# Characterization of Novel POLG Mutations in Mitochondrial Encephalomyopathy: Pathogenic Validation and Comprehensive Genetic Profiling

## TL;DR

A new POLG mutation causing mitochondrial dysfunction is identified in a patient with SANDO syndrome, expanding the genetic understanding of this rare disorder.

## Contribution

A novel pathogenic POLG variant (p.W1099C) is identified and validated for causing mitochondrial dysfunction in SANDO syndrome.

## Key findings

- The novel POLG mutation p.W1099C and known mutation p.L592F both cause elevated ROS and reduced mtDNA copy number in neurons.
- Over 30 POLG mutations are associated with SANDO syndrome, with p.A467T and p.W748S being the most frequent.
- The mean age of onset for SANDO syndrome is 31.6 years.

## Abstract

Mitochondrial encephalomyopathies are multisystem disorders caused by defects in mitochondrial DNA (mtDNA) or nuclear DNA (nDNA). Sensory ataxic neuropathy, dysarthria, and ophthalmoparesis (SANDO) syndrome is a rare manifestation, often associated with POLG mutations. This study identifies a novel POLG mutation in a SANDO patient, validates its pathogenicity, and analyzes the molecular genetics of 61 reported POLG‐SANDO cases.

After obtaining informed consent, the proband underwent neurological examination, electromyography, muscle/nerve biopsies (histochemical/ultrastructural analyses), and genetic testing (whole‐exome sequencing, mtDNA analysis). Pathogenicity of identified POLG variants was assessed in Cas9‐mediated primary neuronal models expressing mutant proteins by measuring reactive oxygen species (ROS) levels and mtDNA copy number (qRT‐PCR, ND1/APP ratio). Literature searches (PubMed, CNKI, Wanfang, and ClinVar) identified reported POLG mutations and clinical features in SANDO.

Clinical and biopsy findings confirmed SANDO syndrome. Genetic analysis revealed compound heterozygous POLG mutations: a novel c.3297G>C (p.W1099C) and a known c.1774C>T (p.L592F). Neurons expressing either mutant exhibited elevated ROS levels (p < 0.05) and reduced mtDNA copy number compared with controls. Literature synthesis identified over 30 SANDO‐associated POLG mutations, with p.A467T (31.2%) and p.W748S (22.1%) being the most frequent. The mean age of onset was 31.6 years.

We identify a novel pathogenic POLG variant (p.W1099C) causing mitochondrial dysfunction via impaired mtDNA maintenance, expanding the SANDO genetic spectrum. Functional studies confirmed both mutations induce mitochondrial dysfunction (elevated ROS and decreased mtDNA Copy Number), validating their pathogenicity. The compiled mutation profile aids diagnosis of this phenotypically heterogeneous, frequently misdiagnosed disorder.

We report a 32‐year‐old female patient with SANDO syndrome (a subtype of mitochondrial encephalomyopathy), who carries two heterozygous POLG mutations: a novel c.3297G>C and a known c.1774C>T. The neuronal models expressing these mutations demonstrated mitochondrial dysfunction, thereby confirming their pathogenicity. Additionally, a summary of POLG mutations related to SANDO is provided to facilitate clinical diagnosis.