Molecular mechanisms underlying secondary mitochondrial dysfunction in patients diagnosed in the 100,000 Genomes Project

Primary mitochondrial diseases (PMDs) are complex neurological and/or multisystem disorders characterised by enormous clinical, biochemical and genetic heterogeneity. These disorders cause severe disability, are frequently life-limiting in early childhood and currently have no curative disease-modifying therapies. Approaching 400 disease genes have been shown to cause PMDs. Known disease mechanisms include mutations in genes encoding subunits and assembly factors of the mitochondrial oxidative phosphorylation enzymes, and disorders of mitochondrial DNA maintenance, protein synthesis, cofactor biosynthesis and lipid metabolism. Recently, in the UK 100,000 genomes project we have identified defects in genes not previously linked to PMDs in children initially suspected to have a PMD. These children presented with clinical features suggestive of mitochondrial disease (complex neurological and/or multisystem manifestations) and had confirmed mitochondrial respiratory chain deficiencies in skeletal muscle biopsies. The genetic defects identified in these children include pathogenic variants in genes involved in RNA metabolism and transcription, DNA repair and calcium signalling, and a proline hydroxylase implicated in the cellular response to hypoxia. The links between these gene defects and mitochondrial dysfunction are not known.

We hypothesise that understanding the molecular mechanisms underlying secondary respiratory chain deficiencies in these rare genetic paediatric neurodegenerative disorders will help to elucidate the pathogenic basis of more common neurodegenerative diseases where secondary mitochondrial dysfunction has been implicated, including Parkinson, Alzheimer and Huntington diseases. These neurodegenerative disorders also lack curative therapies, and targeting the mechanisms leading to secondary respiratory chain dysfunction would be a new therapeutic avenue. Since common downstream pathways may be involved, this research is thus likely to identify novel therapeutic targets for both rare primary paediatric mitochondrial disorders and more common adult neurodegenerative diseases. The proposed project will involve genome-wide transcriptome analysis using RNAseq to identify shared transcriptional differences in primary patient cells, and detailed biochemical functional characterization in neuronal cell lines reprogrammed from patient-derived induced pluripotent stem cells. Nanna Therapeutics have developed a library of small molecule compounds and technologies to assay the effects of these compounds on mitochondrial function using a unique high throughput platform requiring minute samples. We will establish the impact of the identified gene defects on cell biochemistry and physiology and we will use the Nanna Therapeutics assays to screen their library of compounds to identify potential therapeutic hits.