Fractional Sunburn Threshold UVR Doses Generate Equivalent Vitamin D and DNA Damage in Skin Types I-VI but with Epidermal DNA Damage Gradient Correlated to Skin Darkness

Abstract

Public health guidance recommends limiting sun exposure to sub-sunburn levels, but it is unknown whether these can gain vitamin D (for musculoskeletal health) while avoiding epidermal DNA damage (initiates skin cancer). Well-characterized healthy humans of all skin types (I-VI, lightest to darkest skin) were exposed to a low-dose series of solar simulated UVR of 20%-80% their individual sunburn threshold dose (minimal erythema dose). Significant UVR dose responses were seen for serum 25-hydroxyvitamin D and whole epidermal cyclobutane pyrimidine dimers (CPDs), with as little as 0.2 minimal erythema dose concurrently producing 25-hydroxyvitamin D and CPD. Fractional MEDs generated equivalent levels of whole epidermal CPD and 25-hydroxyvitamin D across all skin types. Crucially, we showed an epidermal gradient of CPD formation strongly correlated with skin darkness (r = 0.74, P < 0.0001), which reflected melanin content and showed increasing protection across the skin types, ranging from darkest skin, where high CPD levels occurred superficially, with none in the germinative basal layer, to lightest skin, where CPD levels were induced evenly across the epidermal depth. People with darker skin can be encouraged to use sub-sunburn UVR-exposure to enhance their vitamin D. In people with lighter skin, basal cell damage occurs concurrent with vitamin D synthesis at exquisitely low UVR levels, providing an explanation for their high skin cancer incidence; greater caution is required.

Figure 1

Constitutive melanin level increases with skin type (I–VI), with differing distribution across the epidermis. Sections of skin biopsy samples from volunteers with skin types I–VI were stained for melanin using a modified Warthin-Starry procedure. The percentage of epidermis stained for melanin rose with increasing skin type (I–VI). A gradual decrease in melanin staining was seen with increasing distance from the dermal-epidermal junction, although notable amounts of melanin persisted across the full depth of the epidermis in dark skin types. Scale bar = 50 μm.

Figure 2

Sub-sunburn UVR dose responses are seen for 25(OH)D gain and whole epidermal CPD level across skin types I–VI. Volunteers received an acute UVR exposure of 20%, 40%, 60% and 80% of their individual sunburn threshold dose (MED), and post-UVR serum 25(OH)D change and cutaneous CPD induction outcomes were assessed. A significant response was seen in a mixed-effects regression across the 20%–80% MED dose range for 25(OH)D gain (P < 0.001) and CPD level (P = 0.01), with no influence of skin type (P = 0.23 and 0.63, respectively). There was no evidence of a minimum threshold dose for 25(OH)D gain or CPD induction. Individual data are shown; n = 33–38 per dose for 25(OH)D and n = 11–13 per dose for CPD. 25(OH)D, 25-hydroxyvitamin D; AU, arbitrary unit; CPD, cyclobutane pyrimidine dimer; MED, minimal erythema dose.

Figure 3

Gain in basal layer CPD and 25(OH)D occurs concurrently in lighter but not darker skin types. Assessment of epidermal CPD level in the basal layer alone and serum 25(OH)D was performed in volunteers before and after acute UVR exposures of 20%–80% of their individual sunburn threshold (MED). (a) In lighter skin types (I–III), a simultaneous increase in serum 25(OH)D (n = 16–19 per dose) and CPD (n = 7–9 per dose) in the germinative basal layer was seen across the UVR dose range. (b) In contrast, in darker skin types (IV–VI), although a similar, significant UVR-25(OH)D dose response occurred, basal layer CPD level remained undetectable across the dose range (25(OH)D n = 17–19 per dose; CPD n = 3–5 volunteers per dose). Individual volunteer data are shown. 25(OH)D, 25-hydroxyvitamin D; AU, arbitrary unit; CPD, cyclobutane pyrimidine dimer; MED, minimal erythema dose.

Figure 4

A gradient of CPD formation is seen across the epidermal depth that strongly correlates with skin darkness. (a) Representative images of CPD (red) and DAPI (blue) staining in skin from volunteers with light (skin type I) and dark (skin type V) skin, immediately after a single 80% MED UVR exposure and in corresponding unexposed control skin. The dashed line indicates the dermal-epidermal junction, and the solid line indicates the skin surface. CPD staining varied little with epidermal depth in light skin, whereas dark skin showed a gradient of CPD formation, with strong staining in the upper epidermis and very little in the basal layer after UVR. Scale bar = 50 μm. (b) Total CPD levels were quantified according to epidermal depth by determining the CPD/DAPI ratio within epidermal nuclei along lines perpendicular to the skin surface to generate a CPD gradient value (AU/μm) for each volunteer. The figure shows that the higher skin types had a steeper CPD gradient from skin surface to dermal-epidermal junction than lower skin types and that CPD gradient strongly correlated with skin darkness (100–L*) (r = 0.74, P < 0.0001). Data points represent volunteers’ gradient values, which are shown for individual skin types. The UVR dose was the highest dose received, that is, 60% or 80% MED. AU, arbitrary units; MED, minimal erythema dose; CPD, cyclobutane pyrimidine dimer.

Figure 5

CPD repair is virtually complete at 48 hours with no difference between UVR doses or skin types. Epidermal CPD level was assessed immediately (15 minutes) and 48 hours after UVR. Mean whole epidermal CPD level in darker skin types (IV–VI, n = 3–5 per dose) was similar to that in lighter skin types (I–III; n = 7–9 per dose) at both (a) the lower (20% or 40% MED) UVR dose and (b) the higher (60% and 80% MED) UVR dose with minimal/no CPD detectable 48 hours after UVR exposure. UVR-induced CPD in the basal layer of lighter-skinned volunteers was minimal/undetectable at 48 hours, with CPD in the basal layer of darker skin minimal/undetectable at both time points. Dotted line denotes baseline (unexposed skin); data are mean ± standard error of the mean. AU, arbitrary unit; CPD, cyclobutane pyrimidine dimer; MED, minimal erythema dose.