Endocrine Abstracts

Background: Pathogenic IGFI gene mutations causing childhood growth failure are extremely rare. Only five autosomal recessive mutations, one IGFI copy number variant and two heterozygous frameshift mutations are reported. Heterozygous missense IGFI mutations haven’t previously been described.

Objectives: To identify and functionally characterise a novel missense IGFI variant in a patient with postnatal growth failure and delayed bone age.

Patient and Methods: The patient had a normal birth weight (-0.2 SDS) but height -3.4 SDS and head circumference -1.6 SDS by 10.1 years. Bone age was delayed by 2.5 years. A high peak GH was observed upon glucagon stimulation (17.1mcg/L). Baseline IGF-I levels were low/normal (144micrograms/L; -1.3 SDS) and responded poorly (increase <15micrograms/L) during IGF-I generation testing. Patient DNA was sequenced on our unique short stature gene panel. Functional analysis was performed using immunocytochemistry and furin cleavage assays. HEK293T cells transfected with wildtype and mutant IGFI constructs containing FLAG and HA tags were assessed by confocal microscopy. Whole cell lysates from HEK293T cells transfected with streptomycin-tagged wildtype and mutant IGFI constructs were incubated with furin to assess cleavage at the target site. IGFI constructs designed for the furin assay lacked the signal peptide, preventing their secretion from the cell and enabling them to be harvested from the cell lysate.

Results: We identified a rare novel heterozygous IGFI variant (102813333C>T, c.356G>A, p.R119H; gnoMAD frequency 0.004%) which was predicted damaging by SIFT; CADD score 32. This variant alters the first amino acid of IGF-I E domain, the most critical residue for furin binding, which is highly conserved across species. Furin cleaves pro-IGF-I to mature IGF-I (E domain removal). Functional analysis showed that both wildtype and mutant IGF-1 are able to translocate to the endoplasmic reticulum. Furin cleavage assay showed reduced cleavage for mutant p.R119H IGF-I compared to wildtype.

Conclusions: We report the first missense variant identified at the critical furin cleavage site, impairing the cleavage of pro-IGF-I to mature IGF-I. Our findings suggest the novel variant impairs pro-IGF-I cleavage, reducing mature circulating IGF-I levels. Pro-IGF-I is likely less biologically active than mature IGF-I, resulting in functional IGF-I deficiency and postnatal growth failure.