An imprinted gene network that controls mammalian somatic growth is down-regulated during postnatal growth deceleration in multiple organs

Abstract

In mammals, somatic growth is rapid in early postnatal life but decelerates with age and eventually halts, thus determining the adult body size of the species. This growth deceleration, which reflects declining proliferation, occurs simultaneously in multiple organs yet appears not to be coordinated by a systemic mechanism. We, therefore, hypothesized that growth deceleration results from a growth-limiting genetic program that is common to multiple tissues. Here, we identified a set of 11 imprinted genes that show down-regulation of mRNA expression with age in multiple organs. For these genes, Igf2, H19, Plagl1, Mest, Peg3, Dlk1, Gtl2, Grb10, Ndn, Cdkn1c, and SLC38a4, the declines show a temporal pattern similar to the decline in growth rate. All 11 genes have been implicated in the control of cell proliferation or somatic growth. Thus, our findings suggest that the declining expression of these genes contributes to coordinate growth deceleration in multiple tissues. We next hypothesized that the coordinate decline in expression of these imprinted genes is caused by altered methylation and consequent silencing of the expressed allele. Contrary to this hypothesis, the methylation status of the promoter regions of Mest, Peg3, and Plagl1 did not change with age. Our findings suggest that a set of growth-regulating imprinted genes is expressed at high levels in multiple tissues in early postnatal life, contributing to rapid somatic growth, but that these genes are subsequently downregulated in multiple tissues simultaneously, contributing to coordinate growth deceleration and cessation, thus imposing a fundamental limit on adult body size.

Fig. 1.

Temporal changes in expression of imprinted genes in mice. Changes in gene expression were analyzed by microarray in mouse lung, kidney, and heart from 1 to 8 wk of age. Heat maps were constructed for all known imprinted genes using JMP software ver. 7. Green rectangles represent down-regulated genes, and red rectangles represent up-regulated genes compared with 1-wk-old animals. The color intensity corresponds to the magnitude of the change from baseline (log2 [value at later time point/value at baseline time point]). The dendrogram on the right side of the heat map shows the hierarchal clustering of genes imposed by the software package, which grouped genes with similar expression patterns into 4 major clusters (colored red, green, blue, and yellow).

Fig. 2.

Temporal changes in organ and whole body size in mice. Weight of liver, kidney, lung, and the whole animal, and length of tail and tibia were measured at 1, 4, and 8 wk of age.

Fig. 3.

Concordant temporal changes in a set of imprinted genes in multiple organs. Expression of Plagl1, Mest, Peg3, Dlk1, Gtl2, Igf2, H19, Ndn, Grb10, Cdkn1c, and Slc38a4, were determined by real-time PCR in liver, kidney, and lung at four different time points. Except Slc38a4 in the liver, there is a strikingly consistent decrease in expression from 1 to 4 wk of age in all three organs. E19, embryonic day 19.

Fig. 4.

DNA methylation status of three different imprinted genes did not change with time. A: schematic diagram depicting the relative position of CpGs studied in the DMRs of Plagl1, Mest, and Peg3. B: percent methylation at CpG dinucleotides within the DMRs of Plagl1, Mest, and Peg3 in liver at different ages determined by bisulfite sequencing. There is ∼50% methylation at each site. Although there is some variability among different positions, values at each position show minimal change with age.