Type V OI primary osteoblasts display increased mineralization despite decreased COL1A1 expression.
BACKGROUND
Patients with type V osteogenesis imperfecta (OI) are heterozygous for a dominant IFITM5 c.-14C>T mutation, which adds five residues to the N terminus of bone-restricted interferon-induced transmembrane-like protein (BRIL), a transmembrane protein expressed in osteoblasts. Type V OI skeletal findings include hyperplastic callus formation, ossification of the forearm interosseous membrane, and dense metaphyseal bands.


OBJECTIVE
The objective of this study was to examine the role of osteoblasts in the active mineralization traits of type V OI and the effect of the IFITM5 mutation on type I collagen.


METHODS
We identified eight patients with the IFITM5 c.-14C>T mutation. Cultured osteoblasts from type V OI patients were used to study osteoblast differentiation and mineralization.


RESULTS
We verified the expression and stability of mutant IFITM5 transcripts. In differentiated type V OI primary osteoblasts in culture, the IFITM5 expression and BRIL level is comparable with control. Both early and late markers of osteoblast differentiation are increased in type V OI osteoblasts. Mineralization, assayed by alizarin red staining, was increased in type V OI osteoblasts compared with control. However, type V OI osteoblasts have significantly decreased COL1A1 transcripts in mid- to late differentiation. Type I collagen protein is concomitantly decreased, with decreased cross-linked collagen in matrix and altered appearance of fibrils deposited in culture.


CONCLUSIONS
This study demonstrates that type V OI mineralization has a gain-of-function mechanism at the osteoblast level, which likely underlies the overactive tissue mineralization seen in patients. Decreased type I collagen expression, secretion, and matrix incorporation establish type V OI as a collagen-related defect.
Bone and Extracellular Matrix Branch (A.R., A.S.B., A.M.B., W.A.C., J.C.M.), Eunice Kennedy Shriver National Institute of Child Health and Human Development, and Department of Diagnostic Radiology (S.C.H.), National Institutes of Health Clinical Center, National Institutes of Health, Bethesda, Maryland 20892; Physiology and Experimental Medicine Program (A.H.), Heart Center, Hospital for Sick Children, University of Toronto, Ontario, Canada M5S 3OA4; Division of Diagnostic Imaging (J.S.), Department of Pediatrics, and Division of Clinical and Metabolic Genetics (D.C.), Department of Pediatrics, Hospital for Sick Children, University of Toronto, Toronto, Ontario, Canada M5G 1X8; and The Prenatal Diagnosis and Medical Genetics Program (D.C.), Department of Obstetrics and Gynecology, Mt Sinai Hospital, University of Toronto, Toronto, Ontario, Canada M5G 1Z5.
Adi
A
Reich
Alison S
AS
Bae
Aileen M
AM
Barnes
Wayne A
WA
Cabral
Aleksander
A
Hinek
Jennifer
J
Stimec
Suvimol C
SC
Hill
David
D
Chitayat
Joan C
JC
Marini
D000328:Adult; D000368:Aged; D002114:Calcinosis / Q000235:genetics / Q000378:metabolism / Q000473:pathology*; D002675:Child, Preschool; D024042:Collagen Type I / Q000235:genetics* / Q000378:metabolism; D000090602:Collagen Type I, alpha 1 Chain; D005260:Female; D006801:Humans; D008297:Male; D008565:Membrane Proteins / Q000235:genetics* / Q000378:metabolism; D008875:Middle Aged; D009154:Mutation; D010006:Osteoblasts / Q000378:metabolism / Q000473:pathology*; D010013:Osteogenesis Imperfecta / Q000235:genetics / Q000378:metabolism / Q000473:pathology*; D055815:Young Adult
D024042:Collagen Type I; D000090602:Collagen Type I, alpha 1 Chain; C575746:IFITM5 protein, human; D008565:Membrane Proteins