Endocrine Abstracts

Background: Homozygous mutations in STAT5B result in GH insensitivity and immune dysfunction. Heterozygous dominant negative mutations have not been described.

Aims and objectives: To assess STAT5B sequence in children selected for a phenotype suggestive of Stat5b deficiency. To further characterize genomic STAT5B variants in two families.

Methods: Selection of children from a tertiary Paediatric Endocrine Centre with short stature and biochemical features of GH insensitivity, with additional features of Stat5b deficiency (raised prolactin, frequent infections, lung pathology, or arthritis). Sanger sequencing of STAT5B from genomic DNA. Functional analysis of mutant STAT5B in HEK293 cells.

Results: Five children were selected that fulfilled selection criteria. A mutation in STAT5B was found in one child, whose brother subsequently presented with a similar phenotype. Another family was identified in a separate cohort of short children with features of GH insensitivity. Family 1: the index case grew at −2.9 S.D. from the age of 2 years. Investigations revealed IGF1 <25 ng/ml, IGFBP3 1.29 ng/ml (NR 0.8–3.9), prolactin 265–653 mU/l (NR 59–271), provoked GH-peak 17.3 μg/l, and normal GH-peaks on overnight sampling. A standard and extended three-step IGF1-generation test (2 weeks GH s.c. at 0.7, 1.4, and 2.4 mg/m² per day) showed a poor response. His brother had short stature (−2.9 S.D.), mild speech delay, eczema, undetectable IGF1, a GH peak of 13.9 μg/l and poor response in the IGF1-generation test. Both brothers had elevated IgE concentrations. Family histories were positive for short stature, eczema and transient hyperprolactinaemia. A heterozygous missense variant c.1433C>T (p.Ala478Val) was identified within the conserved STAT5B DNA-binding domain, and segregated with the phenotype. Family 2: male monozygotic twins presented at age 14 yrs with short stature (−5.3 SDS), eczema and a history of mild respiratory infections. Investigations revealed a provoked GH peak of 16.2 μg/l, low IGF1 (56 μg/l) and elevated IgE concentrations. rhIGF1 therapy led to modest catch-up growth. A de novo heterozygous variant (c.530A>C; p.Gln177Pro) was identified. Neither of the STAT5B variants are listed in control databases. Functional evaluation of the FLAG–STAT5B mutants indicated normal protein expression and phosphorylation but severely compromised nuclear translocation or transcriptional function compared to WT. The variants inhibited either translocation and/or transcriptional activity of WT FLAG–STAT5B, suggesting a dominant-negative mode of action.

Conclusion: This is the first description of dominant-negative STAT5B mutations in subjects with short stature and mild GH insensitivity. Eczema may also be related to impaired STAT5B function.