Microsatellite analysis of allelic imbalance in tumour and blood from patients with prostate cancer

Liquid biopsy with cell free DNA: new horizons for prostate cancer

Although prostate cancer (PCa) is one of the most common tumors in European males, the only minimally invasive diagnostic tool in PCa setup is the determination of PSA in serum. Cell-free DNA (cfDNA) has been demonstrated to be helpful for PCa diagnosis but has not yet been integrated into the clinical setting. This review aims to provide a systematic update of cfDNA and its fragmentation patterns in PCa reported in literature published over the last twenty years. Due to the high variability of the scientific methods adopted and a lack of standardized median cfDNA levels, results fluctuate across different studies. These differences may be due to the cfDNA source, the quantification method, or the fragmentation pattern. Blood plasma is the most frequently analyzed biological fluid, but seminal plasma has been reported to contain higher cfDNA concentration due to its vicinity to the tumor origin. CfDNA has been shown to be composed of single-stranded (ssDNA) and double-stranded DNA (dsDNA), so the total cfDNA concentration should be preferred as it corresponds best to the tumor mass. Fluorometry and capillary electrophoresis (CE) may be quick and cost-effective tools for cfDNA assessment in a clinical setting. The greatest future challenge is the elaboration of common guidelines and standardized procedures for diagnostic laboratories performing cfDNA analysis. A multiparametric approach combining the analysis of total cfDNA (both ssDNA and dsDNA), cfDNA fragment length, and specific genetic mutations (ctDNA assessment) is required for optimal future applications of liquid biopsy.

Detection of Loss of Heterozygosity (LOH) Using Circulating Cell-free DNA (cfDNA) by Fluorescence-based Multiplex PCR for Identification of Patients With Prostate Cancer

Several lines of evidence suggest that loss of heterozygosity (LOH) in specific chromosomal regions is a common mechanism for the inactivation of tumor-suppressor genes that are implicated in the pathogenesis of prostate cancer (PCa). Short tandem repeat (STR) sequences are extremely reliable genetic markers for the detection of LOH associated with cancers. Hence, in the current study, we investigated the detection of LOH at 6 STR markers (D8S360, D9S1748, D9S171, D8S137, D6S1631, and THRB) using blood circulating cell-free DNA (cfDNA), which can be used to distinguish PCa from benign prostatic hyperplasia (BPH). A total of 136 individuals were included in the study, 76 male patients diagnosed with PCa (50 male patients with localized PCa and 26 male patients with metastatic PCa) as experimental subjects and 60 male patients with BPH as controls. Circulating cfDNA was extracted from plasma samples and amplified with fluorescence-labeled primers specific for known STR markers. We also evaluated the serum prostate-specific antigen in both groups. Our findings revealed that the frequency of LOH at D8S360, D9S1748, D9S171, D8S137, and D6S1631 was significantly higher in PCa subjects than in controls (P<0.05). Of the 6 STR markers, LOH at D8S360 could discriminate metastatic PCa from localized PCa. We found that 71.05% of patients with PCa and 1.66% of BPH subjects had LOH at least at 3 of the markers in cfDNA. Our findings provide additional evidence to support the hypothesis that analysis of LOH at D8S360, D9S1748, D9S171, D8S137, and D6S1631 STR markers using cfDNA can be applied as a noninvasive diagnostic approach for the detection of PCa.

Feasibility and clinical value of circulating tumor DNA testing in patients with gastric adenocarcinomas

Gastric cancer is one of the leading causes of cancer worldwide, and this trend appears to be rising. Most patients are diagnosed at an advanced stage and thus prognosis is poor. Liquid biopsy, or circulating tumor DNA (ctDNA) testing, is emerging as a promising prognostic and/or predictive biomarker for patients with various types of malignancies. Its value and utility for patients with gastrointestinal malignancies, particularly gastric cancer is still being explored. There is ongoing research in other tumor types to suggest that ctDNA testing can be potentially used to identify tumor specific genomic alterations, predict tumor mutation burden, as well as help assess clinical response. We report on the feasibility and clinical value of ctDNA testing in patients with gastric cancers in a real time clinical context by reporting data on cohort of patients with gastric cancers (including those with gastroesophageal junction adenocarcinomas) treated at our institution.

Notes for developing a molecular test for the full characterization of circulating tumor cells

The proved association between the circulating tumor cell (CTC) levels and the patients' survival parameters has been growing interest to investigate the molecular profile of these neoplastic cells among which hide out precursors capable of initiating a new distant metastatic lesion. The full characterization of the tumor cells in peripheral blood of cancer patients is expected to be of help for understanding and (prospectively) for counteracting the metastatic process. The major hitch that is hampering the successful gaining of this result is the lack of a consensus onto standard operating procedures (SOPs) for performing what we generally define as the "liquid biopsy". Here we review the more recent acquisitions in the analysis of CTCs and tumor related nucleic acids, looking to the main open questions that are hampering their definitive employ in the routine clinical practice.

When Prostate Cancer Circulates in the Bloodstream

Management of patients with prostate cancer is currently based on imperfect clinical, biological, radiological and pathological evaluation. Prostate cancer aggressiveness, including metastatic potential, remains difficult to accurately estimate. In an attempt to better adapt therapeutics to an individual (personalized medicine), reliable evaluation of the intrinsic molecular biology of the tumor is warranted, and particularly for all tumor sites (primary tumors and secondary sites) at any time of the disease progression. As a consequence of their natural tendency to grow (passive invasion) or as a consequence of an active blood vessel invasion by metastase-initiating cells, tumors shed various materials into the bloodstream. Major efforts have been recently made to develop powerful and accurate methods able to detect, quantify and/or analyze all these circulating tumor materials: circulating tumors cells, disseminating tumor cells, extracellular vesicles (including exosomes), nucleic acids, etc. The aim of this review is to summarize current knowledge about these circulating tumor materials and their applications in translational research.

Loss of Heterozygosity in Thyroid Hormone Receptor Beta in Invasive Breast Cancer

Background

Loss of heterozygosity (LOH) on chromosome arm 3p, where the gene of thyroid hormone receptor beta (THRB) is located, has been reported in breast cancer. Although some studies performed in vitro have suggested that THRB could act as a tumor suppressor in breast cancer development, there is still no unequivocal evidence to support this.

Methods

To determine the role of LOH in breast tumor development, the LOH of THRB and its proximal microsatellite markers D3S1293, D3S3659, D3S3700, D3S2307 and D3S2336 was investigated in a genomic region spanning ~3.3 Mb in tumor specimens and in corresponding normal tissues of 74 invasive breast cancer patients. The association was analyzed between LOH in microsatellite markers and clinicopathological characteristics.

Results

LOH was detected in D3S1293 (36.7%), THRB (59.4%), D3S3659 (37.5%) and D3S3700 (55.2%) among the informative cases, while LOH was not detected in D3S2307 and D3S2336. Cases exhibited LOH of 52.8%-71.4% if any 2 markers were combined and analyzed out of the first 4 microsatellite markers. LOH in THRB was associated with negative estrogen receptor (ER), negative progesterone receptor (PR), both negative estrogen receptor and progesterone receptor (HR) and human epidermal growth factor receptor-2 (HER2) and lymph node metastasis (p = 0.0001, p = 0.005, p = 0.001 and p = 0.018). The association with negative PR in LOH in THRB and/or D3S1293 was pronounced (p<0.0001). LOH in D3S3700 showed an association with lymph node metastasis (p = 0.014). This association was enhanced if D3S3700 was combined with THRB or D3S3659 (p = 0.0004, p = 0.0002).

Conclusions

LOH in THRB and its proximal microsatellite markers may play a role in tumorigenesis and development in invasive breast cancer.

Circulating nucleic acids as biomarkers of prostate cancer

Prostate cancer, the most common cancer of western men, requires new biomarkers, especially given that the benefits of PSA testing remain uncertain. Nucleic acids can now be accurately and sensitively detected in human blood. Over the last decade, investigations into utility of circulating cell-free miRNA, DNA and mRNA as novel biomarkers have expanded exponentially. In the near future, they may be routinely used to accurately diagnose cancers, stratify indolent from aggressive disease and inform treatment decisions. However, advancement of such tests into clinical settings is hampered by technical problems with assay specificity and sensitivity, and small study sizes. This review highlights the different forms of circulating nucleic acids and those that show the most potential as viable biomarkers for prostate cancer.

Nucleic acid-based biomarkers in body fluids of patients with urologic malignancies

This review focuses on the promising potential of nucleic acids in body fluids such as blood and urine as diagnostic, prognostic, predictive and monitoring biomarkers in urologic malignancies. The tremendous progress in the basic knowledge of molecular processes in cancer, as shown in the companion review on nucleic acid-based biomarkers in tissue of urologic tumors, provides a strong rationale for using these molecular changes as non-invasive markers in body fluids. The changes observed in body fluids are an integrative result, reflecting both tissue changes and processes occurring in the body fluids. The availability of sensitive methods has only recently made possible detailed studies of DNA- and RNA-based markers in body fluids. In addition to these biological aspects, methodological aspects of the determination of nucleic acids in body fluids, i.e. pre-analytical, analytical and post-analytical issues, are particularly emphasized. The characteristic changes of RNA (differential mRNA and miRNA expression) and DNA (concentrations, integrity index, mutations, microsatellite and methylation alterations) in serum/plasma and urine samples of patients suffering from the essential urologic cancers of the prostate, bladder, kidney and testis are summarized and critically discussed below. To translate the promising results into clinical practice, laboratory scientists and clinicians have to collaborate to resolve the challenges of harmonized and feasible pre-analytical and analytical conditions for the selected markers and to validate these markers in well-designed and sufficiently powered multi-center studies.

Diagnostic value of DNA alteration: loss of heterozygosity or allelic imbalance-promising for molecular staging of prostate cancers

The biological behavior of prostate cancer is uncertain, and therefore, search for molecular prognostic markers associated with disease progression seems to be essential. We performed microsatellite allelotyping of DNA isolated from primary prostate tumors biopsies (prostate adenocarcinoma, PCa). We evaluated the frequency of allelic imbalance (AI), including loss of heterozygosity and/or microsatellite imbalance (LOH/MSI) as well as the association of these DNA alterations with clinicopathological variables. We assessed the significance of LOH/MSI alterations in selected imprinted and non-imprinted chromosomal regions (IR and NIR) in PCa. A total of 50 biopsies of prostate tumor (containing >75 % tumor cells) were histologically examined confirming prostate carcinoma. Microsatellite allelotyping using 16 microsatellite markers linked to the following chromosomal regions: 1p31.2, 3p21.3–25.3, 7q32.2, 9p21.3, 11p15.5, 12q23.2, and 16q22.1 was performed. The incidence of LOH/MSI alterations in prostate tumor cells was the highest for chromosomal regions 7q32.2 and 16q22.1 (31.25 and 26.60 %, respectively), followed by 1p31.2 and 3p21.3–25.3 (26.50 and 17.40 %, respectively). Statistically significant increase in LOH/MSI alterations has been observed for markers: D1S2137 (1p region; p = 0.00032), D9S974 (9p region; p = 0.0017), and D16S3025 (16q region; p = 0.0017). Statistically significant differences in frequency of LOH/MSI alterations in particular chromosomal regions have been found for 1p31.2, 7q32.2 and 16q22.1 (p = 0.027, p = 0.012 and p = 0.031, respectively). We documented statistically significant association between Fractional Allele Loss (FAL) index and advanced tumor stage (p < 0.05). We suggest that genomic instability of LOH/MSI type is a frequent event in prostate carcinogenesis and assessed as FAL index has clinical value for the molecular staging of prostate cancer in (TRUS)-guided prostate biopsy material.

NotI microarrays: novel epigenetic markers for early detection and prognosis of high grade serous ovarian cancer

Chromosome 3-specific NotI microarray (NMA) containing 180 clones with 188 genes was used in the study to analyze 18 high grade serous ovarian cancer (HGSOC) samples and 7 benign ovarian tumors. We aimed to find novel methylation-dependent biomarkers for early detection and prognosis of HGSOC. Thirty five NotI markers showed frequency of methylation/deletion more or equal to 17%. To check the results of NMA hybridizations several samples for four genes (LRRC3B, THRB, ITGA9 and RBSP3 (CTDSPL)) were bisulfite sequenced and confirmed the results of NMA hybridization. A set of eight biomarkers: NKIRAS1/RPL15, THRB, RBPS3 (CTDSPL), IQSEC1, NBEAL2, ZIC4, LOC285205 and FOXP1, was identified as the most prominent set capable to detect both early and late stages of ovarian cancer. Sensitivity of this set is equal to (72 ± 11)% and specificity (94 ± 5)%. Early stages represented the most complicated cases for detection. To distinguish between Stages I + II and Stages III + IV of ovarian cancer the most perspective set of biomarkers would include LOC285205, CGGBP1, EPHB1 and NKIRAS1/RPL15. The sensitivity of the set is equal to (80 ± 13)% and the specificity is (88 ± 12)%. Using this technique we plan to validate this panel with new epithelial ovarian cancer samples and add markers from other chromosomes.

Quantitative analysis of cell-free DNA in the plasma of gastric cancer patients

In the present study, an accurate and reproducible method for quantifying cell-free DNA (cfDNA) in human blood was established and tested for its ability to predict gastric cancer in patients. Using 'Alu81-qPCR' to amplify 81-bp Alu DNA sequences, we first estimated the amount of cfDNA in the serum or plasma of 130 patients with gastric cancer to identify which source of cfDNA is more suitable for the biomarker screening of these patients. The results of Alu81-qPCR revealed that the amount of cfDNA in the plasma was low compared with that in the serum, but was found at similar levels among the samples, indicating that the plasma may be a more suitable source of cfDNA for biomarker screening. For the 54 patients with gastric cancer and the 59 age-matched healthy controls, the mean levels of plasma cfDNA were 2.4-fold higher in the patient group compared with the control group, indicating that plasma cfDNA levels may be useful for predicting patients with gastric cancer. The results of our study suggest that Alu81-qPCR is a more reliable method than other techniques, such as the PicoGreen assay, for quantifying cfDNA in human blood, demonstrating the potential to complement current diagnostic procedures for the management of gastric cancer patients.

Allelic imbalance in oral lichen planus and assessment of its classification as a premalignant condition

OLP is a relatively common immune-mediated mucosal condition with a predilection for middle-aged women. Although classified as a premalignant condition, this classification remains controversial. Using stringent diagnostic criteria, some authors have found that OLP patients are not at increased risk for oral SCC. Credible but limited genetic evidence also indicates that epithelial tissues from OLP patients diagnosed using stringent criteria differs from premalignant or malignant oral lesions but is similar to epithelium from benign oral lesions. To further investigate this genetic line of evidence, biopsy specimens diagnosed as fibroma, OLP, low-grade dysplasia, high-grade dysplasia, and SCC were retrieved from the archives of the Oral Pathology Consultants at the Ohio State University. Using laser capture microdissection, tissue of interest was captured from each case and DNA subsequently extracted. Fluorescently labeled PCR primers were used to amplify DNA at 3 tumor suppressor gene loci (3p14.2, 9p21, and 17p13) and evaluated for LOH or microsatellite instability (MSI). OLP was found to be significantly different from low-grade dysplasia, high-grade dysplasia, and SCC when LOH/MSI was found at more than 1 loci (P = .011, P = .032, P = .003), but not different from benign fibromas (P = .395). In agreement with previous studies, well-documented cases of OLP diagnosed using stringent criteria exhibit a genetic profile more similar to a benign or reactive process than a premalignant/malignant one. These findings do not support the classification of OLP as a premalignant condition.

Genomic profiling of cell-free DNA in blood and bone marrow of prostate cancer patients

PURPOSE

To advance the characterization of tumor-associated cell-free DNA in blood and bone marrow (BM), a rapid profiling method using methylation-specific multiplex ligation-dependent probe amplification (MS-MLPA) was established. MS-MLPA detects genetic and epigenetic aberrations of 37 tumor suppressor genes (TSG) in a single reaction and might, therefore, avoid the cumbersome single gene analyses.

METHODS

The validity of MS-MLPA for using cell-free plasma DNA was assessed by analyzing blood and BM samples of 91 patients with prostate cancer. As reference analyses, the methylation patterns of 4 genes (CD44, E-cadherin, CDKN2A and PTEN) chosen from the TSG set of the MS-MLPA kit were investigated in single reactions by sodium bisulfite DNA sequencing.

RESULTS

Copy number changes and aberrant DNA methylation of 37 circulating TSG could be analyzed in BM and blood of 30 and 13 of the 91 patients, respectively, whereas the DNA content in the remaining samples was too low (<50 ng/μl of eluted DNA). The copy number of 28 of the 37 TSG was altered, and most changes were found for APC, CHFR, TP73 and GSTP1 genes in BM plasma. Statistical evaluations showed an association between copy number changes of TP73 and a positive resection margin of the prostate (p = 0.05). Both MS-MLPA and sodium bisulfite sequencing techniques showed that all genes were unmethylated.

CONCLUSIONS

Our results demonstrate the potential and limitation of MS-MLPA for multiplex characterization of TSG in cell-free plasma DNA as a new non-invasive approach to obtain information on the molecular tumor biology of individual cancer patients.

Cell-free DNA in the blood as a solid tumor biomarker--a critical appraisal of the literature

Circulating cell-free DNA (cfDNA) has been suggested as a cancer biomarker. Several studies assessed the usefulness of quantitative and qualitative tumor-specific alterations of cfDNA, such as DNA strand integrity, frequency of mutations, abnormalities of microsatellites, and methylation of genes, as diagnostic, prognostic, and monitoring markers in cancer patients. Most of the papers that could be evaluated in this review resulted in a positive conclusion. However, methodical diversity without the traceability of data and differently designed and often underpowered studies resulted in divergent results between studies. In addition, the limited diagnostic sensitivity and specificity of cfDNA alterations temper the effusive hope of novel tumor markers, raising similar issues as those for other tumor markers. To validate the actual clinical validity of various cfDNA alterations as potential cancer biomarkers in practice for individual tumor types, the main problems of the observed uncertainties must be considered in future studies. These include methodical harmonization concerning sample collection, processing, and analysis with the traceability of measurement results as well as the realization of well-designed prospective studies based on power analysis and sample size calculations.

Molecular analysis of the polymorphisms of thymidylate synthase on cell-free circulating DNA in blood of patients with advanced colorectal carcinoma

As a key enzyme in folate metabolism, the thymidylate synthase (TS) is important for the synthesis of nucleotides. Its polymorphisms may affect the TS gene expression and the susceptibility for Fluoropyrimidine (FU)-based chemotherapies. In this study, we assessed the relationship between the TS genotypes and clinical outcome to 5-FU-based chemotherapy, and examined whether cell-free circulating DNA is applicable for these molecular analyses. We combined the variable number tandem repeat (VNTR) and single nucleotide (SNP) polymorphisms of the TS promoter and the deletion variants (1494del6) in the 3'UTR with the occurrence of loss of heterozygosity (LOH) at the microsatellite markers D18S59, D18S1140, and D18S976 mapped up- and downstream to the TS locus. Cell-free blood DNA, tumor tissues, and leukocytes of 51 patients with advanced colorectal cancer were used. Genotyping revealed linkage disequilibrium between TS promoter and 3'UTR (p = 0.03) in blood and leukocytes. Inverse associations of the response to therapy with the number of polymorphisms (p = 0.05) and the 494del6 polymorphism (p < 0.02) were detected on serum DNA. The quantification of serum DNA showed significant correlations with the 1494del6 variant (p = 0.006) in tumor tissues and the combined polymorphisms in leukocytes (p < 0.04). In contrast to the low LOH frequency in blood, LOH spanned more than one marker in the primary tumor of the majority of the patients suggesting the loss of the entire TS locus. Our data provide insight into the molecular diversity of the regulation of the TS gene expression. This is the first study that compares multi-variant TS genotypes in different clinical specimens.

Blood and tissue biomarkers in prostate cancer: state of the art

The prevalence of prostate cancer (PCa) is high and increases with age. PCa is the most common cutaneous cancer in American men. Prostate-specific antigen (PSA) screening has impacted the detection of PCa and is directly responsible for a dramatic decrease in stage at diagnosis. Gleason score and stage at the time of diagnosis remain the mainstay to predict prognosis, in the absence of more accurate and reliable tissue or blood biomarkers. Despite extensive research efforts, very few biomarkers of PCa have been introduced to date in clinical practice. Even screening with PSA has recently been questioned. A thorough analysis of all tissue and serum biomarkers in prostate cancer research cannot be easily synthesized, and goes beyond the scope of the present article. Therefore the authors focus here on the most recently reported tissue and circulating biomarkers for PCa whose application in clinical practice is either current or expected in the near future.

Serum DNA can define tumor-specific genetic and epigenetic markers in gliomas of various grades

We evaluated whether cell-free circulating DNA can be used as a noninvasive approach for detection of genetic/epigenetic alterations in brain tumors during the course of the disease. Paired tumor-serum samples from 70 patients with either high-grade astrocytomas (n = 41) or oligodendrogliomas of various grades were analyzed. The median interval between surgery and serum sampling was 1 month (range 0.5-168 months). DNA was extracted from whole blood, serum, and paraffin-embedded tumor sections. Loss of heterozygosity (LOH) in chromosomes 1p, 19q, and 10q was assessed by polymerase chain reaction (PCR)-based microsatellite analysis. The methylation status of O(6)-methyl guanine methyltransferase (MGMT) and phosphatase and tensin homolog promoters was studied by methylation-specific PCR. LOH and/or methylation that could identify DNA as tumor-specific was found in 80.5% of astrocytic tumors and in all oligodendrogliomas. The rate of serum detection of these biomarkers was 51% and 55%, respectively, with specificity around 100%. The rate of serum detection did not differ between low- and high-grade oligodendrogliomas. Statistically significant tumor-serum concordance was found for MGMT methylation in both astrocytic tumors (83%; P < .001) and oligodendroglial tumors (72%; P < .003) and for LOH of 10q (79%; P < .002) and 1p (62%; P < .03) in oligodendrogliomas. We conclude that serum DNA in glial tumors is informative for both LOH and aberrant gene promoter methylation analysis during the course of the disease. The sensitivity is moderate and specificity is high for both low- and high-grade tumors. Future studies should identify a panel of biomarkers that bear the highest potential for clinical application.

The role of cell-free circulating DNA in the diagnosis and prognosis of prostate cancer

The presence of small amounts of circulating DNA in plasma was demonstrated 60 years ago. Since then, cell-free DNA has been tested for quantity, fragmentation pattern, and tumor-specific sequences in patients with various malignancies. Recent studies have shown that cell-free DNA levels are distinctly increased in most patients with prostate cancer (PCA) and that the DNA fragmentation pattern is different from healthy individuals and patients with benign prostate disease. The origin of this circulating DNA remains largely unknown, but it is established that a small fraction of the DNA is derived from the tumor itself, and genetic (allelic imbalances) and epigenetic (DNA methylation) alterations are regularly detected in patients with PCA. The detection of increased DNA levels and tumor-specific DNA sequences may provide diagnostic and prognostic information. The recent findings in the emerging field of cell-free DNA will be discussed.

Cell-free tumor DNA in blood plasma as a marker for circulating tumor cells in prostate cancer

PURPOSE

Circulating cell-free DNA in the blood of cancer patients harbors tumor-specific aberrations. Here, we investigated whether this DNA might also reflect the presence of circulating tumor cells (CTC).

EXPERIMENTAL DESIGN

To identify the source of cell-free DNA in blood, plasma derived from 81 patients with prostate cancer was examined for CTCs and cell-free DNA. An epithelial immunospot assay was applied for detection of CTCs, and a PCR-based fluorescence microsatellite analysis with a panel of 14 polymorphic markers was used for detection of allelic imbalances (AI).

RESULTS

The plasma DNA levels significantly correlated with the diagnosis subgroups of localized (stage M0, n = 69) and metastasized prostate cancer (stage M1, n = 12; P = 0.03) and with the tumor stage of these patients (P < 0.005). AI was found on cell-free DNA in plasma from 45.0% and 58.5% of M0 and M1 patients, respectively. Detection of CTCs showed that 71.0% or 92.0% of the M0 and M1 patients harbored 1 to 40 CTCs in their blood, respectively. The occurrence of CTCs correlated with tumor stage (P < 0.03) and increasing Gleason scores (P = 0.04). Notably, significant associations of the number of CTCs with the AI frequencies at the markers D8S137 (P = 0.03), D9S171 (P = 0.04), and D17S855 (P = 0.02) encoding the cytoskeletal protein dematin, the inhibitor of the cyclin-dependent kinase CDKN2/p16 and BRCA1, respectively, were observed.

CONCLUSIONS

These findings show, for the first time, a relationship between the occurrence of CTCs and circulating tumor-associated DNA in blood, which, therefore, might become a valuable new source for monitoring metastatic progression in cancer patients.

Review: Biological relevance of disseminated tumor cells in cancer patients

The prognosis of cancer patients is largely determined by the occurrence of distant metastases. In patients with primary tumors, this relapse is mainly due to clinically occult micrometastasis present in secondary organs at primary diagnosis but not detectable even with high resolution imaging procedures. Sensitive and specific immunocytochemical and molecular assays enable the detection and characterization of disseminated tumor cells (DTC) at the single cell level in bone marrow (BM) as the common homing site of DTC and circulating tumor cells (CTC) in peripheral blood. Because of the high variability of results in DTC and CTC detection, there is an urgent need for standardized methods. In this review, we will focus on BM and present currently available methods for the detection and characterization of DTC. Furthermore, we will discuss data on the biology of DTC and the clinical relevance of DTC detection. While the prognostic impact of DTC in BM has clearly been shown for primary breast cancer patients, less is known about the clinical relevance of DTC in patients with other carcinomas. Current findings suggest that DTC are capable to survive chemotherapy and persist in a dormant nonproliferating state over years. To what extent these DTC have stem cell properties is subject of ongoing investigations. Further characterization is required to understand the biology of DTC and to identify new targets for improved risk prevention and tailoring of therapy. Our review will focus on breast, colon, lung, and prostate cancer as the main tumor entities in Europe and the United States.