The risk for cutaneous squamous cell carcinomas (SCCs) among epidermolysis bullosa (EB)EB subtypes seems to reflect the severity of the disease itself. Therefore, cSCC occurs very rarely among EB simplex (EBS)S patients (2.6%) ^[1][5], and according to some authors, EBS is not actually related to any higher risk compared to the general population ^[2][6]. On the contrary, higher percentages are indeed registered among KS, JEB, DDEB and RDEB patients. KS and DDEB seem to display a similar risk for cSCC, with a prevalence of 6% for both subtypes ^[1][5]. JEB and RDEB, on the other hand, are the subtypes more frequently related to the development of cSCC, with variable percentages ranging from 6.8% to 16.2% and from 35.4% to 70%, respectively ^[1][3][4][5,7,8].

Unlike the general population, cSCC in EB usually displays a peculiar aggressive clinical behavior ^[5][1]. It generally occurs at younger ages, with a mean age of onset ranging from 32.8 to 36 years old among all EB subtypes, with even earlier peaks of incidence among RDEB patients (29.5 years old) ^[1][3][5,7].

Among all subtypes, RDEB is associated not only with the highest incidence of cSCC and the youngest onset, but also with the worst prognosis ^[6][9]. The cumulative risk for cSCC in RDEB patients is indeed dramatically high, with possible differences according to patients’ age (7.5% by the age of 20 and over 90.1% by the age of 55) ^[7][10] and disease severity (10% by the age of 35 among RDEB patients with and intermediate generalized disease and at least 76.1% among RDEB patients with a severe generalized form) ^[4][8]. Therefore, the cumulative risk of death from cSCC in RDEB patients seems as well to reflect patients’ age, as it is 57.2% by the age of 35 and 87.3% by the age of 45 ^[7][10].

2. Clinical Presentation

In EB patients, cSCC preferably occurs on chronic non-healing ulcers located on bony prominences ^[4][5][8][1,8,11]. Therefore, early diagnosis of SCC can be very difficult since it can present similarly to typical chronic ulceration with scarring and crusting; similarly to burn scar tumors, SCCs in EB patients usually start as an ulcer margin ^[1][5][8][1,5,11]. Among all EB subtypes, limbs are the most commonly affected sites, with a consistent involvement of both lower (54.7%) and upper (30.8%) extremities which becomes prominent in RDEB, accounting for the 91.3–95% of cases ^[1][3][9][5,7,12]. On the other hand, mucosal SCC rarely occurs in EB, with a mean incidence of 8.6% among all variants ^[1][5]. At first diagnosis, ulceration is the most common clinical feature among all EB subtypes (44.9%), with a macroscopic diameter more frequently larger than 2 cm (59.1%) in all groups ^[1][5]. RDEB patients demonstrate similar percentages with ulceration and diameter larger than 2 cm in 30.4% of cases ^[9][12]. Due to its clinical aggressive behavior, in EB patients, cSCC usually presents with multifocal lesions at diagnosis (63.6%) with an average of three tumors per patient in RDEB and two tumors per patient in both JEB and DDEB ^[3][7].

3. Histological Findings

Despite its aggressive clinical behavior, cSCC in EB patients presents as histologically well-differentiated in most cases (55.4–73.9%) ^[1][3][9][5,7,12]. Interestingly, in RDEB, the percentage of well-differentiated cases seems to be even higher, reaching peaks of 91.4% in localized disease and of 85% in metastatic disease ^[1][5]. However, over time, 36% of EB patients may display a certain tendency to shift from well-differentiated forms to poorly differentiated ones, thus leading to a progressive worsening of prognosis ^[3][7]. These cSCCs may chronically involve any area of the skin, esophagus and mouth ^[3][7]. While the diagnosis in poorly differentiated forms of cSCC is quite simple from a dermatopathological point of view, there is some difficulty in the differential diagnosis between well-differentiated forms of cSCC and pseudoepitheliomatous hyperplasia ^[3][10][7,13]. In fact, in the skin of subjects affected by EB, there is a continuous alteration which leads to a certain difficulty in differentiating these different conditions on a morphological basis.

4. Pathogenesis

4.1. Genetic Factors and UV Damage

The exact role of UV exposure in the development of EB-cSCC is still somewhat controversial. While on one hand it has been indeed demonstrated that UV exposure plays only a marginal role in EB-cSCC compared to the general population, on the other hand, in KS, UV-induced skin damage may contribute to tumor onset and progression, as loss of kindlin-1 seems to increase the release of reactive oxygen species, thus sensitizing keratinocytes to solar damage ^[5][1]. However, despite sharing similar genetic profiles with UV-induced cSCC, EB-cSCC displays certain peculiar genetic features which can possibly explain the distinct clinical behavior of these two forms of SCC. First of all, p53 mutations in RDEB-cSCC are more similar to those normally found in other types of cancer such as lung cancer than to those found in UV-cSCC ^[11][14]. On the other hand, KS-cSCC seems to represent once more a distinct entity, as it displays different molecular signatures compared not only to UV-cSCC in the general population, but also to RDEB-cSCC ^[5][1]. Therefore, different genetic profiles may also justify the premature senescent features of keratinocytes isolated from KS-cSCC, thus underlying new possible differences compared to other EB-cSCC ^[5][1].

4.2. The Role of the Microenvironment

Although genetics plays a crucial role in tumorigenesis, the crosstalk between cancer cells and their microenvironment may certainly influence disease progression and clinical outcome as favorable local conditions may justify cancer invasion and distant spreading. In EB-cSCC, the microenvironment seems to play an even more important role compared to other types of cancer, as tumorigenesis in EB patients is deeply related to the impaired wound healing process, local inflammation and ECM inconsistency, especially in RDEB ^[5][12][1,15].

4.3. Altered Wound Healing Process and Fibrosis

Wound healing is a complex and delicate process aimed at re-establishing tissue integrity after injury. It usually involves three phases known as “inflammation”, “new tissue formation” and “remodeling”. Dermal fibroblasts and myofibroblasts play a crucial role in the second phase. Their activation and regulation depend on ECM composition and interaction with other cell types. This delicate phase is strongly impaired in EB patients, especially among RDEB ones, where impaired wound healing clinically leads to a delayed healing process, exuberant fibrosis, retracting scars and ultimately cancer. As UVs do not have such a central role in the pathogenesis of cSCC in EB patients compared to patients without EB, endogenous factors, such as impaired wound healing, seem to play a pivotal role in tumorigenesis, thus possibly explaining why cSCC in these patients preferably occurs on chronic non-healing wounds rather than on sun-exposed areas ^[5][13][1,18]. As for RDEB, it became the perfect model for molecular studies regarding cSCC pathogenesis and aggressive clinical behavior. RDEB is indeed genetically determined by a genetic deficiency of the COL7A1 gene, thus causing a clinically relevant absence of type VII collagen (C7) within the skin ^[5][1]. Therefore, in RDEB patients, the genetically determined loss of C7 directly interferes with wound healing. It has been demonstrated indeed that C7 loss enhances keratinocyte migration and invasion, reduces epithelial differentiation, promotes epithelial–mesenchymal transition and upregulates tumorigenesis and angiogenesis through TGF-β1 signaling ^{[5][12][14][15]}[1,15,19,20]. However, the role of C7 in RDEB tumorigenesis is still controversial. Retroviral transduction of C7 into RDEB patients’ keratinocytes seems to increase cancer cell migration and invasion, although laboratory techniques to restore C7 presence on RDEB skin may lead to excessive C7 concentrations, thus not reflecting physiological conditions and possibly influencing the reliability of results ^[16][21]. On the other hand, some authors demonstrated that cSCCs from RDEB patients express variable percentages of C7, thus suggesting that cSCC in RDEB may arise regardless of C7 skin concentrations ^[17][22]. According to this theory, further investigations seem to suggest that the only tumorigenic domain of C7 is the so-called “N-terminal non-collagenous domain” (NC1); therefore, only C7 expressing this domain can actually drive cancer transition, unrelatedly to its concentration on patients’ skin ^[18][23]. Besides C7 deficiency in RDEB patients, fibrosis due to the altered healing process in all EB patients leads to the formation of a permissive cancer environment, with a progressive fibroblast conversion into carcinoma-associated fibroblasts (CAFs). Indeed, CAFs in both EB cancer and non-cancer skin contribute to microenvironment alteration through the release of multiple cytokines and chemokines, thus also leading to inflammation ^[5][19][1,24]. TGF-β is a crucial regulator of fibroblast differentiation and fibrotic response. For these reasons, it is not surprising that TGF-β also displays an important role in tumorigenesis. TGF-β may indeed promote keratinocyte dedifferentiation, thus facilitating cancer transition ^[12][15]. In RDEB patients, the upregulation of TGF-β signaling seems to be related to C7 depletion, as both pathways are extremely interconnected ^[12][15]. When C7 is deficient, TGF-β expression is increased, thus leading to an enhanced collagen 1 release and a thicker dermis ^[12][15]. Moreover, a stiffer cancer stroma may drive tumor progression through a mechanosensing signaling mediated by β1 integrin, activated focal adhesion kinase (FAK) and phosphoinositide 3-kinase (PI3K) ^[12][15]. Interestingly, TGF-β genes are overexpressed in both cancer and non-cancer RDEB skin, thus indicating once more that RDEB skin intrinsically offers a favorable environment for cancer ^[20][25].

4.4. Inflammation and Local Immune Response

Inflammation is a well-known cancer-promoting factor in both EB and non-EB patients. In particular, recent studies have demonstrated that serum IL-6 levels in RDEB patients are higher compared to those of healthy controls and correlate to disease severity and extent ^[5][1]. In RDEB patients, IL-6 may indeed promote cancer progression through both fibrosis and CAF activation, thus providing an interesting link between systemic inflammation and cancer development ^[5][1]. However, despite a certain degree of systemic inflammation, both generalized and skin-localized immune deficiencies are likely to occur in RDEB patients ^[21][29]. Interestingly, recent works have demonstrated a reduced inflammatory infiltrate in RDEB-cSCC compared to non-RDEB-cSCC. In particular, a significant reduction in CD3+, CD4+ and CD68+ has been demonstrated in RDEB-cSCC compared to primary cSCC in patients without EB, with a further significant reduction in CD3+, CD4+, CD8+ and CD20+ compared to secondary cSCC (postburns and postradiotherapy), thus suggesting that a certain degree of immune tolerance towards cancer antigens is present within RDEB skin ^[21][29].

4.5. Superinfections

Microbial skin superinfections in EB patients seem to contribute to cancer development in many ways. First of all, they can promote skin inflammation, thus enhancing local immune dysregulation ^[5][1]. Furthermore, infections are well-known risk factors for wound-healing failure, and microbial agents on EB skin may worsen an already impaired process and promote cancer in chronic non-healing wounds ^[5][1]. Therefore, an intrinsic major susceptibility to skin Staphylococcus aureus infections has been demonstrated on RDEB skin, regardless of its integrity ^[5][1]. C7 deficiency may indeed destabilize the ECM in lymphoid conduits of the spleen and lymph nodes, thus facilitating bacterial infections ^[5][1]. On the other hand, human papillomavirus (HPV) infections do not seem to be related to cSCC in RDEB patients, despite being well-known causative agents of cutaneous and mucosal SCC in the general population ^[5][1].

5. Biomarkers

Several molecules have been proposed as possible biomarkers for the development of cSCC in RDEB patients. Interestingly, cancer-type SLCO1B3, encoding for a family of anion-transporting polypeptides, has been isolated selectively in RDEB-cSCC cells, and its levels on liquid biopsies of tumor-bearing mice were higher than those of healthy subjects, thus displaying an interesting potential role as a biomarker for cSCC in RDEB patients ^[22][31]. Moreover, further studies focused on possible biomarkers for disease progression and described how serine proteases C_1r and C_1s are significantly overexpressed in advanced cSCC from RDEB patients, suggesting their potential for use not only in predicting disease progression but also as potential targets for new therapeutic approaches in metastatic cSCC ^[23][32].

6. Treatment

The first-line therapy for the majority of EB-cSCCs is surgical excision. Ideally, the tumor should be surrounded by a 2 cm excision margin, although this is often difficult to perform in clinical practice, especially in RDEB patients. Interestingly, although many minimally invasive surgical techniques have been proposed and used in RDEB-cSCCs, there is no clear evidence of their superiority over classical wide surgical excision ^[2][6]. Electrochemotherapy, a local treatment that combines low-dose intralesional or systemic cytotoxic drugs (bleomycin or cisplatin) and the application of high-intensity electric pulses, has been described as a potential treatment in eight patients with favorable results ^[24][25][35,36]. Radiotherapy has also been widely used for both definitive and palliative treatment, as well as to reduce original tumor size, thus facilitating radical surgical excision ^[2][6]. Adjunctive topical therapies may be considered in EB-SCC. Topical 5% imiquimod has been indeed successfully used in a few cases of in situ EB-cSCC, although results are still conflicting and not always promising ^[3][7]. On the other hand, PDT has been successfully used in a few other cases of in situ EB-cSCC, although it was poorly tolerated ^[26][37]. Oral retinoids demonstrated a certain efficacy in the chemoprevention of SCCs in the organ transplant population ^[27][38], although no effects have been demonstrated yet in EB ^[5][28][1,39]. Local recurrences and/or distant metastasis may require other treatments, possibly systemic. Conventional chemotherapy is not commonly used to treat metastatic cSCC in RDEB patients. Agents used in the literature include cisplatin, carboplatin and fluorouracil, although they often result in poor responses, supporting recommendations that risks may outweigh potential benefits ^[2][6]. On the other hand, more and more reports seem to suggest the use of target drugs in metastatic disease, suggesting that cetuximab (an epidermal growth factor receptor (EGFR) inhibitor) may be useful in the treatment of advanced cSCC ^{[2][3][4][6][7][8][9][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37]}[6,7,8,9,10,11,12,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40].