GH deficiency may occur in combination with other pituitary hormone deficiencies and is often referred to as hypopituitarism, panhypopituitarism or multiple pituitary hormone deficiency (MPHD).

The anterior portion of pituitary gland forms from Rathke's pouch around the third week of gestation(14). A number of transcription factors have been identified that are required for normal pituitary development, while others are required for continued normal function (Table 4). Mutations have been identified in the genes for several of these pituitary transcription factors, including LHX3, LHX4, RPX1, PROP1 and POU1F1.

LHX3 is a member of the LIM family of HomeoboX transcription factors. The gene LHX3, is located on 9q34.3, comprises 7 exons (including two alternative exon 1's, 1a and 1b) and encodes a protein of 402 amino acids (Figure 4). LHX3 is expressed in the developing Rathke's pouch and is required for the development of most anterior pituitary cell types, including the somatotrope, the thyrotrope, the lactotrope and the gonadotrope (but notably not the corticotrope). LHX3 binds as a dimer, synergizing with POU1F1 (PIT1). Two unrelated families with MPHD were identified in 2000(15) as harboring mutations in LHX3. The affected members of the family manifested severe growth retardation in association with restricted rotation of the cervical spine. Inheritance is consistent with an autosomal recessive pattern of inheritance and of note one individual was found to have an enlarged pituitary. Recently, a new mutation in LHX3 was described in a child with hypointense pituitary lesion, focal amyotrophy and mental retardation in addition to neck rigidity and growth retardation (103). These clinical findings expand the phenotype associated with mutations in LHX3.

LHX4 also has a critical role in the development of anterior pituitary cells, and is co-expressed with LHX3 in Rathke's pouch in an overlapping but not wholly redundant pattern. Raetzman et al showed overlapping functions with Prop1 in early pituitary development, but also observed that their mechanisms of action were not identical. Lhx4 is necessary for cell survival and Lhx3 expression, with the pituitary hypoplasia seen in Lhx4 mutants actually results from increased cell death and reduced differentiation, directly attributable to loss of Lhx3; while Prop1 mutants exhibit normal cell proliferation and cell survival but show evidence of defective dorsal-ventral patterning(16). In the absence of both of these genes, no specification of corticotropes, gonadotropes or thyrotropes occurs in the anterior lobe. Although both LHX3 and LHX4 are crucial for the development of pituitary gland, LHX3 is expressed at all stages studied, whereas LHX4 expression is transient at 6 weeks of development (104). LHX4 is located on 1q25 and comprises 6 exons spread over a 45 kb genomic region (Figure 5). An intronic splice site mutation has been described in one family, manifesting GH, TSH and ACTH deficiency, along with cerebellar and skull defects. The mutation is transmitted as an autosomal dominant condition, with complete penetrance. Interestingly, a heterozygous mutant mouse model had no discernable phenotype, while homozygous loss of function in the mouse was fatal(17).

RPX1 (Rathke's Pouch HomeoboX) is also referred to as HESX1 (HomeoboX gene expressed in Embryonic Stem cells) and is necessary for the development of the anterior pituitary. RPX1 comprises 4 exons and encodes a protein of 185 amino acids that features both a homeodomain as well as a repressor domain and is located on chromosome 3p21.2 (Figure 6). The extinguishing of HESX1 requires the appearance of another pituitary transcription factor, PROP1. A mutation has been described in two children of a consanguineous union who had optic nerve hypoplasia, agenesis of the corpus callosum and panhypopituitarism, with an apparent autosomal recessive mode of inheritance(18). This Arg --> Cys mutation lies between the repressor and homeodomains but the mutant protein was shown in vitro to be unable to bind to its cognate sequences. A novel homozygous missense mutation (126T) of the critical Engrailed homology repressor domain (eh1) of HESX1 was described in a girl born to consanguineous parents (105). Neuroimaging revealed a thin pituitary stalk with anterior pituitary hypoplasia and an ectopic posterior pituitary. Unlike previous cases, she did not have midline or optic nerve abnormalities. Although 126T mutation did not affect the DNA-binding ability of HESX1, it impaired ability of HESX1 to recruit Groucho-related corepressor, thereby leading to partial loss of repression. Tantalizingly, additional nucleotide variants have been described in individuals with isolated GH deficiency, although it is not convincingly clear that these polymorphisms are actually pathogenic(19,20).

PROP1 (the Prophet of PIT-1) encodes a transcription factor required for the development of most pituitary cell lines, including the somatotrope (GH secretion), lactotrope (prolactin (PRL) secretion), thyrotrope (TSH secretion), and the gonadotrope (FSH and LH secretion). Mutation of PROP1, therefore, results in the deficiency of GH, TSH, PRL, FSH and LH although some individuals with PROP1 mutations have been described with ACTH deficiency(21). Since PROP1 does not appear to be required for the development of the corticotrope cell line, the etiology of ACTH deficiency is unclear. It appears that the ACTH deficiency here is a consequence of the compensatory pituitary hyperplasia that develops over time. Significantly, the degree of TSH deficiency appears to be quite variable, even within mutation-identical individuals, suggesting that the general phenotype associated with PROP1 mutations is also quite variable. PROP1 is encoded by three exons and is located on 5q. Many mutations have been described in PROP1-all inherited in an autosomal recessive manner. Although several studies suggest that mutation of PROP1 is the most common cause of familial MPHD, but is less common in sporadic cases of MPHD (106). Two recurrent mutations have been described, both involving exonic runs of GA tandem repeats (Figure 7). In both cases, the loss of a tandem unit at either locus results in a frameshift and premature termination, and a protein incapable of transactivation.

POU1F1 encodes the POU1F1 transcription factor, also known as PIT1, which is required for the development and function of three major cell lines of anterior pituitary: somatotropes, lactotropes and thyrotropes. Various mutations in the gene encoding POU1F1 have been described, resulting in a syndrome of multiple pituitary hormone deficiency involving GH, PRL and TSH hormones. POU1F1 is located on 3p11 and consists of six exons encoding 291 amino acids (Figure 8). Many mutations of POU1F1 have been described; some are inherited as autosomal recessive and some as autosomal dominant. There is a wide variety of clinical presentation in patients with POUF1 mutations. Generally, GH and prolactin deficiencies are seen early in life. However, TSH deficiency can be highly variable with presentation later in childhood or normal T4 secretion can be preserved into the 3rd decade. (107,108). To date, POU1F1 mutations have been described in a total of 46 patients from 34 families originating in 17 different countries (109). Recessive mutations are generally associated with decreased activation, while dominant mutations have been shown to bind but not transactivate-i.e. act as dominant-negative mutations, rather than through haploinsufficiency. One such mutation is the recurrent Arg271Trp (R271W), located in exon 6, which results from a C --> T transition at a CpG dinucleotide, i.e. a region predisposed to spontaneous mutagenesis. Another interesting mutation is the Lys216Glu mutation of exon 5. This mutation is unique in that the mutant transcription factor activates both the GH and PRL promoters at levels greater than wild-type (i.e. acts as a superagonist), but down-regulates its own (i.e. the POU1F1) promoter-leading to decreased expression of PIT1. R271W is the most frequent mutation of POU1F1. A recent report describing a novel mutational hot spot (E230K) in Maltese patients suggests a founder effect (108). The same group reported two additional novel mutations within POU1F1 gene; an insertion of a single base pair (ins778A) and a missense mutation (R172Q) (108).

SOX3 encodes a single-exon gene SOX3, an HMG box protein, located on the X chromosome (Xq26.3) in all mammals (110). It is believed to be the gene from which SRY, testis–determining gene evolved (111). Based on sequence homology, SOX, however, is more closely related to SOX1 and SOX2, together comprising the SOXB1 subfamily and are expressed throughout the developing CNS (112, 113). In humans, mutations in the SOX3 gene have been implicated in X-linked hypopituitarism and mental retardation. In a single family, a SOX3 gene mutation was shown in affected males who had mental retardation and short stature due to GH deficiency (114). The mutation was an in-frame duplication of 33 bp encoding for an additional 11-alanine, causing an expansion of a polyalanine tract within SOX3. Recently, other mutations including a submicroscopic duplication of Xq27.1 containing SOX3, a novel 7-alanine expansion within the polyalanine tract, and a novel polymorphysm (A43T) in the SOX3 gene were described in males with hypopituitarism. Phenotypes of these patients include severe short stature, anterior pituitary hypoplasia, and ectopic posterior pituitary, colossal abnormalities, and infundibular hypoplasia. Although duplications of SOX3 have been implicated in the etiology of X-linked hypopituitarism with mental retardation, in at least one study, none of the affected individuals had mental retardation or learning difficulties (115). Taken together, the data suggests that SOX3 has a critical role in the development of the hypothalamic-pituitary axis in humans, and mutations in SOX3 gene are associated with X-linked hypopituitarism but not necessarily mental retardation (115).