Endocrine Abstracts

Background: Silver Russell syndrome (SRS) is a heterogeneous disorder characterised by intrauterine and post-natal growth retardation, relative macrocephaly, protruding forehead, feeding difficulties and body asymmetry. Variants in HMGA2 are a rare cause of SRS and despite strong evidence for the crucial role of HMGA2 in growth regulation, the underlying mechanism underlying growth retardation has, thus far, not been elucidated.

Methods: Our cohort of undiagnosed patients with SRS were genotyped. A missense variant, c.166A>G (p.K56E) predicted to abrogate DNA-binding was created by mutagenesis of an N-terminal FLAG tagged-HMGA2 cDNA clone. HMGA2-WT (wild type) and variant expression and nuclear localisation were assessed by immunoblotting and confocal microscopy in HEK293 cells which lack endogenous HMGA2. Markers of the integrated stress response and STAT signalling were probed by immunoblotting. Transcript levels of IGF2 and PLAG1 were quantified in wild-type and variant mRNA.

Results: Our cohort of six heterogeneous SRS patients with variable height SDS (range -3.2 to -3.9) and IGF-1 SDS (range from -1.9 to +4.4) were found to have deleterious variants in HMGA2 with differing functional impacts. Microcephaly appeared to be a highly penetrant and consistent feature in these subjects highlighting the pleiotropic nature of variants in HMGA2 since mutation type often does not predict SRS phenotypic expression. Nuclear localisation was attenuated for all variants except the well-expressed, c.166A>G (p.K56E), that retained functional DNA-binding AT hook domains. Transient expression of this variant in HEK293 cells was associated with downregulation of IGF2 mRNA transcript levels when compared to HMGA2 wild type. PLAG1 levels were unaltered. Immunoblotting revealed significant upregulation of endoplasmic reticulum stress markers, phosphorylated STAT3 and phosphorylated eIF2α (serine 51).

Conclusion: Our data has uncovered a novel, substantial role for HMGA2 in modulating cellular chronic stress responses. Loss of functional HMGA2 leads to STAT3 hyperactivation, an event known to inhibit human growth and concomitant potentiation of eIF2α phosphorylation, a key initiator of macroautophagy. Our in vitro model suggests that HMGA2 is crucial for IGF2 transcription and its absence may potentiate macroautophagy, a response to misfolded proteins.