Asynchronous DNA replication detected by fluorescence in situ hybridisation as a possible indicator of genetic damage in human lymphocytes

Replication timing and nuclear structure

DNA replication proceeds along spatially and temporally coordinated patterns within the nucleus, thus protecting the genome during the synthesis of new genetic material. While we have been able to visualize replication patterns on DNA fibers for 50 years, recent developments and discoveries have provided a greater insight into how DNA replication is controlled. In this review, we highlight many of these discoveries. Of great interest are the physiological role of the replication timing program, cis and trans-acting factors that modulate replication timing and the effects of chromatin structure on the replication timing program. We also discuss future directions in the study of replication timing.

DNA replication timing, genome stability and cancer: late and/or delayed DNA replication timing is associated with increased genomic instability

Normal cellular division requires that the genome be faithfully replicated to ensure that unaltered genomic information is passed from one generation to the next. DNA replication initiates from thousands of origins scattered throughout the genome every cell cycle; however, not all origins initiate replication at the same time. A vast amount of work over the years indicates that different origins along each eukaryotic chromosome are activated in early, middle or late S phase. This temporal control of DNA replication is referred to as the replication-timing program. The replication-timing program represents a very stable epigenetic feature of chromosomes. Recent evidence has indicated that the replication-timing program can influence the spatial distribution of mutagenic events such that certain regions of the genome experience increased spontaneous mutagenesis compared to surrounding regions. This influence has helped shape the genomes of humans and other multicellular organisms and can affect the distribution of mutations in somatic cells. It is also becoming clear that the replication-timing program is deregulated in many disease states, including cancer. Aberrant DNA replication timing is associated with changes in gene expression, changes in epigenetic modifications and an increased frequency of structural rearrangements. Furthermore, certain replication timing changes can directly lead to overt genomic instability and may explain unique mutational signatures that are present in cells that have undergone the recently described processes of "chromothripsis" and "kataegis". In this review, we will discuss how the normal replication timing program, as well as how alterations to this program, can contribute to the evolution of the genomic landscape in normal and cancerous cells.

Epigenetic analyses in blood cells of men suspected of prostate cancer predict the outcome of biopsy better than serum PSA levels

Lymphocytes from the peripheral blood of patients with prostate cancer-the most frequent (noncutaneous) tumor in men-display epigenetic aberrations (altered modes of allelic replication) characteristic of the malignant phenotype. The present study aims to determine whether replication aberrations add certainty to the suspicion of prostate cancer provided by the prostate-specific antigen (PSA) blood test. The allelic replication mode (whether synchronous or asynchronous) was exemplified for RB1 and AML1. These two genes normally exhibit a synchronous mode of allelic replication. Fluorescence in situ hybridization (FISH) replication assay was used for replication analyses. The FISH assays were applied to PHA-stimulated lymphocytes, established from peripheral blood samples of 35 men referred to biopsy due to suspected prostate cancer. Following biopsy 13 out of these 35 men were found positive for prostate malignancy. The FISH assay-showing asynchronous or synchronous RB1 and AML1 replication-was able to predict, respectively, the results of all biopsy-positive men and in 18 out of the 22 biopsy-negative ones. These measurements, distinguishing biopsy-positive from biopsy-negative men, were highly significant (P < 10(-8); 100% sensitivity and 81.8% specificity). Yet, distinguishing between the two groups of men based on the PSA measurements was nonsignificant (P > 0.70). The FISH replication assay applied to peripheral blood lymphocytes of 35 men referred for biopsy significantly predicted the outcome of the pathological examination, more precisely than the serum PSA test. As such, the epigenetic alteration offers a potential noninvasive blood marker, complementary to the PSA, for a preliminary prostate cancer diagnosis.