Abstract
Objective:
To understand whether mitochondrial DNA (mtDNA) mutation load can modify the penetrance of variants in the Parkinson’s disease (PD) causing gene PRKN (Parkin), in heterozygous carriers.
Background:
Homozygous or compound-heterozygous mutations in PD causing genes, including Parkin and PINK1, have proven to disrupt mitochondrial performance. Heterozygous carriers of mutations in these genes are frequent in the population. These variants may predispose to some of the disease symptoms albeit with highly reduced penetrance. Differences in mtDNA mutation load might affect mitochondrial function and thus influence the penetrance of such nuclear variants.
Methods:
Unaffected carriers of heterozygous Parkin mutations were identified in the CHRIS and MICROS cohorts being conducted in South Tyrol/Northern Italy. MtDNA mutation was assessed by deep mtDNA sequencing (coverage=~10,000x) in blood-derived DNA with an Illumina NextSeq kit. Targeted metabolomics was performed in serum using the AbsoluteIDQ® p180 Kit (Biocrates Life Sciences), which includes up to 187 endogenous metabolites capturing metabolites involved in mitochondrial metabolism.
Results:
In a subset of the CHRIS and MICROS studies (n=4,662), a total of n=149 carriers of heterozygous Parkin mutations were identified by genotyping, exome sequencing and MLPA. Mutation carriers cluster in 30 multigenerational families and 8 small multiplex families. mtDNA sequencing in a subset of carriers with either CNVs or a 1-bp truncating deletion in exon9 (n=41), showed significantly more mtDNA variants including both homoplasmic and heteroplasmic variants (>15% frequency) compared to age-matched non-carriers from the same families (n=26, p=0.037). Furthermore, in spent media of iPSC-derived neurons and serum samples of Parkin mutation carriers, we identified metabolites with significant differences in concentration compared to non-carriers, suggesting the presence of a metabolic signature involving mitochondrial shuttles reflecting a dysregulation of mitochondrial function in mutation carriers.
Conclusions:
In line with previous data from animal and cell models, our data suggest an increased number of mtDNA variants and altered mitochondrial function in the presence of Parkin mutations, which might contribute to increased risk of developing mild features of PD in heterozygous mutation carriers.