A novel mutation in the mitochondrial tRNA(Pro) gene associated with late-onset ataxia, retinitis pigmentosa, deafness, leukoencephalopathy and complex I deficiency

Abstract

We present a patient with ataxia, retinitis pigmentosa, dysarthria, neurosensorial deafness, nystagmus and leukoencephalopathy. A novel heteroplasmic G to A transition at nucleotide 15 975 was found, affecting the T arm of the mitochondrial (mt) tRNA(Pro) gene. A biochemical analysis of respiratory chain enzymes in muscle revealed isolated complex I deficiency. This is the fourth pathogenic tRNA(Pro) point mutation to be associated with an mt disorder. The result highlights the importance of molecular dissection of mtDNA in patients with defined mt disorder and confirms the clinical and biochemical heterogeneity associated with tRNA(Pro) mutations.

Figure 1

(a) Direct sequencing of mitochondrial tRNA^Pro gene from the patient's muscle and blood and from the muscle of a control. Electropherograms of nucleotide position 15 967–15 982 showing heteroplasmic C–T transition at position 15 975 in muscle mtDNA and its presence at low levels from blood mtDNA. (b) PCR-RFLP analysis of DNA amplified from the patient's skeletal muscle (M), blood (B), urinary epithelium (U), mouth epithelium (ME), hair root (H) and fibroblasts (F). UC, uncut PCR; MVIII, DNA molecular weight marker VIII. In the presence of the mutation, a 162-base pair (bp) fragment is digested by Tsp509I into two fragments of 112 and 50 bp (see Materials and methods for details). (c) Family pedigree. Mutant load is shown for various tissues in the patient and family members. ND, not detected; trace, mutant bands detectable, but too faint to be quantified (<5%).

Figure 2

Functional consequence of the G15975A mutation. (a) Proposed two-dimensional structure of human mitochondrial tRNA^Pro (reverse) showing the mutation in the TΨC stem and the affected G–C base pair. (b) A closer examination of this region of the tRNA^Pro gene reveals that the base pair involving the nucleotide at position 15 975 is highly conserved throughout the species. The mutated base in the patient is shown underlined in bold.