It has been previously suggested that mutations in the chaperone protein FKBP10
could be responsible for OI type VI (MIM #610968); this was based upon a focal abnormal lamellar pattern that was observed in a single bone biopsy sample from a severe OI patient carrying a mutation in this gene.12
However, no mutations in FKBP10
were identified in our patient cohort that was classified based on the diagnostic requirement for OI type VI of having an excess of unmineralized osteoid tissue, indicative of a mineralization defect in addition to a fish-scale pattern in their bone matrix. To the best of our knowledge, the reported patients with FKBP10
mutations do not have the distinct pathognomonic histological features of OI type VI and instead, have a diagnosis of severe OI.14
In our report, next-generation sequencing of the homozygous region from a large consanguineous family with OI type VI identified a stop mutation in SERPINF1
in patient V-1, and Sanger sequencing of this candidate gene revealed the same stop mutation in the related patient IV-3. We also identified a homozygous 4-bp duplication in an unrelated OI type VI patient. Heterozygosity was confirmed in the parents of all three described cases. At the functional level, qRT-PCR suggests that the mutant transcript undergoes NMD, which is supported by undetectable PEDF levels in the patients' serum. Recently, Becker and colleagues16
mutations in patients diagnosed with severe OI type III, but bone biopsies were not available in these cases and therefore it is not known whether these patients had the bone histological features of OI type VI. Our work suggests that OI type VI is the specific clinical consequence of PEDF loss of function.
PEDF was originally isolated from the conditioned medium of cultured primary human fetal retinal pigment epithelial cells and functions as both an antiangiogenic factor and a neurotrophic and cell differentiation factor.17
In bone, it has also been reported to upregulate osteoprotegerin, which inhibits osteoclast maturation by blocking RANKL-mediated osteoclast precursor proliferation and differentiation.18
Because our patients have loss of function mutations in SERPINF1
, lower osteoprotegerin levels might result in increased numbers of osteoclasts. In accordance with this hypothesis, there is some evidence of increased bone resorption in these patients.14
If loss of PEDF resulted only in an increase in the number of mature osteoclasts, one would expect bisphosphonate therapy to be beneficial in disease management.19
However, OI type VI patients do not appear to respond as well as other OI patients to bisphosphonate therapy, suggesting that PEDF may have additional functions in maintaining bone homeostasis, specifically in the regulation of osteoid mineralization.13
PEDF has been shown to inhibit the downstream actions of vascular endothelial growth factor (VEGF), a protein expressed by chondrocytes during endochondral bone formation.20
VEGF stimulates blood vessel formation and allows the migration of osteoblasts and osteoclasts to the sites of bone deposition.17
However, PEDF is also secreted by osteoblasts and to a lesser extent, osteoclasts.17
Associating with type I collagen in the extracellular matrix, it may serve as a potent angiogenesis inhibitor.21
PEDF binds to type I collagen near the α1β2 and the α1β1 integrin binding site, suggesting that PEDF could alter integrin-collagen interactions, which have been shown to play a role in cell adhesion and, importantly, angiogenesis.21
Because PEDF is a potent antiangiogenic factor, it represents a promising tumor suppressor agent, and the metastases of many tumor types in mouse models are inhibited by infusion of recombinant PEDF. 22-24 Given that recombinant PEDF is available and that OI type VI patients respond poorly to bisphosphonates, systemic infusion of PEDF might constitute a viable therapeutic approach for OI type VI. At the same time, modulation of PEDF in the context of cancer and/or angiogenesis may have off target effects in the skeleton and should be addressed in future clinical trials. Finally, our data suggest that screening for PEDF levels in serum may be useful for diagnosing OI type VI patients.
Taken together, these data identify a previously undescribed mechanism for the pathogenesis of dysregulated bone mineralization in OI and, importantly, suggest that mutations in an extracellular secreted morphogen and/or signaling protein can contribute to a heritable connective tissue disorder.