Mitochondrial DNA and cardiovascular diseases: A long road ahead

Mitochondria are organelles found in nucleated cells. They have the distinctive function of energy production, as they generate most of the adenosine triphosphate (ATP) of a typical cell, and the distinctive feature of having their own DNA. Mitochondrial DNA (mtDNA) is a double-stranded circular molecule of 16,569 base pairs that encodes for 13 oxidative phosphorylation (OXPHOS) genes involved in the process of ATP synthesis and in the generation of reactive oxygen species, the modulation of pH and Ca2+ levels, and the initiation of apoptosis. mtDNA also encodes for 22 transfer RNAs (tRNAs) and two ribosomal RNAs that regulate its transcription and replication. ¹ It differs from nuclear DNA in several ways:

mtDNA is maternally inherited, although in some exceptional cases paternal mtDNA could be passed to offspring. ²

mtDNA is present in multiple copies per cell, and this number varies according to the bioenergetic needs of each particular cell. ³ Thus, diseases caused by mutations in mtDNA generally follow the laws of population genetics, not the Mendelian laws.

mtDNA mutations are heteroplasmic, meaning that some of the mtDNA copies in a cell could be normal and others could present a mutation. In general, heteroplasmic mutations follow a ‘recessive’ inheritance pattern and a high number of mutated copies are required to present the associated clinical phenotype, a threshold that varies across mutations and tissues. Moreover, a mutation could be related to several phenotypes at different thresholds. For example, heteroplasmic patients with the mtDNA 3243A>G mutation can present with diabetes (20–30% mutant load), neuromuscular degenerative disease (50–90% load) or lethal perinatal disease (100% load). ⁴

In direct opposition to the idea that coding for genes with a critical role in cell survival (in this case, OXPHOS) should be highly conserved, mtDNA has a high mutation rate. These mutations appear in three main forms: ⁵ i) point mutations, equivalent to single nucleotide polymorphisms in nuclear DNA, that have been related to Mendelian disorders ¹ such as some skeletal myopathies, ⁶ and complex diseases such as diabetes; ⁷ ii) large-scale deletions, usually sporadic, present only in the proband, and occurring either in the germ line of the mother or in early embryogenesis of the proband; recent reports also suggest that these deletion variants accumulate with aging and could be related to chronic diseases; and iii) secondary defects, in which mtDNA integrity is altered as a consequence of a nuclear DNA mutation in genes encoding for proteins required for the proper maintenance of mtDNA.

mtDNA mutation seems most likely to occur during repair of damage rather than during replication; ⁸ mtDNA damage repair has been associated with obesity and insulin resistance ⁹ and atherosclerosis. ¹⁰

In the European Journal of Preventive Cardiology, Borghini et al. have analysed the relationship between long-term low-dose exposure to ionizing radiation and mtDNA 4977-bp deletion in catheterization laboratory workers (n=147), using a cross-sectional design that also included 74 non-exposed individuals. ¹¹ Ionizing radiation is one of the exposures that can induce the appearance of somatic mutations or the progression of clonal mutated mtDNAs, either by increasing reactive oxidative species or by affecting the functionality of proteins repairing/maintaining mtDNA. The authors note that workers in a catheterization laboratory offer a unique research model to address the health effects of low levels of ionizing radiation exposure. They assessed radiation dose using an objective method (official records of the Health Physics Department) in 28% of the participants and a self-report questionnaire was completed by all participants. The validity of this questionnaire was assessed in the subsample of participants with both objective and self-reported data (Spearman rank correlation coefficient = 0.73).

The authors analysed the association between ionizing radiation and two mtDNA markers (mtDNA4977 deletion and mtDNA copy number) as potential mediators of the health effects of ionizing radiation, testing two hypotheses:

Ionizing radiation exposure increases the number of mtDNA with the 4977 deletion showing this mutation as a biomarker of mitochondrial dysfunction. mtDNA4977 deletion is a frequently studied variant, increases with aging, is a potential marker of mitochondrial dysfunction and has been related to several chronic cardiovascular diseases. ¹² It is a common deletion of 4977 base pairs that affects seven OXPHOS genes and five regulatory tRNAs. Thus, an increase in the number of mtDNA showing this mutation could surpass the required threshold to impact on clinical phenotypes.

Ionizing radiation exposure decreases the number of mtDNA copies as a reflection of lost mtDNA repair capacity. The number of mitochondrial genomes per cell is a marker of mitochondrial dysfunction and reflects the mtDNA repair/maintenance capacity.

The authors report a direct association between ionizing radiation exposure and the number of mtDNA showing the 4977 deletion mutation, confirming their first hypothesis. However, they did not observe a clear pattern of ionizing radiation association with mtDNA copy number: the copy number increased in the low-exposure group, compared with the non-exposed group, and did not differ between the high-exposure and non-exposed groups. In the discussion, the authors argue that this observation could be related to an initial compensatory molecular response to ionizing radiation exposure, a potential explanation that should be explored in other studies.

At this point, mtDNA4977 deletion arises as a potential marker of ionizing radiation exposure in catheterization laboratory workers (and others exposed to this type of radiation). Prospective studies are required to assess the temporal and dose–response relationship between ionizing radiation and mtDNA mutation rate and to analyse the role of these mutations in mediating the health consequences of ionizing radiation exposure. Identification of the mechanisms involved in the appearance of somatic mtDNA mutations and their clonal proliferation could also identify potential therapeutic targets for cardiovascular disease prevention and treatment.

More than 250 pathogenic mtDNA mutations have been reported in humans. The first of these to be described, in 1988, was a point mutation in the MTND4 gene (m.11778A>G) related to a hereditary optic neuropathy (Leber’s neuropathy). ¹³ Most subsequent reports have been related to neurological diseases, but associations with metabolic, ⁹ vascular ¹⁰ and cardiac alterations^14,15 have also been described. mtDNA has been largely ‘forgotten’ in cardiovascular research. The new generation of genomic (and other -omic) technologies and the new methods available to assess mitochondrial functionality ¹⁵ could generate new knowledge on the relevance of the mitochondria in hereditary and complex cardiovascular diseases. As demonstrated by the discovery of the association between nuclear DNA variants and complex cardiovascular diseases, collaboration between groups and between disciplines will be key to success in this new effort to unravel the role of mtDNA in cardiovascular diseases. A long road lies ahead to find new ways to prevent and treat these diseases.