Deletion screening of mitochondrial DNA via multiprimer DNA amplification

De novo double mutation in PAX6 and mtDNA tRNA Lys associated with atypical aniridia and mitochondrial disease

We report on the clinical, molecular and biochemical findings of a patient with the rare event (<4.02 x 10(-9) per generation) of coinciding de novo mutations in the nuclear PAX6 (c.1252-1267del16) and the mitochondrial mt.RNA (Lys) (8347A-->G) genes. The boy suffers from exercise intolerance, ptosis, nystagmus, macular hypoplasia and anterior segment abnormalities evocative of Axenfeld-Rieger anomaly. The PAX6 mutation is predicted to cause haploinsufficiency. The novel mt.RNA (Lys) mutation is located close to the classic myoclonic epilepsy with ragged-red-fibers mutation, but the patient exhibits neither myoclonic epilepsy nor ragged-red-fibers. The degree of mutant mtDNA heteroplasmy, as determined by a very accurate pyrosequencing assay, varies between 31% (muscle) and 38% (fibroblasts). We discuss a potential effect of the PAX6 mutation on the mtDNA mutation rate.

Pyruvate dehydrogenase complex deficiency and altered respiratory chain function in a patient with Kearns-Sayre/MELAS overlap syndrome and A3243G mtDNA mutation

Combined alteration of the pyruvate dehydrogenase complex and respiratory chain function is described in a 21 year-old male patient with overlapping MELAS (mitochondrial encephalomyopathy, lactic acidosis, and 'stroke-like' episodes) and Kearns-Sayre syndrome. Progressive external ophthalmoplegia, pigmentary retinopathy and right bundle branch block were present when he experienced the first 'stroke-like' episode at 18 years old. The A>G tRNALeu(UUR) point mutation at nucleotide 3243 of the mitochondrial DNA was predominant in muscle tissue (79%) and present, but at lower levels in fibroblasts (49%) and blood cells (37%). Biochemical analysis revealed diminished activities of pyruvate dehydrogenase (23%) and respiratory chain complexes I and IV (57%, respectively) in muscle, but normal activities in the fibroblasts. Immunochemical studies of the muscular pyruvate dehydrogenase components showed normal content of E1alpha, E1beta and E2 protein. Molecular screening of the E1alpha gene did not indicate a nuclear mutation. These observations suggest that mitochondrial DNA defects may be associated with altered nuclear encoded enzymes which are actively imported into mitochondria and constitute components of the mitochondrial matrix. Biochemical workup of mitochondrial disorders should not be restricted to the respiratory chain even if mitochondrial DNA mutations are present.

S1 nuclease hybrid analysis of mitochondrial DNA amplified by long-distance PCR: rapid screening for small-scale rearrangements

We report on a method suitable for screening large regions (>3 kb) of mtDNA for structural changes of <500 bp and their localization. Heteroduplexes consisting of a wild-type and a mutant strand are cleaved by S1nuclease when single-stranded loops are present due to deletions or duplications/insertions. This strategy was successfully applied to screen the muscle mtDNA of 20 patients with mitochondrial encephalomyopathies. In three of them, an altered cleavage pattern was observed caused by a homoplasmic 9 bp deletion as shown by subsequent mapping and sequencing studies.

Hypoparathyroidism and deafness associated with pleioplasmic large scale rearrangements of the mitochondrial DNA: a clinical and molecular genetic study of four children with Kearns-Sayre syndrome

In four children with hypoparathyroidism and deafness as initial major manifestations of Kearns-Sayre syndrome, a unique pattern of mitochondrial DNA rearrangements was observed. Hypocalcemic tetany caused by PTH deficiency started between age of 6-13 y and was well controlled by small amounts of 1.25-(OH)2-cholecalciferol. Rearranged mitochondrial genomes were present in blood cells of all patients and consisted of partially duplicated and deleted molecules, created by the loss of 7813, 8348, 8587, and 9485 bp, respectively. The deletions were localized between the origins of replication of heavy and light strands and encompassed at least eight polypeptide-encoding genes and six tRNA genes. Sequence analysis revealed imperfect direct repeats present in all rearrangements flanking the break-points. The duplicated population accounted for 25-53% of the mitochondrial genome and was predominant to the deleted DNA (5-30%) in all cases. The proportions of the mutant populations (30-75%) correlated with the age at onset of the disease. We conclude that, unlike heteroplasmic deletions, pleioplasmic rearrangements may escape selection in rapid-dividing cells, distribute widely over many tissues, and thus cause multisystem involvement. Hypoparathyroidism and deafness might be the result of altered signaling pathway caused by selective ATP deficiency.

Cerebral adrenoleukodystrophy (ALD) in only one of monozygotic twins with an identical ALD genotype

We report on monozygotic twins with different clinical phenotypes of X-linked adrenoleukodystrophy. At the age of 10 years both boys were neurologically asymptomatic. The first cranial magnetic resonance examination showed normal findings in the first twin and parietooccipital demyelination in the second. The latter developed behavioral problems 9 months later, followed by visual impairment and gait ataxia. His cranial magnetic resonance image at the age of 11 years showed progressive demyelination. In contrast, neurological status and magnetic resonance images remained normal in the first twin. The same point mutation in exon 8 of the adrenoleukodystrophy gene (C2203T) was detected in both boys. All genotype examinations were consistent with the diagnosis of monozygotic twins, suggesting that some nongenetic factors may be important for different adrenoleukodystrophy phenotypes.