, with a total length of 51 Mb. High-throughput sequencing was conducted on the captured library, achieving an average sequencing depth of 134.03X. Candidate variant gene loci were identified through bioinformatics analysis.

Sanger sequencing family verification

Based on the high-throughput sequencing results, primers were designed to amplify the target fragments. Genomic DNA was extracted from 50 to 100 µL of EDTA-anticoagulated whole blood using a commercial kit (G3633; Wuhan Servicebio Technology Co., Ltd.). Using these primers, target fragments were amplified by polymerase chain reaction (PCR) in 50 µL reactions containing 25 µL of 2× Fast Pfus PCR Master Mix (G3305; Wuhan Servicebio Technology Co., Ltd.), 1.5 µL of each forward and reverse primer (10 µM; sequences listed in Supplementary Table 1), 2 µL of DNA template, and nuclease-free water to a final volume of 50 µL. The thermocycling conditions consisted of an initial denaturation at 98 °C for 2 min, followed by 30 cycles of denaturation at 98 °C for 20 s, annealing at 55 °C for 20 s, and extension at 72 °C for 10 s, with a final extension at 72 °C for 5 min. Reactions were then held at 16 °C. PCR products were verified by 2% agarose gel electrophoresis and were bidirectionally sequenced on an ABI 3730xl platform (Applied Biosystems, USA) by Wuhan Servicebio Technology Co., Ltd. Sequence chromatograms were analyzed using Chromas v2.6.6 (Technelysium Pty Ltd.) to confirm pathogenic variants identified by high-throughput sequencing.

Detection of CD46 mRNA expression by fluorescence quantitative PCR

A total of 5 ml of peripheral venous blood was collected from the patient, the patient’s mother, and healthy control. Total RNA was extracted using a commercial kit (G3013; Wuhan Servicebio Technology Co., Ltd.) and its concentration/purity assessed spectrophotometrically. cDNA was synthesized from 500 ng − 1 µg of total RNA using the SweScript All-in-One RT SuperMix for qPCR (One-Step gDNA Remover) (G3337; Wuhan Servicebio Technology Co., Ltd.) in a 20 µl reaction volume according to the manufacturer’s instructions. Reverse transcription was performed at 25 °C for 5 min, 42 °C for 30 min, and 85 °C for 5 s using a thermal cycler (ETC811; Beijing Dongsheng Innovation Biotechnology Co., Ltd.). SYBR Green qPCR was performed using the 2× Universal Blue SYBR Green qPCR Master Mix (G3326; Wuhan Servicebio Technology Co., Ltd.) and 0.25 µM (final concentration) of each specific primer (sequences listed in Supplementary Table 2) in a 15 µl reaction volume: 7.5 µl Master Mix, 0.75 µl of each primer (2.5 µM stock), 2 µl cDNA, and nuclease-free water to 15 µl. The amplification protocol consisted of: 95 °C for 30 s; 40 cycles of 95 °C for 15 s and 60 °C for 30 s (annealing/extension); followed by a melting curve analysis (65 °C to 95 °C, 0.5 °C increments with a 5-sec hold/fluorescence acquisition at each step). Three technical replicates were performed per sample. Relative CD46 mRNA expression, normalized to GAPDH, was calculated via the 2^−ΔΔCT method.

Detection of the CD46 protein expression level in the plasma via enzyme-linked immunosorbent assay (ELISA)

A human CD46 ELISA detection kit with a kit specification of 48T was used. The testing instrument used was an enzyme-linked immunosorbent assay analyzer (Rayto RT-6100) (Wuhan Servicebio Technology Co., Ltd.). The testing methods and operational steps were performed according to the instructions provided in the manual.

Genetic investigation results

WES genetic sequencing results

High-throughput WES revealed a homozygous mutation in the first nucleotide of intron 12 of the CD46 gene in the proband (NM_172359.3, c.1127 + 2T > A, gene location 207785684). According to the prediction results from the Rare Disease Data Center (RDDC) RNA splicer algorithms (https://rddc.tsinghua-gd.org/) [15], this alteration in the sequence may lead to a change in the reading frame, exon skipping, and deletion of a 64 bp fragment, causing a frameshift variant (Fig. 3). Consequently, it induces alternative splicing, affects protein coding, and may lead to a splicing-related disease [16]. According to ACMG guidelines [17], this variant is classified as a pathogenic variant (PVS1 + PM2_Supporting + PM3_ Supporting): this variant is a null variant (splicing mutation) that may result in loss of gene function (PVS1); the frequency in the general population database is zero (PM2); and the variant is a homozygous variant in a recessive inheritance gene (PM3).

Fig. 3.

Prediction results of RDDC RNA splicer algorithms: The alteration in the sequence may lead to a change in the reading frame, exon skipping, and deletion of a 64 bp fragment, causing a frameshift variant

Sanger sequencing validation results

The proband harbors a homozygous mutation in the first nucleotide of intron 12 of the CD46 gene (c.1127 + 2T > A), whereas the parents harbor a heterozygous mutation (c.1127 + 2T > A). The pedigree of the proband’s family and Sanger sequencing chromatogram of CD46 are shown in Fig. 4.

Fig. 4.

Pedigree and Sanger sequencing of the pathogenic CD46 splice-site variant (c.1127 + 2T > A). (A) Pedigree showing autosomal recessive inheritance: proband homozygous for c.1127 + 2T > A, parents heterozygous. (B) Sanger chromatograms: Proband (Homozygous): T > A substitution at intron 12 donor site (+ 2 position, NM_172359.3). Parents (Heterozygous): Co-occurrence of wild-type T and mutant A peaks. Functional impact: Disrupts canonical splicing (GT→GA), predicted to cause exon skipping, 64-bp deletion, and frameshift leading to loss-of-function. Classified as pathogenic (PVS1 + PM2_Supporting + PM3_Supporting) per ACMG guidelines

Detection of peripheral blood mRNA via fluorescence PCR

The fold change in CD46 mRNA expression in peripheral whole blood was 0.78 for the proband, 1.62 for the mother, and 1 for the healthy control (Fig. 5A).

Fig. 5.

Peripheral blood mRNA results and plasma CD46 protein expression levels. (A) Fluorescence PCR detection of peripheral blood mRNA in the proband (aHUS), proband’s mother (F-C), and healthy control (NC); (B) Plasma CD46 protein expression levels in the proband (aHUS), proband’s mother (F-C), and healthy control (NC)

Plasma CD46 protein expression levels detected by ELISA

The CD46 protein level in the peripheral blood of the proband was 10.82 ng/ml, that in the mother was 18.06 ng/ml, and that in the healthy control was 22.91 ng/ml (Fig. 5B).

Discussion and conclusions

The association between CD46 gene mutations and aHUS was first reported in 2003 [18]. To date, over 100 different CD46 gene mutations have been identified in patients with aHUS; more than 75% of these mutations are consistent with autosomal dominant inheritance and are found primarily as heterozygous mutations, whereas homozygous mutations are rare. Most mutations are missense mutations, with splice site mutations accounting for less than 10% of all mutations [19, 20]. Studies have shown that in more than 75% of aHUS patients, CD46 mutations lead to reduced expression of the CD46 protein, with heterozygous mutations resulting in an approximately 50% decrease, whereas homozygous mutations result in even lower levels, approximately 0–25%. Insufficient expression of the CD46 protein leads to inadequate control of complement activation. CD46 mutations that result in the expression of normal levels of mutant protein may lead to a decreased ability to bind C3b/C4b, leading to reduced cofactor activity of CD46, which does not prevent excessive complement activation [13, 21, 22]. In this case report, we present a patient diagnosed with aHUS who, despite experiencing frequent recurrences over the past ten years, has never undergone genetic testing. Given the critical role of genetic screening in informing therapeutic decisions and prognostication for this patient, we recommended testing. Due to the extreme genetic heterogeneity and multisystem manifestations associated with aHUS, we selected whole-exome sequencing (WES) as the method of detection. This approach effectively addresses the limitations of multi-gene panels in detecting novel variants while avoiding the substantial costs, analytical complexity, and interpretive challenges associated with whole-genome sequencing (WGS), particularly in non-coding and repetitive regions. WES allows for comprehensive deep-coverage analysis of all protein-coding genes (e.g., CFH, CFI, CFB, C3, CD46), offers greater flexibility for reanalysis compared to targeted panels. Whole-exome sequencing (WES) revealed a rare novel homozygous mutation, c.1127 + 2T > A, in the CD46 gene associated with aHUS. In the family, with the exception of the proband, all the relatives presented normal phenotypes. Sanger sequencing confirmed the mutation site in the proband, while the parents were found to carry a heterozygous mutation, c.1127 + 2T > A, in the CD46 gene, indicating that two of the mutant alleles in the proband were inherited from the parents. According to the ACMG guidelines, the c.1127 + 2T > A variant is predicted to affect the splicing of CD46 transcripts and is classified as pathogenic.

Previously reported splicing mutations in the CD46 gene affect transcription and translation, leading to shorter mature mRNAs that are translated into truncated proteins. These protein variants may lack normal functionality and could impair the production and/or transport of the CD46 protein to the cell surface, resulting in reduced expression [12, 13, 23]. This reduced expression may lead to an inability to inhibit excessive activation of the alternative complement pathway. We suspect that the proband in this case may have a similar condition. Subsequent tests indicated that the proband’s whole blood mRNA levels were significantly decreased, and the CD46 protein levels were only 50% of those in healthy controls, further supporting our suspicion. However, since this variant has not been subjected to single-cell proteomics analysis, we cannot determine the specific changes in protein levels. Additionally, the exact effects of this splicing variant on the structure and function of the protein are unclear. Notably, mutations in the CD46 gene alone are not pathogenic factors for the development of aHUS but rather susceptibility factors. Additional environmental factors, such as infections, medications, malignancies, pregnancy, and transplantation, act as triggers for the disease, which is consistent with the “two-hit hypothesis” [2, 3, 5]. The proband experienced disease onset in childhood and has since experienced seven recurrences of aHUS, with each episode triggered by an infection, further supporting the “two-hit hypothesis” for aHUS development.

Eculizumab, a monoclonal antibody that targets the complement system, specifically inhibits the cleavage of complement protein C5, thereby preventing complement-mediated thrombotic microangiopathy. This pathophysiologically targeted mechanism offers a significant therapeutic advantage over plasma exchange. While plasma exchange primarily functions by supplementing deficient complement regulatory factors and removing mutant complement proteins or autoantibodies, it fails to provide a durable correction of the dysregulated complement activation microenvironment. Prospective, observational, and multicenter studies have confirmed the efficacy and safety of eculizumab in treating atypical aHUS, establishing it as the first-line therapy [24–26]. In this case report, the patient’s most recent episode in April 2025 coincided with the inclusion of eculizumab in China’s national reimbursement drug list (effective January 1, 2024). The substantially improved accessibility and reduced cost facilitated the initiation of eculizumab treatment for the first time in this case. Treatment outcomes demonstrated a rapid increase in platelet counts within 24 h of initiation, achieving normalization within one week (Fig. 2B). Hemoglobin levels and renal function also showed improvement beginning in the first week, reaching complete normalization within two weeks, consistent with findings from eculizumab’s phase II clinical trials [25]. The therapeutic response exceeded the speed historically observed with plasma exchange in this patient’s previous episodes (Fig. 2A), providing supporting evidence for the pivotal role of targeted complement inhibition in the management of aHUS. Notably, when eculizumab is not available, plasma exchange remains the preferred intervention and should be initiated promptly within 24 to 48 h after diagnosis to increase its efficacy [4, 27]. In the present case, during the sixth episode in March 2022, owing to the unavailability of eculizumab, the patient underwent six sessions of plasma exchange during hospitalization, resulting in complete remission and favorable treatment outcomes. However, several studies have questioned the efficacy of plasma exchange specifically in CD46-mutated aHUS. A cohort study conducted in Italy on patients with CD46-related aHUS revealed that 91% of patients in the plasma exchange group achieved remission, whereas 100% of those in the untreated group also experienced remission [28]. A similar finding was reported in another study from France [29], suggesting that plasma exchange may not significantly impact the outcomes of patients with CD46 mutation-related aHUS. A review of the 7 previous episodes in the treatment history of the proband with aHUS revealed that for 3 episodes, the patient was treated with plasma exchange, whereas for the other 4 episodes, he was treated with only symptom-based treatments, such as fresh frozen plasma infusion and anti-infection therapy; however, ultimately, the condition completely resolved after all episodes. These results may be related to CD46 being a membrane-bound protein rather than a circulating protein. Theoretically, plasma exchange would not correct this defect; however, this theory still requires verification through larger prospective studies.

Recurrence risk in aHUS exhibits substantial gene-specific heterogeneity: Mutations in CFI confer relatively lower recurrence rates (10–30%), whereas THBD, CFH or CFH-CFHR1/3 hybrid, and C3 demonstrate intermediate recurrence rates of 30%, 50%, and 50%, respectively. CD46 mutations are associated with high recurrence rates (70–90%). Notably, CFB mutations show 100% recurrence based on limited data; however, given their low prevalence (1–4%), larger cohort studies are warranted to validate this recurrence pattern [4]. Prognostically, patients with CFH mutations have the worst prognosis, with poor long-term outcomes and a very high risk of ESRD or death (70–80%). Patients with mutations in CFI, C3, CFB, and THBD have only slightly better prognoses [28, 30]. In contrast, patients with CD46 mutations typically have a good prognosis; although they experience frequent relapses, spontaneous remission is common. Approximately 90% of these patients maintain normal kidney function, leading to favorable long-term outcomes [23]. Additionally, patients with aHUS associated with CD46 mutations are considered to have a good prognosis after kidney transplantation with a low risk of relapse; this favorable prognosis is because CD46 is expressed on the cell membranes of the kidneys, and the donor kidneys can express normal CD46 [31]. In our case report, the proband experienced disease onset at age 8 and experienced 7 relapses over the past 19 years. Each relapse completely resolved after treatment, resulting in a good prognosis, which is consistent with the results of previous studies. Given the high recurrence propensity of CD46-associated aHUS, we will maintain eculizumab therapy for this patient. Simultaneously, we will establish a multidisciplinary collaborative team comprising specialists in nephrology, hematology, genetic counseling, and critical care medicine to ensure long-term supportive care, prevent infections and other precipitating factors, thereby minimizing relapse risks and optimizing the patient’s long-term prognosis.

Our study presents several principal limitations. Firstly, the absence of renal biopsy precludes histological differentiation between aHUS and potential mimickers, such as IgA nephropathy or C3 glomerulopathy. Secondly, while the reduced expression of CD46 in PBMCs indicates a loss of function, critical assays, including C3b binding affinity and flow cytometry of membrane CD46, were not performed. We acknowledge that the use of PBMCs, while a well-established and practical model for research, cannot fully recapitulate the endothelial-specific pathophysiology of aHUS. It is crucial to emphasize that this approach is more of an expedient than a definitive test for establishing pathogenicity. Functional validation in human endothelial cells would provide more direct and physiologically relevant pathogenic evidence. Furthermore, although whole-exome sequencing identified a novel splice-site mutation, our genetic analysis did not include multiplex ligation-dependent probe amplification (MLPA), which is necessary to rule out large genomic rearrangements or deletions/duplications (such as CFH-CFHR hybrid genes) that are known to contribute to aHUS [4]. These limitations delineate specific pathways for mechanistic and comprehensive genetic validation in our subsequent studies. Future research will prioritize these assays to definitively establish the causal link between the c.1127 + 2T > A variant and impaired complement regulation.

In conclusion, our case highlights the importance of genetic analysis for the definitive diagnosis of genetic complement-mediated aHUS. Patients clinically diagnosed with aHUS should undergo genetic analysis, which can aid in treatment, anticipate disease progression, and predict patient prognosis. Additionally, genetic variations are not the only factors leading to the development of aHUS, and additional environmental factors are needed as disease triggers.

Patient perspective

As a patient with a rare disease, I initially experienced profound isolation. Genetic testing revealed a CD46 mutation (c.1127 + 2T > A), explaining my recurrent aHUS triggered by infections and resolving years of diagnostic uncertainty. Today, I actively engage in treatment adherence and advocate for broader adoption of genetic analysis among patients—it not only informs therapeutic strategies but also alleviates psychological burdens stemming from diagnostic uncertainty. Despite potential future relapses, precision medicine empowers me to confront aHUS with resilience, transforming this journey from solitary struggle to scientifically guided management.

Supplementary Information

Below is the link to the electronic supplementary material.

Abbreviations

Ahus

Atypical hemolytic uremic syndrome

ESRD

End-stage renal disease

TMA

Thrombotic microangiopathy

WES

Whole-exome sequencing

ACMG

American College of Medical Genetics and Genomics

ELISA

Enzyme-linked immunosorbent assay

RDDC

Rare Disease Data Center

Author contributions

L.X.W and G.X.H conceived and designed the study. H.B.J drafted the manuscript. W.X(Xu Wang) contributed to the modification of the manuscript. W.X(Xian Wang), Z.H.C, and M.D.D collected the clinical and follow-up data. All authors reviewed the manuscript.

Funding

This study received funding from the Grants and Funding Anhui Provincial Key Research and Development Project (Grant number 2022e07020057) and the Special Project for Clinical Medical Research Transformation of Key Research and Development Projects in Fuyang City (Grant number FYZDYF2023LCYX002).

Data availability

The datasets generated and/or analysed during the current study are available in the ClinVar repository under accession SCV006064987 and the SRA repository under BioProject PRJNA1261101, further inquiries can be directed to the corresponding author.

Declarations

Ethics approval and consent to participate

The study was performed in accordance with the Declaration of Helsinki and was approved by the ethics committee of the Fuyang People’s Hospital Affiliated with Anhui Medical University (Number: [2025]19), and all participants signed written informed consent forms. Clinical trial number: not applicable. The studies were conducted in accordance with local legislation and institutional requirements.

Consent for publication

Written informed consent was obtained from the patient and his parents for publication of this case report and accompanying images.

Competing interests

The authors declare no competing interests.