Animal Model

Snell mice²⁸ are a well-studied animal model of pituitary function, which arises from a spontaneous single nucleotide mutation in the Pit1 gene that results in the substitution of W261C in the homeodomain, rendering the mutant gene product incapable of DNA binding and hence unable to activate potential target genes.²⁹ Mice heterozygous for this mutation are phenotypically normal. The homozygous offspring of this mutation are dwarf and infertile, and they exhibit loss of three pituitary hormone cell types, GH, PRL, and TSHβ; whereas the gonadotrope and corticotrope cells are unaffected, suggesting that the Pit1 is required for terminal differentiation of the somatotrope, lactotrope, and thyrotrope cell types. In the Pit1^Snell animal model where the Pit1 gene is functionally defective, the initial activation of the Pit1 is unaffected, while the later transcription of the Pit1 gene is altered, resulting in the failed expression of the Pit1 in the adult animal and a dwarf phenotype.^29,30 The Pit1 lineage can be converted to alternative fates before e17.5 but exhibits a cell-autonomous commitment after e17.5, when Pit1 gene regulation shifts from a Pit1-independent early enhancer to a Pit1-autoregulated later enhancer.³¹ Pit1^Jackson is a second mouse model with a defect in the Pit1 gene. The genomic structure of the Pit1 gene, located on chromosome 16, is grossly rearranged in mutant Pit1^Jackson mice, with a phenotype very similar or identical to that of the Pit1^Snell mice.²⁹ In addition, Pit1 mutations result in decreased activity of the insulin/IGF1 pathway, which may result in physiologic homeostasis consequences that favor longevity^32,33 (for reviews see [34–36]).

Related Diseases

The human Pit1 gene has been mapped to chromosome 3. Lesions in Pit1 have been identified as an etiology of CPHD (see Table 8-2). Initial study has revealed a homozygous nonsense mutation R172X in the Pit1 gene in a patient of consanguineous parents with cretinism due to deficiency of GH, PRL, and TSHβ.³⁷ Many cases of CPHD with Pit1 defects have since been reported. It appears that the inheritance of Pit1 mutations in humans is complex, ranging from autosomal recessive to autosomal dominant to imprinting with variable phenotypic penetrance.³⁸ Pituitary gonadotropins and corticotropins are normal in Pit1-defective patients. Deficiency of GH is consistently observed in all Pit1 patients, and deficiency for PRL is observed in most patients, whereas TSHβ deficiency usually has a delayed onset and incomplete penetrance (see Table 8-2). Different backgrounds may be the major contributing factor to the TSH phenotypic variation. Alternatively, however, there exists an embryonic population of thyrotrope termed rostral tip thyrotrope. The expression of this embryonic TSH is not Pit1 dependent, and consequently it may be a contributing element to the TSH phenotypic variation observed in Pit1 patients. The presentation of patients with Pit1 disorders varies considerably. At infancy, they usually have a protruding forehead, depressed facial structures, and a saddled nose, although CPHD is generally not diagnosed until growth retardation due to the deficiencies of GH and thyroid hormone becomes obvious.^39,40

Mechanism

The modular structure of the Pit1 protein can be divided into the transactivation and the DNA-binding domains. The transcriptional activation domain is located in the first 80 amino acids, followed by a POU DNA-binding domain at the C terminus. The POU domain is further divided into a 75-amino acid POU-specific domain, which is conserved among various POU-domain proteins, and a 60-amino acid POU homeodomain with a linker region between them. The POU homeodomain by itself is sufficient for low-affinity DNA binding, although both the POU-specific domain and POU homeodomain are required for specific high-affinity DNA binding of the Pit1-responsive elements. Pit1 protein is able to bind as a monomer in solution to the consensus (A/T)(A/T)TATNCAT site, where N may be any nucleotide; in most cases, however, Pit1 binds DNA as a dimer.⁴¹ Analysis of data derived from a cocrystal study of the Pit1 protein and the PRL proximal promoter Pit1-binding element reveals that the Pit1 protein binds to DNA in a parallel dimer form.^42,43 Pit1 protein wraps around the DNA molecule, with the POU-specific domain and the POU homeodomain binding to the DNA molecule in a perpendicular angle in opposite orientation. The POU-specific domain of one Pit1 molecule interacts with the C terminus of the POU homeodomain of the other Pit1 molecule in a dual composition. In addition, the spacing between the DNA contacts made by the POU-specific domain and the POU homeodomain of each monomer is critical. Compared to the Pit1 binding site in the PRL minimum promoter sequences, two additional base pairs spacing are needed to direct restricted GH gene transcription based on elements of two Pit1 binding sites on the proximal promoter of rat GH locus.⁴⁴

This dimerization interface is a “hot spot” for debilitating mutations. Additional mutations, like in the Pit1^Snell mice, a G-to-T mutation results (W261C) in the third helix of the POU homeodomain, eliminating its DNA-binding ability by altering the contact point of the mutant gene product with the major groove of the responsive elements, causing a dwarf phenotype in an autosomal-recessive fashion. Similarly, several mutations observed in human cases could affect the stability and specificity of this protein-DNA interface.^45,46

As a transcription factor, Pit1 exerts its effects as a component of a transcriptional complex regulated by coactivator and repressor elements. The Pit1 POU domain can associate with coactivator complex of CBP/p300 and P/CAF, both of which possess histone acetylase activity. N-CoR, acting as a corepressor, can bind to the homeodomain of Pit1 and actively suppress transactivation by Pit1; this suppression depends on Sin3, SAP30, and histone deacetylase. Thus the transcriptional activity of Pit1 may be regulated by the competing binding of complexes mediating either acetylation or deacetylation events, resulting in activation or repression, respectively.⁴⁷

In addition to Pit1, the determination of individual pituitary cell types may require other molecules. The estrogen receptor has been implicated in synergistic activation of the PRL gene.^48,49 Members of the ETS family of transcription factors can bind to Pit1 binding sites in the PRL promoter and mediate signals from growth factors and the Ras/mitogen-activated protein kinase pathway.⁵⁰ The transcription factor GATA2 appears to be required for the formation of both thyrotrope and gonadotrope cells; the presence of Pit1 represses the gonadotropic phenotype and promotes the thyrotrope phenotype. Pit1 can inhibit binding of GATA2 to cognate DNA sites important for generation of the gonadotrope phenotype. In contrast, Pit1 leads to synergistic activation with GATA2 on promoters that contain both Pit1 and GATA2 sites.⁵¹

Animal Model

The Prop1 gene was initially identified by a positional cloning strategy in the naturally occurring Ames mouse mutant. The mutant Prop1 allele at the Prop1^Ames locus harbors a point mutation that results in a single amino acid substitution (S83P) in the second helix of the homeodomain, causing altered progression of nascent pituitary gland and subsequent failed expression of Pit1.⁵² Phenotypes of the Prop1^Ames mice are transmitted in an autosomal-recessive fashion; heterozygous mutant mice are normal. Homozygous mutant mice are born grossly normal but develop a proportional dwarfism by the time of weaning.⁵⁶ The adult mutant mice are about half the size of the wild-type animals. The Prop1^Ames mutation caused dysmorphogenesis of Rathke’s pouch at e12.5, with convolution of the lumen and a failure of expression of the Pit1 lineage. The appearance of gonadotrope was delayed, but corticotrope appeared as expected. In contrast to the complete absence of somatotrope, lactotrope, and thyrotrope cells in the Pit1^Snell mouse, the Prop1^Ames mouse pituitary gland contains a small number (<1%)^54,57 of the normal complement of somatotrope cells, as well as a few lactotrope and thyrotrope cells.⁵⁷ Prop1^Ames dwarf mice live twice as long as their wild-type littermates.⁵⁸

Related Diseases

The human Prop1 coding region has three exons separated by two introns and maps to chromosome 5q34. The Prop1 gene encodes a polypeptide of 226 amino acids and contains a short N terminus, a 60-amino acid homeodomain, and a transactivating C terminus. Compared to the mouse homologue, the human Prop1 homeodomain is highly conserved, with only two amino acid substitutions.

Initial reports identified mutations in the human Prop1 gene in patients with short stature in several families. Direct sequencing of polymerase chain reaction (PCR) products of the Prop1 gene revealed that all the affected patients were harboring mutations in both alleles of the Prop1 gene, and their parents were heterozygous for the respective mutations, suggesting that the mutations in the Prop1 gene act in an autosomal-recessive manner, causing CPHD in these patients. All of the affected individuals in this study failed to respond to GHRH, thyrotropin-releasing hormone, and LH-releasing hormone stimulation, suggesting a defect in hormone-secreting cells of the pituitary gland.⁵⁹ Subsequent reports have revealed that Prop1 mutation is a common cause of familial CPHD. These alternations in the Prop1 gene range from point mutation to deletions, affecting structure and integrity in the homeodomain of the Prop1 gene. A 2-bp A301G302 deletion, leading to a frame-shift and the loss of DNA-binding homeodomain and C-terminal transactivation domain of the Prop1 gene product, is the most frequently encountered mutation among these Prop1 patients, representing a mutational “hot spot.”⁶⁰ Individuals with various Prop1 mutations invariably display severe deficiencies for pituitary gonadotropins in addition to the defects of GH, PRL, and TSH levels. In human cases with Prop1 mutations, many adult patients express ACTH at a normal level; however, there are reported cases with a late onset of corticotropin deficiency (see Table 8-2). The expression of ACTH phenotypes is highly heterogeneous; differences in genetic background in these patients may contribute to the discrepancy of this phenotype.^61,62

Mechanism

The Prop1 gene product exerts its actions through binding to the responsive elements of target genes, with the helix-turn-helix motif of the homeodomain providing the contact point for protein-DNA interactions. The fact that most of the naturally occurring mutations of the Prop1 gene are located in the homeodomain suggests that the Prop1 homeodomain is critical for Prop1 function.

In Prop1^Ames mice, examination of the mutant Rathke’s pouch revealed severe dysmorphogenesis, but the pituitary precursor cells were generated. The precursor cells of Rathke’s pouch failed to migrate to form the nascent pituitary gland, leading to an expansion of the luminal structure and lack of expression of a late pituitary differentiation marker, the Pit1 gene. However, proliferation of the mutant precursor cells in the Ames mice continued, resulting in normal-sized pituitary glands.^52,63 In addition to Pit1, both Wnt and Notch pathways are affected in the Prop1^Ames mice.^64,65 Later, persistent expression of Prop1 under control of the αGSU promoter caused decreased gonadotrope differentiation and increased adenomatous hyperplasia,^66,67 indicating that properly extinguishing Prop1 also may be an important later step in paired-like homeodomain-mediated organogenesis.

Prop1 can bind to its site and activate target genes via the C-terminal transactivation domain, whereas the N terminus and the homeodomain of Prop1 possess repression function,^52,68 suggesting that Prop1 can act as a transcriptional activator as well as a repressor. Recent studies of the pituitary-specific inactivation of the β-catenin gene reveal that a Prop1/β-catenin complex acts as transcriptional activator for Pit1 and as a repressor for Hesx1, depending on the associated co-factors.⁶⁹

Phenotypic comparisons have been made between the Prop1-defective patient and the Prop1-mutant Prop1^Ames mouse. Deficiencies of GH, PRL, and TSH are consistently observed in both species. All the patients with Prop1 mutations eventually develop gonadotropin deficiency in their adult lives. In the Prop1^Ames mice, the expression of gonadotropin is observed at birth, but the level of expression of the gonadotropin is reduced to one quarter of that of the wild-type animals.⁵² The expression of ACTH is apparent during development in the Prop1^Ames mouse pituitary, and the level of ACTH in the blood is normal in adults. In human Prop1 patients, cortisol levels are normal at birth, but some patients develop cortisol deficiency later in life.^70–72 The Prop1 mutation may affect all the major cell types in the anterior pituitary gland, including the gonadotrope and the corticotrope (see Table 8-2).

Animal Model

The mouse Hesx1 gene is located on chromosome 14, and targeted deletion of Hesx1 resulted in mice that exhibited variable anterior central nervous system defects with reduced prosencephalon and defective olfactory development.⁷⁵ Hesx1 mutants also have defects in the pituitary gland, with bifurcations in Rathke’s pouch in most cases. By e12.5, multiple oral ectoderm invaginations reflecting pituitary glands are observed in most Hesx1 embryos. Between e13.5 and e15.5, Hesx1 mutants are characterized by a dramatic cellular over-proliferation of all the hormone-producing cell types, leading to a failure of the underlying mesenchyme to condense and form the sphenoid cartilage that separates the pituitary from the oral cavity. In the late stages of pituitary development, the terminal differentiation of the hormone-producing cell types appear normal in most Hesx1 mutants, with overexpression of αGSU, TSHβ, GH, POMC, and Pit1 by e16.5. Earlier in development, there is a delay in the onset of POMC expression both in Rathke’s pouch and in the developing hypothalamus at e12.5, and there also appears to be a dual induction of αGSU expression on both the rostral and caudal sides of Rathke’s pouch. Strikingly, in occasional Hesx1 gene–deleted mice, the initial thickening of oral ectoderm and minimal activation of Lhx3 are observed at e12.5, but the embryos exhibit a complete arrest of pituitary development, and the pituitary gland is absent by e18.5. The discrepancy of incomplete phenotype penetrance in Hesx1 mutants is likely influenced by the actions of the linked modifier genes.^76,77

Related Diseases

The human Hesx1 gene contains four exons separated by three introns, and it maps to chromosome 3p21. The Hesx1 gene encodes a highly conserved polypeptide of 185 amino acids with a 60-amino acid homeodomain at its C terminus. Initial analyses of the Hesx1 mutations carried out in kindreds with septo-optic dysplasia identified a nucleotide transition that resulted in the substitution of R160C (in the third helix of the homeodomain) in two children with CPHD born to a highly consanguineous family. Magnetic resonance imaging revealed an ectopic/undescended posterior pituitary associated with a hypoplastic anterior lobe in these two affected siblings.⁷⁵ None of the heterozygote parents exhibited features of septo-optic dysplasia, consistent with an autosomal-recessive inheritance. Additional mutations (e.g., Q6H, S170L, T181A, I26T, and 306/307InsAG-X) have been found in the coding region of the Hesx1 gene and are associated with variable phenotypes, including hypopituitarism, ranging from isolated GH deficiency to CPHD. It is clear from these reported cases that mutation in the Hesx1 gene can cause pituitary hormone deficiency with variable phenotypes and with incomplete penetrance.^78,79

Mechanism

The Hesx1 gene product can bind to either dimer or monomer DNA sites with high affinity in transient transfection assays.^80,81 Modular structure analysis revealed that in addition to the DNA-binding homeodomain, Hesx1 contains two sequences in the N terminus; one is similar to the eh1 motif found in Drosophila engrailed, and one is similar to the WRPW motif found in several helix-loop-helix proteins, both of which are capable of recruiting the Groucho class of corepressors.^82,83 Both the N-terminal and homeodomain regions of Hesx1 can independently act as repressors. Hesx1 is a strong transcriptional repressor that acts by recruiting the mSin3A/B, HDACs 1 and 2, and the Brg1 complexes to its homeodomain and the TLE corepressor to its eh1 domain. The strong association between Tle1 and Hesx1 is mediated by a highly conserved helical motif (FXLXXIL) present in the Hesx1 N terminus, which can also be found in Nkx, Six, and certain Pax homeodomain factors’ family members.⁸⁴ These recruitments are required and sufficient for the repressive actions of Hesx1 in vivo. Forced persistent expression of Hesx1 and Tle1 resulted in the loss of the Pit1 lineage and a Prop1^Ames-like dysmorphogenesis, while the expression of Prop1 and POMC remained. The mutation in human Hesx1 (R160C) has a dominant negative effect both in vitro and in vivo. This dominant negative activity requires the eh1 repression domain, which is also required for full-length recombinant Hesx1 dimerization in solution. This dominant transcription repressor activity may help to explain the heterozygous phenotypes observed in Hesx1 patients.⁸⁰ Recent identification of a homozygous mutation in the eh1 motif (I26T) in a patient with CPHD has further underlined that Tle association is an integral mechanism for Hesx1 function in vivo.⁸⁵

Hesx1 and Prop1 share a conserved DNA-recognition site. The repression domain in Hesx1 can suppress the transcription activation activity of Prop1. The Hesx1 repressor can heterodimerize with Prop1 and can bind to the palindromic site as homodimers or heterodimers, with Prop1 acting as an activator and Hesx1 as a repressor, to inhibit Prop1 activation function. The expression of Prop1 is elevated in Hesx1-mutant mice, suggesting not only that Hesx1 can repress Prop1 activation function but also that it is required for proper Prop1 expression.⁷⁶ Forced early expression of Prop1 to the uncommitted oral ectoderm blocks the formation of Rathke’s pouch, which results in absence of the anterior pituitary gland with no initial induction of Lhx3 expression, demonstrating that premature expression of Prop1 can block the pituitary organogenesis that phenocopies the effects of Hesx1-gene deletion,¹⁵ in contrast to the Hesx1/Tle1 transgenic mouse with a Prop1^Ames-like phenotype, suggesting that the antagonistic repressor complex can suppress Prop1 activation of expression.⁷⁶ The sequential repression and activation of a common set of regulatory genes may prove to be an underlying strategy in the temporal code of pituitary organ development, with initial repression required for organ commitment and proliferation and subsequent activation required for commitment of specific cell lineages.⁶⁹

Animal Model

Employing a reverse genetic approach, mice with a targeted disruption in the Lhx3 gene were generated. Mice heterozygous for the mutation are apparently normal and fertile, whereas homozygous individuals are stillborn or expire within 24 hours of birth. In these homozygous mice, the hindbrain, spinal cord, and pineal gland are grossly normal, as is the posterior lobe of the pituitary, but the anterior and intermediate lobes of the pituitary are absent. During embryonic development, the mutant animal exhibits a lack of growth in Rathke’s pouch, and pituitary-gland development does not progress beyond the Rathke’s pouch stage. With the exception of residual corticotrope, other anterior pituitary cell types are absent, indicating that Lhx3 is required for the appearance of the somatotrope, lactotrope, thyrotrope, and gonadotrope cell types.⁸⁸

Mice homozygous for the targeted deletion of the Lhx4 gene exhibit an early postnatal death due to a failure of pulmonary maturation.⁸⁷ Lhx4-deleted mice have a well-formed Rathke’s pouch but display incomplete pituitary development following this stage, and the differentiation of pituitary cell types is perturbed. Consequently, by e12.5, there exists a miniature Rathke’s pouch, and by e14.5, the nascent pituitary structure has progressed to a larger pouch, but the anterior lobe is discernible only as a slight thickening in the ventral region. This hypocellularity of the anterior lobe is caused by failure of pituitary precursor cells to survive; large numbers of apoptotic cells are evident throughout the pituitary primordia of Lhx4-mutant mice at e12.5.⁸⁹ In later gestation stages, Rathke’s pouch is hypoplastic, with an enlarged lumen resulting from reduced proliferation of the precursors, and the anterior lobe of the pituitary is reduced in size. Expression analyses have revealed residual amounts of LH- and GNRHR-positive cells at e18.5. Thus, Lhx4 is not required for specification of gonadotrope cells, but it does support the expansion of the cell population. Similarly, all five anterior pituitary–specific cell lineages are present in the Lhx4-mutant pituitary but in dramatically reduced numbers. By contrast, the intermediate-lobe melanotrope cells are undisturbed.

Mice with double deletion of Lhx3 and Lhx4 demonstrated that both genes direct formation of the pituitary gland.⁹⁰ The early formation of the Rathke’s pouch rudiment from pituitary primordium does not depend entirely on the function of either Lhx3 or Lhx4 alone, but together these genes redundantly control the formation of the definitive pouch. Lhx3 also controls a subsequent step of pituitary fate commitment, and in these early stages, Lhx4 appears to act upstream of the Lhx3 and Isl1 genes and is required for expansion of Rathke’s pouch. Therefore, Lhx3 and Lhx4 dictate pituitary gland identity by controlling decision points of organogenesis and regulation of the proliferation and differentiation of pituitary-specific cell lineages.

Related Diseases

Human Lhx3 shares a high degree of homology with its mouse orthologue, exhibiting 94% identity at the amino acid level. Lhx3 is located on human chromosome 9q34 and spans a genomic fragment of at least 6 kb that includes 6 exons.^91,92 In a candidate-gene screen based on pituitary phenotypes observed in a recessive lethal mutation in mice, two mutations in the Lhx3 gene were identified in two unrelated consanguineous pedigrees that display CPHD.⁹³ In one family, affected individuals are homozygous for a Y116C mutation located in the highly conserved LIM2 domain of Lhx3, a domain critical for protein-protein interactions. In the second family, affected individuals are homozygous for a 23-base-pair deletion in an intragenic region, predicting a severely truncated protein that lacks the entire homeodomain and rendering it incapable of DNA binding. Lhx3-defective patients have deficiencies in GH, TSH, PRL, FSHβ, and LHβ, but they display intact levels of ACTH, similar to the endocrine profiles observed in Prop1 patients (see Table 8-2). In addition, these Lhx3-defective patients displayed a rigid cervical spine that restricted their head rotation. More recently, novel 6 mutations have been found in the coding region of the Lhx3 gene.^94,95 All of them are associated with variable phenotype of hypopituitarism. Lhx3 mutations are a rare cause of CPHD involving deficiencies for GH, prolactin, TSH, and LH/FSH in all patients. Whereas most patients have a severe hormone deficiency manifesting after birth, milder forms can be observed, and limited neck rotation is not a universal feature of patients with Lhx3 mutations.

The human Lhx4 gene encodes a 390-amino acid protein that contains two LIM domains and a homeodomain that shares 99% sequence identity with its mouse orthologue. Genomic analysis revealed that the human Lhx4 gene contains 6 exons and is mapped to chromosome 1q25.⁹⁶ In a large consanguineous pedigree of three generations, a G-to-C substitution in the intron preceding exon 5 of Lhx4 generates a mutant protein with perturbed homeodomain, which affects its DNA-binding function. Patients with this disease have short stature with CPHD, which affects GH, thyroxine, and cortisol, as well as cerebellar defects and abnormalities of the sella turcica. This mutant allele is transmitted in a dominant fashion, affecting only the maternal side of the kindred with a high phenotypic penetrance.⁹⁶ More recently, three novel mutations in the Lhx4 gene have been mapped.⁹⁷ All of them affect the DNA-binding domain, and all of them are associated with CPHD.

Mechanism

LIM homeodomain proteins are transcription factors and exert their effects by regulating target gene expression. Lhx3 binds with high affinity to AT-rich DNA sequences (including minor groove interaction) and bends the DNA molecule to an angle of 62 degrees in a model system.⁹² Lhx3 can activate the regulatory regions of pituitary genes, including αGSU, PRL, TSHβ, and Pit1. Lhx3 expression is partially regulated by the Lhx4 gene during pituitary development. At e12.5, only a few cells express Lhx3 in the dorsal-most aspect of the pouch in the Lhx4 mutants. However, the normal pattern of Lhx3 expression, including the dorsal-ventral gradient, is established in Lhx4 mutants by e14.5.⁸⁹

Genetic analysis revealed that Lhx4 interacts with Prop1 to stimulate anterior pituitary lobe expansion. Neither gene is essential for initiating corticotrope specification. However, no POMC or αGSU expression is detected in double-mutant mice at e14.5, suggesting that Prop1 and Lhx4 have overlapping roles in corticotrope and gonadotrope development.⁸⁹ In Hesx1-deleted mutants, the domains of Lhx3 and Prop1 expression are increased, as well as those of FGF8 and FGF10 in the infundibulum, which become expanded rostrally.⁷⁶ These findings indicate that Hesx1 is required for maintaining the proper expression of FGFs, consistent with the notion that Lhx3 expression can be regulated by FGF signaling.

Animal Model

Mice with targeted disruption of the Tbx19 gene have been generated. Mice heterozygous for the mutation are apparently normal. Adult mice homozygous for the mutation have very few ACTH-positive cells in the pituitary, although initial expression of the POMC gene is undisturbed at the Rathke’s pouch stages. These cells are born in normal quantities in mutants but are lost or fail to expand appropriately, suggesting Tbx19 is not required for corticotrope cell commitment but is later important for POMC lineage differentiation. The intermediate-lobe melanotropes in mutant mice are populated by gonadotrope and some Pit1-independent thyrotrope, also indicating that Tbx19 normally represses pituitary gonadotrope differentiation.^101,102

Related Diseases

The human Tbx19 gene shares 94% amino acid identity with that of mouse Tbx19 and maps to chromosome 1q23-q24. Several cases of isolated ACTH deficiency were identified with a nonsense mutation C-to-T transition in exon 6 in Tbx19, resulting in a truncated gene product (R286X).¹⁰³ The transmission of this mutation appears to be recessive. In another case of isolated ACTH deficiency, a heterozygous C-to-T transition in exon 2 of the Tbx19 gene was identified, resulting in a conserved amino acid S128F mutation, suggesting a dominant negative inheritance.⁹⁹ More recent study revealed new mutation in the Tbx19 gene (missense M86R) that did not affect monomer DNA-binding activity per se, but it impaired DNA binding with other DNA-bound proteins, including itself (homodimers) and Pitx1, resulting in congenital isolated ACTH deficiency.¹⁰⁴ Additional mutations in the Tbx19 gene have been shown to be a cause of neonatal death due to neonatal-onset isolated ACTH deficiency.¹⁰³ Tbx19 defects result in POMC deficiency in both humans and mice, establishing Tbx19 as the gene required for effective POMC expression in vivo.^101,102

Mechanism

Tbx19 is a transcriptional regulator, recognizing target genes through its T-box DNA-binding domain. In response to signals elicited by the hypothalamic hormone corticotrope-releasing hormone, Tbx19 functions as an activator of transcription by recruiting SRC/p160 coactivators to its cognate DNA target in the POMC promoter.¹⁰⁵ Tbx19 can synergize with orphan nuclear receptor NGFI-B, serving as part of the transcription regulatory complex on the POMC promoter in response to hormonal stimulation.¹⁰⁶ Transgenic expression of Tbx19 in non-POMC-producing regions of the pituitary gland can cause ectopic POMC expression,⁹⁹ and Tbx19 is an inhibitor of αGSU expression in rostral tip cells, gonadotrope, and thyrotrope, and of TSHβ production in caudomedial thyrotrope.¹⁰⁰ Tbx19 deficiency is permissive for transdifferentiation of cells normally destined to be corticotrope and melanotrope into alternative cell fates, namely, gonadotrope and rostral tip thyrotrope, suggesting a determinative role of Tbx19 in cell lineage specification. Tbx19 defects have no effect on differentiation of Pit1-dependent cell lineages (see Figs. 8-1 and 8-2, and Table 8-2).^101,102