High throughput detection of DNA mutations causing mitochondrial disease
Project: High throughput detection of DNA mutations causing mitochondrial disease
Recent Australian studies show that 1 out of 200 people will carry a mitochondrial genetic defect, although most are still unaware.
The current method in diagnosing suspected mito involves screening for selected known mutations in mitochondrial or DNA. There are more than 100 unidentified genes involved in the structure, functioning and maintenance of mito, resulting in the majority of patients being left with unidentified mutations.
Mito can present itself in many ways, making it challenging to identify whether a disease is mitochondrial or not.
Dr Phillipa Lamont’s research involves analysing DNA samples from mitochondrial patients whose underlying molecule cause of their disease had not yet been elucidated.
- To discover the unknown genes involved in the structure, function and maintenance of mitochondria
- To identify the unknown mutations in mitochondrial or DNA
- To determine the underlying molecule cause of patient’s disease
- To evaluate the ways in which mitochondrial disease manifests itself as a result of a particular mutation
- To investigate whether next generation sequencing techniques should become part of ‘routine’ genetic testing for presumed mito
Having the ability to identify and understand the exact gene and the mutation significantly increases the chances of the patient receiving the most appropriate treatment. The sooner the patient is accurately diagnosed, the sooner they can be treated, decreasing the chance of passing this devastating disease to potential children.
The risk of developing a serious case of mito in the general population is about 1 in 5,000. As more research is undertaken and the ability to diagnosis increases, the incidence will rise.
Without accurate molecular diagnosis, patients are denied access to potential treatment. This project has the potential to increase diagnosis and therefore access to treatment.
Increased knowledge about mito will enable the Mito Foundation to better support and give improved practical information to sufferers and their families.
Ultimately the more we learn about the complexity of mito, the faster we can discover effective treatments and find a cure.
After excluding the ‘common’ mitochondrial DNA mutations at m.3243, m.8993 and m.8334, Phillipa and her team identified rare mitochondrial DNA point mutations in seven patients, and a large 13kb deletion in another.
They have also identified a new nuclear gene responsible for complex III deficiency, found a patient to be homozygous for mutations in POLG, and have completed mtDNA haplotyping on three other patients.
They are continuing to analyse an additional eight exomes.
The adult patients, in whom the project identified rare mtDNA point mutations, had not had any biochemical analysis. This indicates that a suggestive history, plus or minus muscle biopsy findings, may be sufficient to diagnose mito.
The study found less genetic abnormalities in the paediatric cases. The clinical information for these patients was limited and most did not suffer from biochemical abnormalities. This suggests that the likelihood of the patients suffering from mito was less, and next generation sequencing was therefore not as appropriate for those cases as ‘first-line’ investigation.
John* was born six weeks early after a non-elective caesarean section for poor growth as a foetus. From early on, he had significant problems with his liver, with jaundice and progressive enlargement of his liver and spleen. When he was four months old, he had a liver biopsy, which showed moderately severe hepatitis, and it was thought it was due to infection with cytomegalovirus. He had a febrile convulsion aged one year. He was delayed in his walking, not getting onto his feet until over 18 months of age. He still had no speech by 2 years of age. At this time his brother George* was born. He too was a small baby, and also had early problems with his liver, but that righted itself within a couple of months of birth. Unfortunately he went on to develop a severe heart problem, requiring medication. He too had developmental delay, and did not walk until nearly two years of age. Both boys have gone on to have persistent and severe problems with their speech, and poor growth. When George was five years of age, a baby sister Sarah* was born. By this stage it had been decided that the boys had mitochondrial disease, but no genetic cause had been detected. Sarah was born very small, and needed to be ventilated for some weeks. She had very high lactate levels in her blood, and required bicarbonate medication. By 10 months of age, she had much more marked developmental delay than her brothers. Thanks to the Mito Foundation DNA sequencing project, it was discovered that all three children carried a mutation in the DNA molecule, which had been described once before in a patient with a different disease called Leigh’s disease. The mutation is at position 5559 of the circular DNA molecule, and interrupts the gene responsible for a protein that transports an amino acid called tryptophan. Their mother was shown to carry the same mutation, although at a much lower level. This discovery has allowed her to make reproductive decisions about further babies.
Geoffrey* first presented when he was 13 years of age with rapidly deteriorating vision in both eyes over a 4 week period. His vision was measured at 6/60, meaning he was legally blind. However, it began to improve after 3 months, returning to normal by 12 months. He remained well until he was 18 years of age, when he developed rapid deterioration in his hearing bilaterally. A diagnosis of multiple sclerosis was made, and he was given a course of high dose steroids, followed by daily injections of disease-modifying medication. Again, after several months his hearing improved. A repeat MRI of his brain showed abnormalities in the central grey matter of his brain, raising the question of a mitochondrial disease. A muscle biopsy was within normal when examined under microscopy, but enzymatic analysis indicated complex I deficiency. Routine molecular genetic testing for common mitochondrial mutations failed to find any abnormality. His DNA was sent to the Mito Foundation DNA sequencing project, and a novel mutation was found at m.14487. This is positioned within the MTND6 gene, just three basepairs away from one of the commonest sites for mutations causing Leber’s Hereditary Optic Neuropathy. Geoffrey was able to stop the multiple sclerosis injections. When he was last seen both his sight and hearing were normal and he was feeling well.
* not their real names