A novel mutation in IFITM5, encoding BRIL, impairs osteoblast production of PEDF and causes atypical type VI osteogenesis imperfecta

Adi Reich; Charles R Farber; Aileen M Barnes; Patricia Becerra; Frank Rauch; Wayne A Cabral; Alison Bae; Francis H Glorieux; Thomas L Clemens; Joan C Marini

doi:10.1530/boneabs.3.PP187

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Bone Abstracts (2014) 3 PP187 | DOI: 10.1530/boneabs.3.PP187

ECTS2014 Poster Presentations Genetics (11 abstracts)

A novel mutation in IFITM5, encoding BRIL, impairs osteoblast production of PEDF and causes atypical type VI osteogenesis imperfecta

Adi Reich , Charles R Farber ² , Aileen M Barnes ¹ , Patricia Becerra ³ , Frank Rauch ⁴ , Wayne A Cabral ¹ , Alison Bae ¹ , Francis H Glorieux ⁴ , Thomas L Clemens ⁵ & Joan C Marini ¹

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¹Bone and Extracellular Matrix Branch, NICHD/NIH, Bethesda, MD, USA; ²Center for Public Health Genomics and Departments Public Health Sciences, Biochemistry and Molecular Genetics, UVA, Charlottesville, VA, USA; ³Section on Protein Structure and Function, LRCMB, NEI, NIH, Bethesda, MD, USA; ⁴Shriners Hospital for Children and McGill University, Montreal, Quebec, Canada; ⁵Department Orthopaedic Surgery, Johns Hopkins School of Medicine, Baltimore, MD, USA.

Osteogenesis imperfecta (OI) type V is caused by a unique dominant mutation (c.−14C>T) in IFITM5, which encodes BRIL, a transmembrane ifitm-like protein most strongly expressed in osteoblasts, while type VI OI is caused by recessive null mutations in SERPINF1, encoding pigment epithelium-derived factor (PEDF). We identified a 25-year-old woman with severe OI, whose dermal fibroblasts and cultured osteoblasts displayed minimal secretion of PEDF, but whose serum PEDF was in the normal range. Her SERPINF1 sequences were normal despite bone histomorphometry typical of type VI OI, and elevated childhood serum alkaline phosphatase. We performed exome sequencing on the proband, both parents and an unaffected sibling. IFITM5 emerged as the candidate gene from bioinformatics analysis. The de novo IFITM5 mutation was confirmed in one allele of the proband, causing a p.S40L substitution in the BRIL intracellular domain, but was absent in unaffected family members. IFITM5 expression was normal in proband fibroblasts and osteoblasts, as was BRIL protein level in proband osteoblasts on western blot and in permeabilized proband osteoblasts by microscopy. Notably, SERPINF1 expression was decreased in proband osteoblasts and PEDF was barely detectable in conditioned media of proband cells. Expression and secretion of type I collagen was similarly decreased in proband osteoblasts, confirming that the proband’s OI was collagen-related. Osteoblasts with the S40L mutation also had deceased expression of alkaline phosphatase and osteocalcin, as seen in primary PEDF defects. In contrast, osteoblasts from typical type V OI, with five residues added to the N-terminus of BRIL, have increased SERPINF1 expression and PEDF secretion during osteoblast differentiation. Together, these data suggest i) that the type V OI and p.S40L BRIL are gain- and loss-of-function mutations respectively, and ii) that BRIL and PEDF have a relationship that connects the genes for types V and VI OI and their roles in bone mineralization.