Background:
Melanocortins are known to act via the MC1R to increase melanogenesis. MC1R signalling is implicated in melanoma susceptibility and paradoxically in UV damage protection. Existing in vivo models of melanocortin activation rely on the use of exogenous peptides or transgenic systems, requiring multiple daily doses or long and costly model development, respectively.
Aim:
To develop a novel murine model of sustained overexpression of melanocortin variants using intramuscular delivery of DNA minicircles, a next-generation non-viral gene delivery method utilising small plasmids with bacterial parental plasmid elements removed, leading to greater transfection efficiency and reduced immunogenicity.
Methods:
C57BL/6 mice were given intramuscular injections of DNA minicircles encoding melanocortin peptides. A 7-day pilot study was undertaken to confirm if melanocortins could be successfully expressed off the minicircles. A single intramuscular injection of 25μg minicircle DNA was administered. This study was then followed by a 10-week study where mice received 100μg (50 μg/leg) of minicircle DNA. These mice were followed for 10 weeks and ears were removed at the time of sacrifice for melanin extraction and melanin assay analysis of cutaneous pigmentation.
Results:
Increased serum levels of melanocortins were confirmed by ELISA in both studies.
Ear melanin levels were significantly increased in mice receiving melanocortin DNA minicircles as compared to minicircle controls. Sustained expression and pigmentation were seen over the course of the 10-week study.
Conclusion:
These studies support the feasibility of sustained and systemic melanocortin overexpression via non-viral methods. Minicircles are a flexible platform for melanocortin overexpression and cause increases in cutaneous pigment from a single intramuscular injection. Given the central role of MC1R signalling in melanoma and in UV-mediated melanogenesis, this model has potential applications in murine studies of melanoma susceptibility, photoprotection and therapeutic melanocortin agonism in diseases such as erythropoietic protoporphyria, variegate porphyria, xeroderma pigmentosum and vitiligo.