Mammary gland morphogenesis begins during embryonic development and proceeds postnatally through puberty, pregnancy, lactation, and subsequent involution. Most of the development and functional differentiation in the mammary gland, therefore, occurs after birth.- During three major developmental windows—puberty, pregnancy, and involution—the gland undergoes profound morphological and functional changes [1
]. These changes correspond to periods of cell proliferation, apoptosis, and differentiation in conjunction with changes in gene expression patterns [2
] and are regarded as a succession of cell fate determinations [9
]. During the past decades, we have gained knowledge about the numerous signaling pathways involved in establishing these expression patterns and morphological changes, which have been reviewed by Watson and Khaled [10
Epigenetics has been defined as the “stable alterations in gene expression potential that arise during development and proliferation” [11
]. These alterations have been shown among others to be involved in development of the central nervous system [12
], the pancreas [13
], the liver [14
], and the male or female reproductive organs, [15
] and during differentiation of hematopoietic progenitors and T-helper-cells [16
]. Therefore, such epigenetic modifications can be expected to play a role during mammary gland development, as well. Furthermore, epigenetics also may be defined as “the manifestation of a phenotype, which can be transmitted to the next generation of cells or individual, without alterations to the DNA sequence (genotype)” [21
]. In general, epigenetics has been interpreted in the context of changes to the chromatin but could be interpreted more widely to include any external effect on the phenotype (epigenator).
Mammary gland development enables lactation to occur after parturition, and lactation performances in domestic animals have been largely improved in ruminants by genetic selection [22
]. However, the environment during mammary gland development from fetal life to pregnancy and lactation, also can influence lactation in genetically selected animals, thus altering the expected performances of an animal [23
]. The resulting phenotype is, therefore, not only related to the genotype of the animal but might be related to epigenetic modifications of the genome, resulting in a specific epigenotype.
At the biochemical level, epigenetic changes lead to alterations in chromatin conformation. These changes in chromatin are brought about by DNA methylation (DNAme) [24
], histone variants [25
], post-translational modifications of the core and N-terminal tails of histones [26
], non-histone chromatin proteins [27
], and non-coding RNAs (nc RNA) [30
Large-scale chromatin conformation represents another level of epigenetic regulation. Experimental evidence in eukaryotic cells suggests that bending and looping of chromatin facilitates specific genomic interactions over distance [31
]. These interactions may occur between transcription activators bound to enhancers and transcription machinery at the promoter, they can also insulate a gene domain from the action of a repressive chromatin environment.
The mechanisms involved in epigenetics can be summarized in several steps [21
]. First, influences coming from outside the cell, such as a differentiation signal, environmental influences, and nutrition, can be considered as the “epigenator signal,” which is defined by protein-protein interactions. These external signals could generate an “epigenetic initiator signal,” which will determine where the modification will occur. The epigenetic initiator signal can be DNA-binding factors or ncRNAs. The epigenetic state then will be sustained through cell divisions, with the contribution of an “epigenetic maintainer signal” such as histone/DNA modifiers (enzymatic activities that convey modifications) or histone variants. Based on this paradigm, we can see how direct transcriptional regulation and cell memory regulation use similar mechanisms to change chromatin status (chromatin conformation) and affect gene transcription, both in the short term and in the long term.
Chromatin conformation is expected to play a key role in transcriptional regulation during mammary gland development. However, the precise changes in chromatin conformation/compaction involved in mammary gland development and differentiation are not well known. The mammary gland is an excellent model to study these processes because of its postnatal development and differentiation. Its easy access and the possibility of performing tissue reconstitution experiments also offer distinct advantages. Finally, the availability of numerous genetically engineered mouse models renders it an especially attractive model for studying specific changes in chromatin conformation. Despite these advantages, very little is known about chromatin status in the developing mammary gland and how it might integrate the many signaling pathways discovered during the past few decades, which are involved in mammary gland development and function.
Almost four decades ago, Marzluff and McCarty [33
] reported that the acetylation of what we now know as Histone H3 and H4 in mammary tissue was influenced by hormones and that this process was reversible and correlated with RNA transcription. They postulated that the reversible acetylation of histones in mouse mammary explants could play a role in transcriptional regulation by modifying DNA-histone interactions. Similarly, Hohmann and Cole [34
] observed differential lysine incorporation into histone fractions under the influence of lactogenic hormones. They hypothesized that these hormones regulate intracellularly the structure of new chromatin as it is being formed, a process that has been well established in non-mammary developmental systems.
It has taken 3 decades to return our attention to the changes in histone modifications and chromatin during mammary gland development and functional differentiation. This renewed interest is attributable in part to new technologies and the availability of complete genome sequences. These technologies enable us to study chromatin compaction, DNA methylation, and histone post-transcriptional modifications at specific genomic locations and, more recently, on a genome wide scale in more detail and more quantitatively than previously was possible [35
In this review, we summarize what is known about chromatin conformation and epigenetic modifications during normal mammary gland development and functional differentiation as marked by the expression of milk protein genes, such as casein and WAP. Furthermore, we address emerging data on epigenetics in mammary stem and progenitor cells.