Mediator subunit 12 coordinates intrinsic and extrinsic control of epithalamic development

Mediator complex in neurological disease

Neurological diseases, including traumatic brain injuries, stroke (haemorrhagic and ischemic), and inherent neurodegenerative diseases cause acquired disability in humans, representing a leading cause of death worldwide. The Mediator complex (MED) is a large, evolutionarily conserved multiprotein that facilities the interaction between transcription factors and RNA Polymerase II in eukaryotes. Some MED subunits have been found altered in the brain, although their specific functions in neurodegenerative diseases are not fully understood. Mutations in MED subunits were associated with a wide range of genetic diseases for MED12, MED13, MED13L, MED20, MED23, MED25, and CDK8 genes. In addition, MED12 and MED23 were deregulated in the Alzheimer's Disease. Interestingly, most of the genomic mutations have been found in the subunits of the kinase module. To date, there is only one evidence on MED1 involvement in post-stroke cognitive deficits. Although the underlying neurodegenerative disorders may be different, we are confident that the signal cascades of the biological-cognitive mechanisms of brain adaptation, which begin after brain deterioration, may also differ. Here, we analysed relevant studies in English published up to June 2023. They were identified through a search of electronic databases including PubMed, Medline, EMBASE and Scopus, including search terms such as "Mediator complex", "neurological disease", "brains". Thematic content analysis was conducted to collect and summarize all studies demonstrating MED alteration to understand the role of this central transcriptional regulatory complex in the brain. Improved and deeper knowledge of the regulatory mechanisms in neurological diseases can increase the ability of physicians to predict onset and progression, thereby improving diagnostic care and providing appropriate treatment decisions.

MED12-Related (Neuro)Developmental Disorders: A Question of Causality

MED12 is a member of the Mediator complex that is involved in the regulation of transcription. Missense variants in MED12 cause FG syndrome, Lujan-Fryns syndrome, and Ohdo syndrome, as well as non-syndromic intellectual disability (ID) in hemizygous males. Recently, female patients with de novo missense variants and de novo protein truncating variants in MED12 were described, resulting in a clinical spectrum centered around ID and Hardikar syndrome without ID. The missense variants are found throughout MED12, whether they are inherited in hemizygous males or de novo in females. They can result in syndromic or nonsyndromic ID. The de novo nonsense variants resulting in Hardikar syndrome that is characterized by facial clefting, pigmentary retinopathy, biliary anomalies, and intestinal malrotation, are found more N-terminally, whereas the more C-terminally positioned variants are de novo protein truncating variants that cause a severe, syndromic phenotype consisting of ID, facial dysmorphism, short stature, skeletal abnormalities, feeding difficulties, and variable other abnormalities. This broad range of distinct phenotypes calls for a method to distinguish between pathogenic and non-pathogenic variants in MED12. We propose an isogenic iNeuron model to establish the unique gene expression patterns that are associated with the specific MED12 variants. The discovery of these patterns would help in future diagnostics and determine the causality of the MED12 variants.

Mediator's Kinase Module: A Modular Regulator of Cell Fate

Selective gene expression is crucial in maintaining the self-renewing and multipotent properties of stem cells. Mediator is a large, evolutionarily conserved, multi-subunit protein complex that modulates gene expression by relaying signals from cell type-specific transcription factors to RNA polymerase II. In humans, this complex consists of 30 subunits arranged in four modules. One critical module of the Mediator complex is the kinase module consisting of four subunits: MED12, MED13, CDK8, and CCNC. The kinase module exists in variable association with the 26-subunit Mediator core and affects transcription through phosphorylation of transcription factors and by controlling Mediator structure and function. Many studies have shown the kinase module to be a key player in the maintenance of stem cells that is distinct from a general role in transcription. Genetic studies have revealed that dysregulation of this kinase subunit contributes to the development of many human diseases. In this review, we discuss the importance of the Mediator kinase module by examining how this module functions with the more recently identified transcriptional super-enhancers, how changes in the kinase module and its activity can lead to the development of human disease, and the role of this unique module in directing and maintaining cell state. As we look to use stem cells to understand human development and treat human disease through both cell-based therapies and tissue engineering, we need to remain aware of the on-going research and address critical gaps in knowledge related to the molecular mechanisms that control cell fate.

De novo loss-of-function variants in X-linked MED12 are associated with Hardikar syndrome in females

PURPOSE

Hardikar syndrome (MIM 612726) is a rare multiple congenital anomaly syndrome characterized by facial clefting, pigmentary retinopathy, biliary anomalies, and intestinal malrotation, but with preserved cognition. Only four patients have been reported previously, and none had a molecular diagnosis. Our objective was to identify the genetic basis of Hardikar syndrome (HS) and expand the phenotypic spectrum of this disorder.

METHODS

We performed exome sequencing on two previously reported and five unpublished female patients with a clinical diagnosis of HS. X-chromosome inactivation (XCI) studies were also performed.

RESULTS

We report clinical features of HS with previously undescribed phenotypes, including a fatal unprovoked intracranial hemorrhage at age 21. We additionally report the discovery of de novo pathogenic nonsense and frameshift variants in MED12 in these seven individuals and evidence of extremely skewed XCI in all patients with informative testing.

CONCLUSION

Pathogenic missense variants in the X-chromosome gene MED12 have previously been associated with Opitz-Kaveggia syndrome, Lujan syndrome, Ohdo syndrome, and nonsyndromic intellectual disability, primarily in males. We propose a fifth, female-specific phenotype for MED12, and suggest that nonsense and frameshift loss-of-function MED12 variants in females cause HS. This expands the MED12-associated phenotype in females beyond intellectual disability.

Mediator complex subunit Med12 regulates cardiac jelly development and AV valve formation in zebrafish

The molecular mechanism essential for the formation of heart valves involves complex interactions of signaling molecules and transcription factors. The Mediator Complex (MC) functions as multi-subunit machinery to orchestrate gene transcription, especially for tissue-specific fine-tuning of transcriptional processes during development, also in the heart. Here, we analyzed the role of the MC subunit Med12 during atrioventricular canal (AVC) development and endocardial cushion formation, using the Med12-deficient zebrafish mutant trapped (tpd). Whereas primary heart formation was only slightly affected in tpd, we identified defects in AVC development and cardiac jelly formation. We found that although misexpression of bmp4 and versican in tpd hearts can be restored by overexpression of a modified version of the Sox9b transcription factor (harboring VP16 transactivation domain) that functions independent of its co-activator Med12, endocardial cushion development in tpd was not reconstituted. Interestingly, expression of tbx2b and its target hyaluronan synthase 2 (has2) - the synthase of hyaluronan (HA) in the heart - was absent in both uninjected and Sox9b-VP16 overexpressing tpd hearts. HA is a major ECM component of the cardiac jelly and required for endocardial cushion formation. Furthermore, we found secreted phosphoprotein 1 (spp1), an endocardial marker of activated AV endocardial cells, completely absent in tpd hearts, suggesting that crucial steps of the transformation of AV endocardial cells into endocardial cushions is blocked. We demonstrate that Med12 controls cardiac jelly formation Sox9-independently by regulating tbx2b and has2 expression and therefore the production of the glycosaminoglycan HA at the AVC to guarantee proper endocardial cushion development.

Proteomic Characterization of the Extracellular Matrix of Human Uterine Fibroids

Uterine leiomyomas (fibroids) are the most common benign tumors that are associated with increased production of extracellular matrix (ECM). Excessive ECM deposition plays a major role in the enlargement and stiffness of these tumors and contributes to clinical symptoms, such as abnormal bleeding and abdominal pain. However, no study so far has explored the global composition of the ECM of fibroids and normal myometrium. In this study, we performed a systematic ECM enrichment procedure and comparative proteomic analyses to profile the ECM composition of genetically annotated different-sized fibroids (small, medium, and large) and adjacent normal myometrium (ANM). Our matrisome analysis identified a combined total of 108, 126, 126, and 130 unique ECM and ECM-associated proteins with a confidence corresponding to a false discovery rate <1% in ANM and in small, medium, and large fibroids, respectively. The majority of fibroid ECM proteins belong to the core matrisome that includes glycoproteins, collagens, and proteoglycans. Considering that the small-sized fibroids represent the initial stages of leiomyogenesis, we highlighted some of the most abundant and important upregulated ECM proteins in small fibroids (i.e., POSTN, TNC, COL3A1, COL24A1, and ASPN). Furthermore, we revealed 30 unique ECM proteins that exist only in fibroids but that are not present in ANM regardless of MED12 mutation. We propose that some of the proteins identified represent potential novel ECM drug targets that may change the paradigm of fibroid treatment.

Separate roles for Med12 and Wnt signaling in regulation of oxytocin expression

Transcriptional control of oxytocinergic cell development influences social, sexual, and appetite related behaviors and is implicated in disorders such as autism and Prader-Willi syndrome. Mediator 12 (Med12) is a transcriptional coactivator required for multiple facets of brain development including subsets of serotonergic and dopaminergic neurons. We surveyed hormone gene expression within the hypothalamo-pituitary axis of med12 mutant zebrafish embryos with a focus on oxytocin (oxt) expression. Some transcripts, such as oxt, vasopressin (avp) and corticotrophin releasing hormone (crh) are undetectable in the med12 mutant, while others are upregulated or downregulated to varying degrees. In med12 mutants, the expression patterns of upstream transcriptional regulators of oxytocinergic cell development remain largely intact in the pre-optic area, suggesting a more direct influence of Med12 on oxt expression. We show that Med12 is required for Wnt signaling in zebrafish. However, oxt expression is unaffected in Wnt-inhibited embryos indicating independence of Wnt signaling. In fact, overactive Wnt signaling inhibits oxt expression, and we identify a Wnt-sensitive period starting at 24 h post fertilization (hpf). Thus, Med12 and repression of Wnt signaling display critical but unrelated roles in regulating oxt expression.

Summary: Mediator 12, a transcriptional coactivator, greatly enhances Wnt signaling in the developing embryo. Separate from its role in Wnt signaling, Mediator 12 is required for oxytocin expression.

Development and connectivity of the habenular nuclei

Accumulating evidence has reinforced that the habenular region of the vertebrate dorsal forebrain is an essential integrating center, and a region strongly implicated in neurological disorders and addiction. Despite the important and diverse neuromodulatory roles the habenular nuclei play, their development has been understudied. The emphasis of this review is on the dorsal habenular nuclei of zebrafish, homologous to the medial nuclei of mammals, as recent work has revealed new information about the signaling pathways that regulate their formation. Additionally, the zebrafish dorsal habenulae have become a valuable model for probing how left-right differences are established in a vertebrate brain. Sonic hedgehog, fibroblast growth factors and Wingless-INT proteins are all involved in the generation of progenitor cells and ultimately, along with Notch signaling, influence habenular neurogenesis and left-right asymmetry. Intriguingly, a genetic network has emerged that leads to the differentiation of dorsal habenular neurons and, through localized chemokine signaling, directs the posterior outgrowth of their newly emerging axons towards their postsynaptic target, the midbrain interpeduncular nucleus.

MED12 regulates a transcriptional network of calcium-handling genes in the heart

The Mediator complex regulates gene transcription by linking basal transcriptional machinery with DNA-bound transcription factors. The activity of the Mediator complex is mainly controlled by a kinase submodule that is composed of 4 proteins, including MED12. Although ubiquitously expressed, Mediator subunits can differentially regulate gene expression in a tissue-specific manner. Here, we report that MED12 is required for normal cardiac function, such that mice with conditional cardiac-specific deletion of MED12 display progressive dilated cardiomyopathy. Loss of MED12 perturbs expression of calcium-handling genes in the heart, consequently altering calcium cycling in cardiomyocytes and disrupting cardiac electrical activity. We identified transcription factors that regulate expression of calcium-handling genes that are downregulated in the heart in the absence of MED12, and we found that MED12 localizes to transcription factor consensus sequences within calcium-handling genes. We showed that MED12 interacts with one such transcription factor, MEF2, in cardiomyocytes and that MED12 and MEF2 co-occupy promoters of calcium-handling genes. Furthermore, we demonstrated that MED12 enhances MEF2 transcriptional activity and that overexpression of both increases expression of calcium-handling genes in cardiomyocytes. Our data support a role for MED12 as a coordinator of transcription through MEF2 and other transcription factors. We conclude that MED12 is a regulator of a network of calcium-handling genes, consequently mediating contractility in the mammalian heart.

Mediator kinase module and human tumorigenesis

Mediator is a conserved multi-subunit signal processor through which regulatory informatiosn conveyed by gene-specific transcription factors is transduced to RNA Polymerase II (Pol II). In humans, MED13, MED12, CDK8 and Cyclin C (CycC) comprise a four-subunit "kinase" module that exists in variable association with a 26-subunit Mediator core. Genetic and biochemical studies have established the Mediator kinase module as a major ingress of developmental and oncogenic signaling through Mediator, and much of its function in signal-dependent gene regulation derives from its resident CDK8 kinase activity. For example, CDK8-targeted substrate phosphorylation impacts transcription factor half-life, Pol II activity and chromatin chemistry and functional status. Recent structural and biochemical studies have revealed a precise network of physical and functional subunit interactions required for proper kinase module activity. Accordingly, pathologic change in this activity through altered expression or mutation of constituent kinase module subunits can have profound consequences for altered signaling and tumor formation. Herein, we review the structural organization, biological function and oncogenic potential of the Mediator kinase module. We focus principally on tumor-associated alterations in kinase module subunits for which mechanistic relationships as opposed to strictly correlative associations are established. These considerations point to an emerging picture of the Mediator kinase module as an oncogenic unit, one in which pathogenic activation/deactivation through component change drives tumor formation through perturbation of signal-dependent gene regulation. It follows that therapeutic strategies to combat CDK8-driven tumors will involve targeted modulation of CDK8 activity or pharmacologic manipulation of dysregulated CDK8-dependent signaling pathways.

High-resolution melting analysis of MED12 mutations in uterine leiomyomas in Chinese patients

OBJECTIVES

Somatic mutations in mediator complex subunit 12 (MED12) have emerged as a critical genetic change in the development of uterine leiomyomas. Studies, however, have focused largely on cohorts consisting of Caucasian patients. In this study, uterine leiomyomas from Chinese patients were examined for MED12 mutations. In addition, polymerase chain reaction (PCR)-based high-resolution melting analysis (HRMA) was compared with direct sequencing as a potentially more sensitive method for the detection of MED12 mutations.

METHODS

Tissue samples with the pathologies of uterine leiomyoma (n=181) and other endometrial diseases (n=157) were collected from Chinese patients at the Taizhou People's Hospital and Taizhou Polytechnic College (Taizhou City, China). Genomic DNA was prepared from all samples. Both PCR-based HRMA and PCR-based direct sequencing were used to detect MED12 mutations.

RESULTS

PCR-based HRMA and direct sequencing revealed MED12 mutations in 95/181 (52.5%) and 93/181 (51.4%) uterine leiomyomas, respectively. Nearly half of these mutations (46/93) were found in a single codon, codon 131. The coincidence rate between the two methods was 98.9% (179/181) so that no statistically significant difference was evident in the application of the methodologies (χ(2)=0.011, p=0.916). In addition, MED12 mutations were identified in 1/157 (4.17%) case of other endometrial pathologies by both methods.

CONCLUSIONS

MED12 mutations were closely associated with the development of uterine leiomyomas, as opposed to other uterine pathologies in Chinese patients, and PCR-based HRMA was found to be a reliable method for the detection of MED12 mutations.

Neurotransmitter map of the asymmetric dorsal habenular nuclei of zebrafish

The role of the habenular nuclei in modulating fear and reward pathways has sparked a renewed interest in this conserved forebrain region. The bilaterally paired habenular nuclei, each consisting of a medial/dorsal and lateral/ventral nucleus, can be further divided into discrete subdomains whose neuronal populations, precise connectivity, and specific functions are not well understood. An added complexity is that the left and right habenulae show pronounced morphological differences in many non-mammalian species. Notably, the dorsal habenulae of larval zebrafish provide a vertebrate genetic model to probe the development and functional significance of brain asymmetry. Previous reports have described a number of genes that are expressed in the zebrafish habenulae, either in bilaterally symmetric patterns or more extensively on one side of the brain than the other. The goal of our study was to generate a comprehensive map of the zebrafish dorsal habenular nuclei, by delineating the relationship between gene expression domains, comparing the extent of left-right asymmetry at larval and adult stages, and identifying potentially functional subnuclear regions as defined by neurotransmitter phenotype. Although many aspects of habenular organization appear conserved with rodents, the zebrafish habenulae also possess unique properties that may underlie lateralization of their functions.