Molecular and functional effects of the T14709C point mutation in the mitochondrial DNA of a patient with maternally inherited diabetes and deafness.

A heteroplasmic T to C transition at nucleotide position 14709 in the mitochondrial tRNA glutamic acid (tRNA(Glu)) gene has previously been associated with maternally inherited diabetes and deafness (MIDD). To investigate the pathogenic mechanism of the T14709C mutation, we have constructed transmitochondrial cell lines by transferring fibroblasts mitochondria from a patient with the mutation into human cells lacking mitochondrial DNA (mtDNA) (rho degrees cells). Clonal cybrid cell lines were obtained containing various levels of the heteroplasmic mutation, or exclusively mutated or wild-type mtDNA. Measurement of respiratory chain enzymatic activities failed to detect a difference between the homoplasmic mutant and homoplasmic wild-type cybrid cell lines. However, a subtle decrease in the steady-state levels of tRNA(Glu) transcripts in some mutant clones. Our studies suggest that the T14709C mutation is insufficient to lead impairment of mitochondrial function in homoplasmic osteosarcoma cybrid clones, and that we cannot exclude that the T14709C mutation affects mitochondrial function by a yet unidentified mechanism.

Mitochondrial DNA variations in patients with maternally inherited diabetes and deafness syndrome.

Mitochondrial DNA (mtDNA) variants have been implicated in the pathogenesis of diabetes. A mutation in the tRNA leucine gene at position 3243 has been previously reported in mtDNA of maternally inherited diabetes and deafness (MIDD) patients. Because the true prevalence of the mitochondrial origin in diabetes may be underestimated, we searched for potentially diabetogenic anomalies of mtDNA in 9 patients highly suspected of mitochondrial diabetes selected on maternally inheritance and clinical features. In order to detect high levels of mutant DNA, the mtDNA of muscle sample of 2 patients was totally sequenced and the 22 tRNA genes and flanking sequences of 7 patients were analyzed. A new homoplasmic mutation at position 8381 was found in the ATPase 8 gene of mtDNA of a MIDD patient. The prevalence of three homoplasmic variations (G1888A, T4216G, A4917G) was significantly higher in the small group of MIDD patients compared to controls and other subjects groups. This study demonstrated in our patients sample the high frequency of homoplasmic variations, which could play a role by themselves or in combination, in the pathogenesis of diabetes.

Importance of searching for associated mitochondrial DNA alterations in patients with multiple deletions.

Multiple mitochondrial DNA (mtDNA) deletions have been reported in patients with autosomal dominant and recessive disorders. We studied several affected and one non-affected individuals belonging to a pedigree in which the inheritance of the pathological trait was compatible with an autosomique dominant transmission. Affected members had late-onset multisystem disorders with multiple mtDNA deletions in skeletal muscle. But this family presented a striking difference from previously described cases, because none of the patients had progressive external ophthalmoplegia (PEO). We also studied one young boy with a no contributary family history. He had a cerebellar ataxia with PEO and multiple mtDNA deletions in muscle. Molecular analysis revealed that in the first family, repeated sequences were present at the breakpoint junctions, whereas such motifs were not found in the young patient's case. In the first family, we evidenced mtDNA point mutations in clones containing breakpoint junctions and a 9-bp motif triplication in the intergenic COII/tRNA(Lys) region, whereas this sequence is repeated twice in the wild type mtDNA. Our results suggest that multiple deletions observed in the two pedigrees result from different molecular mechanisms and point out the role of repeated sequences in the first pedigree. No mtDNA repair system has been described in mammals so far, but the molecular abnormalities found in the first family suggest that a defect in an mtDNA repair system, homologous to the E. coli MutHLS pathway, could be responsible for such a phenotype.

Cloning and expression analysis of a meiosis-specific MutS homolog: the human MSH4 gene.

The Escherichia coli MutHLS system has been highly conserved throughout evolution. The eukaryotic pathway results in a specialization of MutS homologs that have evolved to play crucial roles in both DNA mismatch repair and meiotic recombination. So far, meiosis-specific genes belonging to this family have only been identified in yeast. In Saccharomyces cerevisiae, MSH4 (MutS homolog 4) is a meiosis-specific protein that is not involved in mismatch correction. This protein is required for reciprocal recombination and proper segregation of homologous chromosomes at meiosis I. We have identified the human MSH4 homolog gene. The predicted amino acid sequence shows 28.7% identity with the S. cerevisiae MSH4 protein. By Northern blot analysis, human MSH4 transcripts are only detectable in testis and in ovary with a lower level of expression. We have mapped MSH4 to human chromosome 1 at band p31 by fluorescence in situ hybridization. The identification of such a gene provides a powerful tool for clarifying the respective roles of MSH and MLH genes in mammalian meiosis.

Early-onset fatal encephalomyopathy associated with severe mtDNA depletion.

UNLABELLED: We studied a 3-month-old girl who was admitted to hospital because of respiratory distress. The clinical course was characterized by a rapidly progressive generalized hypotonia with lactic acidosis and she died at 4 months of age. A muscle biopsy showed few ragged-red fibres and lack of histochemical cytochrome c oxidase reaction in all fibres. Enzyme activities of the respiratory chain complexes containing subunits encoded by the mitochondrial DNA (mtDNA) were markedly decreased. A quantitative Southern blot analysis revealed 99% depletion of mtDNA in muscle and normal amounts in blood. There was no family history and the dizygotic twin sister of the patient was no symptomatic. CONCLUSION: This new case confirms the rapidly fatal evolution associated with severe depletion of muscle mtDNA.

Branchial dysplasia, mental deficiency, club feet, and inguinal herniae: a report of two further cases associated with paucity of interlobular bile ducts.

The observation of two new cases in a previously reported family has brought about a change in the delineation of the syndrome initially defined. To the abnormalities already described (branchial dysplasia, mental deficiency, club feet, inguinal herniae) must be added a paucity of interlobular bile duts; the relationship between this new syndrome and the Alagille syndrome requires reconsideration.

[Biochemical and ultrastructural study of two familial cases of Winchester syndrome]. / Etude biochimique et ultrastructurale de deux cas familiaux de syndrome de Winchester.

Two new familial cases of Winchester syndrome with the characteristic features allowed to be more explicit on a few data of this syndrome. As reported in a previous paper an abnormal oligosaccharide was detected in urine of patients but the pathological significance of this oligosaccharide must be discussed and its finding in patients with Winchester syndrome does not lead to further elucidation of the aetiology of this condition. Cultured fibroblasts were obtained from a skin biopsy performed in thickened area. These cells had a normal level of the hydrolases studied whereas they showed ultrastructural abnormalities with numerous secondary lysosomes and pseudomyelinic figures.

[Expressivity of familial forms of Fraser syndrome]. / Expressivité des formes familiales du syndrome de Fraser.

Two new cases of the Fraser syndrome are presented. The literature review indicates that the clinical expression inside families is very constant with regard to cryptophthalmos, syndactyly or internal malformations. Particularly the fatal issue of severe forms is regularly constant in each of the families recently reported with a detailed autopsy. These conclusion would change the genetic counseling. A lethal form of the disease would recur in the same manner and could be detected by exhibiting the genito-urinary malformations at ultrasonography or even the syndactyly at foetoscopy.