Michael A. Derr

Familial Short Stature Caused by Haploinsufficiency of the Insulin-Like Growth Factor I Receptor due to Nonsense-Mediated Messenger Ribonucleic Acid Decay

BACKGROUND IGF-I, essential for normal human growth in utero and postnatally, mediates its effects through the IGF-I receptor (IGF1R), a widely expressed, cell surface tyrosine kinase receptor. Five cases of heterozygous mutations in the IGF1R gene have been identified in patients with varying degrees of intrauterine and postnatal growth retardation. OBJECTIVE The objective of the study was the analysis of the IGF1R gene in a short-statured patient and his affected family members. PATIENT The male patient, with a height of -3.1 sd score (SDS; aged 12 yr), had normal circulating levels of GH binding protein, IGF-I, and IGF binding protein-3. His mother (-4.6 SDS), one of his siblings (-1.94 SDS), and several other maternal family members were also short statured. RESULTS The patient, his mother, and the short-statured sibling carry a novel heterozygous 19-nucleotide duplication within exon 18 of the IGF1R gene, which introduces a premature termination codon at codon 1106 of the IGF1R open reading frame on one allele. Analyses of the primary dermal fibroblasts derived from the patient and family members indicated that the IGF1R mRNA expressed from the mutant allele was degraded through the nonsense-mediated mRNA decay pathway, resulting in reduced amount of wild-type IGF1R protein and, subsequently, diminished activation of the IGF1R pathway. CONCLUSIONS The mutation results in haploinsufficiency of IGF1R protein due to nonsense-mediated mRNA decay and is associated with familial short stature.

Severe Short Stature Caused by Novel Compound Heterozygous Mutations of the Insulin-Like Growth Factor 1 Receptor (IGF1R)

CONTEXT IGF-I, essential for normal human growth in utero and postnatally, mediates its effects through the IGF-I receptor (IGF1R). More than nine heterozygous mutations, including one compound heterozygous mutation, of the IGF1R gene have been reported in patients with varying degrees of intrauterine and postnatal growth retardation. OBJECTIVE The objective of the study was the analysis of the IGF1R gene in a short-statured patient. PATIENT The male patient, with a height of -5.91 sd score (aged 20.3 yr), had consistently elevated circulating serum concentrations of IGF-I. A diagnosis of antibody-negative insulin-requiring diabetes was made at age 14 yr. His deceased sister was also severely short statured (-3.75 sd score). RESULTS The patient and his sister carried novel, compound heterozygous IGF1R missense mutations, E121K (exon 2) and E234K (exon 3), inherited from the mother and father, respectively. In vitro reconstitution studies demonstrated that neither the E121K nor E234K mutation affected IGF1R prepeptide expression, but levels of the proteolytically cleaved α- and β-subunit were consistently low. As a consequence, each IGF1R variant exhibited significantly reduced IGF-I-induced signal transduction. Correlating to these studies, expression of functional IGF1R and the IGF-I-induced activation of the IGF1R pathway were markedly reduced in the primary dermal fibroblasts established from the patient. CONCLUSIONS Only the second compound heterozygous IGF1R mutations to be identified, the p.E121K/E234K variant is the cause of intrauterine growth retardation and the most severe postnatal growth failure described to date in a patient with IGF1R defects. Whether the mutant IGF1R also contributes to the diabetic phenotype, however, remains to be determined.

Large-Scale Pooled Next-Generation Sequencing of 1077 Genes to Identify Genetic Causes of Short Stature

CONTEXT The majority of patients presenting with short stature do not receive a definitive diagnosis. Advances in genetic sequencing allow for large-scale screening of candidate genes, potentially leading to genetic diagnoses. OBJECTIVES The purpose of this study was to discover genetic variants that contribute to short stature in a cohort of children with no known genetic etiology. DESIGN This was a prospective cohort study of subjects with short stature. SETTING The setting was a pediatric endocrinology and genetics clinics at an academic center. PATIENTS A total of 192 children with short stature with no defined genetic etiology and 192 individuals of normal stature from the Framingham Heart Study were studied. INTERVENTION Pooled targeted sequencing using next-generation DNA sequencing technology of the exons of 1077 candidate genes was performed. MAIN OUTCOME MEASURES The numbers of rare nonsynonymous genetic variants found in case patients but not in control subjects, known pathogenic variants in case patients, and potentially pathogenic variants in IGF1R were determined. RESULTS We identified 4928 genetic variants in 1077 genes that were present in case patients but not in control subjects. Of those, 1349 variants were novel (898 nonsynonymous). False-positive rates from pooled sequencing were 4% to 5%, and the false-negative rate was 0.1% in regions covered well by sequencing. We identified 3 individuals with known pathogenic variants in PTPN11 causing undiagnosed Noonan syndrome. There were 9 rare potentially nonsynonymous variants in IGF1R, one of which is a novel, probably pathogenic, frameshift mutation. A previously reported pathogenic variant in IGF1R was present in a control subject. CONCLUSIONS Large-scale sequencing efforts have the potential to rapidly identify genetic etiologies of short stature, but data interpretation is complex. Noonan syndrome may be an underdiagnosed cause of short stature.

Three Novel IGFALS Gene Mutations Resulting in Total ALS and Severe Circulating IGF-I/IGFBP-3 Deficiency in Children of Different Ethnic Origins

Background/Aims: To date, four mutations in the IGFALS gene have been reported. We now describe two children of different ethnic background with total acid-labile subunit (ALS) and severe circulating IGF-I/IGFBP-3 deficiencies resulting from three novel mutations in the IGFALS gene. Patients/Methods: Serum and DNA of patients were analyzed. Results: Case 1 is a 12-year-old boy of Mayan origin. Case 2 is a 5-year-old girl of Jewish/Eastern European (Polish, Russian, Austrian-Hungarian)/Icelandic/European (French, English) ancestry. The reported cases had moderate short stature (–2.91 and –2.14 SDS, respectively), nondetectable serum ALS and extremely low serum concentrations of IGF-I and IGFBP-3. Case 1 harbored a novel homozygous 1308_1316 dup9 mutation in a highly conserved leucine-rich repeat (LRR) 17 motif of exon 2, representing an in-frame insertion of 3 amino acids, LEL. Case 2 harbored a novel heterozygous C60S/L244F mutation in exon 2, located within a highly conserved LRR 1 and LRR 9, respectively. Conclusions: The identification of additional novel IGFALS mutations, resulting in severe IGF-I/IGFBP-3 and ALS deficiencies, supports IGFALS as a candidate gene of the GH/IGF system, implicated in the pathogenesis of primary IGF deficiency, and represents an important part of its differential diagnosis.

Identification of a Novel Heterozygous IGF1 Splicing Mutation in a Large Kindred with Familial Short Stature

Background/Aims: Insulin-like growth factor (IGF)-I is critical for normal human growth. Extremely rare homozygous mutations of the IGF1 gene severely impair intrauterine growth, intellectual development and postnatal growth. Case/Method: A young male presented with postnatal growth retardation (–4.0 height SDS). His serum IGF-I concentration was low (115 µg/l, –2.21 SDS) and increased minimally to 130 µg/l (–1.82 SDS) on GH therapy, and he was analyzed for defect(s) in the GH-IGF-I axis. Severe short stature could be traced back several generations. Results: From the proband and 4 other severely short-statured family members, two novel, heterozygous, variants were identified in the IGF1 gene: c.207G>A in exon 3 and c.402+1G>C in the donor splice site of intron 4. The IGF1 gene was normal in 11 normal stature family members, and, interestingly, in 5 other short-statured family members. Study of IGF1 mRNA indicated c.402+1G>A induced splicing out of exon 4, leading to a predicted frameshift and protein truncation. Conclusions: A novel heterozygous IGF1 splicing variant is associated with familial short stature in an extended family. Although it remains unclear whether this heterozygous mutation is the cause of the growth failure, the extreme rarity of IGF1 gene defects makes these cases of considerable interest.

A Novel Y332C Missense Mutation in the Intracellular Domain of The Human Growth Hormone Receptor Does Not Alter STAT5b Signaling: Redundancy of GHR Intracellular Tyrosines Involved in STAT5b Signaling

Background: The growth hormone receptor (GHR), upon binding with GH, induces JAK2-mediated phosphorylation of GHR intracellular tyrosines, which then recruit STAT5b. Aberrancies in STAT5b signaling, due to mutations in GHR or STAT5b genes, result in poor responses to GH and severe short stature. Objective: To evaluate and compare the role of a novel Y332C GHR variant identified in a patient with short stature to the other GHR intracellular tyrosines in the GHR-STAT5b signaling process. Results: Recombinant human GHR constructs carrying Y332C or single Y to F changes for each of the 7 intracellular tyrosines did not alter GH-induced GHR-STAT5b signaling in reconstitution studies. However, GH-induced STAT5b activation was specifically abrogated in an hGHR variant in which all 7 tyrosines were inactivated (MYF). When hGHR variants carrying single intracellular tyrosines were evaluated, STAT5b activation was comparable to that of wild-type hGHR only with variants carrying Y534, Y566 or Y627. Conclusion: We provide evidence that in human GHR, 3 intracellular tyrosines are critical and redundant in the GH-induced STAT5b signaling process. This redundancy may explain why an Y332C variant did not alter STAT5b signaling. Identification of missense variants in human GHR intracellular domain should be interpreted with caution and rigorously analyzed.

Current Issues on Molecular Diagnosis of GH Signaling Defects

The growth-promoting effects of GH are mediated primarily by regulating the biosynthesis of insulin-like growth factor (IGF)-1. The binding of circulating GH to the cell surface GH receptor (GHR) initiates signaling cascades, of which the signal transducer and activator of transcription (STAT)-5b pathway has proven, in both rodent models and human case studies, to be the most critical in regulating IGF-1 production. The identification of rare inactivating STAT5B mutations in children, whose severe postnatal growth retardation was associated with GH insensitivity (GHI) and IGF-1 deficiency, confirmed the importance of STAT5b in regulating IGF-1 gene expression. Unlike GHI due to mutations in the GHR gene, patients carrying STAT5B mutations often present with immune dysfunction that can lead to severe, life-threatening infections and chronic pulmonary disease, consistent with the fact that STAT5b is activated by multiple cytokines involved in immunity. The possibility of a STAT5b disorder should be considered, therefore, when children present with chronic infection and/or unexplained pulmonary disease concomitant with severe postnatal growth failure.

The Growth Hormone Receptor (GHR) c.899dupC Mutation Functions as a Dominant Negative: Insights into the Pathophysiology of Intracellular GHR Defects

CONTEXT GH insensitivity (GHI) is a condition characterized by pronounced IGF-I deficiency and severe short stature. We previously identified a novel compound heterozygous GH receptor (GHR) mutation, GHR:p.R229H/c.899dupC, in a patient presenting with GHI. The heterozygous p.R229H (prepeptide) variant was previously associated with GHI despite a lack of adequate functional studies. The novel heterozygous GHR:c.899dupC variant affects the critical JAK2-binding Box 1 region of the GHR intracellular domain; the duplication predicted a frameshift and early protein termination. OBJECTIVE The individual and synergistic effect(s) of the p.R229H and c.899dupC mutations on GHR function(s) were evaluated in reconstitution studies. RESULTS The recombinant human GHR (hGHR):p.R229H variant was readily expressed, and unexpectedly, GH-induced signal transducer and activator of transcription 5b (STAT5b) phosphorylation was comparable to that induced by wild-type hGHR. The truncated, immunodetected hGHR:c.899dupC variant, in contrast, was unresponsive to GH. To mimic a compound heterozygous state, the two variants were coexpressed, and strikingly, the presence of the hGHR:c.899dupC effectively abolished the GH-induced STAT5b activities that were observed with hGHR:p.R229H alone. Furthermore, hGHR:c.899dupC dose-dependently reduced the GH-induced STAT5b activities associated with hGHR:p.R229H. This dominant negative effect was also observed when hGHR:c.899dupC was coexpressed with wild-type hGHR. CONCLUSION The p.R229H variant, contrary to an earlier report, appeared to function like wild-type GHR and, therefore, is unlikely to cause GHI. The c.899dupC variant is a novel dominant negative mutation that disrupted normal GHR signaling and is the cause for the GHI phenotype of the reported patient.

IGF-I in Human Growth: Lessons from Defects in the GH-IGF-I Axis

The IGF system plays a critical role in all phases of human growth, including intrauterine, childhood and pubertal. The importance of IGF-I for both in utero as well as postnatal human growth is highlighted by rare human homozygous IGF1 mutations, which are characterized by intrauterine growth retardation (IUGR), microcephaly, mental retardation and severe postnatal growth failure. Clinical conditions of IGF-I resistance due to mutations in the IGF-I receptor (IGFIR) similarly lead to IUGR and postnatal growth retardation. Postnatal regulation of IGF-I production is predominantly GH dependent. Defects in the GH-IGF-I axis, including mutations in the GHR, STAT5B and IGFALS genes, lead to postnatal IGF deficiency and GH insensitivity. Patients are of normal birth size but present with severe postnatal growth failure, despite normal or elevated levels of GH. Other phenotypic features - immune deficiency for STAT5B defects and insulin insensitivity for IGFALS defects - are of note. Mutations identified have been predominantly recessive. The identification and assessment of genetic defects in the GH-IGF axis has greatly enhanced our understanding of the critical importance of IGF-I in human linear growth. Continued evaluations will facilitate better diagnosis and management of children presenting with abnormal growth and development.

Atypical GH Insensitivity Syndrome and Severe Insulin-Like Growth Factor-I Deficiency Resulting from Compound Heterozygous Mutations of the GH Receptor, Including a Novel Frameshift Mutation Affecting the Intracellular Domain

Background/Aims: GH insensitivity and IGF deficiency may result from aberrations of the GH receptor (GHR). We describe a 4-year-old child with modest growth failure and normal serum concentrations of GH-binding protein (GHBP), but clinical evidence of GH insensitivity. Method: Serum and DNA samples from the proband and his parents were analyzed. Results: The child had a height of –4 SD, elevated serum GH concentrations, abnormally low serum IGF-I and IGFBP-3 concentrations and normal GHBP concentrations. DNA analysis revealed compound heterozygosity for mutations of GHR, including a previously reported R211H mutation and a novel duplication of a nucleotide in exon 9 (899dupC), the latter resulting in a frameshift and a premature stop codon. Treatment with recombinant DNA-derived IGF-I resulted in growth acceleration. Conclusion: Mutations affecting the intracellular domain of the GHR can result in GH insensitivity and IGF deficiency, despite normal serum concentrations of GHBP. The presence of clinical and biochemical evidence of GH resistance is sufficient to consider the possibility of aberrations of the GHR, even in the presence of normal serum GHBP concentrations.