UKRI Medical Research Council
Project: Targeting the cellular metabolism to treat tissue-specific mitochondrial diseases
Although mitochondria are present in almost all eukaryotic cell types, mitochondrial diseases have distinct tissue-specific phenotypes that are poorly understood. Our project focuses on mitochondrial diseases characterised by impaired mitochondrial translation, which can be caused either by mtDNA mutations (especially in mt-tRNAs) or in nuclear encoded proteins that play a role in mitochondrial translation. Mt-tRNA or nuclear gene mutations affect tRNA levels and structure directly or indirectly, leading to a defect of mitochondrial protein synthesis. These mutations can also lead to the presence of free tRNAs without their cognate amino acid (‘uncharged tRNA’), which constitutes a stress signal for cells.
We are planning to investigate whether different tissues have different levels of uncharged tRNAs and whether this correlates with the vulnerability of those tissues for mitochondrial defects. Uncharged tRNA activates the integrated stress response (ISR), a major signalling pathway that allows eukaryotic cells to sense stress and adapt to it. We obtained fibroblasts derived from patients with different mitochondrial translation defects to determine tRNA and ISR levels. Furthermore, we will reprogram the fibroblasts to iPSCs and differentiate those to neurons, muscle cells and cardiomyocytes for these experiments. Next, we will examine what metabolic changes are induced by the activated ISR in these cells.
In one disease studied by our laboratory, reversible infantile respiratory chain deficiency (RIRCD), the metabolic changes induced by the ISR in the muscle of patients allowed a recovery of the patients from severe disease. Our results indicate that the ISR is protective to some cells and damaging to others. We would like to exploit the knowledge gained with RIRCD to induce similar metabolic changes in mitochondrial conditions that are currently not reversible. Modifying the ISR or manipulating key metabolic factors (amino acids, FGF21) may enable us to devise therapeutic options for mitochondrial translation defects in the future.
Wellcome Investigator Award
Project: Exploring novel molecular targets in mitochondrial protein synthesis to develop treatments in mitochondrial disease
Most mitochondrial diseases are disabling, progressive or fatal, affecting the brain, liver, skeletal muscle, heart and other organs. Currently there are no effective cures and treatment is at best symptomatic. Although defective oxidative phosphorylation is the common final pathway, it is unknown why different mtDNA or nuclear mutations result in largely heterogeneous clinical presentations.
The diagnosis in patients with multiple respiratory chain complex defects (~30% of all mitochondrial disease) due to abnormal mitochondrial translation is particularly difficult because of the massive number of nuclear genes involved in intra-mitochondrial protein synthesis. Many of these genes are not yet linked to human disease. Whole exome sequencing (WES) rapidly changed the diagnostic pathway by identifying the primary genetic defect, thus making invasive and complex biochemical testing unnecessary. However, our understanding of the mitochondrial protein synthesis apparatus and expression of mitochondrial proteins in health and disease remains limited, slowing down the development of personalized therapies.
Exploring why defects of mitochondrial protein synthesis or RNA metabolism lead to extremely variable clinical presentations will reveal novel functions of mitochondria in different tissues. Identification of tissue specific targets will provide rationale for developing novel therapies.
MRC Neurosciences and Mental Health Board Grant (MR/N025431/1)
Project: Exosomal protein deficiencies: how abnormal RNA metabolism results in childhood-onset neurological diseases
The importance of RNA metabolism in neurons is highlighted with a rapidly increasing number of mutations in genes/proteins involved in RNA metabolism in human neurogenetic diseases such as spinal muscular atrophy (SMA) and cerebellar Purkinje cell dysfunction (pontocerebellar hypoplasias). Why these diseases specifically affect different types of neuronal cells, such as spinal motor neurons and Purkinje cells is still unknown. Both too much and too little of a certain mRNA species could be deleterious, especially in neurons, but our understanding of the regulatory mechanisms is still limited.
A novel group of RNA-associated neurological disease has been discovered, which is caused by mutations in components of the human exosome. The exosome is a multimeric complex, which consists of nine proteins. Other mRNA binding proteins and enzymes are also associated with the exosome, but little is known about their role in disease mechanisms. Mutations in exosome subunit genes EXOSC3, EXOSC8 and EXOSC9 were reported in patients with pontocerebellar hypoplasia and spinal muscular atrophy (PCH1). The prominent and isolated neurological presentation in these diseases raises the possibility that the exosome is particularly important in neurons. The aim of this project is to further study and characterize the role of the exosome in different types of neurons. We study how mutations in human exosome protein genes alter RNA metabolism, protein-protein interactions and cellular function in different model systems.
Newton Fund (UK/Turkey) MR/N027302/1
Project: New genomics approaches to explore the neurogenetic disease burden of consanguineous marriages in Turkey
Consanguineous marriages are common in Turkey and carry an increased risk of genetic conditions with autosomal recessive inheritance. Many of these conditions affect the nervous system and muscle, leading to severe disability or premature death in children. While each of these conditions is individually rare, there are several hundred genetically defined neurogenetic disease entities resulting in a significant health burden. A large proportion of children with neurogenetic disorders in Turkey remain undiagnosed, and important opportunities for prevention and treatment are missed. Through systematic deep phenotyping of neurogenetic patients from consanguineous families, combined with whole genome sequencing and integration of data in advanced international bioinformatics platforms, we identify new disease genes, support the discovery of modifier genes, and contribute to development of prevention and treatment strategies.
This project is a collaborative award with Dr. Semra Hiz and Dr. Yavuz Oktay at the Dokuz Eylul University.
Lily Foundation – Research Grant
Project: Testing new treatment options in zebrafish models of mitochondrial DNA depletion syndromes
Mitochondria contain multiple copies of their own genetic material (mtDNA) which is replicated independently DNA in the nucleus. Numerous proteins are involved in the mtDNA replication and in providing the building blocks (nucleotides) for the DNA synthesis. Defects in any of them cause mtDNA depletion syndromes (MDDS) characterised by a low amount of mtDNA in the cells. It has been shown that adding a surplus of the starting materials (nucleosides) for production of nucleotides to cells from patients with MDDS can rescue the defect. Our aim is to systematically test this nucleoside supplementation in different zebrafish models of MDDS.