Redundant and non-redundant cytokine-activated enhancers control Csn1s2b expression in the lactating mouse mammary gland

STAT5B leukemic mutations, altering SH2 tyrosine 665, have opposing impacts on immune gene programs

STAT5B is a vital transcription factor for lymphocytes. Here, the function of two STAT5B mutations from human T-cell leukemias: one substituting tyrosine 665 with phenylalanine (STAT5BY665F) and the other with histidine (STAT5BY665H), was interrogated. In silico modeling predicted divergent energetic effects on homodimerization with a range of pathogenicity. In primary T cells in vitro, STAT5BY665F showed gain-of-function, whereas STAT5BY665H demonstrated loss-of-function. Introducing the mutation into the mouse genome illustrated that the gain-of-function Stat5b Y665F mutation resulted in accumulation of CD8+ effector and memory and CD4+ regulatory T cells, altering CD8+/CD4+ ratios. In contrast, STAT5BY665H "knock-in" mice showed diminished CD8+ effector and memory and CD4+ regulatory T cells. In contrast to WT STAT5B, the STAT5BY665F variant displayed greater STAT5 phosphorylation, DNA binding, and transcriptional activity after cytokine activation, whereas the STAT5BY665H variant resembled a null. The work exemplifies how joining in silico and in vivo studies of single nucleotides deepens our understanding of disease-associated variants, revealing structural determinants of altered function, defining mechanistic roles, and, specifically here, identifying a gain-of-function variant that does not directly induce hematopoietic malignancy.

Disease-Associated Mutations of the STAT5B SH2 Domain Regulate Cytokine-Driven Enhancer Function and Mammary Development

Mammary gland development during pregnancy is controlled by lactogenic hormones via the JAK2-STAT5 pathway. Gene deletion studies in mice have revealed the crucial roles of both STAT5A and STAT5B in establishing the genetic programs necessary for the development of mammary epithelium and successful lactation. Several hundred single nucleotide polymorphisms (SNPs) have been identified in human STAT5B, although their pathophysiological significance remains largely unknown. The SH2 domain is vital for STAT5B activation, and this study focuses on the impact of two specific missense mutations identified in T cell leukemias, the substitution of tyrosine 665 with either phenylalanine (Y665F) or histidine (Y665H). By introducing these human mutations into the mouse genome, we uncovered distinct and opposite functions. Mice harboring the STAT5BY665H mutation failed to develop functional mammary tissue, resulting in lactation failure, while STAT5BY665F mice exhibited accelerated mammary development during pregnancy. Transcriptomic and epigenomic analyses identified STAT5BY665H as Loss-Of-Function (LOF) mutation, impairing enhancer establishment and alveolar differentiation, whereas STAT5BY665F acted as a Gain-Of-Function (GOF) mutation, elevating enhancer formation. Persistent hormonal stimulation through two pregnancies led to the establishment of enhancer structures, gene expression and successful lactation in STAT5BY665H mice. Lastly, we demonstrate that Olah, a gene known to drive life-threatening viral disease in humans, is regulated by STAT5B through a candidate four-partite super-enhancer. In conclusion, our findings underscore the role of human STAT5B variants in modulating mammary gland homeostasis and their critical impact on lactation.

Data-driven projections of candidate enhancer-activating SNPs in immune regulation

BACKGROUND

Millions of single nucleotide polymorphisms (SNPs) have been identified in humans, but the functionality of almost all SNPs remains unclear. While current research focuses primarily on SNPs altering one amino acid to another one, the majority of SNPs are located in intergenic spaces. Some of these SNPs can be found in candidate cis-regulatory elements (CREs) such as promoters and enhancers, potentially destroying or creating DNA-binding motifs for transcription factors (TFs) and, hence, deregulating the expression of nearby genes. These aspects are understudied due to the sheer number of SNPs and TF binding motifs, making it challenging to identify SNPs that yield phenotypic changes or altered gene expression.

RESULTS

We developed a data-driven computational protocol to prioritize high-potential SNPs informed from former knowledge for experimental validation. We evaluated the protocol by investigating SNPs in CREs in the Janus kinase (JAK) - Signal Transducer and Activator of Transcription (-STAT) signaling pathway, which is activated by a plethora of cytokines and crucial in controlling immune responses and has been implicated in diseases like cancer, autoimmune disorders, and responses to viral infections. The protocol involves scanning the entire human genome (hg38) to pinpoint DNA sequences that deviate by only one nucleotide from the canonical binding sites (TTCnnnGAA) for STAT TFs. We narrowed down from an initial pool of 3,301,512 SNPs across 17,039,967 nearly complete STAT motifs and identified six potential gain-of-function SNPs in regions likely to influence regulation within the JAK-STAT pathway. This selection was guided by publicly available open chromatin and gene expression data and further refined by filtering for proximity to immune response genes and conservation between the mouse and human genomes.

CONCLUSION

Our findings highlight the value of combining genomic, epigenomic, and cross-species conservation data to effectively narrow down millions of SNPs to a smaller number with a high potential to induce interferon regulation of nearby genes. These SNPs can finally be reviewed manually, laying the groundwork for a more focused and efficient exploration of regulatory SNPs in an experimental setting.

Transcriptional Regulation of Mammary Alveolar Proliferation and Differentiation during Early Pregnancy

Proliferation and differentiation of the mammary gland during pregnancy is regulated by a wide variety of factors. Using gene expression data, we have predicted that the E2F5 transcription factor has a role in the mammary gland during pregnancy. Using CUT&RUN for E2F5 in combination with gene expression data revealed that there were a number of E2F5 target genes associated with gene expression in early pregnancy, suggesting a critical role. This prediction was then functionally tested through analysis of mice with an ablation of E2F5 in the mouse mammary epithelium using a Floxed E2F5 in combination with MMTV-Cre. This revealed a striking delay in alveolar proliferation and differentiation at the early stages of pregnancy. The significance of this effect was lost in both a second pregnancy and by the mid-point of pregnancy in single parous females. Analysis of E2F5 targets revealed overlap but a slight divergence from the consensus E2F sequence and that E2F5 was bound to the promoter of many genes involved in cellular proliferation, including known regulators of pregnancy associated alveolar expansion such as Stat6. However, progesterone stimulation of cells in a reporter assay revealed that it counteracts the repression of Stat6 by E2F5, leading to expression of Stat6. Together these data reveal the complexity of the transcriptional regulation of alveolar proliferation in early pregnancy.

STAT5B SH2 variants disrupt mammary enhancers and the stability of genetic programs during pregnancy

During pregnancy, mammary tissue undergoes expansion and differentiation, leading to lactation, a process regulated by the hormone prolactin through the JAK2-STAT5 pathway. STAT5 activation is key to successful lactation making the mammary gland an ideal experimental system to investigate the impact of human missense mutations on mammary tissue homeostasis. Here, we investigated the effects of two human variants in the STAT5B SH2 domain, which convert tyrosine 665 to either phenylalanine (Y665F) or histidine (Y665H), both shown to activate STAT5B in cell culture. We ported these mutations into the mouse genome and found distinct and divergent functions. Homozygous Stat5bY665H mice failed to form functional mammary tissue, leading to lactation failure, with impaired alveolar development and greatly reduced expression of key differentiation genes. STAT5BY665H failed to recognize mammary enhancers and impeded STAT5A binding. In contrast, mice carrying the Stat5bY665F mutation exhibited abnormal precocious development, accompanied by an early activation of the mammary transcription program and the induction of otherwise silent genetic programs. Physiological adaptation was observed in Stat5bY665H mice as continued exposure to pregnancy hormones led to lactation. In summary, our findings highlight that human STAT5B variants can modulate their response to cytokines and thereby impact mammary homeostasis and lactation.

Cell-specific and shared regulatory elements control a multigene locus active in mammary and salivary glands

Regulation of high-density loci harboring genes with different cell-specificities remains a puzzle. Here we investigate a locus that evolved through gene duplication and contains eight genes and 20 candidate regulatory elements, including one super-enhancer. Casein genes (Csn1s1, Csn2, Csn1s2a, Csn1s2b, Csn3) are expressed in mammary glands, induced 10,000-fold during pregnancy and account for 50% of mRNAs during lactation, Prr27 and Fdcsp are salivary-specific and Odam has dual specificity. We probed the function of 12 candidate regulatory elements, individually and in combination, in the mouse genome. The super-enhancer is essential for the expression of Csn3, Csn1s2b, Odam and Fdcsp but largely dispensable for Csn1s1, Csn2 and Csn1s2a. Csn3 activation also requires its own local enhancer. Synergism between local enhancers and cytokine-responsive promoter elements facilitates activation of Csn2 during pregnancy. Our work identifies the regulatory complexity of a multigene locus with an ancestral super-enhancer active in mammary and salivary tissue and local enhancers and promoter elements unique to mammary tissue.

3D genomics and its applications in precision medicine

Three-dimensional (3D) genomics is an emerging discipline that studies the three-dimensional structure of chromatin and the three-dimensional and functions of genomes. It mainly studies the three-dimensional conformation and functional regulation of intranuclear genomes, such as DNA replication, DNA recombination, genome folding, gene expression regulation, transcription factor regulation mechanism, and the maintenance of three-dimensional conformation of genomes. Self-chromosomal conformation capture (3C) technology has been developed, and 3D genomics and related fields have developed rapidly. In addition, chromatin interaction analysis techniques developed by 3C technologies, such as paired-end tag sequencing (ChIA-PET) and whole-genome chromosome conformation capture (Hi-C), enable scientists to further study the relationship between chromatin conformation and gene regulation in different species. Thus, the spatial conformation of plant, animal, and microbial genomes, transcriptional regulation mechanisms, interaction patterns of chromosomes, and the formation mechanism of spatiotemporal specificity of genomes are revealed. With the help of new experimental technologies, the identification of key genes and signal pathways related to life activities and diseases is sustaining the rapid development of life science, agriculture, and medicine. In this paper, the concept and development of 3D genomics and its application in agricultural science, life science, and medicine are introduced, which provides a theoretical basis for the study of biological life processes.

Cell-specific and shared enhancers control a high-density multi-gene locus active in mammary and salivary glands

Regulation of high-density loci harboring genes with different cell-specificities remains a puzzle. Here we investigate a locus that evolved through gene duplication 1 and contains eight genes and 20 candidate regulatory elements, including a super-enhancer. Five genes are expressed in mammary glands and account for 50% of all mRNAs during lactation, two are salivary-specific and one has dual specificity. We probed the function of eight candidate enhancers through experimental mouse genetics. Deletion of the super-enhancer led to a 98% reduced expression of Csn3 and Fdcsp in mammary and salivary glands, respectively, and Odam expression was abolished in both tissues. The other three casein genes were only marginally affected. Notably, super-enhancer activity requires the additional presence of a distal Csn3 -specific enhancer. Our work identifies an evolutionary playground on which regulatory duality of a multigene locus was attained through an ancestral super-enhancer active in mammary and salivary tissue and gene-specific mammary enhancers.

Cell-specific and shared enhancers control a high-density multi-gene locus active in mammary and salivary glands

Regulation of high-density loci harboring genes with different cell-specificities remains a puzzle. Here we investigate a locus that evolved through gene duplication 1 and contains eight genes and 20 candidate regulatory elements, including a super-enhancer. Five genes are expressed in mammary glands and account for 50% of all mRNAs during lactation, two are salivary-specific and one has dual specificity. We probed the function of eight candidate enhancers through experimental mouse genetics. Deletion of the super-enhancer led to a 98% reduced expression of Csn3 and Fdcsp in mammary and salivary glands, respectively, and Odam expression was abolished in both tissues. The other three casein genes were only marginally affected. Notably, super-enhancer activity requires the additional presence of a distal Csn3 -specific enhancer. Our work identifies an evolutionary playground on which regulatory duality of a multigene locus was attained through an ancestral super-enhancer active in mammary and salivary tissue and gene-specific mammary enhancers.

A cytokine-responsive promoter is required for distal enhancer function mediating the hundreds-fold increase in milk protein gene expression during lactation

During lactation, specialized cells in the mammary gland produce milk to nourish the young. Milk protein genes are controlled by distal enhancers activating expression several hundred-fold during lactation. However, the role of promoter elements is not understood. We addressed this issue using the Csn2 gene, which accounts for 10% of mRNA in mammary tissue. We identified STAT5 and other mammary transcription factors binding to three distal candidate enhancers and a cytokine-response promoter element. While deletion of the enhancers or the introduction of an inactivating mutation in a single promoter element had a marginable effect, their combined loss led to a 99.99% reduction of Csn2 expression. Our findings reveal the essential role of a promoter element in the exceptional activation of a milk protein gene and highlight the importance of analyzing regulatory elements in their native genomic context to fully understand the multifaceted functions of enhancer clusters and promoters.

TF-Prioritizer: a Java pipeline to prioritize condition-specific transcription factors

BACKGROUND

Eukaryotic gene expression is controlled by cis-regulatory elements (CREs), including promoters and enhancers, which are bound by transcription factors (TFs). Differential expression of TFs and their binding affinity at putative CREs determine tissue- and developmental-specific transcriptional activity. Consolidating genomic datasets can offer further insights into the accessibility of CREs, TF activity, and, thus, gene regulation. However, the integration and analysis of multimodal datasets are hampered by considerable technical challenges. While methods for highlighting differential TF activity from combined chromatin state data (e.g., chromatin immunoprecipitation [ChIP], ATAC, or DNase sequencing) and RNA sequencing data exist, they do not offer convenient usability, have limited support for large-scale data processing, and provide only minimal functionality for visually interpreting results.

RESULTS

We developed TF-Prioritizer, an automated pipeline that prioritizes condition-specific TFs from multimodal data and generates an interactive web report. We demonstrated its potential by identifying known TFs along with their target genes, as well as previously unreported TFs active in lactating mouse mammary glands. Additionally, we studied a variety of ENCODE datasets for cell lines K562 and MCF-7, including 12 histone modification ChIP sequencing as well as ATAC and DNase sequencing datasets, where we observe and discuss assay-specific differences.

CONCLUSION

TF-Prioritizer accepts ATAC, DNase, or ChIP sequencing and RNA sequencing data as input and identifies TFs with differential activity, thus offering an understanding of genome-wide gene regulation, potential pathogenesis, and therapeutic targets in biomedical research.

Evaluation of the α-casein (CSN1S1) locus as a potential target for a site-specific transgene integration

Transgenic animals are an important tool in biotechnology, including the production of recombinant proteins in the milk. Traditionally, expression constructs are based on hybrid vectors bearing mammary gland specific regulatory elements from the α-casein (Csn1s1), β-casein (Csn2), whey acidic protein (WAP), or β-lactoglobulin (BLG) genes. Overexpression from the randomly integrated vectors typically provides high levels of expression, but has drawbacks due to unpredictable genome localization. CRISPR-Cas9 targeted transgene integration into the endogenous casein locus could alleviate the need for extensive animal screening to achieve high and reproducible expression levels. We decided to evaluate such a “precise” integration approach, placing the human granulocyte–macrophage colony-stimulating factor (hGMCSF) gene under control of the mouse endogenous alpha-S1-casein (Csn1s1) promoter. We designed two types of transgene integrations: a knock-in in the second exon of the Csn1s1 (INS-GM) and a full-size Csn1s1 replacement with hGMCSF (REP-GM) which was never tested before. The INS-GM approach demonstrated low transgene expression and milk protein levels (0.4% of Csn2 transcripts; 2–11 µg/ml hGMCSF). This was probably caused by the absence of the 3’-polyadenylation signal in the hGMCSF transgene. REP-GM animals displayed high transgene expression, reaching and slightly exceeding the level of the endogenous Csn1s1 (30–40% of Csn2 transcripts), but yielded less hGMCSF protein than expected (0.2–0.5 mg/ml vs 25 mg/ml of Csn1s1), indicating that translation of the protein is not optimal. Homozygous inserts leading to the Csn1s1 knock-out did not have any long standing effects on the animals’ health. Thus, in our experimental design, site-specific transgene integration into the casein locus did not provide any significant advantage over the overexpression approach.