Twinkle-associated familial parkinsonism with Lewy pathology

Mitochondrial dysfunction is a recognised cause of autosomal recessive Parkinson's disease (PD) and may contribute to idiopathic disease.1 Twinkle protein is a DNA helicase coded by the C10orf2 gene which, along with polymerase gamma and other proteins, is responsible for regulating mitochondrial replication. Heterozygous C10orf2 mutations are a recognised cause of chronic progressive ophthalmoplegia (CPEO) and other neurological manifestations, but their relationship with parkinsonism is unclear. Here, we report a case (along with postmortem examination findings) of familial parkinsonism associated with a heterozygous mutation in C10orf2, alongside reviewing previously published cases.

selegiline stopped; rivastigmine and quetiapine started). He was eventually admitted to a nursing home and died approximately 10 years after diagnosis due to a presumed aspiration pneumonia.
Just before his death, further genetic testing revealed a heterozygous variant in the C10orf2 gene on chromosome 10 (c.908 G>A, p.[Arg303Gln]). Bioinformatic analysis predicted that this variant was pathogenic and had a very low  Abbreviations: DAT = dopamine transporter; NR = not reported. All cases had CPEO, which began 1-2 decades before their parkinsonism (3 cases underwent corrective eye surgery), with 1 exception where the patient had childhood-onset ptosis. Similar numbers of males and females were affected, and the majority had a positive family history. Dopamine transporter imaging was universally abnormal when it was performed, whereas 3 of 5 MR brain scans showed white matter abnormalities. Muscle biopsy tended to be abnormal when it was performed (with mitochondrial changes including COX negative fibers and mtDNA deletions), but 1 was normal. Most patients responded to levodopa when it was initiated, although limited information was available on progression of symptoms. Brain postmortem assessment was not performed in any of these cases. a Age at onset of parkinsonism. b One of more first-degree family member.
allele frequency on gnomAD (0.0012%). PD gene panel testing identified no other clinically relevant variants.
Postmortem examination revealed severe, patchy neuronal cell loss in the substantia nigra with evidence of limbic (transitional) stage Lewy body disease according to the Montine classification (figure). 2 In the substantia nigra, alpha-synuclein immunostains labeled Lewy bodies and Lewy neurites, as well as diffusely filled perikarya. Lewy bodies were also present in the locus coeruleus, periaqueductal gray matter, basal temporal neocortex, and cingulate gyrus (figure, C, D, H); but sparse in other regions of the neocortex. There were no abnormalities in the cerebellum. There was no significant deposition of tau, betaamyloid, or TDP-43 proteins.

Discussion
We propose that heterozygous C10orf2 mutations may be a rare cause of parkinsonism and should be considered in patients with a positive family history and/or other features of a mitochondrial disorder (e.g., CPEO). We searched the literature and found 8 previously reported cases of parkinsonism associated with heterozygous C10orf2 mutations (table). [3][4][5][6][7] They are unlikely to be a major contributor to overall PD heritable risk; indeed, C10orf2 does not appear as a risk loci on the most recent meta-analysis of genome-wide association studies. 8 We cannot exclude the possibility that the association may be a coincidence, especially because we were unable to perform segregation analysis. Unfortunately, only fixed brain tissue was available for the postmortem examination, which precluded molecular analysis (such as mtDNA deletion load). In the single reported autopsy case of a patient with heterozygous C10orf2 mutation, there was also significant loss of substantia nigra neurons (although the patient did not have clinical evidence of parkinsonism), but no Lewy bodies were present (unlike our case). 9 We hope that neuropathologic analysis of future cases will help to determine the precise pathologic underpinnings of parkinsonism in heterozygous C10orf2 mutation carriers.
Twinkle protein is important for maintaining mtDNA integrity. In a mouse model expressing mutant Twinkle, there was accelerated accumulation of mtDNA deletions and loss of TH-positive neurons (leading to motor impairment). 10 Patients with biallelic C10orf2 mutations typically present with severe and complex neurologic phenotypes (e.g., infantile-onset spinocerebellar ataxia, epilepsy, sensory polyneuropathy, Perrault syndrome, and adult-onset mitochondrial myopathy) alongside systemic features, but not parkinsonism. The classical pathology in these patients includes severe neuronal loss in the substantia nigra in the absence of Lewy bodies or alpha-synuclein deposition, often accompanied by degeneration of the cerebellar-dentato-olivary system. Further studies are required to explain why most patients with biallelic C10orf2 mutations do not exhibit parkinsonism despite demonstrating severe substantia nigra neuronal loss (which also occurs with biallelic POLG mutations). 9 Heterogeneous neuropathology is a recognized feature of genetic PD associated with mitochondrial dysfunction. A review of autopsy findings in 18 homozygous or compound heterozygous Parkin cases found Lewy bodies in only 6 patients, despite evidence of neuronal loss in the substantia nigra in all cases. 11 In the 2 postmortem cases of biallelic PINK1 mutations, Lewy bodies were present in one 12 and absent in the other. 13 The role of heterozygous Parkin and PINK1 mutations is controversial; however, it is intriguing that the limited number of autopsy studies has shown diffuse Lewy bodies in both groups. This may support the concept of genetic predisposition to PD by the mitochondrial dysfunction caused by these heterozygous states, which is in line with a recent report showing greater Lewy body pathology in older patients with mitochondrial dysfunction due to a range of nuclear and mtDNA genetic defects. 14