© Borgis - Postępy Nauk Medycznych 2/2013, s. 171-174
Mutacje genów kodujących białka podocyta jako przyczyna uszkodzenia kłębuszków nerkowych u dzieci i chorych dorosłych
Genetic mutations of podocyte proteins as underlying mechanism of glomerular diseases in pediatric and adult patients
Department of Nephrology, Kidney Transplantation and Hypertension, The Children’s Memorial Health Institute, Warszawa
Head of Department: prof. Ryszard Grenda, MD, PhD
Różnicowanie się podocytów i tworzenie kłębuszków jest w życiu płodowym poddawane kontroli swoistych genów. Jakiekolwiek zaburzenie tego procesu może skutkować zaburzeniami budowy i czynności kłębuszków. Wczesne ujawnienie się takich zaburzeń po urodzeniu i/lub oporność na leczenie, skłania do podejrzewania podłoża genetycznego u chorych z białkomoczem. Im młodszy jest pacjent, tym wyższe jest ryzyko obecności tła genetycznego glomerulopatii. Częstość występowania podłoża genetycznego steroido-opornego zespołu nerczycowego wynosi 100% u noworodków, 57% u niemowląt, 36% u dzieci, 25% u młodocianych i 14% u chorych dorosłych. Rodzaj mutacji wpływa na rokowanie. W niektórych przypadkach przy długotrwałym stosowaniu cyklosporyny osiągany jest częściowy efekt terapeutyczny, ale w innych – efektu leczenia nie ma wcale. W najcięższych przypadkach wrodzonego zespołu nerczycowego u dzieci z mutacją typu Fin-major, najbardziej agresywnym sposobem postępowania jest usunięcie obu nerek i docelowo transplantacja nerki.
Podocyte differentiation and glomerulogenesis in fetal life is controlled by specific genes and any disturbance of this process may lead to further structural defects with clinical consequences. The genetic background of glomerular disease with predominating proteinuria is suspected mainly due to occurrence early in life and/or resistance to specific pharmacotherapy. The younger is the patient, the higher is risk of genetic background. Incidence of genetic background of steroid-resistant nephrotic syndrome was reported as 100% in newborns, 57% in infants, 36% in children, 25% in adolescents and 14% in adult patients. Long-term outcome might be related to specific type of mutation. Some cases do partially respond to long-term cyclosporine A therapy, while on treatment and some are resistant to any therapy. Bilateral nephrectomy and further renal transplantation is the most aggressive management of cases with severe Fin-major mutation of nephrin – related congenital nephrotic syndrome, seen in young children.
The fetal differentiation and glomerulogenesis is strictly regulated process and many genes are involved in this regulation (1, 2). There is increasing evidence for genetic background of several clinical types of nephrotic syndrome, presented by pediatric, adolescent and adult patients. The major distinctive pattern between specific subgroups of nephrotic syndrome is “syndromic” vs “isolated” type. In “syndromic” pattern renal disease is combined with several extrarenal symptoms, including different types of congenital malformations or malfunctions, present e.g. in Denys-Drash, Frasier’s and nail- patella syndromes or Schimke’s immunoosseous dystrophy. So-called syndromic nephrosis is mainly presented by infants with other co-morbidities, however it should be stressed, that it may be diagnosed also in adolescent or adult patients, as it is in Charcot-Marie-Tooth disease, where there is a link between INF2 gene mutation, formin protein, podocytes and Schwann-cell function (the last causing severe neuropathy) (3). There are no extrarenal symptoms in isolated forms of genetic nephrotic syndrome and clinical symptoms are mainly related to severity of proteinuria.
Genotype – fenotype link
In some cases there is an evidence for link between type of specific gene mutation and severity of relevant nephrotic syndrome. In congenital nephrotic syndrome of Finnish type, so called Fin-major mutation of NPHS1 causes limitation of number of aminoacids in nephrin molecule from 1241 to 90, while in Fin-minor mutation reduces this number from 1241 to 1108. The clinical expression of those two types is significantly different and in Fin-major related cases the child is delivered being critically ill, with huge general oedema, severe dysproteinemia and renal injury. Those patients do not respond to any antiproteinuric treatment and after short time of vigorous supplementation of protein and albumin – bound vital substances, most of the patients must finally undergo bilateral nephrectomy, to avoid life-threatening complications of massive, untreatable urinary loss of protein. Then the patients are dialyzed until successful renal transplantation, which however may be complicated by “recurrence” of the disease due to anti-nephrin antibodies. Patient with Fin-minor mutation present much more mild clinical course and some of them respond to antiproteinuric treatment with ACEi (captopril) and/or indomethacine. Effect of these drugs is limited to lower proteinuria, while on treatment (4, 5).
Specific mutation may also serve as predictive factor for long-term renal outcome. In the study including 117 patients with early – onset non-Finnish nephrotic syndrome, the presence of podocine mutation was related to significantly better renal survival compared to nephrin mutation. The mean time to end-stage renal disease in patients with podocin mutation was 79 months, compared to 32 months in cases with nephrin mutation (p = 0.007)(6). Specific analysis of podocin (NPHS2) has also predictive value in both childhood and adult-onset steroid-resistant nephrotic syndrome. The combined presence of R229Q and A284V allele in adult patients was correlated with lack of response to corticosteroids and immunosuppression, development of focal segmental glomerulosclerosis (FSGS) in biopsy and 8-year progression to end-stage renal disease. On the other hand, it was also correlated with low recurrence rate after renal transplantation (7).
Who should be screened for genetic background of nephrotic syndrome?
In the study including 125 adult and pediatric patients with primarily steroid-resistant nephrotic syndrome, overall 37 (34%) demonstrated relevant podocine mutation. The incidence of mutation was conversely related to the age and was 100% in symptomatic newborns, 57% in infants, 36% in children, 25% in adolescents and 14% in patients with adult-onset nephrotic syndrome (8). Basing on this observation the relevant algorithm of genetic screening in steroid-resistant nephrotic syndrome was released. The authors suggest that regarding the age of disease onset, the first step of screening should be testing for NPHS1 (nephrin) in newborns, NPHS2 (podocin) in infants and children, and specific R229Q polymorphism of podocin in adolescents and adults. Negative result of the first-step screening suggests looking for other mutations, including WT1, TRPC6, ACTN4 and PLCE1 (8).
Does genetically determined nephrotic syndrome respond to any treatment?
In cases of the most severe congenital nephrotic syndrome (and nephrin mutations) the symptomatic treatment is based on aggressive supplementation of protein and albumin-bound hormones and vitamins lost into urine. There is no effect of steroid therapy or immunosuppression. Attempts to reduce heavy proteinuria include use of captopril and indomethacin, and uni- or bilateral nephrectomy, preceding temporary dialysis and further renal transplantation is reported option (9). Steroid-resistance is seen in patients with mutations of podocin. There are specific mutations, which determine the age of disease onset, such as R229Q heterozygotic variant, associated with adult-onset steroid-resistant nephrotic syndrome (10, 11). Patients with steroid-resistance are put on long-term cyclosporine therapy, which does not cure the disease, however may at least partially control the severity of proteinuria (12).
There is increasing evidence, that in genetically determined, steroid-resistant nephrotic syndrome, some other mechanisms of cyclosporine (beyond the effect on interleukin-2 and T-cells) are associated with it’s therapeutic effect. One of them is related to protection of podocyte synaptopodin from cathepsin L – mediated degradation, achieved by diminished dephosphorylation – associated with calcineurine inhibition (by cyclosporine). This stabilizes actin, the important protein of podocyte cytoskeleton and in consequence reduces proteinuria (13, 14). This phenomenon justifies the opinion, that podocyte is a direct target of specific immunosuppressive drugs (15). This also may explain the partial remission achieved with cyclosporine in patients with nephrotic syndrome and WT1 mutation (16). One of the clinical subtypes of genetically determined nephrotic syndrome, which respond to calcineurine inhibition is TRPC6 channel mutation. Defect of this functional channel, present in the podocyte causes steroid-resistant nephrotic syndrome and segmental sclerosis (FSGS) (17). The inhibition of calcineurine by specific drug (cyclosporine) interacts with the TRPC6 channel molecules and improves it’s dysregulation (18, 19).
Despite some effect of cyclosporine on reduction of proteinuria, overall efficacy of this drug in steroid-resistant genetic nephrotic syndromes is limited and in comparison to steroid-resistant non-genetic forms. One multicenter report showed that response to therapy was 68% vs 17% (p = 0.005), in favor to non-genetic cases (20).
Angiotensin receptor-1 (AT1R) blockers may be useful in reducing proteinuria, as they directly prevent a reduction in the expression of the slit diaphragm functional molecules, however thay may serve only as additional supplementary management (21).
Genetic background of nephrotic syndrome should be suspected in young-age – onset disease and primary steroid-resistance (fig. 1).
Fig. 1. Major types and incidence of genetically determined steroid-resistant nephrotic syndrome.
The incidence of genetic background is reversely correlated with age at onset of the disease.
Majority of cases of genetic nephrotic syndromes do not respond to steroids nor immunosuppression in terms of achieving complete remission; in minority of cases partial remission (defined as reduction of proteinuria) may be achieved and maintained with cyclosporine in long-term therapy.
Genetic background determines renal outcome and mutation of nephrin is associated with worse renal survival compared to mutation of podocin.
1. Kreidberg JA: Podocyte differentiation and glomerulogenesis. J Am Soc Nephrol 2003; 14: 806-814.
2. Hildebrand F: Genetic kidney diseases. Lancet 2010; 375: 1287-1295.
3. Boyer O, Nevo F, Plaisier E et al.: INF2 mutations in Charcot-Marie-Tooth disease with glomerulopathy. N Eng J Med 2011; 363: 2377-2388.
4. Holberg C, Antikainen M, Rönnholm K et al.: Management of congenital nephrotic syndrome of Finnish type. Pediatr Nephrol 1995; 9: 87-93.
5. Kuusuniemi A, Qvist E, Sun Y et al.: Plasma exchange and retransplantation in recurrent nephrosis of patients with congenital nephrotic syndrome of the Finnish type (NPHS1). Transplantation 2007; 83: 1316-1323.
6. Machuca E, Benoit G, Nevo F et al.: Genotype-phenotype correlations in non-Finnish congenital nephrotic syndrome. J Am Soc Nephrol 2010; 21: 1209-1217.
7. Santin S, Tazòn-Vega B, Silva I et al.: On behalf of the FSGS Spanish Study Group: Clinical value of NPHS2 analysis in early- and adult onset steroid-resistant nephrotic syndrome. Clin J Am Soc Nephrol 2011; 6: 344-354.
8. Santin S, Bullich G, Tazòn-Vega B et al.: Clinical utility of genetic testing in children and adults with steroid-resistant nephrotic syndrome. Clin J Am Soc Nephrol 2011; 6: 1139-1148.
9. Licht C, Eifinger F, Gharib M et al.: A stepwise approach to the treatment of early onset nephrotic syndrome. Pediatr Nephrol 20001; 14: 1077-1982.
10. Ruf RG, Lichtenberger A, Karle S et al.: Patients with mutation in NPHS2 (podocin) do not respond to standard steroid treatment of nephrotic syndrome. J Am Soc Nephrol 2004; 15: 722-732.
11. Machuca E, Hummel A, Nevo F et al.: Clinical and epidemiologiocal assessment of steroid-resistant nephrotic syndrome associated with the NPHS2 R229Q variant. Kidney Int 2009; 75: 727-735.
12. Cattran DC, Alexopoulos E, Heering P et al.: Cyclosporin in idiopathic glomerular disease associated with the nephrotic syndrome: workshop recommendations. Kidney Int 2007; 12: 1429-1447.
13. Kistler AD, Peev V, Forst AL et al.: Enzymatic disease of the podocyte. Pediatr Nephrol 2010; 25: 1017-1023.
14. Faul C, Donnelly M, Merscher-Gomez S et al.: The actin cytoskeleton of kidney is a direct target of the antiproteinuric effect of cyclosporine A. Nat Med 2008; 14: 931-938.
15. Schönenberger F, Ehrich JH, Haller H, Schiffer M: The podocyte as a direct target of immunosuppressive agents. Nephrol Dial Transplant 2001; 26: 18-24.
16. Stefanidis C, Querfeld U: The podocyte as target: cyclosporine A in the management of the nephrotic syndrome caused by WT1 mutations. Eur J Pediatr 2011; 170: 1377-1383.
17. Walz G: Slit or pore? A mutation of the ion channel TRPC6 causes FSGS. Nephrol Dial Transpl 2005; 20: 1777-1779.
18. Winn MO, Dasalakis N, Sparney RF, Middleton JP: Unexpected role of TRPC6 channel in familial nephrotic syndrome: does it have clinical implications? J Am Soc Nephrol 2006; 17: 378-387.
19. Dryer S, Reiser J: TRPC6 channels and their binding partners in podocytes: a role in glomerular filtration and pathophysiology. Am J Physiol Renal Pysiol 2010; 299: F689-F791.
20. Büscher AK, Kranz B, Büscher R et al.: Immunosuppression and renal outcome in congenital and pediatric steroid-resistant nephrotic syndrome. Clin J Am Soc Nephrol 2010; 5: 2075-2084.
21. Kawasaki H, Suzuki K, Miyachi N et al.: Slit diaphtagm dysfunction in proteinuric states: identification of novel therapeutic targets for nephrotic syndrome. Clin Exp Nephrol 2009; 13: 275-280.