Discovery and functional characterization of novel single-gene causes and genetic modifiers in nephronophthisis
Final Report Abstract
Nephronophthisis (NPH) is an autosomal recessive tubulointerstitial kidney disease that represents the most frequent genetic cause of chronic kidney failure (KF) in children. NPH belongs to the heterogenous group of ciliopathies whose underlying cause is a defect in the structure and/or function of the primary cilium. To date, no specific treatment for NPH is available. Knowing exact disease mechanism is important for targeting certain signaling pathways to develop curative therapeutic approaches. Thus, the aim of my fellowship in the Laboratory of Hereditary Kidney Diseases of Sophie Saunier was to identify novel disease genes for NPH and validate their candidate status with subsequent functional studies to unravel targetable disease mechanism. However, when my fellowship started, the COVID-19 pandemic begun and I switched to a project where I analyzed one of the largest NPH cohorts worldwide collected by the research group. In 600 NPH patients genetic testing identified a total of 788 pathogenic variants in 40 ciliopathy genes, thus adding 168 novel variants to the Human Gene Mutation Database library (HGMD). We could show that the phenotypic variability was in a large part associated with mutation types and genes. Moreover, by applying a ciliary module-based approach, we identified phenotypic clusters improving prognosis of KF onset and pattern of extrarenal involvement. Next, I continued with my project following up on the unresolved NPH cases. I analyzed WES data of clinically diagnosed NPH patients, whose gene panel diagnostics was unable to establish a molecular diagnosis. WES provided a genetic diagnosis in 35% of patients, while the detection rate was higher in patients presenting with early KF, syndromic form of disease and consanguineous parents. Hypomorphic variants e.g., missense variants or variants of unknown significance (VUS) in known ciliopathy genes, as well as genetic variants in phenocopying nephropathies including nephrotic syndrome and focal segmental glomerulosclerosis (FSGS) were identified. Pathogenic variants in systemic disorders including Primary Hyperoxaluria, Shukla-Vernon syndrome, and Cortical dysplasia mimicked extrarenal ciliopathy symptoms. Finally, WES unraveled four potential ciliopathy candidate genes from which one gene was selected for functional studies. In two NPH siblings WES identified novel compound-heterozygous SSBP1 variants. SSBP1 represents a key protein for mitochondrial DNA replication that was already reported to cause optic atrophy with or without concomitant KF. However, SSBP1 was not systematically assessed as nephropathy gene so far and the role of mitochondrial function in tubulointerstitial nephropathy is unknown. Our immunofluorescence studies on kidney biopsy confirmed mitochondrial colocalization of SSBP1 and a significantly lower expression of both in patient. Using CRISPR Cas9 technology we generated compound-heterozygous SSBP1 clones for analysis of mitochondrial function and morphology. In a next step we would like to investigate how the mitochondrial dysfunction causes tubulointerstitial kidney disease, whether it is a cilia-linked mechanism or another pathway.
Publications
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The genetic landscape and clinical spectrum of nephronophthisis and related ciliopathies. Vortrag Jahreskongress Deutsche Gesellschaft für Nephrologie (DGfN). 10/2022 Berlin
Petzold F.
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The genetic landscape and clinical spectrum of nephronophthisis and related ciliopathies. Kidney International, 104(2), 378-387.
Petzold, Friederike; Billot, Katy; Chen, Xiaoyi; Henry, Charline; Filhol, Emilie; Martin, Yoann; Avramescu, Marina; Douillet, Maxime; Morinière, Vincent; Krug, Pauline; Jeanpierre, Cécile; Tory, Kalman; Boyer, Olivia; Burgun, Anita; Servais, Aude; Salomon, Remi; Benmerah, Alexandre; Heidet, Laurence; Garcelon, Nicolas ... & Ziliotis, Marie-Julia
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The genetic landscape and clinical spectrum of nephronophthisis and related ciliopathies. Vortrag Jahreskongress Deutsche Gesellschaft für Pädiatrische Nephrologie (GPN). 05/2023 Marburg
Petzold F.
