Random mitotic segregation of mitochondrial DNA in MELAS syndrome

Mitochondrial DNA sequence characteristics modulate the size of the genetic bottleneck

With a combined carrier frequency of 1:200, heteroplasmic mitochondrial DNA (mtDNA) mutations cause human disease in ∼1:5000 of the population. Rapid shifts in the level of heteroplasmy seen within a single generation contribute to the wide range in the severity of clinical phenotypes seen in families transmitting mtDNA disease, consistent with a genetic bottleneck during transmission. Although preliminary evidence from human pedigrees points towards a random drift process underlying the shifting heteroplasmy, some reports describe differences in segregation pattern between different mtDNA mutations. However, based on limited observations and with no direct comparisons, it is not clear whether these observations simply reflect pedigree ascertainment and publication bias. To address this issue, we studied 577 mother–child pairs transmitting the m.11778G>A, m.3460G>A, m.8344A>G, m.8993T>G/C and m.3243A>G mtDNA mutations. Our analysis controlled for inter-assay differences, inter-laboratory variation and ascertainment bias. We found no evidence of selection during transmission but show that different mtDNA mutations segregate at different rates in human pedigrees. m.8993T>G/C segregated significantly faster than m.11778G>A, m.8344A>G and m.3243A>G, consistent with a tighter mtDNA genetic bottleneck in m.8993T>G/C pedigrees. Our observations support the existence of different genetic bottlenecks primarily determined by the underlying mtDNA mutation, explaining the different inheritance patterns observed in human pedigrees transmitting pathogenic mtDNA mutations.

A wide range of 3243A>G/tRNALeu(UUR) (MELAS) mutation loads may segregate in offspring through the female germline bottleneck

Segregation of mutant mtDNA in human tissues and through the germline is debated, with no consensus about the nature and size of the bottleneck hypothesized to explain rapid generational shifts in mutant loads. We investigated two maternal lineages with an apparently different inheritance pattern of the same pathogenic mtDNA 3243A>G/tRNA^Leu(UUR) (MELAS) mutation. We collected blood cells, muscle biopsies, urinary epithelium and hair follicles from 20 individuals, as well as oocytes and an ovarian biopsy from one female mutation carrier, all belonging to the two maternal lineages to assess mutant mtDNA load, and calculated the theoretical germline bottleneck size (number of segregating units). We also evaluated “mother-to-offspring” segregations from the literature, for which heteroplasmy assessment was available in at least three siblings besides the proband. Our results showed that mutation load was prevalent in skeletal muscle and urinary epithelium, whereas in blood cells there was an inverse correlation with age, as previously reported. The histoenzymatic staining of the ovarian biopsy failed to show any cytochrome-c-oxidase defective oocyte. Analysis of four oocytes and one offspring from the same unaffected mother of the first family showed intermediate heteroplasmic mutant loads (10% to 75%), whereas very skewed loads of mutant mtDNA (0% or 81%) were detected in five offspring of another unaffected mother from the second family. Bottleneck size was 89 segregating units for the first mother and 84 for the second. This was remarkably close to 88, the number of “segregating units” in the “mother-to-offspring” segregations retrieved from literature. In conclusion, a wide range of mutant loads may be found in offspring tissues and oocytes, resulting from a similar theoretical bottleneck size.

Spontaneous event of mitochondrial DNA mutation, A3243G, found in a family of identical twins

A mutation in mitochondrial DNA (mtDNA) A3243G is an important cause of some serious mitochondrial diseases, and maternal inheritance of the mutation has been reported. In order to investigate the heredity of the mutation, we measured the ratio of the mutated mtDNA molecule among 32 families of identical twins. Both twins from one family showed 20.16% and 18.49% mutated molecules, and the level is significantly high in comparison with members of other families and control subjects (0.23-0.86%). Their parents, however, showed normal level of mutated molecules (0.70% and 0.66%). The high-level mutation of the twins may be due to a spontaneous event, which occurred during development of germ line of their mother, or oogenesis of their mother, or during early stage of their development.

Prognosis of symptomatic patients with the A3243G mutation of mitochondrial DNA

BACKGROUND/PURPOSE

The clinical analyses and prognoses of mitochondrial diseases with A3243G mutation are rarely documented in Taiwan. Our study investigated the clinical phenotypes and the outcomes of patients with mitochondrial disease and the A3243G mutation of mtDNA in a Taiwanese population, and compared these with previous reports.

METHODS

We retrospectively studied 22 consecutive patients with mitochondrial disease and the A3243G mutation of mtDNA in Chang Gung Memorial Hospital between 1988 and 2009. All patients underwent a detailed demographic registration, neurological examinations, a muscle biopsy, and mitochondrial DNA analysis. Modified Rankin scale, the presence of recurrent strokes or seizures, critical medical complications, and death were monitored during the follow-up period.

RESULTS

Of the 22 patients, seizures and stroke-like episodes were found in 12 (55%). Visceral involvement, including cardiomyopathy, nephropathy, and pulmonary hypertension, were noted in five patients (23%). Patients with seizures had a high frequency of status epilepticus (92%) and a younger age of onset (21.3±7.2 years). Both the Kaplan-Meier survival analysis and the Cox-regression model showed a marked deterioration in patients with seizures after 7 years of follow-up.

CONCLUSION

Our study found that seizures and status epilepticus are the most important predictive values for a poor outcome in patients with the mtA3243G mutation of mtDNA. Age of onset and visceral organ involvement had no prominent influence on the prognosis. Some medical complications could be well controlled or even reversed after management.

Human mitochondrial diseases: answering questions and questioning answers

Since the first identification in 1988 of pathogenic mitochondrial DNA (mtDNA) mutations, the mitochondrial diseases have emerged as a major clinical entity. The most striking feature of these disorders is their marked heterogeneity, which extends to their clinical, biochemical, and genetic characteristics. The major mitochondrial encephalomyopathies include MELAS (mitochondrial encephalopathy with lactic acidosis and stroke-like episodes), MERRF (myoclonic epilepsy with ragged red fibers), KSS/CPEO (Kearns-Sayre syndrome/chronic progressive external ophthalmoplegia), and NARP/MILS (neuropathy, ataxia, and retinitis pigmentosum/maternally inherited Leigh syndrome) and they typically present highly variable multisystem defects that usually involve abnormalities of skeletal muscle and/or the CNS. The primary emphasis here is to review recent investigations of these mitochondrial diseases from the standpoint of how the complexities of mitochondrial genetics and biogenesis might determine their varied features. In addition, the mitochondrial encephalomyopathies are compared and contrasted to Leber hereditary optic neuropathy, a mitochondrial disease in which the pathogenic mtDNA mutations produce a more uniform and focal neuropathology. All of these disorders involve, at some level, a mitochondrial respiratory chain dysfunction. Because mitochondrial genetics differs so strikingly from the Mendelian inheritance of chromosomes, recent research on the origin and subsequent segregation and transmission of mtDNA mutations is reviewed.