Objective: To investigate whether apparently healthy individuals carrying a heterozygous pathogenic Parkin (PRKN) mutation show molecular phenotypes of mitochondrial dysfunction.

Background: Homozygous or compound-heterozygous mutations in Parkin are the most common cause of autosomal recessive early-onset (< 45 years) familial PD. Heterozygous carriers of Parkin mutations are frequent in the population. The phenotype expressivity in heterozygous mutation carriers is described as extremely variable, including PD symptoms, subtle motor signs, and no signs. Parkin is, together with PINK1, the most prominent monogenic PD gene involved in mitochondrial dysfunction.

Method: Unaffected carriers of heterozygous Parkin mutations were identified in the Cooperative Health Research in South Tyrol (CHRIS) study conducted in Northern Italy, by genotyping, exome sequencing, and digital PCR analyses to detect copy number variants. In lymphoblasts, mitochondrial respiration was measured by using a high-resolution respirometer (Oxygraph-2k; Oroboros Instruments, Innsbruck, Austria), and mitochondrial superoxide (mROS) levels were assessed by flow cytometry with Mitosox red staining. In induced pluripotent stem cell (iPSC)-derived neurons, mitochondrial membrane potential (MMP) and mROS were analyzed using the cell permeant dyes TMRM and Mitosox red.

Results: In a subset of the CHRIS study with exome sequence data available (n=3,603), a total of n=164 carriers of heterozygous Parkin mutations were identified. In lymphoblasts of a subset of 11 carriers of a heterozygous exon 7 deletion, we found an elevation of mitochondrial respiratory activity, suggesting that this might be due to a compensatory increase of mitochondrial activity. This was accompanied by an increase in the production of mROS. In the differentiated neurons, we observed increased mROS levels in two heterozygous mutation carriers as compared to two non-carriers, which is, however, lower than in a patient line with homozygous Parkin mutations (obtained from an independent study), thus suggesting a gene-dosage effect. We also detected alterations in the MMP, which might indicate functional deficits of electron transport and oxidative phosphorylation.

Conclusion: Our data from lymphoblasts and iPSC-neurons derived from heterozygous Parkin mutation carriers suggest a detectable altered mitochondrial function, possibly representing an early indicator for developing mild features of PD.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/molecular-investigation-of-apparently-healthy-heterozygous-parkin-mutation-carriers/