Heterozygous mutation of HESX1 causing hypopituitarism and multiple anatomical malformations without features of septo-optic dysplasia

Appetite- and Weight-Regulating Neuroendocrine Circuitry in Hypothalamic Obesity

Since hypothalamic obesity (HyOb) was first described over 120 years ago by Joseph Babinski and Alfred Fröhlich, advances in molecular genetic laboratory techniques have allowed us to elucidate various components of the intricate neurocircuitry governing appetite and weight regulation connecting the hypothalamus, pituitary gland, brainstem, adipose tissue, pancreas, and gastrointestinal tract. On a background of an increasing prevalence of population-level common obesity, the number of survivors of congenital (eg, septo-optic dysplasia, Prader-Willi syndrome) and acquired (eg, central nervous system tumors) hypothalamic disorders is increasing, thanks to earlier diagnosis and management as well as better oncological therapies. Although to date the discovery of several appetite-regulating peptides has led to the development of a range of targeted molecular therapies for monogenic obesity syndromes, outside of these disorders these discoveries have not translated into the development of efficacious treatments for other forms of HyOb. This review aims to summarize our current understanding of the neuroendocrine physiology of appetite and weight regulation, and explore our current understanding of the pathophysiology of HyOb.

Homozygous HESX1 and COL1A1 Gene Variants in a Boy with Growth Hormone Deficiency and Early Onset Osteoporosis

We report on a patient born to consanguineous parents, presenting with Growth Hormone Deficiency (GHD) and osteoporosis. SNP-array analysis and exome sequencing disclosed long contiguous stretches of homozygosity and two distinct homozygous variants in HESX1 (Q6H) and COL1A1 (E1361K) genes. The HESX1 variant was described as causative in a few subjects with an incompletely penetrant dominant form of combined pituitary hormone deficiency (CPHD). The COL1A1 variant is rare, and so far it has never been found in a homozygous form. Segregation analysis showed that both variants were inherited from heterozygous unaffected parents. Present results further elucidate the inheritance pattern of HESX1 variants and recommend assessing the clinical impact of variants located in C-terminal propeptide of COL1A1 gene for their potential association with rare recessive and early onset forms of osteoporosis.

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.

Genetic causes of hypopituitarism

Hypopituitarism in neonates is rare, but has life-threatening complications if untreated. This review describes the features of hypopituitarism and the evidence for which infants in whom a genetic cause should be suspected. Importantly, neonates are often asymptomatic or present with non-specific symptoms. Hypopituitarism can be due to abnormal gland development as a result of genetic defects, which result from mutations in gene coding for transcription factors which regulate pituitary development. The mutations can be divided into those causing isolated hypopituitarism or those causing syndromes with associated hypopituitarism. The latter involve mutations in transcription factors which regulate pituitary, as well as extra-pituitary development. There is a paucity of evidence as to which patients should be investigated for genetic mutations, but detailed clinical and biochemical phenotyping with magnetic resonance imaging of the pituitary gland could help target those in whom genetic investigations would be most appropriate.

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.

Novel Mutations in HESX1 and PROP1 Genes in Combined Pituitary Hormone Deficiency

BACKGROUND/AIMS

The HESX1 gene is essential in forebrain development and pituitary organogenesis, and its mutations are the most commonly identified genetic cause of septo-optic dysplasia (SOD). The PROP1 gene is involved in anterior pituitary cell lineage specification and is commonly implicated in non-syndromic combined pituitary hormone deficiency (CPHD). We aimed to assess the involvement of HESX1 and PROP1 mutations in a cohort of patients with SOD and CPHD.

METHODS

Six patients with sporadic SOD and 16 patients with CPHD from 14 pedigrees were screened for mutations in HESX1 and PROP1 genes by exon sequencing. Half of the CPHD patients had variable associated clinical characteristics, such as hearing loss, orofacial cleft, kidney disorder or developmental delay. Novel variants were evaluated in silico and verified in SNP databases.

RESULTS

A novel heterozygous p.Glu102Gly mutation in the HESX1 gene and a novel homozygous p.Arg121Thr mutation in the PROP1 gene were detected in 2 pedigrees with CPHD. A small previously reported deletion in PROP1 c.301_302delAG was detected in a separate patient with CPHD, in heterozygous state. No mutations were identified in patients with SOD.

CONCLUSIONS

Our results expand the spectrum of mutations implicated in CPHD. The frequency of 15% of the PROP1 mutations in CPHD was low, likely due to the clinical heterogeneity of the cohort.

Frequency of genetic defects in combined pituitary hormone deficiency: a systematic review and analysis of a multicentre Italian cohort

OBJECTIVE

Combined pituitary hormonal deficiency (CPHD) can result from mutations within genes that encode transcription factors. This study evaluated the frequency of mutations in these genes in a cohort of 144 unrelated Italian patients with CPHD and estimated the overall prevalence of mutations across different populations using a systematic literature review.

MATERIAL AND METHODS

A multicentre study of adult and paediatric patients with CPHD was performed. The PROP1, POU1F1, HESX1, LHX3 and LHX4 genes were analysed for the presence of mutations using direct sequencing. We systematically searched PubMed with no date restrictions for studies that reported genetic screening of CPHD cohorts. We only considered genetic screenings with at least 10 individuals. Data extraction was conducted in accordance with the guidelines set by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA).

RESULTS

Global mutation frequency in Italian patients with CPHD was 2·9% (4/136) in sporadic cases and 12·5% (1/8) in familial cases. The worldwide mutation frequency for the five genes calculated from 21 studies was 12·4%, which ranged from 11·2% in sporadic to 63% in familial cases. PROP1 was the most frequently mutated gene in sporadic (6·7%) and familial cases (48·5%).

CONCLUSION

The frequency of defects in genes encoding pituitary transcription factors is quite low in Italian patients with CPHD and other western European countries, especially in sporadic patients. The decision of which genes should be tested and in which order should be guided by hormonal and imaging phenotype, the presence of extrapituitary abnormalities and the frequency of mutation for each gene in the patient-referring population.

Combined pituitary hormone deficiency: current and future status

Over the last two decades, the understanding of the mechanisms involved in pituitary ontogenesis has largely increased. Since the first description of POU1F1 human mutations responsible for a well-defined phenotype without extra-pituitary malformation, several other genetic defects of transcription factors have been reported with variable degrees of phenotype-genotype correlations. However, to date, despite the identification of an increased number of genetic causes of isolated or multiple pituitary deficiencies, the etiology of most (80-90 %) congenital cases of hypopituitarism remains unsolved. Identifying new etiologies is of importance as a post-natal diagnosis to better diagnose and treat the patients (delayed pituitary deficiencies, differential diagnosis of a pituitary mass on MRI, etc.), and as a prenatal diagnosis to decrease the risk of early death (undiagnosed corticotroph deficiency for instance). The aim of this review is to summarize the main etiologies and phenotypes of combined pituitary hormone deficiencies, associated or not with extra-pituitary anomalies, and to suggest how the identification of such etiologies could be improved in the near future.

Identification of HESX1 mutations in Kallmann syndrome

OBJECTIVE

To determine whether HESX1 mutations are present in patients with idiopathic hypogonadotropic hypogonadism (IHH)/Kallmann syndrome (KS).

DESIGN

Polymerase chain reaction-based DNA sequencing was performed on 217 well-characterized IHH/KS patients. Putative missense mutations were analyzed by sorting intolerant from tolerant (SIFT) and Clustal Ω.

SETTING

Academic medical center.

PATIENT(S)

Two hundred seventeen patients with IHH/KS and 192 controls.

INTERVENTION(S)

Deoxyribonucleic acid was extracted from patients and controls; genotype/phenotype comparisons were made.

MAIN OUTCOME MEASURE(S)

Deoxyribonucleic acid sequence of HESX1, SIFT analysis, and ortholog alignment.

RESULT(S)

Two novel heterozygous missense mutations (p.H42Y and p.V75L) and previously reported heterozygous missense mutation p.Q6H in HESX1 were identified in 3 of 217 patients (1.4%). All were males with KS. Both p.Q6H and p.H42Y were predicted to be deleterious by SIFT, whereas p.V75L was conserved in 8 of 9 species. No other IHH/KS gene mutations were present.

CONCLUSION(S)

HESX1 mutations may cause KS in addition to more severe phenotypes. Our findings expand the phenotypic spectrum of HESX1 mutations in humans, thereby broadening its role in development.

A novel HESX1 splice mutation causes isolated GH deficiency by interfering with mRNA processing

OBJECTIVE

Mutations in HESX1 represent a rare cause of GH deficiency (GHD) associated with a broad spectrum of other anomalies. We searched for causative mutations in a cohort of 244 Italian patients affected by combined and isolated GHD (IGHD).

METHODS

The HESX1 gene-coding region and exon-intron boundaries were screened by denaturing HPLC scanning.

RESULTS

A novel mutation adjacent to the invariant donor splice site of intron 2 (c.357+3G>A) was identified at the heterozygous state in an IGHD patient. The in vitro and in vivo mRNA analysis of the wild-type HESX1 allele revealed the presence of the whole cDNA and two isoforms lacking exon 2 and exons 2-3 respectively. The mutant HESX1 allele yielded only two splicing products, the whole cDNA and the cDNA missing exons 2-3, whereas the mRNA lacking exon 2 was absent. An in vitro assay demonstrated that the exon 2-deleted mRNA, predicting a prematurely truncated protein, is subjected to nonsense-mediated mRNA decay (NMD).

CONCLUSIONS

The c.357+3G>A mutation prevents the generation of one of the alternative isoforms normally produced by the wild-type allele, predicting a truncated HESX1 protein. The mutation is likely to cause IGHD in the heterozygous patient by interfering with the downregulation of HESX1 expression mediated by alternative splicing and NMD. Our results open new insight into the mechanism of HESX1 regulation suggesting that the coupling of alternative splicing and NMD might play a fundamental role in directing the HESX1 expression, and that the alteration of this process might lead to severe consequences.

Septo-optic dysplasia and other midline defects: the role of transcription factors: HESX1 and beyond

Septo-optic dysplasia (SOD) is a highly heterogeneous condition comprising variable phenotypes including midline and forebrain abnormalities, optic nerve and pituitary hypoplasia. Most instances of SOD are sporadic and several aetiologies including drug and alcohol abuse have been suggested to account for the pathogenesis of the condition. However, a number of familial cases have been described with an increasing number of mutations in developmental transcription factors including HESX1, SOX2, SOX3 and OTX2 being implicated in its aetiology. These factors are essential for normal forebrain/pituitary development, and disruptions to these genes could account for the features observed in SOD and other midline disorders. The variable phenotypes observed within the condition are most likely due to the varying contributions of genetic and environmental factors. This review will discuss the current knowledge about SOD. Further study of these and other novel factors may shed light on the complex aetiology of this condition.

Genetic regulation of pituitary gland development in human and mouse

Normal hypothalamopituitary development is closely related to that of the forebrain and is dependent upon a complex genetic cascade of transcription factors and signaling molecules that may be either intrinsic or extrinsic to the developing Rathke's pouch. These factors dictate organ commitment, cell differentiation, and cell proliferation within the anterior pituitary. Abnormalities in these processes are associated with congenital hypopituitarism, a spectrum of disorders that includes syndromic disorders such as septo-optic dysplasia, combined pituitary hormone deficiencies, and isolated hormone deficiencies, of which the commonest is GH deficiency. The highly variable clinical phenotypes can now in part be explained due to research performed over the last 20 yr, based mainly on naturally occurring and transgenic animal models. Mutations in genes encoding both signaling molecules and transcription factors have been implicated in the etiology of hypopituitarism, with or without other syndromic features, in mice and humans. To date, mutations in known genes account for a small proportion of cases of hypopituitarism in humans. However, these mutations have led to a greater understanding of the genetic interactions that lead to normal pituitary development. This review attempts to describe the complexity of pituitary development in the rodent, with particular emphasis on those factors that, when mutated, are associated with hypopituitarism in humans.