A novel heterozygous IGF-1 receptor mutation associated with hypoglycemia

Abstract

Mutation in the insulin-like growth factor-1 receptor (IGF1R) gene is a rare cause for intrauterine and postnatal growth disorders. Patients identified with IGF1R mutations present with either normal or impaired glucose tolerance. None of the cases described so far showed hypoglycemia. We aimed to identify the genetic basis for small for gestational age, short stature and hypoglycemia over three generations in one family. The proband, a 9-year-old male, presented in infancy with recurrent hypoglycemic episodes, symmetric intrauterine growth retardation and postnatal growth retardation. Blood DNA samples from the patient, his parents, a maternal sister and maternal grandmother underwent Sanger sequencing of the IGF1R gene. Primary skin fibroblast cultures of the patient, his mother and age- and sex-matched control donors were used for gene expression and receptor functional analyses. We found a novel heterozygous mutation (c.94 + 1g > a, D1105E) affecting the splicing site of the IGF1R mRNA in the patient, his mother and his grandmother. Primary fibroblast cultures derived from the patient and his mother showed reduced proliferation and impaired activation of the IGF1R, evident by reduced IGF1R and AKT phosphorylation upon ligand binding. In conclusion, the newly identified heterozygous missense mutation in exon 1 of IGF1R (D1105E) results in impaired IGF1R function and is associated with small for gestational age, microcephaly and abnormal glucose metabolism. Further studies are required to understand the mechanisms by which this mutation leads to hypoglycemia.

Keywords: IGF-1 receptor (IGF1R); IUGR; SGA; hypoglycemia; insulin-like growth factor-1 (IGF-1).

Figure 1

Pedigree of the index patient’s (arrow) family and known relatives. Gray symbols indicate subjects with diagnosed type 2 diabetes mellitus; black symbols mark individuals with present or past hypoglycemia. Below the symbols, height SDS is given. Rods – subjects for whom genetic analyses of the IGF1R were performed. Individuals carrying the heterozygous c.94+1g > a IGF1R mutation are indicated by asterisks (*).

Figure 2

Serum IGF-1 levels of the index case, expressed as SDS for age and gender.

Figure 3

Western blot analysis of IGF1R-mediated signaling in IGF1R-mutant patient and mother. Fibroblast cultures of proband, mother and control were serum-starved for 24 h, after which they were treated with IGF-1 (50 ng/mL) for 10 min. At the end of the incubation period, cells were lysed and electrophoresed through SDS-PAGE. Membranes were incubated with antibodies against total- and phospho-IGF1R, AKT and ERK. Tubulin levels were measured as a loading control.

Figure 4

Proliferation assays fibroblasts of patient and mother. Fibroblasts were grown in serum-containing media for 24 h, after which proliferation tests were performed using an XTT assay. Values on the y-axis represent arbitrary units of absorbance. A value of 100% was assigned to the number of cells in control cultures. The bars represent mean ± s.e.m. of three independent experiments, performed each in triplicate samples. *P < 0.05 vs control.