Ординатура / Офтальмология / Английские материалы / Ocular Disease Mechanisms and Management_Levin, Albert_2010

Demographics

Sebaceous cell carcinoma mostly affects older adults with an estimated mean age at diagnosis between 63 and 77 years.10–13 However, it may occur at a much younger age in people with prior history of facial irradiation.14,15 Asia and the Indian subcontinent have a high incidence of sebaceous cell carcinoma. In North America, sebaceous cell carcinoma is primarily seen in people of European descent.3 A possible association with the autosomal–dominant Muir–Torre syndrome with mutations in the mismatch repair genes hMLH-1 and hMLH-2 has been shown in some cases.16

Clinical presentation

Theoretically, sebaceous cell carcinoma can occur anywhere in the body where sebaceous glands are found. However, the ocular adnexa is by far the most common location for this neoplasm, with a vast majority occurring in the meibomian glands and fewer developing in the pilosebaceous glands of Zeis and caruncle.9,17

The most common presentation of sebaceous cell carcinoma is a solitary lid nodule with yellowish discoloration and madarosis, a key clinical feature differentiating it from more common benign lesions such as a chalazion or hordeolum. A recurrent chalazion in an older patient should raise

Box 52.1  Presentation

Pathology

the suspicion for sebaceous cell carcinoma.3 Madarosis is not a requisite, however (Figure 52.1), so a high level of suspicion must be maintained in the proper clinical setting.

The second most common pattern for sebaceous cell carcinoma development is a diffuse pattern with unilateral lid thickening and reactive inflammation which is often mistaken for blepharitis (Figure 52.2). Refractory and unilateral cases should raise suspicion for sebaceous cell carcinoma. A recent discussion of the varied clinical presentations of sebaceous cell carcinoma and the differential diagnosis has been published.3

Pathology

The histopathologic patterns of sebaceous cell carcinoma vary among tumors, making the disease challenging to diagnose (Box 52.2). However, there are certain characteristics that should be looked for in making the diagnosis. Sebaceous carcinoma cells are pleomorphic: they commonly exhibit enlarged nuclei and basophilic cytoplasm that is foamy in appearance due to the presence of fat. Mitotic figures, often with unusual appearance, are common. In well-differentiated carcinomas, vacuolization is common (Figure 52.3), and a comedo pattern can often be seen, showing the tumor cells attempting to reiterate the normal holocrine architecture of sebaceous glands (Figure 52.4). Poorly differentiated carcinomas have large cells, greater pleomorphism, higher mitotic rates, and disorganized architectures (Figure 52.5). Intraepithelial spread, also called pagetoid invasion – an important hallmark of sebaceous cell carcinoma – is known to occur in 44–80% of cases18,19 (Figure 52.6). It is characterized by invasion of tumor cells, individually or in clusters, within the epithelium of the conjunctiva and skin, often eliciting subepithelial chronic inflammation.18,19 Pagetoid invasion often results in skip lesions in which normal epithelial tissue may be found between nests of tumor cells, raising the need for map biopsies to sample the ocular surface.3

The level of differentiation appears to correlate well with the aggressiveness of the tumor: well-differentiated tumors

Figure 52.2  Sebaceous cell carcinoma presenting as diffuse thickening and inflammation, which can lead to misdiagnosis of blepharitis.

Box 52.2  Histology

are typically less aggressive than poorly differentiated tumors. The cytologic appearance of low-grade sebaceous cells is characterized by a cytoplasm containing many vacuoles, little nuclear pleomorphism, and rare mitoses (Figures 52.3 and 52.4). High-grade tumors are more intensely basophilic, exhibiting fewer cytoplasmic vesicles, prominent nucleoli, and more mitotic figures (Figure 52.5).20 Unfortunately, both welland poorly differentiated sebaceous tumor cells can be found within the same tumor (Figures 52.3–52.5 are from the same patient as shown in Figure 52.1). A recent publication attempted to classify histological patterns of sebaceous cell carcinoma into lobular, comedocarcinoma, papillary, and mixed.3 However, the clinical value of such a classification is unclear. In addition, the term seboapocrine carcinoma has been proposed to describe sebaceous tumors with focal glandular pattern of apocrine glands (decapitation secretion).2,21 Such a variety of architectures and tumor configurations highlights the likelihood that these tumors arise from stem cells associated with sebaceous and pilosebaceous glands (see below), but have not been shown to alter prognosis.

The pathologic diagnosis of poorly differentiated sebaceous cell carcinomas can be difficult because epidermal and sebaceous cells are derived from a common precursor and may display sebaceous and epidermal characteristics. For instance, sebaceous cell carcinomas may show areas with

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keratin pearls, intracellular bridges, and dyskeratosis, leading to a misdiagnosis as a poorly differentiated squamous cell carcinoma. In addition, there are spindle cell and basaloid variants of sebaceous cell carcinoma which mimic spindle cell squamous cell carcinoma and basal cell carcinoma, respectively.20 The roles of the Wnt and Notch signaling systems and of the transcriptional regulator Lef-1 in cell type specification will be discussed below.

Given the difficulty in correctly identifying poorly differentiated sebaceous cell carcinomas, several immunohistochemical markers and special stains have been used to aid in diagnosis. Like normal sebaceous cells, the tumor cells contain lipid, which stains red with the oil red-O stain, a histochemical stain which has been performed with frozen sections of tumors for many years.3 Immunohistochemistry staining for human milk fat globule-1 (HMFG1) and epithelial membrane antigen (EMA) has also been used as these antigens are strongly expressed on sebaceous cells.22 Other studies have demonstrated that low-molecular-weight cytokeratins such as Cam5.2 and anti breast carcinomaassociated antigen-225 (BRST-1) are expressed in sebaceous cell carcinoma, but not in basal or squamous cell carcinomas, and may be useful in differentiating these tumors.23–25

Regional metastasis to preauricular, parotid, submandibular, and cervical lymph nodes is known to occur in approximately 30% of cases. Distant metastases of sebaceous cell carcinoma are rare, occurring in the lungs, liver, brain, and bone.3 There are current studies on the usefulness of sentinel lymph node biopsy for assessing regional metastasis, followed by treatment with local lymph node dissection and/ or adjuvant chemotherapy.26–28 In the largest study to date, 10 patients with sebaceous cell carcinoma underwent sentinel lymph node biopsy and two of the 10 demonstrated microscopic evidence of tumor metastasis.28 It remains to be determined whether detection and treatment of lymph node micrometastases will alter the clinical course of the disease.

Disease management

The initial management of sebaceous cell carcinoma depends on several factors, including the index of clinical suspicion and the size of the tumor (Box 52.2). Small tumors for which there is high clinical suspicion should be excised with margin control as the first intervention. In contrast, a full-thickness biopsy of the eyelid is considered a better approach for the initial assessment of large lesions requiring extensive eyelid reconstruction following excision.3,29,30 At the present time there is still considerable controversy as to whether Mohs micrographic surgery, serial excisions with frozen section control, or serial excisions with permanent section control are most effective.31–33 Based on retrospective studies, there is a suggestion that, in experienced hands, traditional excision with permanent section control of the margins provides the best chance of avoiding recurrence.32–34 With pagetoid spread, wide surgical margins are advisable, although the ideal width of clear margins is controversial.35

Given the likelihood of pagetoid invasion, map biopsies of the skin and conjunctiva are essential to determining the extent of the disease and treatment options.36 Historically, extensively positive map biopsies have been an indication for orbital exenteration. However, the development of new

Figure 52.4  Well-differentiated sebaceous cell carcinoma attempting to reiterate the normal sebaceous gland architecture, leading to comedo pattern with holocrine secretion centrally. Note the vacuolization. 200×.

surgical techniques and materials for reconstructing the eyelid and conjunctiva following extensive excision have prompted more localized tumor excision.3,31 In addition to surgery, a number of adjuvant techniques have been employed to supplement surgical excision or to treat local recurrences. Cryotherapy has been used with success as an adjuvant to surgical excision to treat pagetoid invasion of the conjunctiva.31,37 More recently, topical mitomycin C, an alkylating agent that inhibits DNA synthesis, has been advocated as an adjuvant agent to treat pagetoid invasion on the conjunctiva and cornea in a small number of patients.38,39 Further studies are needed to determine the efficacy of these treatments, particularly as pagetoid invasion may extend into adnexal structures, including the lacrimal gland ducts and drainage system.

Orbital exenteration remains the definitive treatment in cases of extensive conjunctival involvement and where there is orbital invasion without evidence of metastases.3,31 In patients with orbital extension who are unable or unwilling to undergo orbital exenteration or who have advanced disease and are seeking palliative measures, irradiation with

at least 55 Gy of radiation has been used with some success.31,40,41 However, the use of radiotherapy for this neoplasm is controversial and surgical excision is preferred.31 Finally, some authors advocate brachytherapy, in which a radioactive plaque is inserted close to the tumor and delivers 150 Gy directly to the area as an alternative to orbital exenteration while protecting much of the surrounding tissue from additional exposure.42 Additional studies will be required to assess its efficacy.

Pathophysiology

The pilosebaceous gland, the bulge, and the role of hair follicle stem cells

The skin and its appendages are critical for animal survival. Among its many biological functions, skin protects animals from dehydration, radiation, trauma, temperature changes, and microbial infections. The adult skin is composed of varied groups of cells from diverse embryologic origins. The

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surface ectoderm forms a layer of progenitor cells that goes on to form stratified epidermis, hair follicles, as well as sebaceous and apocrine glands. The mesoderm contributes the collagen-producing fibroblasts of the dermis, the skin vasculature, the erector pili muscles of the hair follicles, the subcutaneous fat, and immune cells. Neural crest cells contribute melanocytes, sensory nerve endings, and the dermis of the head and face.43

Sebaceous cell carcinomas of the ocular adnexa can derive from both pilosebaceous glands of Zeis and the specialized sebaceous glands of the eyelid margin – the meibomian glands (Box 52.3). Our understanding of the biology of the pilosebaceous gland is significantly greater than that of the meibomian glands, but this understanding can be extrapolated to shed some light on the origins, genetics, and cell biology of eyelid sebaceous carcinomas in general. This extrapolation is based on the self-renewal properties that the holocrine cells of all sebaceous glands share with the progenitor cells of the hair follicle and skin.

The epidermis and hair follicles are renewed throughout life. Hair follicle renewal is achieved through a cycle comprising a growth phase (anagen), a regression phase

Box 52.3  Pathophysiology

(catagen), and a resting phase (telogen). This renewal depends on the presence of progenitor stem cells in an area referred to as “the bulge” in both rodents and humans. The bulge region contains undifferentiated stem cells that maintain a high proliferative capacity and multipotency (similar

to intestinal, corneal, and other clustered stem cell populations; Figure 52.7). It should be noted that the stem cell population of the bulge may have been experimentally overestimated by label-retaining techniques, since rigorous studies of stem cell behavior and label retention revealed that this presumed stem cell characteristic is not universal.44 On the other hand, the multipotency of epidermal stem cells is well established, and use of epidermal stem cells for generating cells with embryonic pluripotency has been published.45,46

The morphology of the bulge is different between mouse and human hair follicles. The murine bulge is a discrete protuberance of the outer root sheath, while the human bulge is in fact just a subtle swelling. However, the biology of the human and murine epidermal and bulge stem cells appears to be quite similar.47 The adult skin epithelium is composed of the pilosebaceous unit and the surrounding interfollicular epidermis (IFE) and its associated apocrine glands. The IFE relies on its own source of progenitor cells to provide for tissue renewal in the absence of injury, while the pilosebaceous bulge contains multipotent stem cells that

Matrix (TA cell)

Dermal papilla

Figure 52.7  Epithelial stem cells are often clustered where they can supply cellular renewal to tissues that require it. These cells are multipotent, and capable of differentiating into a variety of cell types within the proper milieu. Label-retaining cells of the hair follicle reside below the sebaceous gland in a region known as the bulge, which is connected to the arrector pili muscle. During periods of rest, bulge stem cells form the base of the follicle, which is adjacent to the specialized new hair germ. As the germ grows, a proliferative compartment of transient amplifying (TA: matrix) cells engulfs the dermal papilla (DP) at the base. These cells progress to differentiate to form seven concentric shells of discrete cell lineages, which are from outer to inner: the companion layer, the three layers of the inner root sheath, and the three layers of the hair shaft. These differentiated layers are surrounded by the outer root sheath, which extends below the bulge and is thought to contain stem cells that continue to migrate down to the follicle base during the growth phase of the hair cycle. The interfollicular epidermis is a stratified epithelium with a basal layer that contains unipotent progenitor cells and TA cells. Basal cells differentiate upward to form the spinous, granular, and stratum corneum layers of the epidermis. (Modified from Blanpain C, Horsley V, Fuchs E. Epithelial stem cells: turning over new leaves. Cell 2007;128:445–458.)

are activated at the start of each new hair cycle, at the time of injury, and as needed to supply the holocrine cells of the sebaceous gland. During the hair cycle, bulge stem cells are stimulated to migrate out of the stem cell niche, proliferate, and differentiate into the various cell types of the pilosebaceous unit. Although the bulge stem cells are relatively quiescent, they can also be induced to migrate and proliferate by mitogenic stimuli such as phorbol esters (12-O- tetradecanoylphorbol-13-acetate (TPA)). Bulge stem cells appear to be in continuous flux throughout the growth phase of the hair cycle, migrating from the bulge along the basal layer of the outer root sheath where they proliferate and differentiate. The ability of these stem cells to differentiate into multiple cell types of the epidermis, sebaceous glands, and hair follicles has been shown by elegant in vivo and in vitro experiments, including transplantation experiments and clonal analysis.43 In addition, even when bulge cells detach from the basal lamina and undergo early commitment to the hair follicle lineage, the process is reversible, at least in vitro.48

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Genetic and molecular regulation of sebaceous cell carcinoma

Genetic profiling of bulge stem cells using microarray analysis has identified many genes which are expressed at higher levels within this population. Interestingly, 14% of these genes are also expressed at higher levels in other stem cell types such as hematopoietic, neuronal, and embryonic stem cells.48–50 The most interesting of these are genes that belong to the Wnt-β-catenin and the transforming growth factor-β/ bone morphogenic protein (BMP) genetic signaling pathways.48,51 In addition, microarray analysis demonstrated decreased expression of many genes that inhibit proliferation in bulge stem cells, consistent with their relatively quiescent state.48,51 Both the Wnt and the BMP signaling pathways play very important roles in hair follicle morphogenesis and cycling.52–59 Hence, upregulation of Wnt and BMP regulatory components in bulge stem cells strongly suggests that these pathways are critical for proper stem cell function.

The Wnt/β-catenin signaling pathway is conserved throughout the eukaryotic kingdom and has repeatedly been shown to be critical to embryonic and postnatal development.60 It is often referred to as the canonical Wnt pathway and is involved in a variety of human cancers (Figure 52.8). An effector of intercellular adhesion, β-catenin is usually stabilized at the plasma membrane through association with cadherin at adherens junctions via armadillo repeats. Under basal conditions, free cytoplasmic β-catenin is rapidly degraded by the proteosome in a ubiquitin-dependent manner. β-catenin is normally bound by two scaffolding proteins, adenomatous polyposis coli (APC) and axin, which leads to the phosphorylation and ubiquitination of β-catenin, resulting in the subsequent proteosomal degradation.60 Wnts are a large family of cysteine-rich secreted glycoproteins which bind members of the frizzled family of serpentine receptors and a member of the low-density lipoprotein receptor family, Lrp5/6. The binding of Wnt to a frizzled receptor inactivates axin by a mechanism that may involve the binding of disheveled. This inactivates the phosphorylation and ubiquitination of β-catenin, leading to β-catenin stabilization. Stable, free β-catenin is then translocated to the nucleus, where it binds the N-termini of DNA-binding transcription factors of the T-cell factor/leukocyte enhancer factor (Tcf/Lef) family.

There are important lines of evidence showing a direct link between the Wnt system and sebaceous cell carcinomas. The history of the research that revealed this link can provide important insights into the scientific process that will continue to tease out the genetic and cellular processes underlying human adnexal tumors. The first clue for the importance of the Wnt system was the finding that lymphoid enhancer factor Lef-1 is critically important for ectodermal commitment to hair follicle differentiation and the required epithe- lial–mesenchymal interactions.61 Next, mice carrying a stabilized β-catenin were shown to undergo de novo hair morphogenesis, including the formation of new ectopic sebaceous glands in the adult mouse.62 The clinical relevance of this was revealed by studies that identified β-catenin- stabilizing mutations in human pilomatricomas, a common skin tumor.63 Concurrently, Lef-1 and Tcf-3 were found to form transcriptional complexes with β-catenin that were

critically important in regulating cell fate and differentiation commitment during hair follicle development.64 Indeed, when β-catenin is selectively deleted from the developing epidermis and hair follicles using Cre/loxP technology with the keratin-14 promoter driving Cre-recombinase, hair follicle development was aborted in wide patches, and hair follicles that did develop failed to regenerate after the first hair cycle.52 Specifically, skin stem cells failed to develop into hair keratinocytes and associated cells, and instead differentiated into epidermal keratinocytes. In addition, ectopic expression of the diffusible Wnt inhibitor Dickkopf-1 resulted in failure of hair follicle development.54 Hence, the canonical Wnt system was found to be critical for both morphogenesis and maintenance of cell fate commitment. The role of the canonical Wnt pathway in pilosebaceous differentiation and cell fate commitment was further elucidated by experiments showing that dominant negative Lef-1 mutations lead to suppression of hair follicle differentiation while promoting sebocyte differentiation.65

The association of Lef-1 activity, epidermal differentiation, and skin tumors was further revealed by studies of a Lef-1 mutant lacking the amino terminus ( N-Lef-1). This mutant cannot bind to translocated β-catenin, and functions as a disruptor of skin differentiation. Overexpression of N- Lef-1 under the control of the keratin 14 promoter resulted in the formation of dermal cysts and spontaneous skin tumors, most of which exhibited sebaceous differentiation.66 This association between sebaceous differentiation and Lef-1

mutations led to a key study in which human sebaceous tumors were tested for the presence of Lef-1 mutations. The finding that 30% of human sebaceous adenomas carried somatic Lef-1 mutations that interfere with Wnt signaling and act as dominant-negative alleles to reduce Wnt-driven gene expression provided critical links among: (1) the genetic pathways that regulate normal skin development; (2) the genetic signals that control stem cell lineage determination and differentiation in the bulge; and (3) the genetic underpinnings of human sebaceous tumors.67 Further research revealed that Lef-1 has two complementary roles that together promote the development of sebaceous tumors. First, it is involved in lineage specification, and can determine whether randomly mutagenized skin cells will develop into squamous or sebaceous tumors (Figure 52.9). Second, normal Lef-1 function is important for the activation of the checkpoint tumor suppressor genes p53 and p21 via p14ARF induction. Therefore, Lef-1 inactivation can cause failure of tumor suppression activity when progenitor cells accumulate mutations (Figure 52.9).68 It can be concluded that the Wnt pathway in general, and the transcriptional regulator Lef-1 in particular, are likely to be intimately involved in the generation of human sebaceous cell carcinomas of the ocular adnexa.

Like the Wnt signaling pathway, the Hedgehog pathway is a critical regulator of metazoan development and differentiation, often acting in concert with the Wnt-β-catenin pathway. The details of the Hedgehog signaling pathway are

beyond the scope of this chapter, and the literature includes several excellent reviews.69–74 Briefly, Sonic hedgehog (Shh) is a secreted factor that binds the transmembrane receptor Patched (Figure 52.10). In a simplified model of Hedgehog signaling, the absence of Shh ligand activity allows Patched constitutively to inhibit Smoothened (Smo), which results in the ubiquitin-dependent proteolysis of the zinc-finger activator/repressor proteins of the Gli family. In the proteolized form, Gli proteins function as transcriptional repressors. Once Shh binds to Patched, Smoothened is no longer inhibited, leading to stabilization and nuclear translocation of the full-length activator form of Gli. This results in the induction of specific gene expression.

The Hedgehog signaling pathway has essential roles in the proliferation and differentiation of epidermal stem cells,69,75–78 and sebaceous gland development.79,80 In addition, the Hedgehog pathway plays critical roles in the pathogenesis of skin cancer.72,81–84 Specifically, mutations in the Patched and Smoothened genes have been demonstrated in sporadic basal cell carcinomas. Moreover, basal cell nevus syndrome is an autosomal-dominant condition caused by a mutation in the Patched gene, which results in the development of multiple basal cell carcinomas.85,86 The Hedgehog and Wnt signaling pathways share many commonalities, and are also known to interact with and regulate one another, including in the regulation of skin development and disease.53,80,87

Another signaling pathway that likely participates in the pathogenesis of sebaceous tumors is the Notch pathway. Notch signaling is involved in a variety of cellular processes: cell fate specification, differentiation, apoptosis, proliferation, migration, adhesion, angiogenesis, and the epithelial–

mesenchymal transition.88–90 The pathway consists of several Notch receptors that contain extracellular epidermal growth factor (EGF) repeats that bind to transmembrane ligands of the Delta-Serrate-Lag2 (DSL) family (Figure 52.11). Notch receptors also contain negative regulatory regions and a heterodimerization domain that are important in inhibiting Notch activity. As with the Wnt and Hedgehog pathways, canonical Notch signaling requires regulated, ubiquitinmediated proteolysis. The Notch pathway is a complex array of interrelated cellular processes, and for greater detail, the interested reader is directed to one of many outstanding reviews.88–90 The ability of Notch to function as either a tumor suppressor or an oncogene in various contexts reflects the plasticity of this signaling pathway. The clearest example of oncogenic Notch signaling is found in acute lymphoblastic T-cell leukemia (T-ALL), an aggressive neoplasm of T cells. However, Notch can also function as a tumor suppressor, since Notch knockout mice develop basal cell carcinoma with increased levels of both Wnt and Hedgehog signaling,91 and overexpression of the Notch inhibitor MAML1 in mice results in squamous cell carcinoma.92 Finally, both oncogenic and tumor suppressor activities may be related to the roles of Notch in establishing and maintaining a variety of stem cell populations.88,90

In the epidermis, Notch has a critical role in the normal development and regeneration of adnexal tissues. The literature on this subject is growing rapidly and supports a clear interaction between the Notch and Wnt signaling cascades in epidermal growth and differentiation.93 After the initial discoveries showing that Notch is important in skin differentiation,94 Notch expression was found to be modified in basal cell carcinoma.95 Subsequent studies demonstrated

Jagged or delta

Notch

ADAM protease

γ-secretase

NICD

NICD

Figure 52.11  Schematic of canonical Notch signaling. Upon ligand (Jagged or Delta) binding, the Notch transmembrane receptor is cleaved by proteases (ADAM protease and γ-secretase), releasing the Notch intracellular domain (NICD), which can then translocate into the nucleus and associate with the DNA-binding protein RBP-Jk to permit transcription of target genes. (Modified from Blanpain C, Fuchs E. Epidermal stem cells of the skin. Annu Rev Cell Dev Biol 2006;22:339–373.)

that disruption of Notch signaling in mice results in epidermal and corneal hyperplasia followed by the development of skin tumor and heightened sensitivity to mutagenesis.91 Interestingly, these tumors were found to express elevated levels of the Hedgehog transcription factor Gli2 as well as the activation of β-catenin signaling which maintained cells in a poorly differentiated state. Shortly thereafter, Notch was found to regulate the differentiation and commitment of the multiple skin layers. As proliferating cells leave the basal layer, migrate outwardly, and terminally differentiate into spinous, granular, and stratum corneum skin layers, Notch signaling, mediated by the RBP-J and Hes1, pushes cells toward terminal differentiation and induces spinous-specific gene expression.96

More recently, the Notch ligand, JAG1, was identified as an important target of the Wnt/β-catenin signaling pathway, and its expression was particularly localized to stem cells and progenitor cells, where it is important in maintaining the self-renewal properties of these cells.97 Finally, both Notch and Wnt pathways are highly active in precommitment hair follicle cells, and the deletion of JAG1 resulted in the inhibition of the hair growth cycle and the activation of epidermal differentiation. Conversely, activation of Notch resulted in expansion of the pilosebaceous progenitor cell population along with enlargement of sebaceous glands.98 Interestingly, induction by β-catenin of ectopic pilosebaceous development in the adult mouse was blocked by Notch inhibition

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through either JAG1 deletion or by treatment with the Notch inhibitor N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S- phenylglycine t-butyl ester (DAPT).

The recent discovery of increased retinoic acid receptor expression in sebaceous cell carcinoma99,100 adds another layer of complexity to an already complex picture. Retinoid receptors are members of the superfamily of nuclear receptors that include steroid hormone, vitamin D, and thyroid hormone receptors. These receptors serve to regulate gene expression through complex dimerization DNA binding and recruitment of other gene expression regulators. Retinoic acid is a well-known regulator of cell proliferation and differentiation, and vitamin D was found to be particularly important for maintenance of the stem cell population of the hair follicle bulge. Mutations in the vitamin D receptor result in alopecia in both mice and humans, which is caused by the inability of bulge stem cells to regenerate. Vitamin D receptor ablation is associated with biasing hair follicle stem cells toward the sebaceous differentiation pathway, which is strikingly similar to the phenotype seen with impaired Wnt signaling. Indeed, the absence of vitamin D receptor activity results in the inhibition of Lef-1 activity, suggesting that vitamin D, and possibly other member of the nuclear receptor superfamily, are important regulators of the Wnt/Notch/ Hedgehog pathways in skin.101

Summary

From careful and productive research into the normal biology of skin development, we have gained important insights into the processes that are responsible for generating sebaceous cell carcinomas of the ocular adnexa.102 The eyelids contain at least two major anatomic structures that can degenerate into sebaceous cell carcinomas: the ciliaassociated glands of Zeis and the meibomian glands. In both cases normal holocrine function requires the presence of a multipotent stem cell population that serves to regenerate the cells of these structures. It is likely that these populations of multipotent stem cells are responsible for the malignant degeneration that results in eyelid cancer. Maintaining a population of poorly differentiated and multipotentiated cells is tenuous and hence tightly regulated by an incredible array of cellular and genetic pathways. Nevertheless, the three critical signaling pathways in this process appear to be:

(1) the Hedgehog pathway, which controls cellular proliferation; (2) the Notch pathway, which controls cell commitment and differentiation to maintain the cellular balance as skin adnexa develop and mature, and, most importantly; (3) the Wnt pathway, which appears to serve as a master regulator of stem cell maintenance and cell fate, particularly in the case of sebaceous differentiation. It is likely that the natural accumulation of mutations in adnexal stem cells is coupled with the proliferative burden that is placed on these cells by the nature of their functions. It also appears possible that sebaceous-like differentiation is a default fate when progenitor cells are signaled to proliferate, and triggers for epidermal differentiation are lacking. The association between Lef-1 mutations and sebaceous hyperplasia and tumorigenesis is intriguing because of the apparent dual roles of Lef-1 in activating squamous cell differentiation and concurrently serving as an activator of tumor suppressor genes such as