Modifier locus for mitochondrial DNA disease: linkage and linkage disequilibrium mapping of a nuclear modifier gene for maternally inherited deafness.

PURPOSE: To examine the role of the nuclear genome in affecting the phenotypic expression of the simplest model of a mitochondrial DNA disease, maternally transmitted deafness. METHODS: Linkage analysis in families with maternally inherited deafness associated with the homoplasmic A1555G mutation. RESULTS: Significant linkage and linkage disequilibrium on chromosome 8 was identified. CONCLUSIONS: This finding represents the first identification of a modifier locus for a human mitochondrial DNA disease and supports the concept of mitochondrial DNA diseases having complex genetic inheritance. The eventual identification of this modifier gene will provide insights into the pathophysiological pathways determining the clinical expression of mitochondrial DNA diseases, an important step toward diagnostic and therapeutic interventions.

Candidate locus for a nuclear modifier gene for maternally inherited deafness.

Maternally inherited deafness associated with the A1555G mutation in the mitochondrial 12S ribosomal RNA (rRNA) gene appears to require additional environmental or genetic changes for phenotypic expression. Aminoglycosides have been identified as one such environmental factor. In one large Arab-Israeli pedigree with congenital hearing loss in some of the family members with the A1555G mutation and with no exposure to aminoglycosides, biochemical evidence has suggested the role of nuclear modifier gene(s), but a genomewide search has indicated the absence of a single major locus having such an effect. Thus it has been concluded that the penetrance of the mitochondrial mutation appears to depend on additive effects of several nuclear genes. We have now investigated 10 multiplex Spanish and Italian families with 35 members with the A1555G mutation and sensorineural deafness. Parametric analysis of a genomewide screen again failed to identify significant evidence for linkage to a single autosomal locus. However, nonparametric analysis supported the role of the chromosomal region around marker D8S277. The combined maximized allele-sharing LOD score of 3.1 in Arab-Israeli/Spanish/Italian families represents a highly suggestive linkage result. We suggest that this region should be considered a candidate for containing the first human nuclear modifier gene for a mitochondrial DNA disorder. The locus operates in Arab-Israeli, Spanish, and Italian families, resulting in the deafness phenotype on a background of the mitochondrial A1555G mutation. No obvious candidate genes are located in this region.

Inherited susceptibility to aminoglycoside ototoxicity: genetic heterogeneity and clinical implications.

PURPOSE: Aminoglycoside-induced ototoxicity appears to have a genetic susceptibility in some individuals, and the A1555G mutation in the mitochondrial 12S ribosomal RNA gene has been shown to be responsible for this susceptibility in all familial cases. An Italian family with 5 family members who became deaf after aminoglycoside exposure presented to us, and molecular analysis excluded the A1555G mutation. The purpose of this study is to identify the molecular basis for the aminoglycoside susceptibility in this family. PATIENTS AND METHODS: Two sisters and three of their children developed severe to profound high-frequency hearing loss after aminoglycoside exposure. DNA was extracted from the blood of these individuals and their unaffected relatives, and analyzed for mitochondrial DNA mutations. The region around nucleotide 961 was also cloned and individual clones were sequenced. RESULTS: Sequencing of the 12S ribosomal RNA gene revealed a thymidine deletion at position 961, with a complex pattern of sequence around this mutation. Sequencing of individual clones around the 961 mutation demonstrated a varying number of inserted cytosines in different mitochondrial molecules. CONCLUSION: This family establishes the nucleotide 961 thymidine deletion associated with a varying number of inserted cytosines in the mitochondrial 12S ribosomal RNA gene as the second pathogenic mutation that can predispose to aminoglycoside ototoxicity. It demonstrates the clinical relevance of taking a family history before administering aminoglycosides to any patient. In addition, it would be desirable for sporadic patients with aminoglycoside-induced hearing loss to be screened with molecular tests for the presence of the 1555 and 961 mutations. Such screening could significantly decrease the prevalence of aminoglycoside-induced hearing loss.

Hearing loss due to the mitochondrial A1555G mutation in Italian families.

Six Italian families with familial nonsyndromic hearing loss consistent with a maternal pattern of inheritance were analyzed for mitochondrial mutations. The three known mitochondrial mutations associated with nonsyndromic hearing loss were investigated by polymerase chain reaction amplification, followed by restriction fragment length analysis or DNA sequencing. The A7445G mutation and C7472 insertion were not present in either of the families, but the A1555G mutation in the 12S rRNA gene was identified in homoplasmic form in two of the families. In one of the families the onset of hearing loss is congenital, while in the other it starts later in life. The families are from different regions of Italy, and mitochondrial haplotype analysis showed that the mutation arose independently in these two families. This suggests that the A1555G mutation may not be an uncommon cause of hearing loss in Italians, and is clinically important because maternal hearing relatives of patients with the A1555G mutation are at risk for aminoglycoside induced deafness. We discuss potential reasons for the normal phenotype in some relatives with the mutation, and the different onset of hearing loss in the two families.