# Yeast model analysis of novel polymerase gamma variants found in patients with autosomal recessive mitochondrial disease

## TL;DR

Researchers used a yeast model to study new variants of a gene linked to mitochondrial disease, finding that some variants cause severe DNA replication issues.

## Contribution

The study identifies five novel POLG variants and demonstrates their pathogenicity using a yeast model system.

## Key findings

- Three yeast POLG variants caused complete loss of functional mtDNA, showing strong, partially dominant effects.
- Two other variants led to partial mtDNA depletion and increased point mutation frequencies.
- The results correlate with patient symptom severity, confirming these variants as pathogenic.

## Abstract

Replication of the mitochondrial genome depends on the single DNA polymerase (pol gamma). Mutations in the POLG gene, encoding the catalytic subunit of the human polymerase gamma, have been linked to a wide variety of mitochondrial disorders that show remarkable heterogeneity, with more than 200 sequence variants, often very rare, found in patients. The pathogenicity and dominance status of many such mutations remain, however, unclear. Remarkable structural and functional conservation of human POLG and its S. cerevisiae ortholog (Mip1p) led to the development of many successful yeast models, enabling to study the phenotype of putative pathogenic mutations. In a group of patients with suspicion of mitochondrial pathology, we identified five novel POLG sequence variants, four of which (p.Arg869Ter, p.Gln968Glu, p.Thr1053Argfs*6, and p.Val1106Ala), together with one previously known but uncharacterised variant (p.Arg309Cys), were amenable to modelling in yeast. Familial analysis indicated causal relationship of these variants with disease, consistent with autosomal recessive inheritance. To investigate the effect of these sequence changes on mtDNA replication, we obtained the corresponding yeast mip1 alleles (Arg265Cys, Arg672Ter, Arg770Glu, Thr809Ter, and Val863Ala, respectively) and tested their effect on mitochondrial genome stability and replication fidelity. For three of them (Arg265Cys, Arg672Ter, and Thr809Ter), we observed a strong, partially dominant phenotype of a complete loss of functional mtDNA, whereas the remaining two led to partial mtDNA depletion and significant increase in point mutation frequencies. These results show good correlation with the severity of symptoms observed in patients and allow to establish these variants as pathogenic mutations.

The online version of this article (doi:10.1007/s00439-015-1578-x) contains supplementary material, which is available to authorized users.