Intraindividual rDNA copy number variation and methylation in humans
The absolute number of rDNA transcription units can vary by about one power of ten among individuals. Apart from extensive rDNA copy number (CN) variation and instability in many cancers, there is little information on the extent of intraindividual CN variation between normal s. Here we used droplet digital PCR and deep bisulfite sequencing to determine both the absolute rDNA CN and the number of presumably active CN with a hypomethylated (0-10%) promoter region in up to six different s of 13 autopsy probands. In general, the absolute rDNA CN as well as the frequency of the minor A variant were highly similar between s (cerebellum, cerebrum, colon, heart, intestine, kidney, and liver  liver and spleen) of the same individual. However, in some probands absolute CN in one or multiple s was much higher than in the other s., consistent with relaxation/breakdown of the CN control system. The amplified copies were inactivated by promoter methylation and, thus, the number of active CN was largely independent from absolute CN. Collectively, our data suggest that with some notable exceptions absolute and even more active rDNA CN are maintained during development and differentiation in different s of the same individual. Despite the low intraindividual variation active CN appeared to systematically vary between s. Cerebellum, which is characterized by the highest density of non-dividing neuronal cells in the body, consistently exhibited lower active CN than other s. We estimate that a minimum number of > 50 active rDNA TU is required for normal /organ function. Overall design: Deep bisulfite sequencing and droplet digital PCR were used to determine the methylation state of ribosomal DNA core promototer/upstream controls element and 28 rDNA, respectively, in up to 6 different s of 13 autopsy probands