2 LARGE SPANISH PEDIGREES WITH NONSYNDROMIC SENSORINEURAL DEAFNESS AND THE MTDNA MUTATION AT NT-1555 IN THE 12S RIBOSOMAL-RNA GENE - EVIDENCE OF HETEROPLASMY

We describe two unrelated Spanish families with isolated sensorineural hearing loss. In both pedigrees, the deafness was transmitted materna lly, which suggested a mitochondrial DNA (mtDNA) defect. Within the sa me pedigree, some relatives showed aminoglycoside-induced deafness, wh ereas others were not exposed to aminoglycosides before the onset of h earing loss. Molecular genetic analysis in both families showed the A- to-G transition at nt 1555 (A1555G) in the mitochondrial 12S rRNA gene . In one pedigree, the mutation was homoplasmic; in the other, it was heteroplasmic. To assess the frequency of this mutation, we screened 4 2 patients of various ethnic backgrounds with isolated sensorineural h ealing loss; none harbored the A1555G mutation. This is the first repo rt of heteroplasmy in a family with isolated sensorineural deafness as sociated with the A1555G mutation.