Mitochondrial tRNA^Leu(UUR) Mutation at Position 3243 and Symptomatic Polyneuropathy in Type 2 Diabetes

In 1994, we documented a high frequency of complicated posttreatment neuropathy in patients with mitochondrial diabetes associated with tRNA^Leu(UUR) mutation at position 3243′ (MDM3243) (1). Thereafter, experimental studies accumulated evidence that mitochondria are a major culprit in the initiation and development of complications of diabetes through oxidative stress or altered redox changes (2–4). We recently confirmed our previous observation on the association of mitochondrial DNA (mtDNA) mutation with clinical symptoms of diabetic distal polyneuropathy by conducting a large-scale study. A total of 271 Japanese patients with type 2 diabetes at Saiseikai Central Hospital were subjected and divided into two groups. Patients who had leg symptoms not only at the time of this study, but also in their history were regarded as positive and were classified into group 1. Symptomatic neuropathy was assessed by the presence of one, two, or all of the following symptoms over a previous 5-year period: numbness in the feet, pricking sensation in the feet, and deep or burning pain in the legs. Subjects without these subjective symptoms were classified into group 2. The definition of neuropathy was based on criteria for the diagnosis of diabetic polyneuropathy, proposed by the committee for discussing diabetic neuropathy in Japan (5). Detection of the 3243 mtDNA mutation was performed by integrating two methods (PCR /restriction fragment–length polymorphism [RFLP] and allele-specific PCR amplification), which improve the sensitivity of detecting the mutation in leukocytes (6). The detective threshold of finding heteroplasmy degree is as small as 0.2%. The details of this integrated methodology have been described previously (7).

In result, of 271 subjects 11 were found to have the 3243 mtDNA mutation, a frequency of 4.1%. All showed clinical differences from the syndromes of MELAS (mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes), CPEO (chronic progressive external ophthalmoplegia), and MERRF (myoclonic epilepsy associated with ragged red fibers). Among all subjects, 91 were assigned to group 1 and 180 to group 2. In group 2, two were found to have the 3243 mutation, and the frequency of finding the mutation was 1.1%. In group 1, nine were found to have the 3243 mutation, and the frequency of finding the mutation was 9.8%, which was significantly higher than that of group 2 (P < 0.001 by χ² analysis). Thus, the 3243 mtDNA mutation is frequently found among diabetic patients with symptomatic polyneuropathy (group 1). Additionally, patients of group 1 had earlier onset of diabetes and a higher frequency of retinopathy, nephropathy, and insulin therapy (data not shown here).

Oxidative stress may be the leading proposed mechanism for understanding the relation fully, because reactive oxygen species is associated with a number of pathological conditions of diabetes. Low et al. (2) hypothesized that lipid peroxidation under hyperglycemic conditions causes mtDNA mutations that increase oxygen radicals, causing further damage to mitochondrial respiratory chain, ultimately resulting in sensory neuropathy. Interestingly, the 3243 mtDNA mutation itself increases intracellular reactive oxygen species production (8), which may in turn cause secondary somatic mutations in diabetes, making a vicious cycle (9). Therefore, we speculate that in diabetic patients with high oxidative stress, when the effective mechanism for maintaining mitochondrial function is lacking, the vicious cycle of oxidative stress with increase of the 3243 mtDNA mutation may be facilitated. Furthermore, when patients have a certain amount of innate 3243 mtDNA mutation inherited from the mother, the disadvantage renders the patients all the more susceptible to oxidative stress under hyperglycemia, thereby precipitating the vicious cycle and producing symptomatic neuropathy.

The frequency of finding the 3243 mtDNA mutation in this study was 4.1% in total. This frequency is higher than the reported data of other researchers in Japanese subjects (6,10). One reason for this is because Saiseikai Central Hospital is the Diabetes Centers where patients with complications are likely to be referred to from local clinics, thus the hospital bias may have increased the frequency of finding the 3243 mtDNA mutation. The second plausible reason is that in this study, the detective threshold of finding heteroplasmy degree for the 3243 mtDNA mutation is more sensitive than that of other researchers, where the detected threshold is ∼1% (6,10). This highly sensitive methodology (7) decreases the number of overlooked patients carrying a very small degree of heteroplasmy, which in turn leads to the increased frequency of finding the mutation.

In conclusion, sensory neuropathy in diabetes is associated with the 3243 mtDNA mutation. This result of human study supports the recent evidence of experimental studies (2,3). However, further studies are needed to reveal the association of sensory neuropathy with more varieties of mitochondrial DNA abnormalities than the 3243 mutation.