MELAS is a rare form of dementia. MELAS is an abbreviation that stands for Mitochondrial Encephalopathy, Lactic acidosis, and Stroke-like episodes.
What causes MELAS?
MELAS syndrome is caused by mutations in the genetic material (DNA) in the mitochondria. While most of our DNA is in the chromosomes in the cell nucleus, some of our DNA is in another important structure called the mitochondrion (plural: mitochondria).
The mitochondria are located outside the nucleus in the cell’s cytoplasm. Each mitochondrion has a chromosome made of DNA that is quite different from the better known chromosomes in the nucleus.
The mitochondrial chromosome is much smaller; it is round (whereas the chromosomes in the nucleus are normally shaped like rods); there are many copies of the mitochondrial chromosome in every cell; and no matter whether we are male or female, we inherit all of our mitochondrial chromosome from our mother.
Much of the DNA in our mitochondria is used to manufacture proteins involved in the key function of mitochondria — to produce energy and power the cells in our body.
What are the symptoms of MELAS?
As a result of the disturbed function of their cells’ mitochondria, patients with MELAS develop brain dysfunction (encephalopathy) with seizures and headaches, as well as muscle disease with a build-up of lactic acid in the blood (a condition called lactic acidosis), temporary local paralysis (stroke-like episodes), and abnormal thinking (dementia).
How is MELAS diagnosed?
The diagnosis of MELAS is usually suspected on clinical grounds. However, confirmation of the diagnosis usually requires a muscle or brain biopsy.
The muscle biopsy shows characteristic ragged red fibers; a brain biopsy shows stroke-like changes.
When do people with MELAS develop symptoms?
MELAS can affect people at very different times in life, ranging from age 4 to age 40 or more. However, most patients with MELAS syndrome show symptoms before they are 20 years old.
The defect involves the respiratory chain (responsible for energy production). It is caused by a point mutation at nucleotide 3243 mtDNA (A to G translocation) which encodes for transfer RNA for leucine.
It is therefore thought that this abnormality results in abnormal protein manufacture throughout the mitochondrium and affects multiple parts of the respiratory chain.
The exact mechanism notwithstanding, the net result is depletion of NAD+ and NADH+.
This in turn results in a shift to anaerobic metabolism accounting for the build up of lactic acid, and renders the cortex susceptible to neuronal death 1.
As a number of mitochondria are passed in the ovum, not all will have the mutant mtDNA. The percentage of mutated genes will affect the severity of clinical manifestations.
How is MELAS treated?
There is no known treatment for the underlying disease, which is progressive and fatal. Patients are managed according to what areas of the body are affected at a particular time.
Antioxidants and vitamins have been used, but there have been no consistent successes reported.
Are there other mitochondrial diseases?
Yes, mutations (genetic changes) in the mitochondrial chromosome are responsible for a number of other disorders aside from MELAS such as:
an important eye disease called Leber hereditary optic atrophy;
- a type of epilepsy called MERRF which stands for Myoclonus Epilepsy with Ragged Red Fibers; and
- a neuromuscular disease called the Kearns-Sayre syndrome.
- MELAS and all other mitochondrial diseases were entirely enigmatic before it was discovered that they were due to mutations not in regular chromosomes but in the chromosome of the mitochondria.