Mitochondrial DNA Copy Number Drives the Penetrance of Acute Intermittent Porphyria

Abstract

No published study has investigated the mitochondrial count in patients suffering from acute intermittent porphyria (AIP). In order to determine whether mitochondrial content can influence the pathogenesis of porphyria, we measured the mitochondrial DNA (mtDNA) copy number in the peripheral blood cells of 34 patients and 37 healthy individuals. We found that all AIP patients had a low number of mitochondria, likely as a result of a protective mechanism against an inherited heme synthesis deficiency. Furthermore, we identified a close correlation between disease penetrance and decreases in the mitochondrial content and serum levels of PERM1, a marker of mitochondrial biogenesis. In a healthy individual, mitochondrial count is usually modulated to fit its ability to respond to various environmental stressors and bioenergetic demands. In AIP patients, coincidentally, the phenotype only manifests in response to endogenous and exogenous triggers factors. Therefore, these new findings suggest that a deficiency in mitochondrial proliferation could affect the individual responsiveness to stimuli, providing a new explanation for the variability in the clinical manifestations of porphyria. However, the metabolic and/or genetic factors responsible for this impairment remain to be identified. In conclusion, both mtDNA copy number per cell and mitochondrial biogenesis seem to play a role in either inhibiting or promoting disease expression. They could serve as two novel biomarkers for porphyria.

Keywords: PERM1; acute intermittent porphyria; heme; incomplete penetrance; mitochondrial biogenesis; mtDNA copy number.

Figure 1

Quantification of mtDNA copy number by qPCR. The figures display the distribution of values obtained with median, interquartile range, minimum, and maximum values. (a) Comparison between healthy individuals (CTRL) and total AIP cases (Patients). (b) Comparison among healthy individuals, asymptomatic (AS) patients, and symptomatic (S) patients; **** adjusted p < 0.0001.

Figure 2

Relative percentage of the mtDNA content. We calculated the ratio of mtDNA copy number to the mean of controls for all individuals analyzed to determine the mean percentage. The panel presents the mean ± SD of mtDNA percentage values obtained for healthy individuals (CTRL), asymptomatic (AS) patients, and symptomatic (S) patients. The statistical significance for reduction is not reported since the same values were used in Figure 1, and the significance remained the same (p < 0.0001).

Figure 3

Integrity of mtDNA. The ratio of intact copies of mtDNA (ND4 gene) to the total of mtDNA detected (ND1 or CYTB genes) was calculated for all individuals. The results are presented as the mean ± SD. A value close to 1 (0.8–1.2) indicates that no mtDNA damage occurred. There were no significant differences between healthy individuals (CTRL) and patients.

Figure 4

Quantification of heme. The results are graphically presented as the mean ± SD. (a) Comparison was performed between healthy individuals (CTRL) and all AIP cases (patients). (b) Comparisons were performed among healthy individuals, asymptomatic (AS) patients, and symptomatic (S) patients; ns = not significant, * exact p < 0.05, and ** exact p < 0.01.

Figure 5

Quantification of PERM1. The figure shows the distribution of values obtained with median, interquartile range, minimum, and maximum values (a) Comparison was performed between healthy individuals (CTRL) and total AIP cases (patients). (b) Comparisons were made among healthy individuals, asymptomatic (AS) patients, and symptomatic (S) patients; ns = not significant, * p < 0.05, ** p < 0.01, and *** p < 0.001.