IGF-I sensitivity in Silver-Russell syndrome with IGF2/H19 hypomethylation

Clinical and Molecular Heterogeneity of Silver-Russell Syndrome and Therapeutic Challenges: A Systematic Review

BACKGROUND

Silver-Russell syndrome (SRS) is a developmental disorder involving extreme growth failure, characteristic facial features and underlying genetic heterogeneity. As the clinical heterogeneity of SRS makes diagnosis a challenging task, the worldwide incidence of SRS could vary from 1:30,000 to 1:100,000. Although various chromosomal, genetic, and epigenetic mutations have been linked with SRS, the cause had only been identified in half of the cases.

MATERIAL AND METHODS

To have a better understanding of the SRS clinical presentation and mutation/ epimutation responsible for SRS, a systematic review of the literature was carried out using appropriate keywords in various scientific databases (PROSPERO protocol registration CRD42021273211). Clinical features of SRS have been compiled and presented corresponding to the specific genetic subtype. An attempt has been made to understand the recurrence risk and the role of model organisms in understanding the molecular mechanisms of SRS pathology, treatment, and management strategies of the affected patients through the analysis of selected literature.

RESULTS

156 articles were selected to understand the clinical and molecular heterogeneity of SRS. Information about detailed clinical features was available for 228 patients only, and it was observed that body asymmetry and relative macrocephaly were most prevalent in cases with methylation defects of the 11p15 region. In about 38% of cases, methylation defects in ICRs or genomic mutations at the 11p15 region have been implicated. Maternal uniparental disomy of chromosome 7 (mUPD7) accounts for about 7% of SRS cases, and rarely, uniparental disomy of other autosomes (11, 14, 16, and 20 chromosomes) has been documented. Mutation in half of the cases is yet to be identified. Studies involving mice as experimental animals have been helpful in understanding the underlying molecular mechanism. As the clinical presentation of the syndrome varies a lot, treatment needs to be individualized with multidisciplinary effort.

CONCLUSION

SRS is a clinically and genetically heterogeneous disorder, with most of the cases being implicated with a mutation in the 11p15 region and maternal disomy of chromosome 7. Recurrence risk varies according to the molecular subtype. Studies with mice as a model organism have been useful in understanding the underlying molecular mechanism leading to the characteristic clinical presentation of the syndrome. Management strategies often need to be individualized due to varied clinical presentations.

Novel Insights Into the Genetic Causes of Short Stature in Children

Short stature is a common reason for consulting a growth specialist during childhood. Normal height is a polygenic trait involving a complex interaction between hormonal, nutritional and psychosocial components. Genetic factors are becoming very important in the understanding of short stature. After exclusion of the most frequent causes of growth failure, clinicians need to evaluate whether a genetic cause might be taken into consideration. In fact, genetic causes of short stature are probably misdiagnosed during clinical practice and the underlying cause of short stature frequently remains unknown, thus classifying children as having idiopathic short stature (ISS). However, over the past decade, novel genetic techniques have led to the discovery of novel genes associated with linear growth and thus to the ability to define new possible aetiologies of short stature. In fact, thanks to the newer genetic advances, it is possible to properly re-classify about 25–40% of children previously diagnosed with ISS. The purpose of this article is to describe the main monogenic causes of short stature, which, thanks to advances in molecular genetics, are assuming an increasingly important role in the clinical approach to short children.

Sleep disordered breathing in Silver-Russell syndrome patients: a new outcome

OBJECTIVE

Imprinting disorders (ID), such as Prader-Willi syndrome (PWS), are associated with sleep-disordered breathing (SDB). No data are available for Silver-Russell syndrome (SRS), another ID that shares clinical features with PWS, although many patients describe excessive daytime sleepiness, disturbed sleep, and snoring. The aim of this study was to characterize sleep in children with SRS and to evaluate the impact of recombinant growth hormone (rGH) therapy.

METHODS

We performed a retrospective analysis of sleep recordings in 40 patients with molecularly proven SRS (methylation anomaly in 11p15 [n = 32] or maternal uniparental disomy of chromosome 7 [n = 16]). Sleep recordings were either by means of polygraphy or polysomnography (PSG) (n = 16). A total of 34 patients received rGH therapy.

RESULTS

We collected 61 sleep recordings. The mean apnea-hypopnea index (AHI) was 3.4 events/h (0-12.4), with a mean central AHI of 0.5 events/h (0-2.4). SDB was identified in 73.8% (n = 45) of the recordings and was severe in 4.9%. SDB was present in 86.4% of patients before rGH therapy and was severe in 13.6%. AHI worsened for 5 of 12 patients with sleep recordings before and after rGH therapy initiation, reaching mild impairment. The mean rGH dose was 32.3 μg/kg/(12.9-51.4), with a mean insulin-like growth factor 1 plasma level of 1.7 SDS (-1.9 to 6.6).

CONCLUSION

Most patients with SRS present with SDB with an obstructive profile, possibly explained by narrowing of the airways and lymphoid organ hypertrophy. We recommend systematic ear-nose-throat evaluation of SRS patients and PSG if there are clinical anomalies, preferably before initiating rGH therapy, to monitor and adapt the management of patients with SDB.

The Emerging Role of Glucose Metabolism in Cartilage Development

PURPOSE OF REVIEW

Proper cartilage development is critical to bone formation during endochondral ossification. This review highlights the current understanding of various aspects of glucose metabolism in chondrocytes during cartilage development.

RECENT FINDINGS

Recent studies indicate that chondrocytes transdifferentiate into osteoblasts and bone marrow stromal cells during endochondral ossification. In cartilage development, signaling molecules, including IGF2 and BMP2, tightly control glucose uptake and utilization in a stage-specific manner. Perturbation of glucose metabolism alters the course of chondrocyte maturation, suggesting a key role for glucose metabolism during endochondral ossification. During prenatal and postnatal growth, chondrocytes experience bursts of nutrient availability and energy expenditure, which demand sophisticated control of the glucose-dependent processes of cartilage matrix production, cell proliferation, and hypertrophy. Investigating the regulation of glucose metabolism may therefore lead to a unifying mechanism for signaling events in cartilage development and provide insight into causes of skeletal growth abnormalities.

Nonclassical GH Insensitivity: Characterization of Mild Abnormalities of GH Action

GH insensitivity (GHI) presents in childhood with growth failure and in its severe form is associated with extreme short stature and dysmorphic and metabolic abnormalities. In recent years, the clinical, biochemical, and genetic characteristics of GHI and other overlapping short stature syndromes have rapidly expanded. This can be attributed to advancing genetic techniques and a greater awareness of this group of disorders. We review this important spectrum of defects, which present with phenotypes at the milder end of the GHI continuum. We discuss their clinical, biochemical, and genetic characteristics. The objective of this review is to clarify the definition, identification, and investigation of this clinically relevant group of growth defects. We also review the therapeutic challenges of mild GHI.

Systemic signalling and local effectors in developmental stability, body symmetry, and size

Symmetric growth and the origins of fluctuating asymmetry are unresolved phenomena of biology. Small, and sometimes noticeable, deviations from perfect bilateral symmetry reflect the vulnerability of development to perturbations. The degree of asymmetry is related to the magnitude of the perturbations and the ability of an individual to cope with them. As the left and right sides of an individual were presumed to be genetically identical, deviations of symmetry were traditionally attributed to non-genetic effects such as environmental and developmental noise. In this review, we draw attention to other possible sources of variability, especially to somatic mutations and transposons. Mutations are a major source of phenotypic variability and recent genomic data have highlighted somatic mutations as ubiquitous, even in phenotypically normal individuals. We discuss the importance of factors that are responsible for buffering and stabilizing the genome and for maintaining size robustness and quality through elimination of less-fit or damaged cells. However, the important question that arises from these studies is whether this self-correcting capacity and intrinsic organ size controls are sufficient to explain how symmetric structures can reach an identical size and shape. Indeed, recent discoveries in the fruit fly have uncovered a conserved hormone of the insulin/IGF/relaxin family, Dilp8, that is responsible for stabilizing body size and symmetry in the face of growth perturbations. Dilp8 alarm signals periphery growth status to the brain, where it acts on its receptor Lgr3. Loss of Dilp8-Lgr3 signaling renders flies incapable of detecting growth perturbations and thus maintaining a stable size and symmetry. These findings help to understand how size and symmetry of somatic tissues remain undeterred in noisy environments, after injury or illnesses, and in the presence of accumulated somatic mutations.

Short children born small for gestational age outcomes in the era of growth hormone therapy

Small-for-gestational age (SGA) infants are at risk for short and long term medical and metabolic complications. Most SGA infants (85-90%) demonstrate spontaneous catch-up growth, typically in the first year after birth. Although catch-up growth (CUG) is a desired goal, it is important to note if CUG is too rapid the infants are at increased risk for insulin resistance and type 2 diabetes mellitus as they become adults. On the flip side, infants who do not exhibit CUG are also at increased risk of adverse adult outcomes including those for cardiovascular disease, insulin resistance and type 2 diabetes mellitus, neurodevelopmental and cognitive impairments, in addition to adult short stature. Treatment with growth hormone is safe and effective not only in increasing adult height, but also in improving body composition and decreasing metabolic complications. The aims of this review are to summarize the current knowledge on what constitutes "healthy" catch-up growth in children born SGA as well as provide an update on the role of growth hormone treatment for short children born SGA.

New clinical and molecular insights into Silver-Russell syndrome

PURPOSE OF REVIEW

The purpose of review is to summarize new outcomes for the clinical characterization, molecular strategies, and therapeutic management of Silver-Russell syndrome (SRS).

RECENT FINDINGS

Various teams have described the clinical characteristics of SRS patients by genotype. A clinical score for the definition of SRS and for orienting molecular investigations has emerged. Insulin-like growth factor 2 (a major fetal growth factor) has been implicated in the pathophysiology of SRS, as the principle molecular mechanism underlying the disease is loss of methylation of the 11p15 region, including the imprinted insulin-like growth factor 2 gene. Maternal uniparental disomy of chromosome 7 and recently identified rare molecular defects have also been reported in patients with SRS. However, 40% of patients still have no molecular diagnosis.

SUMMARY

The definition of SRS has remained clinical since the first description of this condition, despite the identification of various molecular causes. The clinical issues faced by these patients are similar to those faced by other patients born small for gestational age (SGA), but patients with SRS require specific multidisciplinary management of their nutrition, growth, and metabolism, as they usually present an extreme form of SGA. Molecular analyses can confirm SRS, and are of particular importance for genetic counseling and prenatal testing.

MECHANISMS IN ENDOCRINOLOGY: Novel genetic causes of short stature

The fast technological development, particularly single nucleotide polymorphism array, array-comparative genomic hybridization, and whole exome sequencing, has led to the discovery of many novel genetic causes of growth failure. In this review we discuss a selection of these, according to a diagnostic classification centred on the epiphyseal growth plate. We successively discuss disorders in hormone signalling, paracrine factors, matrix molecules, intracellular pathways, and fundamental cellular processes, followed by chromosomal aberrations including copy number variants (CNVs) and imprinting disorders associated with short stature. Many novel causes of GH deficiency (GHD) as part of combined pituitary hormone deficiency have been uncovered. The most frequent genetic causes of isolated GHD are GH1 and GHRHR defects, but several novel causes have recently been found, such as GHSR, RNPC3, and IFT172 mutations. Besides well-defined causes of GH insensitivity (GHR, STAT5B, IGFALS, IGF1 defects), disorders of NFκB signalling, STAT3 and IGF2 have recently been discovered. Heterozygous IGF1R defects are a relatively frequent cause of prenatal and postnatal growth retardation. TRHA mutations cause a syndromic form of short stature with elevated T3/T4 ratio. Disorders of signalling of various paracrine factors (FGFs, BMPs, WNTs, PTHrP/IHH, and CNP/NPR2) or genetic defects affecting cartilage extracellular matrix usually cause disproportionate short stature. Heterozygous NPR2 or SHOX defects may be found in ∼3% of short children, and also rasopathies (e.g., Noonan syndrome) can be found in children without clear syndromic appearance. Numerous other syndromes associated with short stature are caused by genetic defects in fundamental cellular processes, chromosomal abnormalities, CNVs, and imprinting disorders.