ESPE Abstracts

Background: Silver Russell syndrome (SRS) is genetically heterogenous and around 30% of patients with clinical SRS have no genetic diagnosis. Mutations in HMGA2 have recently been identified causing growth failure and an SRS-like phenotype. Despite strong evidence of the crucial role of HMGA2 in growth across species, the mechanism of action of HMGA2 in human linear growth is unclear.

Objective: Identify and functionally characterise an HMGA2 mutation in a patient with growth failure and SRS features.

Methods: We used custom bioinformatic pipelines to filter genetic data generated from our novel targeted genome short stature gene panel. Comprehensive functional analysis of a novel HMGA2 missense variant was performed using Immunocytochemistry and Electrophoretic Mobility Shift Assays (EMSAs). Our collaborators at The University of Mannitoba, Winnepeg generated novel transgenic mice with targeted mutations of Hmga2 using CRISPR/Cas technology.

Results: We identified several novel heterozygous HMGA2 mutations in patients with short stature and SRS features including the first heterozygous missense HMGA2 mutation c.166A>G, p.(Lys56Glu) in a patient with pre- and post-natal growth failure; low BMI, triangular face, high arched palate and 3/6 NHCSS. The variant was inherited from her mother who had short stature (height -3.5 SDS) and similar facial features. This novel variant was of particular interest as it resides in a critically important region of HMGA2, adjacent to the second AT hook or DNA binding region. This missense variant substitutes lysine, a positively charged amino acid crucial for DNA binding at target sites, for glutamic acid, a negatively charged amino acid. Immunocytochemistry identified that the wild-type and mutant c.166A>G constructs were able to translocate to the nucleus. EMSAs confirmed reduced binding of the mutant c.166A>G HMGA2 protein to target DNA sequences. Transgenic mice harbouring homozygous c.166A>G p.(Lys56Glu) mutations (Hmga2^K56E) displayed dysmorphic facial features similar to the phenotypes observed in SRS children. 3D reconstructed serial micro-CT images showed an overall shortening of the viscerocranium and asymmetry of the skull bones. Hmga2^K56E transgenic mice were fertile but small for gestational age and showed SRS-like dwarfism.

Conclusions: This novel c.166A>G mutation is the first heterozygous missense HMGA2 mutation identified in a patient with growth failure and SRS features. The mutant HMGA2 protein could translocate to the nucleus but binding to DNA target sites was impaired. A novel transgenic Hmga2^K56E mouse model recapitulated the SRS phenotype seen in our patient, confirming the critical functional importance of this amino acid residue.