A recessive mutation resulting in a disabling amino acid substitution (T194R) in the LHX3 homeodomain causes combined pituitary hormone deficiency

Dwarfism in Tibetan Terrier dogs with an LHX3 mutation

Canine pituitary dwarfism in German Shepherd and related dog breeds has been reported to be associated with a 7-bp deletion mutation in intron 5 of the LHX3 gene. This mutation is transmitted as an autosomal recessive trait that results in dwarf dogs with significantly smaller stature and abnormal haircoat, and potentially early death. Phenotypically, affected adult dogs are proportionally dwarfs. These dwarfs also have a soft, woolly puppy coat that fails to transition into the typical adult hair coat, and marked hair loss occurs in some dogs. We report a similar manifestation of dwarfism in Tibetan Terriers with the same LHX3 mutation. Dwarf Tibetan Terrier puppies were born physically normal but failed to gain weight or to grow at the same rate as their normal littermates. The 7-bp deletion mutation of the LHX3 gene was identified in both alleles of 3 Tibetan Terrier dwarfs from 3 litters, which were biologically related. All parents of these dogs are carriers, confirming transmission of dwarfism in an autosomal recessive manner. Recognition and detection of this mutation will help in guiding future breeding plans to eventually eliminate this trait from Tibetan Terriers.

Genetic Anomalies of Growth Hormone Deficiency in Pediatrics

Several different proteins regulate, directly or indirectly, the production of growth hormones from the pituitary gland, thereby complex genetics is involved. Defects in these genes are related to the deficiency of growth hormones solely, or deficiency of other hormones, secreted from the pituitary gland including growth hormones. These studies can aid clinicians to trace the pattern of the disease between the families, start early treatment and predict possible future consequences. This paper highlights some of the most common and novel genetic anomalies concerning growth hormones, which are responsible for various genetic defects in isolated growth and combined pituitary hormone deficiency disease.

Genetics of congenital hypogonadotropic hypogonadism: peculiarities and phenotype of an oligogenic disease

A genetic basis of congenital isolated hypogonadotropic hypogonadism (CHH) can be defined in almost 50% of cases, albeit not necessarily the complete genetic basis. Next-generation sequencing (NGS) techniques have led to the discovery of a great number of loci, each of which has illuminated our understanding of human gonadotropin-releasing hormone (GnRH) neurons, either in respect of their embryonic development or their neuroendocrine regulation as the "pilot light" of human reproduction. However, because each new gene linked to CHH only seems to underpin another small percentage of total patient cases, we are still far from achieving a comprehensive understanding of the genetic basis of CHH. Patients have generally not benefited from advances in genetics in respect of novel therapies. In most cases, even genetic counselling is limited by issues of apparent variability in expressivity and penetrance that are likely underpinned by oligogenicity in respect of known and unknown genes. Robust genotype-phenotype relationships can generally only be established for individuals who are homozygous, hemizygous or compound heterozygotes for the same gene of variant alleles that are predicted to be deleterious. While certain genes are purely associated with normosmic CHH (nCHH) some purely with the anosmic form (Kallmann syndrome-KS), other genes can be associated with both nCHH and KS-sometimes even within the same kindred. Even though the anticipated genetic overlap between CHH and constitutional delay in growth and puberty (CDGP) has not materialised, previously unanticipated genetic relationships have emerged, comprising conditions of combined (or multiple) pituitary hormone deficiency (CPHD), hypothalamic amenorrhea (HA) and CHARGE syndrome. In this review, we report the current evidence in relation to phenotype and genetic peculiarities regarding 60 genes whose loss-of-function variants can disrupt the central regulation of reproduction at many levels: impairing GnRH neurons migration, differentiation or activation; disrupting neuroendocrine control of GnRH secretion; preventing GnRH neuron migration or function and/or gonadotropin secretion and action.

Development of the Pituitary Gland

The development of the anterior pituitary gland occurs in distinct sequential developmental steps, leading to the formation of a complex organ containing five different cell types secreting six different hormones. During this process, the temporal and spatial expression of a cascade of signaling molecules and transcription factors plays a crucial role in organ commitment, cell proliferation, patterning, and terminal differentiation. The morphogenesis of the gland and the emergence of distinct cell types from a common primordium are governed by complex regulatory networks involving transcription factors and signaling molecules that may be either intrinsic to the developing pituitary or extrinsic, originating from the ventral diencephalon, the oral ectoderm, and the surrounding mesenchyme. Endocrine cells of the pituitary gland are organized into structural and functional networks that contribute to the coordinated response of endocrine cells to stimuli; these cellular networks are formed during embryonic development and are maintained or may be modified in adulthood, contributing to the plasticity of the gland. Abnormalities in any of the steps of pituitary development may lead to congenital hypopituitarism that includes a spectrum of disorders from isolated to combined hormone deficiencies including syndromic disorders such as septo-optic dysplasia. Over the past decade, the acceleration of next-generation sequencing has allowed for rapid analysis of the patient genome to identify novel mutations and novel candidate genes associated with hypothalmo-pituitary development. Subsequent functional analysis using patient fibroblast cells, and the generation of stem cells derived from patient cells, is fast replacing the need for animal models while providing a more physiologically relevant characterization of novel mutations. Furthermore, CRISPR-Cas9 as the method for gene editing is replacing previous laborious and time-consuming gene editing methods that were commonly used, thus yielding knockout cell lines in a fraction of the time. © 2020 American Physiological Society. Compr Physiol 10:389-413, 2020.

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.

Heterozygous LHX3 mutations may lead to a mild phenotype of combined pituitary hormone deficiency

LHX3 is an LIM domain transcription factor involved in the early steps of pituitary ontogenesis. We report here functional studies of three allelic variants, including the first heterozygous variant of LHX3 NM_178138.5(LHX3):c.587T>C (p.(Leu196Pro)) that may be responsible for a milder phenotype of hypopituitarism. Our functional studies showed that NM_178138.5(LHX3):c.587T>C (p.(Leu196Pro)) was not able to activate target promoters in vitro, as it did not bind DNA, and likely affected LHX3 function via a mechanism of haplo-insufficiency. Our study demonstrates the possibility that patients with a heterozygous variant of LHX3 may have pituitary deficiencies, with a milder phenotype than patients with homozygous variants. It is thus of vital to propose an optimal follow-up of such patients, who, until now, were considered as not being at risk of presenting pituitary deficiency. The second variant NM_178138.5(LHX3):c.622C>G (p.(Arg208Gly)), present in a homozygous state, displayed decreased transactivating ability without loss of binding capacity in vitro, concordant with in silico analysis; it should thus be considered to affect LHX3 function. In contrast, the NM_178138.5(LHX3):c.929G>C (p.(Arg310Pro)) variant, in a heterozygous state, also predicted as deleterious in silico, proved functionally active in vitro, and should thus still be classified as a variant of unknown significance. Our study emphasizes the need for functional studies due to the limits of software-based predictions of new variants, and the possible association of a pituitary phenotype to heterozygous LHX3 variants.

Two novel LHX3 mutations in patients with combined pituitary hormone deficiency including cervical rigidity and sensorineural hearing loss

BACKGROUND

Congenital combined pituitary hormone deficiency (CPHD) is a rare heterogeneous group of conditions. CPHD-type 3 (CPHD3; MIM# 221750) is caused by recessive mutations in LHX3, a LIM-homeodomain transcription factor gene. The isoforms of LHX3 are critical for pituitary gland formation and specification of the anterior pituitary hormone-secreting cell types. They also play distinct roles in the development of neuroendocrine and auditory systems.

CASE PRESENTATION

Here, we summarize the clinical, endocrinological, radiological and molecular features of three patients from two unrelated families. Clinical evaluation revealed severe CPHD coupled with cervical vertebral malformations (rigid neck, scoliosis), mild developmental delay and moderate sensorineural hearing loss (SNHL). The patients were diagnosed with CPHD3 based on the array of hormone deficiencies and other associated syndromic symptoms, suggestive of targeted LHX3 gene sequencing. A novel missense mutation c.437G > T (p. Cys146Phe) and a novel nonsense mutation c.466C > T (p. Arg156Ter), both in homozygous forms, were found. The altered Cys146 resides in the LIM2 domain of the encoded protein and is a phylogenetically conserved residue, which mediates LHX3 transcription factor binding with a zinc cation. The p. Arg156Ter is predicted to result in a severely truncated protein, lacking the DNA binding homeodomain.

CONCLUSIONS

Considering genotype/phenotype correlation, we suggest that the presence of SNHL and limited neck rotation should be considered in the differential diagnosis of CPHD3 to facilitate molecular diagnosis. This report describes the first LHX3 mutations from Saudi patients and highlights the importance of combining molecular diagnosis with the clinical findings. In addition, it also expands the knowledge of LHX3-related CPHD3 phenotype and the allelic spectrum for this gene.

Genetics of Combined Pituitary Hormone Deficiency: Roadmap into the Genome Era

The genetic basis for combined pituitary hormone deficiency (CPHD) is complex, involving 30 genes in a variety of syndromic and nonsyndromic presentations. Molecular diagnosis of this disorder is valuable for predicting disease progression, avoiding unnecessary surgery, and family planning. We expect that the application of high throughput sequencing will uncover additional contributing genes and eventually become a valuable tool for molecular diagnosis. For example, in the last 3 years, six new genes have been implicated in CPHD using whole-exome sequencing. In this review, we present a historical perspective on gene discovery for CPHD and predict approaches that may facilitate future gene identification projects conducted by clinicians and basic scientists. Guidelines for systematic reporting of genetic variants and assigning causality are emerging. We apply these guidelines retrospectively to reports of the genetic basis of CPHD and summarize modes of inheritance and penetrance for each of the known genes. In recent years, there have been great improvements in databases of genetic information for diverse populations. Some issues remain that make molecular diagnosis challenging in some cases. These include the inherent genetic complexity of this disorder, technical challenges like uneven coverage, differing results from variant calling and interpretation pipelines, the number of tolerated genetic alterations, and imperfect methods for predicting pathogenicity. We discuss approaches for future research in the genetics of CPHD.

Genetic causes of isolated and combined pituitary hormone deficiency

Research over the last 20 years has led to the elucidation of the genetic aetiologies of Isolated Growth Hormone Deficiency (IGHD) and Combined Pituitary Hormone Deficiency (CPHD). The pituitary plays a central role in growth regulation, coordinating the multitude of central and peripheral signals to maintain the body's internal balance. Naturally occurring mutation in humans and in mice have demonstrated a role for several factors in the aetiology of IGHD/CPHD. Mutations in the GH1 and GHRHR genes shed light on the phenotype and pathogenesis of IGHD whereas mutations in transcription factors such as HESX1, PROP1, POU1F1, LHX3, LHX4, GLI2 and SOX3 contributed to the understanding of CPHD. Depending upon the expression patterns of these molecules, the phenotype may consist of isolated hypopituitarism, or more complex disorders such as septo-optic dysplasia (SOD) and holoprosencephaly. Although numerous monogenic causes of growth disorders have been identified, most of the patients with IGHD/CPHD remain with an explained aetiology as shown by the relatively low mutation detection rate. The introduction of novel diagnostic approaches is now leading to the disclosure of novel genetic causes in disorders characterized by pituitary hormone defects.

Atlanto-axial malformation and instability in dogs with pituitary dwarfism due to an LHX3 mutation

BACKGROUND

Canine pituitary dwarfism or combined pituitary hormone deficiency (CPHD) in shepherd dogs is associated with an LHX3 mutation and can lead to a wide range of clinical manifestations. Some dogs with CPHD have neurological signs that are localized to the cervical spine. In human CPHD, caused by an LHX3 mutation, anatomical abnormalities in the atlanto-axial (C1-C2) joint have been described.

OBJECTIVES

To evaluate the presence of atlanto-axial malformations in dogs with pituitary dwarfism associated with an LHX3 mutation and to investigate the degree of similarity between the atlanto-axial anomalies found in canine and human CPHD patients with an LHX3 mutation.

ANIMALS

Three client-owned Czechoslovakian wolfdogs and 1 client-owned German shepherd dog, previously diagnosed with pituitary dwarfism caused by an LHX3 mutation, with neurological signs indicating a cervical spinal disorder.

METHODS

Radiography, computed tomography, and magnetic resonance imaging of the cranial neck and skull, necropsy, and histology.

RESULTS

Diagnostic imaging identified abnormal positioning of the dens axis and incomplete ossification of the suture lines between the ossification centers of the atlas with concurrent atlanto-axial instability and dynamic compression of the spinal cord by the dens axis. The malformations and aberrant motion at C1-C2 were confirmed at necropsy and histology.

CONCLUSIONS AND CLINICAL IMPORTANCE

The atlanto-axial abnormalities of the dwarf dogs resemble those encountered in human CPHD patients with an LHX3 mutation. These findings suggest an association between the LHX3 mutation in dogs with CPHD and atlanto-axial malformations. Consequently, pituitary dwarfs should be monitored closely for neurological signs.

The role of DNA methylation in regulation of the murine Lhx3 gene

LHX3 is a LIM-homeodomain transcription factor with critical roles in pituitary and nervous system development. Mutations in the LHX3 gene are associated with pediatric diseases featuring severe hormone deficiencies, hearing loss, developmental delay, and other symptoms. The mechanisms that govern LHX3/Lhx3 transcription are poorly understood. In this study, we examined the role of DNA methylation in the expression status of the mouse Lhx3 gene. Pituitary cells that do not normally express Lhx3 (Pit-1/0 cells) were treated with 5-aza-2'-deoxycytidine, a demethylating reagent. This treatment leads to activation of Lhx3 gene expression suggesting that methylation contributes to Lhx3 regulation. Treatment of Pit-1/0 pituitary cells with a combination of a demethylating reagent and a histone deacetylase inhibitor led to rapid activation of Lhx3 expression, suggesting possible crosstalk between DNA methylation and histone modification processes. To assess DNA methylation levels, treated and untreated Pit-1/0 genomic DNAs were subjected to bisulfite conversion and sequencing. Treated Pit-1/0 cells had decreased methylation at specific sites in the Lhx3 locus compared to untreated cells. Chromatin immunoprecipitation assays demonstrated interactions between the MeCp2 methyl binding protein and Lhx3 promoter regions in the Pit-1/0 cell line. Overall, this study demonstrates that DNA methylation patterns of the Lhx3 gene are associated with its expression status.

Cell-specific actions of a human LHX3 gene enhancer during pituitary and spinal cord development

The LIM class of homeodomain protein 3 (LHX3) transcription factor is essential for pituitary gland and nervous system development in mammals. In humans, mutations in the LHX3 gene underlie complex pediatric syndromes featuring deficits in anterior pituitary hormones and defects in the nervous system. The mechanisms that control temporal and spatial expression of the LHX3 gene are poorly understood. The proximal promoters of the human LHX3 gene are insufficient to guide expression in vivo and downstream elements including a conserved enhancer region appear to play a role in tissue-specific expression in the pituitary and nervous system. Here we characterized the activity of this downstream enhancer region in regulating gene expression at the cellular level during development. Human LHX3 enhancer-driven Cre reporter transgenic mice were generated to facilitate studies of enhancer actions. The downstream LHX3 enhancer primarily guides gene transcription in α-glycoprotein subunit -expressing cells secreting the TSHβ, LHβ, or FSHβ hormones and expressing the GATA2 and steroidogenic factor 1 transcription factors. In the developing nervous system, the enhancer serves as a targeting module active in V2a interneurons. These results demonstrate that the downstream LHX3 enhancer is important in specific endocrine and neural cell types but also indicate that additional regulatory elements are likely involved in LHX3 gene expression. Furthermore, these studies revealed significant gonadotrope cell heterogeneity during pituitary development, providing insights into the cellular physiology of this key reproductive regulatory cell. The human LHX3 enhancer-driven Cre reporter transgenic mice also provide a valuable tool for further developmental studies of cell determination and differentiation in the pituitary and nervous system.

LHX3 interacts with inhibitor of histone acetyltransferase complex subunits LANP and TAF-1β to modulate pituitary gene regulation

LIM-homeodomain 3 (LHX3) is a transcription factor required for mammalian pituitary gland and nervous system development. Human patients and animal models with LHX3 gene mutations present with severe pediatric syndromes that feature hormone deficiencies and symptoms associated with nervous system dysfunction. The carboxyl terminus of the LHX3 protein is required for pituitary gene regulation, but the mechanism by which this domain operates is unknown. In order to better understand LHX3-dependent pituitary hormone gene transcription, we used biochemical and mass spectrometry approaches to identify and characterize proteins that interact with the LHX3 carboxyl terminus. This approach identified the LANP/pp32 and TAF-1β/SET proteins, which are components of the inhibitor of histone acetyltransferase (INHAT) multi-subunit complex that serves as a multifunctional repressor to inhibit histone acetylation and modulate chromatin structure. The protein domains of LANP and TAF-1β that interact with LHX3 were mapped using biochemical techniques. Chromatin immunoprecipitation experiments demonstrated that LANP and TAF-1β are associated with LHX3 target genes in pituitary cells, and experimental alterations of LANP and TAF-1β levels affected LHX3-mediated pituitary gene regulation. Together, these data suggest that transcriptional regulation of pituitary genes by LHX3 involves regulated interactions with the INHAT complex.

Developmental analysis and influence of genetic background on the Lhx3 W227ter mouse model of combined pituitary hormone deficiency disease

Combined pituitary hormone deficiency (CPHD) diseases result in severe outcomes for patients including short stature, developmental delays, and reproductive deficiencies. Little is known about their etiology, especially the developmental profiles and the influences of genetic background on disease progression. Animal models for CPHD provide valuable tools to investigate disease mechanisms and inform diagnostic and treatment protocols. Here we examined hormone production during pituitary development and the influence of genetic background on phenotypic severity in the Lhx3(W227ter/W227ter) mouse model. Lhx3(W227ter/W227ter) embryos have deficiencies of ACTH, α-glycoprotein subunit, GH, PRL, TSHβ, and LHβ during prenatal development. Furthermore, mutant mice have significant reduction in the critical pituitary transcriptional activator-1 (PIT1). Through breeding, the Lhx3(W227ter/W227ter) genotype was placed onto the 129/Sv and C57BL/6 backgrounds. Intriguingly, the genetic background significantly affected viability: whereas Lhx3(W227ter/W227ter) animals were found in the expected frequencies in C57BL/6, homozygous animals were not viable in the 129/Sv genetic environment. The hormone marker and PIT1 reductions observed in Lhx3(W227ter/W227ter) mice on a mixed background were also seen in the separate strains but in some cases were more severe in 129/Sv. To further characterize the molecular changes in diseased mice, we conducted a quantitative proteomic analysis of pituitary proteins. This showed significantly lower levels of PRL, pro-opiomelanocortin (ACTH), and α-glycoprotein subunit proteins in Lhx3(W227ter/W227ter) mice. Together, these data show that hormone deficiency disease is apparent in early prenatal stages in this CPHD model system. Furthermore, as is noted in human disease, genetic background significantly impacts the phenotypic outcome of these monogenic endocrine diseases.