Mechanisms responsible for dominant expression of human growth hormone gene mutations

Growth hormone - past, present and future

Growth hormone (GH) research and its clinical application for the treatment of growth disorders span more than a century. During the first half of the 20th century, clinical observations and anatomical and biochemical studies formed the basis of the understanding of the structure of GH and its various metabolic effects in animals. The following period (1958-1985), during which pituitary-derived human GH was used, generated a wealth of information on the regulation and physiological role of GH - in conjunction with insulin-like growth factors (IGFs) - and its use in children with GH deficiency (GHD). The following era (1985 to present) of molecular genetics, recombinant technology and the generation of genetically modified biological systems has expanded our understanding of the regulation and role of the GH-IGF axis. Today, recombinant human GH is used for the treatment of GHD and various conditions of non-GHD short stature and catabolic states; however, safety concerns still accompany this therapeutic approach. In the future, new therapeutics based on various components of the GH-IGF axis might be developed to further improve the treatment of such disorders. In this Review, we describe the history of GH research and clinical use with a particular focus on disorders in childhood.

AnaLysis of Expression on human chromosome 21, ALE-HSA21: a pilot integrated web resource

Transcriptome studies have shown the pervasive nature of transcription, demonstrating almost all the genes undergo alternative splicing. Accurately annotating all transcripts of a gene is crucial. It is needed to understand the impact of mutations on phenotypes, to shed light on genetic and epigenetic regulation of mRNAs and more generally to widen our knowledge about cell functionality and tissue diversity. RNA-sequencing (RNA-Seq), and the other applications of the next-generation sequencing, provides precious data to improve annotations' accuracy, simultaneously creating issues related to the variety, complexity and the size of produced data. In this ‘scenario’, the lack of user-friendly resources, easily accessible to researchers with low skills in bioinformatics, makes difficult to retrieve complete information about one or few genes without browsing a jungle of databases. Concordantly, the increasing amount of data from ‘omics’ technologies imposes to develop integrated databases merging different data formats coming from distinct but complementary sources. In light of these considerations, and given the wide interest in studying Down syndrome—a genetic condition due to the trisomy of human chromosome 21 (HSA21)—we developed an integrated relational database and a web interface, named ALE-HSA21 (AnaLysis of Expression on HSA21), accessible at http://bioinfo.na.iac.cnr.it/ALE-HSA21. This comprehensive and user-friendly web resource integrates—for all coding and noncoding transcripts of chromosome 21—existing gene annotations and transcripts identified de novo through RNA-Seq analysis with predictive computational analysis of regulatory sequences. Given the role of noncoding RNAs and untranslated regions of coding genes in key regulatory mechanisms, ALE-HSA21 is also an interesting web-based platform to investigate such processes. The ‘transcript-centric’ and easily-accessible nature of ALE-HSA21 makes this resource a valuable tool to rapidly retrieve data at the isoform level, rather than at gene level, useful to investigate any disease, molecular pathway or cell process involving chromosome 21 genes.

Database URL: http://bioinfo.na.iac.cnr.it/ALE-HSA21/

Gene dosage and gene duplicability

The evolutionary process leading to the fixation of newly duplicated genes is not well understood. It was recently proposed that the fixation of duplicate genes is frequently driven by positive selection for increased gene dosage (i.e., the gene dosage hypothesis), because haploinsufficient genes were reported to have more paralogs than haplosufficient genes in the human genome. However, the previous analysis incorrectly assumed that the presence of dominant abnormal alleles of a human gene means that the gene is haploinsufficient, ignoring the fact that many dominant abnormal alleles arise from gain-of-function mutations. Here we show in both humans and yeast that haploinsufficient genes generally do not duplicate more frequently than haplosufficient genes. Yeast haploinsufficient genes do exhibit enhanced retention after whole-genome duplication compared to haplosufficient genes if they encode members of stable protein complexes, but the same phenomenon is absent if the genes do not encode protein complex members, suggesting that the dosage balance effect rather than the dosage effect is the underlying cause of the phenomenon. On the basis of these and other results, we conclude that selection for higher gene dosage does not play a major role in driving the fixation of duplication genes.

Disruption of exon definition produces a dominant-negative growth hormone isoform that causes somatotroph death and IGHD II

Isolated growth hormone deficiency type II (IGHD II) is characterized by short stature due to dominant-negative mutations of the human growth hormone gene (GH1). Most of the known mutations responsible for IGHD II cause aberrant splicing of GH1 transcripts. We have recently shown that mutations that cause exon 3 skipping and produce a dominant-negative 17.5-kDa isoform in humans also cause a dose-dependent disruption of GH secretory vesicles when expressed in GC cells and transgenic mice. We show here that overexpression of the dominant-negative 17.5-kDa isoform also destroys the majority of somatotrophs, leading to anterior pituitary hypoplasia in transgenic mice. It is, therefore, important to understand the regulation of GH1 splicing and why its perturbation causes IGHD II. We demonstrate that dual splicing enhancers are required to ensure exon 3 definition to produce full-length 22-kDa hormone. We also show that splicing enhancer mutations that weaken exon 3 recognition produce variable amounts of the 17.5-kDa isoform, a result which could potentially explain the clinical variability observed in IGHD II. Non-canonical splicing mutations that disrupt splicing enhancers, such as those illustrated here, demonstrate the importance of enhancer elements in regulating alternative splicing to prevent human disease.