Object: IGF2 is a paternally expressed gene involved in the development of Silver-Russell syndrome (SRS). Here, we report six Japanese patients with IGF2 mutations and compare clinical findings between patients with IGF2 mutations including literature cases and those with H19/IGF2:IG-DMR epimutations.
Patients: We recruited six Japanese patients with IGF2 mutations. The major reason for molecular studies was Ectrodactyly, fetal growth restriction (FGR) and disorder of sex development (DSD) in case 1, multiple congenital anomalies/mental retardation (MCA/MR) in case 2, SRS and DSD in case 3, SRS in case 4, ectrodactyly and FGR in case 5, and FGR and extremely high serum IGF-I value in case 6.
Molecular Studies: We first performed  pyrosequencing-based methylation analysis for SRS-related DMRs in cases 3 and 4,  standard Sanger direct sequencing for IGF1 and IGF1R in case 6, and  array comparative genomic hybridization analysis to examine pathogenic copy-number variants in cases 16, using leukocyte genomic DNA samples. However, no genetic or epigenetic abnormality was identified. Thus, we carried out comprehensive mutation analyses with TruSight One Sequencing Kit, a custom-made HaloPlex Target Enrichment System, and whole exome sequencing.
Results: We identified various IGF2 mutations, i.e., case 1 with a frameshift mutation p.(Leu37Glnfs*31), case 2 with a splice site mutation (c.61G>C) leading to skipping of exon 2, and cases 36 with different missense mutations (p.(Cys70Tyr), p.(Cys71Arg), p.(Cys33Ser), and p.(Cys45Ser)) affecting cysteine residues involved in the S-S bindings. All the mutations resided on the paternally inherited allele, and the mutation of case 6 was present in a mosaic condition. Phenotypic comparison between apparently non-mosaic patients with IGF2 mutations identified to date and multiple patients with H19/IGF2:IG-DMR epimutations revealed that, while overall phenotype was similar between the two groups, IGF2 mutations resulted in SRS with high Netchine-Harbison score (≥ 5/6), low frequency of hemihypoplasia, high frequency of feeding difficulty and/or reduced body mass index, and mild degree of relative macrocephaly, together with occasional development of severe limb malformations, high frequency of cardiovascular anomalies and developmental delay, and low serum IGF-II values.
Conclusion: The results are primarily explained by the H19/IGF2:IG-DMR methylation pattern-dependent paternal IGF2 expression in most tissues and the tissue-specific promoter-dependent biparental IGF2 expression in the brain and liver, and by the mosaic condition of H19/IGF2:IG-DMR epimutations and the non-mosaic condition of IGF2 mutations. The present study indicates that IGF2 mutations are associated with characteristic clinical features.
19 Sep 2019 - 21 Sep 2019