**Case Report: Identification of a Novel Variant (m.8909T**>**C) of Human Mitochondrial** *ATP6* **Gene and Its Functional Consequences on Yeast ATP Synthase**

**Qiuju Ding 1,**† **, Ró˙za Kucharczyk 2,**† **, Weiwei Zhao 1,**† **, Alain Dautant 3 , Shutian Xu 1 , Katarzyna Niedzwiecka 2 , Xin Su 3 , Marie-France Giraud 3 , Kewin Gombeau 3 , Mingchao Zhang 1 , Honglang Xie 1 , Caihong Zeng 1 , Marine Bouhier 3 , Jean-Paul di Rago 3 , Zhihong Liu 1 , Déborah Tribouillard-Tanvier 3,4, \* and Huimei Chen 1, \***


Received: 31 July 2020; Accepted: 17 September 2020; Published: 22 September 2020

**Abstract:** With the advent of next generation sequencing, the list of mitochondrial DNA (mtDNA) mutations identified in patients rapidly and continuously expands. They are frequently found in a limited number of cases, sometimes a single individual (as with the case herein reported) and in heterogeneous genetic backgrounds (heteroplasmy), which makes it difficult to conclude about their pathogenicity and functional consequences. As an organism amenable to mitochondrial DNA manipulation, able to survive by fermentation to loss-of-function mtDNA mutations, and where heteroplasmy is unstable, *Saccharomyces cerevisiae* is an excellent model for investigating novel human mtDNA variants, in isolation and in a controlled genetic context. We herein report the identification of a novel variant in mitochondrial *ATP6* gene, m.8909T>C. It was found in combination with the well-known pathogenic m.3243A>G mutation in mt-tRNALeu . We show that an equivalent of the m.8909T>C mutation compromises yeast adenosine tri-phosphate (ATP) synthase assembly/stability and reduces the rate of mitochondrial ATP synthesis by 20–30% compared to wild type yeast. Other previously reported *ATP6* mutations with a well-established pathogenicity (like m.8993T>C and m.9176T>C) were shown to have similar effects on yeast ATP synthase. It can be inferred that alone the m.8909T>C variant has the potential to compromise human health.

**Keywords:** *MT-ATP6*; m.8909T>C; ATP synthase; nephropathy; oxidative phosphorylation; mitochondrial disease
