Patterns of X chromosome inactivation in sporadic basal cell carcinomas: evidence for clonality

Monoclonal Origin of Anatomically Distinct Basal Cell Carcinomas

Basal cell carcinoma (BCC) is the most common skin tumor in Caucasians. Loss of heterozygosity (LOH) of chromosome 9 (9q22.3) is common in BCCs, and indirectly implies their monoclonal origin. Although BCCs are considered monoclonal tumors, the clonal origin of anatomically distinct tumors in the same patient was rarely investigated. Forty-one anatomically distinct BCCs from 15 female patients were genotyped for LOH at chromosome 9q22.3, and the inactivation pattern of X-chromosome using the androgen receptor polymorphic site. Overall, 11 women with 30 tumors were heterozygous (informative) with either one of the two 9q markers or the androgen receptor polymorphism. LOH of 9q was detected in 20 tumors from seven patients, and in all tumors derived from the same patient, the same allele was lost. An identical X-chromosome inactivation pattern was noted in all four tumors taken from one patient, and in another informative patient, one tumor demonstrated LOH and the other retained it. In five tumors from two patients, there was no evidence of monoclonality using either technique. We conclude that the majority of anatomically distinct BCCs are monoclonal neoplasms and may represent expansion of the same clone.

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.

Molecular cytogenetic analysis of basal cell carcinoma DNA using comparative genomic hybridization

In an attempt to define genomic copy number changes associated with the development of basal cell carcinoma, we investigated 15 sporadic tumors by comparative genomic hybridization. With the incorporation of tissue microdissection and degenerate oligonucleotide primed-polymerase chain reaction we were able to isolate, and then universally amplify, DNA from the tumor type. This combined approach allows the investigation of chromosomal imbalances within a histologically distinct region of tissue. Using comparative genomic hybridization we have observed novel and recurrent chromosomal gains at 6p (47%), 6q (20%), 9p (20%), 7 (13%), and X (13%). In addition comparative genomic hybridization revealed regional loss on 9q in 33% of tested tumors encompassing 9q22.3 to which the putative tumor suppressor gene, Patched, has been mapped. The deletion of Patched has been indicated in the development of hereditary and sporadic basal cell carcinomas. The identification of these recurrent genetic aberrations suggests that basal cell carcinomas may not be as genetically stable as previously thought. Further investigation of these regions may lead to the identification of other genes responsible for basal cell carcinoma formation.

Monoclonality of endocrine tumours: What does it mean?

It is believed and hoped that clonality assignment of tumours predicts long-term behaviour, narrows the search for the putative pathogenic event or events, identifies the extent of involvement of surrounding and distant tissues and assesses the completeness of surgical clearance. It should define optimal therapeutic strategies, such as gene therapy, and the most fruitful directions for research. Because the histological characteristics that are central to establishing prognosis in most neoplasms are of limited value in endocrine tumours, the benefits of interpretation of clonality are potentially even more valuable and far-reaching in these tissues. So, can clonality be accurately defined and, if it can, can the distinction between a polyclonal expansion of cells that are simply 'obeying orders' and the presumed progeny of a single anarchic mutant be usefully made? This review comes to the conclusion that the answer is 'probably not'.

Clonal nature of seborrheic keratosis demonstrated by using the polymorphism of the human androgen receptor locus as a marker

We evaluated the clonality of seborrheic keratoses using a polymorphism due to the random inactivation of one of two X chromosomes in females. Thirty-eight seborrheic keratoses obtained from the skin of females with polymorphism of the human androgen receptor (HUMARA) locus were examined by a fluorescent polymerase chain reaction procedure, which allowed accurate measurement of the peak intensities of each HUMARA allele. The epithelial portion of seborrheic keratosis and normal control epidermis adjacent to the seborrheic keratosis were removed by laser capture microdissection. As biopsied specimens of seborrheic keratoses contained small amounts of normal epidermis, the effect of digestion by a restriction enzyme (HhaI) recognizing the nonmethylated active sites was compared between seborrheic keratoses and normal control epidermis in only five seborrheic keratosis cases. Disappearance or significant reduction in intensity of one of two HUMARA alleles was observed after HhaI digestion in seborrheic keratoses, but not in the normal control epidermis. Although the skewing of the polymorphism was not corrected by the normal control epidermis in the remaining 33 seborrheic keratosis cases, one of two HUMARA peaks practically disappeared after HhaI digestion in 20 of 33 seborrheic keratosis cases. In total, 25 of 38 seborrheic keratoses were considered to be monoclonal. The histologic type of seborrheic keratoses did not affect clonality.

The clonal origin and clonal evolution of epithelial tumours

While the origin of tumours, whether from one cell or many, has been a source of fascination for experimental oncologists for some time, in recent years there has been a veritable explosion of information about the clonal architecture of tumours and their antecedents, stimulated, in the main, by the ready accessibility of new molecular techniques. While most of these new results have apparently confirmed the monoclonal origin of human epithelial (and other) tumours, there are a significant number of studies in which this conclusion just cannot be made. Moreover, analysis of many articles show that the potential impact of such considerations as patch size and clonal evolution on determinations of clonality have largely been ignored, with the result that a number of these studies are confounded. However, the clonal architecture of preneoplastic lesions provide some interesting insights --many lesions which might have been hitherto regarded as hyperplasias are apparently clonal in derivation. If this is indeed true, it calls into some question our hopeful corollary that a monoclonal origin presages a neoplastic habitus. Finally, it is clear, for many reasons, that methods of analysis which involve the disaggregation of tissues, albeit microdissected, are far from ideal and we should be putting more effort into techniques where the clonal architecture of normal tissues, preneoplastic and preinvasive lesions and their derivative tumours can be directly visualized in situ.

Field cancerization, clonality, and epithelial stem cells: the spread of mutated clones in epithelial sheets

There has been considerable debate about the origin of human tumours, whether they arise from a single cell and are clonal populations or whether there needs to be some sort of co-operativity between cells for the neoplastic process to begin. Current theories subscribe to the clonal view, where a series of mutations in one cell begins a process of selection and clonal evolution leading to the development of the malignant phenotype. This review approaches this problem by asking how mutated clones, once established, spread through tissues before becoming overtly invasive. While there is substantial evidence in favour of independent origins of each tumour from a unique mutated clone, there are instances where such clones expand and remain cohesive, often involving a large area of tissue. The main example is the movement of mutated clonal crypts through the colorectal epithelium, by the process of crypt fission. In passing, the clonal architecture of early, pre-invasive lesions is examined, often with some surprising results.