The basal layer in human squamous tumors harbors more UVA than UVB fingerprint mutations: a role for UVA in human skin carcinogenesis

Abstract

We hypothesized that a substantial portion of the mutagenic alterations produced in the basal layer of human skin by sunlight are induced by wavelengths in the UVA range. Using laser capture microdissection we examined separately basal and suprabasal keratinocytes from human skin squamous cell carcinomas and premalignant solar keratosis for both UVA- and UVB-induced adduct formation and signature mutations. We found that UVA fingerprint mutations were detectable in human skin squamous cell carcinomas and solar keratosis, mostly in the basal germinative layer, which contrasted with a predominantly suprabasal localization of UVB fingerprint mutations in these lesions. The epidermal layer bias was confirmed by immunohistochemical analyses with a superficial localization of cyclobutane thymine dimers contrasting with the localization of 8-hydroxy-2'-deoxyguanine adducts to the basal epithelial layers. If unrepaired, these adducts may lead to fixed genomic mutations. The basal location of UVA-rather than UVB-induced DNA damage suggests that longer-wavelength UVR is an important carcinogen in the stem cell compartment of the skin. Given the traditional emphasis on UVB, these results may have profound implications for future public health initiatives for skin cancer prevention.

Figures

Fig. 1.

(A) Regions sampled by LCM for mutational analysis in SK and SCCs. 1, uninflamed upper; 2, uninflamed lower; 3, inflamed upper; 4, inflamed lower; 5, stromal; 6, normal upper; 7, normal lower (note that only regions 6 and 7 sampled in normal control skin). (B) Distribution of UVB fingerprint mutations in SK, SCC, and normal skin. U, upper epidermal regions 1 and 3; L, lower epidermal regions 2 and 4. Normal U and Normal L are the mutations in the upper and lower epidermal regions, respectively, of control samples from non-sun-exposed skin. Region 6 and 7 (normal skin adjacent to the SK or SCC) data are not shown for these regions. (C) Distribution of UVA fingerprint mutations in SK, SCC, and normal skin. (D) Number of cells positive for CPDs as determined by IHC. The minimal number of CPD-positive cells in normal control skin is too small to be visualized. (E) Number of cells positive for 8-oxoG as determined by IHC. (F) Number of cells positive for p53 as determined by IHC.