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Cutaneous lymphomas are hematological malignancies that present in the skin without evidence of extracutaneous disease at the time of diagnosis. This thesis explores the pathogenetic basis of common and rare cutaneous lymphoma entities (i.e. tumor-stage mycosis fungoides, primary cutaneous anaplastic large cell lymphoma, primary cutaneous CD8+ aggressive epidermotropic cytotoxic T‐cell lymphoma, blastic plasmacytoid dendritic cell neoplasm) by studying them using next-generation sequencing approaches. These studies had a special focus on structural genomic alterations (i.e. genomic rearrangements, copy number alterations), since a detailed examination of this type of genetic defects in cutaneous lymphomas using high-resolution techniques had been largely neglected in prior molecular studies. The body of work presented in this thesis expanded the understanding of the pathogenetic basis of four different cutaneous lymphoma entities. Firstly, it showed that structural genomic alterations play important roles in the pathobiology of cutaneous lymphomas. Secondly, it identified recurrently affected genes and cellular processes/pathways in each of the analyzed malignancies. Finally, it provided molecular insights that may be instrumental for the development of novel therapies in the future.

Melanoma is the most aggressive and lethal type of skin cancer since it has the ability to spread to other organs in the body making it harder to control the disease.In this thesis, we aim to explore the degree to which epigenetics play a role in melanoma, namely, inherited and acquired epigenetic alterations in melanoma susceptibility and development.

The incidence of skin carcinomas has been rising over the last decades. Ultraviolet (UV) radiation from the sun is the main exogenous risk factor. The principal premise underlying the present study is that stem cells are the primary targets of UV carcinogenesis. The aim of this study was to identify the main (stem) cell pool that drives this skin carcinogenesis. We studied cells in the interfollicular epidermis (IFE), quiescent stem cells and actively cycling Lgr5+ and Lgr6+ cells. Our data indicated that UV induced squamous cell carcinomas originate from the IFE. Furthermore, quiescent stem cells seem to play an important role in skin carcinogenesis. These cells accumulate DNA damage under chronic low level UV exposure. When we forced these DNA-damage retaining cells to proliferate, persistent (in situ) carcinomas developed. The actively cycling Lgr5+ and Lgr6+ cells did not seem to entail tumor-initiating stem cells in skin tumors induced by UV radiation or chemicals. We did not observe any tumors with substantial clonal expansion of Lgr5+ or Lgr6+ stem cells. Taken together, these results indicate that the continuously proliferating Lgr5+ and Lgr6+ stem cells are less vulnerable to cancerous transformation than quiescent stem cells.

The studies described in this thesis focused on identifying diagnostic and prognostic markers in tumor stage mycosis fungoides (stage IIB) and Sézary syndrome. Sézary syndrome is a type of cutaneous T-cell lymphoma, characterized by an erythroderma, lymphadenopathy and malignant T cells in skin, lymph nodes and blood. Especially in the early stages of disease, it can be difficult to differentiate Sézary syndrome from erythrodermic inflammatory dermatoses. The studies identified immunohistochemical (TOX), immunophenotypic (CD7, CD26), molecular (STAT4, TWIST1, DNM3/ PLS3) and epigenetic (CMTM2) markers that are useful as additional diagnostic markers to discriminate Sézary syndrome from erythrodermic inflammatory dermatoses. Of the molecular markers, PLS3 expression was an important prognostic factor in Sézary syndrome. In addition, it was shown that the number of tumors and time interval between tumor formation differs greatly among patients with mycosis fungoides stage IIB and these differences correlate with survival. Patients that developed four or more tumors during the first 6 months after diagnosis of stage IIB have a poor prognosis.

Primary cutaneous large B-cell lymphomas can be divided into primary cutaneous follicle center lymphoma and primary cutaneous large B-cell lymphoma, leg type. Accurately making the distinction between these two lymphomas at the time of diagnosis is essential for the patient, as there are differences in prognosis (5 year overall survival >95% versus

Skin cancer is the most common type of cancer in fair-skinned populations. Cutaneous squamous cell carcinoma (SCC) comprises about 15% of all skin cancer diagnoses. Treatment associated with the high and rising prevalence of cutaneous SCC puts an increasingly high financial burden on society, marking a pressing need for advancements in skin cancer drug development. For screening of novel therapeutics, representative models of human cutaneous SCC are required. The aim of the research described in this thesis was to develop a representative in vitro model of human SCC for screening therapeutics, without the unnecessary use of animals. To this end, we generated three-dimensional in vitro SCC models in which the malignant epidermal cancer cells were either represented by intact primary human cutaneous or by established, spontaneously immortalized cutaneous SCC cell lines. The dermal microenvironment in our models was seeded with either primary normal human dermal fibroblasts or primary SCC-associated fibroblasts. In verifying human cutaneous SCC representation by these in vitro models, we focused on hyperproliferation, cytological and architectural atypia and invasion as three main features of primary SCC. The in vitro skin cancer models presented in this thesis add to the spectrum of available in vitro models for therapeutic screening.

The research described in the thesis is focused at identifying molecular aberrations contributing to the pathogenesis of CTCL. In search for differences in chromosomal alterations underlying the different clinical behavior and prognosis of patients with mycosis fungoides (MF) and S_zary syndrome (Sz) the DNA copy number alterations of MF were investigated and compared with the previously published profile of Sz. Several alterations of prognostic value were identified. Gene expression data of MF was integrated with DNA copy number alterations to search for genes contributing to tumorigenesis and possible therapeutic targets. With the same purpose gene expression data of MF was compared to normal controls, to identify genes dysregulated otherwise than due to copy number alteration. Additionally miRNA profiles of MF and Sz were elucidated to determine their possible role as gene expression regulator. DNA copy number alterations and gene expression profiles were analyzed of primary cutaneous anaplastic large cell lymphoma and peripheral T cell lymphoma not otherwise specified to gain insight in the molecular mechanisms underlying the different clinical aggressiveness.

The studies presented in this thesis focused on cutaneous CD30-positive lymphoproliferations, particularly on primary cutaneous anaplastic large cell lymphoma (C-ALCL), a distinct type of cutaneous T-cell lymphoma (CTCL). In the initial staging of patients with an anaplastic large cell lymphoma first presenting in the skin, bone marrow examination has limited value (chapter 4). C-ALCL patients usually have an excellent prognosis, although some patients follow an unfavorable course. These patients often have extensive disease with leg involvement (chapter 2). Several phenotypical markers have been reported to aid in the differentiation between different types of cutaneous CD30-positive lymphoproliferations, however, TRAF1, MUM1 and BCL2 are not useful for this purpose (chapter 3). The miRNA expression profile of C-ALCL does show differences with that of tumor stage mycosis fungoides (MF), another type of CTCL, suggesting a different contribution to the pathogenesis of these lymphomas (chapter 5). In patients with transformed MF, several parameters can be used as prognostic factors, including CD30 expression, folliculotropic MF, extent of skin lesions and extracutaneous transformations (chapter 6).

Skin cancer is a serious problem for many organ transplant recipients. Half of them develop skin cancer within 20 years after the transplantation. The main cause of this increased skin cancer risk is thought to be suppression of the immune system, a necessity to prevent rejection of the transplanted organ. This thesis focuses on the effects of immunosuppressive drugs on the responses of skin cells to UV irradiation and how these altered responses would affect UV-induced skin cancer development. The goal was to compare several immunosuppressants and identify the least hazardous one. Human skin cultures and mice were used as experimental models to study short and long term effects in successive stages of tumor development. These studies have yielded several unexpected findings. Foremost, none of the tested immunosuppressants increased skin cancer development in mice when administrated in the diet; remarkably, cyclos porin even delayed tumor onset. And furthermore, seemingly discordant effects of the drugs on different putative stages of tumor development evoked a new perspective on the step-wise process leading up to skin cancer. More specifically and contrary to the prevailing consensus, this study showed that UV-induced early microscopic clusters of skin cells overexpressing the mutant-p53 tumorsuppressor protein are not early stages or precursors of ensuing skin carcinomas with mutant-p53. Surprisingly, some immunosuppressants affected the number of these clusters without corresponding effects on tumor onset.

The term primary cutaneous B-cell lymphomas refers to a heterogeneous group of B-cell non-Hodgkin lymphomas, that present in the skin without evidence of extracutaneous disease at the time of diagnosis. In recent years, there has been considerable debate regarding the classification and terminology of the group of primary cutaneous B-cell lymphomas and different classification schemes were used. In the new WHO-EORTC consensus classification for cutaneous lymphomas published in 2005, three major groups of primary cutaneous B-cell lymphoma are distinguished: primary cutaneous marginal zone B-cell lymphoma and primary cutaneous follicle center lymphoma with a good prognosis, and primary cutaneous diffuse large B-cell lymphoma, leg type with an intermediate prognosis. Studies presented in this thesis focus on three aspects of these primary cutaneous B-cell lymphomas: (1) the clinical usefulness of the WHO-EORTC classification in daily practice, (2) the validity of prognostic parameters reported in previous studies and identified in patient groups classified according to previously used classification schemes and (3) optimal management for the different types of primary cutaneous B-cell lymphomas as defined in the WHO-EORTC classification.