The diagnosis of CAH may seem incredibly daunting for families, with all the information they are given when their child is diagnosed. Boys tend to present in the emergency room when they are around 2 weeks of age with dehydration and vomiting, whereas girls are usually diagnosed shortly after birth, as their genitalia may look more like a boy’s. This can be frightening for families, and this leaflet on differences of sex development is a comforting, useful resource, giving tips to families on how best to talk to friends, family and hospital professionals, and explanations of words that they may never have heard before.

Hydrocortisone (HC) is the preferred glucocorticoid for children with CAH. A short half-life requires dosing three times daily, which is a challenge in infants. Longer-acting steroids have their place but generally only for adults. Despite cortisone being introduced 70 years ago to treat CAH, replicating the cortisol diurnal rhythm remains a challenge.

Patients with NS frequently have proportional postnatal short stature. Birth weight is usually normal, but there is a trend of lower birth length. Short stature (height standard deviation score [SDS] <–2] is observed in approximately 70–80% of children with NS. The growth deficit is more pronounced at pubertal ages due to the delayed puberty commonly observed in these patients (2). Interestingly, patients harbouring mutations in some specific NS-causative genes, mainly SOS1 and SOS2, have more preserved postnatal growth in comparison with other genotypes. As a group, the adult height of non-treated patients with NS is around –2.5 SDS (2).

The exact physiopathology of growth impairment remains unclear and probably involves multiple mechanisms, including reduction in IGF-1 generation and direct effect on the growth plate (1). Nevertheless, it is accepted that human recombinant growth hormone (rhGH) treatment can increase the short-term height velocity and improve the adult height of these patients (3, 4). Growth hormone therapy is licensed for NS by the US Food & Drug Administration (5) but not by the European Medicines Agency. The total increment in height SDS after long-term rhGH therapy ranges from 0.6 to 1.7 in different studies.

Early studies suggested that patients with PTPN11 mutations could have a lower short-term growth response to rhGH treatment (6), although these findings have not been reproduced by other studies (3). Only three studies evaluated the effect of rhGH therapy on adult height regarding the patients’ genotype. In these studies, the majority of the patients harbour the PTPN11 mutation and no clear difference in height gain is observed regarding genotypes.

Insights into the genetic mutations that lead to familial DP have come from sequencing genes within the gonadotropin-releasing hormone (GnRH) pathway known to cause pubertal failure. Recently, via next generation sequencing, mutations in HS6ST1, GNRHR, IL17RD, SEMA3A, TACR3 and TAC3 [2] have been found in patients with DP and spontaneous onset of puberty. One persuasive hypothesis is that a single deleterious mutation may lead to a phenotype of DP, whilst two or more mutations may be required to cause absent puberty, for example in congenital hypogonadotropic hypogonadism (cHH).

By extension, other pathways related to GnRH neuronal development and function have been explored in the search for the genetic basis of DP. Mutations in IGSF10 provoke a dysregulation of GnRH neuronal migration during embryonic development, which first presents in adolescence as DP without previous constitutional delay in growth [3]. Pathogenic IGSF10 mutations leading to disrupted IGSF10 signalling potentially result in reduced numbers or mistimed arrival of GnRH neurons at the hypothalamus; this produces a functional defect in the GnRH neuroendocrine network and thus an increased “threshold” for the onset of puberty.

Upstream transcriptional regulators of GnRH signalling, such as KISS1, OCT2, TTF1, YY1 and EAP1, which act as a pubertal “brake” through a balance of activating and repressive inputs, have been proposed as attractive candidates for the pathogenesis of DP. EAP1 is known to contribute to the initiation of female puberty through transactivation of the GnRH promoter, and mutations in EAP1 have very recently been found in families with self-limited DP [4]. New evidence has also identified small non-coding RNAs important for the murine critical period (or mini-puberty). The microRNA (miR)-200/429 family and miR-155 both act as epigenetic up-regulators of GnRH transcription [5], whilst miR-7a2 has been demonstrated to be essential for normal hypothalamic–pituitary–gonadal function, with deletion in mice leading to cHH.

Administration of GH to the patient’s fibroblasts failed to produce the expected increase in IGF-I, leading the researchers to focus on the signalling pathways between these two molecules. This revealed a homozygous missense mutation in STAT5b, which rendered the protein incapable of activation by GH, despite being stably expressed.

Activated STAT5b in turn starts a signalling cascade that leads to increased transcription of IGF-I, IGF binding protein 3 and ALS. Together these molecules form a ternary complex, in which ALS stabilises the binding of IGF-I to IGF binding protein 3 and extends the half-life of circulating IGF-I. As the majority (80–85%) of IGF-I in the body is in this complex form, it was thought to be the main pathway through which GH exerts its effects.³

However, the case of a boy who entirely lacked ALS challenged this notion. The patient had a frameshift point mutation, which prevented production of ALS; he therefore had no ternary complexes and his plasma level of IGF-I was more than 5 SD below the average.

Yet, although the patient was referred to Horacio Domené (Ricardo Gutiérrez Children’s Hospital, Buenos Aires, Argentina) and colleagues partly for growth restriction, this was less severe than might be expected, at 2.05 SD below average. The team speculated that growth in their patient had been sustained by free or locally produced IGF-I, implying that total circulating IGF-I levels are less critical than previously believed.

Large-scale genome-wide association studies in the general population have highlighted hundreds of potential loci associated with the timing of puberty. Few genes at these loci have been shown to be causal in DP, although several loci are in or near to genes implicated in rare disorders of puberty (LEPR, GNRH1, KISS1 and TACR3) and pituitary function (POU1F1, TENM2 and LGR4), and two imprinted central precocious puberty genes (MKRN3 and DLK1). Genes involved in BMI control including FTO were also seen, and rare heterozygous variants in FTO have been identified in families with self-limited DP with extreme low BMI and maturational delay in growth.

Evidence for the role of the IGF-I receptor in pre- and postnatal growth came from Steven Chernausek (Cincinnati Children’s Hospital Medical Center, Ohio, USA) and co-workers, who sequenced the gene in 51 children with heights more than 2 SD below average.⁵ They identified two children, who both had intrauterine and postnatal growth restriction and both had mutations in the IGF-I receptor gene, one resulting in reduced IGF-I receptor function and the other in a reduced number of IGF-I receptors on the fibroblasts.

Although closely related to IGF-I, and well-recognised as a major influence on intrauterine growth, IGF-II has been considered less important for postnatal growth. However, a very recent paper detailed how a paternally inherited IGF-II nonsense mutation resulted in Silver–Russell syndrome.⁶

Thomas Eggermann (University Hospital, Aachen, Germany) and co-researchers found their patients lacked mutations known to cause Silver–Russell syndrome, but detected the IGF-II mutation using exome and Sanger sequencing. Two of the three affected children were very small at birth but responded to later growth hormone treatment and achieved an adult height close to the third centile; the third did not respond and had continued poor growth.

Another aspect of these case studies was the finding of other, non-growth symptoms that the researchers attributed to the mutations in the GH/IGF-I axis, implying pleiotropic actions of this pathway. For example, the boy with truncated IGF-I had, among other symptoms, profound sensorineural deafness and mental retardation, the girl with mutated STAT5b had a compromised immune response, and the Silver–Russell patients had symptoms including severe postnatal feeding problems, delayed development, mental retardation and hypotonia.

Thus, the genetic basis of DP is highly heterogeneous, with gene defects producing pathogenic mechanisms that act from early foetal life into adolescence, all converging on a common pathway of pubertal delay.

1. Howard SR. Genes underlying delayed puberty. Mol Cell Endocrinol 2018; doi: 10.1016/j.mce.2018.05.001
2. Zhu J, Choa RE, Guo MH, et al. A Shared Genetic Basis for Self-Limited Delayed Puberty and Idiopathic Hypogonadotropic Hypogonadism. J Clin Endocrinol Metab 2015; doi: 10.1210/jc.2015-1080
3. Howard SR, Guasti L, Ruiz-Babot G, et al. IGSF10 mutations dysregulate gonadotropin-releasing hormone neuronal migration resulting in delayed puberty. EMBO Mol Med 2016; doi: 10.15252/emmm.201606250
4. Mancini A, Howard SR, Cabrera CP, et al. EAP1 regulation of GnRH promoter activity is important for human pubertal timing. Hum Mol Genet  2019; doi: 10.1093/hmg/ddy451
5. Messina A, Langlet F, Chachlaki K, et al. A microRNA switch regulates the rise in hypothalamic GnRH production before puberty. Nat Neurosci 2016; doi: 10.1038/nn.4298

Laron dwarfism was first described in 1966, as an autosomal recessive syndrome with many clinical similarities to isolated GH deficiency. The characteristic low levels of IGF-I despite increased levels of GH implied a role for the GH receptor, but it was not until 1989 that researchers provided direct evidence of this hypothesis. Michel Goossens (CHU Henri Mondor, Créteil, France) and colleagues identified a likely culprit mutation in the GH receptor gene of a family with consanguineous parents with normal phenotypes who had four children, all affected by Laron dwarfism.¹

All of the children were homozygous for a single base-pair substitution resulting in serine being present at a position in the extracellular domain of the GH receptor where phenylalanine is invariably found across multiple species. This, combined with the lack of GH-binding activity in the study patients, led the team to conclude that the mutation was probably causative, providing the first direct evidence for the role of the GH receptor in growth.

In contrast to this study, which actively sought the genetic basis for a condition, another two landmark reports detail more serendipitous findings; direct evidence for the involvement of STAT5 and the IGF-I receptor in growth signalling both emerged from researchers’ efforts to identify the cause of short stature in an individual child, while the surprising findings of another case study challenged the importance of the acid-labile subunit (ALS) in GH/IGF-I signalling.

In one case, Ron Rosenfeld (Oregon Health and Science University, Portland, USA) and co-researchers studied a girl whose height was 7.5 standard deviations (SD) below the average for her age, of 16.5 years.² She had elevated GH levels and low IGF-I levels, consistent with GH resistance, yet her GH receptor gene had no mutations.

Administration of GH to the patient’s fibroblasts failed to produce the expected increase in IGF-I, leading the researchers to focus on the signalling pathways between these two molecules. This revealed a homozygous missense mutation in STAT5b, which rendered the protein incapable of activation by GH, despite being stably expressed.

Activated STAT5b in turn starts a signalling cascade that leads to increased transcription of IGF-I, IGF binding protein 3 and ALS. Together these molecules form a ternary complex, in which ALS stabilises the binding of IGF-I to IGF binding protein 3 and extends the half-life of circulating IGF-I. As the majority (80–85%) of IGF-I in the body is in this complex form, it was thought to be the main pathway through which GH exerts its effects.³

However, the case of a boy who entirely lacked ALS challenged this notion. The patient had a frameshift point mutation, which prevented production of ALS; he therefore had no ternary complexes and his plasma level of IGF-I was more than 5 SD below the average.

Yet, although the patient was referred to Horacio Domené (Ricardo Gutiérrez Children’s Hospital, Buenos Aires, Argentina) and colleagues partly for growth restriction, this was less severe than might be expected, at 2.05 SD below average. The team speculated that growth in their patient had been sustained by free or locally produced IGF-I, implying that total circulating IGF-I levels are less critical than previously believed.

A third case study supplied direct evidence that IGF-I is crucial not just for postnatal growth but also for intrauterine growth. The report featured a 15.8-year-old boy who had a birth length 5.4 SD below average and continued to have severe growth retardation throughout childhood, so that, by the time of the report, his height was 6.9 SD below average.⁴

This patient had elevated GH levels and undetectable serum IGF-I, and Adrian Clark and colleagues, from St Bartholomew’s Hospital in London, UK, found that this was caused by a mutation of the IGF-I gene itself. Their patient was missing exons 4 and 5, resulting in a truncated IGF-I protein.

Evidence for the role of the IGF-I receptor in pre- and postnatal growth came from Steven Chernausek (Cincinnati Children’s Hospital Medical Center, Ohio, USA) and co-workers, who sequenced the gene in 51 children with heights more than 2 SD below average.⁵ They identified two children, who both had intrauterine and postnatal growth restriction and both had mutations in the IGF-I receptor gene, one resulting in reduced IGF-I receptor function and the other in a reduced number of IGF-I receptors on the fibroblasts.

Although closely related to IGF-I, and well-recognised as a major influence on intrauterine growth, IGF-II has been considered less important for postnatal growth. However, a very recent paper detailed how a paternally inherited IGF-II nonsense mutation resulted in Silver–Russell syndrome.⁶

Thomas Eggermann (University Hospital, Aachen, Germany) and co-researchers found their patients lacked mutations known to cause Silver–Russell syndrome, but detected the IGF-II mutation using exome and Sanger sequencing. Two of the three affected children were very small at birth but responded to later growth hormone treatment and achieved an adult height close to the third centile; the third did not respond and had continued poor growth.

Another aspect of these case studies was the finding of other, non-growth symptoms that the researchers attributed to the mutations in the GH/IGF-I axis, implying pleiotropic actions of this pathway. For example, the boy with truncated IGF-I had, among other symptoms, profound sensorineural deafness and mental retardation, the girl with mutated STAT5b had a compromised immune response, and the Silver–Russell patients had symptoms including severe postnatal feeding problems, delayed development, mental retardation and hypotonia.

Postnatal growth is not regulated only by the GH/IGF-I axis. A notable example outside of this pathway is the SHOX (short stature homeobox-containing) gene, a transcription factor that is expressed in the growth plate.

SHOX was first described in 1997, by Gudrun Rappold (Heidelberg University, Germany) and colleagues.⁷ The cause of short stature in girls with Turner syndrome had at this time been attributed to monosomy of genes in the sex chromosomes, and narrowed down to the pseudoautosomal region. By studying patients with partial monosomy of this region and adults’ heights ranging from normal to more than 3 SD below average, the team refined this to a 170 kb section, in which they discovered SHOX.

Besides explaining the short stature of Turner syndrome patients, SHOX may also account for a small percentage of idiopathic short stature; the researchers discovered a mutation in one of 91 such patients tested.

And SHOX is just the tip of the iceberg, according to Dauber. He says that although the reviewed papers have provided “a tremendous amount of insight” into the GH/IGF-I axis, “there’s much more to growth than that one biological pathway.”

Illustrating this, Dauber points out that all reported patients with mutations in the GH/IGF axis number only a few hundred worldwide.

“But if you look at the number of kids who come to be evaluated by an endocrinologist, even with heights below minus 3 standard deviations or so, there are tens of thousands of those and we really only explain a tiny percentage of those with bona fide mutations in the GH/IGF axis.”

This suggests that most patients with idiopathic short stature will prove to have mutations in genes that are not directly involved in this pathway.

Not that Dauber believes the influence of the GH/IGF pathway is fully mapped out; he sees research in that area starting to focus more on mutations in regulatory regions of genes, and in epigenetics.

But he says: “I think that the challenge for endocrinologists in the coming years is going to be how to learn and think about the defects in growth that are outside that classic hormonal pathway.”

A complicating factor is the large grey area between polygenic short stature – caused by the cumulative effect of the thousands of variants with, at most, a millimetre effect on height – and monogenic short stature – caused by a mutation in a specific critical gene.

“The long-term challenge is going to be finding therapies that don’t affect the GH/IGF axis but do affect growth,” says Dauber.

Such therapies are coming into play, however; Dauber gives the example of the NPR2 gene, which encodes a receptor (natriuretic protein receptor B) that acts directly in the growth plate and is essential for endochondral ossification. An NPR2 agonist is currently in development for patients with achondroplasia, he says.

“That’s a totally different type of growth-promoting therapy, in a disorder that’s more commonly handled by geneticists than endocrinologists.”

1. Amselem S, Duquesnoy P, Attree O, et al. Laron dwarfism and mutations of the growth hormone-receptor gene. N Engl J Med 1989; 321: 989–995

   http://www.nejm.org/doi/full/10.1056/NEJM198910123211501
2. Kofoed EM, Hwa V, Little B, et al. Growth hormone insensitivity associated with a STAT5b mutation. N Engl J Med 2003; 349: 1139–1147

   http://www.nejm.org/doi/full/10.1056/NEJMoa022926
3. Domené HM, Bengolea SV, Martínez AS, et al. Deficiency of the circulating insulin-like growth factor system associated with inactivation of the acid-labile subunit gene. N Engl J Med 2004; 350: 570–577

   http://www.nejm.org/doi/full/10.1056/NEJMoa013100
4. Woods KA, Camacho-Hübner C, Savage MO, Clark AJ. Intrauterine growth retardation and postnatal growth failure associated with deletion of the insulin-like growth factor I gene. N Engl J Med 1996; 335: 1363–1367

   http://www.nejm.org/doi/full/10.1056/NEJM199610313351805
5. Abuzzahab MJ, Schneider A, Goddard A, et al. IGF-I receptor mutations resulting in intrauterine and postnatal growth retardation. N Engl J Med 2003; 349: 2211–2222

   http://www.nejm.org/doi/full/10.1056/NEJMoa010107
6. Begemann M, Zirn B, Santen G, et al. Paternally inherited IGF2 mutation and growth restriction. N Engl J Med 2015; 373:349–356

   http://www.nejm.org/doi/full/10.1056/NEJMoa1415227
7. Rao E, Weiss B, Fukami M, et al. Pseudoautosomal deletions encompassing a novel homeobox gene cause growth failure in idiopathic short stature and Turner syndrome. Nat Genet 1997; 16: 54–63

   http://www.nature.com/ng/journal/v16/n1/pdf/ng0597-54.pdf
8. Picture courtesy of Cincinnati Children’s Hospital Medical Center

   http://www.cincinnatichildrens.org/default.htm