Association of deletion and homoplasmic point mutation of the mitochondrial DNA in an ocular myopathy

Homoplasmy of the G7444A mtDNA and heterozygosity of the GJB2 c.35delG mutations in a family with hearing loss

OBJECTIVE

Mitochondrial mutations have been shown to be responsible for syndromic as well as non-syndromic hearing loss. The G7444A mitochondrial DNA mutation affects COI/the precursor of tRNA(Ser(UCN)), encoding the first subunit of cytochrome oxidase. Here we report on the first Greek family with the G7444A mitochondrial DNA mutation.

METHODS

Clinical, cytogenetic, and molecular methods were employed in this study.

RESULTS

We describe the high variability of phenotypes among three family members harboring the G7444A mutation and also the frequent GJB2 c.35delG mutation of the nuclear genome in heterozygosity. Their phenotypes ranged from normal hearing to deafness, while the proband presented with several other symptoms.

CONCLUSIONS

The G7444A mitochondrial DNA mutation has been reported in only a few cases worldwide, alone or in cosegregation with other mitochondrial DNA mutations, but to our knowledge, never before in coexistence with the GJB2 c.35delG mutation.

Long PCR analysis of human gamete mtDNA suggests defective mitochondrial maintenance in spermatozoa and supports the bottleneck theory for oocytes

The long PCR and the Southern blot techniques were used to study mitochondrial DNA (mtDNA) in 94 sperm samples, and in 35 oocytes collected from 12 women. The sperm samples were classified in two sets: 37 samples from normal subjects, and 57 samples from patients with oligoasthenospermia. In both sets, most of the spermatozoan mitochondria had multiple mtDNA deletions. The rate of mtDNA mutation, which appears unexpectedly high, considering the short life span of the spermatozoa, may be due to impaired maintenance during differentiation. In contrast, despite the long life span of oocytes and the extended meiotic period, oocyte mitochondria showed few mtDNA rearrangements. However, mitochondria in oocytes from a given donor revealed considerable mutational heterogeneity. This supports the bottleneck theory of rapid segregation of mtDNA genotypes during early oogenesis. The long PCR technique, which allows analysis of the entire mitochondrial genome, provides new information on mtDNA instability in human gametes. Our findings suggest that mtDNA maintenance differs in the two types of gametes.

Recent developments in the molecular genetics of mitochondrial disorders

Rapid progress has been made in the identification of mitochondrial DNA mutations which are typically associated with diseases of the nervous system and muscle. The well established mitochondrial disorders are maternally inherited and males and females are equally affected. An exception is Leber's hereditary optic atrophy (LHON) which is observed much more frequently in males than in females. There are three common point mutations in LHON which can be homoplasmic or heteroplasmic. In mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS) most mutations are single base changes and lie within the tRNA-Leu gene. Point mutations in myoclonic epilepsy with ragged red fibres (MERRF) usually occur within the tRNA-Lys gene but mutations of the tRNA-Leu gene are also observed. MELAS and MERRF mutations are heteroplasmic and there is considerable clinical overlap between these diseases. Point mutations within the ATPase6 gene result in either neuropathy, ataxia and retinitis pigmentosa (NARP) or in Leigh's syndrome. The latter occurs if the mutation is present in the majority of mitochondria (extreme heteroplasmy). Finally, mitochondrial DNA deletions are the cause underlying Kearns-Sayre syndrome (KSS). Apart from the well-established mitochondrial diseases, there is increasing evidence that mitochondrial mutations may also play a role in the neurodegenerative disorders Parkinson, Alzheimer and Huntington disease. The complex I defect found in Parkinson disease is especially interesting in this respect. However, no causative mitochondrial mutation has as yet been established in any of these three common disorders.

Exhaustive scanning approach to screen all the mitochondrial tRNA genes for mutations and its application to the investigation of 35 independent patients with mitochondrial disorders

To gain a better understanding of the molecular basisof mitochondrial (mt) encephalomyopathies, a highly heterogeneous condition, we developed a denaturing gradient gel electrophoresis-based approach that allows rapid and exhaustive screening for mutations of all 22 mt tRNA-encoding genes and their flanking regions in large cohorts of patients. This method, that detects heteroplasmy (i.e. co-existence of mutant and wild-type mtDNA species in various ratios) directly, was applied to the investigation of 35 independent patients with a disease phenotype compatible with a mitochondrial encephalomyopathy. Twenty-five of the 35 patients investigated displayed a sequence variation in at least one tRNA gene. A total of 46 different sequence variations (41 point mutations, four short insertions and one short deletion), among which 20 are new, were characterized. Forty of them were present in a homoplasmic state, whereas six were heteroplasmic. Twenty-two were located in tRNA genes, among which 10 are new homoplasmic or heteroplasmic sequence variations; 24 were located in flanking regions (12 in mRNA-encoding genes, seven of them leading to missense sequence variations; two in rRNA genes; and 10 in non-coding regions). This study demonstrates (i) the high frequency of homoplasmic tRNA gene sequence variations in our patient sample, and (ii) the existence of several polymorphic sites in tRNA gene regions that may be helpful for defining haplogroups in different populations. It relies on a screening method that can now be applied easily to other population samples.

MELAS- and Kearns-Sayre-type co-mutation [corrected] with myopathy and autoimmune polyendocrinopathy

A 35-year-old woman with features of Kearns-Sayre syndrome consisting of progressive ptosis, ophthalmoparesis, mitochondrial myopathy, and pigmentary retinopathy also had autoimmune polyglandular syndrome type 11 (Addison's disease, autoimmune insulin-dependent diabetes mellitus, Hashimoto's thyroiditis, and primary ovarian failure). There was no history of similarly affected relatives. Analysis of muscle mitochondrial DNA (mtDNA) revealed a 2,532-bp deletion of the type seen in Kearns-Sayre syndrome as well as a heteroplasmic A3243G mutation in the tRNA-Leu(UUR) gene of the type seen in mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes (MELAS). The patient's blood and her mother's blood harbored the A3243G mutation but not the deletion, and the maternal grandmother's blood had neither mutation. In muscle, the species of mtDNA harboring the deletion was exclusively associated with the species harboring the A3243G mutation, suggesting that the point mutation predisposed to the large-scale deletion. The mtDNA species with both mutations accounted for 88% of total muscle mtDNA. Other and as yet unrecognized point mutations in mtDNA might also be associated with, and possible causally related to, large-scale mtDNA deletions.

[Chronic progressive external ophthalmoplegia with mitochondrial anomalies. Clinical, histological, biochemical and genetic analysis (9 cases)]

We report the clinical signs and histological findings in nine patients with mitochondrial ocular myopathies. There were four males and five females. Of age ranging from 47 to 82 years. A more often asymetrical ptosis was in all cases of chronic progressive external ophtalmoplegia (CPEO), but muscle weakness in limbs was not usual. The prognosis in this group was good, but ubidecarenone (150 mg/d) used for two cases, did not improve ophtalmoplegia. The serum creatine kinase was normal in eight of nine cases and electromyography showed myopathic changes in three cases. Histoenzymatic analysis of the muscle biopsy and biochemical studies of mitochondria isolated from the muscle sample demonstrated mitochondrial myopathy associated with partial deficiency of complexes I and/or IV of the electron transfer chain. One of seven patients studied had single deletion by Southern blot analysis, in a heteroplasmic state and another an A-->G transition at position 3243 within the mitochondrial tRNA leu (UUR) gene. Chronic progressive external ophtalmoplegia, without large deletion, may have abnormality in other coding regions of mt DNA such as tRNA, rRNA or protein genes.