LncRNA expression and increased DNA methylation in a distal region upstream from the alphaS1 casein gene after once daily milking
Résumé
Milking regulates both milk secretion and synthesis and its frequency in ruminants has been shown to have long lasting effects on milk production. The mechanisms involve a number of genes that are differentially expressed (1, 2). In this study, we hypothesized that a decrease in milking frequency induces methylation of CpG located within regulatory regions of genes expressed in the mammary gland. This hypothesis has been challenged on a candidate gene, the αS1-casein (CSN1S1 - a major milk protein) for which a distal regulatory region had been previously described (3, 4). We used an original animal model which allows studying epigenetic alterations within identical genetic and environment backgrounds. Milk production from the left and right udeer halves of eight heifers were studied over three periods of 7 days each. During the first and last periods, both udder halves were milked twice daily for 5 days, whereas during the second period, the right and left udder halves were differentially milked twice and once a day, respectively. Once daily milking (ODM) reduced the accumulation levels in the mammary gland of the main milk protein mRNAs (RNASeq), which represent up to 70% of mRNA expressed during lactation. It slightly increased the methylation level of the whole genome in the mammary gland (75.9%), as compared to that observed after twice daily milking (TDM); (74.6%, P=0.04); (LUMA). Simultaneously, it decreased the CSN1S1 mRNA level by 50%. The methylation level of four CpG sites located close to the distal regulatory region of this gene was evaluated in relation to gene expression. After ODM, the methylation level of the three more distal CpGs (relative to the transcription start point of the CSN1S1 gene) is higher as compared to that observed after TDM (P<0.025, P~0.05, and P=0.025, respectively), (pyrosequencing or sequencing of cloned DNA fragments after bisulfite conversion). It is however much lower that that observed at mid pregnancy when the gene is expressed to very low level or in the liver which does not express the gene. Variations in DNA methylation of those three CpG sites may partly explain the two fold lower. CSN1S1 gene expression observed after TDM, ODM did not modify the methylation level of the fourth CpG site but this level was much lower than that observed at mid pregnancy or in the liver. This CpG is located within a low affinity binding site for Stat5 and 16nt upstream of a second high affinity binding site for Stat5. The fact that its methylation profile is not altered after ODM may explain why CSN1S1 gene transcription is not more deeply reduced by this treatment. Furthermore, these two Stat5 binding sites may play a role in the transcription of an antisense non-coding RNA which occurs nearby, which is specific of the mammary gland and is not affected by ODM. References: 1 Littlejohn et al. (2010) Physiol Genomics 41:21-32. 2 Galio et al. (submitted for publication). 3. Vanselow et al. (2006) J Mol Endocrinol. 37:463-77. 4 Singh et al. (2012) Animal 6:375-81.