Optically Guided High-Frequency Ultrasound to Differentiate High-Risk Basal Cell Carcinoma Subtypes: A Single-Centre Prospective Study.

BACKGROUND: Basal cell carcinoma (BCC) is the most common type of skin cancer in the Caucasian population. Currently, invasive biopsy is the only way of establishing the histological subtype (HST) that determines the treatment options. Our study aimed to evaluate whether optically guided high-frequency ultrasound (OG-HFUS) imaging could differentiate aggressive HST BCCs from low-risk tumors. METHODS: We conducted prospective clinical and dermoscopic examinations of BCCs, followed by 33 MHz OG-HFUS imaging, surgical excision, and a histological analysis. We enrolled 75 patients with 78 BCCs. In total, 63 BCCs were utilized to establish a novel OG-HFUS risk classification algorithm, while 15 were employed for the validation of this algorithm. The mean age of the patients was 72.9 ± 11.2 years. Histology identified 16 lesions as aggressive HST (infiltrative or micronodular subtypes) and 47 as low-risk HST (superficial or nodular subtypes). To assess the data, we used a one-sided Fisher's exact test for a categorical analysis and a Receiver Operating Characteristic (ROC) curve analysis to evaluate the diagnostic accuracy. RESULTS: OG-HFUS distinguished aggressive BCC HSTs by their irregular shape (p < 0.0001), ill-defined margins (p < 0.0001), and non-homogeneous internal echoes (p = 0.004). We developed a risk-categorizing algorithm that differentiated aggressive HSTs from low-risk HSTs with a higher sensitivity (82.4%) and specificity (91.3%) than a combined macroscopic and dermoscopic evaluation (sensitivity: 40.1% and specificity: 73.1%). The positive and negative predictive values (PPV and NPV, respectively) for dermoscopy were 30.2% and 76.8%, respectively. In comparison, the OG-HFUS-based algorithm demonstrated a PPV of 94.7% and an NPV of 78.6%. We verified the algorithm using an independent image set, n = 15, including 12 low-risk and 3 high-risk (high-risk) with two blinded evaluators, where we found a sensitivity of 83.33% and specificity of 91.66%. CONCLUSIONS: Our study shows that OG-HFUS can identify aggressive BCC HSTs based on easily identifiable morphological parameters, supporting early therapeutic decision making.

Real-World Experience with Cemiplimab Treatment for Advanced Cutaneous Squamous Cell Carcinoma-A Retrospective Single-Center Study.

BACKGROUND: The systemic treatment of advanced cutaneous squamous cell carcinoma (cSCC) has seen significant developments in recent years. The anti-PD1 inhibitor cemiplimab has demonstrated efficacy in clinical trials, but real-world data are still limited. Here, we aimed to evaluate the efficacy and the safety of cemiplimab in a real-world clinical setting. METHODS: A retrospective analysis was carried out for all patients who received at least two doses of cemiplimab at our department between February 2020 and January 2023. Progression-free survival (PFS), overall survival (OS), the objective response rate (ORR), the disease control rate (DCR) and adverse events (AEs) were evaluated. RESULTS: Twenty-five patients were included with a median age of 78 (65-82) years. The median treatment duration was 48 (16-72) weeks. Five (20%) patients were immunocompromised. Sixteen patients (64%) developed AEs, including 36% serious AEs (SAEs) of grade ≥ 3. Six patients (24%) were withdrawn from treatment due to the occurrence of AEs. Among the 25 patients, 52% showed an objective response (3 complete and 10 partial responses), 76% had controlled disease and 24% experienced progression. Among the five immunocompromised patients, the ORR was 60%, while the DCR was 80%. CONCLUSIONS: This retrospective real-world study revealed that locally advanced or metastatic cSCC could be effectively treated with cemiplimab even in elderly, polymorbid and immunocompromised patients.

Correlation of systemic involvement and presence of pathological skin calcification assessed by ex vivo nonlinear microscopy in Pseudoxanthoma elasticum.

Pseudoxanthoma elasticum (PXE (OMIM 264800)) is an autosomal recessive connective tissue disorder mainly caused by mutations in the ABCC6 gene. PXE results in ectopic calcification primarily in the skin, eye and blood vessels that can lead to blindness, peripheral arterial disease and stroke. Previous studies found correlation between macroscopic skin involvement and severe ophthalmological and cardiovascular complications. This study aimed to investigate correlation between skin calcification and systemic involvement in PXE. Ex vivo nonlinear microscopy (NLM) imaging was performed on formalin fixed, deparaffinized, unstained skin sections to assess the extent of skin calcification. The area affected by calcification (CA) in the dermis and density of calcification (CD) was calculated. From CA and CD, calcification score (CS) was determined. The number of affected typical and nontypical skin sites were counted. Phenodex + scores were determined. The relationship between the ophthalmological, cerebro- and cardiovascular and other systemic complications and CA, CD and CS, respectively, and skin involvement were analyzed. Regression models were built for adjustment to age and sex. We found significant correlation of CA with the number of affected typical skin sites (r = 0.48), the Phenodex + score (r = 0.435), extent of vessel involvement (V-score) (r = 0.434) and disease duration (r = 0.48). CD correlated significantly with V-score (r = 0.539). CA was significantly higher in patients with more severe eye (p = 0.04) and vascular (p = 0.005) complications. We found significantly higher CD in patients with higher V-score (p = 0.018), and with internal carotid artery hypoplasia (p = 0.045). Significant correlation was found between higher CA and the presence of macula atrophy (ß = - 0.44, p = 0.032) and acneiform skin changes (ß = 0.40, p = 0.047). Based on our results, the assessment of skin calcification pattern with nonlinear microscopy in PXE may be useful for clinicians to identify PXE patients who develop severe systemic complications.

Optically Guided High-Frequency Ultrasound Shows Superior Efficacy for Preoperative Estimation of Breslow Thickness in Comparison with Multispectral Imaging: A Single-Center Prospective Validation Study.

Melanoma is the most aggressive form of skin cancer that is known for its metastatic potential and has an increasing incidence worldwide. Breslow thickness, which determines the staging and surgical margin of the tumor, is unavailable at initial diagnosis. Novel imaging techniques for assessing Breslow thickness lack comparative data. This study evaluates optically guided high-frequency ultrasound (OG-HFUS) and multispectral imaging (MSI) for preoperative estimation of Breslow thickness and staging. We enrolled 101 patients with histologically confirmed primary melanoma and categorized them based on tumor thickness. Optically guided 33 MHz HFUS and MSI were utilized for the assessment. Our MSI-based algorithm categorized melanomas into three subgroups with a sensitivity of 62.6%, specificity of 81.3%, and fair agreement (κ = 0.440, CI: 0.298-0.583). In contrast, OG-HFUS demonstrated a sensitivity of 91.8%, specificity of 96.0%, and almost perfect agreement (κ = 0.858, CI: 0.763-0.952). OG-HFUS performed better than MSI in estimating Breslow thickness, emphasizing its potential as a valuable tool for melanoma diagnosis and patient management. OG-HFUS holds promise for enhancing preoperative staging and treatment decision-making in melanoma.

Quantitative Multispectral Imaging Differentiates Melanoma from Seborrheic Keratosis.

Melanoma is a melanocytic tumor that is responsible for the most skin cancer-related deaths. By contrast, seborrheic keratosis (SK) is a very common benign lesion with a clinical picture that may resemble melanoma. We used a multispectral imaging device to distinguish these two entities, with the use of autofluorescence imaging with 405 nm and diffuse reflectance imaging with 525 and 660 narrow-band LED illumination. We analyzed intensity descriptors of the acquired images. These included ratios of intensity values of different channels, standard deviation and minimum/maximum values of intensity of the lesions. The pattern of the lesions was also assessed with the use of particle analysis. We found significantly higher intensity values in SKs compared with melanoma, especially with the use of the autofluorescence channel. Moreover, we found a significantly higher number of particles with high fluorescence in SKs. We created a parameter, the SK index, using these values to differentiate melanoma from SK with a sensitivity of 91.9% and specificity of 57.0%. In conclusion, this imaging technique is potentially applicable to distinguish melanoma from SK based on the analysis of various quantitative parameters. For this application, multispectral imaging could be used as a screening tool by general physicians and non-experts in the everyday practice.

Autofluorescence Imaging of the Skin Is an Objective Non-Invasive Technique for Diagnosing Pseudoxanthoma Elasticum.

Pseudoxanthoma elasticum (PXE) is a rare multisystemic autosomal recessive connective tissue disease. In most cases, skin manifestations of PXE are the first to develop, followed later by severe ocular and cardiovascular complications. In our present study, in addition to dermoscopy, we introduced novel techniques, autofluorescence (AF) and diffuse reflectance (DR) imaging for the assessment of affected skin sites of five PXE patients. PXE-affected skin areas in most skin sites showed a previously observed pattern upon dermoscopic examination. With the novel imaging, PXE-affected skin lesions displayed high AF intensity. During our measurements, significantly higher mean, minimum and maximum AF intensity values were found in areas of PXE-affected skin when compared to uninvolved skin. Conversely, images acquired with the use of 660 and 940 nm illumination showed no mentionable difference. Our results demonstrate that AF imaging may be used in the in vivo diagnostics and quantification of the severity of the skin lesions of PXE patients. In addition, it is a safe, fast and cost-effective diagnostic method. AF imaging may be also used to objectively monitor the efficacy of the possible novel therapeutic approaches of PXE in the future.

Multispectral Imaging Algorithm Predicts Breslow Thickness of Melanoma.

Breslow thickness is a major prognostic factor for melanoma. It is based on histopathological evaluation, and thus it is not available to aid clinical decision making at the time of the initial melanoma diagnosis. In this work, we assessed the efficacy of multispectral imaging (MSI) to predict Breslow thickness and developed a classification algorithm to determine optimal safety margins of the melanoma excision. First, we excluded nevi from the analysis with a novel quantitative parameter. Parameter s' could differentiate nevi from melanomas with a sensitivity of 89.60% and specificity of 88.11%. Following this step, we have categorized melanomas into three different subgroups based on Breslow thickness (≤1 mm, 1-2 mm and >2 mm) with a sensitivity of 78.00% and specificity of 89.00% and a substantial agreement (κ = 0.67; 95% CI, 0.58-0.76). We compared our results to the performance of dermatologists and dermatology residents who assessed dermoscopic and clinical images of these melanomas, and reached a sensitivity of 60.38% and specificity of 80.86% with a moderate agreement (κ = 0.41; 95% CI, 0.39-0.43). Based on our findings, this novel method may help predict the appropriate safety margins for curative melanoma excision.

Emergency dermatology: Three-month experience from an Italian academic outpatient clinic during lockdown for COVID-19 pandemic.

Dermatology is a field of medicine where urgent cases occur commonly. However, access to specialized emergency dermatology services is very limited. Following the declaration of the COVID-19 pandemic, the cessation of all elective dermatology visits was widely urged. Accordingly, in Italy, a country severely affected by the pandemic, various measures were applied and the care at university clinics was limited to urgent cases. Here we retrospectively analyzed data of patients who presented at an Italian academic outpatient clinic reserved only for emergency cases. In total, 252 patients (109 males and 143 females) with a mean age of 55.25 ± 20.99 years were cared for at our clinic during a three-month period. We classified 10 patients (4%) as real emergency cases. Pityriasis rosea was diagnosed in three patients. Many patients sought care for skin cancer screening. In 131 patients (52%) dermoscopic skin examinations were performed. In 39 patients (15%), actinic keratosis or nonmelanoma skin cancer was detected, while melanoma was diagnosed in three patients, two of which were proven later as in situ melanoma. About 111 patients (44%) visited our clinic for other, nonurgent skin diseases. Our results imply that many patients felt that their skin problems required immediate attention, even if this could not be justified. Melanoma care may be considered an emergency care for its highly malignant potential and the possibility of rapid spreading. Adequately taken photos with a dermoscope may be readily read without the presence of specialist in the emergency room to prevent unnecessary delay in diagnosing oncologic skin diseases.

Nonlinear optical microscopy is a novel tool for the analysis of cutaneous alterations in pseudoxanthoma elasticum.

Pseudoxanthoma elasticum (PXE, OMIM 264800) is a rare autosomal recessive disorder with ectopic mineralization and fragmentation of elastin fibers. It is caused by mutations of the ABCC6 gene that leads to decreased serum levels of inorganic pyrophosphate (PPi) anti-mineralization factor. The occurrence of severe complications among PXE patients highlights the importance of early diagnosis so that prompt multidisciplinary care can be provided to patients. We aimed to examine dermal connective tissue with nonlinear optical (NLO) techniques, as collagen emits second-harmonic generation (SHG) signal, while elastin can be excited by two-photon excitation fluorescence (TPF). We performed molecular genetic analysis, ophthalmological and cardiovascular assessment, plasma PPi measurement, conventional histopathological examination, and ex vivo SHG and TPF imaging in five patients with PXE and five age- and gender-matched healthy controls. Pathological mutations including one new variant were found in the ABCC6 gene in all PXE patients and their plasma PPi level was significantly lower compared with controls. Degradation and mineralization of elastin fibers and extensive calcium deposition in the mid-dermis was visualized and quantified together with the alterations of the collagen structure in PXE. Our data suggests that NLO provides high-resolution imaging of the specific histopathological features of PXE-affected skin. In vivo NLO may be a promising tool in the assessment of PXE, promoting early diagnosis and follow-up.

Voluntary exercise improves murine dermal connective tissue status in high-fat diet-induced obesity.

Obesity is a risk factor for several cardiovascular and metabolic diseases. Its influence on the skin is less obvious, yet certain negative effects of adipose tissue inflammation on the dermis have been suggested. Excess weight is closely associated with sedentary behavior, so any increase in physical activity is considered beneficial against obesity. To investigate the effects of obesity and physical exercise on the skin, we established a mouse model in which mice were kept either on a high-fat diet or received standard chow. After the two groups achieved a significant weight difference, physical exercise was introduced to both. Animals were given the opportunity to perform voluntary exercise for 40 min daily in a hamster wheel for a period of 8 weeks. We evaluated the status of the dermis at the beginning and at the end of the exercise period by in vivo nonlinear microscopy. Obese mice kept on high-fat diet lost weight steadily after they started to exercise. In the high-fat diet group, we could detect significantly larger adipocytes and a thicker layer of subcutaneous tissue; both changes started to normalize after exercise. Nonlinear microscopy revealed an impaired collagen structure in obese mice that improved considerably after physical activity was introduced. With the ability to detect damage on collagen structure, we set out to address the question whether this process is reversible. With the use of a novel imaging method, we were able to show the reversibility of connective tissue deterioration as a benefit of physical exercise.

Sodium ascorbate kills Candida albicans in vitro via iron-catalyzed Fenton reaction: importance of oxygenation and metabolism.

AIM: Ascorbate can inhibit growth and even decrease viability of various microbial species including Candida albicans. However the optimum conditions and the mechanism of action are unclear. Materials/methodology: Candida albicans shaken for 90 min in a buffered solution of ascorbate (90 mM) gave a 5-log reduction of cell viability, while there was no killing without shaking, in growth media with different carbon sources or at 4°C. Killing was inhibited by the iron chelator 2,2'-bipyridyl. Hydroxyphenyl fluorescein probe showed the intracellular generation of hydroxyl radicals. RESULTS/CONCLUSION: Ascorbate-mediated killing of C. albicans depends on oxygenation and metabolism, involves iron-catalyzed generation of hydroxyl radicals via Fenton reaction and depletion of intracellular NADH. Ascorbate could serve as a component of a topical antifungal therapy.

Diet-induced obesity skin changes monitored by in vivo SHG and ex vivo CARS microscopy.

Obesity related metabolic syndrome and type 2 diabetes have severe consequences on our skin. Latest developments in nonlinear microscopy allow the use of noninvasive, label free imaging methods, such as second harmonic generation (SHG) and coherent anti-Stokes Raman scattering (CARS), for early diagnosis of metabolic syndrome-related skin complications by 3D imaging of the skin and the connective tissue. Our aim was to study effects of various types of diet-induced obesity in mice using these methods. We examined mice on different diets for 32 weeks. The collagen morphology was evaluated four times in vivo by SHG microscopy, and adipocytes were examined once at the end of experiment by ex vivo CARS method. A strong correlation was found between the body weight and the adipocyte size, while we found that the SHG intensity of dermal collagen reduces considerably with increasing body weight. Obese mice on high-fat diet showed worse results than those on high-fat - high-fructose diet. Animals on high-fructose diet did not gain more weight than those on ordinary diet despite of the increased calorie intake, but their collagen damage was nonetheless significant. Obesity and high sugar intake damages the skin, mainly the dermal connective tissue and subcutaneous adipose tissue, which efficiently can be monitored by in vivo SHG and ex vivo CARS microscopy.

Photosensitivity of murine skin greatly depends on the genetic background: clinically relevant dose as a new measure to replace minimal erythema dose in mouse studies.

Artificial UV irradiation of murine skin is a frequently used method for testing photosensitivity, study carcinogenesis and photoprotective effects of different compounds. However, doses of UV radiation and mouse strains used in experiments vary greatly. The genetic background of mice may influence the photosensitivity as melanin content, pigmentation and hair cycle parameters are dissimilar. Doses of UV are often expressed in relation to the minimal erythema dose (MED) that was not necessarily determined for the given strain. We set out to standardize the method of measuring photosensitivity in three commonly used mouse strains, C57BL/6N, Balb/c and SKH-1. We found that MED may not be determined for some strains as erythema development in mice with diverse genotypes differs greatly. We measured the oedema response in vivo and ex vivo by using OCT. Given the strain-specific variability of erythema, we introduced Clinically Relevant Dose (CRD) as a new term to replace MED in experiments, to describe the lowest dose that triggers a perceptible skin reaction in mice. Not only the CRD but the proportion of erythema and oedema were different in strains examined. C57BL/6N mice display skin reactions at the lowest UVB dose, while SKH-1 hairless mice show changes, mostly oedema, after higher doses of UVB. The cellular composition and skin thickness were examined by histopathology. IL-1beta and IL-6 levels in skin correlated with the increasing doses of UVB. Despite the variations in the degree of erythema and oedema, no major differences in cytokine expressions were seen among various strains of mice.

In vivo second-harmonic generation and ex vivo coherent anti-stokes raman scattering microscopy to study the effect of obesity to fibroblast cell function using an Yb-fiber laser-based CARS extension unit.

Nonlinear microscopy techniques are being increasingly used to perform in vivo studies in dermatology. These methods enable us to investigate the morphology and monitor the physiological process in the skin by the use of femtosecond lasers operating in the red, near-infrared spectral range (680-1,300 nm). In this work we used two different techniques that require no labeling: second harmonic generation (SHG) for collagen detection and coherent anti-Stokes Raman scattering (CARS) to assess lipid distribution in genetically obese murine skin. Obesity is one of the most serious public health problems due to its high and increasing prevalence and the associated risk of type 2 diabetes and cardiovascular diseases. Other than these diseases, nearly half of patients with diabetes mellitus suffer from dermatological complications such as delayed wound healing, foot ulcers and several other skin changes. In our experiment we investigated and followed the effects of obesity on dermal collagen alterations and adipocyte enlargement using a technique not reported in the literature so far. Our results indicate that the in vivo SHG and ex vivo CARS imaging technique might be an important tool for diagnosis of diabetes-related skin disorders in the near future.

Whole genome transcriptional profiling identifies novel differentiation regulated genes in keratinocytes.

Keratinocyte differentiation plays a pivotal role in the epidermal barrier. Single keratinocyte differentiation genes have already been studied, but many important constituents of this process may have been missed so far. Gene expression profiling by microarray was carried out in cultured normal human epidermal keratinocytes undergoing confluence-induced differentiation to find novel differentiation genes. Candidate gene lists were established and genes of potential dermatological interest were validated by quantitative reverse transcription polymerase chain reaction and immunohistochemical analysis. Some of these points lead to the identification of counter-regulation of heme oxygenase and biliverdin reductase as well as glutaredoxin and glutathione reductase indicative of potential novel redox signaling in differentiating human keratinocytes. Others indicate a strong concert down-regulation of interleukin-1 signaling at previously unidentified levels during keratinocyte differentiation. We believe that identified genes contribute to a more comprehensive understanding of the complicated epidermal differentiation process and lead to better understanding of dermatological diseases.

Novel sphingolipid derivatives promote keratinocyte differentiation.

Sphingolipids are important components of the water permeability barrier of the skin. Moreover, ceramides were also shown to influence keratinocyte differentiation and regulate cellular signalling. A confluence-induced differentiation model of normal human keratinocytes was established to allow evaluation of pro- and anti-differentiation effects of exogenous compounds. The effects of phytosphingosine (PS), sphingosine (SO), sphinganine (SA) and their hexanoyl (-C6), stearoyl (-C18) and salicyl (-SLC) derivatives, C12-alkylamine-salicylate (C12-SLC), salicylate (SLC) along with vitamin D3 (VD3) and retinol as control substances were tested in this system. Cytotoxicity assays were carried out to optimize the incubation conditions of compounds and whole genome expression changes were monitored by DNA-microarray on days 0, 1 and 4. Geometric means of gene expression levels of a subset of known keratinocyte differentiation-related genes were calculated from the microarray data to compare effects of the sphingolipid derivatives. Compound treatment-induced transcriptional changes were analysed by the ExPlain software (BIOBASE GmbH). Five of the assayed substances (SA, SO-C6, PS-C6, SO-SLC, PS-SLC) were found to be potent promoters of keratinocyte differentiation compared with VD3, and C12-SLC revealed potential anti-differentiation properties. ExPlain analysis found a different regulatory profile in the computed transcriptional networks of the sphingoid bases versus their -C6 and especially -SLC derivatives suggesting that the change in their keratinocyte differentiation modifying potential is due to a unique effect of the covalent attachment of the salicylic acid. Taken together, these results demonstrate the gene regulatory potential of sphingolipid species that could be valuable for dermatological or cosmetic applications.

UVB induces a biphasic response of HIF-1alpha in cultured human keratinocytes.

Hypoxia in the skin is important in chronic degenerative dermo-epidermal changes, inflammation, photoageing and carcinogenesis. In these processes, vascular endothelial growth factor (VEGF) plays a crucial role and is known to be affected by ultraviolet radiation (UVR). Hypoxia-inducible factor-1 (HIF-1) closely regulates the expression of VEGF in several experimental settings. We set out to study the impact of acute UVB irradiation on the level of HIF-1 as a major regulator of hypoxia-induced genes. Effects of UVB exposure on HIF-1alpha expression were investigated in HaCaT cells after a single irradiation by Western blots. Downstream target gene expression was measured by quantitative real-time polymerace chair reaction (PCR). UVB treatment resulted in an initial decrease of the HIF-1alpha protein level followed by a subsequent prolonged increase. If cells were exposed to additional UVB irradiation, another decrease in HIF-1alpha was provoked, similar to the original effect. The observed changes followed a strict timeline and were dose-dependent. The role of the PI3K/AKT pathway was examined. No change in the total level of AKT after UVB treatment was seen; however, its phosphorylation level was found to be markedly higher. In accordance with these observations, wortmannin, an inhibitor of PI3-kinase effectively blocked the UVB-induced increase in HIF-1alpha. In agreement with previous findings, UVB irradiation increased VEGF and haem oxygenase-1 mRNA levels determined by quantitative real-time PCR. It is concluded that changes in HIF-1alpha expression underlie the alterations in expression of VEGF upon UVB irradiation. Our findings indicate the involvement of PI3K in UVB-mediated HIF-1alpha upregulation.

Antigen-specific immunity does not mediate acute regression of UVB-induced p53-mutant clones.

Chronic irradiation of human or murine epidermis with ultraviolet B (UVB) induces clones of p53-mutant keratinocytes. Clones precede and parallel the induction of carcinomas, suggesting that they are an early stage of UVB carcinogenesis. In the absence of UVB, these clones rapidly regress. For UVB-induced murine skin tumors and papillomas, regression is known to involve antigen-specific immunity. To determine whether antigen-specific immunity influences the creation, expansion, or regression of p53-mutant clones, we studied Rag1 knockout mice deficient in the recombination activating gene 1 required for development of B, alphabetaT, gammadeltaT, and natural killer T cells. Since tissue homeostasis could affect proliferation or persistence of clones, we also examined the effect of Rag1 on UVB-induced hyperplasia and apoptosis. Mice were irradiated with UVB daily for 7-11 weeks to create p53-mutant clones, and then retained in the absence of UV. After UV ended, epidermal thickness decreased and p53-mutant clones observed in the epidermal sheets regressed, with no significant differences between Rag1(-/-) and wild type. During the initial chronic UVB irradiation, increasing irradiation time increased both the number and size of p53-mutant clones, with no significant difference between genotypes. We conclude that antigen-specific immunity is not involved in the initiation, expansion, or acute regression of p53-mutant clones.

Inactivating E2f1 reverts apoptosis resistance and cancer sensitivity in Trp53-deficient mice.

The E2f1 transcription factor, which regulates genes required for S-phase entry, also induces apoptosis by transcriptional and post-translational mechanisms. As E2f1 is inducible by DNA damage we investigated its importance in vivo in ultraviolet (UV)-induced apoptosis, a protective mechanism that prevents the epidermis from accumulating UV-induced mutations. Contrary to expectation, E2f1-/- mice demonstrated enhanced keratinocyte apoptosis after UVB exposure, whereas apoptosis was suppressed by epidermis-specific overexpression of human E2F1. Apoptosis induced by -radiation was also repressed by E2f1. E2f1-/-;Trp53-/- double knockout mice exhibited the elevated UVB-induced apoptosis of E2f1-/- alone, rather than the profound apoptosis defect seen in Trp53-/- mice, indicating that Trp53 (p53) lies functionally upstream of E2f1. Transfecting E2F1 into E2f1-/-;Trp53-/- primary fibroblasts suppressed UVB-induced apoptosis and this suppression was relieved by Trp53. The double knockout also reverted the abnormal sex ratio and early-onset tumours of Trp53-/- mice. These results imply that E2f1 functions as a suppressor of an apoptosis pathway that is initiated by DNA photoproducts and perhaps genetic abnormalities; p53 relieves this suppression.