and erythema.11,12,13 The degree of erythema is directly correlated
to degree of DNA damage.14
Our study was performed to determine whether erythema
from a controlled dose of UV exposure demonstrates a circadian
pattern in humans as well. We also aimed to determine
whether these results were accompanied by changes in protein
expression relevant to DNA damage, such as xeroderma pigmentosum,
complementation group A (XPA), and cyclobutane
pyrimidine dimers (CPD). XPA is a protein involved with recognizing
damaged sites for nucleotide excision repair (NER) while
CPD is a form of DNA damage most commonly induced by UV
exposure. Both XPA and CPD are indicative of UV-induced DNA
damage in the cell and the cell’s efforts towards repair.15,16
RESULTS
7 subjects completed the study: 2 Fitzpatrick Skin Type (FST) II, 4 FST
III, and 1 FST IV. Sleep duration averaged 6.5-8 hours with a mean
of 7.3 hours.
Similar to mice, our human subjects showed increased erythemal
response to UVR in the AM compared to the UVR in the PM. Expression
levels of both XPA and CPD were found to be significantly
elevated in the biopsy samples irradiated in the AM compared to
the biopsy samples irradiated in the PM.
Mean Erythema Dose (MED)
The mean erythema dose (MED) was performed to determine
the subject’s sensitivity to simulated solar radiation (SSR). MED
is defined as the lowest dose of UVR that produces a change in
erythema equivalent to 2.5 units in the a* axis by colorimetry.21
Visual MED showed increased erythema on skin exposed to SSR
in the morning versus the afternoon in each subject. Figure 1
shows the results of visual MED for a female subject of FST III.
On the right panel (AM MED), there was visible full circle erythema
at 62 seconds, with faint but discernible pink circles at 40
and 50 seconds. In contrast, on the left side (PM MED), it took
longer, 78 seconds to see a full circle erythema. Lower visual
MED equates to less SSR required to produce the same amount
of erythema. This trend of decreased MED in the AM versus the
PM was seen in all patients.
Quantitative data on MED via chromameter readings are shown
in Figure 2. The y-axis represents the MED in mJ/cm2. The subject
data along the x-axis are presented according to the order
of enrollment in the study. In every subject, the MED was lower
in the AM; therefore, it took a lower dose of SSR to produce
erythema in the morning. Paired t-testing of the MED values in
AM vs PM showed the difference to be statistically significant
(P = 0.03). Figure 3 shows the mean of quantitative MED for
all 7 subjects with SSR exposure in the AM versus the PM. The
