Novel Mitochondrial DNA Length Variants and Genetic Instability in a Family with Diabetes and Deafness

Introduction

Genetic analysis has revealed a large number of mtDNA mutations that associate with distinct multisystem disorders including maternally inherited diabetes and deafness (MIDD), mitochondrial encephalomyopathies, cancer, and with the ageing process ([Van den Ouweland et al., 1992]; [Wallace, 1999]; [Maassen and Kadowaki, 1996]; [Fliss et al., 2000]; [Michikawa et al., 1999]; [Hattori et al., 2005]). The total gene content of human mtDNA consists of 13 protein-coding genes, all involved in oxidative phosphorylation, and the 2 rRNA and 22 tRNA genes required to synthesize these proteins within the mitochondrial matrix. mtDNA occurs as a multicopy genome and shows complex segregation patterns during oogenesis and further differentiation ([Jacobs et al., 2000]; [Battersby et al., 2003]). Because a single cell may contain up to several thousands of copies of the genome, a mutation can exist in a homoplasmic or a biplasmic state in which mixtures of wild-type and mutant mtDNA are present in variable proportions. The latter heteroplasmic situation is often found associated to disease. The third sporadically occurring multiplasmic state, in which multiple genetic variants are present, have been found in polycytidine tracts at nt301, nt961, and nt16189 ([Tagliabracci et al., 2001]; [Marchington et al., 1996]; [Bendall and Sykes, 1995]; [Casano et al., 1999]).

Maternally inherited diabetes and deafness is clinically characterized by an early-middle age onset of diabetes, sensorineural hearing loss of high-frequency tones, progressive insulin secretory defect, absence of islet-cell antibodies, and general absence of obesity ([Maassen et al., 2001]; [Maassen et al., 2004]). The A3243G mutation is the most predominant mtDNA mutation in diabetic subjects and accounts between 0.2 to 2 % of the diabetic population in Europe and Japan ([Kadowaki et al., 1994]; [Maassen and Kadowaki, 1996]), while the maturity-onset-diabetes-of-the-young genes belong to the most abundant nuclear diabetes genes ([Stride and Hattersley, 2002]). Several other mutations in mtDNA have been found in diabetic subjects with low frequency ([Maassen et al., 2005]). Finally, apparently silent mtDNA variants may also contribute to the pathogenesis of the diabetic phenotype ([Poulton et al., 2002]), especially when additional features of mitochondrial diabetes, like deafness, are present ([Choo-Kang et al., 2002]). Here we report the presence of novel multiple heteroplasmic length variants together with additional point mutations in a family with diabetes and hearing disturbances. We provide evidence that cybrid cells containing the mtDNA from the maternal line display a reduced fitness under conditions of proliferative competition.

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