Intragenomic variability and extended sequence patterns in the mutational signature of ultraviolet light

Mutational signatures can reveal properties of underlying mutational processes and are important when assessing signals of selection in cancer. Here, we describe the sequence characteristics of mutations induced by ultraviolet (UV) light, a major mutagen in several human cancers, in terms of extended (longer than trinucleotide) patterns as well as variability of the signature across chromatin states. Promoter regions display a distinct UV signature with reduced TCG > TTG transitions, and genome-wide mapping of UVB-induced DNA photoproducts (pyrimidine dimers) showed that this may be explained by decreased damage formation at hypomethylated promoter CpG sites. Further, an extended signature model encompassing additional information from longer contextual patterns improves modeling of UV mutations, which may enhance discrimination between drivers and passenger events. Our study presents a refined picture of the UV signature and underscores that the characteristics of a single mutational process may vary across the genome.

Intragenomic variability and extended sequence patterns in the mutational signature of ultraviolet light

ABSTRACTMutational signatures can reveal properties of underlying mutational processes and are important when assessing signals of selection in cancer. Here we describe the sequence characteristics of mutations induced by ultraviolet (UV) light, a major mutagen in several human cancers, in terms of extended (longer than trinucleotide) patterns as well as variability of the signature across chromatin states. Promoter regions display a distinct UV signature with reduced TCG>TTG transitions, and genome-wide mapping of UVB-induced DNA photoproducts (pyrimidine dimers) showed that this may be explained by decreased damage formation at hypomethylated promoter CpG sites. Further, an extended signature model encompassing additional information from longer patterns improves modeling of UV mutation rate, which may enhance discrimination between drivers and passenger events. Our study presents a refined picture of the UV signature and underscores that the characteristics of a single mutational process may vary across the genome.

Skin Retention of Sorbates from an After Sun Formulation for a Broad Photoprotection

Overexposure to sunlight is widely accepted as the underlying cause of cutaneous melanoma. UV radiation induces the formation of DNA photoproducts that, if unrepaired, can induce carcinogenic mutations. Recent data indicate that sorbates can be useful to widen the protection against UV radiation by acting as a triplet-state quencher in the melanocyte. The aim of the present work was to prepare an after sun formulation containing ethylsorbate or sorbic acid in order to take advantage of the triplet-state quenching activity of these molecules and protect the skin from UV-induced damages. Ethylsorbate and sorbic acid were characterized in terms of solubility and partition coefficient, and their transdermal permeation and skin accumulation were studied in vitro from simple solutions and in the presence of cyclodextrins (alpha and hydroxypropylbeta) as a complexing agent. The goal was to reduce as much as possible sorbates permeation while sustaining their skin levels. The obtained results indicated that the addition of alphacyclodextrins determined a 6-folds (ethylsorbate ) or 4-folds (sorbic acid) reduction of the transdermal permeation. Sorbic acid and alphacyclodextrin (1:1 molar ratio) were then formulated in an after sun vehicle using 1.5% hyaluronic acid (sodium salt) as a thickener and hydrating agent. The addition of hyaluronic acid gave rise to a formulation with good cosmetic properties and good sorbate (0.2–0.3 µmol/cm2) skin levels (stratum corneum + viable epidermis) and thus a potential protection against post-exposure UV damage.

Structures of the antibody 64M-5 Fab and its complex with dT(6–4)T indicate induced-fit and high-affinity mechanisms

DNA photoproducts with (6–4) pyrimidine–pyrimidone adducts produced by ultraviolet light are mutagenic and carcinogenic. The crystal structures of the anti-(6–4) photoproduct antibody 64M-5 Fab and of its complex with dT(6–4)T were determined at 2.5 and 2.0 Å resolution, respectively. A comparison between the dT(6–4)T-liganded and unliganded structures indicates that the side chain of His93L is greatly rotated and shifted on binding to dT(6–4)T, leading to the formation of an electrostatic interaction with the phosphate moiety of dT(6–4)T, which shows a remarkable induced fit. Based on a comparison of the dT(6–4)T-liganded structures of the 64M-5 and 64M-2 Fabs, the electrostatic interaction between the side chain of His93L in 64M-5 and the phosphate moiety of dT(6–4)T is lost for Leu93L in 64M-2, while Arg90L in 64M-5 instead of Gln90L in 64M-2 stabilizes the conformation of complementarity-determining region (CDR) L3. These differences contribute to the higher affinity of 64M-5 for dT(6–4)T compared with that of 64M-2.

Small RNA-mediated repair of UV-induced DNA lesions by the DNA DAMAGE-BINDING PROTEIN 2 and ARGONAUTE 1

As photosynthetic organisms, plants need to prevent irreversible UV-induced DNA lesions. Through an unbiased, genome-wide approach, we have uncovered a previously unrecognized interplay between Global Genome Repair and small interfering RNAs (siRNAs) in the recognition of DNA photoproducts, prevalently in intergenic regions. Genetic and biochemical approaches indicate that, upon UV irradiation, the DNA DAMAGE-BINDING PROTEIN 2 (DDB2) and ARGONAUTE 1 (AGO1) of Arabidopsis thaliana form a chromatin-bound complex together with 21-nt siRNAs, which likely facilitates recognition of DNA damages in an RNA/DNA complementary strand-specific manner. The biogenesis of photoproduct-associated siRNAs involves the noncanonical, concerted action of RNA POLYMERASE IV, RNA-DEPENDENT RNA POLYMERASE-2, and DICER-LIKE-4. Furthermore, the chromatin association/dissociation of the DDB2-AGO1 complex is under the control of siRNA abundance and DNA damage signaling. These findings reveal unexpected nuclear functions for DCL4 and AGO1, and shed light on the interplay between small RNAs and DNA repair recognition factors at damaged sites.

Dewar valence isomers, the third type of environmentally relevant DNA photoproducts induced by solar radiation

Dewar valence isomers are mutagenic DNA lesions produced by photoisomerisation of (6-4) photoproducts. Their efficient formation upon exposure to a combination of UVB and UVA makes them biologically and environmentally relevant DNA damage.