Background: Sp7, also known as Osterix (Osx), is a zinc-finger transcription factor, essential for osteoblast differentiation and bone formation. While bone formation is absent in Osx knockout mice, human loss of function mutations lead to impaired bone formation and cause recessive osteogenesis imperfecta, type XII (OMIM 613849 ).
Case: The 5-year-old son of non-consanguineous parents presented with multiple long bone fractures due to bone fragility, scoliosis, remarkably thickened calvarium, craniosynostosis, and heterogeneity of long bone cortical thickness. Elevated serum alkaline phosphatase and Tartrate-resistant acid phosphatase (TRAP5b) indicated an increased bone turnover, while linear growth and psychomotor development remained unaffected.
Results: Iliac crest biopsy confirmed increased bone turnover with elevated osteoblast number, osteoid thickness, osteoclast number, and decreased bone matrix mineralization. Exome sequencing, confirmed by Sanger sequencing, on the proband and his parents, identified a de novo rare missense mutation in SP7 (c.926C>G:p.S309W) which was predicted to be deleterious by multiple silico analyses.
To test the impact of the SP7 variant, we infected mouse primary chondrocytes and mesenchymal stem cells with retrovirus expressing GFP (as a negative control), wild-type (WT) SP7, or the S309W SP7 variant, followed by induction of osteoblast differentiation. S309W SP7 resulted in an aberrant expression profile: elevated endogenous Sp7 and Col1a1 and decreased Sox9 and Col2a1 mRNA as compared both to GFP and WT, suggesting accelerated osteoblast differentiation, while decreased expression of Mmp13, Ibsp, and Bglap indicated impaired bone matrix formation.
Heterozygous knock-in mice carrying the S309W variant showed perinatal lethality, but a small number of mice were recovered. Micro-CT showed increased bone thickness in clavicles, ribs, and long bones. Trabecular bone density was decreased at the metaphyses but increased at the diaphyses. Marrow space was almost completely absent in the long bones in a mosaic heterozygous 20-week mouse.
Discussion: A previously reported frameshift mutation in SP7 in humans caused recessive osteogenesis imperfecta, and biallelic knockout in mice caused lack of bone formation. In contrast, our patient showed a markedly different dominant phenotype, with evidence of elevated osteoblast numbers and heterogeneously increased bone formation but defective osteoblast function. Mice with this mutation also showed a dominant phenotype with increased bone formation. Taken together, the findings suggest that the SP7 variant in this patient is not a simple loss-of-function mutation causing a failure of osteoblast differentiation but instead causes more complex alterations in osteoblast differentiation.
19 - 21 Sep 2019
European Society for Paediatric Endocrinology