Mitochondrial-DNA nucleotides G4298A and T10010C as pathogenic mutations: the confirmation in two new cases Marco Crimi a, * , Sara Galbiati a , Monica Sciacco a , Andreina Bordoni a , Maria Grazia Natali c , Monica Raimondi d , Nereo Bresolin a,b , Giacomo Pietro Comi a a Dipartimento di Scienze Neurologiche, Universita ` degli Studi di Milano, IRCCS Ospedale Maggiore Policlinico, Centro Dino Ferrari and Centro di Eccellenza per le malattie Neuro-Degenerative (CEND) Via F. Sforza 35, 20122 Milan, Italy b IRCCS ‘E. Medea’, Bosisio Parini (LC), Italy c Istituto Scientifico San Raffele, Milan, Italy d Ospedale Civico di Lugano, Lugano, Switzerland Received 6 June 2003; received in revised form 28 November 2003; accepted 5 February 2004 Abstract Mitochondrial encephalomyopathies are highly variable clinically and at the genetic level. In practice, when the mitochondrial DNA (mtDNA) of any mitochondrial-patient is sequenced, a very high number of variations are noted. The vast majority of these differences are simply polymorphisms, that is, non-pathologic, homoplasmic sequence variations; however, when a heteroplasmic variant is detected (co-existence of two different populations in the same tissue) this is clinically significant. We identified two different heteroplasmic mutations in the mtDNA of two subjects: G4298A in the tRNA Ala (Alanine) gene and T10010C in the tRNA Gly (Glycine), both of which have been reported previously. This work confirms the pathogenicity of these mutations and helps define the correlation between genotype and phenotype. q 2004 Elsevier B.V. and Mitochondria Research Society. All rights reserved. Keywords: Mitochondrial (mt)DNA; tRNA gene; Encephalomyopathy and point mutation 1. Introduction Mitochondria are the ATP-generating organelles in mammalian cells and contain their own DNA, which is maternally inherited. The human mtDNA has been the object of intensive investigation since 1981, when its complete sequence was determined. In the past decade, over 100 different mtDNA point mutations affecting both protein-coding and RNA genes have been catalogued. Particularly, there have been increasing reports of neurological disorders harbour- ing a mutation in transfer RNA genes. Both normal and mutated mtDNA may co-exist within patient tissues (heteroplasmy). The phenotypic expression of a pathologic mutation depends on the amount of mutated mtDNA in each tissue and on the tissue- specific threshold level, beyond which the presence of the mutation becomes clinically expressed. Definitive diagnosis comes from the detection of specific molecular defects. 1567-7249/$20.00 q 2004 Elsevier B.V. and Mitochondria Research Society. All rights reserved. doi:10.1016/j.mito.2004.02.004 Mitochondrion 3 (2004) 279–283 www.elsevier.com/locate/mito * Corresponding author. Tel.: þ39-02-5503-3817; fax: þ 39-02- 5032-0430. E-mail address: marcreamy@tiscali.it (M. Crimi).