MATERNAL INHERITANCE OF MITOCHONDRIAL DNA MUTATIONS

Some mitochondrial encephalomyopathies that may be caused by mtDNA mutations and are subject to the rules of maternal inheritance are MERFF, MELAS, NARP, PEO and MILS.

In some syndromes, mtDNA mutations tend to occur spontaneously -- that is, the mutation isn't present in the mother or the father but has, instead, occurred very early in the development of the embryo. This is often the case for KSS, PEO and Pearson, three diseases that result from a type of mtDNA mutation called a deletion (specific portions of the DNA are missing) or mtDNA depletion (a general shortage of mtDNA). These types of spontaneously acquired mutations aren't usually passed to the next generation.

A third kind of mitochondrial disease inheritance is a combination of nuclear and mtDNA defects. This type of disease is inherited in a Mendelian fashion, indicating the involvement of a nuclear gene, but is also characterized by mtDNA deletions. In this case, the mtDNA deletions occur because there's a "breakdown in communication" between the nuclear and mitochondrial DNA.

An example of this type of disease is MNGIE. Recently, Columbia University researcher Michio Hirano and colleagues identified the nuclear gene involved in MNGIE. The gene codes for a protein called thymidine phosphorylase that may be involved in regulating the building blocks of mitochondrial DNA