Renal-Limited Thrombotic Microangiopathy in Infants: A Case Series

Abstract

Rationale & Objective

Thrombotic microangiopathy (TMA), an intractable disease, can present in a renal-limited mode. Data on infantile-onset renal-limited TMA (RL-TMA) is lacking. This study describes 5 cases of RL-TMA in infants.

Study Design

Case series.

Setting & Participants

Five patients with infantile-onset RL-TMA who were admitted and/or followed up at Nanfang Hospital, Southern Medical University, from January 2024 to June 2025.

Analytical Approach

Clinical data of 5 patients were collected. Amino acid conservation was analyzed by Clustal Omega, the secondary structure of the mutants was predicted by PolyPhen-2 and PHYRE2, and tertiary structure was predicted by AlphaFold3 and ChimeraX.

Results

All patients presented with acute nephritic syndrome with hematuria and proteinuria with onset in infancy, but without hemolytic anemia or thrombocytopenia. Corticosteroid treatment was attempted in 3 patients with no response. Renal pathology revealed microthrombi; segmental endothelial cells that swelled with widening of the subendothelial gap and/or mesangiolysis were observed. C3 heterozygous missense variant (c.2184 C>T, p.Cys728X), CD46 homozygous variant c.614A>G (p.Glu205Gly), and CFI heterozygous variant c.610A>G (p.Met204Val) were detected in 3 patients, respectively. Possible activation of alternative complement pathway (AP) was indicated in all 4 tested patients. Complete remission was achieved in 2 patients, and partial response was observed in another 2 patients who received eculizumab therapy. The patient who harbored the CFI variation and did not receive eculizumab therapy progressed to kidney failure at 11 months of age.

Limitations

Functional verification of genetic variants of C3, CD46 and CFI in this study were not implemented. AP activation-related factors were not checked in patient 5.

Conclusions

RL-TMA can occur in infancy, which has not been reported previously. Activation of the AP may be the common cause of infantile-onset RL-TMA. Its diagnosis and therapy are challenging. Promising outcomes can be achieved with eculizumab therapy.

Plain-Language Summary

In this study, 5 infants presented with acute nephritic syndrome with apparent and persistent hematuria, which is uncommon in congenital/idiopathic nephrotic syndrome. Renal pathology revealed thrombotic microangiopathy (TMA) changes, but hemolytic anemia or thrombocytopenia were absent in these patients. By exclusion of infectious diseases and immune-mediated renal diseases, and with the test results of alternative complement pathway components and genetic sequencing, a possible diagnosis of complement-mediated renal-limited TMA was considered. Rapid improvement was achieved in all 4 patients treated with eculizumab. Renal-limited TMA is rare in children and has not been reported in infants. Renal biopsy, genetic sequencing, and testing of alternative complement pathway-related factors may be highly valuable for precision diagnosis and therapy in patients with suspected TMA.

Thrombotic microangiopathy (TMA) is characterized by microvascular hemolytic anemia, thrombocytopenia, and ischemic organ damage. Kidney is the most frequently involved organ.¹ More than half of cases are caused by abnormalities of the alternative complement pathway (AP), a condition that is termed complement-mediated TMA (CM-TMA), formerly called atypical hemolytic uremic syndrome (aHUS). TMA has a remarkable clinical heterogeneity.² Thrombocytopenia or hemolytic anemia may be absent, which is known as incomplete or partial TMA^3,4; if both are absent with only kidney involvement, it is termed renal-limited-TMA (RL-TMA).⁵ RL-TMA has a severe long-term prognosis similar to that of systemic TMA.^5,6 RL-CM-TMA exhibits more aggressive disease progression than those of non–complement-mediated RL-TMA.⁷ Early treatment, such as complement inhibitor therapy, might substantially improve the outcomes of patients with RL-CM-TMA.^7,8 However, it is challenging to diagnose RL-TMA, especially in children for whom renal biopsy is not commonly performed, which can lead to delayed treatment and a poor prognosis.¹

Currently, RL-TMA is mainly reported in adults and infrequently in children.⁹ This article summarizes the clinical characteristics and the effects of eculizumab on the outcomes of 5 infants with RL-TMA.

Methods

Clinical data were collected from patients with infantile-onset RL-TMA who were admitted and/or followed up at Nanfang Hospital, Southern Medical University, from January 2024 to September 2025. The data included demographic data; initial symptoms; peripheral blood cell count; stool tests; urine analysis; tests for renal function; autoantibodies, including antinuclear antibodies, anti-double-stranded DNA antibodies, and antineutrophil cytoplasmic antibodies; tests for intrauterine infection, including HIV and syphilis; tests for serum concentrations of complement 3 (C3), complement 4 (C4), and total complement; and AP activation-related tests including the serum concentrations of complement factor H (CFH), factor I (CFI), factor P (CFP), anti-CFH antibody, anti-CFI antibody, C3a, C5a, Ba, serum and urine C5b-9, and serum ADAMTS13 (von Willebrand factor protease) antibody and its activity. Renal pathology and whole exome sequencing results were also collected. Bioinformatics analysis of genetic variations was performed: amino acid conservation was analyzed using Clustal Omega (https://www.ebi.ac.uk/jdispatcher/msa/clustalo), the secondary structure was predicted by PolyPhen-2 (http://genetics.bwh.harvard.edu/pph2/) and PHYRE2 (https://www.sbg.bio.ic.ac.uk/∼phyre2/html/page.cgi?id=index), and the tertiary structure of the proteins were predicted by AlphaFold3 and ChimeraX. This study was approved by the Ethics Committee of Nanfang Hospital, Southern Medical University (number NFEC-2025-221), and informed consent was obtained from the guardians of all participants.

Clinical Data

Five patients with infantile-onset RL-TMA, 3 boys and 2 girls, were included in the study (Table 1). All were individuals of Chinese Han ethnicity from nonconsanguineous parents with no positive family history. All patients presented with nephritic syndrome, hematuria, and proteinuria at an onset age of 2-8 months. Intrauterine infection and other infectious diseases were excluded. Autoantibodies and stool tests were all negative. Serum ADAMTS13 activity and ADAMTS13 antibody levels were normal in 3 tested patients.

Table 1.

Clinical Data of the 5 Patients With Infantile-Onset Renal-Limited Complement-Mediated Thrombotic Microangiopathy

Patients	P1	P2	P3	P4	P5
Sex	F	F	M	M	M
Age of onset (mo)	2	6	2	8	6
Age of diagnosis (mo)	5	7	6	96	9
Urine RBC (0-18/μL)	2,442	7,477	547	9,020	319
UPCR (0-0.2 mg/mg)	64.5	16.9	65.3	12.4	5.4
Hb (g/L)	77	108	109	100	99
PLT (×109/L)	533	938	280	372	675
Scr (μmol/L)	14	16	107	150	332
BUN (mmol/L)	5.1	5.8	2.4	13.4	16.1
Cys-C (mg/L)	1.62	1.32	0.73	2.46	4.6
LDH	243	320	213	260	344
ALB (g/L)	10	28.9	22.5	21.1	26.6
CH50 (U/mL)	20	56	22	57	ND
C3 (g/L)	1.06	1.47	0.95	1.81	0.64
C4 (g/L)	0.17	0.24	0.26	0.34	0.15
ADAMTS13 antibody	Negative	Negative	Negative	ND	ND
ADAMTS13 activity (%)	84.80	102.7	78.63	ND	ND
Gene mutation	C3 c.2184 C>T (p.Cys728X) heterozygous (maternal)	Negative	CD46 c.614A>G (p.Glu205Gly) homozygous (heterozygous in father)	Negative	CFI c.610A>G (p.Met204Val) heterozygous
Treatment	Ecu, CS, Tac	Ecu, CS, MMF	Tac, CS, MMF, Ecu	Tac, CS, Ecu, MMF	Tac, HD, PE
Outcome	CR	CR	PR	PR	Kidney failure

Values in parentheses were the referece ranges.

Abbreviations: ADAMTS13, von Willebrand factor protease; ALB, albumin; BUN, blood urea nitrogen; C, complement; CH50, total complement; CR, complete remission; CS, corticosteroid; Cys-C, cystatin C; Ecu, eculizumab; eGFR, estimated glomerular filtration rate; GBM, glomerular basement membrane; Hb, hemoglobin; HD, hemodialysis; LDH, lactate dehydrogenase; MMF, mycophenolate mofetil; ND, not detected; PE, plasma exchange; PLT, platelet; PR, partial remission; RBC, red blood cells; Scr, serum creatinine; Tac, tacrolimus; UPCR, urinary protein-creatinine ratio.

Patient 1, a girl, was delivered by cesarean section at 38 weeks, 4 days gestational age because of breech pregnancy, with a birth weight (BW) of 3,150 g. She presented with edema in her lower limbs at age 2 months, 21 days. Urine tests revealed red blood cell (RBC) count was 21-2,442 cells/μL, she had proteinuria (3+∼4+), a urinary protein-creatinine ratio (UPCR) of 117.3 mg/mg, and a serum albumin (ALB) concentration of 10.8 g/L. Her hemoglobin (Hb) concentration was 113 g/L and platelet (PLT) count 533 × 10⁹/L. No RBC fragments were observed. Her serum creatinine (Scr) concentration was 27 μmol/L, lactate dehydrogenase 243 U/L, C3 1.05 g/L, and C4 0.16 g/L. The patient was managed with ALB infusion and diuresis because of her severe edema, as well as captopril. Renal pathology (at age 4 months) revealed focal segmental glomerulosclerosis (Fig 1A); segmental capillary lumens were narrow and occlusive with microthrombi formation (Fig 1A). Thickening of the arteriolar wall, renal tubular atrophy, and RBC casts were observed. Immunofluorescence (IF) staining revealed only IgM+. Electron microscopy (EM) revealed diffuse foot process effacement and segmental endothelial cell swelling and detachment from the basement membrane, without electron-dense deposits (Fig 1B). She was referred to our hospital at age 5 months. Her blood pressure (BP) was 97/53 mm Hg, and AP activation-related tests revealed elevated concentrations of urine C5b-9/urine creatinine 112.16 μg/mmol (0-3.94 μg/mmol). A heterozygous variant in C3 (c.2184 C>T, p.Cys728X) was identified, inherited from her mother, who was asymptomatic. Eculizumab (300 mg) was administered, and her hematuria decreased rapidly from 2,319 to 131 cells/μL within 4 days (Fig 2A). Afterward, she received methylprednisolone pulse therapy (0.15 g every other day for 3 days), and then prednisone (2 mg/kg/d) was administered and tapered down gradually. She received another 2 doses of eculizumab during the next 2 months (1 dose per month). Tacrolimus (trough concentration 3.0 μg/L) with losartan was added after the third dose of eculizumab because of the remaining proteinuria 2+∼3+. Her hematuria and proteinuria were negative 2 months and 5 months after the first dose of eculizumab, respectively (Fig 2A). Re-examination of AP-related factors at age 12 months revealed increased anti-CFI antibody, which was 136.15 relative units (RU)/mL (≤36.63 RU/mL) and which didn’t detect previously. Rechecked AP-related factors at 19 months revealed an even higher anti-CFI antibody titer (493.28 ng/mL). Tacrolimus was then substituted with mycophenolate mofetil (MMF) 0.25 g/d. She is in complete remission at age 20 months at the time of this writing.

Figure 1.

Renal pathology of 5 infants with renal-limited complement-mediated thrombotic microangiopathy. (A-B) Patient 1. (A) Microthrombi were observed in some segments of capillary lumens (black arrowhead) (LM, Masson + silver staining; original magnification, ×400; scale bar: 100 μm). (B) Swelling of endothelial cells in a few segments that separated from the glomerular basement membrane (black arrow) (EM; original magnification, ×2,500; scale bar: 5 μm). (C-D) Patient 2. (C) Partial blockage of glomerular capillaries and microthrombi (black arrowhead) (LM, Masson staining; original magnification, ×200; scale bar: 100 μm). (D) Diffuse fusion of glomerular foot processes, endothelial swelling (black arrow), and microthrombi (black arrowhead) (EM; original magnification, ×2,000; scale bar: 2 μm). (E-F) Patient 3. (E) Focal segmental glomerulosclerosis (black square) (LM, HE staining; scale bar: 100 μm). (F) Segmental endothelial cell swelling (black arrow) (EM; original magnification, ×3,000; scale bar: 2 μm). (G-H) Patient 4. (G) Microthrombi in capillaries (black arrowhead) (LM, HE staining; original magnification, ×100; scale bar: 200 μm). (H) Vacuolar degeneration of endothelial cells (white arrowhead) and thickening and stratification of the basement membrane (EM; original magnification, ×2,000; scale bar: 0.5 μm). (I-J) Patient 5. (I) Segmental endothelial cell proliferation, irregular arrangement of capillaries, and shrinkage of the capillary lumen (white arrow) (EM; original magnification, ×3,000; scale bar: 5 μm). (J) Swelling of endothelial cells and widening of the subendothelial gap (black arrow) (EM; original magnification, ×3,000; scale bar: 5 μm). Abbreviations: HE, hematoxylin and eosin; EM, electron microscopy; LM, light microscopy.

Figure 2.

Changes in urine red blood cells (RBCs) and the urinary protein-creatinine ratio (UPCR) in patients 1 to 4 receiving eculizumab therapy. (A) Patient 1. Urine RBC decreased dramatically, and UPCR decreased gradually after the first dose of eculizumab. (B) Patient 2. Urine RBC and UPCR decreased to normal after the second and third doses of eculizumab. (C) Patient 3. Urine RBC and UPCR decreased after the second and third doses of eculizumab. (D) Patient 4. Urine RBC and UPCR decreased gradually after the first and second doses of eculizumab. Abbreviations: MMF, mycophenolate mofetil; MP, methylprednisolone.

Patient 2 was delivered by cesarean section at 37 weeks, 3 days gestational age because of a prolapsed umbilical cord, with a BW of 3,000 g. She was observed to have edema at age 6 months. Urine analysis revealed RBC 200 cells/μL, urine protein 3+, serum ALB 19.1 g/L, cholesterol 7.79 mmol/L, Hb 113 g/L, and PLT 644 × 10⁹/L. No RBC fragments were observed. Her condition did not improve after corticosteroid treatment, and she was referred to our hospital at age 7 months. Her BP was 68/49 mm Hg, urine protein 3+, UPCR 33.9 mg/mg, RBC 7,477 cells/μL, serum ALB 23 g/L, Scr 16 μmol/L, lactate dehydrogenase 389 U/L, C3 1.47 g/L, C4 0.24 g/L, and total hemolytic complement 56 U/mL. Renal biopsy revealed segmental sclerosis in 2 of 28 glomeruli and microthrombi (Fig 1C). IF staining revealed only IgM ±. EM revealed segmental widening of the subendothelial space and microthrombi (Fig 1D). AP activation-related tests revealed an increased CFP concentration of 102.24 μg/ml (4-54 μg/mL). No related pathogenic gene variants were identified. She received a dose of eculizumab (300 mg), which rapidly relieved her hematuria and proteinuria (Fig 2B). She received another dose of eculizumab during the second month, and her hematuria decreased to 38 cells/μL and her UPCR to 0.38 mg/mg. She was administered MMF and prednisone. She achieved complete remission at age 12 months, and prednisone was discontinued. Rechecked CFP at 1 year and 4 months was 11.25 μg/mL. At the time of this writing, she is aged 22 months with MMF being tapered down.

Patient 3, a boy, was born with full-term normal delivery and a BW of 3,200 g. He was noted to have proteinuria when he had pneumonia at age 2 months. Hematuria and proteinuria were confirmed at age 4 months when he was brought to the hospital again because of fever. Urine RBC was 636 cells/μL, proteinuria 3+, and UPCR 65.9 mg/mg. His Hb concentration was 109 g/L, and PLT were 280 × 10⁹/L. No RBC fragments were observed. His serum ALB was 13.7g/L, Scr 107∼428 μmol/L, lactate dehydrogenase 213 U/L, C3 1.46 g/L, and C4 0.40 g/L. He received corticosteroid therapy for 2 months with no response. He was then referred to our hospital at age 6 months with BP 80/53 mm Hg. Renal biopsy at 7 months of age revealed focal segmental glomerulosclerosis (Fig 1E), IF staining showed only IgM+, and EM showed segmental endothelial cell swelling (Fig 1F) without electron-dense deposits. The patient received tacrolimus combined with oral prednisone (7.5 mg every day) for 2 months without improvement, after which tacrolimus was discontinued. At 9 months of age, his AP activation-related tests revealed increased concentration of C5a 12.3 ng/mL (<10 ng/mL), Ba 1,025.7 ng/mL (221.3-727 ng/ml), and anti-CFH antibody 251.55 arbitrary units (AU)/mL (≤96.11 AU/mL), which indicated the activation of AP. Whole exome sequencing revealed a homozygous variant of CD46 c.614A>G (p.Glu205Gly); his father harbored a heterozygous variant, but his mother had no variant at this locus. A run of homozygosity was present at this locus, indicating the possibility of uniparental disomy. Eculizumab was administered. Unfortunately, he developed a severe respiratory infection after that, and his hematuria and proteinuria worsened. No causative microorganism was identified. After his recovery, he received the second and third doses of eculizumab during the next 2 months (1 dose per month). His urine hematuria and proteinuria improved gradually (Fig 2C). However, his renal function did not improve, and eculizumab was discontinued and MMF administered. The boy is 23 months old at the time of this writing, with urine RBC 141 cells/μL and UPCR 1.6 mg/mg.

Patient 4, a boy, had a normal full-term delivery and a BW of 2,900 g. Growth retardation was observed at age 3 months. Nephritic syndrome was observed at 8 months of age, with urine RBC 601-2,970 cells/μL and urine protein ±∼2+. His Hb concentration was 100 g/L and PLT 497 × 10⁹/L. No RBC fragments were detected. His serum ALB concentration was 39.1 g/L and C3 0.72 g/L. Renal pathology at 10 months of age revealed microthrombi (Fig 1G) and mild segmental widening of the mesangium and multiple focal interstitial fibrosis without arteriole wall thickening. IF revealed only IgM+, and collagen IV α3 and α5 staining were negative. EM revealed swelling of epithelial cells and uneven thickness and stratification of the dense layer of the glomerular basement membrane (GBM) without electron-dense deposits (Fig 1H). He did not have a positive familial history, and his eye and hearing examinations were normal. Whole exome sequencing did not identify any related genetic variations, including COL4A3∼A6 variants. He was then administered benazepril. He was admitted to our hospital at 3 years of age because of growth retardation. At age 9 years, his BP 97/56 mm Hg, urine RBC 1,980 cells/μL, UPCR 12.24 mg/mg, Hb 107 g/L, PLT 284 × 10⁹/L, Scr 150 μmol/L, and ALB 19.9 g/L. He was then tested for AP activation factors. Increased Ba >2,060 ng/mL (221.3-727 ng/mL) and an anti-CFI antibody titer of 67.94 RU/mL were detected, which indicated the activation of AP. He received 2 doses of eculizumab at 1-month intervals, together with MMF and prednisone (1 mg/kg/d). His urine RBC decreased to 348 cells/μL, UPCR to 5.34 mg/mg (Fig 2D), and Scr to 120 μmol/L. A second renal biopsy was suggested but was not accepted by his parents, nor was the continuation of eculizumab therapy.

Patient 5, a boy, conceived by in vitro fertilization, had a full-term, normal delivery and BW 2,800 g. He had eyelid edema at age 6 months. His BP was 86/59 mm Hg, urine RBC 3+, urine protein 3+, UPCR 5.7 mg/mg; Hb concentration 99 g/L, PLT 675 × 10⁹/L, no fragmented RBCs, C3 0.64 g/L, C4 0.15 g/L, serum ALB 26.6 g/L, and Scr 15 μmol/L. Methylprednisolone (12 mg/d) was administered for more than half a month without improvement. He then received tacrolimus at age 7 months, with no response. Renal pathology at 9 months revealed focal segmental glomerulosclerosis, and no microthrombus was observed. Mild mesangial proliferation and occasional mesangiolysis were observed. IF staining was negative. EM revealed segmental capillary shrinkage, uneven thickness and layering of the GBM (Fig 1I), endothelial cell swelling and proliferation, and widening of the subendothelial space without electron-dense deposits (Fig 1J). He had a fever at 9 months with decreased urine output and increased Scr (332 μmol/L). The boy progressed to kidney failure and received peritoneal dialysis at 11 months. Whole exome sequencing revealed a de novo variant in CFI c.610A>G (p.Met204Val) and a novel nonsense variant in COL4A5 c.4357C>T (p.Gln1453X). The child did not undergo AP activation-related testing.

Discussion

In this study, we report 5 patients who developed acute nephritis syndrome in infancy, with apparent microscopic hematuria and proteinuria. Infectious diseases, autoimmune diseases, possible pathogenic variants of congenital nephrotic syndrome, ADAMTS13 activity defects, and Shiga toxin of Escherichia coli-related HUS were all excluded. Four children who underwent complement factor tests indicated the possible activation of AP. The other child, whose AP activation-related factors were not checked, carried a CFI variant. Renal pathology of these 5 patients revealed acute or chronic TMA changes. Without secondary causes being identified and owing to the lack of hemolysis, thrombocytopenia, and extrarenal organ involvement, RL-CM-TMA was considered. Eculizumab was administered to 4 of the patients. Two patients who were in the early stage achieved complete remission. The other 2, who were in the disease progression stage, also improved, with decreased hematuria and proteinuria. One patient who had a CFI variation and did not receive eculizumab therapy rapidly progressed to kidney failure.

RL-TMA was first identified in transplanted kidneys in 1994 by Hochstetler et al¹⁰ and then in native kidneys as well. Maisons et al⁵ reported that patients with RL-TMA had lower rates of chronic dialysis, kidney transplantation, and mortality. However, the long-term prognosis of RL-CM-TMA was poor and similar to that of systemic TMA patients. In a study by Sallée et al,⁴ there were no significant differences in outcome between partial/incomplete TMA and systemic TMA. Of the 5 children with RL-TMA reported in our study, 2 cases were progressed to CKD, and 1 has already progressed to kidney failure, indicating poor renal outcome of these patients.

Owing to the absence of systemic manifestations, the diagnosis of RL-TMA mainly relies on renal pathology. Typical TMA changes during the acute disease phase, such as swelling of endothelial cells, widening of the subendothelial space, mesangiolysis, and microthrombi formation, are observed.¹¹ Intramural fibrin and mucinous intimal thickening are also visible in arterioles and interlobular arteries. Renal pathology in the later phase is usually atypical, includes mesangial hyperplasia, insertion, and double contour signs, along with thickening and lamination of the GBM, and onion-like intimal changes in arterioles. In this study, segmental swelling of endothelial cells, widening of the subendothelial space, mesangiolysis, and/or microthrombi were observed in all patients. Interestingly, both patients 4 and 5 who had no positive family history presented uneven thickness and delamination of the GBM resembling the GBM changes of Alport syndrome. Patient 4 showed negative GBM staining for type IV collagen α3 and α5, and pathogenic gene variants characteristic of Alport syndrome were not detected. Patient 5 harbored a COL4A5 variant. However, both patients presented with nephrotic range proteinuria and hematuria during infancy. Patient 4 had positive anti-CFI antibody and aberrant AP activation; he achieved significant improvement after eculizumab therapy. Patient 5 progressed to renal failure rapidly within 1 year of age. All of these do not coincide with Alport syndrome.^12,13 GBM damage may be secondary to TMA injury.

Abnormal activation of AP can serve as either a direct etiology or a synergistic pathogenic factor with secondary causes in RL-TMA, similar to that in systemic TMA. AP abnormalities encompass both inherited and acquired conditions. Genetic factors include mutations in genes encoding complement factors, such as C3 and CFB, or regulatory proteins (eg, CFH or CFI).¹⁴ Acquired CM-TMA is primarily mediated by anti-CFH antibody, and a minority of cases are mediated by anti-CFI antibody.¹⁵ Anti-CFH antibody has been detected in 40%-60% of pediatric aHUS patients.16, 17, 18, 19 In this study, 4 tested patients exhibited possible abnormal activation of AP. Patient 3 was positive for anti-CFH antibody, patient 1 and patient 4 were positive for anti-CFI antibody, and patient 2 had elevated serum concentration of CFP.

Three patients harbored suspected pathogenic variants of C3, CD46, and CFI, respectively, that have not been reported previously. The C3 heterozygous missense variant (c.2184 C>T, p.Cys728X) in patient 1 is in a highly conservative locus (Fig 3A). The mutation is predicted to cause premature termination of the protein, generating a truncated C3 protein without an α chain (Fig 4A and B). C3 variants may be associated with infantile aHUS.¹⁵ In this patient, the C3 variant originated from the child’s asymptomatic mother. The C3 variant p.Cys728X is considered to cause a loss of function, and there are a few reports that C3 loss-of-function variants can also lead to aHUS, although the mechanism is unclear.²⁰ Additionally, the patient in this study had positive anti-CFI antibody, which may also contribute to the pathogenesis of TMA, together with the increased urine C5b-9, indicating the possible activation of AP. The patient’s symptoms remitted rapidly after eculizumab treatment. Given the presence of anti-CFI antibody, it is not clear what the relative contribution of the genetic or the acquired factor was in this patient.

Figure 3.

Conservation analysis secondary structure of the pathogenesis of variants of C3, CD46, and CFI. (A) The amino acid at the locus of C3 c.2184 C>T (p.Cys728X) in patient 1 is highly conserved. (B) The amino acid at the locus of CD46 c.614A>G (p.Glu205Gly) in patient 3 is not conserved. (C) Secondary structure prediction showing that the β fold near the CD46 c.614A>G (p.Glu205Gly) variant site become longer. (D) Amino acid at the CFI c. 610A>G (p.Met204Val) in patient 5 is not conserved. (E) Secondary structure prediction revealed that the CFI c.610A>G (p.Met204Val) variant could cause shortening of the β fold in this area.

Figure 4.

Tertiary structure of the pathogenesis of variants of C3, CD46, and CFI. (A) Wild-type C3 protein. (B) The C3 heterozygous missense variant (c.2184 C>T, p.Cys728X) in patient 1 is predicted to cause a truncated C3 protein without α chain. (C) Wild-type CD46 protein. (D) The tertiary structure prediction of CD46 c.614A>G (p.Glu205Gly) shows a change in the spatial conformation between amino acids 287-339 (in blue). (E) Wild-type CFI protein. (F) The tertiary structure prediction of CFI c.610A>G (p.Met204Val) shows a missed β-sheet between amino acids 476-478, a shortened α-helix, and a shape change between amino acids 1-25 (in blue). (G) The conformation of wild-type CFI binding to C3b; the blue structure represents C3b, and the multicolored structure represents CFI. (H) The conformation of CFI with p.Met204Val variant binding to C3b in patient 5. (G) The conformation of wild-type CFI binding to C3b; the blue structure represents C3b, and the multicolored structure represents CFI. (H) The conformation of CFI with p.Met204Val variant binding to C3b in P5.

A CD46 homozygous variant, c.614A>G (p.Glu205Gly), was detected in patient 3. Further analysis revealed a run of homozygosity at this locus, indicating the possibility of dimorphism. CD46 is a transmembrane glycoprotein that facilitates CFI degradation of C3b and C4b by inhibiting the formation of C3 convertase. Genetic defects in CD46 can lead to abnormal complement activation and cause aHUS²¹ and recurrent microvascular disease.²² Most CD46 variants are heterozygous, resulting in reduced cell membrane expression (approximately 75%) or loss of function (approximately 25%).²³ Variants located in the complement control protein domain are associated with aHUS and typically result in reduced expression or functional abnormalities of CD46.^24,25 The p.Glu205Gly variant of patient 3 was located at the third complement control protein domain, a relatively conservated locus (Fig 3B). This variant causes the acidic hydrophilicity of glutamic acid to be replaced by the neutral hydrophobicity of glycine, which may lead to elongation of the nearby β-fold (Fig 3C) and affect protein function. Polyphen-2 prediction suggest the variant is probably damaging with a score of 1.000. The tertiary structure prediction showed a change in the spatial conformation between amino acids 287 and 339 (Fig 4C and D). The child had early disease onset at 2 months of age. In addition, he had increased concentrations of anti-CFH antibody, C5a, and Ba, indicating abnormal activation of AP. His hematuria and proteinuria decreased after eculizumab therapy, confirming that abnormal AP activation was involved in his disease pathogenesis.

A CFI heterozygous variant, c.610A>G (p.Met204Val), was identified in patient 5. CFI prevents the formation of C3 and C5 convertases by cleaving C3b and C4b.²⁶ Approximately 4%-10% of aHUS patients carry CFI mutations²⁷; of these, 50% progress rapidly after disease onset, usually entering kidney failure or dying within 2 years, and only one-third of the patients achieve complete remission.²⁸ The locus of the variant in patient 5 is not conserved (Fig 3D). However, secondary structure prediction revealed a shortened β-sheet (Fig 3E). Polyphen-2 prediction suggests the variant is benign with a score of 0.084. Tertiary structure prediction showed a missed β-sheet between amino acids 476 and 478, a shortened α-helix, and a shape change between amino acids 1 and 25 (Fig 4E and F); this CFI variant would alter the tertiary structure when binding to C3b (Fig 4G and H). Therefore, we speculated that the CFI variant might be pathogenic. The child had a decreased serum C3 concentration, but C4 was normal, which is in accordance with AP activation. He did not undergo testing for AP activation-related factors and did not receive eculizumab therapy. The early onset and his renal pathology, which did not conform to Alport syndrome, suggested TMA. These characteristics, combined with the patient progressing to kidney failure at 11 months of age, are consistent with CFI variant-associated TMA.

There was no pathogenic gene variant identified in patient 2 and patient 4. Patient 2 exhibited an increase CFP concentration at disease onset, which returned to normal in remission. CFP level has been previously reported to be increased in IgA nephropathy and rheumatoid arthritis but decreased in C3 nephropathy.^29,30 Although CFP can prolong the half-life and the activity of C3 convertase, the significance of higher CFP at the disease onset in patient 2 remains unclear. In patient 4, positive anti-CFI antibody and increased Ba levels indicated the possible activation of AP. Both children received eculizumab therapy, which alleviated the hematuria and proteinuria.

In conclusion, although RL-TMA is not uncommon in adults, reports of RL-TMA in children are rare, and there is currently no report of infantile RL-TMA. Owing to the lack of systemic manifestations, the diagnosis of RL-TMA is challenging, particularly in infants. Apparent and persistent glomerular hematuria with variable levels of proteinuria is a clinical feature of nephritic syndrome and is a clue directing to the diagnosis of TMA. It is rare for congenital nephrotic syndrome to present with apparent and persistent glomerular hematuria. Owing to the rarity of postinfection glomerulonephritis or immune complex-mediated nephritis in infancy, when an acute nephritis syndrome is encountered in infants, TMA-associated kidney injury should be considered, even if there is no hematologic involvement. In this scenario, renal biopsy and testing of AP activation-related factors, as well as screening of pathogenic gene variants, are highly valuable for precision diagnosis and therapy. The complement inhibitor eculizumab can substantially improve the outcome of RL-CM-TMA at the early stage of the disease.