Review Question - QID 107022

The mother and the child both experienced symptoms of mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS). The difference in the symptoms is due to heteroplasmy.

Mitochondrial diseases are inherited only from females with affected mitochondria. The mother will pass it onto all of her offspring (both males and females affected at equal frequency), because sperm do not contribute mitochondria to the zygote. Mitochondrial diseases result from defects in electron transport/oxidative phosphorylation processes. The diseases will present as neuropathies and/or myopathies. Lactic acidosis may be present, and if a muscle biopsy is taken, ragged, red muscle fibers can be seen on a Gömöri trichrome stain (Figure A). The disease may present differently depending on the percentage of mitochondria affected and the cell type containing the diseased mitochondria. This variable expression is termed heteroplasmy. Some examples of mitochondrial diseases are MELAS, Leber hereditary optic neuropathy, and myoclonic epilepsy with ragged, red muscle fibers (MERRF).

DiMauro and Hirano review MELAS stating that onset typically begins in childhood (between 2-10 years of age) with normal psychomotor development and they often are of short stature. Initial symptoms are seizures, frequent headaches and vomiting, anorexia, exercise intolerance, and proximal limb weakness. The seizures that are due to MELAS are accompanied by "stroke-like episodes of transient hemiparesis or cortical blindness". The damage caused by these stroke-like episodes eventually accumulate and can result in brain damage affecting the motor, vision, and cognitive function.

DiMauro and Hirano discuss the management of MELAS. Although there is no specific treatment for the disease, each of the symptoms are treated individually. For example for hearing loss, cochlear implants are used; as for the seizures, they can be controlled with anticonvulsant medications. For prevention patients can be given coenzyme Q10 or idebenone, which has shown some success in prevention of some of the symptoms.

Figure A demonstrates a Gömöri trichrome stain of a muscle biopsy taken from an individual with MELAS. The arrows indicates red ragged fibers, which are an accumulation of diseased mitochondria.

Incorrect answers:
Answer 1: Codominance refers to both alleles being expressed such as the AB blood type.
Answer 2: Incomplete penetrance occurs when an individual has the mutant genotype, but does not have the diseased phenotype.
Answer 3: Pleiotropy is when a single mutation has diverse effects on several organ systems.
Answer 4: Anticipation refers to changes in disease presentation in succeeding generations; there is typically an increase severity and earlier onset.