The keratins are the typical intermediate filament proteins of epithelia, showing an outstanding degree of molecular diversity. Heteropolymeric filaments are formed by pairing of type I and type II molecules. In humans 54 functional keratin genes exist. They are expressed in highly specific patterns related to the epithelial type and stage of cellular differentiation. About half of all keratins—including numerous keratins characterized only recently—are restricted to the various compartments of hair follicles. As part of the epithelial cytoskeleton, keratins are important for the mechanical stability and integrity of epithelial cells and tissues. Moreover, some keratins also have regulatory functions and are involved in intracellular signaling pathways, e.g. protection from stress, wound healing, and apoptosis. Applying the new consensus nomenclature, this article summarizes, for all human keratins, their cell type and tissue distribution and their functional significance in relation to transgenic mouse models and human hereditary keratin diseases. Furthermore, since keratins also exhibit characteristic expression patterns in human tumors, several of them (notably K5, K7, K8/K18, K19, and K20) have great importance in immunohistochemical tumor diagnosis of carcinomas, in particular of unclear metastases and in precise classification and subtyping. Future research might open further fields of clinical application for this remarkable protein family.

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The human keratins: biology and pathology

Aged humans sustain a high rate of epithelial cancers such as carcinomas of the breast and colon, whereas mice carrying common tumour suppressor gene mutations typically develop soft tissue sarcomas and lymphomas. Among the many factors that may contribute to this species variance are differences in telomere length and regulation. Telomeres comprise the nucleoprotein complexes that cap the ends of eukaryotic chromosomes and are maintained by the reverse transcriptase, telomerase. In human cells, insufficient levels of telomerase lead to telomere attrition with cell division in culture and possibly with ageing and tumorigenesis in vivo. In contrast, critical reduction in telomere length is not observed in the mouse owing to promiscuous telomerase expression and long telomeres. Here we provide evidence that telomere attrition in ageing telomerase-deficient p53 mutant mice promotes the development of epithelial cancers by a process of fusion-bridge breakage that leads to the formation of complex non-reciprocal translocations--a classical cytogenetic feature of human carcinomas. Our data suggest a model in which telomere dysfunction brought about by continual epithelial renewal during life generates the massive ploidy changes associated with the development of epithelial cancers.

Telomere dysfunction promotes non-reciprocal translocations and epithelial cancers in mice

A striking link exists between advanced age and increased incidence of cancer. Here I review how several of the age-related molecular and physiological changes might act in concert to promote cancer, and in particular epithelial carcinogenesis. Experimental data indicate that the aged, cancer-prone phenotype might represent the combined pathogenetic effects of mutation load, epigenetic regulation, telomere dysfunction and altered stromal milieu. Further verification of the role of these effects should in turn lead to the design of effective therapeutics for the treatment and prevention of cancer in the aged.

The age of cancer

Chromosome aberrations in human solid tumors are hallmarks of gene deregulation and genome instability. This review summarizes current knowledge regarding aberrations, discusses their functional importance, suggests mechanisms by which aberrations may form during cancer progression and provides examples of clinical advances that have come from studies of chromosome aberrations.

Chromosome aberrations in solid tumors.

Keratin expression in human tissues and neoplasms Keratin filaments constitute type I and type II intermediate filaments (IFs), with at least 20 subtypes named keratin 1-20. Since certain keratin subtypes are only expressed in some normal human tissues but not others, and vice versa, various tissues have been subclassified according to the pattern of keratin staining. Simple epithelia generally express the simple epithelial keratins 7, 18, 19, and 20, while complex epithelia express complex epithelial keratins 5/6, 10, 14, and 15. When an epithelium undergoes malignant transformation, its keratin profile usually remains constant. The constitution and expression patterns of keratin filaments in human epithelial neoplasms are complex and often distinctive. In this article, we first briefly review the molecular and cell biology of keratin filaments. We then focus on the expression patterns of keratin filaments in various human neoplasms.

Keratin expression in human tissues and neoplasms.

There is strong evidence that ductal carcinoma in situ (DCIS) represents a precursor lesion of invasive breast cancer. In order to analyse specific chromosomal alterations of DCIS, 38 paraffin-embedded specimens of DCIS and six associated invasive carcinomas were examined by means of comparative genomic hybridization (CGH). Losses of 16q material were seen almost exclusively in well- and intermediately-differentiated DCIS. These two subgroups differed in the average number of genetic imbalances, 2·5 and 5·5 respectively. Additionally, a higher frequency of gains of 1q and losses of 11q material was seen in intermediately-differentiated in contrast to well-differentiated DCIS. Poorly-differentiated DCIS displayed a higher frequency of amplifications (17q12, 11q13) and a higher average rate of genetic imbalances (7·1). Analysis of adjacent invasive breast carcinoma revealed a genetic pattern almost identical to the one seen in the DCIS counterpart. These data characterize DCIS as a genetically far-advanced, heterogeneous lesion and as a direct precursor of invasive breast cancer. Copyright © 1999 John Wiley & Sons, Ltd.

Comparative genomic hybridization of ductal carcinoma in situ of the breast-evidence of multiple genetic pathways.

Invasive breast cancer shows a wide range of morphological differentiation, associated with differences in prognosis, but as yet, the underlying genetic mechanisms cannot be accounted for. In order to establish a model of the possible progression from the different subtypes of ductal carcinoma in situ (DCIS) to invasive breast cancer, 77 selected cases of invasive breast cancer representing distinct morphological subtypes were investigated by means of comparative genomic hybridization (CGH). There was a high degree of genetic homology between tubular and tubulo-lobular carcinoma and well-differentiated DCIS, and between ductal invasive carcinoma G3 and poorly differentiated DCIS. Highly differentiated invasive breast cancers were characterized by a loss of 16q and a low average number of aberrations per case. In high-grade tumours, losses of this chromosomal region were seen with a much lower frequency in cases with evidence of an aneuploid tumour status. These data demonstrate the close genetic similarity of well-, intermediately, and poorly differentiated DCIS and distinct morphological types of invasive breast carcinoma, providing further evidence that DCIS is a direct precursor lesion of invasive breast cancer and that various evolutionary genetic pathways exist.

Different genetic pathways in the evolution of invasive breast cancer are associated with distinct morphological subtypes.

Aims:

This study was performed to determine the diagnostic value of keratin 5/6 (CK 5/6) immunophenotyping on routinely processed breast tissues.

Methods and results:

Six hundred and ninety-nine breast lesions, including normal tissues as well as benign and malignant lesions in 321 formalin-fixed, paraffin-embedded samples from 158 different patients were investigated immunohistochemically, following wet autoclave pre-treatment for antigen retrieval. In normal breast tissues, both myoepithelial and luminal epithelial cells expressed CK 5/6 in varying amounts. While myoepithelial immunoreactivity was most pronounced in the duct system, luminal epithelial immunoreactivity was strongest in the terminal duct lobular units. In ductal hyperplasias (DH), luminal epithelial cells predominantly revealed CK 5/6 immunoreaction. In contrast, neoplastic epithelial cells in atypical ductal and lobular hyperplasias (ADH and ALH) lacked such an expression, whereas in ductal in-situ carcinomas (DCIS) and in infiltrating ductal carcinomas 3.7% and 7.7%, of the cases respectively, showed positive immunostaining for CK 5/6.

Conclusions:

Immunophenotyping of keratin 5/6 expression can be helpful in the diagnosis of atypical hyperplasias and in-situ carcinomas of the breast. It is particularly valuable in the differential diagnosis between benign and atypical proliferative lesions.

Cytokeratin 5/6 immunohistochemistry assists the differential diagnosis of atypical proliferations of the breast.

Telomerase, a cellular reverse transcriptase, has been detected in the majority of human malignant tumors, where it provides an escape mechanism from proliferative limitations due to progressive telomere erosion with each cell division. In this study, we used a non-radioactive telomeric repeat amplification protocol (TRAP) with an internal telomerase assay standard for the detection and semiquantitative analysis of 98 single frozen sections of normal breast tissue and benign and malignant breast lesions on an automated laser-fluorescence sequencer. Telomerase activity was detected in 36 of 40 (90%) infiltrating breast carcinomas, whereas no activity was found in nonmalignant breast tissues including blunt duct adenosis, papilloma, ductal hyperplasia and atypical ductal hyperplasia. However, telomerase activity was detected in 59% of ductal in situ carcinomas, suggesting that telomerase reactivation is an early event in breast carcinogenesis. We found a positive correlation between telomerase activity levels and cell proliferation determined by MIB1 immunostaining. No correlation, however, could be demonstrated between telomerase activity and other known breast cancer prognostic indicators. Telomerase activity was also detected in 60% of fibroadenomas indicating that careful interpretation of analysis of telomerase activity in fine needle aspirates is required, since low telomerase activity may not necessarily be an indicator of malignancy in breast tissue.

Friedrich Otterbach

Papers

Comparative genomic hybridization of ductal carcinoma in situ of the breast-evidence of multiple genetic pathways.

Different genetic pathways in the evolution of invasive breast cancer are associated with distinct morphological subtypes.

Cytokeratin 5/6 immunohistochemistry assists the differential diagnosis of atypical proliferations of the breast.

Telomerase activity in human proliferative breast lesions.