Collapsing glomerulopathy (CG) is a rare glomerular disease and its familial form is even rarer. CG and non-collapsing forms of focal segmental glomerulosclerosis may both be caused by pathogenic variants in the same genes, but there is less information on genetics of the former disease. We hypothesized that different hits (viral infection and genetic variants) may be involved in the development of a familial CG here described. We performed renal and etiological routine evaluation, PVB19 serology, genetic tests including whole-exome analysis and dosage of serum thrombomodulin (THBD) in two siblings with CG, one healthy sister, and their mother. The THBD gene variant p.A43T in homozygosity was identified in the proband and her affected brother, both with CG. The same mutation was identified in their mother in heterozygosity. THBD levels were elevated in the serum of both affected siblings. They also had PVB19 positive serology and the G1 high-risk apolipoprotein L1 (APOL1) alleles in homozygosity. Their healthy sister had no PVB19-positive serology and no THBD nor APOL1 gene variants. In this case of familial CG, THBD, and APOL1 gene variants, and a previous PVB19 infection may be associated with the development of CG in a multihit process. In addition, the p.A43T THBD variant, identified in the affected siblings, has never been previously described in homozygosis, pointing to a likely autosomal recessive CG trait caused by this gene mutation.

Collapsing focal segmental glomerulosclerosis (FSGS), also known as collapsing glomerulopathy (CG), is a rare glomerular disease that clinically presents with acute kidney injury and nephrotic-range proteinuria. In addition, CG usually has a progressive course and poor response to treatment [1]. CG can be primary, but it is also attributed to several etiologies, as viral infections (HIV, cytomegalovirus, Epstein-Barr-Virus, parvovirus B19 (PVB19), SARS-CoV-2, and others), drugs, severe ischemia [1], thrombotic microangiopathy [2], and other causes. In addition, it is also known that the G1 and G2 risk variants of the apolipoprotein L1 (APOL1) gene increase the risk of developing CG [1]. It is of note that a familial presentation of CG was already described [3, 4], but it is even more uncommon than sporadic forms. THBD is an endothelial glycoprotein that is present in all blood vessels, and variants in THBD gene often translate into quantitative and qualitative abnormalities of this glycoprotein [5]. It was observed that patients with FSGS during active stages have higher levels of soluble THBD, as well as of other markers of endothelial dysfunction when compared with controls [6]. High-risk G1 and G2 variants of the APOL1 gene, predominantly found in people of African descent, increase the risk of developing CG. They are also associated with other kidney diseases, including other forms of FSGS [7]. Here, we present a case of familial CG that was submitted to genetic evaluation and discuss the possible contribution of different factors (genetic variants and infectious agent) in the development of such a phenotype of glomerular disease. Two family members with biopsy-proven CG presented both APOL1 and THBD mutations, as well as previous PVB19 infection.

We performed genetic tests on four members (including two members with biopsy-proven CG) of a two-generation family (Fig. 1) that was screened for renal involvement and is assisted by the Division of Nephrology (Section of Glomerular Diseases) of Federal University of São Paulo (UNIFESP). The proband was a 35-year-old white woman that was diagnosed with CG after presenting edema of inferior limbs. Her kidney biopsy (Fig. 2) revealed in light microscopy CG, showing collapsed capillary loops and podocyte hypertrophy/hyperplasia, microcystic tubular dilation (with positive intratubular PAS protein casts), mild tubular atrophy, and interstitial fibrosis as well as mild arteriosclerosis. Immunofluorescence microscopy was negative for IgA, IgG, IgM, C1q, C3, and fibrinogen. Her parents were consanguineous relatives, and she reported a family history of chronic kidney disease of unspecified etiology (father and paternal grandfather). During the investigation of her disease, it was found that the oldest brother, 38 years old, had the same histological pattern of glomerular disease and required dialysis immediately after diagnosis. His kidney biopsy also was compatible with CG and presented partial glomerular sclerosis, tubular atrophy with areas of ectasia, marked and diffused interstitial fibrosis, focal tubulointerstitial nephritis, and moderate arterial intimal fibrous hyperplasia. Immunofluorescence microscopy was negative for IgA, IgG, IgM, C1q, C3, and fibrinogen. Associated diseases that could be related to the development of glomerulopathies were investigated: ANA, anti-dsDNA, and ANCA were negative; C3, C4, and CH50 levels were in the normal range; HBsAg, anti-HCV e anti-HIV, syphilis serologies were non-reagent. Evaluation of PVB19 infection was performed in the proband, also in her brother with GC and chronic kidney disease stage 5, and healthy sister, being observed positive serology (IgG) only in the index patient and her affected brother. The proband underwent immunosuppressive treatments (oral and IV corticosteroids, IV cyclophosphamide, and oral mycophenolate), at the beginning of the follow-up, before admission in our service, with no improvement in renal function. Subsequently, she was submitted exclusively to treatment with furosemide 40 mg, amlodipine 5 mg, and atorvastatin 20 mg/day, maintaining after 3.5 years of follow-up serum creatinine of 2.66 mg/dL, creatinine clearance of 30.2 mL/min/1.73 m2, 24-h proteinuria of 1.1 g, and serum albumin 4.9 g/dL.

Fig. 1.

Pedigree of two siblings with genetic collapsing FSGS showing the affected members and closest relatives. The arrow indicates the index patient.

Fig. 1.

Pedigree of two siblings with genetic collapsing FSGS showing the affected members and closest relatives. The arrow indicates the index patient.

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Fig. 2.

Light microscopy from the proband. a Segmental collapse of the glomerular capillaries surrounded by hypertrophy/hyperplasia of podocytes (arrow) (PAS – ×400). b Tubulointerstitial damage characterized by tubular dilatation and microcyst formation, tubular atrophy, interstitial fibrosis, and inflammation (PAS – ×100). c Segmental glomerulosclerosis and adhesion to Bowman’s capsule (PAS – ×400). d Collapse of the glomerular tuft (Jones’ silver staining – ×400).

Fig. 2.

Light microscopy from the proband. a Segmental collapse of the glomerular capillaries surrounded by hypertrophy/hyperplasia of podocytes (arrow) (PAS – ×400). b Tubulointerstitial damage characterized by tubular dilatation and microcyst formation, tubular atrophy, interstitial fibrosis, and inflammation (PAS – ×100). c Segmental glomerulosclerosis and adhesion to Bowman’s capsule (PAS – ×400). d Collapse of the glomerular tuft (Jones’ silver staining – ×400).

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Peripheral blood samples were collected for genomic DNA extraction and gene sequencing. Genomic DNA was extracted and then analyzed by whole-exome sequencing (WES) of the proband and her siblings. We also performed Sanger Sequencing to confirm genetic variant (Fig. 3). WES was performed on the Illumina platform according to manufacturer instructions. We analyzed genes from three panels related to nephrotic syndrome, kidney disease, and complement deficiencies in the exome performed in the three family members, as shown in online supplementary Table 1 (for all online suppl. material, see https://doi.org/10.1159/000536244). Exome sequences were filtered for variants by Varstation software and all candidate variants were validated by Sanger sequencing and then submitted to familial segregation. For variant interpretation, the American College of Medical Genetics and the Association for Molecular Pathology (ACMG/AMP) guidelines [8] were used. Final classification was obtained using the VarSome platform [9]. The genetic evaluation of the patient and her relatives found variants in the THBD and APOL1 genes. Out of the three panels analyzed, we found 20 potentially pathogenic mutations, however, only two of them – THBD and APOL1 genes – segregated in affected individuals (online suppl. Table 2). In the proband and her affected brother, the p.A43T variant (rs1800576) was found in homozygosity in the THBD gene. This variant was found in heterozygosity in the patients’ mother and was not present in the proband healthy sister. Based on ACMG criteria, p.A43T was classified as a variant of unknown significance. Furthermore, this variant was found in low frequency in population frequency databases. In gnomAD, the frequency in heterozygosity was 0.002% and in ABraOM the frequency was 0.001. This variant was not found in homozygosity in these databases. In addition to the THDB variant investigation, we also performed the determination of soluble THBD, which levels were shown to be high only in the index patient and her affected brother (Table 1).

Fig. 3.

Genetic variant in THBD detected in the family with CG. Detection of variants in THBD by direct Sanger Sequencing. a Wild sequence. b p.A43T in homozygosis. c p.A43T in heterozygosis.

Fig. 3.

Genetic variant in THBD detected in the family with CG. Detection of variants in THBD by direct Sanger Sequencing. a Wild sequence. b p.A43T in homozygosis. c p.A43T in heterozygosis.

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Table 1.

Genetic, clinical, and laboratory data of the evaluated family members

GenerationSubjectGenderVariant THBDVariant APOL1Parvovirus B19 infectionSerum THBD, pg/mLSerum creatinine, mg/dLSerum albumin, g/dLUrinary protein/creatinine
Female p.A43T in heterozygosis G1 in heterozygosis No 5,967.78 0.75 4.5 0.00 
II Female No variants detected G1 in heterozygosis No 6,521.47 0.92 4.6 0.00 
II Male p.A43T in homozygosis G1 in homozygosis Yes 22,904.16 8.90 4.80 0.94 
II Female p.A43T in homozygosis G1 in homozygosis Yes 12,597.42 2.02 4.60 1.03 
GenerationSubjectGenderVariant THBDVariant APOL1Parvovirus B19 infectionSerum THBD, pg/mLSerum creatinine, mg/dLSerum albumin, g/dLUrinary protein/creatinine
Female p.A43T in heterozygosis G1 in heterozygosis No 5,967.78 0.75 4.5 0.00 
II Female No variants detected G1 in heterozygosis No 6,521.47 0.92 4.6 0.00 
II Male p.A43T in homozygosis G1 in homozygosis Yes 22,904.16 8.90 4.80 0.94 
II Female p.A43T in homozygosis G1 in homozygosis Yes 12,597.42 2.02 4.60 1.03 

In addition, in the APOL1 gene, we found the G1 high-risk alleles (homozygosity) in both affected individuals (G1 = rs73885319 and rs60910145). The proband’s mother presents G1 variants in heterozygosity and the proband’s sister does not present the G1 haplotype.

Familial presentation of CG was initially described in 2003 by Avila-Casado et al. [3]. A familial pattern of CG is a particularly rare entity that, according to prior reports [4], may be caused by mutations in some of the same genes etiologically related to the development of non-collapsing forms of FSGS [4]. An increased risk of developing FSGS is attributed to susceptibility genes, to which genetic or environmental “hits” are added leading to overt manifestation of glomerular disease [10]. In the present study, we evaluated the genotype-phenotype correlation in four members of a two-generation family with two individuals, proband and her brother, presenting CG. The affected individuals presented a rare variant in the THBD, high-risk variants in APOL1, and positive serology to PVB19. In this family, a multihit process of disease development appears to underline the presentation of the CG phenotype, as widely discussed in previous reports [10, 11]. The proband and her affected brother present a rare variant in THBD gene (p.A43T) in homozygosity. This genetic variant was never described in homozygosis, as in the present case. THBD encodes THBD, an endothelial glycoprotein which has anti-inflammatory, cytoprotective, and anticoagulant properties, and is expressed in all blood vessels [6] and on the surface of all endothelial cells [12]. The p.A43T was previously associated with risk of myocardial infarction in men, atherosclerosis [13], and sagittal sinus thrombosis [14]. In addition, variants in THBD have been related to atypical hemolytic-uremic syndrome [5, 12], and mutations that impair the function of THBD occur in about 5% of such patients [12]. THBD mutations were also described in C3 glomerulopathy [15]. Delvaeye et al. [12] found p.A43T variant in heterozygosity in a patient with atypical hemolytic-uremic syndrome which progressed to chronic renal failure. The authors suggest that THBD provides protection against complement activation; therefore, variants in THBD could be associated with activation of the alternative complement pathway. It was established that A43 is positioned on the surface of the molecule in the lectin-like domain of THBD, where it could potentially bind to proteins in the circulation [16]. We measured serum THBD, detecting high levels in both affected patients with CG, but not in their healthy sister. Similarly, high serum THBD, as well as high levels of other markers of endothelial dysfunction, have been reported in active FSGS (not CG in particular) when compared to healthy controls [6]. In addition, we found high-risk variants in APOL1 gene in the proband and her affected brother, both having as a possibly additional etiological factor a previous PVB19 infection. A higher prevalence of PVB19 DNA in kidney biopsies and peripheral blood of CG patients was already shown [17]. As widely discussed in previous reports [10, 11], once again a multihit process of disease development appears to underline the presentation of the CG phenotype, and in particular in this case of a familial CG phenotype. In the present study, in addition to the genetic mutations identified related to CG, it is possible that previous infection by PVB19 had an add-on role in the development of such disease. In fact, association between PVB19 (as well as other viruses, e.g., HIV, CMV, EBV, SARS-CoV2) infection and CG was already reported [18]. In the family here evaluated IgG serology for PVB19 was positive only in the two affected siblings, and not in the healthy sister. This was also observed by Avila-Casado et al. [3] in their report of familial CG. Since a similar MHC haplotype was observed in affected and non-affected members of that family, such authors concluded that the environment plays an important role in the development of the disease. It has been also hypothesized that association of PVB19 and APOL1 (seen as a susceptibility gene) mutations may lead to the development of CG. For instance, Besse et al. [11] described CG in patients with APOL1 high-risk alleles and acute PVB19 infection, in whom this infection was considered as a potential “second hit” in a genetic favorable background. In conclusion, considering the data presented here, a pathogenic variant in THBD seems to be the cause of familial CG, with the association of PVB19 and APOL1 in a multihit process, composed of environmental and genetic factors.

G.M.K. and J.B.P. received grants from CNPq (National Council for Scientific and Technological Development). We also thank Dr. Luiz Moura who kindly provided the light microscopy and immunofluorescence of the kidney biopsy.

All procedures performed in studies involving human participants were in accordance with the ethical standards of the Institutional and/or National Research Committee at which the studies were conducted (CEP/UNIFESP approval number 0602/09) and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards. Written informed consent was obtained from all participants included in the study for publication of the details of their medical case and any accompanying images.

The authors declare no competing interests.

This study was supported by grants from FAPESP (processes numbers 2019/05266-5, 2020/14635-1).

Michelle Tiveron Passos Riguetti, Patrícia Varela-Calais, Danilo Euclides Fernandes, José Francisco da Silva Franco, and Beatriz Ribeiro Nogueira: data analysis, drafting the article, and final approval of the version to be published. João Bosco Pesquero and Gianna Mastroianni Kirstajn: conception and design of the study, analysis, and interpretation of data, drafting the article, and final approval of the version to be published.

Additional Information

Michelle Tiveron Passos Riguetti, Patrícia Varela-Calais, and Danilo E. Fernandes contributed equally to this work and share first authorship. João B. Pesquero and Gianna Mastroianni-Kirsztajn contributed equally to this work and share senior authorship.

The datasets generated during and/or analyzed during the current study are available from the authors on reasonable request.

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