Mutations in LHX3 result in a new syndrome revealed by combined pituitary hormone deficiency

How clinicians add to knowledge of development

Studies of human birth defects and developmental disorders have made major contributions to our understanding of development. Rare human syndromes have allowed identification of important developmental genes, and revealed mechanisms such as uniparental disomy and unstable trinucleotide repeats that were not suspected from animal studies. Some aspects of development, in particular cognitive development, can only be studied in human beings. Basic developmental mechanisms are very highly conserved across a very wide range of animals, making for a rich interplay between animal and human studies. Often, clinical studies identify a gene, or suggest a hypothesis, that can then be investigated in animals.

DNA testing in patients with GH deficiency at the time of transition

Over the last 10 years, major advances in the understanding of pituitary gland development in the mouse have led to the identification of mutations in a number of genes that then lead to delineation of the phenotype of growth hormone deficiency (GHD), either in isolation (IGHD) or in combination with a number of other hormone deficiencies (CPHD) or syndromic features (e.g., septo-optic dysplasia, SOD). The genetic abnormalities include mutations within: (1) Hesx1 (IGHD, SOD or CPHD); (2) Lhx3 (CPHD with preservation of cortisol secretion and a short stiff neck); (3) Lhx4 (GH, TSH and ACTH deficiency with cerebellar hypoplasia); (4) Prop1 (variable CPHD often associated with pituitary masses); (5) POU1F1 (GH, prolactin and TSH deficiency); (6) GHRHR (IGHD) and (7) GH1 (IGHD). There can be variations in inheritance, phenotype and penetrance patterns. Nevertheless, establishing the genetic diagnosis can help in predicting the evolution of the phenotype and in genetic counselling. Therefore, for these reasons it is recommended that all patients with GHD should undergo testing for genetic mutations within the genes associated with IGHD, CPHD and SOD.

An outline of inherited disorders of the thyroid hormone generating system

To date, various genetic defects impairing the biosynthesis of thyroid hormone have been identified. These congenital heterogeneous disorders result from mutations of genes involved in many steps of thyroid hormone synthesis, storage, secretion, delivery, or utilization. In contrast to thyroid dyshormonogenesis, the elucidation of the underlying etiology of most cases of thyroid dysgenesis is much less understood. It is suggested that genetic factors might play a role in some cases of thyroid dysgenesis and the best candidate genes involved are those encoding transcription factors known to play a role in the embryonic development of the thyroid gland. Moreover, discordance for thyroid dysgenesis is the rule for monozygotic twins as recently reported and this may result from epigenetic phenomena, early somatic mutations, or postzygotic events. In the final part of this review the molecular defects involved in proteins that transport thyroid hormone in the circulation are described: thyroxine-binding globulin (TBG), transtiretin and albumin, that may be associated with altered thyroid function tests and other pathologic conditions such as amyloidotic polyneuropathy.

Human and mouse TPIT gene mutations cause early onset pituitary ACTH deficiency

Tpit is a highly cell-restricted transcription factor that is required for expression of the pro-opiomelanocortin (POMC) gene and for terminal differentiation of the pituitary corticotroph lineage. Its exclusive expression in pituitary POMC-expressing cells has suggested that its mutation may cause isolated deficiency of pituitary adrenocorticotropin (ACTH). We now show that Tpit-deficient mice constitute a model of isolated ACTH deficiency (IAD) that is very similar to human IAD patients carrying TPIT gene mutations. Through genetic analysis of a panel of IAD patients, we show that TPIT gene mutations are associated at high frequency with early onset IAD, but not with juvenile forms of this deficiency. We identified seven different TPIT mutations, including nonsense, missense, point deletion, and a genomic deletion. This work defines congenital early onset IAD as a relatively homogeneous clinical entity caused by recessive transmission of loss-of-function mutations in the TPIT gene.

Genetic characterization of growth hormone deficiency and resistance: implications for treatment with recombinant growth hormone

Growth failure can be caused by deficient growth hormone production or action. The genes involved in pituitary development, somatotrope function, as well as growth hormone synthesis, secretion, and action have recently been characterized in considerable detail. Familial growth failure has played an important role in identifying these genes, and a large number of mutations adversely affecting the development and function of the growth hormone/insulin-like growth factor axis have been discovered. Inactivating mutations leading to growth retardation in humans have been identified in several pituitary transcription factor genes (HESX1, PITX2, LHX3, PROP1, POU1F1) as well as in genes encoding the growth hormone-releasing hormone receptor (GHRH-R), the G(s) protein alpha subunit (GNAS1), growth hormone itself (GH-1), the growth hormone receptor (GHR), and in a single case each, the insulin-like growth factor I (IGF-I) and the IGF-I receptor. Mutations in pituitary transcription factors cause developmental abnormalities of the pituitary and deficiency of multiple pituitary hormones [growth hormone (GH), prolactin (Prl), thyrotropin (TSH) and lutropin/follitropin (LH/FSH)]. Most of the syndromes respond well to therapy with recombinant GH; exceptions are antibody-mediated resistance in GHD type IA (not all patients) and cases of Laron syndrome (GHR deficiency). Such patients respond to IGF-I therapy. This review summarizes the molecular genetics, functional defects, phenotypes, diagnostic considerations and therapeutic aspects of syndromes associated with mutations in the relevant genes.

Genes that fashion the pituitary gland

The pituitary gland with its distinct populations of hormone-producing cells is formed from precursor cells that are created when epithelial cells from the prospective diencephalon about the roof of the oral cavity during early development of the mammalian embryo. Successive steps of assembly during development lead from a rudimentary pouch to a definitive structure called Rathke's pouch, and finally to the pituitary gland with its anterior, intermediate and posterior lobes. The proliferation and differentiation of highly specialized cells from their precursors is directed by signaling cascades and transcriptional events that are being dissected by genetic and molecular approaches. Our current state of knowledge in this regard allows us to use the pituitary as a paradigm to delineate general principles of organogenesis in the mammalian organism.

Current approaches for deciphering the molecular basis of combined anterior pituitary hormone deficiency in humans

This review focuses on the general strategies currently used to decipher the molecular bases of combined pituitary hormone deficiency (CPHD) of genetic origin. By summarizing illustrative approaches that turned out to be successful for identifying an increasing number of genes involved in CPHD in the human, this article consider predictable obstacles specific to the investigation of these rare and heterogeneous conditions, while underlining the previously unsuspected roles of several of these genes during the development of extrapituitary structures.

Familial combined pituitary hormone deficiency caused by PROP-1 gene mutation. Growth patterns and MRI studies in untreated subjects

BACKGROUND

Mutations of the prophet of PIT-1 (PROP-1), a paired-like homeodomain transcription factor which is responsible for early embryonic pituitary development, have recently been reported as a cause of combined pituitary hormone deficiency.

METHODS

We describe the phenotype, long-term auxological data and MRI findings in two families with 4 affected members, all of whom have a mutation of the PROP-1 gene. GH, TSH, PRL, LH and FSH were completely deficient in all patients.

RESULTS

ACTH deficiency was not diagnosed until the 3rd or 4th decades of life. Pituitary MRI showed an empty sella in 2 subjects, but unspecific tissue accumulation resembling a pituitary mass lesion in another patient. The affected boy from family II who was continuously treated with all the necessary hormones reached the familial target height. However, the 3 subjects in family I were only treated sporadically (GH treatment lasting from 1 to 3 years).

CONCLUSION

Despite this insufficient therapy, final height was in the lower normal range. Longitudinal growth continued up to the age of 40 years in these subjects.

The spectrum of hypopituitarism caused by PROP1 mutations

Mutations in the PROP1 gene are responsible for a high proportion of cases of multiple or combined anterior pituitary hormone deficiencies in humans. The physical and hormonal phenotypes of affected individuals are not uniform. The diagnosis is seldom considered during the first year of life. Growth failure is usually evident later in childhood. Deficiency of growth hormone (GH) tends to precede deficiency of thyroid-stimulating hormone (TSH). While most affected individuals fail to enter puberty without sex hormone replacement, some enter puberty but then develop pubertal arrest with a loss of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) responses to GnRH. Partial deficiency of corticotrophin (ACTH) is a late finding. Imaging of the pituitary may disclose either a small anterior pituitary gland or an intrapituitary mass. The mechanisms responsible for delayed loss of hormone production and the occasional overgrowth of the pituitary represent important areas for future research.

Molecular basis of combined pituitary hormone deficiencies

Pituitary gland commitment from oral ectoderm occurs in response to inductive signals from the neuroepithelium of the ventral diencephalon. Invagination of the oral ectoderm leads to the creation of Rathke's pouch. Intensified cell proliferation within Rathke's pouch results in formation of the anterior pituitary lobe. Subsequently, highly differentiated cell types arise sequentially due to overlapping, but distinct, spatial and temporal patterns of signaling molecules and transcription factors. Mutations in some of the pituitary-specific transcription factors have been identified in patients with hypopituitarism, confirming the role of these factors in pituitary development.

Perspective: genetic defects in the etiology of congenital hypothyroidism

Congenital hypothyroidism affects about 1:3000 to 1:4000 infants and may be caused by defects in thyroidal ontogeny or hormone synthesis. The impressive advances in molecular genetics led to the characterization of numerous genes that are essential for normal development and hormone production of the hypothalamic-pituitary-thyroid axis. Mutations in many of these genes now provide a molecular explanation for a subset of the sporadic and familial forms of congenital hypothyroidism. Defects in one of the multiple steps required for normal hormone synthesis account for about 10% of cases with congenital hypothyroidism. They are typically recessive and therefore more common in inbred families. In the vast majority of patients, congenital hypothyroidism is sporadic and associated with thyroid dysgenesis, a spectrum of developmental defects, which includes the absence of detectable thyroid tissue, ectopic tissue, and thyroid hypoplasia. The molecular defects known to date only explain a minority of these cases and include mutations in the paired box transcription factor PAX8, and the thyroid transcription factors TTF1 and TTF2. It is likely that a further subset of patients with thyroid dysgenesis have defects in other transacting proteins or elements of the signaling pathways controlling growth and function of thyrocytes. In other instances, thyroid dysgenesis may be a polygenic disease or have a multifactorial basis. Aside from providing fundamental insights into the ontogeny and the pathophysiology of the thyroid, the characterization of the molecular basis of congenital hypothyroidism may have growing importance for genetic testing and counseling in the future.

Genetic disorders of human growth

Growth in humans is a complex process, controlled at numerous levels and by a myriad of factors. These factors may act centrally or peripherally and may be hormones, receptors, or transcription factors. Many of these probably are still unknown. The factors that are discussed here include those that act on the developing pituitary gland (transcription factors including LHX3, HESX1, PROP1, and PIT-1); those that regulate the normal activity of the pituitary (hypothalamic hormones such as GHRH, Ghrelin [growth hormone secretagoguel and somatostatin); those factors coming from the pituitary (essentially growth hormone [GH]); and the downstream modulators, transducers, and effectors of GH (including the GH receptor/GH binding protein, insulin-like growth factor-I and -II, their receptors, and their binding factors). What is becoming increasingly clear is the role of genetics in determining stature. This review discusses the most clinically relevant factors, with an emphasis on ontogeny, genetic inheritance, and clinical presentation.

GnRH neuronal development: insights into hypogonadotrophic hypogonadism

Pulsatile secretion of the hypothalamic decapeptide gonadotrophin-releasing hormone (GnRH) regulates activity of the pituitary-gonadal reproductive axis. Defects of this neuroendocrine axis necessarily result in hypogonadotrophic hypogonadism. In many vertebrate species studied, the main population of GnRH neurones originates extracranially within the olfactory system. In humans, both olfactory and GnRH systems are affected in Kallmann's syndrome--resulting in isolated hypogonadotrophic hypogonadism (IHH) combined with anosmia (loss of sense of smell). Familial IHH is also caused by other genetic conditions, which prevent GnRH from activating luteinizing hormone/follicle-stimulating hormone release from pituitary gonadotrophs. However, many cases of IHH have no defined chromosomal abnormality and, in the absence of pedigree analysis, studying the biological mechanisms controlling migration of GnRH neurones through the olfactory system into the developing central nervous system might reveal additional genetic pathways that play a role in the pathogenesis of IHH.

A case of growth hormone and gonadotropin deficiency associated with unilateral anophthalmia, microphallus, cryptorchidism, and mental retardation

We report a rare case of growth hormone and gonadotropin deficiency associated with dysmorphic features. A 16-year-old boy had left anophthalmia, microphallus, bilateral cryptorchidism, and mental retardation. His chromosomal karyotype was normal, 46, XY. Endocrinological studies revealed growth hormone and gonadotropin deficiency, attributed to hypothalamic dysfunction. Magnetic resonance imaging scan of the head showed a hypoplastic pituitary gland, decreased high intensity signals in the pituitary posterior lobe, absence of the left eye, and a hypoplastic left optic nerve with no abnormality of the pituitary stalk, corpus callosum, or septum pellucidum. Although not completely consistent with the features of septo-optic dysplasia (SOD), his condition was considered within the spectrum of SOD. Despite similarities to the Hesx1 knockout mouse, a model of human SOD, mutation analyses revealed no mutations or polymorphisms in coding regions of any exons or intron-exon boundaries of the HESX1 gene. Further genetic studies of this patient may improve understanding of molecular mechanisms involved in pituitary development.

Transcription factors in pituitary development

Many homeodomain transcription factors having a distinct temporal and spatial expression pattern have been described in the developing anterior pituitary gland. By interacting with each other, as well as with other extrinsic and intrinsic signals, they control cell determination, cell differentiation and eventually maintenance of cell function which is most important for the life long secretion of the pituitary derived hormones in an appropriate manner. The different phenotypes, as mainly studied in the mouse, may help to analyse the consequences of disruption of a known or yet unknown individual transcription factor in humans. Therefore, to study the different steps in morphogenesis will shed light onto developmental processes which will open a most fascinating time not only for basic scientists, biologist but also for clinicians.

Molecular effects of novel mutations in Hesx1/HESX1 associated with human pituitary disorders

The homeobox gene Hesx1/HESX1 has been implicated in the establishment of anterior pattern in the central nervous system (CNS) in a number of vertebrate species. Its role in pituitary development has been documented through loss-of-function studies in the mouse. A homozygous missense point mutation resulting in a single amino acid substitution, Arg160Cys (R160C), is associated with a heritable form of the human condition of septo-optic dysplasia (SOD). We have examined the phenotype of affected members in this pedigree in more detail and demonstrate for the first time a genetic basis for midline defects associated with an undescended or ectopic posterior pituitary. A similar structural pituitary abnormality was observed in a second patient heterozygous for another mutation in HESX1, Ser170Leu (S170L). Association of S170L with a pituitary phenotype may be a direct consequence of the HESX1 mutation since S170L is also associated with a dominant familial form of pituitary disease. However, a third mutation in HESX1, Asn125Ser (N125S), occurs at a high frequency in the Afro-Caribbean population and may therefore reflect a population-specific polymorphism. To investigate the molecular basis for these clinical phenotypes, we have examined the impact of these mutations on the regulatory functions of HESX1. We show that Hesx1 is a promoter-specific transcriptional repressor with a minimal 36 amino acid repression domain which can mediate promoter-specific repression by suppressing the activity of homeodomain-containing activator proteins. Mutations in HESX1 associated with pituitary disease appear to modulate the DNA-binding affinity of HESX1 rather than its transcriptional activity. Wild-type HESX1 binds a dimeric homeodomain site with high affinity (Kd 31 nM) whilst HESX1(S170L) binds with a 5-fold lower activity (Kd 150 nM) and HESX1(R160C) does not bind at all. Although HESX1(R160C) has only been shown to be associated with the SOD phenotype in children homozygous for the mutation, HESX1(R160C) can inhibit DNA binding by wild-type HESX1 both in vitro and in vivo in cell culture. This dominant negative activity of HESX1(R160C) is mediated by the Hesx1 repression domain, supporting the idea that the repression domain is implicated in interactions between homeodomain proteins. Our data suggest a possible molecular paradigm for the dominant inheritance observed in some pituitary disorders.

Identification of porcine Lhx3 and SF1 as candidate genes for QTL affecting growth and reproduction traits in swine

The distal portion of the long arm of porcine chromosome 1 has been shown to harbour several quantitative trait loci affecting growth and reproductive traits in swine. In order to identify potential candidate genes that might underlie these effects, a comparative mapping analysis was undertaken to define the extent of orthologous segments of human chromosome 9. A microsatellite associated with heat shock protein (HSP) A5 was used to define the proximal boundary of the quantitative trait loci (QTL) region, which suggests the human orthologue of the gene(s) responsible for the observed effects lies between HSPA5 and the q arm telomere of human chromosome 9. Examination of this region revealed two candidate genes with known roles in production of hormones essential to growth and reproductive function. The steroidogenic factor 1 and Lhx3 LIM homeodomain transcription factor genes were mapped to 123 and 155 cM, respectively, of the Sus scrofa chromosome 1 (SSC1) linkage group, placing both genes within the confidence interval for the observed QTL. To further evaluate Lhx3, we examined the expression profile during porcine embryonic development. Low levels were detected at early embryonic stages, when development of the nervous system is proceeding. A transient increase in expression level is observed during the time of pituitary organogenesis and again at the time of differentiation of anterior pituitary cells, with relatively high levels of expression persisting in the adult pituitary gland. This ontology is consistent with Lhx3 being a candidate gene for the QTL.

Syndromic short stature in patients with a germline mutation in the LIM homeobox LHX4

Studies of genetically engineered flies and mice have revealed the role that orthologs of the human LIM homeobox LHX4 have in the control of motor-neuron-identity assignment and in pituitary development. Remarkably, these mouse strains, which bear a targeted modification of Lhx4 in the heterozygous state, are asymptomatic, whereas homozygous animals die shortly after birth. Nevertheless, we have isolated the human LHX4 gene, as well as the corresponding cDNA sequence, to test whether it could be involved in developmental defects of the human pituitary region. LHX4, which encodes a protein 99% identical to its murine counterpart, consists of six coding exons and spans >45 kb of the q25 region of chromosome 1. We report a family with an LHX4 germline splice-site mutation that results in a disease phenotype characterized by short stature and by pituitary and hindbrain (i.e., cerebellar) defects in combination with abnormalities of the sella turcica of the central skull base. This intronic mutation, which segregates in a dominant and fully penetrant manner over three generations, abolishes normal LHX4 splicing and activates two exonic cryptic splice sites, thereby predicting two different proteins deleted in their homeodomain sequence. These findings, which elucidate the molecular basis of a complex Mendelian disorder, reveal the fundamental pleiotropic role played by a single factor that tightly coordinates brain development and skull shaping during head morphogenesis.

Computational analysis of human disease-associated genes and their protein products

The complete genome sequences for human, Drosophila melanogaster and Arabidopsis thaliana have been reported recently. With the availability of complete sequences for many bacteria and archaea, and five eukaryotes, comparative genomics and sequence analysis are enabling us to identify counterparts of many human disease genes in model organisms, which in turn should accelerate the pace of research and drug development to combat human diseases. Continuous improvement of specialized protein databases, together with sensitive computational tools, have enhanced the power and reliability of computational prediction of protein function.

Role of the LIM domains in DNA recognition by the Lhx3 neuroendocrine transcription factor

LIM homeodomain transcription factors regulate many aspects of development in multicellular organisms. Such factors contain two LIM domains in their amino terminus and a DNA-binding homeodomain. To better understand the mechanism of gene regulation by these proteins, we studied the role of the LIM domains in DNA interaction by Lhx3, a protein that is essential for pituitary development and motor neuron specification in mammals. By site selection, we demonstrate that Lhx3 binds at high affinity to an AT-rich consensus DNA sequence that is similar to sequences located within the promoters of some pituitary hormone genes. The LIM domains reduce the affinity of DNA binding by Lhx3, but do not affect the specificity. Lhx3 preferentially binds to the consensus site as a monomer with minor groove contacts. The Lhx3 binding consensus site confers Lhx3-dependent transcriptional activation to heterologous promoters. Further, DNA molecules containing the consensus Lhx3 binding site are bent to similar angles in complexes containing either wild type Lhx3 or Lhx3 lacking LIM domains. These data are consistent with Lhx3 having the properties of an architectural transcription factor. We also propose that there are distinct classes of LIM homeodomain transcription factors in which the LIM domains play different roles in modulating interactions with DNA sites in target genes.

An isoform-specific inhibitory domain regulates the LHX3 LIM homeodomain factor holoprotein and the production of a functional alternate translation form

The LHX3 LIM homeodomain transcription factor is required for pituitary development and motor neuron specification. The Lhx3 gene encodes two isoforms, LHX3a and LHX3b, that differ in their amino-terminal sequences. Humans and mice with defective Lhx3 genes are deficient in gonadotrope, lactotrope, somatotrope, and thyrotrope pituitary cells. We show that, whereas Lhx3b is highly expressed in these Lhx3-dependent cell types, high levels of Lhx3a expression are restricted to alpha glycoprotein subunit-expressing thyrotropes and gonadotropes. Cross-species comparison reveals the LHX3b-specific domain is more conserved than the LHX3a-specific domain. We demonstrate that the LHX3b-specific domain is a transferable inhibitor that reduces gene activation and DNA binding by homeodomain proteins. In addition, we identify a novel LHX3 protein (M2-LHX3) and determine that this molecule is generated by an internal translation initiation codon. The LHX3a- and LHX3b-specific coding sequences regulate differential usage of this internal start codon. Further, we identify the major activation domain of LHX3 in the carboxyl terminus of the molecule. M2-LHX3 is active because it retains this domain and binds DNA better than LHX3a or LHX3b. Other LIM homeodomain genes, including Lhx4, generate similar truncated proteins. These studies describe how transcriptional regulatory genes can generate multiple functional proteins.

PROP1 gene screening in patients with multiple pituitary hormone deficiency reveals two sites of hypermutability and a high incidence of corticotroph deficiency

Alterations of the gene encoding the pituitary transcription factor PROP1 were associated with congenital forms of multiple pituitary hormone deficiencies in several families. Among 23 patients with multiple pituitary hormone deficiencies screened for a PROP1 gene abnormality, nine belonging to eight unrelated families had homozygous PROP1 gene defects. All mutations were located in exon 2 and affected only two different sites: a homozygous AG deletion at codons 99/100/101 (n = 5); homozygous point mutations affecting codon 73: R73C (n = 2) or R73H (n = 1), and a R73C/R99X double-heterozygous mutation (n = 1). R73H and R99X were never described. All patients were born to unaffected parents, and consanguinity was documented in two patients. They had complete GH, LH-FSH, and TSH deficiencies and normal basal levels of PRL. Delayed ACTH deficiency was diagnosed in four of nine patients. At magnetic resonance imaging the anterior pituitary was hypoplastic in seven patients and hyperplastic in two. This study found two novel mutations (R73H and R99X) and underlines the high incidence of PROP1 gene alterations in patients with multiple pituitary hormone deficiencies. A corticotroph deficiency was frequently observed in association with GH, TSH, and gonadotropin deficiencies and should be carefully sought during follow-up.

Molecular evolution of the homeodomain family of transcription factors

The homeodomain family of transcription factors plays a fundamental role in a diverse set of functions that include body plan specification, pattern formation and cell fate determination during metazoan development. Members of this family are characterized by a helix-turn-helix DNA-binding motif known as the homeodomain. Homeodomain proteins regulate various cellular processes by specifically binding to the transcriptional control region of a target gene. These proteins have been conserved across a diverse range of species, from yeast to human. A number of inherited human disorders are caused by mutations in homeodomain-containing proteins. In this study, we present an evolutionary classification of 129 human homeodomain proteins. Phylogenetic analysis of these proteins, whose sequences were aligned based on the three-dimensional structure of the homeodomain, was performed using a distance matrix approach. The homeodomain proteins segregate into six distinct classes, and this classification is consistent with the known functional and structural characteristics of these proteins. An ancestral sequence signature that accurately describes the unique sequence characteristics of each of these classes has been derived. The phylogenetic analysis, coupled with the chromosomal localization of these genes, provides powerful clues as to how each of these classes arose from the ancestral homeodomain.

Genetic defects in thyroid hormone synthesis

Thyroid hormone synthesis requires a normally developed thyroid gland, a properly functioning hypothalamic-pituitary-thyroid axis, and sufficient iodine intake. This article focuses on genetic defects in this axis. Defects that are primarily of developmental origin are discussed in our associated article in this issue. Defects in hormone synthesis usually are associated with the development of a goiter, provided that the bioactivity and action of thyrotropin (TSH) are not impaired. In contrast, hypoplasia of the gland may be caused by developmental defects, bioinactive TSH, or resistance to TSH at the level of the receptor or its signaling pathway. At the other end of the spectrum, hyperthyroidism may result from gain of function mutations in genes regulating growth and function.

Transcription factors in pituitary gland development and their clinical impact on phenotype

OBJECTIVE

Several genes encoding homeobox transcription factors are important for normal pituitary gland development and eventually for hormone production.

METHODS AND RESULTS

By interacting with each other on the basis of a distinct temporal and spatial expression pattern, as well as with other extrinsic and intrinsic signals, they control cell determination, cell differentiation and eventually maintenance of cell function which is most important for the lifelong secretion of the pituitary-derived hormones in an appropriate manner. The different phenotypes, as mainly studied in the mouse, may help to analyze the consequences of disruption of a known or yet unknown individual transcription factor in humans.

CONCLUSION

Therefore, to study the different steps in morphogenesis will shed light onto developmental processes which will open a most fascinating time not only for basic scientists and biologists, but also for clinicians seeing all these patients.

Inherited disorders of the gonadotropin hormones

Pulsatile GnRH acts at the GnRH receptor on gonadotropes to stimulate gonadotropin gene expression, hormone synthesis and secretion. The pituitary gonadotropins, LH and FSH, stimulate steroid production and gametogenesis in males and in females. Gonadotropin production thus requires the normal development and function of hypothalamic GnRH-producing neurons and pituitary gonadotrope cells. Genes involved in gonadotrope development and/or gene expression include SF1, DAX1, KAL, GNRHR, PC1, HESX1, LHX3, PROP1, LH beta, and FSH beta. Given the complex control of gonadotropin biosynthesis and secretion, it is not surprising that genetic abnormalities have been identified at several of these steps. Some of the mutations that will be reviewed include: (1) SF1 and DAX1-orphan nuclear receptors that are expressed at multiple levels throughout the reproductive axis; (2) KAL-X-linked Kallmann syndrome, where there is abnormal development of hypothalamic GnRH-producing neurons; (3) PC1-causing abnormal processing of GnRH and GNRHR mutations that impair action at the GnRH receptor; (4) HESX1, LHX3, PROP1-abnormal development/function of the gonadotrope cell lineage; (5) LH beta and FSH beta-mutations in the gonadotropin genes that cause structural abnormalities in the hormones. Although all of these gene defects lead to gonadotropin deficiency, each disorder is associated with unique phenotypic or hormonal features. Characterization of the molecular basis of gonadotropin deficiency is useful for directing therapy and for genetic counseling. Identification of these mutations also provides insight into the pathways that govern reproduction.

A point mutation in the LIM domain of Lhx3 reduces activation of the glycoprotein hormone alpha-subunit promoter

Lhx3, a member of the LIM homeodomain family of transcription factors, is required for development of the pituitary in mice. A recent report has described a point mutation in the human LHX3 gene that is associated with a combined pituitary hormone disorder. The mutation is predicted to lead to the replacement of a tyrosine residue with a cysteine in the second LIM domain of LHX3. We have characterized the effects of this point mutation (Y114C) when analyzed in the context of the mouse Lhx3 coding sequence. Mobility shift assays demonstrated that the Lhx3 Y114C mutant is capable of binding DNA, although a decrease in the formation of a specific complex was observed. Transfection assays using an expression vector for either full-length Lhx3 or a GAL4-Lhx3 LIM domain fusion provided evidence that the Lhx3 Y114C mutant has a decreased ability to stimulate transcription. In particular, a GAL4-Lhx3 Y114C LIM mutant was unable to support Ras responsiveness of a modified glycoprotein hormone alpha-subunit reporter gene. Protein interaction studies suggest that the Y114C mutation may modestly reduce binding to the POU transcription factor, Pit-1. Interestingly, the Y114C mutation essentially abrogated binding to the putative co-activator/adapter, selective LIM-binding protein. The findings provide insights into the mechanisms mediating transcriptional activation by Lhx3 and suggest that the observed phenotype of the human mutation probably involves reduced transcriptional activity of the mutant LHX3.

LHX3 transcription factor mutations associated with combined pituitary hormone deficiency impair the activation of pituitary target genes

The Lhx3 LIM homeodomain transcription factor is critical for pituitary gland formation and specification of the anterior pituitary hormone-secreting cell types. Two mutations in LHX3, a missense mutation changing a tyrosine to a cysteine and an intragenic deletion that results in a truncated protein lacking the DNA-binding homeodomain, have been identified in humans. These mutations were identified in patients with retarded growth and combined pituitary hormone deficiency and also abnormal neck and cervical spine development. For both the LHX3a and LHX3b isoforms, we compared the ability of wild type and mutant LHX3 proteins to trans-activate pituitary genes, bind DNA recognition elements, and interact with partner proteins. The tyrosine missense mutation inhibits the ability of LHX3 to induce transcription from selected target genes but does not prevent DNA binding and interaction with partner proteins such as NLI and Pit-1. Mutant LHX3 proteins lacking a homeodomain do not bind DNA and do not induce transcription from pituitary genes. These studies demonstrate that mutations in the LHX3 isoforms impair their gene regulatory functions and support the hypothesis that defects in the LHX3 gene cause complex pituitary disease in humans.

Genomic structure, chromosomal localization and expression of the human LIM-homeobox gene LHX5

The LIM-homeobox gene Lhx5 plays an essential role in the regulation of neuronal differentiation and migration during development of the central nervous system. Mice lacking Lhx5 function show severely disorganized brain morphology and are impaired in cognition and motor coordination. In this study, we characterized the cDNA and genomic organization of the human LHX5 gene and analyzed its expression and chromosomal location. The human gene was found to contain five exons encoding a protein composed of 402 amino acids that is 98.8% identical to mouse Lhx5. By reverse transcriptase polymerase chain reaction, LHX5 transcripts were detected in fetal brain and in various regions of the adult central nervous system including the spinal cord, the thalamus, and the cerebellum. Fluorescence in situ hybridization mapped the LHX5 gene to chromosome 12, position 12q24.31-24.32. These results provide a framework for future analysis of possible association of human hereditary disorders with mutations in LHX5.

Partial transcriptome of the developing pituitary gland

The anterior lobe of the pituitary gland is composed of five hormone-producing cell types and develops from Rathke's pouch, an invagination of oral ectoderm. In mice, rapid cell proliferation occurs in the pouch from embryonic day 12.5 (e12.5) to e14.5, preceding the appearance of most hormone transcripts. Cell-type-specific commitment probably occurs prior to e14.5, but cell differentiation can be demonstrated only by detection of hormone transcripts. Although several transcription factors critical for pouch expansion are known, few of their target genes have been identified. To identify putative transcription factor target genes and cell-type-specific markers, we used differential display PCR analysis of RNA prepared from e12.5 and e14.5 Rathke's pouches. We present an expression profile of the developing pituitary gland including 83 transcripts, 40% of which are novel. The tissue distribution, cell specificity, and developmental regulation were determined for a subset of the transcripts.

Homozygous deletion in the coding sequence of the c-mer gene in RCS rats unravels general mechanisms of physiological cell adhesion and apoptosis

The RCS rat presents an autosomal recessive retinal pigment epithelium dystrophy characterized by the outer segments of photoreceptors being phagocytosis-deficient. A systematic genetic study allowed us to restrict the interval containing the rdy locus to that between the markers D3Mit13 and D3Rat256. We report the chromosomal localization of the rat c-mer gene in the cytogenetic bands 3q35-36, based on genetic analysis and radiation hybrid mapping. Using a systematic biocomputing analysis, we identified two strong related candidate genes encoding protein tyrosine kinase receptors of the AXL subfamily. The comparison of their expression patterns in human and mice tissues suggested that the c-mer gene was the best gene to screen for mutations. RCS rdy- and RCS rdy+ cDNAs were sequenced. The RCS rdy- cDNAs carried a significant deletion in the 5' part of the coding sequence of the c-mer gene resulting in a shortened aberrant transcript encoding a 20 amino acid peptide. The c-mer gene contains characteristic motifs of neural cell adhesion. A ligand of the c-mer receptor, Gas6, exhibits antiapoptotic properties.

The genetic basis of infertility in men

Subfertility in men is a heterogeneous syndrome, its pathophysiology remaining unknown in the majority of affected men. A large number of genes and loci are associated with sterility in experimental animals, but the human homologues of most of these genes have not been characterized. A British study suggested that, in a large proportion of men with idiopathic infertility, the disorder is inherited as an autosomal recessive trait; this provocative hypothesis needs confirmation. Because normal germ cell development requires the temporally and spatially co-ordinated expression of a number of gene products at the hypothalamic, pituitary and testicular levels, it is safe to predict that a large number of autosomal, as well as X- and Y-linked, genes will probably be implicated in different subsets of male subfertility.