Tumor clonality and its biological significance

Monoclonality in normal epithelium and in hyperplastic and neoplastic lesions of the breast

The clonal nature of neoplastic lesions such as invasive breast cancer and ductal carcinoma in situ (DCIS) has been widely proven by several proliferative, genetic or other malignancy-associated markers. The aim of this study is to clarify whether benign hyperplastic lesions such as ductal hyperplasia of usual type (DH) and papilloma can be distinguished from neoplastic lesions such as DCIS by X-chromosome inactivation analysis. Clonal analysis was performed using a polymerase chain reaction-based assay for non-random X-chromosome inactivation of the human androgen receptor gene (HUMARA). Formalin-fixed and paraffin-embedded archival tissue of ten DCIS, sixteen DH, nine papillomas, and seven normal terminal ductal lobular units (TDLUs) was laser-microdissected to avoid contamination with surrounding tissue. All of the cases analysed revealed a monoclonal origin. Furthermore, in one of these cases, opposite X chromosomes were inactivated within the same breast. X-linked inactivation analysis clearly demonstrates that, at least in the breast, monoclonality is not restricted to neoplastic processes. The data support the hypothesis that the mammary gland is organized into distinct stem cell-derived monoclonal patches and that TDLUs are monoclonal in origin. Any proliferative lesion arising within such a pre-existing clonal patch should therefore be clonal, irrespective of whether it originates from one or more patch cells. Thus, X-chromosome inactivation analysis cannot be considered a valid method for distinguishing between neoplastic and hyperplastic breast lesions.

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.

Monoclonality in fibroadenomas with complex histology and phyllodal features

Fibroadenoma is a common cause of benign breast masses in young women. These women have a slightly increased risk of subsequent breast cancer, particularly if their tumors have complex histologic patterns. We assessed monoclonality in fibroadenomas and correlated the results with histologic analysis. We performed a clonal analysis of 52 fibroadenomas from 43 patients using X-chromosome inactivation studies. The cases included fibroadenomas with complex and simple histology. Areas examined were predominantly stroma but epithelium was also present. DNA was isolated from paraffin-embedded tissue and was subjected to polymerase chain reaction amplification of the human androgen receptor gene with and without predigestion of the DNA with Hha 1. If a monoclonal process was identified, the epithelial and stromal components were subsequently microdissected and reanalyzed. 36/43 (83.7%) women were heterozygous. We studied 45 tumors in these 36 informative women. 1/20 (5% complex fibroadenomas and 1/25 (4%) simple fibroadenomas were monoclonal. The epithelial component of both monoclonal fibroadenomas was polyclonal. The one monoclonal simple fibroadenoma was also the only one with mixed features to contain a phyllodes component. In this case, monoclonality was found in the stroma of both the fibroadenoma and phyllodes regions. Monoclonality has been previously associated with phyllodes phenotype, but not with fibroadenomas, except for 3 fibroadenomas that recurred as phyllodes tumor. We report that monoclonality may also be seen occasionally in complex fibroadenomas, and was found in a tumors with mixed fibroadenoma/phyllodes features without clinical recurrence for 4 years.

Cytogenetic analyses of a murine carcinoma cell line and six metastatic derivatives with different degrees of radioresistability

We reported that a murine carcinoma (DEN3) an its six pulmonary metastases (M2, M4C, M4D, M4E, M4F, and M6) exhibited different degrees of radioresistability (In Vitro Cell. Dev. Biol.26:222-228; 1990). While the M2, M4C, M4E, and M4F cultured cells survived up to 2.5 Gy, the cells of DEN3 and M6 tolerated up to 5.0 Gy, and the M4D cells could withstand up to 10.0 Gy of X-irradiation. In the present investigation, the cytogenetic features of these cell lines were examined: (a) to determine the degree of cytogenetic heterogeneity among these cell lines, and (b) to investigate whether any association between the cytogenetic anomaly and the degree of radioresistability could be established. Heterogeneous cytogenetic aberrations were detected in all of the above lines. Karyotype analysis of the M4D and M6 cell lines displayed both numerical and structural abnormalities. The gain and loss of chromosomal copies were observed. Structural aberrations, such as translocation and deletion appeared in both cell lines. However, correlation between the cytogenetic abnormality and the degree of radioresistability was not demonstrated except for a dramatic reduction in one or more copies of the X-chromosome that occurred in 86% and 93% of the M6 and M4D cells, respectively. The results suggest heterogeneous cytogenetic aberrations among these cell lines and a possible association between the loss of X-chromosome and radioresistability of these tumor cells.

A discrete-time, multi-type generational inheritance branching process model of cell proliferation

Mammalian cell populations, such as tumors, may contain subpopulations differing in parameters such as cell lifetimes, even if the populations are derived from single cells. The mode of inheritance of cell lifetimes has previously been the subject of experimental and mathematical investigation. To obtain data on cell lifetimes over more cell generations then previously available, Axelrod et al. [Cell Prolif. 26:235-249(1988)] measured the number of cells in primary colonies and secondary colonies derived form the primary colonies. The experimental results indicated large variance of cells per colony and highly significant correlations between the numbers of cells in primary and secondary colonies. To mathematically model these results we derive, for previously uninvestigated multi-type Galton-Watson branching process models, the covariance of the cell counts in the primary and secondary colonies. As a result, we are able to successfully model the data with two subpopulations having differing proliferation rates, in which the proliferation rate of a daughter cell is primarily determined by the proliferation rate of its mother. Interestingly, simulations display a trade-off between high values of variances and correlation coefficients. The values obtained from experiment are located on the boundary of the region attainable by simulation.

Integrin alpha V beta 3 expression by bone-residing breast cancer metastases

Breast cancer metastasis to bone is a multistep process requiring attachment of tumor cells to the bone and bone marrow environment. The precise adhesion molecules involved in skeletal homing of breast cancer to bone are unknown but likely include integrins. We investigated the expression of vitronectin receptor (alpha V beta 3) by breast cancer cells residing in bone because this heterodimer mediates osteoclast-bone recognition. We used immunohistochemistry and in situ hybridization in a systematic study of 22 bone biopsies containing breast cancer metastases and available samples of corresponding primary tumors and normal breast and compared alpha V beta 3, alpha 2 beta 1, and alpha B beta 5 integrin expression. The results showed that alpha V beta 3 was strongly expressed by normal breast epithelium and was decreased in some and strongly expressed in other primary invasive breast carcinomas. In contrast, this integrin heterodimer was abundant in all breast cancer cells metastatic to bone. In situ hybridization revealed high levels of steady-state mRNA corresponding to sites of protein expression; alpha 2 beta 1 was weakly expressed in both primary and metastatic tumors, and alpha V beta 5 was not detected. Our results showed an overexpression of alpha V beta 3 by bone-residing breast cancer cells and suggest either subclonal selection of alpha V beta 3-expressing tumor cell populations or upregulation of alpha V beta 3 in the bone microenvironment.

Monoclonal antibody E6G6 recognizes glycolipids as a differentiation antigen in Shope carcinoma cells

Monoclonal antibody E6G6 preferentially reacted with well differentiated and non-tumorigenic Shope carcinoma cell lines but hardly reacted with undifferentiated and tumorigenic lines as determined by enzyme-linked immunosorbent assay, cytofluorometry and the complement dependent cytotoxicity test. Immunohistochemical analysis of the primary carcinoma tissue revealed that the antibody intensely stained keratinizing carcinoma cells with marked shrinkage whereas it did not stain normal epidermal keratinizing rabbit cells or benign tumor (Shope papilloma) cells, suggesting that the antigen expression in vivo was closely correlated with terminal differentiation of the carcinoma cells. Thin layer chromatography immunostaining revealed that the antigen was comprised of several neutral glycolipids and that differentiated cells contained about ten times more antigen than undifferentiated cells.

Clonal analysis of human adrenocortical carcinomas and secreting adenomas

OBJECTIVES

Adrenocortical tumours in man are characterized mainly on biochemical, anatomical and histological grounds which establish their secretory pattern and, with some uncertainty, their benign or malignant nature. To study further these tumours and eventually to shed some light on their pathogenesis, we determined their clonal composition.

METHODS

Clonal composition was determined by X-chromosome inactivation analysis on tumour and leucocyte DNA using three markers: M27 beta, phospho-glycero-kinase (PGK) and hypoxanthine-phosphoribosyl transferase (HPRT) with 88, 33 and 27% heterozygosity rates respectively.

PATIENTS

Clonal analysis was performed on 25 tumours from 19 heterozygous female patients: four had a carcinoma, 14 had a single secreting adenoma, and one had autonomous bilateral macronodular hyperplasia with Cushing's syndrome (seven adenomas examined).

RESULTS

The malignant tumours had patterns indicative of monoclonality. The single adenomas displayed contrasting results with patterns indicative of monoclonality in eight cases, and patterns indicative of polyclonality in six cases; monoclonal adenomas were larger and had a higher prevalence of nuclear pleomorphism than the apparently polyclonal adenomas. In the patient with bilateral macronodular hyperplasia, different clonal patterns were present in different adenomas: whereas a clear monoclonal pattern was observed in the three adenomas of the right gland, in which the active X-allele was not always the same, in two interpretable adenomas of the left gland, a moderately skewed pattern suggested a partial monoclonal component.

CONCLUSIONS

These data show that adrenocortical carcinomas are monoclonal and suggest that adenomas may arise from a single cell or from more than one cell under the putative action of local growth factors. In adenomas, which until now had appeared homogeneous, this genetic heterogeneity may reflect different pathophysiological mechanisms or it may represent different stages of a common multistep process exceptionally occurring in a single patient with bilateral macronodular hyperplasia.

Multiple pre-neoplastic events and clonal selection of radiation induced mouse thymic lymphomas shown by TCR gene rearrangements

After split-dose irradiation, pre-lymphoma cells develop from a tumor-specific surface antigen TL-2+ thymocyte subpopulation. To analyze the clonality of pre-lymphoma cells, various numbers of TL-2+ thymocytes from a single irradiated mouse were intra-thymically injected to Thyl congenic recipient mice. The incidence of donor type thymic lymphoma(s) was subsequently examined in a group of recipient mice. We chose several lymphomas derived from a single donor mouse and analyzed the TCR gene rearrangements and V(D)J junctional diversity as genetic markers of clonality. These results indicate multiple initial neoplastic events and clonal selection into lymphoma.

Intraepithelial neoplasia, surrogate endpoint biomarkers, and cancer chemoprevention

Neoplasia is a progression of molecular, cellular, and tissue changes starting with a critical cell mutation and advancing by clonal evolution, involving further multiple mutations and expanding mutated clones. This process is characterized by five general stages: latency, focal growth of normal-appearing but disorganized cells, abnormal-appearing cells (dysplasia), microinvasion, and finally, metastasis. The two driving forces of neoplastic progression in an epithelium are mutagenesis and mitogenesis. These forces frequently occur concurrently, produced by exposure of the epithelium to environmental and endogenous mutagens and mitogens. The major strategy of chemoprevention is to block the effects of both mutagens and mitogens during the early stages of predysplasia and dysplasia. Surrogate endpoint biomarkers (SEBs) are tissue, cellular, and molecular changes that correlate with the later development of cancer. Because of the savings in cost, labor, and time, SEBs are urgently needed to replace the use of cancer incidence reduction as the endpoint for chemopreventive agent clinical trials. The advent of computer-assisted cytometry allows each of the seven basic criteria of dysplasia to be individually assayed as an SEB. Since the dysplastic changes that characterize intraepithelial neoplasia are embodied in the causal pathway to invasive neoplasia, they are already validated as predictors of cancer incidence. More attention should be paid to the quality control of SEB assays, including control of variation in cell composition of tissue samples, assay protocol, instrumentation used, and observer performance. The dose-response relationship between a known chemopreventive agent and the SEB should also be evaluated. The Division of Cancer Prevention and Control, National Cancer Institute, has begun a program to test chemopreventive agents in short-term Phase II clinical trials using dysplasia-based SEBs. The SEBs are assayed, when possible, by computerized cytometry. Trials are being conducted for oral leukoplakia, cutaneous actinic keratosis, superficial bladder cancer, pulmonary metaplasia/dysplasia, cervical dysplasia (CIN III), and adenomatous colonic polyps.

The natural history of intraepithelial neoplasia: relevance to the search for intermediate endpoint biomarkers

The development of carcinomas, defined as invasive epithelial neoplasms, is preceded by a preinvasive stage termed intraepithelial neoplasia that typically lasts for years. Intraepithelial neoplasia is the target tissue for the action of chemopreventive agents and the site where biomarkers frequently develop. The term "dysplasia" refers to the morphological alterations that characterize intraepithelial neoplasia and, according to many authors, consists of seven basic changes that are the same for the majority of epithelia. These are increased nuclear size, abnormal nuclear shape, increased nuclear stain uptake, nuclear pleomorphism (increased variation in size, shape, and stain uptake), increased mitoses, abnormal mitoses, and disordered or absent differentiation. Clonal evolution appears to begin early in the neoplastic process during intraepithelial neoplasia. The use of intraepithelial neoplasia as an intermediate endpoint biomarker requires that effective chemopreventive agents cause it to regress. Two examples are the regression of dysplastic oral leukoplakia produced by beta-carotene and the regression of colonic polyps in familial polyposis patients following treatment with the nonsteroidal antiinflammatory drug sulindac. There is a critical need to identify and develop biomarkers that correlate with the appearance and regression of intraepithelial neoplasia.

Chronic mitoinhibition during promotion of hepatocarcinogenesis

We have reported previously that orotic acid (OA), a precursor for pyrimidine nucleotide biosynthesis, is able to promote carcinogenic process in both liver and duodenum of rats. The present study investigates the possible role of mitoinhibitory effects of OA as being responsible for its promotional effects. Male Fischer 344 rats were given a semisynthetic basal diet (BD) or a diet containing 1% OA for four weeks coupled with 2/3 partial hepatectomy (PH), and all animals were then continued on BD for an additional four weeks. This protocol is known to exert a promoting effect on the initiated rat liver. Livers were perfused, and the labeling index (LI) of isolated cultured hepatocytes was monitored. Hepatocytes isolated from livers of rats fed a BD or 1% OA exhibited in vitro an LI of 39 +/- 2 and 24 +/- 1%, respectively. The lowered in vitro LI was seen even upon exposure to epidermal growth factor (EGF) (67 +/- 2% in OA-treated livers compared to 91 +/- 2% in hepatocytes from control rat liver). A similar four-week exposure to OA coupled with PH decreased hepatic DNA synthesis induced by a choline-deficient diet in vivo by about 50%. These results indicate that OA is able to decrease the response of normal hepatocytes to growth factors and suggest a possible mechanism of chronic differential mitoinhibition as a basis for promotion induced by OA.

Rat model of the human "Triton" tumor: direct genetic evidence for the myogenic differentiation capacity of schwannoma cells using the mutant neu gene as a cell lineage marker

Spontaneous myogenic differentiation was observed in 2 out of 15 cases when cells from schwannomas induced in the offspring of BDIX rats by transplacental exposure to N-ethyl-N-nitrosourea (EtNU) were grown in monolayer culture following fluorescence-activated cell sorting with monoclonal antibody (Mab) 217c. Myotubes and numerous mononucleated cells no longer expressed the Schwann cell antigens 217c and S-100 protein, but rather revealed the presence of desmin, the alpha-sarcomeric form (alpha-sr) of actin, and the cell surface antigen specified by Mab RB21-7, a 250 kD glycoprotein sharing an epitope with the neural cell adhesion molecule (N-CAM). Subcutaneous reimplantation of such cells into syngeneic animals led to the appearance of tumors composed of both S-100 positive Schwann cells and desmin and alpha-sr-actin positive rhabdomyoblasts, thus closely resembling the human "Triton" tumor. With the use of the polymerase chain reaction and allele-specific oligonucleotide hybridization, DNA isolated from individual myotubes was analyzed for the presence of a T----A transversion mutation at nucleotide 2012 of the neu gene, which is diagnostic of EtNU-induced rat schwannomas. All of the amplified DNA isolates contained the mutant neu allele, thus providing direct genetic proof for the capacity of mammalian neuroectodermal cells for myogenic differentiation.

Cytogenetic heterogeneity of genetically marked and metastatically competent "dominant" tumor cell clones

Examination of tumors usually shows them to consist of phenotypically and clonally heterogeneous cell subpopulations. On the other hand, previous studies from our laboratory have provided compelling evidence for the rapid evolution, or overgrowth, of single "dominant" clones during the course of primary tumor growth. Thus in one such study, syngeneic CBA mice were injected with a mixture of 50-100 different genetically tagged clones of a mouse mammary carcinoma called SP1. The vast majority of these clones were non-metastatic. The different clones were tagged by random integrations of foreign DNA using calcium phosphate-mediated transfection of the plasmid pSV2neo, the resultant primary tumors were found to consist of a single dominant clone, called B5, which was also shown to be metastatically competent. The detection of single dominant clones such as B5 in primary tumors can be reconciled with the concept of tumor cell heterogeneity if it could be shown that the dominant clone was in fact heterogeneous for other genetic or phenotypic markers, i.e., was homogeneous only for the plasmid-based genetic marker used for its detection. To study this question, we examined the karyotypes of several sublines of B5, two derived from a primary advanced tumor and one from a spontaneous lung metastasis. We indeed found evidence to support the existence of marked cellular heterogeneity within and between the three sublines examined. Thus, while all three retained common cytogenetic markers, each also expressed unique markers. Moreover, karyotypic heterogeneity within a given subline was observed. Thus the concept of clonal dominance of primary tumors by metastatically competent cell subpopulations is not incompatible with the concept of the cellular heterogeneity of tumors. The implications of the results are discussed.

Clinically nonfunctioning pituitary tumors are monoclonal in origin

Clinically nonfunctioning pituitary adenomas are benign neoplasms comprising approximately 25-30% of pituitary tumors. Little is known about the pathogenesis of pituitary neoplasia. Clonal analysis allows one to make the important distinction between a polyclonal proliferation in response to a stimulatory factor versus a monoclonal expansion of a genetically aberrant cell. We investigated the clonal origin of pituitary tumors using X-linked restriction fragment length polymorphisms at the phosphoglycerate kinase and hypoxanthine phosphoribosyl-transferase genes. Restriction enzymes were used to distinguish maternal and paternal X-chromosomes, and combined with a methylation-sensitive restriction enzyme to analyze allelic X-inactivation patterns in six pituitary adenomas. All six tumors showed a monoclonal pattern of X-inactivation. These data indicate that nonfunctioning pituitary adenomas are unicellular in origin, a result consistent with the hypothesis that this tumor type is due to somatic mutation.

Cell fusion in tumor development and progression: occurrence of cell fusion in primary methylcholanthrene-induced tumorigenesis

Definitive evidence for the occurrence of cell fusion in tumorigenesis was sought in methylcholanthrene-induced sarcomas. This was approached by using allophenic mice generated from strains differing for electrophoretic variants of the ubiquitous, dimeric enzyme glucose phosphate isomerase, with fusion assessed by heterodimer formation. Eight-three carefully trimmed primary tumor samples (from 23 individual tumors in allophenic mice) were analyzed, as were 1,140 clones derived from them. In all primary tumor samples, zymograms exhibited one GPI homopolymeric band. Expression of a hybrid band (indicative of a fusion event) was not observed in these samples. However, 9 (0.8%) of the tumor clones demonstrated a distinct and reproducible hybrid band which was uniformly lost upon recloning. Our data suggest that cell fusion, although uncommon, occurs in the clonogenic cell fraction during primary MCA tumorigenesis and is followed rapidly by chromosome segregation.