Endocrine Abstracts

Case history: Two unrelated male patients were referred for evaluation of short stature. The first patient aged 16.5 years, had a birth weight of 2.6 kg at term (BWSDS -2.4), height 153 cm (HSDS -3.2) at referral and normal BMI SDS of 0.6. He had early postnatal hypoglycemia, which was conservatively managed, but no other significant clinical history. He had relative macrocephaly and disproportionate short stature. His mother was also short with a similar phenotype (height 147.6 cm, HSDS -2.4). The second patient aged 14.6 years, had a normal BW of 3.7 kg at term (BWSDS 0.2), height at referral was 155 cm (SDS -2.7) and BMI SDS was -1.5. There were no dysmorphic features.

Investigations: Baseline serum analyses were unremarkable for both patients. A skeletal survey of the first patient showed borderline mesomelic shortening in the upper limbs, no evidence of Madelung deformity and a slight hyperlordosis. His IGF-I was 501ng/ml (SDS +2.15), and GH binding protein (GHBP) was 467pM (NR 154-1073pM). The second patient had a high peak GH level (57.5µg/l), low IGF-I (<25ng/ml; -3.0 SDS) and elevated GHBP of 3366pM (NR 154-1073pM).

Results and treatment: Genetic analysis performed using our custom short stature whole genome panel identified two novel heterozygous GHR variants (c.876-15T>G (MUT1) and c.902T>G (MUT2)). Segregation studies confirmed MUT1 was maternally inherited and MUT2 arose de-novo. In vitro splicing assays confirmed both GHR variants activate the same alternative splice acceptor site resulting in abnormal splicing and exclusion of 26 base pairs of GHR exon 9. Western blotting confirmed both variants produced truncated GHR proteins which exerted a dominant negative (DN) effect with blunted GHR signalling. Comprehensive in vitro characterisation using NanoBiT complementation assays revealed both mutant GHR dimers exhibited increased cell surface expression and GHBP production compared to wildtype (WT) GHR. This resulted in GH sequestration and reduction in its availability to bind/signal via WT GHRs leading to short stature. RhIGF-1 treatment could not be initiated in these patients due to their advanced bone age at the time of diagnosis.

Conclusions and Discussion points: We identified two novel DN GHR variants which expand the GHI spectrum. Heterozygous defects in the intracellular domain of GHR should be considered in cases with a mild-moderate (non-classical) GHI phenotype. Patients with non-classical GHI have a varied phenotype which makes clinical assessment challenging. Early incorporation of genetic analysis in the assessment of short stature enhances diagnosis and enables timely access to treatment.