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Recent evidence suggests that the presence of more than one pathogenic mutation in a single patient is more common than previously anticipated. One of the challenges hereby is to dissect the contribution of each gene mutation, for which animal models such as Drosophila can provide a valuable aid. Here, we identified three families with mutations in ADD3, encoding for adducin-γ, with intellectual disability, microcephaly, cataracts and skeletal defects. In one of the families with additional cardiomyopathy and steroid-resistant nephrotic syndrome (SRNS), we found a homozygous variant in KAT2B, encoding the lysine acetyltransferase 2B, with impact on KAT2B protein levels in patient fibroblasts, suggesting that this second mutation might contribute to the increased disease spectrum. In order to define the contribution of ADD3 and KAT2B mutations for the patient phenotype, we performed functional experiments in the Drosophila model. We found that both mutations were unable to fully rescue the viability of the respective null mutants of the Drosophila homologs, hts and Gcn5, suggesting that they are indeed pathogenic in flies. While the KAT2B/Gcn5 mutation additionally showed a significantly reduced ability to rescue morphological and functional defects of cardiomyocytes and nephrocytes (podocyte-like cells), this was not the case for the ADD3 mutant rescue. Yet, the simultaneous knockdown of KAT2B and ADD3 synergistically impaired kidney and heart function in flies as well as the adhesion and migration capacity of cultured human podocytes, indicating that mutations in both genes may be required for the full clinical manifestation. Altogether, our studies describe the expansion of the phenotypic spectrum in ADD3 deficiency associated with a homozygous likely pathogenic KAT2B variant and thereby identify KAT2B as a susceptibility gene for kidney and heart disease in ADD3-associated disorders., Author summary Genetic diseases with complex syndromic constellations may be caused by mutations in more than one gene. Most examples studied so far describe genetic interactions of known disease genes, suggesting that a large number of multilocus diseases remain unexplored. Assessment of mutation pathogenicity can be achieved using animal models. One main advantage of using Drosophila is that it allows easy in vivo gene manipulation in cell types that are relevant for the disease. Here, we report the pathogenicity of ADD3 mutations in three families with intellectual disability, microcephaly, cataracts and skeletal defects. Moreover, we provide evidence that the renal and cardiac phenotypes in one of the families could be unmasked by a homozygous variant in the lysine acetyltransferase encoding KAT2B gene. In Drosophila, this variant resulted not only in decreased viability, but also in functional defects in cardiomyocytes and nephrocytes, the latter being similar to mammalian podocytes. Our study implicates KAT2B as a susceptibility gene for steroid-resistant nephrotic syndrome (SRNS) and cardiomyopathy and emphasizes the importance of protein acetylation in kidney and heart function.