Polypharmacy and sun exposure: Implications for mitochondrial DNA deletions in skin

Most somatic cells contain many copies of mitochondrial DNA (mtDNA). Because of both the high copy number and the lack of repair mechanisms available to mtDNA, damage to it largely goes unrepaired, and can accumulate over time. Large scale deletions are a recognised type of damage sustained by mtDNA as a consequence of exposure to the ultraviolet light in sunlight. A group of patients were identified as having abnormally high levels of either a 4977 base pair deletion (mtDNA⁴⁹⁷⁷) or 3895 base pair deletion (mtDNA³⁸⁹⁵), in mtDNA from sun exposed skin or skin suspected to be a non-melanoma skin cancer, but not in their non-sun exposed skin biopsies. In three of the four cases, skin cancer was ruled out due to histological testing. Additional factors from these patients' medical histories were studied, and it was noted that they shared diagnoses for multiple pathologies common to an older population, and that they were being treated with the same or related pharmaceuticals, including some that had been known to cause dermal side effects. Investigation into the biochemistry underlying the symptoms, the effects of sun exposure and side effects of the prescribed pharmaceuticals revealed a possible synergistic relationship leading to the localised high levels of mtDNA deletions.