Instability in 18S and 5.8S rDNA copy numbers and 18S:5.8S ratios in longitudinally-collected human DNA samples detected by monochrome multiplex qPCR

Abstract

Ribosomal DNA encodes the structural RNAs of the ribosomes. Ribosomal DNA instability is a major contributor to aging in yeast, but its role in human aging and longevity is largely unknown. Human 45S rDNA encodes the 18S, 5.8S, and 28S ribosomal RNAs; ranges in copy number from 60 to > 800 per cell, distributed as tandem repeats along the short arms of the five acrocentric chromosomes (p13, p14, p15, p21, and p22); and is prone to frequent homologous and non-homologous recombination. Here we present two multiplex quantitative PCR assays, one for 18S rDNA normalized to the single copy gene beta-globin (HBB), and the other for 5.8S rDNA normalized to the single copy gene albumin (ALB). Longitudinally-collected pairs of DNAs from bloods drawn approximately 16 years apart from 40 females and 39 males, aged < 1 to 77 years at first blood draw, from the Utah CEPH families were assayed. Ribosomal DNA copy number varied over a four-fold range between subjects and increased approximately 14% across the lifespan. Longitudinal within-individual gains in copy number up to +68% and losses down to -25% were observed, while repeated assays of single DNA samples varied approximately +/- 10%. While 18S and 5.8S rDNA tended to be gained and lost together, the 18S:5.8S ratio was also unstable longitudinally, with increases up to +19% and decreases down to -15% observed. The 18S:5.8S ratio at second draw, relative to that at first draw, increased significantly across the lifespan in males, but not in females. To our knowledge this is the first report of within-individual longitudinal changes in the human 18S and 5.8S rDNA copy numbers. These assays will facilitate investigations of the biology of ribosomal RNA genes and their roles in health and disease across the human lifespan.