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Clinical Research
Open Access

Combined Complement Gene Mutations in Atypical Hemolytic Uremic Syndrome Influence Clinical Phenotype

Elena Bresin, Erica Rurali, Jessica Caprioli, Pilar Sanchez-Corral, Veronique Fremeaux-Bacchi, Santiago Rodriguez de Cordoba, Sheila Pinto, Timothy H.J. Goodship, Marta Alberti, David Ribes, Elisabetta Valoti, Giuseppe Remuzzi, Marina Noris and on behalf of the European Working Party on Complement Genetics in Renal Diseases
JASN March 2013, 24 (3) 475-486; DOI: https://doi.org/10.1681/ASN.2012090884
Elena Bresin
*Clinical Research Center for Rare Diseases, “Aldo e Cele Daccò,” Mario Negri Institute for Pharmacological Research, Ranica, Bergamo, Italy;
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Erica Rurali
*Clinical Research Center for Rare Diseases, “Aldo e Cele Daccò,” Mario Negri Institute for Pharmacological Research, Ranica, Bergamo, Italy;
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Jessica Caprioli
*Clinical Research Center for Rare Diseases, “Aldo e Cele Daccò,” Mario Negri Institute for Pharmacological Research, Ranica, Bergamo, Italy;
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Pilar Sanchez-Corral
†Hospital Universitario La Paz and Centro de Investigacion Biomedica en Enfermedades Raras, Madrid, Spain;
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Veronique Fremeaux-Bacchi
‡Assistance Publique-Hopitaux de Paris, Hôpital Europeen Georges-Pompidou, Service d’Immunologie Biologique, Paris, France;
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Santiago Rodriguez de Cordoba
§Centro de Investigaciones Biològicas and Centro de Investigacion Biomedica en Enfermedades Raras, Madrid, Spain;
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Sheila Pinto
†Hospital Universitario La Paz and Centro de Investigacion Biomedica en Enfermedades Raras, Madrid, Spain;
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Timothy H.J. Goodship
‖Institute of Genetic Medicine, Newcastle University, Newcastle upon Tyne, United Kingdom;
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Marta Alberti
*Clinical Research Center for Rare Diseases, “Aldo e Cele Daccò,” Mario Negri Institute for Pharmacological Research, Ranica, Bergamo, Italy;
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David Ribes
¶Department of Nephrology, Centre Hospitalier Universitaire de Toulouse, Toulouse, France; and
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Elisabetta Valoti
*Clinical Research Center for Rare Diseases, “Aldo e Cele Daccò,” Mario Negri Institute for Pharmacological Research, Ranica, Bergamo, Italy;
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Giuseppe Remuzzi
*Clinical Research Center for Rare Diseases, “Aldo e Cele Daccò,” Mario Negri Institute for Pharmacological Research, Ranica, Bergamo, Italy;
††Department of Nephrology and Dialysis, Azienda Ospedaliera, Ospedali Riuniti di Bergamo, Italy
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Marina Noris
*Clinical Research Center for Rare Diseases, “Aldo e Cele Daccò,” Mario Negri Institute for Pharmacological Research, Ranica, Bergamo, Italy;
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Abstract

Several abnormalities in complement genes reportedly contribute to atypical hemolytic uremic syndrome (aHUS), but incomplete penetrance suggests that additional factors are necessary for the disease to manifest. Here, we sought to describe genotype–phenotype correlations among patients with combined mutations, defined as mutations in more than one complement gene. We screened 795 patients with aHUS and identified single mutations in 41% and combined mutations in 3%. Only 8%–10% of patients with mutations in CFH, C3, or CFB had combined mutations, whereas approximately 25% of patients with mutations in MCP or CFI had combined mutations. The concomitant presence of CFH and MCP risk haplotypes significantly increased disease penetrance in combined mutated carriers, with 73% penetrance among carriers with two risk haplotypes compared with 36% penetrance among carriers with zero or one risk haplotype. Among patients with CFH or CFI mutations, the presence of mutations in other genes did not modify prognosis; in contrast, 50% of patients with combined MCP mutation developed end stage renal failure within 3 years from onset compared with 19% of patients with an isolated MCP mutation. Patients with combined mutations achieved remission with plasma treatment similar to patients with single mutations. Kidney transplant outcomes were worse, however, for patients with combined MCP mutation compared with an isolated MCP mutation. In summary, these data suggest that genotyping for the risk haplotypes in CFH and MCP may help predict the risk of developing aHUS in unaffected carriers of mutations. Furthermore, screening patients with aHUS for all known disease-associated genes may inform decisions about kidney transplantation.

Hemolytic uremic syndrome (HUS) is a rare disease of microangiopathic hemolysis, thrombocytopenia, and renal failure.1,2 The most common form in children is associated with infection by certain strains of Escherichia coli, which produce Shiga-like toxins.3 This form has a good prognosis.1 There are rarer atypical forms (aHUS), not associated with Shiga-like toxins-producing bacteria, that have a worse outcome, with up to 50% of cases progressing to end stage renal failure (ESRF) and 10%–15% dying during the acute phase.1,4

Inherited defects that determine uncontrolled activation of the alternative complement pathway have been well documented in aHUS patients.2,5,6 Research in recent years has identified more than 120 different mutations, accounting for around 40%–60% of cases, in the genes encoding complement factor H (CFH),7–9 membrane cofactor protein (MCP),10–13 complement factor I (CFI),14–16 C3,17 complement factor B (CFB),18,19 CFH-related 5 (CFHR5),20 and thrombomodulin (THBD).20,21 In addition, anti-CFH autoantibodies have been described mostly in children that lack CFHR1 and CFHR3 because of a deletion of the corresponding genes.22–26 Novel genetic abnormalities of CFHR1, CFHR3, and CFHR4 and genomic rearrangement between CFH and CFHR1 have recently been described.27,28

Incomplete penetrance of aHUS has been reported in mutation carriers,12,29–31 indicating that complement gene mutations confer predisposition to develop aHUS, with additional genetic and/or environmental hits necessary for disease manifestation.7,32,33 In keeping with this hypothesis, patients with mutations in more than one complement gene (combined gene mutations) have been described.20,29,34,35 This study was designed to (1) determine the frequency of combined complement gene mutations among four cohorts of aHUS patients; (2) compare short- and long-term outcomes, response to plasma treatment, and outcome of kidney transplantation among patients carrying mutations in different gene combinations; and (3) compare clinical parameters in patients carrying combined mutations versus patients with mutations in a single complement gene. Thanks to a joint effort by the European Working Party on Complement Genetics in Renal Diseases, we genotyped almost 800 aHUS patients for aHUS-associated genes, identifying 27 patients with combined gene mutations.

Results

Patients and Mutations

We undertook mutation screening of CFH, MCP, CFI, C3, and CFB for 795 aHUS patients (including probands and affected relatives) from four independent cohorts: the International Registry (n=274, 58% from Italy, 15% from other European countries, 14% from North America, 8% from the Middle East, 2% from South America, 2% from Africa, and 1% from Asia) and French (n=214), Spanish (n=191), and UK (n=116) cohorts.

Twenty-seven patients with combined mutations in CFH, MCP, CFI, C3, and CFB were identified (27/795; 3.4%) (Figure 1 and Table 1). Seven patients carried combined mutations in CFH and MCP, four patients carried combined mutations in CFH and CFI, two patients carried combined mutations in CFH and C3, nine patients carried combined mutations in MCP and CFI, one patient carried combined mutations in MCP and C3, one patient carried combined mutations in CFI and C3, one patient carried combined mutations in CFI and CFB (double-mutated), and two patients carried combined mutations in CFH, MCP, and CFI (triple-mutated) (Tables 1 and 2). Mutations in a single gene were found in 323/795 (40.6%) patients (single-mutated) (Table 2). Considering overall patients carrying single and combined mutations, we found 350/795 (44%) mutated patients. We noticed that several MCP and CFI mutations were only found combined with mutations in other genes, suggesting a low pathogenic potential requiring another genetic abnormality to induce aHUS (Figure 1).

Figure 1.
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Figure 1.

Localization of the mutations in CFH, MCP, CFI, C3, and CFB that were found in 27 patients with combined gene mutations and patients with single mutations from the four cohorts. Gene variations found in patients as the sole mutation are in gray, changes found only combined with other mutations are in yellow, and changes found both as single or combined mutations are in light blue.

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Table 1.

Summary of patients with combined mutations

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Table 2.

Prevalence of patients with single and combined mutations in CFH, MCP, CFI, C3, and CFB in the four cohorts

Consistently, 22.6% and 27% of patients with either MCP or CFI mutations versus only 8%–10% of patients with CFH, C3, or CFB mutations showed mutations in other genes (Table 2). This observation indicates that single mutations in CFH, C3, or CFB alone are more likely to be sufficient to induce aHUS than single mutations in MCP or CFI.

All patients with combined mutations were Caucasians, with the exception of FRE60 from North Africa ancestry (Table 1 and Supplemental Figure 1). Fifteen patients had familial HUS (10 families), whereas twelve patients had sporadic aHUS (Table 1 and Supplemental Figure 1). Analysis of combined-mutated probands (n=22) (Supplemental Figure 1) and their available relatives (n=79; of whom five double- and four single-mutated subjects were affected) (Supplemental Figure 1) revealed a progressive significant increase of penetrance across subjects with mutations in one, two, or three genes (Figure 2). Among 35 single-mutated subjects, only 4 subjects were affected (Supplemental Figure 1); 25 of 40 double-mutated subjects developed aHUS (63%; ranging from 40% to 66% in subjects with mutations in different gene combinations) (Supplemental Figure 2), whereas both triple-mutated subjects were affected (Supplemental Figure 1). In patients and their relatives, we genotyped two single-nucleotide polymorphisms (SNPs) in CFH (rs3753394, c.1-332C>T and rs1065489, c.2808G>T, p.E936D) that tag the disease risk haplotype CFH-H37,32 and one SNP in MCP (rs7144, c.*897T>C) that tags the MCPggaac risk haplotype32 (Table 1 and Supplemental Figure 1). The presence of at least one copy of both risk haplotypes significantly increased disease penetrance in double-mutated subjects, although penetrance still remained incomplete (Figure 2).

Figure 2.
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Figure 2.

Impact of the number of risk haplotypes in CFH (CFH-H3 targeted by rs3753394, c.1-332C>T and rs1065489, c.2808G>T p.E936D) and MCP (MCPggaac targeted by rs7144, c.*897T>C) on aHUS penetrance in single- (carrying mutations in one complement gene), double- (carrying mutations in two different genes), or triple-mutated subjects (carrying mutations in three different genes). Penetrance in each subgroup was calculated by the ratio between the number of carriers affected and the total number of carriers. We considered only pedigrees for which at least one relative other than the proband was genotyped for the mutations and haplotypes (marked with an asterisk in Supplemental Figure 1). In the table below the graph, we reported the number of affected subjects (bold)/the number of carriers for each group. The chi-squared or Fisher exact test was used for statistical analysis as appropriate.

Case Reports of Familial Cases

Clinical history and pedigrees of familial forms are reported in Supplemental Material.

Clinical Findings

Patients with CFH, MCP, CFI, C3, or CFB mutations combined with mutations in other genes were subgrouped as CFH-combined (n=15), MCP-combined (n=19), CFI-combined (n=17), C3-combined (n=4), and CFB-combined (n=1), respectively. According to this classification, we included each patient with mutations in two or three genes in two or three subgroups, respectively (e.g., a patient with CFH and CFI mutations appears in both the CFH-combined and the CFI-combined subgroups). No significant differences were found in disease presentation, outcome, and response to plasma among each subgroup, with the exception of a higher frequency of triggering/underlying conditions in the CFH-combined versus the CFI-combined subgroups (Table 3). Transplant outcome was comparable among subgroups, with 44%–50% of grafts lost at 3 years post-transplant (Table 3). We found a higher prevalence of low C3 levels in the C3-combined versus the CFI- and MCP-combined subgroups and a higher prevalence of low CFI levels in the CFI-combined versus the CFH-combined subgroup (Table 3).

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Table 3.

Clinical and biochemical data of subgroups of patients with combined mutations from the four cohorts

Thereafter, clinical data of patients with combined mutations (n=27) were compared with data from patients with mutations in a single gene (CFH, MCP, CFI, C3, or CFB) from the same cohorts for which clinical data were available (260 of 3239,10,12,15,17–19,29,30,34,36–44; unpublished data) (Table 4). No significant difference was observed in disease presentation, outcome, and response to plasma between overall combined-mutated and overall single-mutated patients (Table 4). Graft outcome at 3 years in the overall 10 combined-mutated patients receiving a kidney transplant (14 grafts) was similar to graft outcome in the overall 60 transplanted single-mutated patients (77 grafts), with 50% versus 65% of graft loss at 3 years, respectively (Table 4).

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Table 4.

Clinical and biochemical data of combined- and single-mutated patients from the four cohorts

We then compared clinical data from subgroups with CFH- (n=15), MCP- (n=19), CFI- (n=17), C3- (n=4), or CFB- (n=1) combined mutations with data from the corresponding subgroups of single-mutated patients (CFH, n=148; MCP, n=40; CFI, n=27; C3, n=36; CFB, n=9). Disease presentation and short- and long-term outcomes were not significantly different among patients with CFH-, CFI-, or C3-combined mutations and patients with mutations only in CFH, CFI, or C3, respectively (Table 4). However, MCP combined-mutated patients had a worse long-term outcome than MCP single-mutated patients (Table 4). Indeed, at 3 years, 50% of the former patients lost renal function, whereas despite repeated recurrences, only 18.5% of the latter patients developed ESRF (Table 4). Interestingly, a triggering event was reported at the first episode in only 25% of CFI combined-mutated patients, whereas all CFI single-mutated patients developed aHUS after a triggering event (Table 4).

No significant difference in plasma efficacy was observed in patients with CFH- or C3-combined mutations (70% and 80% remission of plasma-treated episodes, respectively) versus patients with CFH or C3 mutations alone (62.5% and 57% remission) (Table 4). However, plasma treatment induced remission in 79% of episodes in patients with CFI-combined mutations versus only 30% of episodes in CFI single-mutated patients. Remission was achieved in 70% and 95% of plasma-treated episodes in MCP combined- and single-mutated patients, respectively (Table 4).

Transplanted patients carrying either CFH- or CFI-combined mutations showed a trend to have a better prognosis than patients with CFH or CFI mutations alone (Table 4). In MCP combined-mutated patients, 6 of 12 transplanted kidneys were lost within 3 years post-transplant, and 4 cases were a recurrence (Table 4 and Supplemental Table 1). However, only 3 of 13 grafts were lost in MCP single-mutated patients (1 graft for aHUS recurrence)12 (Table 4).

Two patients with combined MCP/CFI mutations (Supplemental Table 1) and nine single-mutated patients (six patients with CFH, one patient with MCP, one patient with CFI, and one patient with C3 mutations40–42; unpublished data) received post-transplant plasma prophylaxis; in all but one (with a single CFH mutation41), the graft function was preserved at 3-year follow-up. No combined-mutated patient received Eculizumab post-transplant, whereas four single-mutated patients (three patients with CFH and one patient with C3 mutations alone) were given Eculizumab prophylaxis; all had good transplant outcome at the last follow-up (unpublished data).41,43,44

In patients with combined mutations receiving calcineurin inhibitors, five of eight grafts failed within 3 years post-transplant (three cases were lost for recurrence), whereas all three grafts in patients not receiving calcineurin inhibitors were functioning (Supplemental Table 1).

Discussion

Reports in large patient numbers have described the clinical course, response to therapy, and transplant outcome in aHUS patients with single-gene mutations.20,29,35,45 A few cases with mutations in more than one complement gene have been reported,20,29,33–35,46 with 3%–12% of prevalence.20,29 Here, we report the results of a collaborative study undertaken by the European Working Party on Complement Genetics in Renal Diseases that included 795 aHUS patients screened for known disease-associated genes. We identified 27 patients with combined mutations, accounting for 3.4% of aHUS cases. In a previous report,20 three of eight cases of combined mutations were accounted for by THBD and/or CFHR5 mutations. THBD, CFHR5, and other CFHRs genes were not systematically screened in the cohorts included here, and this process may have led to an underestimation of the prevalence of combined mutations. The present and previously published findings20,29,33–35 indicate that all aHUS-associated genes should be screened in a patient who presents with aHUS. In those patients in whom a single mutation has already been identified previously, then additional screening may be necessary if all the aHUS-associated genes have not been analyzed. Likewise, as additional genes are identified in the future, it may be necessary to screen these new genes in current patients.

Of note, only 8%–10% of patients with CFH, C3, or CFB mutations carried abnormalities in other genes, suggesting that mutations in CFH, C3, or CFB alone may be sufficient to cause aHUS. In contrast, ∼25% of patients with a mutation in MCP or CFI had a second or third mutation in other complement genes. This observation is consistent with previous data34 that 5 of 23 aHUS patients with CFI mutations carried at least one additional genetic risk factor, such as an MCP, CFH, or C3 mutation.

It is generally accepted that complement gene mutations confer predisposition to aHUS rather than directly causing the disease.2,33 Here, we found low aHUS penetrance in subjects with single-gene mutations, whereas penetrance was higher, but still incomplete, in carriers of mutations in two genes; complete penetrance was observed in the two subjects with mutations in three genes.

Control of complement is performed by a network of plasma and membrane-associated regulatory proteins that restricts complement activation to the surface of microorganisms.47 The case of patients with mutations in three genes indicates that the concurrence of multiple genetic susceptibility factors involving plasma- and membrane-associated regulators is required to impair protection to host tissues enough to induce aHUS. Polymorphisms and haplotype blocks in CFH and MCP increase risk of aHUS.32,33 Consistently, we found that the presence of at least one copy of both risk haplotypes, CFH-H3 and MCPggaac, significantly increased disease penetrance in carriers of mutations in two genes, although penetrance still remained incomplete.

Recently, the concomitant presence of anti-CFH antibodies on the background of homozygous CFHR1–3 deletion and mutations in CFH, MCP, C3, or more commonly, CFI has been reported in aHUS.22,34 Here, we quantified CFHR1–3 copy number and anti-CFH antibodies in combined-mutated patients, but none of them was either homozygous for the deletion or had anti-CFH antibodies.

The association of triggering/precipitating events with aHUS onset has been emphasized in patients with single-gene mutations.29,30,48 The same occurred in most patients with combined mutations described here, in whom aHUS manifested on triggering conditions (mainly infections), indicating that environmental factors are critical determinants of HUS development.

Bienaime et al.34 documented that patients with CFI mutations and complete CFHR1 deletion had a worse prognosis than patients with only a CFI mutation. Here, we found that the concomitant presence of mutations in other genes did not modify the disease prognosis in patients with CFI mutations versus patients with CFI mutations alone. A comparable outcome was also observed in patients with combined and single CFH or C3 mutations. However, we observed that the presence of mutations in other genes was associated with a more severe phenotype in MCP-mutated patients with a higher incidence of ESRF than in patients with MCP mutations alone. Altogether, these results would indicate that mutations in CFH, CFI, or C3 exert a dominant effect on disease phenotype in patients with combined MCP mutations.

Previous data have shown that most patients carrying single-gene mutations undergo remission on plasma treatment.1,29,35,49,50 Here, we show that the same occurs in combined-mutated patients. Notably, patients with CFI-combined mutations showed a higher rate of response to plasma than patients with a single CFI mutation; however, the long-term outcome in the two groups was identical.

The finding that patients with MCP-combined mutations had a lower rate of remission after plasma treatment than patients with single MCP mutations possibly reflects the less severe disease phenotype in the latter group. Indeed, in patients with single MCP mutations, remission was generally obtained with or without plasma treatment.29 In the few described patients with single MCP mutations that had very severe disease course, plasma given during the acute episode or to prevent recurrences did not influence the natural course of the disease.14,40

We wish to emphasize that our analysis of response to plasma therapy is limited, because it was based on retrospective data; because of the rarity of combined-mutated patients, it necessarily included cases from several centers, where different plasma therapy protocols may have been used.

Previous data emphasized that kidney transplantation alone in aHUS is compromised by the risk of recurrence, especially in patients with mutations in genes encoding circulating complement proteins.51,52 Consistent with published data,53 here, we found a high prevalence of post-transplant recurrence in patients with combined mutations. The impact of calcineurin inhibitor use on incidence of recurrences is still a matter of debate. Some authors showed that early use of cyclosporine increases the risk, but others denied it.52 The data presented here, showing that, in combined-mutated patients, graft loss for recurrence clustered in the subgroup receiving calcineurin inhibitors, may support the former possibility.

Notably, we observed a high incidence of graft failure for recurrence in the MCP-combined group, contrasting with the good graft outcome among MCP single-mutated patients (1 of 13 graft failure for recurrence).10,12,29,40,52 It is plausible that the concomitant presence of genetic abnormalities leading to dysfunction in circulating proteins (CFH and/or CFI), which could not be corrected by an isolated kidney transplant, contributed to the higher risk of post-transplant recurrences in the MCP-combined group. The observation that two patients with combined MCP/CFI mutations who received intensive plasma prophylaxis had an uneventful post-transplant course would confirm previous data,42,51 showing a beneficial impact of such a regimen to prevent post-transplant recurrences.

In summary, the cases presented here underline the complexity of aHUS genetics. Complement gene mutations may have disparate consequences, ranging from highly pathogenic mutations associated with complete disease penetrance and unfavorable outcome to variants that cause aHUS only combined with other mutations and/or CFH and MCP risk haplotypes.

We would recommend that aHUS patients are screened for all known disease-associated genes. CFH-H3 and MCPggaac risk haplotypes should be also checked, because they impact disease penetrance and phenotype in mutation carriers. The latter point would be of help for genetic counseling in patients’ relatives; indeed, carriers of both combined mutations and risk haplotypes could have a higher likelihood to develop aHUS in life than subjects carrying only combined mutations.

If a novel sequence variant is identified in a patient, it should be looked for in at least 300–400 healthy ethnically matched controls to distinguish disease-causing mutations from rare nonpathogenic variants. Screening should not be stopped after finding a mutation to avoid missing other genetic susceptibility factors influencing disease phenotype. This recommendation particularly applies to patients with MCP or CFI mutations, because they have a higher probability of also carrying mutation in another gene than patients with CFH or C3 mutations. Importantly, in MCP-mutated patients, the presence of combined mutations highly impacts the outcome and risk of post-transplant recurrence versus patients with MCP mutations alone.

The HUS mutation database (www.fh-hus.org) is currently being updated to provide information on whether a given mutation was found alone or combined with other mutations to help optimize genetic screening.

Screening should also include anti-CFH antibodies to rapidly identify patients who need immunosuppressive therapies and intensive plasma exchange to taper the antibody titer,26 THBD and CFHRs gene sequencing, and Multiplex Ligation-Dependent Probe Amplification to identify deletions and rearrangements in CFH-CFHRs.

Recently, the humanized anti-C5 monoclonal antibody Eculizumab has been effectively administered to aHUS patients to induce disease remission, treat, or prevent post-transplant recurrences,41,54–56 and its efficacy has been confirmed in two controlled trials in plasma-resistant and -dependant patients.57,58 However, this drug is not universally available at present, and probably, it will not be in the future because of the high cost and need for chronic treatment. A careful genetic characterization, allowing prediction of disease phenotype and risk of post-transplant recurrences, could help selection of those patients who may need and benefit most from Eculizumab. In particular, it could be useful to exclude from Eculizumab therapy patients with single MCP mutations who are reported to have a good post-transplant outcome52 or patients with anti-CFH antibodies who may benefit from plasma exchange, steroids, or Rituximab.26,59

Concise Methods

Patients

aHUS was diagnosed in all patients included in this study based on microangiopathic hemolytic anemia and thrombocytopenia defined on the basis of hematocrit (Ht) less than 0.3 (30%), hemoglobin (Hb) level less than 100 g/L (10 g/dl), serum lactate dehydrogenase (LDH) level greater than 460 U/L, undetectable haptoglobin level, fragmented erythrocytes in the peripheral blood smear, and platelet count less than 150×109/L (150,000/µl)60 associated with acute renal failure. Familial aHUS was diagnosed when two or more members of the same family were affected by the disease at least 6 months apart and exposure to a common triggering infectious agent was excluded. Sporadic aHUS was diagnosed when one or more episodes of the disease manifested in a subject with no familial history of the disease. At least 300 healthy ethnically matched controls were also genotyped for each variant found in the patients. Only variants found in aHUS patients but not in any of 600 chromosomes were considered mutations. The only exception was the A353V variant in MCP, which was considered a mutation, despite that it was found in 0.7% of 978 chromosomes, because published functional data indicated that the mutant protein is defective in controlling complement activation on cell surface.61 The variants P50A (rs144082872, minor allele frequency [MAF]=0.000, 1/10,758 alleles), H183R (rs75612300, MAF=0.002), I357M (rs200881135, MAF=0.000, 1/2,184 alleles), and P553S (rs113460688, MAF=0.000, 1/2,184 alleles) in CFI are reported in the National Center for Biotechnology Information SNP (http://www.ncbi.nlm.nih.gov) or the 1000 Genomes (http://browser.1000genomes.org/index.html) database as ultra-rare single-nucleotide variations, but they were not found in any of our controls and were included among mutations.

Subjects carrying mutations in two or three different complement genes were defined double- or triple-mutated, respectively (combined-mutated). Subjects carrying heterozygous, double heterozygous, or homozygous mutations in a single gene were called single-mutated. All participating centers have institutional review board approval for the studies included in this report. Informed consent was provided according to the Declaration of Helsinki.

Complement Profile Assessment

Serum concentrations of C3 and C4 were evaluated by kinetic nephelometry. CFH serum levels were measured by radial immunodiffusion assay (The Binding Site, Birmingham, United Kingdom)62 in the International Registry and UK cohorts and ELISA in the French and Spanish cohorts1,9,38; CFI serum levels were measured by ELISA.15 For each laboratory, the normal ranges were set as mean ±2 SD of the values recorded in healthy subjects. Values up the higher limit of the normal ranges were considered as high, whereas values below the lower limit of the normal ranges were considered as low.

Genetic Analyses

Genomic DNA was extracted from peripheral blood leukocytes according to standard procedures. PCR products of the coding sequence and the intronic flanking regions of CFH, MCP, CFI, C3, and CFB genes were genotyped by automatic DNA sequencing.

The reference nucleotide sequence of all genes starts from the codon +1 corresponding to the initial Met residue and includes the signal peptide sequence.

CFH-CFHRs deletion and rearrangements were detected by SALSA MLPA P236-A1 Kit (MRC Holland).

Statistical Analyses

Differences in biochemical and clinical data among subgroups of patients with combined gene mutations and between patients with single and combined mutations were analyzed by the chi-squared test or Fisher exact test (the latter for comparisons when the expected values in at least one cell of a contingency table were less than five) as appropriate. The differences were considered statistically significant at P values≤0.05.

Disclosures

None.

Acknowledgments

The authors are deeply indebted to Drs. R. Maranta, R. Piras, and A. Sorosina for invaluable contribution made by sequencing. We also thank Dr. A. Chianca for helping us with statistical analyses. We thank M. Lena and S. Gamba for the management of patients and biological samples. We thank Dr. G. Barbano, Dr. E. Verrina, and Dr. J.A. Listman for providing detailed clinical information on families #024 and #130 and the French clinicians participating to this study: Drs. E. Boulanger, P. Niaudet, B. Boudaillez, S. Gie, M. Tsimaratos, B. Moulin, F. Fakhouri. and Ch. Loirat for the French Society of Pediatric Nephrology. We also thank Drs. M. Cabello, M. Espinosa, C. Fernández, M. Anton, J.M. Cruzado, and K. Soto for providing clinical data of patients from the Spanish registry. We thank clinicians and patients for their membership of and support to the International Registry of Recurrent and Familial Hemolytic Uremic Syndrome/Thrombotic Thrombocytopenic Purpura (HUS/TTP).

This work was supported by grants from Telethon Project GGP07193, Compagnia di San Paolo (Torino, Italy), Fondazione ART per la Ricerca sui Trapianti ART ONLUS (Milano, Italy), Fondazione Aiuti per la Ricerca sulle Malattie Rare ARMR ONLUS (Bergamo, Italy), Progetto Alice ONLUS (Milano, Italy), and European Union Seventh Framework Programme FP7-EURenOmics project number 305608. P.S.-C. and S.R.d.C. are supported by the Spanish Ministerio de Economía y Competitividad (SAF2010–26583 and PS09/00268), the Comunidad de Madrid (S2010/BMD-2316), and the Fundación Renal Iñigo Alvarez de Toledo. V.F.-B. is supported by the Delegation Regionale a la Recherche Clinique, Assistance Publique-Hopitaux de Paris Grant PHRC AOM 08198, and ANR Grant Genopat 2009. T.H.J.G. is supported by United Kingdom Medical Research Council Grant G0701325.

The results of this paper have been presented in part as oral communication at the 2012 Annual Meeting of the American Society of Nephrology in San Diego, CA (November 1–4, 2012).

Footnotes

  • G.R. and M.N. contributed equally to this work.

  • Published online ahead of print. Publication date available at www.jasn.org.

  • This article contains supplemental material online at http://jasn.asnjournals.org/lookup/suppl/doi:10.1681/ASN.2012090884/-/DCSupplemental.

  • Copyright © 2013 by the American Society of Nephrology

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Journal of the American Society of Nephrology: 24 (3)
Journal of the American Society of Nephrology
Vol. 24, Issue 3
March 2013
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Combined Complement Gene Mutations in Atypical Hemolytic Uremic Syndrome Influence Clinical Phenotype
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Combined Complement Gene Mutations in Atypical Hemolytic Uremic Syndrome Influence Clinical Phenotype
Elena Bresin, Erica Rurali, Jessica Caprioli, Pilar Sanchez-Corral, Veronique Fremeaux-Bacchi, Santiago Rodriguez de Cordoba, Sheila Pinto, Timothy H.J. Goodship, Marta Alberti, David Ribes, Elisabetta Valoti, Giuseppe Remuzzi, Marina Noris, on behalf of the European Working Party on Complement Genetics in Renal Diseases
JASN Mar 2013, 24 (3) 475-486; DOI: 10.1681/ASN.2012090884

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Combined Complement Gene Mutations in Atypical Hemolytic Uremic Syndrome Influence Clinical Phenotype
Elena Bresin, Erica Rurali, Jessica Caprioli, Pilar Sanchez-Corral, Veronique Fremeaux-Bacchi, Santiago Rodriguez de Cordoba, Sheila Pinto, Timothy H.J. Goodship, Marta Alberti, David Ribes, Elisabetta Valoti, Giuseppe Remuzzi, Marina Noris, on behalf of the European Working Party on Complement Genetics in Renal Diseases
JASN Mar 2013, 24 (3) 475-486; DOI: 10.1681/ASN.2012090884
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  • Genetic Disorders of the Glomerular Filtration Barrier
  • Unraveling the Effect of a Potentiating Anti-Factor H Antibody on Atypical Hemolytic Uremic Syndrome-Associated Factor H Variants
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  • Use of Highly Individualized Complement Blockade Has Revolutionized Clinical Outcomes after Kidney Transplantation and Renal Epidemiology of Atypical Hemolytic Uremic Syndrome
  • Synergistic effects of ADAMTS13 deficiency and complement activation in pathogenesis of thrombotic microangiopathy
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  • Genetic Analysis of 400 Patients Refines Understanding and Implicates a New Gene in Atypical Hemolytic Uremic Syndrome
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  • A Patient with Hemolytic Uremic Syndrome and Kidney Failure
  • Recurrent structural variation, clustered sites of selection, and disease risk for the complement factor H (CFH) gene family
  • Statistical Validation of Rare Complement Variants Provides Insights into the Molecular Basis of Atypical Hemolytic Uremic Syndrome and C3 Glomerulopathy
  • Maternal and Fetal Outcomes of Pregnancies in Women with Atypical Hemolytic Uremic Syndrome
  • Hemolytic Uremic Syndrome in Pregnancy and Postpartum
  • Eculizumab cessation in atypical hemolytic uremic syndrome
  • HUS and atypical HUS
  • Atypical hemolytic uremic syndrome
  • All Things Complement
  • High-Throughput Genetic Testing for Thrombotic Microangiopathies and C3 Glomerulopathies
  • The genetic fingerprint of susceptibility for transplant-associated thrombotic microangiopathy
  • Recovery of renal function after long-term dialysis and resolution of cardiomyopathy in a patient with aHUS receiving eculizumab
  • Kidney Disease Caused by Dysregulation of the Complement Alternative Pathway: An Etiologic Approach
  • HUS and the case for complement
  • Characterization of a New DGKE Intronic Mutation in Genetically Unsolved Cases of Familial Atypical Hemolytic Uremic Syndrome
  • Mapping interactions between complement C3 and regulators using mutations in atypical hemolytic uremic syndrome
  • A Novel Atypical Hemolytic Uremic Syndrome-Associated Hybrid CFHR1/CFH Gene Encoding a Fusion Protein That Antagonizes Factor H-Dependent Complement Regulation
  • Factors determining penetrance in familial atypical haemolytic uraemic syndrome
  • New functional and structural insights from updated mutational databases for complement factor H, Factor I, membrane cofactor protein and C3
  • Dynamics of complement activation in aHUS and how to monitor eculizumab therapy
  • Factors Influencing Treatment of Atypical Hemolytic Uremic Syndrome
  • Complement Factor B Mutations in Atypical Hemolytic Uremic Syndrome--Disease-Relevant or Benign?
  • How I treat: the clinical differentiation and initial treatment of adult patients with atypical hemolytic uremic syndrome
  • The importance of genetic mutation screening to determine retransplantation following failed kidney allograft from recurrent atypical haemolytic ureamic syndrome
  • Comprehensive Genetic Analysis of Complement and Coagulation Genes in Atypical Hemolytic Uremic Syndrome
  • Abnormalities in the alternative pathway of complement in children with hematopoietic stem cell transplant-associated thrombotic microangiopathy
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