A novel homozygous missense mutation in the insulin receptor gene results in an atypical presentation of Rabson-Mendenhall syndrome

INSR novel mutations identified in a Chinese family with severe INSR-related insulin resistance syndromes: A case report

RATIONALE

Severe insulin receptor gene (INSR)-related insulin resistance syndromes (SIR) include Donohue syndrome (DS), Rabson-Mendenhall syndrome (RMS), and type A insulin resistance. The incidence of DS is about 1 in 4 million births. We identified novel INSR mutations (c.2246delG and c.2646 + 5G > A) in a patient with SIR, which expanded the variant spectrum and helped to improve the understanding of the diagnosis and treatment of this condition.

PATIENT CONCERNS

A 10-year-old Chinese boy was admitted to the hospital for deepening skin color. He presented with growth retardation, peculiar facial features, acanthosis nigricans, hypertrichosis, extremely high insulin levels, fasting hypoglycemia, and postprandial hyperglycemia, Whole-exome gene testing suggested compound heterozygous mutations in INSR (c.2246delG and c.2646 + 5G > A).

DIAGNOSIS

The diagnosis was SIR. What's more, based on the phenotypic and biographical results, this child did not present typical RMS and DS but rather an intermediate phenotype between the 2 conditions.

INTERVENTIONS

On the basis of a sensible diet and exercise, the patient was prescribed metformin (250 mg) at breakfast and lunch, which was increased to 500 mg after 1 month.

OUTCOMES

After 2 months of treatment, the patient's glycated hemoglobin (HbA1c) levels decreased to 6% but his insulin resistance did not improve significantly.

LESSONS

In children who are not obese but with severe insulin resistance, growth retardation, hirsutism, and hyperglycemia, genetic testing should be performed for early diagnosis, active treatment, and follow-up.

Insulin-Responsive Transcription Factors

The hormone insulin executes its function via binding and activating of the insulin receptor, a receptor tyrosine kinase that is mainly expressed in skeletal muscle, adipocytes, liver, pancreatic β-cells, and in some areas of the central nervous system. Stimulation of the insulin receptor activates intracellular signaling cascades involving the enzymes extracellular signal-regulated protein kinase-1/2 (ERK1/2), phosphatidylinositol 3-kinase, protein kinase B/Akt, and phospholipase Cγ as signal transducers. Insulin receptor stimulation is correlated with multiple physiological and biochemical functions, including glucose transport, glucose homeostasis, food intake, proliferation, glycolysis, and lipogenesis. This review article focuses on the activation of gene transcription as a result of insulin receptor stimulation. Signal transducers such as protein kinases or the GLUT4-induced influx of glucose connect insulin receptor stimulation with transcription. We discuss insulin-responsive transcription factors that respond to insulin receptor activation and generate a transcriptional network executing the metabolic functions of insulin. Importantly, insulin receptor stimulation induces transcription of genes encoding essential enzymes of glycolysis and lipogenesis and inhibits genes encoding essential enzymes of gluconeogenesis. Overall, the activation or inhibition of insulin-responsive transcription factors is an essential aspect of orchestrating a wide range of insulin-induced changes in the biochemistry and physiology of insulin-responsive tissues.

Immediate-early transcriptional response to insulin receptor stimulation

Insulin binding to the insulin receptor triggers intracellular signaling cascades involving the activation of protein and lipid kinases. As a result, multiple biological functions of the cells are changed. Here, we analyzed the regulation and signaling cascades leading to insulin-induced activation of the stimulus-responsive transcription factors. For the analyses, we used chromatin-embedded reporter genes having a cellular nucleosomal organisation, and fibroblasts expressing human insulin receptors (HIRcB cells). The results show that stimulation of the insulin receptor induced the expression of the transcription factor Egr-1. Attenuation of Egr-1 promoter activation was observed following expression of a dominant-negative mutant of the ternary complex factor Elk-1. These data were corroborated by experiments showing that insulin receptor stimulation increased the transcriptional activation potential of Elk-1. In addition, the transcriptional activity of AP-1 was significantly elevated in insulin-stimulated HIRcB cells. Expression of the dominant-negative mutant of Elk-1 reduced insulin-induced activation of AP-1, indicating that Elk-1 controls both serum response element and AP-1-regulated transcription. Moreover, we show that stimulation of the insulin receptor activates cyclic AMP response element (CRE)-controlled transcription, involving the transcription factor CREB. Insulin-induced transcription of Elk-1 and CREB-controlled reporter genes was attenuated by overexpression of MAP kinase phosphatase-1 or a constitutively active mutant of calcineurin A, indicating that both phosphatases are part of a negative feedback loop for reducing insulin-mediated gene transcription. Finally, we show that expression of the adenoviral protein E1A selectively reduced CRE-mediated transcription following stimulation of the insulin receptor. These data indicate that insulin-regulated transcription of CRE-containing genes is under epigenetic control.

Insulin Receptor Trafficking: Consequences for Insulin Sensitivity and Diabetes

Insulin receptor (INSR) has been extensively studied in the area of cell proliferation and energy metabolism. Impaired INSR activities lead to insulin resistance, the key factor in the pathology of metabolic disorders including type 2 diabetes mellitus (T2DM). The mainstream opinion is that insulin resistance begins at a post-receptor level. The role of INSR activities and trafficking in insulin resistance pathogenesis has been largely ignored. Ligand-activated INSR is internalized and trafficked to early endosome (EE), where INSR is dephosphorylated and sorted. INSR can be subsequently conducted to lysosome for degradation or recycled back to the plasma membrane. The metabolic fate of INSR in cellular events implies the profound influence of INSR on insulin signaling pathways. Disruption of INSR-coupled activities has been identified in a wide range of insulin resistance-related diseases such as T2DM. Accumulating evidence suggests that alterations in INSR trafficking may lead to severe insulin resistance. However, there is very little understanding of how altered INSR activities undermine complex signaling pathways to the development of insulin resistance and T2DM. Here, we focus this review on summarizing previous findings on the molecular pathways of INSR trafficking in normal and diseased states. Through this review, we provide insights into the mechanistic role of INSR intracellular processes and activities in the development of insulin resistance and diabetes.

Mecasermin in Insulin Receptor-Related Severe Insulin Resistance Syndromes: Case Report and Review of the Literature

Mutations in the insulin receptor (INSR) gene underlie rare severe INSR-related insulin resistance syndromes (SIR), including insulin resistance type A, Rabson–Mendenhall syndrome and Donohue syndrome (DS), with DS representing the most severe form of insulin resistance. Treatment of these cases is challenging, with the majority of DS patients dying within the first two years of life. rhIGF-I (mecasermin) has been reported to improve metabolic control and increase lifespan in DS patients. A case report and literature review were completed. We present a case involving a male patient with DS, harbouring a homozygous mutation in the INSR gene (c.591delC). Initial rhIGF-I application via BID (twice daily) injection was unsatisfactory, but continuous subcutaneous rhIGF-I infusion via an insulin pump improved weight development and diabetes control (HbA1c decreased from 10 to 7.6%). However, our patient died at 22 months of age during the course of a respiratory infection in in Libya. Currently available data in the literature comprising more than 30 treated patients worldwide seem to support a trial of rhIGF-I in SIR. rhIGF-I represents a treatment option for challenging SIR cases, but careful consideration of the therapeutic benefits and the burden of the disease is warranted. Continuous application via pump might be advantageous compared to single injections.