Three families with 'de novo' m.3243A > G mutation

Abstract

The m.3243A > G mutation is the most prevalent, disease-causing mitochondrial DNA (mtDNA) mutation. In a national cohort study of 48 families harbouring the m.3243A > G mutation, we identified three families in which the mutation appeared to occur sporadically within these families. In this report we describe these three families. Based on detailed mtDNA analysis of three different tissues using two different quantitative pyrosequencing assays with sensitivity to a level of 1% mutated mtDNA, we conclude that the m.3243A > G mutation has arisen de novo in each of these families. The symptomatic carriers presented with a variety of symptoms frequently observed in patients harbouring the m.3243A > G mutation. A more severe phenotype is seen in the de novo families compared to recent cohort studies, which might be due to reporting bias. The observation that de novo m.3243A > G mutations exist is of relevance for both diagnostic investigations and genetic counselling. Firstly, even where there is no significant (maternal) family history in patients with stroke-like episodes, diabetes and deafness or other unexplained organ dysfunction, the m.3243A > G mutation should be screened as a possible cause of the disease. Second, analysis of maternally-related family members is highly recommended to provide reliable counselling for these families, given that the m.3243A > G mutation may have arisen de novo.

Keywords: Genetic counselling; Inheritance; MELAS, mitochondrial myopathy, encephalopathy, lactate acidosis and stroke-like episodes; MERRF, myoclonic epilepsy with ragged-red fibres; MIDD, maternally inherited diabetes and deafness; Maternally inherited diabetes and deafness (MIDD); Mitochondrial myopathy, encephalopathy, lactate acidosis and stroke-like episodes (MELAS); m.3243A > G mutation; mtDNA, mitochondrial DNA.

Fig. 1

a–c Pedigrees of the three patients with a de novo mutation. Fathers are not indicated in the pedigrees. The patients with a detectable m.3243A > G mutation load are indicated as black squares and circles. The patients that were tested using pyrosequencing as earlier described by Lowik et al. (2005) are indicated in light grey shading; the patients that were additionally tested using the more sensitive pyrosequencing assay described by Alston et al. (2011) are indicated with darker grey shading. a: Family 1; the proband is indicated as individual III-2. Family members in whom the mutation was undetectable in urinary epithelial cells, leucocytes and buccal cells are; II-2, II-3, II-4, II-5, II-7, II-9, III-1, III-3, III-4, III-5, III-6, III-7, III-8, III-9, III-10, IV-6. No further family members were available to participate in the study. Family members II-3 and III-1, mother and sister of the proband respectively, were screened for m.3243 A > G heteroplasmy in urinary epithelial cells using the more sensitive pyrosequencing assay. b: Family 2; the proband is indicated as patient IV-1 however individual III-3 (the proband's asymptomatic mother) is considered to be the de novo case following carrier testing. The m.3243A > G mutation was undetectable in urinary epithelial cells, leucocytes and buccal cells of relatives denoted II-1, II-3, II-5, III-4 and III-7. The other family members did not participate in the study. Heteroplasmy was tested in urinary epithelial cells from individual II-1, the proband's mother, using the more sensitive pyrosequencing assay. c: Family 3; the proband is indicated as patient II-4. Tested family members in whom the mutation was undetectable in urinary epithelial cells, leucocytes and buccal cells are; I-1, II-1 and II-2. In all three family members, m.3243A > G mutation load in urinary epithelial cells was tested using the more sensitive pyrosequencing assay. Patient II-3 died at the age of 4 months. Post mortem muscle and liver biopsies were screened and no evidence of the m.3243A > G mutation was reported.