Mosaic pattern of maternal and paternal keratinocyte clones in normal human epidermis revealed by analysis of X-chromosome inactivation

Nonendocrine mechanisms of sex bias in rheumatic diseases

Rheumatic diseases affect a wide range of individuals of all ages, but the most common diseases occur more frequently in women than in men, at ratios of up to ten women to one man. Despite a growing number of studies on sex bias in rheumatic diseases, sex-specific health care is limited and sex specificity is not systematically integrated into treatment regimens. Women and men differ in three major biological points: the number of X chromosomes per cell, the type and quantities of sex hormones present and the ability to be pregnant, all of which have immunological consequences. Could a greater understanding of these differences lead to a new era of personalized sex-specific medicine? This Review focuses on the main genetic and epigenetic mechanisms that have been put forward to explain sex bias in rheumatic diseases, including X chromosome inactivation, sex chromosome aneuploidy and microchimerism. The influence of sex hormones is not discussed in detail in this Review, as it has been well described elsewhere. Understanding the sex-specific factors that contribute to the initiation and progression of rheumatic diseases will enable progress to be made in the diagnosis, treatment and management of all patients with these conditions.

Damage at the root of cell renewal--UV sensitivity of human epidermal stem cells

BACKGROUND

The epidermis harbors adult stem cells that reside in the basal layer and ensure the continuous maintenance of tissue homeostasis. Various studies imply that stem cells generally possess specific defense mechanisms against several forms of exogenous stress factors. As sun exposition is the most prevalent impact on human skin, this feature would be of particular importance in terms of sensitivity to UV-induced DNA damage.

OBJECTIVE

To investigate whether human epidermal stem cells are susceptible to UV-induced DNA damage and subsequent functional impairment.

METHODS

A method to isolate human epidermal stem cells from suction blister epidermis was established and validated. Volunteers were treated with solar-simulated irradiation on test areas of the forearm and stem cells were isolated from suction blister material of this region. DNA damage was analyzed by staining for cyclobutane thymidine dimers. The functional consequences of UV-induced damages were assessed by colony forming efficiency assays and gene expression analyses.

RESULTS

Compared to an unirradiated control, stem cells isolated from areas that were exposed to solar-simulated radiation showed significantly more DNA lesions. Although the number of stem cells was not reduced by this treatment, a functional impairment of stem cells could be shown by reduced colony forming efficiency and altered gene expression of stem cell markers.

CONCLUSIONS

Despite their essential role in skin maintenance, epidermal stem cells are sensitive to physiological doses of UV irradiation in vivo.

Cellular automata and integrodifferential equation models for cell renewal in mosaic tissues

Mosaic tissues are composed of two or more genetically distinct cell types. They occur naturally, and are also a useful experimental method for exploring tissue growth and maintenance. By marking the different cell types, one can study the patterns formed by proliferation, renewal and migration. Here, we present mathematical modelling suggesting that small changes in the type of interaction that cells have with their local cellular environment can lead to very different outcomes for the composition of mosaics. In cell renewal, proliferation of each cell type may depend linearly or nonlinearly on the local proportion of cells of that type, and these two possibilities produce very different patterns. We study two variations of a cellular automaton model based on simple rules for renewal. We then propose an integrodifferential equation model, and again consider two different forms of cellular interaction. The results of the continuous and cellular automata models are qualitatively the same, and we observe that changes in local environment interaction affect the dynamics for both. Furthermore, we demonstrate that the models reproduce some of the patterns seen in actual mosaic tissues. In particular, our results suggest that the differing patterns seen in organ parenchymas may be driven purely by the process of cell replacement under different interaction scenarios.

Sun-induced nonsynonymous p53 mutations are extensively accumulated and tolerated in normal appearing human skin

Here we demonstrate that intermittently sun-exposed human skin contains an extensive number of phenotypically intact cell compartments bearing missense and nonsense mutations in the p53 tumor suppressor gene. Deep sequencing of sun-exposed and shielded microdissected skin from mid-life individuals revealed that persistent p53 mutations had accumulated in 14% of all epidermal cells, with no apparent signs of a growth advantage of the affected cell compartments. Furthermore, 6% of the mutated epidermal cells encoded a truncated protein. The abundance of these events, not taking into account intron mutations and mutations in other genes that also may have functional implications, suggests an extensive tolerance of human cells to severe genetic alterations caused by UV light, with an estimated annual rate of accumulation of ∼35,000 new persistent protein-altering p53 mutations in sun-exposed skin of a human individual.

Analysis of tissue-specific gene expression using laser capture microdissection

Epithelial tissues exhibit optimal conditions for studying cellular differentiation since the differentiation status of a single cell can be determined by its distance to the basal membrane. For that reason Laser Capture Microdissection (LCM) may serve as a perfect tool to compare the characteristics of cells that have been collected from different strata of the epithelium. However, as cell boundaries are not visible in untreated tissue sections, samples have to be stained to allow for sufficient structural orientation. This usually results in a considerable reduction of RNA content in the dissected specimen. To circumvent this problem, we have established a modified hematoxylin/eosin staining protocol that concurrently allows visualization of important structures and the subsequent isolation of sufficient RNA amounts to be used for linear amplification and quantitative analyses.

Tiers of clonal organization in the epidermis: the epidermal proliferation unit revisited

As one of the most proliferative tissues in adult mammals, the epidermis is a good example of the precise regulation necessary between stem cell self-renewal and differentiation. The epidermis is derived from ectodermal progenitor cells and contains three distinct classes of cells: epidermal stem cells which are capable of infinite rounds of cell division; their immediate descendants, transient amplifying cells, which are capable of numerous but finite rounds of cell division; and finally, non-dividing, differentiating cells (Aberdam in Cell and Tissue Research 331:103-107, 2008). This proliferative hierarchy must be tightly regulated both temporally and spatially during epidermal development and homeostasis in order to prevent uncontrolled growth leading to hyperproliferative states and/or tumorigenesis. Historically, the most basic unit of epidermal proliferation has been described as the epidermal proliferation unit (EPU). The EPU, as originally characterized by Christophers, Potten and Mackenzie, is a proliferation unit consisting of approximately 10 basal cells with a clonogenic cell in the center and overlaid by the suprabasal and corneocyte progeny (reviewed in Potten, C. S. (1974). The epidermal proliferative unit: the possible role of the central basal cell. Cell and Tissue Kinetics, 7(1), 77-88). Numerous researchers have identified this classical EPU structure, consisting of approximately 20 cells, in a variety of mammalian skin sources. Recently however, lineage analyses have provided evidence for much larger clonal epidermal units consisting of hundreds to thousands of cells. Furthermore, cutaneous mosaicism as well as a variety of cutaneous pathologies indicate that clonal areas extend to whole patches of mammalian skin many centimeters across. In this review we revisit four decades of experimental evidence and put forward a model of clonal units derived from multiple classes of epidermal progenitors ranging from the largest and most primitive units, clonal ectodermal units, to epidermal stem cell units, and finally, to the most basic structural unit, the EPU.

Lyonization pattern of normal human nails

To examine the X-inactivation patterns of normal human nails, we performed the human androgen receptor gene assay of DNA samples extracted separately from each finger and toe nail plates of nine female volunteers. The X-inactivation pattern of each nail was unique and constant for at least 2 years. The frequency of nails with one of the two X-chromosomes exclusively inactivated was 25.9%. In the nails composed of two types of cells with either one X-chromosome inactivated, the two cell types were distributed in patchy mosaics. These findings suggest that the composition of precursor cells of each nail is maintained at each site at least through several cycles of regeneration time, and that the nail plate has a longitudinal band pattern, each band consisting of cells with only one of the two X-chromosomes inactivated. Using the frequency of nails with one of two X-chromosomes exclusively inactivated, we estimated the number of progenitor cells that gave rise to the nail plate during development to be about 3, under the assumption that the process follows the binominal distribution model. A strong correlation observed among the big, index and little fingers, and among the corresponding toes suggests an interesting interpretation concerning their morphogenetic process.

An integrated systems biology approach to understanding the rules of keratinocyte colony formation

Closely coupled in vitro and in virtuo models have been used to explore the self-organization of normal human keratinocytes (NHK). Although it can be observed experimentally, we lack the tools to explore many biological rules that govern NHK self-organization. An agent-based computational model was developed, based on rules derived from literature, which predicts the dynamic multicellular morphogenesis of NHK and of a keratinocyte cell line (HaCat cells) under varying extracellular Ca++ concentrations. The model enables in virtuo exploration of the relative importance of biological rules and was used to test hypotheses in virtuo which were subsequently examined in vitro. Results indicated that cell-cell and cell-substrate adhesions were critically important to NHK self-organization. In contrast, cell cycle length and the number of divisions that transit-amplifying cells could undergo proved non-critical to the final organization. Two further hypotheses, to explain the growth behaviour of HaCat cells, were explored in virtuo-an inability to differentiate and a differing sensitivity to extracellular calcium. In vitro experimentation provided some support for both hypotheses. For NHKs, the prediction was made that the position of stem cells would influence the pattern of cell migration post-wounding. This was then confirmed experimentally using a scratch wound model.

Single base instability is promoted in vulvar lichen sclerosus

Single base substitution mutations in codons 248 and 273 of TP53 and codon 12 Kirsten-ras (KRAS) are commonly found in human carcinomas. To determine whether these mutations also occur in normal and inflamed tissues from which carcinomas arise, we utilized the ultra-sensitive polymerase chain reaction/restriction endonuclease/ligase chain reaction mutation assay. Ninety samples of genital skin, including lichen sclerosus (LS) affected skin, adjacent normal and non-adjacent normal, were assayed. Mutations were detected in 103 of 349 assays and consisted of KRAS G34A, G34T, G35A, and TP53 C742T, G818C, C817T, and G818A mutations. Mutant prevalence varied from 1 to 20 per 10(6) wild-type cells. Mutations occurred significantly more frequently in LS (78/224 (35%)) than adjacent normal (20/88 (23%)) and non-adjacent normal genital skin (5/38 (13%)). KRAS G34A mutation was relatively common to all classes of specimen, whereas TP53 gene C742T and G818C mutations were significantly more frequent in LS than normal genital skin. In matched samples, immunohistochemistry evaluation of p53 protein expression revealed the presence of epidermal p53 clones in LS whose presence and number significantly correlated with the presence of TP53 C742T and G818C mutations. Based on these results, it appears oncogenic point mutations occur in normal genital skin, and are selected for in LS.

Mechanism of UV-related carcinogenesis and its contribution to nevi/melanoma

Melanoma consists 4-5 % of all skin cancers, but it contributes to 71-80 % of skin cancers deaths. UV light affects cell and tissue homeostasis due to its damaging effects on DNA integrity and modification of expression of a plethora of genes. DNA repair systems protect cells from UV-induced lesions. Several animal models of melanoma have been developed (Xiphophorus, Opossum Monodelphis domestica, mouse models and human skin engrafts into other animals). This review discusses possible links between UV and genes significantly related to melanoma but does not discuss melanoma genetics. These include oncogenes, tumor suppressor genes, genes related to melanocyte-keratinocyte and melanocyte-matrix interaction, growth factors and their receptors, CRH, ACTH, α-MSH, glucocorticoids, ID1, NF-kappaB and vitamin D3.

The X chromosome and systemic sclerosis

PURPOSE OF REVIEW

Similar to the majority of autoimmune rheumatic diseases, systemic sclerosis is characterized by a striking female predominance superimposed on a predisposing genetic background. At least two genetic mechanisms have been proposed that play a role in susceptibility to systemic sclerosis; firstly the maintenance of immune tolerance via genes on the X chromosomes and, secondly, fetal microchimerism. Based on these lines of evidence, experimental efforts have been most recently dedicated to investigating the role of X chromosome abnormalities (i.e. monosomy rates and inactivation patterns) in autoimmunity. We will review herein the most recent data on the role of the X chromosome in systemic sclerosis onset and discuss the potential implications.

RECENT FINDINGS

Women with systemic sclerosis manifest an enhanced rate of X monosomic cells in peripheral blood compared with healthy age-matched women. Furthermore, a severely skewed X chromosome inactivation pattern is found in women with systemic sclerosis.

SUMMARY

These observations, reproduced in other female-predominant autoimmune diseases, strongly support the role of the X chromosome in conferring susceptibility to tolerance breakdown and open novel scenarios to emphasize the unknown etiopathogenesis of systemic sclerosis. The implications of these findings will be discussed.

Genetic mosaicism in basal cell carcinoma

Human basal cell cancer (BCC) shows unique growth characteristics, including a virtual inability to metastasize, absence of a precursor stage and lack of tumour progression. The clonal nature of BCC has long been a subject for debate because of the tumour growth pattern. Despite a morphologically multifocal appearance, genetic analysis and three-dimensional reconstructions of tumours have favoured a unicellular origin. We have utilized the X-chromosome inactivation assay in order to examine clonality in 13 cases of BCC. Four parts of each individual tumour plus isolated samples of stroma were analysed following laser-assisted microdissection. In 12/13 tumours, the epithelial component of the tumour showed a monoclonal pattern suggesting a unicellular origin. Surprisingly, one tumour showed evidence of being composed of at least two non-related monoclonal clones. This finding was supported by the analysis of the ptch and p53 gene. Clonality analysis of tumour stroma showed both mono- and polyclonal patterns. A prerequisite for this assay is that the extent of skewing is determined and compensated for in each case. Owing to the mosaic pattern of normal human epidermis, accurate coefficients are difficult to obtain; we, therefore, performed all analyses both with and without considering skewing. This study concludes that BCC are monoclonal neoplastic growths of epithelial cells, embedded in a connective tissue stroma at least in part of polyclonal origin. The study results show that what appears to be one tumour may occasionally constitute two or more independent tumours intermingled or adjacent to each other, possibly reflecting a local predisposition to malignant transformation.

Ultraviolet radiation and skin cancer: molecular mechanisms

Every living organism on the surface of the earth is exposed to the ultraviolet (UV) fraction of the sunlight. This electromagnetic energy has both life-giving and life-endangering effects. UV radiation can damage DNA and thus mutagenize several genes involved in the development of the skin cancer. The presence of typical signature of UV-induced mutations on these genes indicates that the ultraviolet-B part of sunlight is responsible for the evolution of cutaneous carcinogenesis. During this process, variable alterations of the oncogenic, tumor-suppressive, and cell-cycle control signaling pathways occur. These pathways include (a) mutated PTCH (in the mitogenic Sonic Hedgehog pathway) and mutated p53 tumor-suppressor gene in basal cell carcinomas, (b) an activated mitogenic ras pathway and mutated p53 in squamous cell carcinomas, and (c) an activated ras pathway, inactive p16, and p53 tumor suppressors in melanomas. This review presents background information about the skin optics, UV radiation, and molecular events involved in photocarcinogenesis.

Organization of stem cells and their progeny in human epidermis

Renewal of epidermis is achieved by an ordered replication of stem cells and transit amplifying cells followed by terminal differentiation. In mouse epidermis, renewal is organized around highly ordered structures termed epidermal proliferative units (EPU), each generated by a single stem cell. It has been difficult to apply these concepts to the human epidermis where the basal layer is undulating and the strata have variable thickness. For example, it is unclear whether stem cells in human epidermis are located at the base of rete ridges or overlying the tip of dermal papilla. Data are available to support both views. To gain a better understanding of EPU organization in human skin, we have genetically marked xenografts of human foreskin with a lentivirus encoding a fluorescent marker protein and have mapped labeled columns of cells over a 28-wk period. By following these columns to their origin in the epidermis we have been able to determine that stem cells are dispersed along the basal compartment. The widths of these columns do vary considerably, with the narrowest originating from cells located in the base of the rete ridge. These findings provide new insights into the dynamics of epidermal renewal in human glabrous skin.

The density of epidermal p53 clones is higher adjacent to squamous cell carcinoma in comparison with basal cell carcinoma

BACKGROUND

It is well accepted that ultraviolet radiation from the sun can induce and promote growth of skin tumours. Skin cancer develops as a consequence of multiple genetic hits, where an initial, important step includes proliferation of cells susceptible to malignant transformation. Foci of morphologically normal epidermal keratinocytes overexpressing p53 protein are common in chronically sun-exposed skin. Such foci have previously been shown to represent expanding clones of p53-mutated keratinocytes. Although several characteristics concerning epidermal p53 clones remain to be resolved, an important role in skin carcinogenesis is anticipated. The density of epidermal p53 clones in human skin is largely unknown.

OBJECTIVES

To compare the occurrence of epidermal p53 clones in skin surrounding cancers with that in skin surrounding benign melanocytic naevi. To assess the influence of age on frequency and size of epidermal p53 clones in human facial skin.

METHODS

We have analysed the number and sizes of epidermal p53 clones in skin specimens from patients with squamous cell carcinoma (SCC), basal cell carcinoma (BCC) and benign melanocytic naevi. Cases included normal facial skin from four different age groups. Tissue sections were immunohistochemically stained and the presence of p53 clones was recorded. Approximately 1.4 m of epidermis from a total of 112 biopsies was analysed.

RESULTS

We found 128 epidermal p53 clones in biopsy specimens from 112 patients. The results showed that the number and size of p53 clones increase with age. In normal skin adjacent to SCC p53 clones were significantly more numerous and greater in size in comparison with those in normal skin both adjacent to benign naevi and adjacent to BCC. Interestingly, normal skin in the close vicinity of BCC and melanocytic naevi showed similar results regarding both number and size of epidermal p53 clones.

CONCLUSIONS

Our findings suggest a connection between development of epidermal p53 clones and SCC.

Role of melanoma chondroitin sulphate proteoglycan in patterning stem cells in human interfollicular epidermis

Human interfollicular epidermis is renewed by stem cells that are clustered in the basal layer in a patterned, non-random distribution. Stem cells can be distinguished from other keratinocytes by high expression of β1 integrins and lack of expression of terminal differentiation markers; they divide infrequently in vivo but form actively growing colonies in culture. In a search for additional stem cell markers, we observed heterogeneous epidermal expression of melanoma chondroitin sulphate proteoglycan (MCSP). MCSP was expressed by those keratinocytes with the highest β1 integrin levels. In interfollicular epidermis, expression was confined to non-cycling cells and,in culture, to self-renewing clones. However, fluorescence-activated cell sorting on the basis of MCSP and β1 integrin expression gave no more enrichment for clonogenic keratinocytes than sorting for β1 integrins alone. To interfere with endogenous MCSP, we retrovirally infected keratinocytes with a chimera of the CD8 extracellular domain and the MCSP cytoplasmic domain. CD8/MCSP did not affect keratinocyte proliferation or differentiation but the cohesiveness of keratinocytes in isolated clones or reconstituted epidermal sheets was greatly reduced. CD8/MCSP caused stem cell progeny to scatter without differentiating. CD8/MCSP did not alter keratinocyte motility but disturbed cadherin-mediated cell-cell adhesion and the cortical actin cytoskeleton, effects that could be mimicked by inhibiting Rho. We conclude that MCSP is a novel marker for epidermal stem cells that contributes to their patterned distribution by promoting stem cell clustering.

Minimal evidence of transdifferentiation from recipient bone marrow to parenchymal cells in regenerating and long-surviving human allografts

Liver, small intestine, and heart allografts in residence for 4 days to 16 years were analyzed by simultaneous XY fluorescent in situ hybridization to search for evidence of the recently described process of transdifferentiation of recipient bone marrow stem cells to allograft parenchymal cells. These studies were carried out in an effort to find conditions associated with maximal levels of engraftment or expansion of the recipient parenchymal cells. Despite prolonged survival up to 16 years, regeneration after severe preservation injury or use of split livers, only rare, isolated and tentatively identified recipient hepatocytes were detected in liver allografts. In intestinal allografts, despite survival of up to 8 years and extensive mucosal regeneration because of severe damage from acute rejection, there was no crypt replacement by recipient epithelial cells. In cardiac allografts, no recipient myocytes were detected despite recipient survival for 2-3 days and 3-4 weeks after myocardial infarcts at 5 and 8 years after transplantation. Parenchymal cell transdifferentiation from recipient bone marrow stem cells was rare to nonexistent in severely injured, regenerating, and long-surviving allografts. The rare isolated recipient parenchymal cells tentatively identified did not appear to behave as stem cells: they did not form clusters and did not increase with time after transplantation. Because of the extremely low frequency, interpretation was difficult. Regardless of these results, a more vigorous search for conditions that promote transdifferentiation is warranted.

Control of epidermal stem cell clusters by Notch-mediated lateral induction

Stem cells in the basal layer of human interfollicular epidermis form clusters that can be reconstituted in vitro. In order to supply the interfollicular epidermis with differentiated cells, the size of these clusters must be controlled. Evidence suggests that control is regulated via differentiation of stem cells on the periphery of the clusters. Moreover, there is growing evidence that this regulation is mediated by the Notch signalling pathway. In this paper, we develop theoretical arguments, in conjunction with computer simulations of a model of the basal layer, to show that regulation of differentiation is the most likely mechanism for cluster control. In addition, we show that stem cells must adhere more strongly to each other than they do to differentiated cells. Developing our model further we show that lateral-induction, mediated by the Notch signalling pathway, is a natural mechanism for cluster control. It can not only indicate to cells the size of the cluster they are in and their position within it, but it can also control the cluster size. This can only be achieved by postulating a secondary, cluster wide, differentiation signal, and cells with high Delta expression being deaf to this signal.

The molecular genetics of the genodermatoses: progress to date and future directions

The Human Genome Mapping Project and allied rapid advances in genetic technology over the past decade have facilitated accurate association of allelic variations in several genes with specific skin phenotypes. Currently the genetic bases of the majority of the more common genodermatoses have been elucidated. In scientific terms this work has been extraordinarily successful and has yielded many new biological insights. These advances, although exciting, have yet to be translated into direct benefit for patients with these diseases. Genetic counselling has been greatly aided by gene identification, by the better understanding of genotype-phenotype correlation and by the disclosure of unexpected genetic mechanisms in some families. Knowledge of the molecular basis of these disorders has also been vital in enabling DNA-based prenatal diagnosis in several conditions and DNA-based preimplantation diagnosis has been used in a selected few. While this successful period of gene mapping is now nearing completion, progress towards the next goal, that of developing therapeutic strategies based on the knowledge of these underlying genetic mechanisms, has proven frustratingly slow. Despite the ready access to the skin compared with solid internal organs, the challenges of cutaneous gene therapy are legion and many technical issues need to be surmounted to enable gene replacement or modification of gene expression to have a useful role in these disorders. In this article we make a comprehensive review of progress to date in gene identification, genotype-phenotype correlation, prenatal diagnosis and cutaneous gene therapy, and we examine future directions for research in this field.

A novel, essential control for clonality analysis with human androgen receptor gene polymerase chain reaction

The most widely used technique for determining clonality based on X-chromosome inactivation is the human androgen receptor gene polymerase chain reaction (PCR). The reliability of this assay depends critically on the digestion of DNA before PCR with the methylation-sensitive restriction enzyme HpaII. We have developed a novel method for quantitatively monitoring the HpaII digestion in individual samples. Using real-time quantitative PCR we measured the efficiency of HpaII digestion by measuring the amplification of a gene that escapes X-chromosome inactivation (XE169) before and after digestion. This method was tested in blood samples from 30 individuals: 2 healthy donors and 28 patients with myelodysplastic syndrome. We found a lack of XE169 DNA reduction after digestion in the granulocytes of two myelodysplastic syndrome patients leading to a false polyclonal X-chromosome inactivation pattern. In all other samples a significant reduction of XE169 DNA was observed after HpaII digestion. The median reduction was 220-fold, ranging from a 9.0-fold to a 57,000-fold reduction. Also paraffin-embedded malignant tissue was investigated from two samples of patients with mantle cell lymphoma and two samples of patients with colon carcinoma. In three of these cases inefficient HpaII digestion led to inaccurate X-chromosome inactivation pattern ratios. We conclude that monitoring the efficiency of the HpaII digestion in a human androgen receptor gene PCR setting is both necessary and feasible.

Estimation of size of clonal unit for keratinocytes in normal human skin

OBJECTIVE

It has been suggested that keratinocyte (KC) stem cells reside at the epicenter of a clonal population of cells. To estimate the territory or surface area covered by a single stem-cell-derived KC population in human skin, clonal skin maps were created from 3 healthy adult women and from normal skin of a psoriatic patient.

DESIGN

Two hundred fifty-eight punch biopsy samples of various sizes (ranging from 2 to 8 mm in diameter) were analyzed for clonality employing X chromosome inactivation patterns at the human androgen receptor gene (HUMARA) locus. DNA was isolated and clonality established by significant decrease of either maternal or paternal X chromosome band patterns following restriction enzyme digestion, polymerase chain reaction amplification, and gel electrophoresis.

RESULTS

Fifty-three (41%) of 128 two-mm biopsies were clonal, whereas only 6 (14%) of 43 three-mm, 5 (14%) of 36 four-mm, and 3 (8%) of 35 five-mm biopsies revealed a clonal population of KCs. By contrast, in 5 different biopsies from a psoriatic patient, including 4- or 5-mm sizes, all but 1 were clonal; even an 8-mm biopsy contained a clonal population of KCs. Mantel-Haenszel chi(2) analysis revealed a P value of.001, reflecting a strong trend in probability for presence of a single clone of KCs as related to size of the biopsy sample. By sequentially analyzing 30 contiguous 2-mm biopsy samples within a given strip of skin, 10 clonal domain changes, as reflected in maternal versus paternal switches, were observed.

CONCLUSIONS

These results provide direct evidence of a clonal population of KCs in normal and psoriatic lesion-free skin, and indicate that a clonal epidermal unit of KCs frequently can be detected in small biopsies (2 mm), but that in normal skin sampling, overlapping clones are apparently present in larger (ie, 4-5-mm) biopsies, producing nonclonal patterns. The clonal domain of progeny in normal skin has a rather limited territorial boundary (2 mm in diameter). However, in lesion-free skin from a psoriatic patient, there may be clonal expansion of KCs due to perturbation in epidermopoiesis and/or stem cell distribution.

Clonality analysis of synchronous lesions of cervical carcinoma based on X chromosome inactivation polymorphism, human papillomavirus type 16 genome mutations, and loss of heterozygosity

One of the most common forms of carcinoma in women, cervical invasive squamous cell carcinoma (CIC), often coexists with multiple lesions of cervical intraepithelial neoplasia (CIN). CIC and CIN show heterogeneity with respect to both histopathology and biology. To understand the causes, origin, and model of progression of cervical carcinoma, we assessed the clonality of a case with multiple synchronous lesions by analyzing X chromosome inactivation polymorphism, human papillomavirus type 16 (HPV16) sequence variation/mutations, and loss of heterozygosity (LOH). Microdissection was performed on 24 samples from this case, representing the entire lesional situation. The combination of different X chromosome inactivation patterns, two HPV16 point mutations, and LOH at three genomic microsatellite loci, led to the identification of five different "monoclonal" lesions (CIN II, CIN III, and invasive carcinoma nests) and five different "polyclonal" areas (CIN II and normal squamous epithelium). This finding indicated that CIC can originate from multiple precursor cells, from which some clones might progress via multiple steps, namely via CIN II and CIN III, whereas others might develop independently and possibly directly from the carcinoma precursor cells. Our results also supported the view that HPV16 as a "field factor" causes cervical carcinoma, which is probably promoted by the loss of chromosomal material as indicated by the LOH.

Persistent p53 mutations in single cells from normal human skin

Epidermal clones of p53-mutated keratinocytes are abundant in chronically sun-exposed skin and may play an important role in early development of skin cancer. Advanced laser capture microdissection enables genetic analysis of targeted cells from tissue sections without contamination from neighboring cells. In this study p53 gene mutations were characterized in single cells from normal, chronically sun-exposed skin. Biopsies were obtained from skin subjected to daily summer sun and skin totally protected from the sun by blue denim fabric. Using laser capture microdissection, 172 single-cell samples were retrieved from four biopsies and analyzed using single-cell polymerase chain reaction and direct DNA sequencing. A total of 14 different mutations were identified in 26 of 99 keratinocytes from which the p53 gene could be amplified. Mutations displayed a typical UV signature and were detected in both scattered keratinocytes and in a small cluster of p53-immunoreactive keratinocytes. This minute epidermal p53 clone had a diameter of 10 to 15 basal cells. Two missense mutations were found in all layers of epidermis within the p53 clone. The presented data show that p53 mutations are common in normal skin and that a clone of keratinocytes with a mutated p53 gene prevailed despite 2 months of total protection from ultraviolet light.