Natural History and Clinicopathological Associations of TRPC6-Associated Podocytopathy

Abstract

Key Points:

1. We conducted a clinical, genetic, and pathological analysis on 64 cases from 39 families with TRPC6-associated podocytopathy (TRPC6-AP).

2. Analysis of 37,542 individuals excluded a major contribution of loss-of-function variants to TRPC6-AP, legitimating current drug discovery approaches.

3. This study identifies key features of disease that can help intervention studies design and suggests similarities between TRPC6-AP and primary FSGS.

Background: Understanding the genetic basis of human diseases has become integral to drug development and precision medicine. Recent advancements have enabled the identification of molecular pathways driving diseases, leading to targeted treatment strategies. The increasing investment in rare diseases by the biotech industry underscores the importance of genetic evidence in drug discovery and approval processes. Here we studied a monogenic Mendelian kidney disease, TRPC6-associated podocytopathy (TRPC6-AP), to present its natural history, genetic spectrum, and clinicopathological associations in a large cohort of patients with causal variants in TRPC6 to help define the specific features of disease and further facilitate drug development and clinical trials design.

Methods: The study involved 64 individuals from 39 families with TRPC6 causal missense variants. Clinical data, including age of onset, laboratory results, response to treatment, kidney biopsy findings, and genetic information, were collected from multiple centers nationally and internationally. Exome or targeted sequencing was performed, and variant classification was based on strict criteria. Structural and functional analyses of TRPC6 variants were conducted to understand their effect on protein function. In-depth reanalysis of light and electron microscopy specimens for nine available kidney biopsies was conducted to identify pathological features and correlates of TRPC6-AP.

Results: Large-scale sequencing data did not support causality for TRPC6 protein-truncating variants. We identified 21 unique TRPC6 missense variants, clustering in three distinct regions of the protein, and with different effects on TRPC6 3D protein structure. Kidney biopsy analysis revealed FSGS patterns of injury in most cases, along with distinctive podocyte features including diffuse foot process effacement and swollen cell bodies. Most patients presented in adolescence or early adulthood but with ample variation (average 22, SD ±14 years), with frequent progression to kidney failure but with variability in time between presentation and kidney failure.

Conclusions: This study provides insights into the genetic spectrum, clinicopathological associations, and natural history of TRPC6-AP.

Clinical Trial registry name and registration number:: A Study to Test BI 764198 in People With a Type of Kidney Disease Called Focal Segmental Glomerulosclerosis, NCT05213624.

Graphical abstract

Figure 1

TRPC6 variants identified in this study. Individual variants are arranged by frequency on the y axis, with the number of patients with each variant depicted on the x axis. The most common variants were R985C, E897K, N143S, R175W, and N110H.

Figure 2

TRPC6 variants clustering on protein domains and structural mechanisms of TRPC6 gain-of-function variants. Above, linearized TRPC6 protein with known domains and positional representation of variants identified in population controls (A) and cases (B). Below, one protomer of the TRCP6 tetramer (protein data bank ID: 7DXF) is represented in gray cartoon. In each panel, D890 and E144 that coordinate a Ca²⁺ ion at the CBS1 site are labeled. This Ca²⁺-coordinating site acts as a sensor that inhibits TRCP6 Ca²⁺ conductance in the setting of high intracellular Ca²⁺ concentration. The consequences of selected mutations are here modeled on the TRCP6 structure to demonstrate how these missense variants may lead to gain-of-function by disturbing the CBS1 site. Wild-type residues are in gray. Mutant residues are in magenta. (C) van der Waals contacts between E144 and a tyrosine in an adjacent α helix help maintain E144 in a favorable conformation for Ca²⁺ coordination. The Y173C mutation ablates these contacts, increasing the degree of freedom of E144 and likely weakening affinity for Ca²⁺ at the CBS1 site. (D) The CBS1 site is composed of coordinating residues from both more N-terminal (E144) and C-terminal (D890) regions of TRPC6. Ca²⁺ coordination at CBS1 requires a close approach of α-helices from these two regions. The P112R mutation introduces a steric clash between an N-terminal bundle of α-helices and a C-terminal α helix. This clash will push the α helices apart and likely weaken Ca²⁺ coordination at the nearby CBS1 site. (E) V177M introduces a large amino acid into the hydrophobic core of a bundle of α helices, disturbing the packing of these helices and the nearby Ca²⁺-coordination site. (F) The H145R mutation adjacent to E144 substitutes a large, positively charged residue for a smaller polar but uncharged residue. This will cause both steric clashes and electrostatic repulsions that will likely disturb the CBS1 site.

Figure 3

Representative kidney biopsy images of cases with TRPC6-AP. Light and electron microscopy: The histologic and electron microscopy features of four different kidney biopsies are illustrated. (A) A low-power view shows FSGS lesions of the usual (NOS) type involving two glomeruli, accompanied by tubular atrophy, interstitial fibrosis, and chronic inflammation (periodic acid–Schiff, original magnification ×200). (B) The lesions of FSGS exhibit segmental sclerosis and hyalinosis with loss of overlying podocytes and adhesion to Bowman's capsule involving portions of the glomerulus not located at the tubular or vascular poles, consistent with FSGS (NOS) variant (JMS, original magnification ×400). (C) A glomerulus with normal size and cellularity has prominent enlargement and swelling of the podocyte cell bodies (JMS, original magnification ×600). (D) In this electron microscopy figure, the podocyte primary processes appear swollen, blunted, and disordered with irregular profiles. A podocyte cell body has dilatation of endoplasmic reticulum (original magnification, ×5000). (E) A different biopsy with approximately 80% foot process effacement also has dysmorphic bulbous primary processes inserting at right angles directly onto the outer glomerular basement membrane (original magnification ×6000). JMS, Jones methenamine silver; NOS, not otherwise specified; TRPC6-AP, TRPC6-associated podocytopathy.

Figure 4

Natural history and clinical correlations. (A) Histogram depicting frequencies of age of onset among the affected patients. The distribution is skewed toward younger age of onset, with most patients presenting in adolescence or early adulthood. There is a significant number of patients with childhood-onset disease, but this is likely overrepresented in our cohort as one of our recruiting centers is a children's hospital. (B) Proportion of patients, stratified by sex, who are known to have developed kidney failure. Both male and female patients had significant progression, with 63% of male and 64% of female patients developing kidney failure. The true number is likely even higher, owing to incomplete follow-up. (C) Kidney survival analysis from time to clinical recognition of disease; on average, 50% of individuals reached kidney failure after 15 years from clinical recognition of disease, without difference among males, females, or carriers of specific variants. (D) The time to kidney failure, depicting the number of years from disease onset (or initial clinical recognition) to development of kidney failure. The time to kidney failure was notably short for most of the cohort, with male patients having a nonstatistically significant faster progression time than female patients (P = 0.17). There were no significant differences between the clusters of pathogenic variants, nor among individuals with the most common variants.