Recent advances on the relationship between the DLK1 system and central precocious puberty

Precocious puberty, as a common pediatric endocrine disease, can be divided into central precocious puberty (CPP) and peripheral precocious puberty (PPP), even though most cases of precocious puberty are diagnosed as CPP. According to its etiology, CPP can be further divided into organic and idiopathic CPP. However, the mechanisms of idiopathic CPP have not yet been fully elucidated. Currently, four genes, including the kisspeptin gene (KISS1), the kisspeptin receptor gene (KISS1R), the makorin ring finger protein 3 (MKRN3), and the Delta-like non-canonical Notch ligand 1 (DLK1), have been implicated in CPP cases, of which DLK1 has been determined to represent a key, recently found CPP-related gene. In this review, we will not only highlight the latest discoveries on the relationship between the DLK1 system and CPP but also explore the involvement of the system as well as the Notch signaling pathway in CPP occurrence.

Screening of patients born small for gestational age with the Silver-Russell syndrome phenotype for DLK1 variants

Silver-Russell syndrome (SRS) is a rare imprinting disorder associated with prenatal and postnatal growth retardation. Loss of methylation (LOM) on chromosome 11p15 is observed in 40 to 60% of patients and maternal uniparental disomy (mUPD) for chromosome 7 (upd(7)mat) in ~5 to 10%. Patients with LOM or mUPD 14q32 can present clinically as SRS. Delta like non-canonical Notch ligand 1 (DLK1) is one of the imprinted genes expressed from chromosome 14q32. Dlk1-null mice display fetal growth restriction (FGR) but no genetic defects of DLK1 have been described in human patients born small for gestational age (SGA). We screened a cohort of SGA patients with a SRS phenotype for DLK1 variants using a next-generation sequencing (NGS) approach to search for new molecular defects responsible for SRS. Patients born SGA with a clinical suspicion of SRS and normal methylation by molecular testing at the 11p15 or 14q32 loci and upd(7)mat were screened for DLK1 variants using targeted NGS. Among 132 patients, only two rare variants of DLK1 were identified (NM_003836.6:c.103 G > C (p.(Gly35Arg) and NM_003836.6: c.194 A > G p.(His65Arg)). Both variants were inherited from the mother of the patients, which does not favor a role in pathogenicity, as the mono-allelic expression of DLK1 is from the paternal-inherited allele. We did not identify any pathogenic variants in DLK1 in a large cohort of SGA patients with a SRS phenotype. DLK1 variants are not a common cause of SGA.

Functional characterization of DLK1/MEG3 locus on chromosome 14q32.2 reveals the differentiation of pituitary neuroendocrine tumors

Pituitary neuroendocrine tumors (PitNETs) represent the neoplastic proliferation of the anterior pituitary gland. Transcription factors play a key role in the differentiation of PitNETs. However, for a substantial proportion of PitNETs, the etiology is poorly understood. According to the transcription data of 172 patients, we found the imprinting disorders of the 14q32.2 region and DLK1/MEG3 locus associated with the differentiation of PitNETs. DLK1/MEG3 locus promoted somatotroph differentiation and inhibited tumor proliferation in PIT1(+) patients, furthermore, the level of DLK1 played a critical role in the trend of somatotroph or lactotroph differentiation. Anti-DLK1 monoclonal antibody blockade or siMEG3 both indicated that the DLK1/MEG3 significantly promoted the synthesis and secretion of GH/IGF-1 and inhibited cell proliferation. In addition, loss of DLK1 activated the mTOR signaling pathway in high DLK1-expressing and PIT1(+) GH3 cell lines, a mild effect in the low DLK1-expressing and PIT1(+) MMQ cell lines and no effect in the PIT1(-) ATT20 cell line. These findings emphasize that expression at the DLK1/MEG3 locus plays a key role in the differentiation of PitNETs, especially somatotroph adenomas, and provide potential molecular target data for patient stratification and treatment in the future.

Imprinting disorders in humans: a review

PURPOSE OF REVIEW

Mammals have two complete sets of chromosomes, one from each parent with equal autosomal gene expression. Less than one percentage of human genes are imprinted or show expression from only one parent without changing gene structure, usually by DNA methylation, but reversible in gametogenesis. Many imprinted genes affect fetal growth and development accounting for several human disorders reviewed in this report.

RECENT FINDINGS

Disorders include Prader-Willi and Angelman syndromes, the first examples of imprinting errors in humans, chromosome 15q11.2-q13.3 duplication, Silver-Russell syndrome, Beckwith-Weidemann syndrome, GNAS gene-related inactivation disorders (e.g. Albright hereditary osteodystrophy), uniparental chromosome 14 disomy, chromosome 6q24-related transient neonatal diabetes mellitus, parent of origin effects in 15q11.2 BP1-BP2 deletion (Burnside-Butler) syndrome and 15q11-q13 single gene imprinted disorders.

SUMMARY

Periconceptional and intrauterine life can be influenced by environmental factors and nutrition impacting DNA methylation. This process not only alters development of the fetus, but pregnancy complications may result from large fetal size. Epigenetic processes control imprinted gene functions and regulation with susceptibility to diseases as described. A better understanding of these processes will impact on care and treatment of affected individuals.

Genomic Imprinting and Physiological Processes in Mammals

Complex multicellular organisms, such as mammals, express two complete sets of chromosomes per nucleus, combining the genetic material of both parents. However, epigenetic studies have demonstrated violations to this rule that are necessary for mammalian physiology; the most notable parental allele expression phenomenon is genomic imprinting. With the identification of endogenous imprinted genes, genomic imprinting became well-established as an epigenetic mechanism in which the expression pattern of a parental allele influences phenotypic expression. The expanding study of genomic imprinting is revealing a significant impact on brain functions and associated diseases. Here, we review key milestones in the field of imprinting and discuss mechanisms and systems in which imprinted genes exert a significant role.

14q32 and let-7 microRNAs regulate transcriptional networks in fetal and adult human erythroblasts

In humans, fetal erythropoiesis takes place in the liver whereas adult erythropoiesis occurs in the bone marrow. Fetal and adult erythroid cells are not only produced at different sites, but are also distinguished by their respective transcriptional program. In particular, whereas fetal erythroid cells express γ-globin chains to produce fetal hemoglobin (HbF), adult cells express β-globin chains to generate adult hemoglobin. Understanding the transcriptional regulation of the fetal-to-adult hemoglobin switch is clinically important as re-activation of HbF production in adult erythroid cells would represent a promising therapy for the hemoglobin disorders sickle cell disease and β-thalassemia. We used RNA-sequencing to measure global gene and microRNA (miRNA) expression in human erythroblasts derived ex vivo from fetal liver (n = 12 donors) and bone marrow (n = 12 donors) hematopoietic stem/progenitor cells. We identified 7829 transcripts and 402 miRNA that were differentially expressed (false discovery rate <5%). The miRNA expression patterns were replicated in an independent collection of human erythroblasts using a different technology. By combining gene and miRNA expression data, we developed transcriptional networks which show substantial differences between fetal and adult human erythroblasts. Our analyses highlighted the miRNAs at the imprinted 14q32 locus in fetal erythroblasts and the let-7 miRNA family in adult erythroblasts as key regulators of stage-specific erythroid transcriptional programs. Altogether, our results provide a comprehensive resource to prioritize genes that may modify clinical severity in red blood cell (RBC) disorders, or genes that might be implicated in erythropoiesis by genome-wide association studies of RBC traits.

Transcriptional profiling at the DLK1/MEG3 domain explains clinical overlap between imprinting disorders

Imprinting disorders (IDs) often affect growth in humans, leading to diseases with overlapping features, regardless of the genomic region affected. IDs related to hypomethylation of the human 14q32.2 region and its DLK1/MEG3 domain are associated with Temple syndrome (TS14). TS14 is a rare type of growth retardation, the clinical signs of which overlap considerably with those of Silver-Russell syndrome (SRS), another ID related to IGF2 down-regulation at 11p15.5 region. We show that 14q32.2 hypomethylation affects expression, not only for genes at this locus but also for other imprinted genes, and especially lowers IGF2 levels at 11p15.5. Furthermore, expression of nonimprinted genes is also affected, some of which are also deregulated in SRS patients. These findings highlight the epigenetic regulation of gene expression at the DLK1/MEG3 domain. Expression profiling of TS14 and SRS patients highlights common signatures, which may account for the clinical overlap observed between TS14 and SRS.

Epigenetics of Circadian Rhythms in Imprinted Neurodevelopmental Disorders

DNA sequence information alone cannot account for the immense variability between chromosomal alleles within diverse cell types in the brain, whether these differences are observed across time, cell type, or parental origin. The complex control and maintenance of gene expression and modulation are regulated by a multitude of molecular and cellular mechanisms that layer on top of the genetic code. The integration of genetic and environmental signals required for regulating brain development and function is achieved in part by a dynamic epigenetic landscape that includes DNA methylation, histone modifications, and noncoding RNAs. These epigenetic mechanisms establish and maintain core biological processes, including genomic imprinting and entrainment of circadian rhythms. This chapter will focus on how the epigenetic layers of DNA methylation and long, noncoding RNAs interact with circadian rhythms at specific imprinted chromosomal loci associated with the human neurodevelopmental disorders Prader-Willi, Angelman, Kagami-Ogata, and Temple syndromes.

MicroRNA expression is altered in lateral septum across reproductive stages

MicroRNAs (miRNAs) inhibit RNA targets and may contribute to postpartum central nervous system (CNS) gene expression changes, although this has never been tested. In the present study, we directly evaluated miRNA levels using RNA sequencing during reproduction in female mice in the lateral septum (LS). We found the reliable and robust changes of miRNAs away from the virgin stage at the three other stages, namely pregnant, day 1 postpartum, and day 8 postpartum. For a given miRNA that was significantly different from the virgin condition in more than one group, the direction of change was always the same. Overall, we identified 32 upregulated miRNAs and 25 downregulated miRNAs that were consistently different from the virgin state. 'Arm switching' occurs for miR-433-3 and miR-7b. Unexpectedly, a third of upregulated miRNAs (relative to virgin) were highly localized within the 12qF1 region of chromosome 12 that includes the Dlk1-Dio3 gene cluster implicated in stem cell and neuronal differentiation. Over 1500 genes were targeted by multiple upregulated miRNAs with about 100 genes targeted by five or more miRNAs. Over 1000 genes were targeted by multiple downregulated miRNAs with about 50 genes targeted by five or more miRNAs. Half of the target genes were regulated by up and downregulated miRNAs, indicating homeostatic regulation. Transcriptional regulation was the most enriched pathway for genes linked to up or down regulated miRNAs. Other enriched pathways included protein kinase activity (e.g., MAP kinase), CNS development, axon guidance, neurotrophin signaling, neuron development/differentiation, and neurogenesis. Previously published postpartum LS gene expression changes were enrichment for LS miRNA targets, as expected. Surprisingly, postpartum gene expression changes from other regions were also enriched against LS miRNA targets, suggesting a core group of miRNAs may act across the CNS during reproduction. Together, we directly examine miRNAs and find significant alterations in the postpartum brain.