Identification of new genes involved in hereditary steroid-resistant nephrotic syndrome using next generation sequencing and in vivo functional characterization in drosophila melanogaster

Abstract : Nephrotic syndrome (NS) is a kidney disease characterized by disruption of the glomerular filtration barrier and the massive loss of proteins into the urine. Although in the majority of cases treatment with steroids leads to remission of the disease, in 15-20% of cases the disease is not responsive to this therapy and is classified as steroid-resistant nephrotic syndrome (SRNS). SRNS is a clinical condition with high morbidity leading to progressive renal failure as well as multiple metabolic and cardiovascular complications. Extensive research over the last 20 years has identified more than 40 SRNS causing genes that are crucial for function of the podocyte, a highly specialized kidney epithelial cell. However, the mutated gene is still unknown in about half of the familial cases. We have used exome sequencing to identify new genes mutated in SRNS. In order to prove the pathogenicity of the identified mutations we used the Drosophila model, assessing defects of fly viability and the structure and function of nephrocytes, podocyte like-cells. My thesis work consists of two projects. Firstly, we identified biallelic mutations in a new candidate gene, SGPL1, encoding the sphingosine 1- phosphate lyase, in individuals presenting SRNS with facultative adrenal insufficiency, ichthyosis, neurological defects and immunodeficiency. SGPL1 is the main catabolic enzyme of sphingolipids, irreversibly degrading sphingosine 1-phosphate into phosphoethanolamine and hexadecenal. In flies, these mutations were shown to decrease viability, induce nephrocyte defects and lead to the accumulation of sphingoid bases due to the loss of SGPL1 catabolic activity. Together, these results indicate that the identified SGPL1 mutations are pathogenic and cause a new syndromic form of SRNS. Moreover, in a second project, we defined the contribution of homozygous mutations found in two different genes, ADD3 and KAT2B, to a complex phenotype found in affected individuals from one consanguineous family. These individuals presented with neurological defects, cataracts, mild skeletal defects, cardiomyopathy and SRNS. ADD3 encodes adduciny, an F-actin capping protein that also links the actin cytoskeleton to the spectrin based membrane skeleton, while KAT2B encodes the lysine acetyltransferase 2B, mainly known for acetylation of histones and modulation of transcriptional programs. We found additional nonrelated patients carrying only biallelic ADD3 mutations that presented a partially overlapping syndrome but with no cardiac or renal manifestations. In the Drosophila model we found that both ADD3 and KAT2B mutations impaired fly viability and that the ADD3 mutation also impaired fly motor function. However, only the KAT2B mutation induced functional defects in Drosophila heart and nephrocytes. Altogether, these results suggest that ADD3 mutations are responsible for a neurological phenotype with facultative cataracts and skeletal defects while the KAT2B mutation induces heart and kidney defects. These results highlight the Drosophila as a good in vivo model to test the pathogenicity of the mutations found in SRNS candidate genes.
Keywords : Genetic
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Sara De Almeida Gonçalves. Identification of new genes involved in hereditary steroid-resistant nephrotic syndrome using next generation sequencing and in vivo functional characterization in drosophila melanogaster. Urology and Nephrology. Université Sorbonne Paris Cité, 2017. English. ⟨NNT : 2017USPCB030⟩. ⟨tel-02180575⟩

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