POLG R964C and GBA L444P mutations in familial Parkinson's disease: Case report and literature review

Abstract Polymerase gamma (POLG) is an enzyme responsible for the replication and repair of mitochondrial DNA. Mutations in POLG may cause variable clinical manifestations, including parkinsonism, epilepsy, cerebellar ataxia, neuropathy, and progressive external ophthalmoplegia. However, mutations of this gene are rare in patients with typical Parkinson's disease (PD). We report a man (current age: 59 years) without any underlying disease presenting with right‐hand tremor at the age of 39 years, followed by slow movement, rigidity, and postural instability. He developed motor fluctuation and levodopa‐induced dyskinesia 8 years later. At the age of 58 years, cognitive decline and visual hallucination ensued; he was institutionalized thereafter. We used multiplex ligation‐dependent probe amplification, which demonstrated no large deletions or duplications of relevant PD genes. Next, targeted sequencing panel covering 51 genes causative for PD was applied for the proband; it revealed a heterozygous missense substitution R964C in POLG and a heterozygous missense substitution L444P in GBA. The patient's father, who had been diagnosed as having PD and type 2 diabetes mellitus at the age of 70 years, demonstrated identical mutations. This is the first report of familial PD combined with POLG R964C and GBA L444P mutations. Two pathogenic gene mutations potentially cause double hit in pathological neurodegeneration. This finding extends our understanding of the PD genotype–phenotype correlation.


| C A S E PRE S ENTATI ON
A man (current age: 59 years), without any underlying disease, presented with a right-hand tremor at the age of 39 years, followed by loss of facial expression, slow movement, rigidity, and postural instability. He also had rapid eye movement sleep behavior disorder (RBD), but no hyposmia or orthostatic dizziness. At   We could not obtain his mother's DNA sample because the patient had not been in contact with his mother and sister for many years. His younger brother died in a traffic accident without any history of parkinsonian symptoms. The rest of his family members did not have any extrapyramidal symptom, epilepsy, myopathy, or ataxia. Figure 1 presents the family pedigree of the patient's family.
Genomic DNA was extracted from peripheral venous blood lymphocytes of the patient and his father. Next, multiplex ligationdependent probe amplification was used to detect large deletions or duplication in the DNA (Jeuken, Cornelissen, Boots-Sprenger, Gijsen, & Wesseling, 2006). We then used target exome sequencing with a TruSeq Custom Amplicon Low Input panel (Illumina) to determine the 51 PD-causative genes mutation sites in patients (Deng et al., 2018;Lill, 2016;Puschmann, 2017). Target regions of patients' blood genomic DNA were amplified with specific primers, ligated of adaptors to the amplified PCR products, and finally generated the mutation is categorized as a likely pathogenic variant for PD according to the scoring rule (Richards et al., 2015). Moreover, GBA L444P is categorized as a pathogenic gene in the ClinVar database.

| D ISCUSS I ON
Thus, our patient and his father demonstrated a typical presentation of idiopathic PD, with two mutation sites in GBA and POLG.
GBA L444P is a known risk factor for PD; in a study, it was shown to increase PD risk by 10 times (Sidransky et al., 2009). POLG mutations are linked to a wide range of systemic or neurological diseases (Stumpf et al., 2013). Although this gene mutation could rarely lead to parkinsonism, R964C has never been reported in association with parkinsonism thus far.
The R964C mutation is located in the pol domain ( Figure 4).
Four studies have mentioned R964C mutation so far (Table 2).
Homozygous R964C mutation can present as early ovarian failure or nucleotide reverse transcriptase inhibitor toxicity when anti-human immunodeficiency virus-1 medication is taken (Bailey, Kasiviswanathan, Copeland, & Anderson, 2009;Chen et al., 2018;Yamanaka et al., 2007). In two other studies, both compound heterozygous mutations at R964C and A862T were identified and revealed to be associated with ataxia, epilepsy, and intellectual disability (Table 2) (Stricker et al., 2009;Wong et al., 2008).
According to its biochemical effect, R964C missense mutation can significantly reduce the catalytic efficiency compared with its wild type (Bailey et al., 2009 Moreover, accumulating evidence has indicated that harboring more than two mutational loci in two alleles may cause a synergetic effect, leading to early neurodegeneration (Cady et al., 2015;Giri, Zhang, & Lü, 2016). Also, polygenic factors contribute to the impairment of mitochondrial replication and repair may result in PD (Gaare et al., 2018). We suspect that these two gene mutations could both influence repairing mitochondria and increase oxidative stress causing early neurodegeneration.
In our patient's family, only one patient developed YOPD, whereas his father developed late-onset PD. No literature has reported PD in POLG R964C mutation. Furthermore, the same mutations could reveal variable presentations, suggesting that epigenetic or environmental factors, as well as other modifiers may influence the clinical manifestation.

| CON CLUS ION
We reported a first familial PD of combined POLG R964C and GBA L444P mutations. This finding extends our understanding of the PD genotype-phenotype correlation.

CO N FLI C T O F I NTE R E S T
The authors declare no conflict of interest.