When are mitochondria inoperable

Energy crisis in the cell - on the trail of disease genes with exome sequencing

A new method makes it possible for the first time to quickly find the genetic causes of diseases of the energy metabolism and thus open up new therapies for those affected.

The heart, brain and muscles need a lot of energy. If the energy metabolism of the mitochondria is disturbed, these organs are particularly often affected. Energy metabolism disorders are rare diseases that affect around one in 5,000 people. They show very different clinical pictures and symptoms, which complicates their diagnosis. The most common causes of diseases of the energy metabolism are rare defects in the genetic make-up. “As a rule, however, a large number of genes are possible causes, sometimes more than 100 of them. That makes an accurate molecular diagnosis very difficult, time-consuming and expensive, "describes Dr. Holger Prokisch from the Institute for Human Genetics at the Technical University of Munich. However, without a precise diagnosis, those affected cannot be treated in a targeted manner. That should change now. As part of a project funded by the Federal Ministry of Education and Research (BMBF), Dr. Prokisch and his colleagues from the “Network Mitochondrial Diseases” showed that modern methods of DNA analysis make it possible to find the genetic causes of diseases of the energy metabolism quickly, efficiently and inexpensively.

30 million possibilities - one hit

A case study: a patient has suffered from an energy metabolism disorder since birth. Difficulty breathing, heart problems and acidity in the body are some of the symptoms. Doctors know from tissue samples that a certain enzyme for generating energy within the mitochondrial respiratory chain is significantly less active than in healthy people. But the genetic cause is so far unknown. “We started looking for it using exome sequencing,” explains Dr. Prokish. The researchers do not sequence the entire genome of the patient, but limit themselves to the exome, i.e. the area of ​​the human genome that codes for proteins or other functional products. The exome only makes up about one percent of all human DNA, but is estimated to be responsible for more than 85 percent of all genetic diseases. “While one gene after the other is examined in routine diagnostics, all genes can be examined at once with exome sequencing. This new method doesn't take much longer than analyzing a gene, and it doesn't cost much more either. The challenge lies more in the analysis of the data obtained, ”describes Dr. Prokish. Because if you look at the 30 million base pairs of the exome, the scientists find around 12,000 positions in each person, which differ from person to person. Around 6,500 of these mutated positions lead to a change in the encoded protein. "If you then filter out all the frequent variants, assuming that they are not relevant to the disease, around 500 individual variants remain that are actually specific for the examined person, i.e. in our case for the patient with the energy metabolism disorder," says Dr . Prokish.

But which of these mutations is responsible for the disease? Many rare diseases are inherited recessively, which means that both copies of a gene must be affected for the disease to break out. If you are looking for genes of which two different mutated variants can be found - one from the mother and one from the father - around 30 genes still come into question as the cause of the disease. “At this point, we benefited from the patient's tissue sample in our search. It had shown that there is an enzymatic defect in the mitochondrial respiratory chain, ”says Dr. Prokish. Since only one of the 30 possible genes codes for a protein that fulfills a task in the mitochondria, the cause was quickly found: The gene responsible for the disease of the energy metabolism is called acyl-CoA dehydrogenase 9, or ACAD9 for short. The proof: If the scientists in the laboratory lock the intact ACAD9 gene into the diseased cells, they are fully functional again. "So far it was not even known that ACAD9 had a function in the respiratory chain," said Dr. Prokish.

Targeted treatment can only be given if you know the cause

In this specific case, the molecular diagnosis provided the scientists with an idea for a therapeutic approach: The changes in the ACAD9 gene presumably lead to the protein folding incorrectly. "This probably also hampers the incorporation of riboflavin, ie vitamin B2, into the enzyme that is important for the activity of ACAD9," explains Dr. Prokish. In the laboratory, the energy metabolism of the defective cells can actually be increased by adding riboflavin. "Our patient is now being treated with a high dose of riboflavin and we hope that this treatment will help him."

Conclusion: With the help of exome sequencing, the scientists of the BMBF network have not only succeeded in finding a new disease-relevant gene for disorders of the energy metabolism. Knowledge of the genetic cause now also opens up a new therapeutic option for those affected.

Contact Person:
Dr. Holger Prokisch
Technical University of Munich
Institute of Human Genetics
Trogerstrasse 32
81675 Munich
Tel .: 089 3187-2890
Fax: 089 3187-3297
Email: [email protected]