ADCK4 “reenergizes” nephrotic syndrome

Steroid-resistant nephrotic syndrome has a poor prognosis and often leads to end-stage renal disease development. In this issue of the JCI, Ashraf and colleagues used exome sequencing to identify mutations in the aarF domain containing kinase 4 (ADCK4) gene that cause steroid-resistant nephrotic syndrome. Patients with ADCK4 mutations had lower coenzyme Q₁₀ levels, and coenzyme Q₁₀ supplementation ameliorated renal disease in a patient with this particular mutation, suggesting a potential therapy for patients with steroid-resistant nephrotic syndrome with ADCK4 mutations.

Nephrotic syndrome (NS) is the most common primary glomerular disease in children. All children with NS share similar clinical manifestations, such as edema, and biochemical abnormalities, including proteinuria, hypoalbuminemia, and hyperlipidemia; however, their clinical courses vary greatly (1). The majority of children presenting with idiopathic NS respond to steroid therapy. Unfortunately, more than 20 percent of patients will fail to respond to steroid treatment. These patients with steroid-resistant NS (SRNS) usually progress to end-stage renal disease, necessitating dialysis or renal transplantation. Identifying patients who will respond to steroids and understanding the underlying disease pathogenesis is a critical challenge for disease management. Light microscopic lesions in children with SRNS overlap with those in children with steroid-sensitive NS. These changes range from focal segmental glomerulosclerosis and diffuse mesangial sclerosis to minimal change disease.

Podocyte foot process effacement observed by electron microscopy is the sine qua non of NS, indicating the role of these cells in NS development. Podocytes or glomerular epithelial cells cover the outer surface of the glomerular basement membrane and, along with the underlying endothelial cells, represent the glomerular filtration barrier. Mutations of more than 20 genes have been found to be associated with NS development in humans (Figure 1). Interestingly, all 24 genes associated with NS development are localized to the podocyte, further confirming that podocytes are essential in maintaining the glomerular filtration barrier. Mutations associated with NS can be divided into a few main groups. For example, mutations of podocyte actin cytoskeleton proteins are frequently found in adult SRNS or focal segmental glomerulosclerosis (2, 3). Podocytes appear to be dynamic cell types, and foot process remodeling is critical to maintaining the filtration barrier. Foot processes are connected by a specialized adherens junction, the slit diaphragm. Over the last decade the slit diaphragm emerged as an important signaling platform (4). Mutations of slit diaphragm- and slit diaphragm-associated proteins represent another major group causing SRNS, especially in children (5). Molecular studies indicate that actin and slit mutations usually alter motility and morphology in vitro (6), causing foot process effacement in vivo. Morphological changes, including the foot process effacement and the associated NS, can be reversible in humans. Progressive glomerulosclerosis and end-stage renal disease develops when podocytes are lost due to apoptosis or detachment (7, 8). Podocytes are unable to divide, so podocyte loss is associated with maladaptive hypertrophy and activation of repair pathways, further propagating podocyte damage and causing glomerulosclerosis (9, 10).

Figure 1

The products of the genes whose mutations have been found to cause SRNS reside in the podocyte. Mutations of these genes lead to abnormal or absent protein products. The complex ultrastructural nature of the podocyte makes it difficult to judge the exact function of each of these proteins on a molecular level and thus classify the different mutations. Slit diaphragm–associated (SD-associated) mutations (purple) include nephrin (NPHS1), podocin (NPHS2), PLCε1 (PLCE1), CD2-associated protein (CD2AP), transient receptor potential channel 6 (TRPC6), and protein tyrosine phosphatase receptor type O (PTPRO), and ADCK4 (orange). Cytoskeleton and foot process actin network–associated mutations (red) include α-actinin-4 (ACTN4), inverted formin 2 (INF2), myosin 1E (MYO1E), Rho-GDP-dissociation inhibitor 1 (ARHGDIA), and Rho-GTPase–activating protein 24 (ARHGAP24). Nuclear- or trascription-associated mutations (gray) include Wilm’s tumor protein (WT1), LIM/homeobox transcription factor 1β (LMX1B), ATP-driven annealing helicase (SMARCAL1), and nuclear RNA export factor 5 (NXF5). Glomerular basement membrane–associated (GMB-associated) mutations (pink) include laminin-β2 (LAMB2), integrin-β4 (ITGB4), and integrin-α3 (ITGA3). Mitochondia-associated mutations (brown) include parahydroxybenzoate-polyprenyl transferase (COQ2), ubiquinone biosynthesis monooxygenase (COQ6), decaprenyl-diphosphate synthase subunit 2 (PDSS2), mitochondrially encoded tRNA leucine 1 (MTTL1), and ADCK4. Lysosome-associated mutations (maroon) include scavenger receptor class B, member 2 (SCARB2).

Whole-exome sequencing approaches revolutionized the identification of rare monogenic coding mutations associated with a strong penetrance (11). Exones represent only 1.5% of the genome. Using next-generation sequencing, we can achieve sufficient coverage to identify specific mutations in coding regions.

In this issue of the JCI, Ashraf and colleagues coupled homozygosity mapping with whole-exome sequencing in patients with SRNS to identify a unique disease-causing gene candidate: ADCK4 (12). This gene is located on chromosome 19 and encodes the aarF domain containing kinase 4. The authors identified 11 different mutations in ADCK4 in 15 individuals from 8 nonrelated SRNS families. These mutations affected conserved amino acids. To determine whether ADCK4 mutations truly cause NS, the authors reproduced the nephrosis phenotype in animal models. Knockdown of the ADCK4 ortholog in zebrafish resulted in the characteristic edema, proteinuria, and microscopic changes of NS. In addition to the clinical phenotypes, the authors also noted podocyte foot process effacement and disorganization of the filtration slit, similar to that observed in patients with NS, indicating that ADCK4 mutations are causally linked to NS.

The most frequently mutated SRNS-associated proteins are localized to the podocyte slit-associated signaling platform or the slit-associated actin cytoskeleton (Figure 1); however, ADCK4 is slightly unusual. Ashraf and colleagues found that it localizes not only to the foot processes, but also to the mitochondria in rat podocytes (Figure 2). Additionally, they determined that ADCK4 interacts with COQ6 and COQ7 proteins, which are involved in the coenzyme Q₁₀ (CoQ₁₀) biosynthesis pathway. Levels of CoQ₁₀ were decreased in individuals with ADCK4 mutations, further suggesting that ADCK4 is involved in CoQ₁₀ biosynthesis. CoQ₁₀ is an oil-soluble, vitamin-like substance present in most eukaryotic cells, primarily in the mitochondria. It is a component of the electron transport chain and participates in aerobic cellular respiration, generating energy in the form of ATP. Ninety-five percent of the human body’s energy is generated this way. Therefore, those organs with the highest energy requirements, such as the heart, liver, and kidney, have the highest CoQ₁₀ concentrations. CoQ₁₀ also acts as an antioxidant.

Figure 2

Role of ADCK4 in podocyte-dependant maintenance of the glomerular filter. The foot processes of WT podocytes are connected by a specialized adherens junction, the slit diaphragm. The complex interaction between podocytes and the glomerular capillaries prevents large molecules like albumin from escaping into Bowman’s capsule. In WT podocytes, ADCK4 (orange ovals) resides in mitochondria and in podocyte foot processes. ADCK4 mutations are associated with foot process effacement and albuminuria (NS). CoQ₁₀ supplementation reversed the phenotypic changes.

The mitochondria and CoQ₁₀ are gaining momentum as key players in the development of renal disease. Mutations in three CoQ₁₀ biosynthetic genes, COQ6, COQ2, and prenyl diphosphate synthase, subunit 2 (PDSS2), have been associated with SRNS due to dysfunctional mitochondria in podocytes (13–15). In mice with PDSS2 mutations, administration of CoQ₁₀ decreased albuminuria and interstitial nephritis (16). CoQ₁₀ also ameliorated albuminuria and mitochondrial changes in an experimental model of type 2 diabetes (db/db mice) (17, 18). Importantly, Ashraf et al. also showed that CoQ₁₀ was able to reverse the change in the podocyte migratory phenotype induced by ADCK4, indicating that mitochondrial dysfunction or reactive oxygen species generation is upstream of actin cytoskeleton changes. These changes appear to be clinically relevant, as the authors describe an isolated case of a patient with an ADCK4 mutation who greatly benefited from CoQ₁₀ supplementation. Low CoQ₁₀ levels have been described in patients with different disease conditions, including hypertension, cancer, migraine, congestive heart disease (CHF), and Parkinson’s disease. A clinical trial to test the effectiveness of CoQ₁₀ in CHF is still ongoing. Unfortunately, the trial testing the role of CoQ₁₀ in Parkinson’s disease was stopped prematurely because of ineffectiveness. The study by Ashraf et al. indicates that CoQ₁₀ supplementation may prove a successful therapy for focal segmental glomerulosclerosis in patients carrying ADCK4 mutations.

Although polygenic diseases are more common than single-gene disorders, studies of monogenic diseases provide an invaluable opportunity to learn about the underlying molecular mechanisms of specific disease conditions. Mechanistic insight obtained from patients with rare forms of monogenic disease often can be applied for common disease conditions. Monogenic studies identified the podocyte as the key cell type causally related to NS development. The study by Ashraf et al. is especially significant, showing the critical role of ADCK4, podocyte mitochondria, and CoQ₁₀, which are potentially upstream of slit signaling and actin dynamics in SRNS. This represents a fresh new direction in NS and podocyte research. Future studies shall examine the role of podocyte mitochondria and CoQ₁₀ in polygenic and secondary NS cases as well.

This work was supported by NIH R01DK076077, the Juvenile Diabetes Research Foundation, and the American Diabetes Association.

Address correspondence to: Katalin Susztak, Room 405B Clinical Research Building, 415 Curie Boulevard, Philadelphia, Pennsylvania 19104, USA. Phone: 215.898.2009; Fax: 215.898.0189; E-mail: ksusztak@mail.med.upenn.edu.

Conflict of interest: Work in the laboratory of Katalin Susztak is funded by Boehringer Ingelheim.

Reference information: J Clin Invest. 2013;123(12):4996–4999. doi:10.1172/JCI73168.

See the related article at ADCK4 mutations promote steroid-resistant nephrotic syndrome through CoQ10 biosynthesis disruption.

1. D’Agati VD, Kaskel FJ, Falk RJ. Focal segmental glomerulosclerosis. N Engl J Med. 2011;365(25):2398–2411.
   View this article via: PubMed CrossRef Google Scholar
2. Reiser J, et al. TRPC6 is a glomerular slit diaphragm-associated channel required for normal renal function. Nat Genet. 2005;37(7):739–744.
   View this article via: PubMed CrossRef Google Scholar
3. Winn MP, et al. A mutation in the TRPC6 cation channel causes familial focal segmental glomerulosclerosis. Science. 2005;308(5729):1801–1804.
   View this article via: PubMed CrossRef Google Scholar
4. Kestilä M, et al. Positionally cloned gene for a novel glomerular protein — nephrin — is mutated in congenital nephrotic syndrome. Mol Cell. 1998;1(4):575–582.
   View this article via: PubMed CrossRef Google Scholar
5. Brown EJ, et al. Mutations in the formin gene INF2 cause focal segmental glomerulosclerosis. Nat Genet. 2010;42(1):72–76.
   View this article via: PubMed CrossRef Google Scholar
6. George B, Holzman LB. Signaling from the podocyte intercellular junction to the actin cytoskeleton. Semin Nephrol. 2012;32(4):307–318.
   View this article via: PubMed CrossRef Google Scholar
7. Wiggins RC. The spectrum of podocytopathies: a unifying view of glomerular diseases. Kidney Int. 2007;71(12):1205–1214.
   View this article via: PubMed CrossRef Google Scholar
8. Susztak K, Raff AC, Schiffer M, Böttinger EP. Glucose-induced reactive oxygen species cause apoptosis of podocytes and podocyte depletion at the onset of diabetic nephropathy. Diabetes. 2006;55(1):225–233.
   View this article via: PubMed CrossRef Google Scholar
9. Niranjan T, et al. The Notch pathway in podocytes plays a role in the development of glomerular disease. Nat Med. 2008;14(3):290–298.
   View this article via: PubMed CrossRef Google Scholar
10. Gödel M, et al. Role of mTOR in podocyte function and diabetic nephropathy in humans and mice. J Clin Invest. 2011;121(6):2197–2209.
    View this article via: JCI PubMed CrossRef Google Scholar
11. Hildebrandt F, et al. A systematic approach to mapping recessive disease genes in individuals from outbred populations. PLoS Genet. 2009;5(1):e1000353.
    View this article via: PubMed CrossRef Google Scholar
12. Ashraf S, et al. ADCK4 mutations promote steroid-resistant nephrotic syndrome through CoQ₁₀ biosynthesis disruption. J Clin Invest. 2013;123(12):5179–5189.
    View this article via: JCI CrossRef Google Scholar
13. Diomedi-Camassei F, et al. COQ2 nephropathy: a newly described inherited mitochondriopathy with primary renal involvement. J Am Soc Nephrol. 2007;18(10):2773–2780.
    View this article via: PubMed CrossRef Google Scholar
14. Heeringa SF, et al. COQ6 mutations in human patients produce nephrotic syndrome with sensorineural deafness. J Clin Invest. 2011;121(5):2013–2024.
    View this article via: JCI PubMed CrossRef Google Scholar
15. López LC, et al. Leigh syndrome with nephropathy and CoQ10 deficiency due to decaprenyl diphosphate synthase subunit 2 (PDSS2) mutations. Am J Hum Genet. 2006;79(6):1125–1129.
    View this article via: PubMed CrossRef Google Scholar
16. Saiki R, et al. Coenzyme Q10 supplementation rescues renal disease in Pdss2kd/kd mice with mutations in prenyl diphosphate synthase subunit 2. Am J Physiol Renal Physiol. 2008;295(5):F1535–F1544.
    View this article via: PubMed CrossRef Google Scholar
17. Sourris KC, et al. Ubiquinone (coenzyme Q10) prevents renal mitochondrial dysfunction in an experimental model of type 2 diabetes. Free Radic Biol Med. 2012;52(3):716–723.
    View this article via: PubMed CrossRef Google Scholar
18. Persson MF, et al. Coenzyme Q10 prevents GDP-sensitive mitochondrial uncoupling, glomerular hyperfiltration and proteinuria in kidneys from db/db mice as a model of type 2 diabetes. Diabetologia. 2012;55(5):1535–1543.
    View this article via: PubMed CrossRef Google Scholar