Canonical WNT Signaling Regulates the Pituitary Organizer and Pituitary Gland Formation

Nucleoredoxin regulates WNT signaling during pituitary stem cell differentiation

Nucleoredoxin (Nxn) encodes a multi-functional enzyme with oxidoreductase activity that regulates many different signaling pathways and cellular processes in a redox-dependent manner. Rare NXN mutations are reported in individuals with recessive Robinow syndrome, which involves mesomelic skeletal dysplasia, short stature, craniofacial dysmorphisms, and incompletely penetrant heart and palate defects. Here we report that Nxn is expressed in the ventral diencephalon and developing pituitary gland, and that Nxn deficient mice have pituitary dysmorphology and craniofacial abnormalities that include defects in the skull base and cleft palate. Nxn mutant mice exhibit reduced WNT signaling and reduced differentiation of pituitary stem cells into hormone-producing cells. These results suggest patients with Robinow syndrome could benefit from evaluation by endocrinologists for pituitary structural imaging and hormone insufficiency.

Biological pathways leading to septo-optic dysplasia: a review

BACKGROUND

The precise etiology of septo-optic dysplasia (SOD) remains elusive, to date a complex interaction between genetic predisposition and prenatal exposure to environmental factors is believed to come into play. Being SOD such a heterogeneous condition, disruption of many developmental steps in the early forebrain development might occur. The knowledge of genes possibly determining SOD phenotype should be improved, therefore in this review the authors attempt to highlight the genetic pathways and genes related to this clinical condition.

MAIN BODY

Literature search was conducted and updated in November 2023, using PubMed and Google Scholar to identify primary research articles or case reports with available full text using the following search string "case reports," "humans," "septo-optic dysplasia," "optic nerve hypoplasia," with a recognized genetic diagnosis. Moreover, a review of genetic pathways with an involvement in SOD etiology was conducted. This review thus represents the authors' perspective based on selected literature. The several pathways presented might be already associated to other disease phenotypes and interplay with genes and pathways known to have a role in SOD determination. Those pathways may converge and thus, the implicated genes may function as cascading regulators at multiple levels.

CONCLUSION

The present data suggest that genes other than HESX1, SOX2, SOX3, and OTX2 might be investigated in candidate individuals with a clinical diagnosis of SOD corresponding to the presence of at least two diagnostic criteria, particularly in the presence of additional syndromic anomalies.

Rathke's cleft cyst: From history to molecular genetics

A Rathke's cleft cyst (RCC) is a remnant of the embryologic Rathke's pouch and a common pituitary lesion. A true RCC is lined with ciliated cuboidal or columnar epithelia with occasional goblet cells and squamous metaplasia. A RCC is frequently diagnosed incidentally through magnetic resonance imaging and computed tomography of the brain or pituitary gland. Presentation can range from an asymptomatic clinical picture to a rapidly progressive disease. RCC are located most often in the sellar and suprasellar regions and a careful differential diagnosis is crucial, especially to exclude craniophryngioma. Recent studies illuminate novel molecular mechanisms and markers for understanding the pathogenesis of RCC. PROP-1, a paired-like homeodomain transcription factor, controls pituitary ontogeny and its high expression induces RCCs. Both transgenic mouse models and immunohistochemical analysis of human RCCs indicate that the leukemia inhibitory factor is involved in pathogenesis. The expression of cytokeratins 8 and 2 in RCCs, but not in craniopharyngiomas, and the presence of beta-catenin mutations in many craniopharyngiomas, but not in RCCs, help with the differential diagnosis. For asymptomatic and small RCCs, observation is appropriate, with serial magnetic resonance imaging and hormonal investigation depending on the patient's clinical status. Surgical resection may be required for symptomatic RCC and recurrence rates are generally low. For patients with a recurrence, stereotactic radiosurgery is an effective approach with low risk.

Nucleoredoxin regulates WNT signaling during pituitary stem cell differentiation

Nucleoredoxin (Nxn) encodes a multi-functional enzyme with oxidoreductase activity that regulates many different signaling pathways and cellular processes in a redox-dependent manner. Rare NXN mutations are reported in individuals with recessive Robinow syndrome, which involves mesomelic skeletal dysplasia, short stature, craniofacial dysmorphisms, and incompletely penetrant heart and palate defects. Here we report that Nxn is expressed in the ventral diencephalon and developing pituitary gland, and that Nxn deficient mice have pituitary dysmorphology and craniofacial abnormalities that include defects in the skull base and cleft palate. Nxn mutant mice exhibit reduced WNT signaling and reduced differentiation of pituitary stem cells into hormone-producing cells. These results suggest patients with Robinow syndrome could benefit from evaluation by endocrinologists for pituitary structural imaging and hormone insufficiency.

Glycoprotein hormone subunit alpha 2 (GPHA2): A pituitary stem cell-expressed gene associated with NOTCH2 signaling

NOTCH2 is expressed in pituitary stem cells and is necessary for stem cell maintenance, proliferation, and differentiation. However, the pathways NOTCH2 engages to affect pituitary development remain unclear. In this study, we hypothesized that glycoprotein hormone subunit A2 (GPHA2), a corneal stem cell factor and ligand for the thyroid stimulating hormone receptor (TSHR), is downstream of NOTCH2 signaling. We found Gpha2 is expressed in quiescent pituitary stem cells by RNAscope in situ hybridization and scRNA seq. In Notch2 conditional knockout pituitaries, Gpha2 mRNA is reduced compared with control littermates. We then investigated the possible functions of GPHA2. Pituitaries treated with a GPHA2 peptide do not have a change in proliferation. However, in dissociated adult pituitary cells, GPHA2 increased pCREB expression and this induction was reversed by co-treatment with a TSHR inhibitor. These data suggest GPHA2 is a NOTCH2 related stem cell factor that activates TSHR signaling, potentially impacting pituitary development.

Single-cell analysis of mesenchymal cells in permeable neural vasculature reveals novel diverse subpopulations of fibroblasts

BACKGROUND

In the choroid plexus and pituitary gland, vasculature is known to have a permeable, fenestrated phenotype which allows for the free passage of molecules in contrast to the blood brain barrier observed in the rest of the CNS. The endothelium of these compartments, along with secretory, neural-lineage cells (choroid epithelium and pituitary endocrine cells) have been studied in detail, but less attention has been given to the perivascular mesenchymal cells of these compartments.

METHODS

The Hic1CreERT2 Rosa26LSL-TdTomato mouse model was used in conjunction with a PdgfraH2B-EGFP mouse model to examine mesenchymal cells, which can be subdivided into Pdgfra+ fibroblasts and Pdgfra- pericytes within the choroid plexus (CP) and pituitary gland (PG), by histological, immunofluorescence staining and single-cell RNA-sequencing analyses.

RESULTS

We found that both CP and PG possess substantial populations of distinct Hic1+ mesenchymal cells, including an abundance of Pdgfra+ fibroblasts. Within the pituitary, we identified distinct subpopulations of Hic1+ fibroblasts in the glandular anterior pituitary and the neurosecretory posterior pituitary. We also identified multiple distinct markers of CP, PG, and the meningeal mesenchymal compartment, including alkaline phosphatase, indole-n-methyltransferase and CD34.

CONCLUSIONS

Novel, distinct subpopulations of mesenchymal cells can be found in permeable vascular interfaces, including the CP, PG, and meninges, and make distinct contributions to both organs through the production of structural proteins, enzymes, transporters, and trophic molecules.

Re-analysis of gene mutations found in pituitary stalk interruption syndrome and a new hypothesis on the etiology

Background

Pituitary stalk interruption syndrome (PSIS) is a complex clinical syndrome characterized by varied pituitary hormone deficiencies, leading to severe manifestations across multiple systems. These include lifelong infertility, short stature, mental retardation, and potentially life-threatening pituitary crises if not promptly diagnosed and treated. Despite extensive research, the precise pathogenesis of PSIS remains unclear. Currently, there are two proposed theories regarding the pathogenic mechanisms: the genetic defect theory and the perinatal injury theory.

Methods

We systematically searched English databases (PubMed, Web of Science, Embase) and Chinese databases (CNKI, WanFang Med Online, Sinomed) up to February 24, 2023, to summarize studies on gene sequencing in PSIS patients. Enrichment analyses of reported mutated genes were subsequently performed using the Metascape platform.

Results

Our study included 37 articles. KEGG enrichment analysis revealed mutated genes were enriched in the Notch signaling pathway, Wnt signaling pathway, and Hedgehog signaling pathway. GO enrichment analysis demonstrated mutated genes were enriched in biological processes such as embryonic development, brain development, axon development and guidance, and development of other organs.

Conclusion

Based on our summary and analyses, we propose a new hypothesis: disruptions in normal embryonic development, partially stemming from the genetic background and/or specific gene mutations in individuals, may increase the likelihood of abnormal fetal deliveries, where different degrees of traction during delivery may lead to different levels of pituitary stalk interruption and posterior lobe ectopia. The clinical diversity observed in PSIS patients may result from a combination of genetic background, specific mutations, and variable degrees of traction during delivery.

Pituitary stem cells: past, present and future perspectives

Pituitary cells that express the transcription factor SOX2 are stem cells because they can self-renew and differentiate into multiple pituitary hormone-producing cell types as organoids. Wounding and physiological challenges can activate pituitary stem cells, but cell numbers are not fully restored, and the ability to mobilize stem cells decreases with increasing age. The basis of these limitations is still unknown. The regulation of stem cell quiescence and activation involves many different signalling pathways, including those mediated by WNT, Hippo and several cytokines; more research is needed to understand the interactions between these pathways. Pituitary organoids can be formed from human or mouse embryonic stem cells, or from human induced pluripotent stem cells. Human pituitary organoid transplantation is sufficient to induce corticosterone release in hypophysectomized mice, raising the possibility of therapeutic applications. Today, pituitary organoids have the potential to assess the role of individual genes and genetic variants on hormone production ex vivo, providing an important tool for the advancement of exciting frontiers in pituitary stem cell biology and pituitary organogenesis. In this article, we provide an overview of notable discoveries in pituitary stem cell function and highlight important areas for future research.

Heterozygous variants in SIX3 and POU1F1 cause pituitary hormone deficiency in mouse and man

Congenital hypopituitarism is a genetically heterogeneous condition that is part of a spectrum disorder that can include holoprosencephaly. Heterozygous mutations in SIX3 cause variable holoprosencephaly in humans and mice. We identified two children with neonatal hypopituitarism and thin pituitary stalk who were doubly heterozygous for rare, likely deleterious variants in the transcription factors SIX3 and POU1F1. We used genetically engineered mice to understand the disease pathophysiology. Pou1f1 loss-of-function heterozygotes are unaffected; Six3 heterozygotes have pituitary gland dysmorphology and incompletely ossified palate; and the Six3+/-; Pou1f1+/dw double heterozygote mice have a pronounced phenotype, including pituitary growth through the palate. The interaction of Pou1f1 and Six3 in mice supports the possibility of digenic pituitary disease in children. Disruption of Six3 expression in the oral ectoderm completely ablated anterior pituitary development, and deletion of Six3 in the neural ectoderm blocked the development of the pituitary stalk and both anterior and posterior pituitary lobes. Six3 is required in both oral and neural ectodermal tissues for the activation of signaling pathways and transcription factors necessary for pituitary cell fate. These studies clarify the mechanism of SIX3 action in pituitary development and provide support for a digenic basis for hypopituitarism.

Integrated analysis of Wnt signalling system component gene expression

Wnt signalling controls patterning and differentiation across many tissues and organs of the developing embryo through temporally and spatially restricted expression of multi-gene families encoding ligands, receptors, pathway modulators and intracellular components. Here, we report an integrated analysis of key genes in the 3D space of the mouse embryo across multiple stages of development. We applied a method for 3D/3D image transformation to map all gene expression patterns to a single reference embryo for each stage, providing both visual analysis and volumetric mapping allowing computational methods to interrogate the combined expression patterns. We identify territories where multiple Wnt and Fzd genes are co-expressed and cross-compare all patterns, including all seven Wnt paralogous gene pairs. The comprehensive analysis revealed regions in the embryo where no Wnt or Fzd gene expression is detected, and where single Wnt genes are uniquely expressed. This work provides insight into a previously unappreciated level of organisation of expression patterns, as well as presenting a resource that can be utilised further by the research community for whole-system analysis.

Summary: A systematic analysis of integrated expression patterns of Wnt signalling pathway component-encoding genes and canonical pathway read-out, spatially mapped in 3D to mouse embryo models identifies co-expression territories.

Comparing Two Neurodevelopmental Disorders Linked to CK2: Okur-Chung Neurodevelopmental Syndrome and Poirier-Bienvenu Neurodevelopmental Syndrome—Two Sides of the Same Coin?

In recent years, variants in the catalytic and regulatory subunits of the kinase CK2 have been found to underlie two different, yet symptomatically overlapping neurodevelopmental disorders, termed Okur-Chung neurodevelopmental syndrome (OCNDS) and Poirier-Bienvenu neurodevelopmental syndrome (POBINDS). Both conditions are predominantly caused by de novo missense or nonsense mono-allelic variants. They are characterized by a generalized developmental delay, intellectual disability, behavioral problems (hyperactivity, repetitive movements and social interaction deficits), hypotonia, motricity and verbalization deficits. One of the main features of POBINDS is epilepsies, which are present with much lower prevalence in patients with OCNDS. While a role for CK2 in brain functioning and development is well acknowledged, these findings for the first time clearly link CK2 to defined brain disorders. Our review will bring together patient data for both syndromes, aiming to link symptoms with genotypes, and to rationalize the symptoms through known cellular functions of CK2 that have been identified in preclinical and biochemical contexts. We will also compare the symptomatology and elaborate the specificities that distinguish the two syndromes.

Cell transcriptomic atlas of the non-human primate Macaca fascicularis

Studying tissue composition and function in non-human primates (NHPs) is crucial to understand the nature of our own species. Here we present a large-scale cell transcriptomic atlas that encompasses over 1 million cells from 45 tissues of the adult NHP Macaca fascicularis. This dataset provides a vast annotated resource to study a species phylogenetically close to humans. To demonstrate the utility of the atlas, we have reconstructed the cell-cell interaction networks that drive Wnt signalling across the body, mapped the distribution of receptors and co-receptors for viruses causing human infectious diseases, and intersected our data with human genetic disease orthologues to establish potential clinical associations. Our M. fascicularis cell atlas constitutes an essential reference for future studies in humans and NHPs.

A severe case of Bosch-Boonstra-Schaaf optic atrophy syndrome with a novel description of coloboma and septo-optic dysplasia, owing to a start codon variant in the NR2F1 gene

Bosch-Boonstra-Schaaf optic atrophy syndrome (BBSOAS) is a rare congenital syndrome characterized by a range of phenotypes including optic atrophy and intellectual disability among other features. Pathogenic variants in the NR2F1 (nuclear receptor subfamily 2 group F member 1) gene have been linked to this condition. A recent report has shown that pathogenic variants in the start codon lead to decreased expression of the NR2F1 protein and a relatively mild phenotype, similar to that seen in whole gene deletions, and due to the lack of the dominant negative effect. Here we describe a severe case of BBSOAS with an initiation codon missense variant. The developmental delay, seizures, optic atrophy are in keeping with features observed in this condition, however this is the first report to describe colobomas and septo-optic dysplasia as associated features potentially extending the phenotype linked to BBSOAS. In addition, this is the first description of a severe phenotype linked to a de novo missense variant in the start codon of the NR2F1 gene.

Pituitary stem cells

The anterior pituitary is derived from Rathke's pouch precursors, which differentiate into specific hormone-secreting cell lineages. Sustained low postnatal and adult pituitary cell turnover is governed by stem/progenitor cells that undergo slow mitotic activity and give rise to hormone-secreting cells in response to physiological demands and feedback loops. Pituitary cell populations exhibit stem cell properties, which include stem cell marker expression, non-hormone expression, and the ability to self-renew and to potentially differentiate into any of five hormone-secreting cell lineages. Specific signaling pathways underlie differentiated pituitary cell development and regulation. Several validated pituitary stem cell models have been reported and have the potential for functional regeneration of pituitary hormone-secreting cell functions.

Compound heterozygous GLI3 variants in siblings with thyroid hemiagenesis

PURPOSE

Thyroid hemiagenesis (THA) is an inborn absence of one thyroid lobe of largely unknown etiopathogenesis, affecting 0.05-0.5% population. The aim of the study was an identification of genetic factors responsible for thyroid maldevelopment in two siblings with THA.

METHODS

We evaluated a three-generation THA family with two sisters presenting the disorder. Proband (Patient II:3) was diagnosed at the age of 45 due to neck asymmetry. Left lobe agenesis and nontoxic multinodular goiter were depicted. Proband's sister (Patient II:6) was euthyroid, showed up at the age of 39 due to neck discomfort and left-sided THA was demonstrated. Affected individuals were subjected to whole-exome sequencing (WES) (Illumina, TruSeq Exome Kit) and all identified variants were evaluated for pathogenicity. Sanger sequencing was used to confirm WES data and check segregation among first-degree relatives.

RESULTS

In both siblings, a compound heterozygous mutations NM_000168.6: c.[2179G>A];[4039C>A] (NP_000159.3: p.[Gly727Arg];[Gln1347Lys]) were identified in the GLI3 gene, affecting exon 14 and 15, respectively. According to the American College of Medical Genetics, variants are classified as of uncertain significance, and were found to be very rare (GnomAD MAF 0.007131 and 0.00003187). The segregation mapping and analysis of relatives indicated causativeness of compound heterozygosity.

CONCLUSIONS

We demonstrated for the first time a unique association of THA phenotype and the presence of compound heterozygous mutations p.[Gly727Arg];[Gln1347Lys] of GLI3 gene in two siblings.

Pituitary stem cells produce paracrine WNT signals to control the expansion of their descendant progenitor cells

In response to physiological demand, the pituitary gland generates new hormone-secreting cells from committed progenitor cells throughout life. It remains unclear to what extent pituitary stem cells (PSCs), which uniquely express SOX2, contribute to pituitary growth and renewal. Moreover, neither the signals that drive proliferation nor their sources have been elucidated. We have used genetic approaches in the mouse, showing that the WNT pathway is essential for proliferation of all lineages in the gland. We reveal that SOX2⁺ stem cells are a key source of WNT ligands. By blocking secretion of WNTs from SOX2⁺ PSCs in vivo, we demonstrate that proliferation of neighbouring committed progenitor cells declines, demonstrating that progenitor multiplication depends on the paracrine WNT secretion from SOX2⁺ PSCs. Our results indicate that stem cells can hold additional roles in tissue expansion and homeostasis, acting as paracrine signalling centres to coordinate the proliferation of neighbouring cells.

Developmental Genes and Malformations in the Hypothalamus

The hypothalamus is a heterogeneous rostral forebrain region that regulates physiological processes essential for survival, energy metabolism, and reproduction, mainly mediated by the pituitary gland. In the updated prosomeric model, the hypothalamus represents the rostralmost forebrain, composed of two segmental regions (terminal and peduncular hypothalamus), which extend respectively into the non-evaginated preoptic telencephalon and the evaginated pallio-subpallial telencephalon. Complex genetic cascades of transcription factors and signaling molecules rule their development. Alterations of some of these molecular mechanisms acting during forebrain development are associated with more or less severe hypothalamic and pituitary dysfunctions, which may be associated with brain malformations such as holoprosencephaly or septo-optic dysplasia. Studies on transgenic mice with mutated genes encoding critical transcription factors implicated in hypothalamic-pituitary development are contributing to understanding the high clinical complexity of these pathologies. In this review article, we will analyze first the complex molecular genoarchitecture of the hypothalamus resulting from the activity of previous morphogenetic signaling centers and secondly some malformations related to alterations in genes implicated in the development of the hypothalamus.

Copy Number Variants Contributing to Combined Pituitary Hormone Deficiency

Combined pituitary hormone deficiency represents a disorder with complex etiology. For many patients, causes of the disease remain unexplained, despite usage of advanced genetic testing. Although major and common transcription factors were identified two decades ago, we still struggle with identification of rare inborn factors contributing to pituitary function. In this report, we follow up genomic screening of CPHD patient cohort that were previously tested for changes in a coding sequences of genes with the use of the whole exome. We aimed to find contribution of rare copy number variations (CNVs). As a result, we identified genomic imbalances in 7 regions among 12 CPHD patients. Five out of seven regions showed copy gains whereas two presented losses of genomic fragment. Three regions with detected gains encompassed known CPHD genes namely LHX4, HESX1, and OTX2. Among new CPHD loci, the most interesting seem to be the region covering SIX3 gene, that is abundantly expressed in developing brain, and together with HESX1 contributes to pituitary organogenesis as it was evidenced before in functional studies. In conclusion, with the use of broadened genomic approach we identified copy number imbalances for 12 CPHD patients. Although further functional studies are required in order to estimate its true impact on expression pattern during pituitary organogenesis and CPHD etiology.

Haploinsufficiency of NKX2-1 in Brain-Lung-Thyroid Syndrome with Additional Multiple Pituitary Dysfunction

INTRODUCTION

Heterozygous mutations or haploinsufficiency of NKX2-1 are associated with the brain-lung-thyroid syndrome incorporating primary hypothyroidism, respiratory distress, and neurological disturbances.

CASE PRESENTATION

We report a patient presenting in the neonatal period with multiple pituitary hormone deficiency including central hypothyroidism and hypoadrenalism, growth hormone deficiency, undetectable gonadotrophins, and a small anterior pituitary on MRI. CGH microarray revealed haploinsufficiency for NKX2.1 and during subsequent follow-up, she has exhibited the classic triad of brain-lung-thyroid syndrome with undetectable tissue on thyroid ultrasonography. Whilst the role of NKX2-1 is well described in murine pituitary development, this report constitutes the first description of multiple pituitary dysfunction in humans associated with the syndrome and haploinsufficiency NKX2-1.

CONCLUSION

The report highlights a potential need for pituitary screening in patients with established brain-lung-thyroid syndrome and implicates NKX2.1 in human pituitary disease.

Okur-Chung neurodevelopmental syndrome in a patient from Spain

Okur-Chung neurodevelopmental syndrome (OCNS, MIM#617062) is a rare autosomal dominant syndrome related to CSNK2A1 mutations. It is characterized by intellectual disability, hypotonia, feeding and speech difficulties, dysmorphic features, and multisystem involvement. To date, less than 30 patients with OCNS have been described in detail in the literature, primarily in Asian populations. Here, we report a 5-year-old Spanish female with OCNS arising from a novel CSNK2A1 mutation c.149A>G, p.Tyr50Cys. Although her clinical features were compatible with OCNS syndrome, magnetic resonance imaging unexpectedly showed a duplication of the pituitary gland, a clinical finding not previously related to any known genetic condition. Other novel signs were an absence of the olfactory bulbs and multiple duplications of cervical vertebrae. We suggest that the midline abnormalities may be a significant part of this condition and lead to diagnostic suspicion. However, further descriptions are needed.

Stem Cells, Self-Renewal, and Lineage Commitment in the Endocrine System

The endocrine system coordinates a wide array of body functions mainly through secretion of hormones and their actions on target tissues. Over the last decades, a collective effort between developmental biologists, geneticists, and stem cell biologists has generated a wealth of knowledge related to the contribution of stem/progenitor cells to both organogenesis and self-renewal of endocrine organs. This review provides an up-to-date and comprehensive overview of the role of tissue stem cells in the development and self-renewal of endocrine organs. Pathways governing crucial steps in both development and stemness maintenance, and that are known to be frequently altered in a wide array of endocrine disorders, including cancer, are also described. Crucially, this plethora of information is being channeled into the development of potential new cell-based treatment modalities for endocrine-related illnesses, some of which have made it through clinical trials.

All Together Now: Modeling the Interaction of Neural With Non-neural Systems Using Organoid Models

The complex development of the human nervous system has been traditionally studied using a combination of animal models, human post-mortem brain tissue, and human genetics studies. However, there has been a lack of experimental human cellular models that would allow for a more precise elucidation of the intricate dynamics of early human brain development. The development of stem cell technologies, both embryonic and induced pluripotent stem cells (iPSCs), has given neuroscientists access to the previously inaccessible early stages of human brain development. In particular, the recent development of three-dimensional culturing methodologies provides a platform to study the differentiation of stem cells in both normal development and disease states in a more in vivo like context. Three-dimensional neural models or cerebral organoids possess an innate advantage over two-dimensional neural cultures as they can recapitulate tissue organization and cell type diversity that resemble the developing brain. Brain organoids also provide the exciting opportunity to model the integration of different brain regions in vitro. Furthermore, recent advances in the differentiation of non-neuronal tissue from stem cells provides the opportunity to study the interaction between the developing nervous system and other non-neuronal systems that impact neuronal function. In this review, we discuss the potential and limitations of the organoid system to study in vitro neurological diseases that arise in the neuroendocrine and the enteric nervous system or from interactions with the immune system.

Pituitary Transcription Factor Mutations Leading to Hypopituitarism

Congenital pituitary hormone deficiency is a disabling condition. It is part of a spectrum of disorders including craniofacial midline developmental defects ranging from holoprosencephaly through septo-optic dysplasia to combined and isolated pituitary hormone deficiency. The first genes discovered in the human disease were based on mouse models of dwarfism due to mutations in transcription factor genes. High-throughput DNA sequencing technologies enabled clinicians and researchers to find novel genetic causes of hypopituitarism for the more than three quarters of patients without a known genetic diagnosis to date. Transcription factor (TF) genes are at the forefront of the functional analysis of novel variants of unknown significance due to the relative ease in in vitro testing in a research lab. Genetic testing in hypopituitarism is of high importance to the individual and their family to predict phenotype composition, disease progression and to avoid life-threatening complications such as secondary adrenal insufficiency.This chapter aims to highlight our current understanding about (1) the contribution of TF genes to pituitary development (2) the diversity of inheritance and phenotype features in combined and select isolated pituitary hormone deficiency and (3) provide an initial assessment on how to approach variants of unknown significance in human hypopituitarism. Our better understanding on how transcription factor gene variants lead to hypopituitarism is a meaningful step to plan advanced therapies to specific genetic changes in the future.

Regulation of Pituitary Progenitor Differentiation by β-Catenin

The pituitary gland is a critical organ that is necessary for many physiological processes, including growth, reproduction, and stress response. The secretion of pituitary hormones from specific cell types regulates these essential processes. Pituitary hormone cell types arise from a common pool of pituitary progenitors, and mutations that disrupt the formation and differentiation of pituitary progenitors result in hypopituitarism. Canonical WNT signaling through CTNNB1 (β-catenin) is known to regulate the formation of the POU1F1 lineage of pituitary cell types. When β-catenin is deleted during the initial formation of the pituitary progenitors, Pou1f1 is not transcribed, which leads to the loss of the POU1F1 lineage. However, when β-catenin is deleted after lineage specification, there is no observable effect. Similarly, the generation of a β-catenin gain-of-function allele in early pituitary progenitors or stem cells results in the formation of craniopharyngiomas, whereas stimulating β-catenin in differentiated cell types has no effect. PROP1 is a pituitary-specific transcription factor, and the peak of PROP1 expression coincides with a critical time point in pituitary organogenesis-that is, after pituitary progenitor formation but before lineage specification. We used a Prop1-cre to conduct both loss- and gain-of-function studies on β-catenin during this critical time point. Our results demonstrate that pituitary progenitors remain sensitive to both loss and gain of β-catenin at this time point, and that either manipulation results in hypopituitarism.

Molecular Mechanisms Governing Embryonic Differentiation of Pituitary Somatotropes

Pituitary somatotropes secrete growth hormone (GH), which is essential for normal growth and metabolism. Somatotrope defects result in GH deficiency (GHD), leading to short stature in childhood and increased cardiovascular morbidity and mortality in adulthood. Current hormone replacement therapies fail to recapitulate normal pulsatile GH secretion. Stem cell therapies could overcome this problem but are dependent on a thorough understanding of somatotrope differentiation. Although several transcription factors, signaling pathways, and hormones that regulate this process have been identified, the mechanisms of action are not well understood. The purpose of this review is to highlight the known players in somatotrope differentiation while emphasizing the need to better understand these pathways to serve patients with GHD.