Joanna Poulton

Human mitochondrial DNA deletions associated with mutations in the gene encoding Twinkle, a phage T7 gene 4-like protein localized in mitochondria

The gene products involved in mammalian mitochondrial DNA (mtDNA) maintenance and organization remain largely unknown. We report here a novel mitochondrial protein, Twinkle, with structural similarity to phage T7 gene 4 primase/helicase and other hexameric ring helicases. Twinkle colocalizes with mtDNA in mitochondrial nucleoids. Screening of the gene encoding Twinkle in individuals with autosomal dominant progressive external ophthalmoplegia (adPEO), associated with multiple mtDNA deletions, identified 11 different coding-region mutations co-segregating with the disorder in 12 adPEO pedigrees of various ethnic origins. The mutations cluster in a region of the protein proposed to be involved in subunit interactions. The function of Twinkle is inferred to be critical for lifetime maintenance of human mtDNA integrity.

Exome sequencing identifies ACAD9 mutations as a cause of complex I deficiency

An isolated defect of respiratory chain complex I activity is a frequent biochemical abnormality in mitochondrial disorders. Despite intensive investigation in recent years, in most instances, the molecular basis underpinning complex I defects remains unknown. We report whole-exome sequencing of a single individual with severe, isolated complex I deficiency. This analysis, followed by filtering with a prioritization of mitochondrial proteins, led us to identify compound heterozygous mutations in ACAD9, which encodes a poorly understood member of the mitochondrial acyl-CoA dehydrogenase protein family. We demonstrated the pathogenic role of the ACAD9 variants by the correction of the complex I defect on expression of the wildtype ACAD9 protein in fibroblasts derived from affected individuals. ACAD9 screening of 120 additional complex I–defective index cases led us to identify two additional unrelated cases and a total of five pathogenic ACAD9 alleles.

Mutations at the mitochondrial DNA polymerase (POLG) locus associated with male infertility

Human mitochondrial DNA polymerase, encoded by POLG, contains a polyglutamine tract encoded by a CAG microsatellite repeat. Analysis of POLG genotypes in different populations identified an association between absence of the common, ten-repeat allele and male infertility typified by a range of sperm quality defects but excluding azoospermia.

Clinical and Molecular Genetic Analysis of 19 Wolfram Syndrome Kindreds Demonstrating a Wide Spectrum of Mutations in WFS1

Wolfram syndrome is an autosomal recessive neurodegenerative disorder characterized by juvenile-onset diabetes mellitus and progressive optic atrophy. mtDNA deletions have been described, and a gene (WFS1) recently has been identified, on chromosome 4p16, encoding a predicted 890 amino acid transmembrane protein. Direct DNA sequencing was done to screen the entire coding region of the WFS1 gene in 30 patients from 19 British kindreds with Wolfram syndrome. DNA was also screened for structural rearrangements (deletions and duplications) and point mutations in mtDNA. No pathogenic mtDNA mutations were found in our cohort. We identified 24 mutations in the WFS1 gene: 8 nonsense mutations, 8 missense mutations, 3 in-frame deletions, 1 in-frame insertion, and 4 frameshift mutations. Of these, 23 were novel mutations, and most occurred in exon 8. The majority of patients were compound heterozygotes for two mutations, and there was no common founder mutation. The data were also analyzed for genotype-phenotype relationships. Although some interesting cases were noted, consideration of the small sample size and frequency of each mutation indicated no clear-cut correlations between any of the observed mutations and disease severity. There were no obvious mutation hot spots or clusters. Hence, molecular screening for Wolfram syndrome in affected families and for Wolfram syndrome-carrier status in subjects with psychiatric disorders or diabetes mellitus will require complete analysis of exon 8 and upstream exons.

Decrease of 3243 A→G mtDNA Mutation from Blood in MELAS Syndrome: A Longitudinal Study

It is widely held that changes in the distribution of mutant mtDNAs underlie the progressive nature of mtDNA diseases, but there are few data documenting such changes. We compared the levels of 3243 A-->G mutant mtDNA in blood at birth from Guthrie cards and at the time of diagnosis in a blood DNA sample from patients with mitochondrial encephalopathy, lactic acidosis, and strokelike episodes (MELAS) syndrome. Paired blood DNA samples separated by 9-19 years were obtained from six patients with MELAS. Quantification of mutant load, by means of a solid-phase minisequencing technique, demonstrated a decline (range 12%-29%) in the proportion of mutant mtDNA in all cases (P=.0015, paired t-test). These results suggest that mutant mtDNA is slowly selected from rapidly dividing blood cells in MELAS.

Duplications of mitochondrial DNA in mitochondrial myopathy

Restriction enzyme analysis was done on total cellular DNA extracted from whole blood in two patients with mitochondrial myopathy and multisystem involvement and their families. The two patients had an abnormal mitochondrial genome with a large (about 8 kb) duplication present in several tissues. Normal mitochondrial DNA (mtDNA) was also present, but within each maternal lineage the abnormal mitochondrial genome was confined to clinically affected individuals. This observation, together with the failure of extensive population surveys to identify such abnormalities of mtDNA, suggests that these mutations cause the disease.

Risk of developing a mitochondrial DNA deletion disorder

BACKGROUND Pathogenic mitochondrial DNA (mtDNA) mutations are found in at least one in 8000 individuals. No effective treatment for mtDNA disorders is available, making disease prevention important. Many patients with mtDNA disease harbour a single pathogenic mtDNA deletion, but the risk factors for new cases and disease recurrence are not known. METHODS We did a multicentre study of 226 families in which a single mtDNA deletion had been identified in the proband, including patients with chronic progressive external ophthalmoplegia, Kearns Sayre syndrome, or Pearsons syndrome. We studied the relation between maternal age and the risk of unaffected mothers having an affected child, and determined the recurrence risks among the siblings and offspring of affected individuals. FINDINGS We noted no relation between maternal age and the risk of unaffected mothers having children with an mtDNA deletion disorder. None of the 251 siblings of the index cases developed clinical features of mtDNA disease. Risk of recurrence among the offspring of affected women was 4.11% (95% CI 0.86-11.54, or one in 117 to one in nine births). Only one of the mothers who had an affected child had a duplication of mtDNA in skeletal muscle. INTERPRETATION Unlike nuclear chromosomal rearrangements, incidence of mtDNA deletion disorders does not increase with maternal age, and unaffected mothers are unlikely to have more than one affected child. Affected women were previously thought to have a negligible chance of having clinically affected offspring, but the actual risk is, on average, about one in 24 births.

A common mitochondrial DNA variant is associated with insulin resistance in adult life

Summary Mitochondrial DNA is maternally inherited. Mitochondrial DNA mutations could contribute to the excess of maternal over paternal inheritance of non-insulin-dependent diabetes mellitus (NIDDM). We therefore investigated the relationship between this variant, insulin resistance and other risk factors in a cohort which had been well characterised with respect to diabetes. Blood DNA was screened from 251 men born in Hertfordshire 1920–1930 in whom an earlier cohort study had shown that glucose tolerance was inversely related to birthweight. The 16 189 variant (T- > C transition) in the first hypervariable region of mitochondrial DNA was detected using the polymerase chain reaction and restriction digestion. DNA analysis showed that 28 of the 251 men (11 %) had the 16 189 variant. The prevalence of the 16 189 variant increased progressively with fasting insulin concentration (p < 0.01). The association was independent of age and body mass index and was present after exclusion of the patients with NIDDM or impaired glucose tolerance. We found that insulin resistance in adult life was associated with the 16 189 variant. This study provides the first evidence that a frequent mitochondrial variant may contribute to the phenotype in patients with a common multifactorial disorder. [Diabetologia (1998) 41: 54–58]

Is the Bottleneck Cracked

Since the development of molecular diagnosis of mtDNA disease, there has been increasing pressure on clinical geneticists for genetic counseling of this uniquely difficult group. However, these advances have revolutionized neither prognostication nor prenatal diagnosis of mitochondrial diseases. This inability to predict risk of affected offspring in mtDNA diseases is largely due to the uniparental inheritance of multiple copies of mtDNA. Recently, however, the prospects for prenatal diagnosis of mtDNA diseases have taken a turn for the better.

A common mitochondrial DNA variant associated with susceptibility to dilated cardiomyopathy in two different populations

Idiopathic dilated cardiomyopathy is a recognised manifestation of mitochondrial disease due to specific mitochondrial (mt) DNA mutations. However, whether mtDNA polymorphisms predispose to sporadic dilated cardiomyopathy is not known. We analysed two populations with this disorder for a general mtDNA variant (T16189C), previously implicated in susceptibility to type 2 diabetes. We noted an increased frequency of the polymorphism in both populations compared with controls (p=0.002). The polymorphism occurred on different mtDNA backgrounds, suggesting that it might be a functional variant. This association of an mtDNA variant with increased susceptibility to dilated cardiomyopathy provides evidence for a mitochondrial cause in sporadic disease.