Ribosomal genes in inbred mouse strains: Interstrain and intrastrain variation of copy number and extent of methylation

Quantitative dot hybridization was used to estimate the rDNA copy number in brain tissues of five inbred mouse strains (AKR/JY, NZB/B1OrlY, CBA/CaLacY, 101/HY, and 129/JY), which were obtained from the collection of the Research Center of Biomedical Technologies (Y). In each strain, 9–12 mice aged 1–2 months were examined. The rDNA copy number per diploid genome in strains AKR (range 105–181, mean ± SD 136 ± 27) and NZB (129–169, 148 ± 12) was significantly lower than in strains CBA (172–267, 209 ± 31), 101 (179–270, 217 ± 30), and 129 (215–310, 264 ± 33). Mice of strain NZB were relatively homogeneous in this trait (CV = 8.1%). Strains AKR, CBA, 101, and 129 displayed significant between-group differences, CV varying from 12.5 to 19.9%. The same DNA specimens were digested with MspI or HpaII and used to estimate the extent of methylation of the 28S rDNA region. Regardless of the strain, all mice could be classed into two groups. One group (20 mice) had a methylated fraction accounting for less than 8% of rDNA and included all nine mice of strain NZB, seven out of nine mice of strain 101, and three out of ten mice of strain 129. In the other group (29 mice of strains AKR, CBA, 101, and 109), the methylated fraction varied from 18 to 38%. A possible role of methylation and the genome dosage of ribosomal genes in phenotypic variation (quantitative trait variation) of inbred mouse strains is discussed.