Exhaustive scanning approach to screen all the mitochondrial tRNA genes for mutations and its application to the investigation of 35 independent patients with mitochondrial disorders.

To gain a better understanding of the molecular basisof mitochondrial (mt) encephalomyopathies, a highly heterogeneous condition, we developed a denaturing gradient gel electrophoresis-based approach that allows rapid and exhaustive screening for mutations of all 22 mt tRNA-encoding genes and their flanking regions in large cohorts of patients. This method, that detects heteroplasmy (i.e. co-existence of mutant and wild-type mtDNA species in various ratios) directly, was applied to the investigation of 35 independent patients with a disease phenotype compatible with a mitochondrial encephalomyopathy. Twenty-five of the 35 patients investigated displayed a sequence variation in at least one tRNA gene. A total of 46 different sequence variations (41 point mutations, four short insertions and one short deletion), among which 20 are new, were characterized. Forty of them were present in a homoplasmic state, whereas six were heteroplasmic. Twenty-two were located in tRNA genes, among which 10 are new homoplasmic or heteroplasmic sequence variations; 24 were located in flanking regions (12 in mRNA-encoding genes, seven of them leading to missense sequence variations; two in rRNA genes; and 10 in non-coding regions). This study demonstrates (i) the high frequency of homoplasmic tRNA gene sequence variations in our patient sample, and (ii) the existence of several polymorphic sites in tRNA gene regions that may be helpful for defining haplogroups in different populations. It relies on a screening method that can now be applied easily to other population samples.