Browsing by Author "Burkhalter, Martin D."

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    The analysis of heterotaxy patients reveals new loss-of-function variants of GRK5
    (SpringerNature, 2016-09-13) Lessel, Davor; Muhammad, Tariq; Tena, Teresa Casar; Moepps, Barbara; Burkhalter, Martin D.; Hitz, Marc-Phillip; Toka, Okan; Rentzsch, Axel; Schubert, Stephan; Schalinski, Adelheid; Bauer, Ulrike M. M.; Kubisch, Christian; Ware, Stephanie M.; Philipp, Melanie; Department of Pediatrics, IU School of Medicine
    G protein-coupled receptor kinase 5 (GRK5) is a regulator of cardiac performance and a potential therapeutic target in heart failure in the adult. Additionally, we have previously classified GRK5 as a determinant of left-right asymmetry and proper heart development using zebrafish. We thus aimed to identify GRK5 variants of functional significance by analysing 187 individuals with laterality defects (heterotaxy) that were associated with a congenital heart defect (CHD). Using Sanger sequencing we identified two moderately frequent variants in GRK5 with minor allele frequencies <10%, and seven very rare polymorphisms with minor allele frequencies <1%, two of which are novel variants. Given their evolutionarily conserved position in zebrafish, in-depth functional characterisation of four variants (p.Q41L, p.G298S, p.R304C and p.T425M) was performed. We tested the effects of these variants on normal subcellular localisation and the ability to desensitise receptor signalling as well as their ability to correct the left-right asymmetry defect upon Grk5l knockdown in zebrafish. While p.Q41L, p.R304C and p.T425M responded normally in the first two aspects, neither p.Q41L nor p.R304C were capable of rescuing the lateralisation phenotype. The fourth variant, p.G298S was identified as a complete loss-of-function variant in all assays and provides insight into the functions of GRK5.
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    Imbalanced mitochondrial function provokes heterotaxy via aberrant ciliogenesis
    (American Society for Clinical Investigation, 2019-05-16) Burkhalter, Martin D.; Sridhar, Arthi; Sampaio, Pedro; Jacinto, Raquel; Burczyk, Martina S.; Donow, Cornelia; Angenendt, Max; Competence Network for Congenital Heart Defects Investigators; Hempel, Maja; Walther, Paul; Pennekamp, Petra; Omran, Heymut; Lopes, Susana S.; Ware, Stephanie M.; Philipp, Melanie; Pediatrics, School of Medicine
    About 1% of all newborns are affected by congenital heart disease (CHD). Recent findings identify aberrantly functioning cilia as a possible source for CHD. Faulty cilia also prevent the development of proper left-right asymmetry and cause heterotaxy, the incorrect placement of visceral organs. Intriguingly, signaling cascades such as mTor that influence mitochondrial biogenesis also affect ciliogenesis, and can cause heterotaxy-like phenotypes in zebrafish. Here, we identify levels of mitochondrial function as a determinant for ciliogenesis and a cause for heterotaxy. We detected reduced mitochondrial DNA content in biopsies of heterotaxy patients. Manipulation of mitochondrial function revealed a reciprocal influence on ciliogenesis and affected cilia-dependent processes in zebrafish, human fibroblasts and Tetrahymena thermophila. Exome analysis of heterotaxy patients revealed an increased burden of rare damaging variants in mitochondria-associated genes as compared to 1000 Genome controls. Knockdown of such candidate genes caused cilia elongation and ciliopathy-like phenotypes in zebrafish, which could not be rescued by RNA encoding damaging rare variants identified in heterotaxy patients. Our findings suggest that ciliogenesis is coupled to the abundance and function of mitochondria. Our data further reveal disturbed mitochondrial function as an underlying cause for heterotaxy-linked CHD and provide a mechanism for unexplained phenotypes of mitochondrial disease.