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Background  Mutations in the two  MT-RNR  genes in mitochondrial DNA can cause hearing impairment that presents with variable severity and age of onset. In order to study the prevalence of mutations in  MT-RNR1  and  MT-RNR2  genes among Finnish children, we studied a ten-year cohort of hearing impaired children born in Northern Finland.    Methods  We studied children, who had been born in Northern Finland in 1993–2002 and who had been ascertained to have hearing impairment by 31 December 2007. Samples from 103 children were sequenced in order to find mutations in the  MT-RNR1  and  MT-RNR2  genes.    Results  One child harboured the pathogenic m.1555A > G mutation in  MT-RNR1  suggesting a frequency of 4.4/100,000 in the Finnish paediatric population. In addition, eight rare variants and 13 polymorphisms were found in  MT-RNR1  and  MT-RNR2  genes. Five of the rare variants were deemed to be haplogroup-specific polymorphisms rather than putative pathogenic mutations, while the remaining three variants have been reported in various haplogroups. Among them m.990 T > C occurs at a conserved site.    Conclusions  The presence of m.990 T > C variant in various haplogroups and the rather high degree of conservation at this site suggest that this transition is a pathogenic rather than homoplasic neutral variant. Identification of further patients with m.990 T > C and segregation analysis in their families should help in determining the pathogenic potential of this variant.

Mutations in the two ribosomal RNA genes in mitochondrial DNA among Finnish children with hearing impairment