Rescue of pituitary function in a mouse model of isolated growth hormone deficiency type II by RNA interference

Targeting GH-1 splicing as a novel pharmacological strategy for growth hormone deficiency type II

Isolated growth hormone deficiency type II (IGHD II) is a rare genetic splicing disorder characterized by reduced growth hormone (GH) secretion and short stature. It is mainly caused by autosomal dominant-negative mutations within the growth hormone gene (GH-1) which results in missplicing at the mRNA level and the subsequent loss of exon 3, producing the 17.5-kDa GH isoform: a mutant and inactive GH protein that reduces the stability and the secretion of the 22-kDa GH isoform, the main biologically active GH form. At present, patients suffering from IGHD II are treated with daily injections of recombinant human GH (rhGH) in order to reach normal height. However, this type of replacement therapy, although effective in terms of growth, does not prevent the toxic effects of the 17.5-kDa mutant on the pituitary gland, which may eventually lead to other hormonal deficiencies. As the severity of the disease inversely correlates with the 17.5-kDa/22-kDa ratio, increasing the inclusion of exon 3 is expected to ameliorate disease symptoms. This review focuses on the recent advances in experimental and therapeutic strategies applicable to treat IGHD II in clinical and preclinical contexts. Several avenues for alternative IGHD II therapy will be discussed including the use of small interfering RNA (siRNA) and short hairpin RNA (shRNA) constructs that specifically target the exon 3-deleted transcripts as well as the application of histone deacetylase inhibitors (HDACi) and antisense oligonucleotides (AONs) to enhance full-length GH-1 transcription, correct GH-1 exon 3 splicing and manipulate GH pathway.

Rescue of Isolated GH Deficiency Type II (IGHD II) via Pharmacologic Modulation of GH-1 Splicing

Isolated GH deficiency (IGHD) type II, the autosomal dominant form of GHD, is mainly caused by mutations that affect splicing of GH-1. When misspliced RNA is translated, it produces a toxic 17.5-kDa GH isoform that reduces the accumulation and secretion of wild-type-human GH (wt-hGH). Usually, isolated GHD type II patients are treated with daily injections of recombinant human GH in order to maintain normal growth. However, this type of replacement therapy does not prevent toxic effects of the 17.5-kDa GH isoform on the pituitary gland, which can eventually lead to other hormonal deficiencies. Here, we tested the possibility to restore the constitutive splicing pattern of GH-1 by using butyrate, a drug that mainly acts as histone deacetylase inhibitor. To this aim, wt-hGH and/or different hGH-splice site mutants (GH-IVS3+2, GH-IVS3+6, and GH-ISE+28) were transfected in rat pituitary cells expressing human GHRH receptor (GHRHR) (GC-GHRHR). Upon butyrate treatment, GC-GHRHR cells coexpressing wt-hGH and each of the mutants displayed increased GH transcript level, intracellular GH content, and GH secretion when compared with the corresponding untreated condition. The effect of butyrate was most likely mediated by the alternative splicing factor/splicing factor 2. Overexpression of alternative ASF/SF2 in the same experimental setting, indeed, promoted the amount of full-length transcripts thus increasing synthesis and secretion of the 22-kDa GH isoform. In conclusion, our results support the hypothesis that modulation of GH-1 splicing pattern to increase the 22-kDa GH isoform levels can be clinically beneficial and hence a crucial challenge in GHD research.

Isolated growth hormone deficiency (GHD) in childhood and adolescence: recent advances

The diagnosis of GH deficiency (GHD) in childhood is a multistep process involving clinical history, examination with detailed auxology, biochemical testing, and pituitary imaging, with an increasing contribution from genetics in patients with congenital GHD. Our increasing understanding of the factors involved in the development of somatotropes and the dynamic function of the somatotrope network may explain, at least in part, the development and progression of childhood GHD in different age groups. With respect to the genetic etiology of isolated GHD (IGHD), mutations in known genes such as those encoding GH (GH1), GHRH receptor (GHRHR), or transcription factors involved in pituitary development, are identified in a relatively small percentage of patients suggesting the involvement of other, yet unidentified, factors. Genome-wide association studies point toward an increasing number of genes involved in the control of growth, but their role in the etiology of IGHD remains unknown. Despite the many years of research in the area of GHD, there are still controversies on the etiology, diagnosis, and management of IGHD in children. Recent data suggest that childhood IGHD may have a wider impact on the health and neurodevelopment of children, but it is yet unknown to what extent treatment with recombinant human GH can reverse this effect. Finally, the safety of recombinant human GH is currently the subject of much debate and research, and it is clear that long-term controlled studies are needed to clarify the consequences of childhood IGHD and the long-term safety of its treatment.

Phenotype-genotype correlations in congenital isolated growth hormone deficiency (IGHD)

Isolated growth hormone deficiency (IGHD) may be congenital, often due to genetic mutations, or acquired as a result of other factors such as cranial irradiation. The commonest genes implicated in its genetic etiology are those encoding growth hormone (GH1) and the receptor for GH-releasing hormone (GHRHR). Rarely, IGHD may be caused by mutations in transcription factors (HESX1, SOX3, OTX2) or be the first presentation before the development of other pituitary hormone deficiencies. IGHD has been classified in four genetic forms (type IA, IB, II and III). Despite the increasing number of genes implicated in the etiology of IGHD, mutations in known genes account only for a small percentage of cases; therefore, other as yet unidentified factors may be implicated in its etiology. Although there is no strict genotype/phenotype correlation in patients with IGHD, there are some emerging patterns that may guide us towards a genetic diagnosis of the condition. There is increasing understanding that the phenotype of patients with IGHD is highly variable and sometimes even evolving, dictating the need for long term follow-up in these cases.

Pharmacologic correction of dominant-negative GH1 deficiency causing mutations

PURPOSE

Dominant-negative growth hormone gene (GH1) mutations cause familial isolated growth hormone deficiency type II (IGHD II), which is characterized by GH deficiency, occasional multiple anterior pituitary hormone deficiencies, and anterior pituitary hypoplasia. We have previously shown that 17.5-/22-kDa GH1 transcript ratios correlate with the severity of the IGHD II phenotype. We hypothesized that different pharmaceutical agents could affect the GH1 transcript ratio by modulating alternative splicing.

METHODS

We exposed peripheral blood mononuclear cells from IGHD II patients and unaffected family members to different pharmacologic agents and then determined the 17.5-/22-kDa transcript ratios by real-time PCR.

RESULTS

Dexamethasone and digoxin significantly increased the 17.5-/22-kDa transcript ratio, while sodium butyrate and 5-iodotubericidin significantly decreased the ratio.

CONCLUSION

Since we have previously shown that the ratio of the 17.5-/22-kDa GH1 transcripts correlates with severity of the IGHD II phenotype, our findings here suggest that selected previously unconsidered agents could possibly reduce the severity of IGHD II, while other agents could possibly exacerbate the disease phenotype.

Genetic causes and treatment of isolated growth hormone deficiency-an update

Isolated growth hormone deficiency is the most common pituitary hormone deficiency and can result from congenital or acquired causes, although the majority of cases are idiopathic with no identifiable etiology. Known genes involved in the genetic etiology of isolated growth hormone deficiency include those that encode growth hormone (GH1), growth-hormone-releasing hormone receptor (GHRHR) and transcription factor SOX3. However, mutations are identified in a relatively small percentage of patients, which suggests that other, yet unidentified, genetic factors are involved. Among the known factors, heterozygous mutations in GH1 remain the most frequent cause of isolated growth hormone deficiency. The identification of mutations has clinical implications for the management of patients with this condition, as individuals with heterozygous GH1 mutations vary in phenotype and can, in some cases, develop additional pituitary hormone deficiencies. Lifelong follow-up of these patients is, therefore, recommended. Further studies in the genetic etiology of isolated growth hormone deficiency will help to elucidate mechanisms implicated in the control of growth and may influence future treatment options. Advances in pharmacogenomics will also optimize the treatment of isolated growth hormone deficiency and other conditions associated with short stature, for which recombinant human growth hormone is a licensed therapy.

Isolated GH deficiency type II: knockdown of the harmful Delta3GH recovers wt-GH secretion in rat tumor pituitary cells

Isolated GH deficiency type II (IGHD II) is the autosomal dominant form of GHD. In the majority of the cases, this disorder is due to specific GH-1 gene mutations that lead to mRNA missplicing and subsequent loss of exon 3 sequences. When misspliced RNA is translated, it produces a toxic 17.5-kDa GH (Delta3GH) isoform that reduces the accumulation and secretion of wild-type-GH. At present, patients suffering from this type of disease are treated with daily injections of recombinant human GH in order to maintain normal growth. However, this type of replacement therapy does not prevent toxic effects of the Delta3GH mutant on the pituitary gland, which can eventually lead to other hormonal deficiencies. We developed a strategy involving Delta3GH isoform knockdown mediated by expression of a microRNA-30-adapted short hairpin RNA (shRNA) specifically targeting the Delta3GH mRNA of human (shRNAmir-Delta3). Rat pituitary tumor GC cells expressing Delta3GH upon doxycycline induction were transduced with shRNAmir-Delta3 lentiviral vectors, which significantly reduced Delta3GH protein levels and improved human wild-type-GH secretion in comparison with a shRNAmir targeting a scrambled sequence. No toxicity due to shRNAmir expression could be observed in cell proliferation assays. Confocal microscopy strongly suggested that shRNAmir-Delta3 enabled the recovery of GH granule storage and secretory capacity. These viral vectors have shown their ability to stably integrate, express shRNAmir, and rescue IGHD II phenotype in rat pituitary tumor GC cells, a methodology that opens new perspectives for the development of gene therapy to treat IGHD patients.

A molecular basis for variation in clinical severity of isolated growth hormone deficiency type II

CONTEXT

Dominant-negative GH1 mutations cause familial isolated growth hormone deficiency type II (IGHD II), which is characterized by GH deficiency, occasional multiple anterior pituitary hormone deficiencies, and anterior pituitary hypoplasia. The basis of the variable expression and progression of IGHD II among relatives who share the same GH1 mutation is poorly understood.

OBJECTIVE

We hypothesized that the cellular ratios of mutant/normal GH1 transcripts would correlate with the severity of the IGHD II phenotype. We determined the relative amounts of mutant and normal GH1 transcripts in cell lines and correlated transcript ratios with severity.

DESIGN AND PATIENTS

Members of the same IGHD II kindred were genotyped for the GH1 E3+1 G/A mutation by DNA sequencing. Ratios of their 17.5-kDa (mutant)/22-kDa (normal) GH1 transcripts were determined in cultured lymphocytes (CLs), and these ratios were correlated with height sd scores obtained before GH replacement therapy.

RESULTS

Ratios of 17.5-/22-kDa GH1 transcripts in CLs from family members with the same IGHD II mutation correlated with differences in their height SD scores.

CONCLUSIONS

Our findings suggest that expression levels of both the mutant and normal GH1 allele are important in the pathogenesis of IGHD II, that the ratio of mutant/normal transcripts may be a predictive marker of the penetrance and severity of IGHD II, and that CLs may be useful as surrogates to study GH1 transcript expression of subjects whose anterior pituitary cells are not available.

Truncating and missense BMPR2 mutations differentially affect the severity of heritable pulmonary arterial hypertension

BACKGROUND

Autosomal dominant inheritance of germline mutations in the bone morphogenetic protein receptor type 2 (BMPR2) gene are a major risk factor for pulmonary arterial hypertension (PAH). While previous studies demonstrated a difference in severity between BMPR2 mutation carriers and noncarriers, it is likely disease severity is not equal among BMPR2 mutations. We hypothesized that patients with missense BMPR2 mutations have more severe disease than those with truncating mutations.

METHODS

Testing for BMPR2 mutations was performed in 169 patients with PAH (125 with a family history of PAH and 44 with sporadic disease). Of the 106 patients with a detectable BMPR2 mutation, lymphocytes were available in 96 to functionally assess the nonsense-mediated decay pathway of RNA surveillance. Phenotypic characteristics were compared between BMPR2 mutation carriers and noncarriers, as well as between those carriers with a missense versus truncating mutation.

RESULTS

While there was a statistically significant difference in age at diagnosis between carriers and noncarriers, subgroup analysis revealed this to be the case only for females. Among carriers, there was no difference in age at diagnosis, death, or survival according to exonic location of the BMPR2 mutation. However, patients with missense mutations had statistically significant younger ages at diagnosis and death, as well as shorter survival from diagnosis to death or lung transplantation than those with truncating mutations. Consistent with this data, the majority of missense mutations were penetrant prior to age 36 years, while the majority of truncating mutations were penetrant after age 36 years.

CONCLUSION

In this cohort, BMPR2 mutation carriers have more severe PAH disease than noncarriers, but this is only the case for females. Among carriers, patients with missense mutations that escape nonsense-mediated decay have more severe disease than those with truncating mutations. These findings suggest that treatment and prevention strategies directed specifically at BMPR2 pathway defects may need to vary according to the type of mutation.

Isolated growth hormone deficiency type II caused by a point mutation that alters both splice site strength and splicing enhancer function

A heterozygous single base mutation in the human growth hormone (GH) gene (GH-1) was identified in a family presenting with isolated GH deficiency type II (IGHD II). Affected individuals have a guanine to adenine transition at the first nucleotide of exon 3 (E3+1 G-->A) that results in exon skipping and production of a dominant-negative 17.5-kDa isoform. We show that the mechanistic basis for exon skipping is due to the unique position of this mutation because it weakens the 3' splice site and simultaneously disrupts a splicing enhancer located within the first seven bases of exon 3. A G-->T mutation at this same position not only affects splicing but also results in a premature stop codon for those transcripts that include exon 3. Thus, mutations that alter the first nucleotide of exon 3 illustrate the various mechanisms by which changes in sequence can cause disease: splice site selection, splicing enhancer function, messenger RNA decay, missense mutations, and nonsense mutations. For IGHD II, only exon skipping leads to production of the dominant-negative isoform, with increasing skipping correlating with increasing disease severity.