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What lies in-between: C3 glomerulopathy with non-hemolytic renal microangiopathy and an ultra-rare C3 variant

Open AccessPublished:December 02, 2022DOI:https://doi.org/10.1016/j.amjms.2022.10.004

      Abstract

      We report a 36-year-old female with mixed nephritic-nephrotic syndrome and recurrent pancreatitis. Kidney biopsy showed a crescentic membranoproliferative glomerulonephritis with dominant C3 staining on immunofluorescence (IF) but only scant deposits on electron microscopy (EM) and instead, evidence of severe acute and chronic microangiopathy - endothelial swelling, sub-endothelial fluff, and segmental basement membrane remodeling. Her serum C3 was normal, Factor Ba, and serum Membrane attack complex (sMAC) levels were elevated, and Properdin was low. Genetic testing revealed a heterozygous ultra rare C3 variant of unknown significance (c.4838G>T, p.Gly1613Val) as well as a heterozygous deletion of CFHR3-CFHR1. She showed an initial response to terminal complement blockade with eculizumab, but her renal disease progressed in the next year. Notably, our patient never demonstrated microangiopathic hemolysis, yet pancreatitis of unclear etiology recurred periodically. Our case suggests the existence of a “C3G/aHUS overlap” clinicopathologic syndrome and highlights the challenges of treating complement-mediated kidney disease.

      Introduction

      C3 glomerulopathy (C3G) and atypical hemolytic uremic syndrome (aHUS) are considered clinically and histologically distinct entities, although both result from alternative complement pathway (AP) dysregulation.
      • De Vriese A.S.
      • Sethi S.
      • Van Praet J.
      • Nath K.A.
      • Fervenza F.C.
      Kidney Disease Caused by Dysregulation of the Complement Alternative Pathway: An Etiologic Approach.
      The pathophysiologic distinction between C3G and aHUS is thought to be in the location of AP dysfunction, which has been primarily observed at the level of C3 convertase (fluid phase) in C3G and the endothelial cell surface (solid phase) in aHUS.
      • De Vriese A.S.
      • Sethi S.
      • Van Praet J.
      • Nath K.A.
      • Fervenza F.C.
      Kidney Disease Caused by Dysregulation of the Complement Alternative Pathway: An Etiologic Approach.
      ,
      • Goodship T.H.
      • et al.
      Atypical hemolytic uremic syndrome and C3 glomerulopathy: conclusions from a “Kidney Disease: Improving Global Outcomes” (KDIGO) Controversies Conference.
      Both diseases, however, share common genetic variants in the proteins of the AP,
      • De Vriese A.S.
      • Sethi S.
      • Van Praet J.
      • Nath K.A.
      • Fervenza F.C.
      Kidney Disease Caused by Dysregulation of the Complement Alternative Pathway: An Etiologic Approach.
      ,
      • Servais A.
      • et al.
      Acquired and genetic complement abnormalities play a critical role in dense deposit disease and other C3 glomerulopathies.
      • Ankawi G.A.
      • Clark W.F.
      Atypical haemolytic uremic syndrome (aHUS) and membranoproliferative glomerulonephritis (MPGN), different diseases or a spectrum of complement-mediated glomerular diseases?.
      • Fremeaux-Bacchi V.
      • et al.
      Genetics and outcome of atypical hemolytic uremic syndrome: a nationwide French series comparing children and adults.
      • Bu F.
      • et al.
      High-Throughput Genetic Testing for Thrombotic Microangiopathies and C3 Glomerulopathies.
      and members of the same family (with the same variant) may have different clinical phenotypes.
      • Servais A.
      • et al.
      Acquired and genetic complement abnormalities play a critical role in dense deposit disease and other C3 glomerulopathies.
      ,
      • Fremeaux-Bacchi V.
      • et al.
      Genetics and outcome of atypical hemolytic uremic syndrome: a nationwide French series comparing children and adults.
      Some authors have, therefore, advocated for the common grouping of C3G and aHUS as disorders of AP.
      • De Vriese A.S.
      • Sethi S.
      • Van Praet J.
      • Nath K.A.
      • Fervenza F.C.
      Kidney Disease Caused by Dysregulation of the Complement Alternative Pathway: An Etiologic Approach.
      Conversely, even though the timely use of the C5 monoclonal antibody eculizumab has been shown to improve renal outcomes in aHUS, the exact role of complement blockade in C3G remains to be established.
      • Legendre C.M.
      • et al.
      Terminal complement inhibitor eculizumab in atypical hemolytic-uremic syndrome.
      We present a unique case with overlapping clinical and histologic features that lie “in-between” the distinct C3G and aHUS phenotypes. We show the functional complement dysfunction and genetic predisposition in this case, as well as her response to various treatments over 1.5 years.

      Case presentation

      Initial presentation

      A 36-year-old woman with a history of prior alcohol use presented to our institution with a 1-day history of oliguria and abdominal pain. Based on available records from outside facilities, her past medical history was notable for an episode of stage 3 acute kidney injury (AKI) one year prior to her current presentation (peak serum creatinine [SCr] 3.5 mg/dL). Notably, this first AKI episode was associated with pancreatitis (first known episode in the setting of active alcohol use), hypertension, proteinuria (>300 mg/dL), and hematuria (>100 red blood cells per high-power field [RBC/hpf]). Incidentally, hypertension, proteinuria, and hematuria were first recorded one month before this first AKI episode, at which time SCr was 1 mg/dL. She did not have a known family history of kidney disease or autoimmune disease. She was treated with intravenous fluids (IVF) and bowel rest for this first AKI, which was presumed to be secondary to alcohol-induced pancreatitis. Serum creatinine had improved to 2.4 mg/dL at discharge. The patient was subsequently lost to follow-up, but one month prior to her presentation at our institution, she was seen in the outpatient clinic and started on hydrochlorothiazide (HCTZ), lisinopril, and metoprolol for uncontrolled hypertension.
      At the initial evaluation for the present-day episode (second AKI episode; Day 0), her blood pressure (BP) was 139/82 mmHg, and epigastric tenderness was noted. Labs showed SCr 2.6 mg/dL, serum lipase 445 IU/L [8-78], hemoglobin 9.2 g/dL [11.6-15.6], platelet count 595,000/μL [160-370,000], urinalysis with >100 RBC/hpf, and urine protein to creatinine ratio (UPC) of 6.35 g/g [<0.15]. High-resolution computed tomography of the abdomen was without acute findings. On ultrasound, both kidneys had normal cortical thickness, echogenicity, and size (both measuring 11.1 cm). The patient was again diagnosed with pancreatitis-related acute kidney injury (AKI) and admitted for IVF and bowel rest. The patient insisted that she had stopped alcohol use one-year prior. Important laboratory findings are further detailed in Table 1.
      Table 1Serologic, complement and genetic testing at baseline and over time.
      A) ChemistryInitial AdmissionTwo months LaterNormal Range
      Basic metabolic panel
      Sodium134138136-145 mmol/L
      Potassium4.44.63.5-5.1 mmol/L
      Chloride10310898-107 mmol/L
      Bicarbonate191422-29 mmol/L
      BUN24287-19 mg/dL
      Serum creatinine2.62.20.55-1.02 mg/dL
      Calcium8.37.78.4-10.2 mg/dL
      Additional chemistry
      Magnesium1.81.51.8-2.6 mg/dL
      Phosphorus2.21.6-2.6 mg/dL
      Albumin2.52.33.5-5 g/dL
      Lipase44530008-78 U/L
      LDH258332125-220 U/L
      Liver function tests
      Alkaline Phosphatase64690-150 U/L
      AST17145-34 U/L
      ALT10110-55 U/L
      Total Bilirubin0.20.10.0-1.4 mg/dL
      Venous blood gas
      pH7.147.36-7.44
      pCO25034-46 mmHg
      HCO316.422-26 mmol/L
      Base excess-11.8- 2.5-2.5 mmol/L
      B) HematologyInitial AdmissionTwo months LaterNormal Range
      Cell counts
      Leukocytes10.7233.5-10.5 × 103/uL
      Hemoglobin9.28.911.6-15.6 g/dL
      Hematocrit282734-46%
      MCV919380-97fL
      Platelet count595534160-370 × 103/uL
      Anemia work-up
      Haptoglobin26435-250 mg/dL
      Corrected reticulocyte count1.70.5-2%
      Iron2850-175 ug/dL
      % Saturation1716-50%
      TIBC165250-450 mcg/dL
      Transferrin132180-382 mg/dL
      Ferritin1785-204 ng/mL
      Folate9
      Value obtained 8 months after initial admission.
      7-31.4 ng/mL
      C) Complement TestingInitial Admission
      Initially, C3Nef according to C3CSA testing was negative, but low positive according to IFE testing; sMAC and Factor Ba was elevated reflect overactivity of terminal complement pathway and alternate pathway C3 convertase respectively. The significance of elevated C5, CH50 and properdin is unclear.
      Two months Later
      On repeat testing 2 months after initial admission, sMAC elevated slightly reflecting continued activation of complement cascade.
      Normal Range
      C3Nef (C3CSA)16%10<20%
      C5Nef(C5CSA)14%<20%
      Net Activity Report16.40%<7.5%
      sMAC0.490.40<0.3 mg/L
      C3c1.1<2.0 mg/L
      Complement Bb1.11.1<2.2 mg/L
      Factor B43.636.822-50 mg/dL
      Complement Ba>6.806.1<1.2 mg/L
      Complement C312883-193 mg/dL
      Complement C43115-57 mg/dL
      Complement C521.610-21 mg/dL
      CH5019660-144 CAE units
      Properdin8.010-33 mg/L
      D) Other immunologic/infection testingInitial AdmissionTwo months LaterNormal Range
      ANA1:80; speckledNegative
      Anti-dsDNA70-50 IU/L
      ENA
      Extractable Nuclear Antigen Antibodies = ENA. Includes SM/RNP, Smith, SSA, and SSV antibodies. Negative results indicated reflects negative result for all antibodies stated.
      NegativeNegative
      Rheumatoid factor<150-29 IU/mL
      Antiphospholipid Ab
      Antiphospholipid antibodies include lupus anticoagulant, anti-beta-2-glycoprotein1, and anti-cardiolipin antibodies.
      NegativeNegative
      ANCANegativeNegative
      Anti-GBM00-19 AU/mL
      SPEPNo monoclonal spikeNegative
      Serum FLC
      • Kappa15.10.33-1.94
      • Lambda14.10.57-2.63
      • Kappa/lambda1.070.26-1.95
      Serum IgG Subclass 4259
      Result from pancreatitis workup drawn 19 months after initial admission
      240-1118 mg/dL
      Hepatitis B surface AgNegativeNegative
      Hepatitis B core AgNegativeNegative
      Hepatitis C antibodyNegativeNegative
      HIV antibody/antigenNegativeNegative
      Urine cultureNegativeNegative
      Quantiferon-TB GoldNegativeNegative
      E) Genetic Testing
      Genetic testing for CFH, CFI, MCP (CD46), CFB, CFHR5, C3, THBD, DGKE, PLG, ADAMTS13, MMACHC genes and functional complement testing were analyzed at the Molecular Otolaryngology & Renal Research Laboratory at the University of Iowa.
      Results
      Gene
      C3NM_000064 c.4838G>T, p.Gly1613Val(Variant of unknown significance
      Ann ultra-rare heterozygous variant of C3 was found with valine replacing glycine at nucleotide number 4838. Predicted pathogenic by 4 out of 6 available pathogenicity algorithms. GERP++RS (conserved, score 2.4), PhyloP (conserved, score 2.360000), Polyphen2 HDIV (possible damaging, score 0.0626), and mutation taster (disease causing, score 0.0650414).
      )

      Minor allele frequency: 0.00% in African American and 0.01% in European Americans
      As reported in Exome Variant Sever
      .
      CFH-CFHR5del[CFHR3-CFHR1] (Heterozygous deletion
      Multiple ligation dependent probe amplification (MLPA) identified CFH-CFHR5 gene had a heterozygous deletion of CFHR3-CFHR1. This is common copy number variant and is found in the general population.
      )
      BUN, blood urea nitrogen; LDH, lactate dehydrogenase; AST, aspartate aminotransferase; ALT alanine aminotransferase; MCV, Mean corpuscular volume; TIBC, total iron binding capacity; C3Nef denotes C3 nephritic factor; C5Nef, C5 nephritic factor; IFE, immunofixation electrophoresis; sMAC, soluble membrane attack complex; ANA, antinuclear antibody; anti-dsDNA, anti-double stranded DNA antibody; ANCA, antineutrophil cytoplasmic antibody; anti-GBM, anti-glomerular basement membrane antibody; SPEP, Serum Protein Electrophoresis; FLC, Free Light Chains; Ag, antigen.
      a Value obtained 8 months after initial admission.
      b Initially, C3Nef according to C3CSA testing was negative, but low positive according to IFE testing; sMAC and Factor Ba was elevated reflect overactivity of terminal complement pathway and alternate pathway C3 convertase respectively. The significance of elevated C5, CH50 and properdin is unclear.
      c On repeat testing 2 months after initial admission, sMAC elevated slightly reflecting continued activation of complement cascade.
      d Extractable Nuclear Antigen Antibodies = ENA. Includes SM/RNP, Smith, SSA, and SSV antibodies. Negative results indicated reflects negative result for all antibodies stated.
      e Antiphospholipid antibodies include lupus anticoagulant, anti-beta-2-glycoprotein1, and anti-cardiolipin antibodies.
      f Result from pancreatitis workup drawn 19 months after initial admission
      g Genetic testing for CFH, CFI, MCP (CD46), CFB, CFHR5, C3, THBD, DGKE, PLG, ADAMTS13, MMACHC genes and functional complement testing were analyzed at the Molecular Otolaryngology & Renal Research Laboratory at the University of Iowa.
      h Ann ultra-rare heterozygous variant of C3 was found with valine replacing glycine at nucleotide number 4838. Predicted pathogenic by 4 out of 6 available pathogenicity algorithms. GERP++RS (conserved, score 2.4), PhyloP (conserved, score 2.360000), Polyphen2 HDIV (possible damaging, score 0.0626), and mutation taster (disease causing, score 0.0650414).
      I As reported in Exome Variant Sever
      J Multiple ligation dependent probe amplification (MLPA) identified CFH-CFHR5 gene had a heterozygous deletion of CFHR3-CFHR1. This is common copy number variant and is found in the general population.

      Initial exploration including kidney biopsy

      The patient's abdomen pain improved by hospital day 3, but SCr continued to worsen. A kidney biopsy was performed on day 4 (SCr 7.5 mg/dL), revealing a glomerulonephritis with focal crescents, mild-moderate mesangial hypercellularity, and expansion on light microscopy, dominant C3 (2-3+) in the mesangium and along capillary walls on immunofluorescence (IF), but only sparse electron-dense deposits on electron microscopy (EM). These deposits varied considerably in location and were distributed all over the glomerulus, including the mesangial, subendothelial, intra-membranous, and sub-epithelial sites. In addition, there was evidence of severe acute as well as chronic microangiopathy on EM (Fig. 1a). Fig. 1 provides further details regarding kidney biopsy findings. Serology for autoimmune disorders and paraproteinemia was found to be normal (Table 1). Functional and genetic tests to evaluate the alternate complement pathway were also sent prior to beginning therapy and several weeks later confirmed AP dysfunction and a heterozygous ultra-rare C3 variant and a heterozygous deletion in CFHR3-CFHR1 (Table 1).
      Fig 1
      Fig. 1Renal pathology before eculizumab: A) Periodic acid–Schiff (PAS) staining showing a glomerulus with cellular crescent. B) Immunofluorescence staining with granular 2-3+ C3 staining globally in the mesangium and segmentally on capillary walls. C3 staining was also seen segmentally on Bowman's capsule and focally on tubular basement membranes. IgA, IgG, IgM, C1q and fibrin were not detected. C&D) Electron microscope images showing rare subendothelial and subepithelial electron-dense deposit, severe endothelial swelling, sub-endothelial fluff, segmental basement membrane remodeling, and severe foot process effacement.

      Initial treatment (Fig. S1)

      Pulse dose steroids were started on hospital day 6 and plasma exchange on day 7. One volume plasma was exchanged with fresh frozen plasma for 7 sessions over two weeks. The patient was also started on oral cyclophosphamide 150 mg once daily and transitioned to 60 mg daily oral prednisone following the pulse doses. Serum creatinine peaked at 8.4 mg/dL on day 7, then started to improve and was 3.73 mg/dL at discharge (day 14). One month later, SCr further improved (2.51 mg/dL), but the patient developed mucositis, and cyclophosphamide had to be switched to mycophenolate mofetil (MMF; 1000 mg twice a day).
      The patient was again readmitted for pancreatitis/AKI one month following discharge (third known episode) while still on MMF and high-dose prednisone. On admission, lipase was 3000 U/L,SCr was 2.2 mg/dL (peaking at 4.02 mg/dL three days later), and UPC was 6.34 g/g. Other than conservative medical management with IVF and bowel rest, high dose prednisone (60 mg daily) was continued, and eculizumab (in the standard aHUS adult dosing) was started instead of MMF, as the latter was presumed to have failed in controlling the complement dysfunction. Functional complement testing was repeated prior to the start of eculizumab (Table 1). At discharge, SCr was 3.76 mg/dL, and high dose prednisone was continued in the form of a slow taper (40 mg once daily at discharge to 5 mg once daily dose over 3 months). At 6 months after starting eculizumab and while on prednisone 5 mg once daily, the patient was noted to be in partial renal remission (SCr 1.41 mg/dL, UPC 2.97 g/g).

      Subsequent course

      Over time, the patient started to have difficulty with adherence as she was experiencing headaches and nausea with her eculizumab infusions. She had recurrence of her pancreatitis/AKI episodes, which seemed to correlate to her missed eculizumab infusions about three months after the start of this agent and three months prior to the time of partial renal remission (Fig. S1). Six months following the partial renal remission, SCr increased to 2.5 mg/dL, urinalysis showed 21-50 RBC/hpf, and UPC was 5.83 g/g. A prednisone taper was repeated (Fig. S1), and a second kidney biopsy was performed to guide therapy.

      Second kidney biopsy

      Light microscopy showed worsening global (41/59) glomerulosclerosis, but no mesangial or endocapillary hypercellularity was noted. Immunofluorescence continued to show dominant C3 (3+) along the capillary walls. On electron microscopy, the majority of the sample was sclerosed, with the 1/4 viable glomerulus showing sparse electron-dense deposits in mesangium and mesangial matrix expansion (Fig. 1 and 2B). Given the lack of significant inflammatory activity on the biopsy, prednisone was slowly tapered back down to 5 mg daily. Eculizumab and standard management of chronic kidney disease (CKD) were continued.
      Fig 2
      Fig. 2Renal pathology after eculizumab: A) Trichrome stain showing severe interstitial fibrosis and tubular atrophy (40 X). B) Periodic acid–Schiff (PAS) stain showing severe diffuse global glomerulosclerosis (100 X). C) PAS stain showing a glomerulus with segmental sclerosis but notably no active glomerulonephritis or microangiopathy (200 X). D) Electron microscope image showing sparse electron-dense deposits in the mesangium.

      Pancreatitis workup (Table 1 and Fig. S2)

      Initially attributed to alcohol use, her pancreatitis workup was performed only later in the course of her disease as recurrent episodes of acute pancreatitis continued despite the cessation of alcohol use. The patient saw gastroenterology approximately one and a half years after her initial presentation, and a sphincterotomy was performed. The details of the pancreatitis workup are provided in Fig. S2. The etiology of her pancreatitis, however, remained unclear as episodes of pancreatitis continued even after sphincterotomy in the setting of ongoing poor adherence with eculizumab.

      Discussion

      We present a case of C3G with several unique aspects:(1) a distinctive clinicopathologic phenotype of C3G with partial features of aHUS – severe renal microangiopathy clinically and on histology, episodes of recurrent pancreatitis - but notably, microangiopathic hemolytic anemia was not observed; (2) this clinicopathologic phenotype occurred in the setting of a unique complement genotype (a heterozygous ultra-rare C3 variant of unknown significance and a heterozygous common copy number variation CHFR3-1 deletion); (3) furthermore, following a brief renal response to ongoing terminal complement blockade, our case showed both clinical and pathologic progression in her renal disease, highlighting some of the challenges of treating C3G.
      Our patient was found to have histologically proven C3 glomerulopathy (Fig. 1a) with functional complement studies showing alternate and terminal complement dysfunction (persistently elevated Ba, reduced Properdin, and elevated serum membrane attack complex [sMAC] levels; Table 1[Section C]). She did have normal serum C3 levels, which can be seen in 25-50% of C3G.
      • Zhang Y.
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      Defining the Complement Biomarker Profile of C3 Glomerulopathy.
      ,
      • Medjeral-Thomas N.R.
      • et al.
      C3 Glomerulopathy: Clinicopathologic Features and Predictors of Outcome.
      The presence of severe microangiopathy (endothelial swelling, sub-endothelial fluff) was, however, an atypical finding and more frequently associated with atypical HUS.
      Secondary aHUS in glomerulonephritides has been described in the literature.
      • Manenti L.
      • et al.
      Atypical haemolytic uraemic syndrome with underlying glomerulopathies. A case series and a review of the literature.
      Of the 6 cases described by Manenti et al, interestingly, half (3/6) had C3G.
      • Manenti L.
      • et al.
      Atypical haemolytic uraemic syndrome with underlying glomerulopathies. A case series and a review of the literature.
      We also found two reports of aHUS developing at months to years following a childhood diagnosis of MPGN and one report of an eight-year-old developing concurrent clinical aHUS with MPGN on biopsy.
      • Ankawi G.A.
      • Clark W.F.
      Atypical haemolytic uremic syndrome (aHUS) and membranoproliferative glomerulonephritis (MPGN), different diseases or a spectrum of complement-mediated glomerular diseases?.
      ,
      • Janssen van Doorn K.
      • Dirinck E.
      • Verpooten G.A.
      • Couttenye M.M
      Complement factor H mutation associated with membranoproliferative glomerulonephritis with transformation to atypical haemolytic uraemic syndrome.
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      • Mehta K.
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      Atypical hemolytic uremic syndrome with membranoproliferative glomerulonephritis.
      In the post-renal transplant setting, a case of concomitant aHUS-MPGN presenting in a renal allograft recipient following a pulmonary infection and a case of post-transplant TMA in a native kidney C3G have been described.
      • Lorcy N.
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      • Vigneau C.
      Three kidneys, two diseases, one antibody?.
      ,
      • Wen J.
      • et al.
      C3 glomerulonephritis and thrombotic microangiopathy of renal allograft after pulmonary infection in a male with concomitant two complement factor I gene variations: a case report.
      All these cases underscore that the distinction between aHUS and C3G may not be absolute. To our knowledge, ours is the first reported case of AP dysfunction showing a clinical phenotype of a crescentic C3G with scant deposits, severe renal microangiopathy, and potential extra-renal aHUS manifestation of pancreatitis but no evidence of hemolysis.
      The unique C3 variant (c.4838G>T p.Gly1613Val) demonstrated by our patient is extremely rare in the general population (MAF 0.01%).

      NHLBI GO Exome Sequencing Project. Vol. 2021 (University of Washington).

      It is predicted to be pathogenic by multiple computational algorithms (Table 1), but its functional effects require further study. Solely on the basis of the location of the missense variant, the amino acid change from Glycine to Valine would occur in the critical C-terminal complement domain (CTC/C345C) of C3/C3b, which is one of the two sites on C3b involved in the binding of Factor B to C3b to generate the proconvertase C3bB.
      • Ricklin D.
      • Reis E.S.
      • Mastellos D.C.
      • Gros P.
      • Lambris J.D.
      Complement component C3 - The “Swiss Army Knife” of innate immunity and host defense.
      The CTC is also involved in the binding of Factor D to C3Bb, resulting in the cleavage and release of Ba and generation of the final C3bBb convertase.
      • Ricklin D.
      • Reis E.S.
      • Mastellos D.C.
      • Gros P.
      • Lambris J.D.
      Complement component C3 - The “Swiss Army Knife” of innate immunity and host defense.
      In addition, regulators of complement activation (RCA) such as Factor H compete for the Factor B/Bb binding site on the CTC to either accelerate the C3 convertase decay themselves (by displacing Bb) and/or to help bind Factor I and complement receptor 1 (CR1), which then result in degradation of C3b to iC3b or C3dg. C3 gain of function variants have previously been linked to both C3G and aHUS.
      • De Vriese A.S.
      • Sethi S.
      • Van Praet J.
      • Nath K.A.
      • Fervenza F.C.
      Kidney Disease Caused by Dysregulation of the Complement Alternative Pathway: An Etiologic Approach.
      ,
      • Brackman D.
      • et al.
      Thrombotic microangiopathy mimicking membranoproliferative glomerulonephritis.
      Multiple heterozygous missense variants in the CTC domain, most notably at amino acid positions of p.C1518R, p.L1549M, p.S1619R, and p.V1658A, have been linked with aHUS primarily but also with C3G.
      • Osborne A.J.
      • et al.
      Statistical Validation of Rare Complement Variants Provides Insights into the Molecular Basis of Atypical Hemolytic Uremic Syndrome and C3 Glomerulopathy.
      A functional analysis of three missense variants in the CTC domain of the C3 gene (p.R1591G, p.G1606D and p.S1619R)
      • Mohlin F.C.
      • Gros P.
      • Mercier E.
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      • Blom A.M.
      Analysis of C3 Gene Variants in Patients With Idiopathic Recurrent Spontaneous Pregnancy Loss.
      found significant resultant C3/AP dysfunction with each. There was, however, considerable variation in the effects on C3/AP activity between these closely located single AA changes. The C3 hemolytic activity was notably impaired in p.R1591G but unchanged in p.G1606D and increased in p.S1619R. The p.R1591G variant also resulted in increased cleavage of Factor B and thus increased Bb. Two of the variants (p.R1591G and p.G1606D) also showed impaired degradation of the variant C3 convertase by Factor I regardless of the RCA used as well as impaired binding to several RCAs and/or Factor I
      • Mohlin F.C.
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      • Blom A.M.
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      . Despite these supporting data, as well as an extremely low minor allele frequency (Table 1), the C3 variant identified in our patient remains yet, “of uncertain significance” until further functional and familial studies on this exact variant are performed.
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      Standards and guidelines for the interpretation of sequence variants: a joint consensus recommendation of the American College of Medical Genetics and Genomics and the Association for Molecular Pathology.
      Homozygous deletions in CHFR3-CFHR1 are known to result in the development of deficiencies of CFHR proteins and CFH autoantibody-positive (DEAP)–hemolytic uremic syndrome
      • Blanc C.
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      while heterozygous deletions are a common copy number variation.
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      • et al.
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      Heterozygous deletions in CHFR3-CFHR1, however, have also been linked to an increased risk of aHUS.
      • Zipfel P.F.
      • et al.
      Deletion of complement factor H-related genes CFHR1 and CFHR3 is associated with atypical hemolytic uremic syndrome.
      How the heterozygous CFHR3-CFHR1 deletion interacts with this unique C3 variant is currently not known. Given the young age and the unique genotype of our patient, exploration of the effect of these genetic variations identified on her presentation with a C3G/aHUS overlap syndrome is warranted. Genotype functional studies and pedigree/linkage analysis of future C3G/aHUS cases are needed to help to confirm our suspicions and improve our understanding of the clinical phenotype spectrum as well as the genotype-phenotype correlations in complement-mediated kidney diseases.
      The role of terminal complement blockade in C3G remains unclear. Despite the initial partial renal remission, our patient's renal disease did progress. The findings of the second kidney biopsy suggest that this progression was a result of CKD progression rather than a burn-out C3G, as neither active glomerulonephritis nor active endothelial injury was seen. Patients with high sMAC and early disease (without glomerular scarring) have been noted to have a more favorable response to eculizumab in C3G.
      • Ankawi G.A.
      • Clark W.F.
      Atypical haemolytic uremic syndrome (aHUS) and membranoproliferative glomerulonephritis (MPGN), different diseases or a spectrum of complement-mediated glomerular diseases?.
      ,
      • Vivarelli M.
      • Emma F.
      Treatment of C3 glomerulopathy with complement blockers.
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      • et al.
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      Patient response may still vary based on the degree of upstream dysfunction of the AP.
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      • et al.
      Eculizumab for dense deposit disease and C3 glomerulonephritis.
      Our patient displayed elevated levels of sMAC, but did have significant glomerular scarring even on the first kidney biopsy and had hematuria/proteinuria for at least one year prior to the start of any immunosuppressive therapy. Larger and prospective studies for identifying optimal timing and biomarkers of response for both immunosuppression and terminal complement blockade are needed for C3G.

      Conclusion

      In conclusion, our case highlights and adds to the current evidence for the existence of a clinical syndrome of C3G/aHUS overlap. Our patient presented with recurrent flares of glomerulonephritis and pancreatitis, but there was no evidence of hemolysis. On renal pathology, a crescentic C3G with scant deposits and severe renal microangiopathy were noted. This unique clinicopathologic phenotype occurred in the presence of an ultra-rare heterozygous C3 missense variant (c.4838G>T, p.Gly1613Val) and a heterozygous deletion of CHFR3-CFHR1. Our case emphasizes a subset of patients with C3G where terminal complement blockade may be useful, as well as the need to control complement C3G/aHUS in a timely manner. The optimal treatment of complement-mediated kidney diseases remains to be established.

      Disclosures

      Financial Disclosure

      All the authors report no financial disclosures.

      Declaration of Competing Interest

      All the authors report no financial or non-financial conflict of interest.

      Appendix. SUPPLEMENTARY MATERIALS

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