title: A novel IFITM5 mutation in severe atypical osteogenesis imperfecta type VI impairs osteoblast production of pigment epithelium-derived factor.
abstract: Osteogenesis imperfecta (OI) types V and VI are caused, respectively, by a unique dominant mutation in IFITM5, encoding BRIL, a transmembrane ifitm-like protein most strongly expressed in the skeletal system, and 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 level was in the normal range. SERPINF1 sequences were normal despite bone histomorphometry consistent with 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, and was corroborated by membership in a murine bone co-expression network module containing all currently known OI genes. The de novo IFITM5 mutation was confirmed in one allele of the proband, resulting in a p.S40L substitution in the intracellular domain of BRIL but was absent in unaffected family members. IFITM5 expression was normal in proband fibroblasts and osteoblasts, and BRIL protein level was similar to control in differentiated proband osteoblasts on Western blot and in permeabilized mutant osteoblasts by microscopy. In contrast, SERPINF1 expression was decreased in proband osteoblasts; PEDF was barely detectable in conditioned media of proband cells. Expression and secretion of type I collagen was similarly decreased in proband osteoblasts; the expression pattern of several osteoblast markers largely overlapped reported values from cells with a primary PEDF defect. In contrast, osteoblasts from a typical case of type V OI, with an activating mutation at the 5'-terminus of BRIL, have increased SERPINF1 expression and PEDF secretion during osteoblast differentiation. Together, these data suggest that BRIL and PEDF have a relationship that connects the genes for types V and VI OI and their roles in bone mineralization.
authors: Charles R
authors: CR
authors: Farber
authors: Adi
authors: A
authors: Reich
authors: Aileen M
authors: AM
authors: Barnes
authors: Patricia
authors: P
authors: Becerra
authors: Frank
authors: F
authors: Rauch
authors: Wayne A
authors: WA
authors: Cabral
authors: Alison
authors: A
authors: Bae
authors: Aaron
authors: A
authors: Quinlan
authors: Francis H
authors: FH
authors: Glorieux
authors: Thomas L
authors: TL
authors: Clemens
authors: Joan C
authors: JC
authors: Marini
mesh_terms: D000328:Adult; D000469:Alkaline Phosphatase / Q000378:metabolism; D000595:Amino Acid Sequence; D001483:Base Sequence; D002454:Cell Differentiation / Q000235:genetics; D002648:Child; D002675:Child, Preschool; D024042:Collagen Type I / Q000378:metabolism; D018076:DNA, Complementary / Q000235:genetics; D059472:Exome / Q000235:genetics; D005136:Eye Proteins / Q000096:biosynthesis* / Q000378:metabolism; D005260:Female; D053263:Gene Regulatory Networks; D006801:Humans; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics*; D008969:Molecular Sequence Data; D009154:Mutation / Q000235:genetics*; D009414:Nerve Growth Factors / Q000096:biosynthesis* / Q000378:metabolism; D010006:Osteoblasts / Q000378:metabolism*; D015675:Osteocalcin / Q000378:metabolism; D010013:Osteogenesis Imperfecta / Q000000981:diagnostic imaging / Q000235:genetics*; D055550:Protein Stability; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D011859:Radiography; D015203:Reproducibility of Results; D017422:Sequence Analysis, DNA; D015843:Serpins / Q000096:biosynthesis* / Q000378:metabolism; D055815:Young Adult; D000099174:Pigment Epithelium-Derived Factor
chemical_list: D000469:Alkaline Phosphatase; D024042:Collagen Type I; D018076:DNA, Complementary; D005136:Eye Proteins; D008565:Membrane Proteins; D009414:Nerve Growth Factors; D015675:Osteocalcin; D012333:RNA, Messenger; D015843:Serpins; D000099174:Pigment Epithelium-Derived Factor; C575746:IFITM5 protein, human