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Mitochondrial DNA mutations and Genetic Counseling

© Laura Ruth

You many have heard of mitochondrial DNA before and/or observed a disease in a family that is only transmitted through the females. Mitochondrial DNA is a specific type of DNA that is located in particular compartments (a mitochondria) in the body's tissues. Amazingly, one cell may have a mix of normal and mutant mitochondrial DNA. Although the laboratory methods for detecting a mitochondrial DNA mutation are accurate, the mixture of normal and mutant DNA levels between cells and limited access to human tissue makes predicting the effect of the mutation in the mitochondrial DNA on a person difficult. If prediction of the effect of the mutation is difficult, then the genetic counseling for the particular disease and mutation is also difficult.

A recent prenatal case of a woman with external ophthalmoplegia is a genetic counseling example of dealing with a mitochondrial mutation. Downward sloping eyes, eye pigment degeneration, ataxia, heart problems, and increased cerebrospinal fluid characterize progessive external opthalmoplegia (PEO). A first time pregnant woman, age 26, was first diagnosed with external ophthalmoplegia at the age of 25.

This particular woman showed increased right eye color and a mild hearing deficit. Although prenatal diagnosis of mitochondrial mutations is not common, after genetic counseling, the woman opted for chorionic villus sampling (CVS). Neither the chorionic villus sample, the post-natal cord blood nor the peripheral blood sample from the child at age six months showed any signs of mutated mitochondrial DNA. Future studies investigating the mitochondrial DNA of children of mothers who carry mitochondrial DNA mutations might provide information which would help in predicting the prenatal risk of a mitochondrial DNA genetic disease.

Reference:

Graff, C., Wredenberg, A., Silva, J.P., Bui, T.-H., Borg, K., Larsson, N.-G., "Complex genetic counselling and prenatal analysis in a woman with external opthalmoplegia and deleted mtDNA," Prenatal Diagnosis, 20: 426-431, 2000.

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