6.4. mtDNA mutations in patients with occipital stroke

Four patients with occipital stroke harboured mutations that were not found among the controls, and three of these represented rare mtDNA genotypes, two of which could not be found in 480 Finnish controls. Two of the mutations were in transfer RNA genes. The 4295A>G mutation in tRNA^Ile has been considered aetiologically related to cardiomyopathy in a seven-month-old child (Merante et al. 1996) and the 8296A>G mutation in tRNA^Lys has previously been associated with diabetes mellitus and hearing impairment. This mutation is thought to explain 0.9 % of diabetes mellitus cases and 2.3 % of those of sensorineural hearing impairment in Japan (Kameoka et al. 1998a, 1998b).

Heteroplasmy has been considered one criterion for the pathogenicity of a point mutation. Both 4295A>G and 8296A>G have been shown to be heteroplasmic by restriction analysis and by dot blot hybridisation, respectively (Merante et al. 1996, Kameoka et al. 1998a). In the light of experience with the apparent heteroplasmy of transition 5656A>G, we checked the heteroplasmy of the two tRNA mutations by subcloning the PCR amplified fragments encompassing them. The 8296A>G mutation turned out to be homoplasmic in blood, but both the mutant genome and the wild type genome were detected in subclones containing a segment encompassing nt 4295. The wild type genome was found only in 2 out of 79 clones, however, suggesting that the degree of heteroplasmy in the blood of the patient was approximately 97 %.

Other point mutations found only in patients have to be considered against the background of the genotype in the network. Patients #148 and #282 (IV) represent unique genotypes in the total of 480 genotypes in the Finnish population. Although a synergistic mutation in LHON and other point mutations leading to amino acid replacements were found in patient #148, it is hard to evaluate their significance for the pathogenesis of occipital stroke because of the unique haplotype. The homoplasmic transition 5773G>A in patient #148 is interesting even though it was also found in a healthy control. It is an invariant nucleotide (Sprinzl et al. 1989) in the pseudouridine loop of tRNA^Cys, and was found to exist in two distinct branches of the haplogroup U network.

Patient #153 harboured the mutation 1850T>C in the gene coding for 16S rRNA. This mutation was located in a dinucleotide pair that is highly conserved between species, but its pathogenic nature is not known. Interestingly, the homoplasmic 1555A>G transition in the 12S rRNA gene has been associated with aminoglycoside-induced or spontaneous non-syndromic progressive deafness. It has been postulated to elongate the region in which the tRNA binds to the ribosome, thus facilitating the binding of aminoglycosides and potentiating their effects on the fidelity of mRNA translation (Prezant et al. 1993).