Family with two novel in cis INSR gene variants: Phenotypic evolution with age

Abstract.

Pathogenic variants in the insulin receptor gene (INSR) cause a broad spectrum of metabolic disorders, from severe insulin resistance to milder phenotypes such as hyperinsulinemic hypoglycemia. We describe a preterm female neonate with persistent, asymptomatic hyperinsulinemic hypoglycemia, characterized by markedly elevated insulin and C-peptide levels with suppressed ketones. Genetic testing revealed two novel, maternally inherited heterozygous INSR variants in cis, c.3541A>C p.(Thr1181Pro) and c.3566A>C p.(Tyr1189Ser), located in the tyrosine kinase domain. The infant remains clinically stable without pharmacological treatment and demonstrates normal growth and development. Her 7-yr-old brother, carrying the same variants, is asymptomatic but shows relatively increased insulin responses during oral glucose tolerance testing, while the mother developed insulin-resistant diabetes in early adulthood. This family highlights the age-dependent phenotypic variability associated with the same INSR variants, ranging from neonatal hyperinsulinemic hypoglycemia to childhood hyperinsulinemia with euglycemia and adult insulin resistance. These observations expand the clinical spectrum of heterozygous INSR variants and underscore the influence of residual receptor activity, age, and metabolic demands on disease expression. Early genetic testing in infants with even mild hyperinsulinemic hypoglycemia is important for prognosis, family screening, and long-term follow-up to detect evolving insulin resistance.

Highlights

● Novel in cis INSR variants show age-dependent metabolic effects.

● Phenotype ranged from neonatal hypoglycemia to adult diabetes.

● Genetic diagnosis guided family screening and clinical surveillance.

Introduction

The insulin receptor gene (INSR) encodes a transmembrane glycoprotein, essential for mediating the biological actions of insulin in insulin-responsive tissues such as skeletal muscles, adipose tissue and liver (1, 2). Upon activation, the receptor triggers intracellular signaling pathways that regulate glucose uptake, glycogen synthesis, lipid metabolism and cell growth. Pathogenic variants in INSR gene can impair receptor function and cause insulin resistance of variable severity. To date, INSR variants have been predominantly linked to three Mendelian disorders: the milder type A insulin resistance typically with autosomal dominant inheritance, and the more severe autosomal recessive Rabson–Mendenhall (RMS) and Donohue (DS) syndromes (3, 4).

Type A insulin resistance is estimated to affect about 1 in 100,000 individuals, and manifests later in life with variable insulin resistance, acanthosis nigricans, and hyperandrogenism, typically without obesity or lipoatrophy (2, 5). On the other hand, RMS and DS are exceedingly rare (~1:1,000,000) (4), but present early in life with profound hyperinsulinemia, growth failure, dysmorphic features, and severe metabolic complications, often leading to early mortality (6, 7). While INSR gene variants are classically associated with insulin resistance and these syndromes are well characterized, milder presentations with hyperinsulinemic hypoglycemia remain uncommon, suggesting a broader spectrum and variable expressivity of receptor dysfunction (8,9,10,11).

We present a preterm neonate with persistent hyperinsulinemic hypoglycemia carrying two novel, in cis heterozygous INSR variants. The patient presented in the neonatal period with asymptomatic hypoglycemia, representing an atypical presentation of INSR dysfunction. This case underscores the importance of considering INSR variants in infants with hyperinsulinemic hypoglycemia and expands the genetic landscape of insulin resistance.

Case Presentation

Ethics statement

Written informed consent was obtained from the parents for the genetic testing and for the publication of the clinical data included in this report of all participating family members.

Proband (Neonate)

The proband is a female preterm infant born at 32 + 2 weeks’ gestation from a dichorionic twin pregnancy to a mother with gestational diabetes and autoimmune thrombocytopenia. Delivery was performed by cesarean section due to maternal thrombocytopenia. Birth weight was 1270 g and Apgar scores were 4 and 8 at 1 and 10 min, respectively. The infant required respiratory support for five days (SNIPPV for 72 h and CPAP for 21 h), after which she maintained adequate oxygenation on room air.

During the first hour of life, she developed transient hypoglycemia (blood glucose: 38 mg/dL), which promptly resolved after a single intravenous glucose bolus (121 mg/dL). This episode was initially attributed to maternal gestational diabetes. On the 15^th day of life, routine laboratory testing again revealed low preprandial glucose levels, confirmed by repeated measurements. Continuous glucose monitoring was initiated, and nutritional support was intensified (switch to preterm formula, fortification with Maxijoule, and frequent feeding every 2 h). Despite these interventions, euglycemia could not be consistently maintained, with glucose values fluctuating between 22 mg/dL and 269 mg/dL.

A critical sample obtained during a hypoglycemic episode (22 mg/dL) on the 32^nd day revealed marked hyperinsulinemia (insulin 1148 mU/L, measured using a commercial chemiluminescent immunoassay, with a reference range for fasting insulin 3–25 mU/L and low cross-reactivity with proinsulin, less than 2–10%), elevated serum C-peptide levels (18.21 ng/mL), low ketones, and appropriately increased cortisol (30.8 µg/dL). A repeat insulin level measurement remained markedly elevated (532.3 mU/L). These findings confirmed the diagnosis of hyperinsulinemic hypoglycemia and genetic testing for hyperinsulinemic hypoglycemia genes was requested for the infant, her twin brother, her older brother and her parents.

Enteral feeding continued, initially with Maxijoule fortification, which was later discontinued because of excessive weight gain and intermittent hyperglycemia. Intravenous glucose infusion was introduced on the 34^th day of life, reaching a maximum rate of 14 mg/kg/min, and was gradually tapered off; intravenous glucose was discontinued completely by day 67. At the 68^th day of life, the infant was transferred to the pediatric department for ongoing monitoring while awaiting genetic results.

During this hospitalization, serial pre- and postprandial glucose monitoring documented multiple episodes of asymptomatic pre-prandial hypoglycemia, all of which resolved with feeding. A second critical sample during hypoglycemia (48 mg/dL) again demonstrated profound hyperinsulinemia (172.93 mIU/L).

The infant remained clinically stable without further need for intravenous glucose and was discharged after one week of hospitalization. At 8 mo of chronological age, she demonstrated normal growth and neurodevelopment, without recurrence of symptomatic hypoglycemia or need for pharmacologic therapy.

Mother

The proband’s mother had gestational diabetes requiring insulin therapy and autoimmune thrombocytopenia treated with intravenous immunoglobulin during pregnancy. Following genetic diagnosis, she underwent metabolic evaluation: HbA1c was 6.9%, and an oral glucose tolerance test (OGTT) yielded pathological results, consistent with insulin-resistant diabetes (Table 1: A). She displayed no overt clinical signs of insulin resistance (acanthosis nigricans, hirsutism, or menstrual irregularities). On specialist evaluation, metformin therapy was suggested. The mother was adopted, therefore, grandparental or siblings’ medical history and genotypes are unavailable.

Table 1. Oral glucose tolerance tests of mother (A) and sibling (B).

Sibling (7-yr-old brother)

The proband’s 7-yr-old brother carries the same INSR gene variants as the proband. He is asymptomatic, with normal body mass index, and evaluation, including an OGTT and HbA1c measurement, was performed solely as part of family screening (Table 1: B). Notably, OGTT revealed unexpectedly elevated insulin levels relative to the corresponding glucose concentrations, while HbA1c was normal (5.2%). No history of neonatal hypoglycemia or other metabolic disorders was reported.

Genetic analysis

Genetic analysis of the proband was performed at the Laboratory of Molecular Endocrinology, First Department of Pediatrics, “Aghia Sophia” Children’s Hospital, employing a targeted NGS panel including genes associated with Hyperinsulinemic Hypoglycemia and Monogenic Diabetes. Two heterozygous variants in exon 20 of the INSR gene were identified in the proband: the variant, c.3541A>C, p.(Thr1181Pro), classified as likely pathogenic (LP) according to the ACMG criteria, PM2 supporting, PP1 supporting, PP3 strong, PP4 supporting and the c.3566A>C, p.(Tyr1189Ser) classified as variant of uncertain significance (VUS), according to the ACMG criteria, PM2 supporting, PP1 supporting, PP3 moderate, PP4 supporting. Segregation analysis by targeted Sanger sequencing revealed maternal inheritance of both variants (in cis). Proband’s twin brother was carrying the wild type alleles, whereas the 7-yr-old brother was heterozygote for the same variants (Fig. 1). Both variants are considered novel as they have not been recorded in population databases and have not been previously reported in the literature related to neonatal hyperinsulinemic hypoglycemia thus highlighting a potential novel genotype-phenotype correlation.

Fig. 1.

Family tree. Squares denote males, circles denote females, the arrow indicates the proband.

Discussion

The family described herein illustrates the age-dependent clinical expression of the same INSR gene variants across three distinct life stages: the neonatal period, childhood, and adulthood. In the proband, these variants manifested early as asymptomatic hyperinsulinemic hypoglycemia during the neonatal period, representing an atypical and mild presentation of insulin receptor dysfunction. Despite the markedly low glucose level recorded during one episode, the infant remained clinically asymptomatic. Clinical manifestations of neonatal hypoglycemia do not always correlate with the absolute glucose concentration, and the duration of hypoglycemia is also an important determinant of symptom development (12). In our case, the hypoglycemia was promptly corrected, while the infant was managed in a neonatal intensive care setting with close monitoring. In addition, repeated exposure to hypoglycemia may attenuate neurogenic responses, a phenomenon described as hypoglycemia-associated autonomic failure (13), which has been mainly described in older children and adults, and may partly explain the absence of symptoms in our patient. In childhood, the same genetic defect was associated with euglycemia but relatively elevated insulin responses during an OGTT, suggesting insulin resistance. In adulthood, this phenotype presented as insulin-resistant diabetes, as observed in the mother. Together, these findings highlight a dynamic, evolving phenotype in which partial insulin receptor dysfunction initially presents with hyperinsulinemic hypoglycemia, transitions through a compensated stage, and ultimately culminates in insulin resistance and diabetes later in life.

The insulin receptor (INSR), a transmembrane tyrosine kinase, mediates insulin signaling and maintains glucose homeostasis (14). Variants in INSR gene can variably reduce receptor activity, resulting in a spectrum of metabolic effects from mild hyperinsulinemia to severe insulin resistance (15, 16). Complete loss-of-function mutations, recessively inherited, are classically associated with severe insulin resistance syndromes, such as Rabson–Mendenhall and Donohue syndromes, which present early in life with profound metabolic derangements (16). In contrast, heterozygous INSR gene variants may retain partial receptor activity and give rise to milder and more heterogeneous phenotypes, including hyperinsulinemia with preserved or fluctuating glucose levels. Consistent with this concept, although insulin is a key fetal growth factor, the proband’s birth weight was only mildly reduced for gestational age. This observation may reflect residual activity of the insulin receptor associated with heterozygous variants in the INSR gene, allowing partial preservation of insulin signaling during fetal development. Moreover, fetal growth is regulated by multiple pathways, including IGF-1–mediated signaling, which may partly compensate for reduced insulin receptor function. In addition, the maternal gestational diabetes, through increased transplacental glucose delivery and consequent stimulation of fetal insulin secretion, may also have contributed to the relatively preserved fetal growth.

A mild autosomal dominant form of hyperinsulinemic hypoglycemia associated with heterozygous INSR mutations was described by Højlund et al. in 2004 (11), in one of the earliest well-characterized reports of this atypical phenotype, involving affected family members with inappropriate hyperinsulinemia and recurrent hypoglycemia despite preserved or only mildly impaired glucose tolerance. Subsequent reports have described reactive or fasting hypoglycemia and delayed progression to insulin resistance or diabetes in individuals with heterozygous INSR gene variants (8, 10, 17,18,19,20,21). Our case, featuring two novel maternally inherited heterozygous INSR variants, p.(Thr1181Pro) and p.(Tyr1189Ser), located within the tyrosine kinase domain of the insulin receptor, further expands this phenotypic continuum, demonstrating how previously unreported variants can manifest across distinct life stages. Variant p.(Thr1181Pro) has been classified as LP and is probably consistent with impaired insulin signaling, contributing to the neonate’s hyperinsulinemic hypoglycemia. However, it is not possible to determine the contribution of the p.(Tyr1189Ser), classified as variant of unknown significance, to patient’s phenotype. In any case the effect of these 2 variants identified in cis and located within the tyrosine kinase domain of the insulin receptor, is most probably a dominant negative effect, which explains the mild and variable family phenotypes. Similarly to our case, a family carrying the INSR gene heterozygous variant p.(Met1180Lys) (c.3539T>A), also located within the tyrosine kinase domain and closely positioned to the variants described here in, has been reported to present similar phenotypic spectrum of neonatal hypoglycemia in the proband, gestational diabetes in the mother and type 2 diabetes mellitus in the maternal grandmother (19).

A limitation in our case is that anti-insulin receptor antibodies were not assessed in the mother. Given her history of autoimmune thrombocytopenia, a contribution from Type B insulin resistance mediated by such antibodies, which can manifest as either hypoglycemia or hyperglycemia (22), cannot be entirely excluded. Measurement of these antibodies in this patient would have been helpful to clarify any potential autoimmune contribution. Nevertheless, the identification of two novel heterozygous INSR variants provides a clear genetic explanation for the phenotypes observed in the proband, sibling, and mother.

Clinically, neither the neonate nor the 7-yr-old sibling required pharmacologic therapy, as hypoglycemia was mild or asymptomatic and glucose homeostasis was largely maintained. Metformin therapy was proposed for the mother, consistent with current recommendations for insulin-resistant diabetes, but was not initiated to allow observation of the natural course of the condition. Published management strategies for INSR‑related hyperinsulinemic hypoglycemia include supportive feeding protocols and diazoxide in resistant cases, as illustrated by recent neonatal reports (10), while metformin remains a first-line therapy for insulin resistance in adults, adjusted according to individual metabolic characteristics (9, 23). Long-term follow-up is recommended to monitor progression of metabolic complications and to adjust therapy as needed.

The family we have described highlights the critical role of genetic testing in infants presenting with hyperinsulinemic hypoglycemia, even when clinical manifestations are mild or intermittent. Identifying INSR gene variants early can shape prognosis, guide monitoring, and help anticipate progression to insulin resistance or diabetes later in life. Children carrying heterozygous INSR mutations may appear metabolically compensated during childhood, but as illustrated by the sibling in our report, subtle abnormalities in insulin secretion can be detected using OGTT. Therefore, long-term follow-up with regular metabolic assessment is recommended, including monitoring of fasting glucose, HbA1c, and insulin levels. Such an approach enables early detection of evolving insulin resistance and allows timely intervention with lifestyle or pharmacologic therapy as appropriate.

Conclusions

This report shows that the two novel in cis heterozygous INSR gene variants, p.(Thr1181Pro) and p.(Tyr1189Ser), can lead to an evolving metabolic phenotype over time, from asymptomatic hyperinsulinemic hypoglycemia in the neonatal period, through adequate glycemic regulation despite elevated insulin levels in childhood, to insulin-resistant diabetes in adulthood. Early genetic testing can help identify affected individuals and guide family screening. Regular long-term follow-up is important to detect evolving metabolic changes and to plan interventions if needed. These observations expand the understanding of INSR-related disorders and highlight the value of personalized monitoring and management across the lifespan.

Conflict of interests

All authors declare that they have no conflicts of interest.