Nucleotide sequence of an unequal sister chromatid exchange site in a mouse myeloma cell line

Human rDNA and Cancer

In human cells, each rDNA unit consists of the ~13 kb long ribosomal part and ~30 kb long intergenic spacer (IGS). The ribosomal part, transcribed by RNA polymerase I (pol I), includes genes coding for 18S, 5.8S, and 28S RNAs of the ribosomal particles, as well as their four transcribed spacers. Being highly repetitive, intensively transcribed, and abundantly methylated, rDNA is a very fragile site of the genome, with high risk of instability leading to cancer. Multiple small mutations, considerable expansion or contraction of the rDNA locus, and abnormally enhanced pol I transcription are usual symptoms of transformation. Recently it was found that both IGS and the ribosomal part of the locus contain many functional/potentially functional regions producing non-coding RNAs, which participate in the pol I activity regulation, stress reactions, and development of the malignant phenotype. Thus, there are solid reasons to believe that rDNA locus plays crucial role in carcinogenesis. In this review we discuss the data concerning the human rDNA and its closely associated factors as both targets and drivers of the pathways essential for carcinogenesis. We also examine whether variability in the structure of the locus may be blamed for the malignant transformation. Additionally, we consider the prospects of therapy focused on the activity of rDNA.

Variability of Human rDNA

In human cells, ribosomal DNA (rDNA) is arranged in ten clusters of multiple tandem repeats. Each repeat is usually described as consisting of two parts: the 13 kb long ribosomal part, containing three genes coding for 18S, 5.8S and 28S RNAs of the ribosomal particles, and the 30 kb long intergenic spacer (IGS). However, this standard scheme is, amazingly, often altered as a result of the peculiar instability of the locus, so that the sequence of each repeat and the number of the repeats in each cluster are highly variable. In the present review, we discuss the causes and types of human rDNA instability, the methods of its detection, its distribution within the locus, the ways in which it is prevented or reversed, and its biological significance. The data of the literature suggest that the variability of the rDNA is not only a potential cause of pathology, but also an important, though still poorly understood, aspect of the normal cell physiology.

Direct-repeat analysis of chromatid interactions during intrachromosomal recombination in mouse cells

Homologous intrachromosomal recombination between linked genes can involve interactions that are either intramolecular (intrachromatid) or intermolecular (sister chromatid). To assess the relative proportions of chromatid interactions, we report studies of intrachromosomal recombination in mouse L cells containing herpes simplex virus thymidine kinase genes in two alternative configurations of direct repeats. By comparing products of reciprocal exchanges between these two configurations, we conclude that the majority of interactions that give rise to crossover products involve unequally paired sister chromatids after DNA replication. Analyses of an additional class of crossover products that involve discontinuous associated gene conversion suggest that these recombination events involve a heteroduplex DNA intermediate.

Direct-repeat analysis of chromatid interactions during intrachromosomal recombination in mouse cells

Homologous intrachromosomal recombination between linked genes can involve interactions that are either intramolecular (intrachromatid) or intermolecular (sister chromatid). To assess the relative proportions of chromatid interactions, we report studies of intrachromosomal recombination in mouse L cells containing herpes simplex virus thymidine kinase genes in two alternative configurations of direct repeats. By comparing products of reciprocal exchanges between these two configurations, we conclude that the majority of interactions that give rise to crossover products involve unequally paired sister chromatids after DNA replication. Analyses of an additional class of crossover products that involve discontinuous associated gene conversion suggest that these recombination events involve a heteroduplex DNA intermediate.