Low incidence of Ha-ras oncogene mutations in human epidermal tumors

Src is activated by the nuclear receptor peroxisome proliferator-activated receptor β/δ in ultraviolet radiation-induced skin cancer

Although non-melanoma skin cancer (NMSC) is the most common human cancer and its incidence continues to rise worldwide, the mechanisms underlying its development remain incompletely understood. Here, we unveil a cascade of events involving peroxisome proliferator-activated receptor (PPAR) β/δ and the oncogene Src, which promotes the development of ultraviolet (UV)-induced skin cancer in mice. UV-induced PPARβ/δ activity, which directly stimulated Src expression, increased Src kinase activity and enhanced the EGFR/Erk1/2 signalling pathway, resulting in increased epithelial-to-mesenchymal transition (EMT) marker expression. Consistent with these observations, PPARβ/δ-null mice developed fewer and smaller skin tumours, and a PPARβ/δ antagonist prevented UV-dependent Src stimulation. Furthermore, the expression of PPARβ/δ positively correlated with the expression of SRC and EMT markers in human skin squamous cell carcinoma (SCC), and critically, linear models applied to several human epithelial cancers revealed an interaction between PPARβ/δ and SRC and TGFβ1 transcriptional levels. Taken together, these observations motivate the future evaluation of PPARβ/δ modulators to attenuate the development of several epithelial cancers.

Low incidence of EGFR and HRAS mutations in cutaneous squamous cell carcinomas of a German cohort

Epidermal growth factor receptor (EGFR) is highly expressed in squamous cell carcinoma (SCC). The response of patients with lung cancer to EGFR inhibitors is significantly associated with the presence of EGFR mutations. Although these drugs have already been used for the treatment of advanced cutaneous SCC, the knowledge about EGFR mutations in this cancer is limited to one previous study in the US population. We analysed the presence of EGFR and concomitant HRAS mutations in a German cohort of 31 patients with cutaneous SCC by direct sequencing of EGFR and SNaPshot analysis of concomitant RAS mutations. We found a low prevalence of EGFR mutations (1/31; 3%) and HRAS mutations (1/31; 3%). The detected P741L EGFR mutation was proven to be somatic. Our results indicate that both EGFR and HRAS mutations are rare events in the carcinogenesis of cutaneous SCC, and therefore, only a small subgroup of patients will benefit from the screening for EGFR mutations in the run-up to targeted therapies with EGFR inhibitors.

Viral DNA detection and RAS mutations in actinic keratosis and nonmelanoma skin cancers

BACKGROUND

Actinic keratosis (AK) is a well-established precancerous skin lesion that has the potential to progress to squamous cell carcinoma (SCC). Basal cell carcinoma (BCC) is a locally aggressive slowly growing tumour that rarely metastasizes. A number of viruses have been proposed to play a role in the development of nonmelanoma skin cancers (NMSC), but the most plausible evidence to date suggests that cutaneous human papillomavirus (HPV) is the key instigating factor.

OBJECTIVES

To evaluate the prevalence of HPV, cytomegalovirus (CMV), herpes simplex virus (HSV) and Epstein-Barr virus (EBV) and investigate their relationship with the presence of RAS gene mutations in cutaneous lesions obtained from nonimmunosuppressed patients.

METHODS

HPV, CMV, HSV and EBV detection was performed using polymerase chain reaction (PCR) in skin biopsies (26 AK, 12 SCC and 15 BCC samples) that were collected from immunocompetent patients. The RAS mutation incidence was also investigated in all cutaneous lesions by use of PCR/restriction fragment length polymorphism and direct DNA sequencing.

RESULTS

Seventeen out of 53 (32%) skin lesions were found to be positive for HPV DNA. The highest incidences of HPV infection were five of 15 (33%) in BCC and four of 12 (33%) in SCC specimens. The HPV incidence was eight of 26 (31%) in AK and eight of 53 (15%) in normal skin tissue. Twelve out of 53 (23%) skin lesions were CMV-positive. The highest incidence of CMV infection was six of 15 (40%), observed in BCC specimens. The CMV incidence was two of 26 (8%) in AK and four of 12 (33%) in SCC. No normal skin biopsy was found to be positive for CMV. All cutaneous samples were negative for HSV and EBV DNA, as assessed by our PCR-based assays. Only three samples, one AK (4%), one BCC (6%) and one SCC (8%), were found to carry a G>T transversion at the second position of HRAS codon 12. Both HRAS mutant SCC and BCC biopsies were HPV- and CMV-positive, as well.

CONCLUSIONS

HPV DNA is detected in NMSC, AK and normal skin biopsies. Our results also indicate that CMV is involved in NMSC at higher levels than in premalignant lesions, whereas the virus was not detected in normal skin biopsies. HSV and EBV do not appear to be involved in the pathogenesis of cutaneous lesions. Moreover, we suggest that the HRAS codon 12 mutation is not a very common event in AK or NMSC. Finally, both viral infection and HRAS activation appear to represent independent factors in the aetiology of NMSC, samples of which were obtained from immunocompetent patients.

Fos cooperation with PTEN loss elicits keratoacanthoma not carcinoma, owing to p53/p21WAF-induced differentiation triggered by GSK3β inactivation and reduced AKT activity

To investigate gene synergism in multistage skin carcinogenesis, the RU486-inducible cre/lox system was employed to ablate Pten function (K14.cre/Δ5Ptenflx) in mouse epidermis expressing activated Fos (HK1.Fos). RU486-treated HK1.Fos/Δ5Ptenflx mice exhibited hyperplasia, hyperkeratosis and tumours that progressed to highly differentiated keratoacanthomas, rather than to carcinomas, owing to re-expression of high p53 and p21WAF levels. Despite elevated MAP kinase activity, cyclin D1 and cyclin E2 overexpression, and increased AKT activity that produced areas of highly proliferative papillomatous keratinocytes, increasing levels of GSK3β inactivation induced a novel p53/p21WAF expression profile, which subsequently halted proliferation and accelerated differentiation to give the hallmark keratosis of keratoacanthomas. A pivotal facet to this GSK3β-triggered mechanism centred on increasing p53 expression in basal layer keratinocytes. This increase in expression reduced activated AKT expression and released inhibition of p21WAF, which accelerated keratinocyte differentiation, as indicated by unique basal layer expression of differentiation-specific keratin K1 alongside premature filaggrin and loricrin expression. Thus, Fos synergism with Pten loss elicited a benign tumour context where GSK3β-induced p53/p21WAF expression continually switched AKT-associated proliferation into differentiation, preventing further progression. This putative compensatory mechanism required the critical availability of normal p53 and/or p21WAF, otherwise deregulated Fos, Akt and Gsk3β associate with malignant progression.

Low prevalence of p53, p16(INK4a) and Ha-ras tumour-specific mutations in low-graded actinic keratosis

BACKGROUND

Ultraviolet radiation induces DNA damage and is the major risk factor for the development of non-melanoma skin cancer (NMSC). Different mutation rates of p53, p16(INK4a) and Ha-ras in cutaneous squamous cell carcinoma (SCC) and the earlier stage actinic keratosis (AK) have been reported.

OBJECTIVES

To assess the presence of missense mutations in hotspot exons of p53, p16(INK4a) and Ha-ras in low-graded AK.

PATIENTS/METHODS

Cryo-biopsies of 75 sun-exposed AK lesions and 75 sun-shielded areas of normal skin from 75 AK patients were analysed to identify mutations in p53 (exons 7 and 8), p16(INK4a) (exon 2) and Ha-ras (exon 1) using polymerase chain reaction (PCR) followed by direct sequencing. As a representative subset of the specimens, ten mutation-negative AK were also micro-dissected in order to exclude the possibility that additional mutations were undetected.

RESULTS

Eight missense and one nonsense point mutations were found in the 75 AK lesions examined (12%), of which seven (9%) were tumour-specific (i.e. present in AK lesions only) and two (3%) were p16(INK4a) mutations (i.e. also detected in normal skin). Three of the tumour-specific mutations (42%) were cytosine (C) to thymine (T) transitions at pyrimidine-rich sequences. Tumour-specific mutations were identified in 1% of p16(INK4a) (exon 2), 1% of Ha-ras (exon 1) and at a higher rate of 7% in p53 (exons 7 and 8), including one nonsense mutation.

CONCLUSIONS

The evaluation of a large number of AK specimens in this study have found a low gene mutation rate in low-graded AK lesions. p53 mutations rather than p16(INK4a) and/or Ha-ras mutations may be an early event in the development of AK to cutaneous SCC.

RasGRP1 overexpression in the epidermis of transgenic mice contributes to tumor progression during multistage skin carcinogenesis

RasGRP1 is a guanine nucleotide exchange factor for Ras, activated in response to the second messenger diacylglycerol and its ultrapotent analogues, the phorbol esters. We have previously shown that RasGRP1 is expressed in mouse epidermal keratinocytes and that transgenic mice overexpressing RasGRP1 in the epidermis under the keratin 5 promoter (K5.RasGRP1) are prone to developing spontaneous papillomas and squamous cell carcinomas, suggesting a role for RasGRP1 in skin tumorigenesis. Here, we examined the response of the K5.RasGRP1 mice to multistage skin carcinogenesis, using 7,12-dimethylbenz(a)anthracene as carcinogen and 12-O-tetradecanoylphorbol-13-acetate (TPA) as tumor promoter. We found that whereas tumor multiplicity did not differ between transgenic and wild-type groups, the transgenic tumors were significantly larger than those observed in the wild-type mice (wild-type, 4.58 +/- 0.25 mm; transgenic, 9.83 +/- 1.05 mm). Histologic analysis further revealed that squamous cell carcinomas generated in the transgenic mice were less differentiated and more invasive than the wild-type tumors. Additionally, 30% of the transgenic mice developed tumors in the absence of initiation, suggesting that RasGRP1 overexpression could partially substitute for the initiation step induced by dimethylbenz(a)anthracene. In primary keratinocytes isolated from K5.RasGRP1 mice, TPA stimulation induced higher levels of Ras activation compared with the levels measured in the wild-type cells, indicating that constitutive overexpression of RasGRP1 in epidermal cells leads to elevated biochemical activation of endogenous Ras in response to TPA. The present data suggests that RasGRP1 participates in skin carcinogenesis via biochemical activation of endogenous wild-type Ras and predisposes to malignant progression in cooperation with Ras oncogenic signals.

Non-melanoma skin cancer: what drives tumor development and progression?

Non-melanoma skin cancer, i.e. basal cell carcinoma (BCC) and squamous cell carcinoma (SCC) are the most frequent tumors and their number is still increasing worldwide. Furthermore, immunosuppression in organ transplant patients strongly contributes to the increase in skin cancer incidence--being 65-250 times more frequent than in the general population. Often these patients suffer from a second and third lesion and the severity of these tumors is linked to their number. SCCs in transplant recipients also appear to be more aggressive. They tend to grow rapidly, show a higher rate of local recurrences and metastasize in 5-8% of the patients (all reviewed in Ref. 2). This largely differs from BCCs which are more frequent in the general population--at a ratio of 4:1 as compared with SCCs--but the number is only increased by a factor of 10 in transplant recipients. This may suggest that 'dormant' SCC precursor cells/lesions are present at a high frequency in the population but they are well controlled by the immune system. BCC, on the other hand, may be less dependent on immune surveillance thereby underlining its different etiology. While for BCC development the genetic hallmark is abrogation of the ptch-sonic hedgehog pathway, little is known about the causal alterations of SCCs. However, the complexity of the genetic alterations (numerical and structural aberration profiles) in SCCs argues for several levels of genomic instability involved in the generation and progression of skin cancer.

Basal cell carcinoma: a dermatopathological and molecular biological update

The ideal classification of basal cell carcinoma (BCC) should be able to identify subtypes which correlate with clinical behaviour and treatment requirements. Unfortunately, however, such a classification has yet to be defined. In the interim, the currently most favoured classification is one based predominantly on histological growth pattern. This classification contributes to the useful concept of low- and high-risk histological subtypes of BCC. The latter are characterized by an increased probability of subclinical extension and/or incomplete excision and/or aggressive local invasive behaviour and/or local recurrence. The Royal College of Pathologists has published a minimum dataset for the histopathological reporting of BCC and this has been written to be compatible with the British Association of Dermatologists' management guidelines. Growth patterns to be reported include nodular, superficial, infiltrative/morphoeic and micronodular types, together with differentiation when of severely atypical or malignant squamous type (basosquamous carcinoma). Deep and peripheral excision margins will be reported to be either involved or clear. The latter will include a comment of a clearance of less than 1 mm for close margins and a measured distance in whole millimetres for other excisions. Clinical assessment and histology remain the 'gold standard' for evaluating BCC and cancers in general. However, in the postgenomic era emphasis is changing from the gathering and archiving of genomic data to its analysis and use in guiding clinical practice. In this context, a current goal is to define cancer phenotype in terms of molecular abnormalities and use this as a new gold standard. One way to assess whether this goal is being achieved for BCC is to determine whether our knowledge of its molecular pathology has any relevance to the minimum dataset for histological reporting. Knowledge of BCC molecular pathology has been fuelled by the recent discovery that deregulation of the Hedgehog (Hh) signalling pathway, a key player in embryonic patterning, appears to be fundamental to tumour growth. But despite accrual of a large amount of data concerning Hh pathway molecular alterations in neoplasia, little is known about the functional consequences of these changes in BCC, how they lead to tumour development, or how they relate to non-Hh pathway alterations such as TP53 mutation. Recent work suggests that the cellular localization of beta-catenin gives a degree of credence to the growth pattern classification of BCC. Furthermore, it is possible that beta-catenin may have a pathogenetic role in the invasive behaviour of BCC. This review draws on current evidence to discuss these issues and assess whether they are relevant to the minimum dataset.

Comparative analysis of ras proto-oncogene mutations in selected mammalian tumors

Point mutations within ras proto-oncogenes are frequently detected in human malignancies and in different types of experimentally induced tumors in animals. In contrast to findings in experimental animal models of carcinogenesis, little is known about the incidence of ras mutations in naturally occurring animal tumors. In the present study, we investigated whether point mutations, particularly within the mutational hot-spot codons 12, 13, and 61, occur at comparable frequencies in human malignancies and spontaneously occurring tumors in other mammalian species. Two hundred seventy-nine of the most frequent canine and feline neoplasms were analyzed for changes in mutational hot-spot regions of the N-, Ki-, and Ha-ras genes. DNA fragments from exons 1 and 2 of all three ras genes were amplified by polymerase chain reaction, and the presence of point mutations was assessed by single-strand conformation polymorphism analysis and direct sequencing of amplified products. Only one sample, a case of canine melanoma, exhibited an Ha-ras mutation. Thus, our data strongly suggested that ras mutations at the hot-spot loci are apparently very rare and do not play a major role in the pathogenesis of the spontaneously occurring canine and feline tumors investigated. These observations were in marked contrast to those in experimental rodent models of carcinogen-induced mammary and skin tumors that described a consistent association with Ha- or Ki-ras activation. The role of ras oncogene activation in related human malignancies therefore cannot be readily inferred from studies of experimental carcinogenesis in animal models.

Developmental genes and cancer: role of patched in basal cell carcinoma of the skin

Many genes originally identified because of their role in embryonic development are also important in postnatal control of cell growth and differentiation. Mutations in some of these genes have been shown to cause cancer. Basal cell carcinoma (BCC) of the skin is the most common cancer in humans. More than 750000 new cases are diagnosed annually, and the incidence is rising. BCCs are slow-growing, locally invasive tumors that rarely metastasize but can result in extensive morbidity through local recurrence and tissue destruction. Epidemiologic studies suggest that sunlight (particularly UVB radiation) is a strong risk factor for BCC formation, although other factors are also involved. The nevoid basal cell carcinoma syndrome (NBCCS), a rare genetic disorder, is characterized by predisposition to BCCs and other tumors as well as to a wide range of developmental defects. NBCCS maps to chromosome 9q22.3, and loss of heterozygosity at this site in both sporadic and hereditary BCCs suggests that it functions as a tumor suppressor. The gene for NBCCS was recently cloned and is the human homologue of the Drosophila gene "patched." Genetic studies in Drosophila show that patched is part of the hedgehog signaling pathway, which is important in determining embryonic patterning and cell fate in multiple structures of the developing embryo. Human patched is mutated in both hereditary and sporadic BCCs, and inactivation of this gene is probably a necessary, if not sufficient, step for BCC formation. Delineation of the biochemical pathway in which patched functions may lead to rational medical therapy for BCCs and possibly for other tumors associated with NBCCS.

Laboratory probing of oncogenes from human liquid and solid specimens as markers of exposure to toxicants

Recent discoveries regarding the mechanistic role of oncogenes and tumor suppressor genes in cancer development have opened a new era of molecular diagnosis. It has been observed repeatedly that genetic lesions serve as tumor markers in a broad variety of human cancers. The ras gene family, consisting of three related genes, H-ras, K-ras, and N-ras, acquires transforming activity through amplification or mutation in many tissues. If not all, then most types of human malignancies have been found to contain an altered ras gene. Because the ras oncogenes actively participate in both early and intermediate stages of cancer, several highly specific and sensitive approaches have been introduced to detect these genetic alterations as biomarkers of exposure to carcinogens. There is also mounting evidence that implicate chemical-specific alterations of the p53 tumor suppressor gene detected in most human tumors. Therefore, it seems a reliable laboratory approach to identify both altered p53 and ras genes as biomarkers of human chronic or intermittent exposure to toxicants in a variety of occupational settings.

Differential allele loss on chromosome 9q22.3 in human non-melanoma skin cancer

Familial predisposition to basal cell carcinoma (BCC) and squamous cell carcinoma (SCC) of the skin are apparent in the autosomal dominant syndromes naevoid basal cell carcinoma syndrome (NBCCS) and multiple self-healing squamous epitheliomata (MSSE) respectively. The gene responsible for NBCCS has been proposed to be a tumour-suppressor gene and is mapped to the same 2 Mb interval on 9q22.3 as the MSSE gene ESS1. In an attempt to further map the NBCCS gene, we have examined loss of heterozygosity (LOH) in 16 sporadic BCCs and two familial BCCs using microsatellite markers located within the candidate gene region. The overall frequency of LOH observed was 67% in the BCCs and partial or interstitial deletions were found in eight tumours, with the highest LOH frequency at markers D9S280, D9S287 and D9S180. To determine if the same genomic region also shows frequent LOH in tumours with a squamous phenotype, we have examined 11 SCCs, four actinic keratoses and 13 cases of Bowen's disease for LOH at 9q22.3. An overall LOH frequency of 50% was observed at D9S180, and occurred in all types of squamous tumours. In contrast, a much lower LOH frequency of only 6% was found at the D9S287 locus. Our observation of different patterns of LOH at 9q22.3 in sporadic BCCs and SCCs implies that more than one tumour-suppressor gene might be located in this genomic region.

Mutations in ras oncogenes: rare events in ultraviolet B radiation-induced mouse skin tumorigenesis

The activation of ras proto-oncogenes by point mutation in a broad spectrum of clinical malignancies and experimentally induced tumors suggests their critical role in cancer induction. To determine whether the activation of ras proto-oncogenes by point mutation also contributes to ultraviolet B radiation (UVB)-induced skin tumorigenesis and whether this event is responsible for the different tumorigenic potentials of UVB radiation in different mouse strains, we analyzed the skin tumors induced by UVB in SKH-1 hairless and C3H mice for specific mutations in the Ha-, Ki-, and N-ras oncogenes. With the same UVB irradiation protocol, the latency period for tumor appearance was longer in C3H mice than in SKH-1 hairless mice. In addition, tumor incidence and multiplicity were also significantly higher (P<0.001, chi square and Wilcoxon rank sum tests) in SKH-1 hairless mice compared with C3H mice. None of the 30 skin tumor specimens (15 from each mouse strain) analyzed by polymerase chain reaction (PCR) amplification of specific codons followed by dot-blot hybridization with specific probes contained mutation in codons 13 of Ha-ras; 12, 13, and 61 of Ki-ras; or 12 and 13 of N-ras. However, three of the 15 tumors in SKH-1 hairless mice showed either a G35-->A or G35-->T transition at second position of Ha-ras codon 12. Interestingly, one of these tumors (with a G35-->A transition) also harbored an A182-->G mutation at second position of Ha-ras codon 61. None of the tumors from C3H mice showed mutations in codons 12 or 61 of the Ha-ras oncogene. With regard to codon 61 of the N-ras oncogene, six tumors from SKH-1 hairless mice and 10 tumors from C3H mice showed an A183-->T transversion. While G35-->A or G35-->T transition detected by PCR and dot-blot hybridization was confirmed by sequencing, the mutations identified similarly at codon 61 in either the Ha- or N-ras oncogenes could not be verified by sequencing of PCR-amplified products subcloned into plasmid vectors. With the exception of the low incidence of Ha-ras oncogene mutations at codon 12 in SKH-1 hairless mouse skin tumors induced by UVB, the striking absence of mutations in the Ha-, Ki-, and N-ras oncogenes in UVB-induced mouse skin tumors suggests that ras oncogene mutations are rare and thus are not an initiating event in photocarcinogenesis.

Analysis of p53 gene mutations and loss of heterozygosity for loci on chromosome 9q in basal cell carcinoma

To determine the role of the p53 gene in the pathogenesis of basal cell carcinoma (BCC), we screened mutations of the gene in 11 cases of BCCs using the polymerase chain reaction (PCR) and single-stranded conformation polymorphism (SSCP). However, in all the coding exons of the gene analysed, no evidence suggesting the mutations were obtained. On the other hand, in 2 of 5 informative cases of our BCCs (40%) we found loss of heterozygosity (LOH) for loci on chromosome 9q31 which is linked to the Gorlin syndrome, that predisposes to BCC. Therefore, we suggest that a putative tumor suppressor gene on the region of 9q, but not p53 gene, plays a critical role in the pathogenesis of BCC, independent of race.

7,12-Dimethylbenz[a]anthracene-induced mouse keratinocyte malignant transformation independent of Harvey ras activation

Independent clones of mouse keratinocytes initiated in vitro gave rise to tumor phenotypes typical of mouse skin multistage carcinogenesis and histologically similar to human tumors of the skin, and head and neck. High-molecular-weight genomic DNAs isolated from two 7,12-dimethylbenz[a]anthracene (DMBA)-initiated murine epithelial carcinoma cell lines and one papilloma cell line were examined for transforming activity by transfection into NIH3T3 cells. DNAs from each of these cell lines resulted in the formation of foci morphologically unlike foci containing an activated c-Ha-ras oncogene. Following polymerase chain reaction amplification of the c-Ha-ras gene, Xba I restriction analysis and oligonucleotide differential hybridization did not detect 61st, 12th, or 13th codon mutations. Southern and Northern analysis confirmed that the normal c-Ha-ras gene was not activated by amplification or overexpression. These results provide evidence that 7,12-dimethylbenz[a]anthracene-induced malignant transformation of murine keratinocytes occurred independent of point mutations associated with c-Ha-ras activation. The absence of an activated c-Ha-ras oncogene in these cell lines distinguishes our model from other mouse models of carcinogenesis and may provide a model for functional genetic changes during initiation and progression of human epithelial cancers.