In a 42-year-old man (S1) with autosomal recessive progressive external ophthalmoplegia with mitochondrial DNA deletions-2 (PEOB2; 616479), Reyes et al. (2015) identified compound heterozygous mutations in the RNASEH1 gene: a c.424G-A transition (c.424G-A, NM_002936.4) in exon 4, resulting in a val142-to-ile (V142I) substitution at a highly conserved residue in the catalytic domain, and a c.469C-T transition in exon 4, resulting in an arg157-to-ter (R157X; 604123.0002) substitution in the catalytic domain. The mutations, which were found by whole-exome sequencing, segregated with the disorder in the family and were extremely rare (frequency of less than 0.01%) in the ExAC database. Screening of the RNASEH1 gene in a cohort of 40 genetically undefined patients with a similar disorder identified another unrelated man (S2) who was compound heterozygous for the V142I mutation and a c.554C-T transition in exon 5, resulting in an ala185-to-val (A185V; 604123.0003) substitution at a highly conserved residue in the catalytic domain. A third proband (S3) was homozygous for the V142I mutation. In vitro functional expression assays in E. coli showed that the V142I mutant had about 40% residual activity, the A185V mutant had about 20% residual activity, and the R157X mutant had negligible residual activity, consistent with a loss of function. Skin fibroblasts from patient S1 showed decreased RNASEH1 mRNA transcripts, suggesting nonsense-mediated mRNA decay of the truncating mutation, and virtual absence of the RNASEH1 protein on Western blot analysis, suggesting instability of both variants. Patient cells grew more slowly on galactose medium compared to controls, and had decreased mitochondrial membrane potential as well as abnormal perinuclear aggregation of fragmented mitochondria, suggesting mitochondrial dysfunction. Analysis of mtDNA replication intermediates was consistent with a defect in the removal of RNA, resulting in slower replication.
