Structure and in vivo psoralen DNA crosslink repair activity of mycobacterial Nei2.

Mycobacterium smegmatis Nei2 is a monomeric enzyme with AP ß-lyase activity on single-stranded DNA. Expression of Nei2, and its operonic neighbor Lhr (a tetrameric 3'-to-5' helicase), is induced in mycobacteria exposed to DNA damaging agents. Here, we find that nei2 deletion sensitizes M. smegmatis to killing by DNA inter-strand crosslinker trimethylpsoralen but not to crosslinkers mitomycin C and cisplatin. By contrast, deletion of lhr sensitizes to killing by all three crosslinking agents. We report a 1.45 Å crystal structure of recombinant Nei2, which is composed of N and C terminal lobes flanking a central groove suitable for DNA binding. The C lobe includes a tetracysteine zinc complex. Mutational analysis identifies the N-terminal proline residue (Pro2 of the ORF) and Lys51, but not Glu3, as essential for AP lyase activity. We find that Nei2 has 5-hydroxyuracil glycosylase activity on single-stranded DNA that is effaced by alanine mutations of Glu3 and Lys51 but not Pro2. Testing complementation of psoralen sensitivity by expression of wild-type and mutant nei2 alleles in ∆nei2 cells established that AP lyase activity is neither sufficient nor essential for crosslink repair. By contrast, complementation of psoralen sensitivity of ∆lhr cells by mutant lhr alleles depended on Lhr's ATPase/helicase activities and its tetrameric quaternary structure. The lhr-nei2 operon comprises a unique bacterial system to rectify inter-strand crosslinks.IMPORTANCEThe DNA inter-strand crosslinking agents mitomycin C, cisplatin, and psoralen-UVA are used clinically for the treatment of cancers and skin diseases; they have been invaluable in elucidating the pathways of inter-strand crosslink repair in eukaryal systems. Whereas DNA crosslinkers are known to trigger a DNA damage response in bacteria, the roster of bacterial crosslink repair factors is incomplete and likely to vary among taxa. This study implicates the DNA damage-inducible mycobacterial lhr-nei2 gene operon in protecting Mycobacterium smegmatis from killing by inter-strand crosslinkers. Whereas interdicting the activity of the Lhr helicase sensitizes mycobacteria to mitomycin C, cisplatin, and psoralen-UVA, the Nei2 glycosylase functions uniquely in evasion of damage caused by psoralen-UVA.

Clarithromycin attenuates airway epithelial-mesenchymal transition in ovalbumin-induced asthmatic mice through modulation of Kv1.3 channels and PI3K/Akt signaling.

Airway epithelial-mesenchymal transition (EMT) is the important pathological feature of airway remodeling in asthma. While macrolides are not commonly used to treat asthma, they have been shown to have protective effects on the airways, in which mechanisms are not yet fully understood. This study aims to investigate the impact of clarithromycin on airway EMT in asthma and its potential mechanism. The results revealed an increase in Kv1.3 expression in the airways of ovalbumin (OVA)-induced asthmatic mice, with symptoms and pathological changes being alleviated after treatment with the Kv1.3 inhibitor 5-(4-phenoxybutoxy)psoralen (PAP-1). Clarithromycin was found to attenuate airway epithelial-mesenchymal transition through the inhibition of Kv1.3 and PI3K/Akt signaling. Further experiments in vitro confirmed that PAP-1 could mitigate EMT by modulating the PI3K/Akt signaling in airway epithelial cells undergoing transformation into mesenchymal cells. These findings confirmed that clarithromycin might have a certain protective effect on asthma-related airway remodeling and represent a promising treatment strategy.

Psoralen Isolated from the Roots of Dorstenia psilurus Welw. Modulate Th1/Th2 Cytokines and Inflammatory Enzymes in LPS-Stimulated RAW 264.7 Macrophages.

Dorstenia psilurus is a widely used plant spice in traditional African medicine to treat pain-related conditions. However, the anti-inflammatory mechanisms underlying this activity and the main active ingredients of D. psilurus have not yet been fully characterized. This study aimed to isolate and identify the main active anti-inflammatory constituents of the D. psilurus extract and to investigate the underlying anti-inflammatory mechanisms in murine macrophages. Chromatographic techniques and spectroscopic data were used for compound isolation and structure elucidation. The Griess reagent method and the ferrous oxidation-xylenol orange assay were used to evaluate the inhibition of NO production and 15-lipoxygenase activity, respectively. Cyclooxygenase activity was assessed using the fluorometric COX activity assay kit, and Th1/Th2 cytokine measurement was performed using a flow cytometer. The results indicated that the extract and fractions of D. psilurus inhibit NO production and proliferation of RAW 264.7 macrophage cells. Bioguided fractionation led to the identification of psoralen, a furocoumarin, as the main bioactive anti-inflammatory compound. Psoralen inhibited NO production and 15-lipoxygenase activity and reduced pro-inflammatory Th1 cytokines (IFN-γ, TNF-α, and IL-2) while increasing the secretion of anti-inflammatory cytokines (IL-4, IL-6, and IL-10) in activated RAW 264.7 macrophage cells. The encouraging results obtained in this study suggest that psoralen-based multiple modulation strategies could be a useful approach to address the treatment of inflammatory diseases.

In Vivo Genomic Supercoiling Mapping Using Psora-seq.

Supercoiling is a fundamental property of DNA that governs all strand opening reactions, including DNA replication, transcription, and homologous recombination. However, traditional genomic supercoiling assays are difficult and lack sensitivity. Building on prior assays using the DNA intercalator psoralen, we developed a supercoil mapping assay that is robust and sensitive to a wide range of supercoiling while requiring only commercially available reagents and common laboratory equipment. This method, psoralen affinity purification with genomic sequencing (Psora-seq), utilizes biotinylated psoralen and streptavidin-conjugated magnetic beads to facilitate efficient pull-down of psoralen-bound DNA, followed by deep sequencing to identify and quantify supercoiling at 1 kb resolution. Psora-seq overcomes two major bottlenecks associated with existing psoralen pull-down assays, inefficient photo-binding of psoralen-bound molecules, and poor recovery of cross-linked DNA.

The effects of CYP2B6 inactivators on the metabolism of ciprofol.

Ciprofol is a novel short-acting intravenous anaesthetic developed in China that is mainly metabolized by cytochrome P450 2B6 (CYP2B6) and uridine diphosphate glucuronosyltransferase 1A9 (UGT1A9). Currently, insufficient evidence is available to support drug‒drug interactions between ciprofol and CYP2B6 inactivators. Here, we established a high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) method to assess the concentration of ciprofol and investigated the effects of psoralen and clopidogrel on the metabolism of ciprofol in liver microsomes and rats. In rat and human liver microsomes, the median inhibitory concentration (IC50) values of psoralen were 63.31 µmol·L-1 and 34.05 µmol·L-1, respectively, showing mild inhibitory effects on ciprofol metabolism, whereas the IC50 values of clopidogrel were 6.380 µmol·L-1 and 2.565 µmol·L-1, respectively, with moderate inhibitory effects. SD rats were randomly divided into three groups: psoralen (27 mg·kg-1), clopidogrel (7.5 mg·kg-1), and the same volume of 0.5% carboxy methyl cellulose. After 7 days, all rats were injected with 2.4 mg·kg-1 ciprofol. Compared with the control group, the AUC and MRT values of ciprofol in the psoralen and clopidogrel groups were significantly greater, whereas the CL values were significantly lower. In addition, the durations of loss of righting reflex (LORR) in the psoralen and clopidogrel groups were 16.1% and 23.0% longer than that in the control group, respectively. In conclusion, psoralen and clopidogrel inhibit ciprofol metabolism to different degrees and prolong the duration of LORR in rats.

The efficacy of long-term psoralen plus ultraviolet A and low-dose interferon-a combination therapy in mycosis fungoides: A literature review.

BACKGROUND/PURPOSE: Interferon (IFN)-a is often used in combination with psoralen plus ultraviolet A (PUVA) in patients with mycosis fungoides (MF) refractory to skin-targeted therapies in early or advanced stages. The main objective is to evaluate the effectiveness of combined PUVA and low-dose IFN-α-2a therapy in patients with early- and advanced-stage MF. METHODS: Sixty-eight patients who received a combination of PUVA twice or thrice a week and INF-a 3 MU thrice a week for at least 3 months were reviewed retrospectively. The treatment response was evaluated as complete remission (CR), partial remission, stable disease, or progression. RESULTS: At the initiation, the majority of patients (66.2%) had early-stage disease. In 27.9% of cases, this was the initial treatment administered following the diagnosis of MF. The median duration of combination therapy was 11 months. Complete remission was achieved in 45.6% of the patients with an overall response rate of 60.3%. The mean duration of response was 5 months. Complete remission was statistically significantly higher in early-stage patients (p < .05). No statistically significant correlation was observed between CR and gender, histopathological features, or laboratory parameters. In patients with CR, 80% experienced relapse, significantly higher in early-stage patients (p < .05). However, there was no significant difference in disease-free survival between early and advanced stages (p > .05). CONCLUSIONS: The study results indicated that PUVA + low-dose INF-a combination therapy was more effective in the early stage than in the advanced stage. Additionally, there was a high relapse rate after the cessation of treatment in patients who achieved CR.

Psoralen protects neurons and alleviates neuroinflammation by regulating microglial M1/M2 polarization via inhibition of the Fyn-PKCδ pathway.

Microglia-mediated neuroinflammation is closely associated with many neurodegenerative diseases. Psoralen has potential for the treatment of many diseases, however, the anti-neuroinflammatory and neuroprotective effects of psoralen have been unclear. This study investigated the anti-neuroinflammatory and neuroprotective effects of psoralen and its regulation of microglial M1/M2 polarization. The LPS-induced mice model was used to test anti-neuroinflammatory effects, regulatory effects on microglia polarization, and neuroprotective effects of psoralen in vivo. The LPS-induced BV2 model was used to test the anti-neuroinflammatory effects and the regulatory effects and mechanisms on microglial M1/M2 polarization of psoralen in vitro. PC12 cell model induced by conditioned medium of BV2 cells was used to validate the protective effects of psoralen against neuroinflammation-induced neuronal damage. These results showed that psoralen inhibited the expression of iNOS, CD86, and TNF-α, and increased the expression of Arg-1, CD206, and IL-10. These results indicated that psoralen inhibited the M1 microglial phenotype and promoted the M2 microglial phenotype. Further studies showed that psoralen inhibited the phosphorylation of Fyn and PKCδ, thereby inhibiting activation of the MAPKs and NF-κB pathways and suppressing the expression of pro-inflammatory cytokines in microglia. Furthermore, psoralen reduced oxidative stress, neuronal damage, and apoptosis via inhibition of neuroinflammation. For the first time, this study showed that psoralen protected neurons and alleviated neuroinflammation by regulating microglial M1/M2 polarization, which may be mediated by inhibition of the Fyn-PKCδ pathway. Thus, psoralen may be a potential agent in the treatment of neuroinflammation-related diseases.

KCNE4-dependent modulation of Kv1.3 pharmacology.

The voltage-dependent potassium channel Kv1.3 is a promising therapeutic target for the treatment of autoimmune and chronic inflammatory disorders. Kv1.3 blockers are effective in treating multiple sclerosis (fampridine) and psoriasis (dalazatide). However, most Kv1.3 pharmacological antagonists are not specific enough, triggering potential side effects and limiting their therapeutic use. Functional Kv are oligomeric complexes in which the presence of ancillary subunits shapes their function and pharmacology. In leukocytes, Kv1.3 associates with KCNE4, which reduces the surface abundance and enhances the inactivation of the channel. This mechanism exerts profound consequences on Kv1.3-related physiological responses. Because KCNE peptides alter the pharmacology of Kv channels, we studied the effects of KCNE4 on Kv1.3 pharmacology to gain insights into pharmacological approaches. To that end, we used margatoxin, which binds the channel pore from the extracellular space, and Psora-4, which blocks the channel from the intracellular side. While KCNE4 apparently did not alter the affinity of either margatoxin or Psora-4, it slowed the inhibition kinetics of the latter in a stoichiometry-dependent manner. The results suggested changes in the Kv1.3 architecture in the presence of KCNE4. The data indicated that while the outer part of the channel mouth remains unaffected, KCNE4 disturbs the intracellular architecture of the complex. Various leukocyte types expressing different Kv1.3/KCNE4 configurations participate in the immune response. Our data provide evidence that the presence of these variable architectures, which affect both the structure of the complex and their pharmacology, should be considered when developing putative therapeutic approaches.

Functionalizing DNA Origami by Triplex-Directed Site-Specific Photo-Cross-Linking.

Here, we present a cross-linking approach to covalently functionalize and stabilize DNA origami structures in a one-pot reaction. Our strategy involves adding nucleotide sequences to adjacent staple strands, so that, upon assembly of the origami structure, the extensions form short hairpin duplexes targetable by psoralen-labeled triplex-forming oligonucleotides bearing other functional groups (pso-TFOs). Subsequent irradiation with UVA light generates psoralen adducts with one or both hairpin staples leading to site-specific attachment of the pso-TFO (and attached group) to the origami with ca. 80% efficiency. Bis-adduct formation between strands in proximal hairpins further tethers the TFO to the structure and generates "superstaples" that improve the structural integrity of the functionalized complex. We show that directing cross-linking to regions outside of the origami core dramatically reduces sensitivity of the structures to thermal denaturation and disassembly by T7 RNA polymerase. We also show that the underlying duplex regions of the origami core are digested by DNase I and thus remain accessible to read-out by DNA-binding proteins. Our strategy is scalable and cost-effective, as it works with existing DNA origami structures, does not require scaffold redesign, and can be achieved with just one psoralen-modified oligonucleotide.

Colon-targeted oral nanoliposomes loaded with psoralen alleviate DSS-induced ulcerative colitis.

Oral administration, while convenient, but complex often faces challenges due to the complexity of the digestive environment. In this study, we developed a nanoliposome (NLP) encapsulating psoralen (P) and coated it with chitosan (CH) and pectin (PT) to formulate PT/CH-P-NLPs. PT/CH-P-NLPs exhibit good biocompatibility, superior to liposomes loaded with psoralen and free psoralen alone. After oral administration, PT/CH-P-NLPs remain stable in the stomach and small intestine, followed by a burst release of psoralen after reaching the slightly alkaline and gut microbiota-rich colon segment. In the DSS-induced ulcerative colitis of mice, PT/CH-P-NLPs showed significant effects on reducing inflammation, mitigating oxidative stress, protecting the integrity of the colon mucosal barrier, and modulating the gut microbiota. In conclusion, the designed nanoliposomes demonstrated the effective application of psoralen in treating ulcerative colitis.

Thio and Seleno-Psoralens as Efficient Triplet Harvesting Photosensitizers for Photodynamic Therapy.

The Psoralen (Pso) molecule finds extensive applications in photo-chemotherapy, courtesy of its triplet state forming ability. Sulfur and selenium replacement of exocyclic carbonyl oxygen of organic chromophores foster efficient triplet harvesting with near unity triplet quantum yield. These triplet-forming photosensitizers are useful in Photodynamic Therapy (PDT) applications for selective apoptosis of cancer cells. In this work, we have critically assessed the effect of the sulfur and selenium substitution at the exocyclic carbonyl (TPso and SePso, respectively) and endocyclic oxygen positions of Psoralen. It resulted in a significant redshifted absorption spectrum to access the PDT therapeutic window with increased oscillator strength. The reduction in singlet-triplet energy gap and enhancement in the spin-orbit coupling values increase the number of intersystem crossing (ISC) pathways to the triplet manifold, which shortens the ISC lifetime from 10-5 s for Pso to 10-8 s for TPso and 10-9 s for SePso. The intramolecular photo-induced electron transfer process, a competitive pathway to ISC, is also considerably curbed by exocyclic functionalizations. In addition, a maximum of 115 GM of two-photon absorption (2PA) with IR absorption (660-1050 nm) confirms that the Psoralen skeleton can be effectively tweaked via single chalcogen atom replacement to design a suitable PDT photosensitizer.

Psoralen and Isopsoralen, Two Estrogen-Like Natural Products from Psoraleae Fructus, Induced Cholestasis via Activation of ERK1/2.

With the increasing use of oral contraceptives and estrogen replacement therapy, the incidence of estrogen-induced cholestasis (EC) has tended to rise. Psoralen (P) and isopsoralen (IP) are the major bioactive components in Psoraleae Fructus, and their estrogen-like activities have already been recognized. Recent studies have also reported that ERK1/2 plays a critical role in EC in mice. This study aimed to investigate whether P and IP induce EC and reveal specific mechanisms. It was found that P and IP increased the expression of esr1, cyp19a1b and the levels of E2 and VTG at 80 µM in zebrafish larvae. Exemestane (Exe), an aromatase antagonist, blocked estrogen-like activities of P and IP. At the same time, P and IP induced cholestatic hepatotoxicity in zebrafish larvae with increasing liver fluorescence areas and bile flow inhibition rates. Further mechanistic analysis revealed that P and IP significantly decreased the expression of bile acids (BAs) synthesis genes cyp7a1 and cyp8b1, BAs transport genes abcb11b and slc10a1, and BAs receptor genes nr1h4 and nr0b2a. In addition, P and IP caused EC by increasing the level of phosphorylation of ERK1/2. The ERK1/2 antagonists GDC0994 and Exe both showed significant rescue effects in terms of cholestatic liver injury. In conclusion, we comprehensively studied the specific mechanisms of P- and IP-induced EC and speculated that ERK1/2 may represent an important therapeutic target for EC induced by phytoestrogens.

[Pharmacokinetics of 11 active components of Psoraleae Fructus in normal and diabetic rats].

A total of 11 active ingredients including psoralen, isopsoralen, bakuchiol, bavachalcone, bavachinin, corylin, coryfolin, isobavachalcone, neobavaisoflavone, bakuchalcone, and corylifol A from Psoraleae Fructus in the plasma samples of diabetic and normal rats were simultaneously determined by UHPLC-MS/MS. The pharmacokinetic parameters were calculated to elucidate the pharmacokinetic profiles of coumarins, flavonoids, and monoterpene phenols in normal and diabetic rats. The rat model of type 2 diabetes mellitus(T2DM) was induced by a high-sugar and high-fat diet combined with injection of 1% streptozotocin every two days. The plasma samples were collected at different time points after the rats were administrated with Psoraleae Fructus. The proteins in the plasma samples were precipitated by ethyl acetate, and the plasma concentrations of the 11 components of Psoraleae Fructus were determined by UHPLC-MS/MS. The pharmacokinetic parameters were calculated by DAS 3.0. The results showed that the pharmacokinetic beha-viors of 8 components including psoralen, isopsoralen, bakuchiol, and bavachinin from Psoraleae Fructus in both female and male mo-del rats were significantly different from those in normal rats. Among them, the coumarins including psoralen, isopsoralen, and corylin showed lowered levels in the blood of both female and male model rats. The flavonoids(bavachinin, corylifol A, and bakuchalcone) and the monoterpene phenol bakuchiol showed decreased levels in the female model rats but elevated levels in the male model rats. It is suggested that the dosage of Psoraleae Fructus should be reasonably adjusted for the patients of different genders at the time of clinical administration.

Spectroscopic view on the interaction between the psoralen derivative amotosalen and DNA.

Psoralens are eponymous for PUVA (psoralen plus UV-A radiation) therapy, which inter alia can be used to treat various skin diseases. Based on the same underlying mechanism of action, the synthetic psoralen amotosalen (AMO) is utilized in the pathogen reduction technology of the INTERCEPT® Blood System to inactivate pathogens in plasma and platelet components. The photophysical behavior of AMO in the absence of DNA is remarkably similar to that of the recently studied psoralen 4'-aminomethyl-4,5',8-trimethylpsoralen (AMT). By means of steady-state and time-resolved spectroscopy, intercalation and photochemistry of AMO and synthetic DNA were studied. AMO intercalates with a higher affinity into A,T-only DNA (KD = 8.9 × 10-5 M) than into G,C-only DNA (KD = 6.9 × 10-4 M). AMO covalently photobinds to A,T-only DNA with a reaction quantum yield of ΦR = 0.11. Like AMT, it does not photoreact following intercalation into G,C-only DNA. Femto- and nanosecond transient absorption spectroscopy reveals the characteristic pattern of photobinding to A,T-only DNA. For AMO and G,C-only DNA, signatures of a photoinduced electron transfer are recorded.

Psoralen: a narrative review of current and future therapeutic uses.

Psoralen is a family of naturally occurring photoactive compounds found in plants that acquire potential cytotoxicity when activated by specific frequencies of electromagnetic waves. Psoralens penetrate the phospholipid cellular membranes and insert themselves between the pyrimidines of deoxyribonucleic acid (DNA). Psoralens are initially biologically inert and acquire photoreactivity when exposed to certain classes of electromagnetic radiation, such as ultraviolet light. Once activated, psoralens form mono- and di-adducts with DNA, leading to marked cell apoptosis. This apoptotic effect is more pronounced in tumor cells due to their high rate of cell division. Moreover, photoactivated psoralen can inhibit tyrosine kinase signaling and influence the immunogenic properties of cells. Thus, the cytotoxicity of photoactivated psoralen holds promising clinical applications from its immunogenic properties to potential anti-cancer treatments. This narrative review aims to provide an overview of the current understanding and research on psoralen and to explore its potential future pharmacotherapeutic benefits in specific diseases.

Discovery of psoralen as a quorum sensing inhibitor suppresses Pseudomonas aeruginosa virulence.

Pseudomonas aeruginosa is a common opportunistic pathogen with growing resistance and presents heightened treatment challenges. Quorum sensing (QS) is a cell-to-cell communication system that contributes to the production of a variety of virulence factors and is also related to biofilm formation of P. aeruginosa. Compared to traditional antibiotics which kill bacteria directly, the anti-virulence strategy by targeting QS is a promising strategy for combating pseudomonal infections. In this study, the QS inhibition potential of the compounds derived from the Traditional Chinese Medicines was evaluated by using in silico, in vitro, and in vivo analyses. The results showed that psoralen, a natural furocoumarin compound derived from Psoralea corylifolia L., was capable of simultaneously inhibiting the three main QS regulators, LasR, RhlR, and PqsR of P. aeruginosa. Psoralen had no bactericidal activity but could widely inhibit the production of extracellular proteases, pyocyanin, and biofilm, and the cell motilities of the model and clinical P. aeruginosa strains. RNA-sequencing and quantitative PCR analyses further demonstrated that a majority of QS-activated genes in P. aeruginosa were suppressed by psoralen. The supplementation of psoralen could protect Caenorhabditis elegans from P. aeruginosa challenge, especially for the hypervirulent strain PA14. Moreover, psoralen showed synergistic antibacterial effects with polymyxin B, levofloxacin, and kanamycin. In conclusions, this study identifies the anti-QS and antibiofilm effects of psoralen against P. aeruginosa strains and sheds light on the discovery of anti-pseudomonal drugs among Traditional Chinese Medicines. KEY POINTS: • Psoralen derived from Psoralea corylifolia L. inhibits the virulence-related phenotypes of P. aeruginosa. • Psoralen simultaneously targets the three core regulators of P. aeruginosa QS system and inhibits the expression of a large part of downstream genes. • Psoralen protects C. elegans from P. aeruginosa challenge and enhances the susceptibility of P. aeruginosa to antibiotics.

Synthesis and Antibacterial Evaluation of Novel Psoralen Derivatives against Methicillin-Resistant Staphylococcus aureus (MRSA).

Today, the bacterial infections caused by multidrug-resistant pathogens seriously threaten human health. Thereby, there is an urgent need to discover antibacterial drugs with novel mechanism. Here, novel psoralen derivatives had been designed and synthesized by a scaffold hopping strategy. Among these targeted twenty-five compounds, compound ZM631 showed the best antibacterial activity against methicillin-resistant S. aureus (MRSA) with the low MIC of 1 µg/mL which is 2-fold more active than that of the positive drug gepotidacin. Molecular docking study revealed that compound ZM631 fitted well in the active pockets of bacterial S. aureus DNA gyrase and formed a key hydrogen bond binding with the residue ASP-1083. These findings demonstrated that the psoralen scaffold could serve as an antibacterial lead compound for further drug development against multidrug-resistant bacterial infections.

Effects of psoriasis and psoralen exposure on the somatic mutation landscape of the skin.

Somatic mutations are hypothesized to play a role in many non-neoplastic diseases. We performed whole-exome sequencing of 1,182 microbiopsies dissected from lesional and nonlesional epidermis from 111 patients with psoriasis to search for evidence that somatic mutations in keratinocytes may influence the disease process. Lesional skin remained highly polyclonal, showing no evidence of large-scale spread of clones carrying potentially pathogenic mutations. The mutation rate of keratinocytes was similarly only modestly affected by the disease. We found evidence of positive selection in previously reported driver genes NOTCH1, NOTCH2, TP53, FAT1 and PPM1D and also identified mutations in four genes (GXYLT1, CHEK2, ZFP36L2 and EEF1A1) that we hypothesize are selected for in squamous epithelium irrespective of disease status. Finally, we describe a mutational signature of psoralens-a class of chemicals previously found in some sunscreens and which are used as part of PUVA (psoralens and ultraviolet-A) photochemotherapy treatment for psoriasis.