Elevated Fibroblast Growth Factor Signaling Is Critical for the Pathogenesis of the Dwarfism in Evc2/Limbin Mutant Mice

Abstract

Ellis-van Creveld (EvC) syndrome is a skeletal dysplasia, characterized by short limbs, postaxial polydactyly, and dental abnormalities. EvC syndrome is also categorized as a ciliopathy because of ciliary localization of proteins encoded by the two causative genes, EVC and EVC2 (aka LIMBIN). While recent studies demonstrated important roles for EVC/EVC2 in Hedgehog signaling, there is still little known about the pathophysiological mechanisms underlying the skeletal dysplasia features of EvC patients, and in particular why limb development is affected, but not other aspects of organogenesis that also require Hedgehog signaling. In this report, we comprehensively analyze limb skeletogenesis in Evc2 mutant mice and in cell and tissue cultures derived from these mice. Both in vivo and in vitro data demonstrate elevated Fibroblast Growth Factor (FGF) signaling in Evc2 mutant growth plates, in addition to compromised but not abrogated Hedgehog-PTHrP feedback loop. Elevation of FGF signaling, mainly due to increased Fgf18 expression upon inactivation of Evc2 in the perichondrium, critically contributes to the pathogenesis of limb dwarfism. The limb dwarfism phenotype is partially rescued by inactivation of one allele of Fgf18 in the Evc2 mutant mice. Taken together, our data uncover a novel pathogenic mechanism to understand limb dwarfism in patients with Ellis-van Creveld syndrome.

Achondroplasia: Development, pathogenesis, and therapy

“…Treatment of primary chondrocytes with PTH(1-34) suppressed expression of Fgfr3 (Zhang et al, 2016) as did injection of PTH(1-34) in vivo (Karuppaiah et al, 2016). Although not investigated in chondrocytes, Fgfr3 expression was induced by hypoxia in a transcriptional and HIF1α-dependent manner in bladder cancer cells (Blick et al, 2013).…”

A Ciliary Protein EVC2/LIMBIN Plays a Critical Role in the Skull Base for Mid-Facial Development

“…In this study, we found loss of function of Evc2 results in premature closure of the ISS before P8 in Evc2 global mutants and in Evc2 Wnt1 mutants. We previously reported that loss of function of Evc2 results in severe chondrodysplastic dwarfism; however, we never observed premature closure of appendicular growth plates through adult stages ( Zhang et al, 2016a ). Those data suggest that a function of Evc2 to maintain cartilage is different between appendicular growth plates and synchondroses.…”

“…We and others have used both Evc and Evc2 mutant mice to characterize different aspects of abnormal development ( Ruiz-Perez et al, 2007 ; Caparros-Martin et al, 2013 ; Zhang et al, 2015 ). Generating our own Evc2 mutant mice enabled us to characterize the pathological mechanisms leading to abnormal appendicular bone and hypomorphic enamel development ( Zhang et al, 2015 , 2016a , b ). Our subsequent studies further demonstrated the expression of Evc2 in the mid-facial regions ( Badri et al, 2016a ) and mid-facial defects in Evc2 global mutant mice ( Badri et al, 2016b ).…”

Molecular and Cellular Pathogenesis of Ellis-van Creveld Syndrome: Lessons from Targeted and Natural Mutations in Animal Models

“…This is further supported by comparisons of mRNA levels of Gli1 and Ptch1 between multiple tissues between control and Evc or Evc2/Limbin mutant mice [ 10 , 25 ]. As pointed out by Zhang et al [ 25 ], the partially compromised, instead of largely attenuated, Hedgehog signaling is the likely reason leading to normal lips, palate and neural tube development in humans affected by EVC and in Evc or Evc2/Limbin mutant mice [ 9 , 10 , 25 ], as summarized in Table 1 . On the other hand, simply believing that the partially compromised Hedgehog signaling is the reason leading to the abnormal phenotypes in mutant mice or signs in humans may lead to an inaccurate understanding of the pathological mechanism.…”

“…It should be noted that multiple lines of evidence suggest that EVC or EVC2/LIMBIN loss of function only partially compromises Hedgehog signaling [ 9 , 11 , 25 ]. This critical detail is supported by both in vitro molecular studies and in vivo phenotypic studies.…”