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In subjects where the mutation causing the mitochondrial disease has been identified, whether it concerns mtDNA or nuclear DNA, the subsequent family screening can be performed through a simple analysis of DNA carried out from a blood test or, especially in the case of mutations in the mtDNA, from urinary sediment.

It is important to underline that mtDNA mutations are frequently associated with a considerable variability in symptoms within the same family, and the quantitative measurement of the mutation may have a prognostic value.

Next to patients with severe symptoms, there are often maternal relatives who have low or very low mutation percentage, they have only minor symptoms or they are completely asymptomatic. The prognosis for these individuals, however, is sometimes complicated by the different distribution of the mutation in different tissues.

For these reasons, genetic counseling for mitochondrial diseases is particularly difficult, and must be undertaken in a specialized center. General guidelines can be provided anyway.

1. Male carriers of mtDNA mutations do not transmit the disease to their children.
2. Similarly, and apart from very exceptional cases, women with PEO or Kearns-Sayre syndrome, in which a single deletion of mitochondrial DNA is proved, do not transmit the disease to their children.
3. In families where a point mutation in mitochondrial DNA is documented, all women in the maternal line should be considered at risk of having an affected child.
4. In families carrying pathogenic mutations in a nuclear gene, the risk of recurrence follows Mendel's law and it will vary predictably depending on whether it is a dominant, recessive or X-linked character.

Prenatal diagnosis is reliable only when nuclear DNA mutations are concerned.

For mtDNA point mutations, the prognosis is not easily formulated, as the percentage of mutated DNA present in amniocytes or chorionic villi may not correspond to that present in other fetal tissues. Moreover, this percentage may change during both intrauterine development and after birth.

Our Center provides “outpatient services for genetic counseling”.